United States
          Environmental Protection
             Office of Research and
             Washington DC 20460
September 2001
Performing Quality Flow
Measurements at
Mine Sites

                                                                March 2001
Performing Quality Flow Measurements
                     at Mine Sites
                   Science Applications International Corporation
                     Environment and Health Sciences Group
                           Idaho Falls, ID 83401
                        EPA Contract No. 68-C-98-006
                           Task Order 14 & 29
                       Sustainable Technology Division
                  National Risk Management Research Laboratory
                           Cincinnati, Ohio 45268
                  National Risk Management Research Laboratory
                      Office of Research and Development
                     U.S. Environmental Protection Agency
                           Cincinnati, OH 45268

Quality Flow Measurements at Mine Sites
       The U.S. Environmental Protection Agency through its Office of Research and Development
funded and managed the compilation of information included in this guidebook under Contract
69-C-98-006 to Neptune and Co. It has been subjected to the Agency's peer and administrative
review and has been approved for publication as an EPA document. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.

Quality Flow Measurements at Mine Sites
       The U.S. Environmental Protection Agency is charged by Congress with protecting the
Nation's land, air, and water resources. Under a mandate of national environmental laws, the Agency
strives to formulate and implement actions leading to a compatible balance between human activities
and the ability of natural systems to support and nurture life. To meet this mandate, EPA's research
program is providing data and technical support for solving environmental problems today and building
a science knowledge base necessary to manage our ecological resources wisely, understand how
pollutants  affect our health, and prevent or reduce environmental risks in the future.

       The National Risk Management Research Laboratory is the Agency's center for investigation
of technological and management approaches for preventing and reducing risks from pollution that
threatens human health and the environment.  The focus of the Laboratory's research program is on
methods and their cost-effectiveness for prevention and control of pollution to air, land, water, and
subsurface resources; protection of water quality in public water systems; remediation of contaminated
sites, sediments and ground water; prevention and control of indoor air pollution; and restoration of
ecosystems. NRMRL collaborates with both public and private sector partners to foster technologies
that reduce the cost of compliance and to anticipate emerging problems.  NRMRL's research provides
solutions to environmental problems by: developing and promoting technologies that protect and
improve the environment; advancing scientific and engineering information to support regulatory and
policy decisions; and providing the technical support and information transfer to ensure implementation
of environmental regulations and strategies at the national, state, and community levels.

       This publication has been produced as part of the Laboratory's strategic long-term research
plan.  It is published and made available by EPA's Office of Research and Development to assist the
user community and to link researchers with their clients.

                                           E. Timothy  Oppelt, Director
                                           National Risk Management Research Laboratory

Quality Flow Measurements at Mine Sites
       Accurate flow measurement data is vital to research, monitoring, and remediation efforts at
mining sites. This guidebook has been prepared to provide a summary of information relating to the
performance of flow measurements, and how this information can be applied at mining sites.
Information presented in this guidebook includes the theory, methods, selection criteria for these
methods, and quality assurance/quality control (QA/QC) guidance for performing flow measurements at
mining sites.

Quality Flow Measurements at Mine Sites
                              TABLE OF CONTENTS






             2.1.1   Gage Datum	3
             2.1.2   HEAD 	4
             2.1.3   STAGE	4
             2.1.4   VELOCITY	4
             2.1.5   Discharge	7
             2.2.1   Continuity Equation	8
             2.2.2   Energy Equation	9
             2.2.3   Momentum Equation	11

      3.1     GAGES	13
             3.1.1   General Criteria for Selecting Sites to Establish Gages	13
             3.1.2   Non-Recording Gages	14
             3.1.3   Recording Gages	16
      3.2     WEIRS	18
             3.2.1   Siting Criteria for Weirs	19
             3.2.2   Sharp-Crested Weirs	19
             3.2.3   Broad-Crested Weirs	24
      3.3     FLUMES	25
             3.3.1   Siting Criteria for Flumes  	26
             3.3.2   Long-Throated Flumes	27

Quality Flow Measurements at Mine Sites
             3.3.3  Short-Throated Flumes  	29

      3.4    CURRENT METERS	34
             3.4.1  Siting Criteria for Current Meters	34
             3.4.2  Types of Current Meters  	34
             3.4.3  Methods to Determine Flow Velocity	36
             3.4.4  Computing Stream Discharge	37
             3.5.1  Siting Criteria for Acoustic Velocity Meters  	38
             3.5.2  Types of Acoustic Velocity Meters	39
             3.6.1  Siting Criteria and Sources of Error 	40
             3.6.2  Types of Tracers	41
             3.6.3  Methods to Determine Stream Discharge  	43

             4.3.1  Measuring Discharge From Adits	59
             4.3.2  Measuring Discharge From Tailings Ponds	61
             4.3.3  Measuring Discharge in Remote Areas with Unstable Channel
                      Conditions 	62

        MEASUREMENTS  	63
               ASSURANCE PROJECT PLAN	63
      5.2    QA/QC COMPONENTS 	63
             5.2.1  QA Management	63
             5.2.2  Quality Assurance Objectives	64
             5.2.3  Measurement and Data Acquisition	69
             5.2.4  Assessment and Oversight  	70
             5.2.5  Data Validation, Usability, and Reporting	72



Quality Flow Measurements at Mine Sites
                                    LIST OF FIGURES
Figure 2-1.  Typical Open Channel Velocity Profile	6
Figure 2-2.  Typical Velocity Distributions for Several Channel Profiles	6
Figure 2-3.  Graphical Representation of the Terms in Bernoulli's Equation for an Open
              Channel	9
Figure 2-4.  Typical Specific Force plus Momentum Curve	11
Figure 3-1. Sharp-crested Weirs	19
Figure 3-2. Flat-crested, Long-throated Flume  	27
Figure 3-3.  Configuration and Proportions for Parshall Flumes	29
Figure 3-4.  Configuration and Proportions of Type Hs, H, and Ffl Flumes	33
Figure 3-5.  Lateral Mixing and Longitudinal Dispersion Patterns and Changes in
              Concentration Distributions from a Single, Slug Injection of a Tracer	40
                                    LIST OF TABLES
Table 2-1. Typical Values for Manning's n	5
Table 4-1. Water Measurement Device Selection Guidelines	52
Table 4-2. Water Measurement Device Selection Criteria  	57
Table 4-3. Water Measurement Device Selection; Ranking of Factors	58
Table 5-1. Accuracy and Precision Considerations for Mine Flow Measurement Methods	67
Table 5-2. Sensitivity Limitations for Different Mine Flow Measurement Methods	69
Table 5-3. Example QC Objectives for Specific Measurements	70
Table 5-4. Typical QC Objectives for Specific Measurement Device	70

Quality Flow Measurements at Mine Sites
                                 LIST OF ACRONYMS
Acoustic Doppler Current Profilers
Acoustic Doppler Velocimeters
Acoustic Velocity Meter
Best Management Practices
Bureau of Reclamation
Clean Air Act
Comprehensive Environmental Response, Compensation, and Liability Act
Cubic Feet per Second
Cubic Meters per Second
Clean Water Act
Energy Grade Line
U.S. Environmental Protection Agency
Feasibility Studies
Feet per Second
Square Feet
Gallons per minute
Hydraulic Grade Line
Liters per Second
Laser Doppler Velocimeters
lithium bromide
Method Detection Limits
Mine Waste Technology Program
Sodium Bromide
Sodium Chloride
National Environmental Policy Act
National Pollutant Discharge Elimination System
U.S. Natural  Resource Conservation Service
National Risk Management Research Laboratory
Quality assurance
Quality Assurance/Quality Control
Quality Assurance Project Plan
Quality Control

Quality Flow Measurements at Mine Sites
                            LIST OF ACRONYMS (continued)
RI/FS            Remedial Investigations/Feasibility Studies
RIs              Remedial Investigations
RPD             Relative Percent Difference
RSD             Relative Standard Deviation
SCS             Soil Conservation Service
SME            Society of Mining Metallurgy and Exploration
TMDL           Total Maximum Daily Loads
USGS            United States Geological Survey

Quality Flow Measurements at Mine Sites

      Mining projects have historically impacted the quantity, velocity, and timing of surface water flow
by altering natural drainage patterns and the infiltration/runoff relationships in a watershed. Mine
structures, such as waste rock dumps, tailings impoundments, leach facilities, and process facilities
potentially impact natural stream systems through discharges of storm water and wastewater,
impounding water, changing the character of gaining and losing stream reaches through mine
dewatering, mining through stream channels and flood plains, and by diverting, re-routing, and
channelizing streams. Waste dumps, tailings impoundments, mine pits and other facilities often become
permanent features of the post-mining landscape that can cause fundamental changes in the  physical
characteristics and behavior of a watershed (O'Hearn,  1997). Measurement of mine facility
discharges, therefore, are critical to understanding environmental impacts created by post-mining


      The U.S. Environmental Protection Agency (EPA) is responsible for ensuring that mining
operations do not adversely impact the environment during construction, operation, or following mine
closure. EPA's authority is granted under provisions of the National Environmental Policy Act
(NEPA), the Clean Water Act (CWA), and the Clean Air Act (CAA).  The EPA National  Risk
Management Research Laboratory (NRMRL), Sustainable Technology Division, houses the Mine
Waste Technology Program (MWTP). As a part of this program, both basic and applied research
projects are conducted relating to the treatment of wastes associated with or generated by mines.
Environmental monitoring programs, as well as research projects, require accurate measurements of
discharges from mine facilities and of stream flows both above and below mine sites. Accurate flow
measurements provide the foundation for successful environmental monitoring programs and research
designed to minimize, reduce or prevent pollutant loading from mine facilities.  Accurate  flow
measurements are critical to analyze potential short- and long-term impacts to site hydrology, water
quality, aquatic biota and ecosystems.

      The purpose of this guidebook is to outline and provide comparative information of different
methods used to measure stream flow or mine facility discharges. This guidebook presents  tools
appropriate for selecting measurement methods appropriate for the needed application.  The guidebook
is designed for personnel and engineers responsible for developing plans of operation, conducting site
characterization studies and research of hydrologic and water quality systems, designing hydrologic
control structures, designing Best Management Practices (BMPs), or developing environmental
monitoring programs at mine sites.

Quality Flow Measurements at Mine Sites

      The guidebook is divided into five separate sections.  Section 2.0 of this guidebook describes
basic concepts related to flowing water and measurement. This section has been provided because a
general understanding of these concepts is extremely important in choosing an appropriate method to
obtain flow measurements, determining appropriate sites for measurement, and in implementing the
method in the field.  Section 3.0 provides descriptions of accepted methods used to measure flow,
describes siting criteria for measurements, and provides useful information for understanding when to
use and or not to use a particular measurement.  Section 4.0 provides a comparison of methods,
including selection criteria, thereby summarizing some of the information in Section 3.0 in order to assist
with selection of the appropriate measurement method. Section 4.3 provides a discussion of some
typical scenarios at mine sites where flow measurements are required and recommends methods which
could be used. A discussion of Quality Assurance/Quality Control (QA/QC), specific to mine flow
measurements, is provided in Section 5.0. Appendix A provides background information describing
types of mine facilities, potential impacts to site hydrology, water quality, and aquatic resources, and the
importance of flow measurements to other EPA programs and site characterization studies. This
appendix has been included because background knowledge of typical mine features, waste facilities
and dumps, and process operations is important when outlining objectives for flow and water quality
studies and in choosing flow measurement methods.

Quality Flow Measurements at Mine Sites

      Open channel flow is defined as flow in any channel where the liquid flows with a free surface.
Open channel flow is not under pressure; gravity is the only force that can cause flow in open channels
and a progressive decline in water surface elevation always occurs as the flow moves downstream
(BOR, 1997). Examples of open channel flow at mine sites include: rivers, streams, creeks, discharges
from tailings ponds, and other uncovered conduits. Closed channels,  such as adits, tunnels, and
ventilation shafts, can be treated as open channels when flowing partially full and not under pressure.

      The purpose of Section 2.0 is to briefly introduce the reader to basic terminology and concepts
related to open channel flow to allow for a simpler and more accurate presentation of the flow
measurement techniques and methods in Section 3.0. A working knowledge of these concepts and
relationships is extremely important in selecting the appropriate measurement tool as well as siting,
calibrating, and collecting data from the chosen measurement device.  This section is included as a
supplement to this document and offers background information that some readers may already
understand. It is not meant as  a complete discussion of each concept but more as a summary
discussion. Cited references offer more in depth explanations if desired.


      Most open channel water measuring devices or methods calculate stream discharge from a
combination of head, stage, and velocity measurements with respect to a common reference point.
These terms are described below:

2.1.1  Gage Datum

      Gage datum is a common elevation selected as a reference point for subsequent measurements
and calculations pertaining to gage installation, calibration, and operation. The datum may be a
recognized datum (e.g., mean sea level) or an arbitrary datum chosen for the convenience of measuring
gage heights in relatively low numbers. When using an arbitrary datum, the datum selected for gage
operations should be below the zero flow, or no flow elevation to eliminate the possibility of negative
gage heights (Buchanan and Somers, 1982).

      A permanent datum should be maintained to ensure that the gage-height record uses the  same
datum for the  life of a gage.  The permanent datum can be maintained by establishing two or three
reference marks that are independent of the gage.  The reference marks are periodically checked to
make sure the datum is fixed in the same location.  Establishing reference marks independent of the
gage allows the datum to be reestablished if the gage is damaged or destroyed (Buchanan and Somers,

Quality Flow Measurements at Mine Sites
2.1.2 HEAD

      Head is an engineering term frequently used in water measurement equations and practice.
Under open channel flow conditions, head is the difference in elevation, relative to a specific datum,
between the water surface elevation at locations upstream and downstream of the water measurement
location. The resulting pressure on the fluid at the downstream point is expressible as the elevational
difference, or head.  Head can also be expressed in terms of differences in pressure.

2.1.3 STAGE

      Stage is the height of the water surface above an established datum plane.  Stage measurements
are often used or incorporated into calculations to determine stream discharge within a particular
channel reach. Stage is typically measured with a staff gage, a fixed scale measuring device installed in
a primary  measuring device (Section 3.0) or in an open channel reach where the channel configuration
and channel geometry is well-defined. Staff gages are often mounted vertically; however, greater
accuracy can be  obtained by inclining the staff so that the graduations are larger for a given change in
water surface elevation (Grant and Dawson, 1997).  The water surface elevation (i.e., stage) read from
the staff gage is commonly called the gage-height.


      The Manning equation relates velocity to total bed resistance or friction to calculate flow velocity
(F).  The equation balances the gravitational acceleration of water in an inclined, open channel against
surface area and  bed roughness.  The Manning equation is intuitively appealing because of its simple

        V  =  flow velocity, feet per second
        n  =  Manning roughness coefficient
        R  =  hydraulic radius, feet
        S  =  longitudinal slope, feet per foot

      Manning's n is a dimensionless number that defines the flow resistance of a unit of bed surface.
Resistance is a function of particle  size, bed shape, and constructional bed forms (e.g., ripples).

Quality Flow Measurements at Mine Sites
Manning's n incorporates many physical factors including the channel roughness, irregularity of the
channel cross section, channel alignment and bends, vegetation, sedimentation, scouring, and channel
obstructions (Barfield et al, 1981).  Table 2-1 presents a list of commonly used values for Manning's n.

                          Table 2-1. Typical Values for Manning's n
              Type and Description of Conduits
Earth bottom, rubble sides
Drainage ditches, large, no vegetation
<2.5 hydraulic radius
2.5 - 4.0 hydraulic radius
4.0-5.0 hydraulic radius
>5.0 hydraulic radius
Small drainage ditches
Stony bed, weeds on bank
Straight and uniform
Winding, sluggish



(A) Clean, straight bank, full stage, no rifts or deep pools
(B) Same as (A) but some weeds and stones
(C) Winding, some pools and shoals, clean
(D) Same as (C), lower stages, more ineffective slopes and
(E) Same as (C), some weeds and stones
(F) Same as (D), stony sections
(G) Sluggish river reaches, rather weedy or with very deep
(H) Very weedy reaches


Quality Flow Measurements at Mine Sites
SOURCE: Barfield et al. (1981)

      Hydraulic radius (R) can be approximated for parabolic channels where the water surface width
is »than the depth of the water as:
                                           R= -d

where d is the average depth. For channel geometries that approximate trapezoidal or rectangular
cross sections and where the bottom width is »than the average depth, R is approximately equal to
average depth (d). The hydraulic radius of a stream with a triangular cross section can be
approximated as 0.5d (Barfield et al, 1981).  Uncertainties associated with the Manning equation can
be minimized by basing the variables on accurately measured data. Specifically, the channel cross
section should be surveyed to obtain accurate measurements of width,  depth, and hydraulic radius.

      The actual distribution of flow velocity is generally quite complex. Open channel flow is often
laminar or near-laminar, with the different layers moving at different velocities.  Flow velocity at the
contact point with the channel boundary is low (Barfield et al., 1981).  Typically, the highest velocity
flow is located in the center of the flow channel and slightly below the water surface. Figures 2-1 and
2-2 present typical velocity profile and a typical vertical velocity distributions under open channel flow
conditions.  A general knowledge of velocity distributions is extremely important in  evaluating and
selecting a method of flow measurement.  Sites with irregular or complicated channel geometries, such
as meanders or riffle areas, can cause a decrease in measurement accuracy when using methods that
rely on velocity measurements to calculate discharge.  These methods and factors associated with the
proper siting for measurements are described in Section 3.0.
                             /   /   /    f  f  f    iff  r
                      Figure 2-1.  Typical Open Channel Velocity Profile
                                     (Barfield et al.,  1981)

Quality Flow Measurements at Mine Sites

                                    trrtflulor  Chono*l
           Figure 2-2. Typical Velocity Distributions for Several Channel Profiles
                                    (Barfield et al., 1981)
2.1.5 Discharge

      Discharge is the volume of water per unit time flowing past a set point or station.  Open channel
discharge is commonly reported in cubic feet per second (cfs) or cubic meters per second (cms).
Gallons per minute (gpm) is the common unit of measure used in studies to evaluate and predict mine
site water balances. Gpm is also the common measurement unit for reporting industrial and wastewater
treatment plant discharges.  Many discharge measurements made at mine sites and from mine facilities,
therefore, are converted to gpm in order to evaluate their association with the water balance for the

Quality Flow Measurements at Mine Sites
      A series of discharge measurements made at a gaging station is often used to define a discharge
rating curve for a site.  A discharge rating curve may be a simple relationship between stage and
discharge, or a more complicated relationship that includes stage, slope, rate of stage change, and other
factors (Carter and Davidian, 1968). Most methods and devices for measuring flow are designed to
calculate stream discharge.

      Discharge  (Q) is generally expressed in cfs and is calculated from:

                                              Q" VA

where V is the average flow velocity at a cross section, in feet per second (ft/s), and A is the area of
that cross section, in square feet (ft2) (Barfield et al., 1981).  The Manning equation (Section 2.1.4) can
be used to estimate average flow velocity (F).  To obtain discharge (0 in units besides cfs, the
constant of 1.49 in the Manning equation can be changed to 669 if discharge (Q) will be reported in
gpm, or 1.00 for liters per second (1/s) (Grant and Dawson, 1997). The primary measuring devices
presented in Section 3.0, however, provide more accurate estimates of discharge than can be obtained
using the above equation combined with estimates of average velocity obtained by the Manning


      Open channel flow occurs under one of three possible flow conditions: sub-critical; critical; or
super-critical. Three basic relationships govern open channel flow: the continuity equation, the
momentum equation, and the energy equation.  Each of the relationships is briefly described in the
following sub-sections. The reader is encouraged to consult some of the hydrology and hydraulic
engineering texts listed in the reference section for more information.

2.2.1 Continuity Equation

      The continuity equation is a simple mass balance and can be written as:

                                 Inflow* Outflow* AStorage
where inflow represents the volume or rate of flow across an upstream cross-section during time t and
outflow is the volume or rate of flow across a downstream cross section during time t. The change in
storage (Astorage) is the rate or volume at which water is accumulating or diminishing within the section.

Quality Flow Measurements at Mine Sites
2.2.2 Energy Equation

      The energy equation, also known as Bernoulli's theorem or equation, is given by:

       V   = average flow velocity, feet per second
       g    = gravitational constant, 32.2 feet per second squared
       y    = depth of flow, feet
       z    = elevation of the channel bottom above some datum point, feet
       p    = pressure, pounds per square foot
       a    = unit weight of water, 62.4 pounds per cubic foot
       hL !_2 = represents the energy loss between section 1 and 2, feet

      Bernoulli's equation, which represents an energy balance between two points along a channel, is
graphically depicted in Figure 2-3.
                                                              L 1-2
              Figure 2-3.  Graphical Representation of the Terms in Bernoulli's
                               Equation for an Open Channel
                                   (Barfield et al., 1981)

Quality Flow Measurements at Mine Sites
      Each complete term in the equation has the units of length and each term is associated with a
"head." The term V2/2g is the velocity head, y + z is the elevation head, and p/a is the pressure head
(Barfield et al, 1981).  Total energy is the sum of the velocity head, pressure head, and elevation head
and is represented by the energy grade line (EGL) in Figure 2-3. The hydraulic grade line (HGL) is the
sum of the elevation head and the pressure head.  In open channel flow, the free water surface is
exposed to the atmosphere so the pressure head is equal to zero. The water surface is represented by
the HGL under open channel flow conditions.

      Bernoulli's equation can be used to derive the Froude number (Fr) for a given stream reach.  The
Froude number is a dimensionless number defining the type or degree of water turbulence in a reach.
The Froude number, the ratio of the inertia force to the force of gravity, can be used to distinguish
between sub-critical, critical and super-critical flows. Critical flow occurs when the Froude number is
unity (i.e., 1), inertia! forces are equal to gravitational forces. Critical flow is unstable, tending towards
one of the other two conditions.  A Froude number less than unity (i.e., inertial forces are less than
gravitational forces) indicates sub-critical flow. Sub-critical flow is laminar with each element of fluid
moving in approximate parallel paths with uniform velocity. A Froude number greater than unity (i.e.,
inertial forces are greater than gravitational forces) indicates super-critical flow.  Super-critical flow is
turbulent, characterized by breaking surface waves and increased resistance to flow.  The Froude
number associated with a given reach has important implications for flow measurement, and  sediment
loading, transport, and erosion.  Flow measurements are typically taken in reaches with sub-critical or
critical flow. Very few methods can accurately measure stream discharge in stream reaches with super-
critical flow. The Froude number (Fr) can be calculated from:
where V is velocity (fps), g is the gravitational constant, and hm is hydraulic mean depth (ft). Open
channel flow measurement generally requires Fr of the approach flow to be less than 0.5.  Sub-critical
approach flows avoid wave action that could hinder or prevent accurate flow readings (BOR, 1997).

      Experience and knowledge regarding sub-critical, critical, and super-critical flow are extremely
important in determining a method for measuring discharge and in siting a location for measurement.
For example, water measurement flumes function best when flow is forced through the flume at a depth
where flow is critical. At critical depth, discharge can be measured using one upstream head
measurement station. Moreover, calibration of weirs and flumes is simplified because these
measurement devices have one unique head value for each discharge (BOR, 1997).

Quality Flow Measurements at Mine Sites
2.2.3 Momentum Equation

      The momentum principle states that the sum offerees in a given direction is equal to the change in
momentum in that direction. M is a constant that represents the specific force plus momentum.  In a
short reach where frictional resistance is insignificant and the channel slope is small (i.e., the sine of the
channel slope approaches zero), M can be derived from (Barfield et al, 1981):
       M =  specific force plus momentum constant
       y  =  depth of flow, feet
        V  =  average flow velocity, feet per second
       g  =  gravitational constant, 32.2 feet per second squared
       q  =  flow rate per unit of width, ft2/sec or cfs/ft

      Figure 2-4 is a graphical representation of a plot of depth (y) versus M for a constant q. Every
Mhas two possible depths and a definite minimum. At the minimum M(yc on Figure 2-4), specific
energy is minimum and only a single flow depth occurs. This condition is referred to as critical flow (y ).
                                    Mc   M
                  Figure 2-4. Typical Specific Force plus Momentum Curve
                                    (Barfield et al., 1981)

Quality Flow Measurements at Mine Sites
Super-critical flow (y2) occurs when depth is less than critical and velocity is greater than the critical
condition. When depth is greater than critical and velocity is less than critical the flow is termed sub-
critical (yi) (Roberson et al, 1988). As previously noted, experience and knowledge regarding sub-
critical, critical, and super-critical flow are extremely important in determining a method for measuring
stream discharge.

Quality Flow Measurements at Mine Sites

      The stage (i.e., height) of water in a stream can be readily measured at some point along a stream
reach with a staff gage or water level recorder.  Conversion of stage to a discharge rate, or quantity of
flow per unit time, can then be accomplished by stream gaging or the use of precalibrated flow
measurement devices. Section 3.1 presents the use of gages for measuring and recording stage in
relation to calculating discharge.  Sections 3.2 and 3.3 describe the use of pre-calibrated weirs and
flumes, including the appropriate use for each measurement method, and describes how the
measurements are made.  The remaining sections describe non-structural flow measurement devices,
such as current meters, acoustic flow meters, and the application of tracers and dye dilution methods.
Each section, in addition to a description of the measurement devices, discusses siting criteria for the
particular devices, the importance of site selection for obtaining accurate measurements, and the
applicability of the method.

3.1   GAGES

      The gage is the part of the water measurement structure (e.g., weir, flume) that measures stage
height, or stage height and velocity, for input into the equation to calculate discharge.  Gages can also
exist separate from a physical structure. A staff gage, for example, could be attached to a bridge piling
and use the channel itself as the measuring structure.

      Gages are either non-recording or recording types. Recording gages keep track of stage levels
at preset intervals and non-recording gages require an observer to read stage height from the gage.
Non-recording gages have the advantage of low initial costs and relatively simple installation. However,
non-recording gages require an field observer to take regular measurements. Over time, accuracy and
precision may be enhanced with a recording  gage. Recording gages eliminate the need for regular field
observation and allow collection of reliable, long-term stage  records. Non-recording gages are
discussed in Section 3.1.2 and recording gages in Section 3.1.3.

3.1.1 General Criteria for Selecting Sites to Establish Gages

      Water measurement sites are typically selected after a thorough reconnaissance of the area;
including examination of geologic, topographic, and other maps of the area.  Tentative sites are marked
on maps and the stream characteristics are examined in the field. Carter and Davidian (1968) outline
basic factors to be considered when  choosing a site for measuring flow.  These basic factors  are
general siting considerations that are applicable to all measurement approaches discussed in this

Quality Flow Measurements at Mine Sites
            Channel characteristics should be such that a fixed and permanent relationship can be
            established between stage and discharge. Good measurement sites are often located in
            reaches with critical or sub-critical flow (e.g., immediately upstream of riffles or falls).

            Sites that experience backwater conditions should be avoided. Poor measurement
            conditions occur when backwater submerges measurement structures.  Backwater
            conditions can result from downstream tributaries, lakes, channel constrictions, tailings
            ponds, storm water ponds or other sources.

      •      The channel cross section should be known or measurable by survey, and
            geomorphologically  stable to ensure quantification of an accurate stage-discharge

            Flow should not bypass the site in ground water (i.e., intermittent conditions), or side
            channels. Measurement errors can also occur in reaches where significant flow occurs
            through alluvial  gravels in the substrate.  This is often a problem at mine sites because
            mines are often  located in remote mountainous areas where channel substrates often are
            composed of large cobbles and gravels. Under these conditions, accurate flow
            measurements can be obtained using dye-dilution or tracers methods (see Section 3.6).
      ••     The site should be accessible under reasonably foreseeable flow and weather conditions.

3.1.2 Non-Recording Gages

      A reliable record of stage can be obtained through systematic observation of a non-recording
gage by a trained observer. Factors to consider when making the  choice to rely on  a non-recording
gage include:

      ••     Site accessibility.
      ••     Weather.
            Length of data collection.
      • •     Regularity of data collection.
      ••     Hydrologic variability.

      Generally, a recording gage should be selected for use at mine sites, with a non-recording back-
up, unless the site is easily accessed under all weather conditions.  Non-recording gages could be used
if only periodic measurements are necessary or cost is a driving factor.  Site access to remote gages at
mine sites is often difficult. Non-recording gages will tend to miss storm driven, peak flow, or other
unique events unless the observer happens to be taking readings when the event occurs.  It is often

Quality Flow Measurements at Mine Sites
impractical in remote mine areas to travel to important sites to obtain flow measurements during storm

      Common types of non-recording gages are staff, wire-weight, float-tape, and electric-tape.
These gages are described in detail below.

Staff Gage

      A staff gage is used to read stage height and is a common component of most gaging stations or
measurement programs.  The United States Geological Survey (USGS) vertical staff gage is considered
the standard. The USGS  staff gage is made from one or more porcelain enameled iron sections.  Each
section is 4 inches wide and 3.4 feet long with measurements every 0.02 feet.

      Staff gages are installed in either a vertical or inclined alignment. Vertical staff gages are
commonly used as reference gages in stilling wells or as a backup gage situated in the channel outside
the stilling well. Vertical staff gages can be installed on bridge pilings or other permanent, fixed
structures in the river channel, providing that channel geometry is understood. Knowledge of the
channel cross-section allows stage height, as measured on the staff gage, to be converted to discharge
using either Manning's equation, or using a specific stage-discharge relationship if one has been
developed. Inclined staff gages are  constructed from heavy timber and securely attached to some
permanent foundation. Inclined staff gages are less susceptible to damage by floods and floating debris
because they are flush against the streambank (Buchanan and Somers,  1982).

Wire-Weight Gage

      The standard type A wire-weight gage consists of a drum wound with a single layer of cable, a
bronze weight attached to the end of the cable, a graduated disc, and a Veeder counter.  The disc is
graduated in tenths and hundredths of a foot and is permanently connected to the  Veeder counter.  The
bronze weight is lowered until it touches the water surface. Stage is measured as  a combined reading
of the counter  and the graduated disc (Buchanan and Somers, 1982).

Float-Tape Gage

      The float-tape gage consists of a float, a graduated steel tape, a counterweight, and a pulley.  The
float is attached to one end of the graduated steel tape and the counterweight is attached to the other.
The float is typically a 10-inch diameter copper float that rests on the water surface and is kept in place
by the counterweight. Float-tape gages are commonly found inside stilling wells (Buchanan and
Somers, 1982).

Quality Flow Measurements at Mine Sites
Electric-Tape Gage

      Electric-tape gages consist of a graduated steel tape fastened to a weight, a reel for the tape, a
battery, and a voltmeter or buzzer.  The tape is lowered until it contacts the water surface.  Contact
with the water surface completes the electronic circuit and produces a signal to the voltmeter or buzzer.
Electric-tape gages are typically used for measuring stage height in stilling wells or shallow ground water
wells.  These gages are occasionally used outside and can be particularly useful if oil is floating on the
water surface.  The electric-tape gage can be used to measure the oil/water interface due to the fact
that oil is a dielectric (Buchanan and  Somers,  1982).

3.1.3 Recording Gages

      Recording gages automatically  track changes in the water surface with respect to time, eliminating
the need for regular site visits to read the gage. Recording gages can also be relied upon to capture
more variability in the range of discharges, including extreme events, because water level is being
continuously recorded or recorded at  regular intervals. The two common types of water stage
recorders are analogue or graphic, and digital.   The  analogue recorder has been used extensively since
the early part of the twentieth century; however, digital recorders are becoming increasingly common.
While the digital recorder is replacing the analogue, neither system is foolproof. Both systems should
be installed, with the analogue as a back-up, at particularly important or sensitive sites (British
Columbia,  1998).

      Water stage recorders can either be connected to a float located in a stilling well or to a bubbler
or submersible pressure sensor. The stilling well is fastened to the channel bottom or water
measurement device; intake pipes ensure that the water level in the stilling well is equal to the water
level in the channel or measuring device.  Stilling wells are used, instead of measuring stage directly off
the water surface, to protect the stage recorder and minimize fluctuations in the water surface caused by
wind and waves.  The bottom of the stilling well must be lower than the minimum anticipated stage and
the top above the maximum expected stage. Intakes should be properly sized to prevent lag during
rapid stage changes and prevent velocity-head  effects in the stilling well. Bubbler or submersible
pressure sensors do not require the use of a stilling well. These devices are not affected by small
fluctuations in water surface elevation caused by wind or waves because they rely on pressure (i.e.,
head) measurements taken inside the  water column.  Accuracy of these devices, however, can be
affected by changes in barometric pressure.

Digital Recorder

      Digital recorders use electronic sensors and data loggers to record and store water level
information in database ready digital format. Data can be downloaded  from the data logger to a


Quality Flow Measurements at Mine Sites
personal computer for easier, faster, and more accurate compilation of recorded values. Digital
recorder technology has progressed to the point where data on water quality and meteorology, in
addition to water level, can be collected with a single recorder (British Columbia, 1998).  Digital
recorders can also be set up for remote access via telephone or the internet. Several varieties of digital
recorders are available, including ultrasonic level sensors, submersible pressure sensors, and pressure
measurement sensors or bubbler gages.

      Ultrasonic sensors send out a series of sound waves through the air, the sound waves strike the
water surface and bounce back to the sensor. Total transit time from the sensor to the water surface
and back is related to the distance traveled and water stage.  Ultrasonic sensors are non-invasive,
requiring no physical contact with the channel being measured (British Columbia, 1998).  This can be
advantageous in situations where periodic flooding might carry away a conventional gage station  or
water quality concerns (e.g., pH extremes or metals) might affect the longevity or reliability of the
recording device.  Accuracy can be affected if environmental conditions (i.e., temperature,  pressure,
humidity) change the travel speed of the sound wave.

      Submersible pressure sensors measure water stage with a pressure transducer mounted at  a fixed
depth in the water column. The sensor transmits an analogue or digital signal to the data logger through
underwater conductors.  A submersible pressure sensor also, generally, has a vent tube that allows the
sensor to equilibrate itself to changes in barometric pressure. Submersible pressure sensors are
relatively inexpensive, easy to install, and accurate. Analogue sensors can have an accuracy as good as
0.1 percent and digital sensors can be as accurate as 0.02 percent or better.  Submersible pressure
sensors  typically have to be replaced if leaks develop; generally the electronics will be  damaged beyond
repair (British Columbia, 1998).  Submersible pressure sensors are not affected by wind, turbulence,
floating foam, or debris.

      Pressure measurement, or bubbler, sensors are highly accurate digital sensors which measure the
gas pressure required to generate a bubble at the end of a submerged orifice line.  The pressure
required to create the bubble is proportional to the water head above the orifice. Bubbler sensors cost
less than submersible digital sensors and the only  component in the water is the low cost orifice line.
The sensor and the pressure source, nitrogen tank or battery compressor, are located in a shelter
outside  the channel. The bulky pressure tank is the main disadvantage to bubbler sensors; accuracy is
similar to submersible digital sensors (British Columbia, 1998). Bubbler sensors are also not affected
by wind, turbulence, floating foam, or debris.

Graphic Recorder

      Graphic recorders chart a continuous trace of water stage with respect to time. Stage is
recorded on a strip-chart or drum recorder with a pen or pencil attached to the gage-height element.


Quality Flow Measurements at Mine Sites
Most graphic recorders can record an unlimited range by a stylus reversing device or by unlimited
rotation of the drum.  Strip-chart recorders can be operated for several months between servicing.
These types of recorders are extremely practical and cost effective at mine sites that often require
monitoring of stream flow at remote locations in upper portions of a watershed or on important
tributaries entering a site. Drum recorders need to be serviced weekly (Buchanan and Somers, 1982).

3.2   WEIRS

      Weirs are the simplest, least expensive, and probably the most common type of device for
measuring open channel flow. A weir is simply an obstruction or dam built across an open channel.
The weir basin is formed by partial impoundment of water behind the weir face.  The impoundment is
only partial because water will continue to flow over the weir crest, the top  edge  of the weir plate.
Typically water flows over the weir crest through a notch cut in the center of the  weir crest. The notch
can be V-shaped, rectangular, or trapezoidal (Grant and Dawson, 1997). Weirs are named for either
the shape of the weir notch, as in 'sharp-crested weirs', or for the shape of the flow control section, as
in 'broad-crested weirs'. Both types are discussed in subsequent sections.

      Weirs can be temporary or permanent measurement fixtures. Portable sharp-crested weirs may
be used to measure small flows in earthen channels or lined tunnels. A simple weir for measuring flows
in small earthen channels can be constructed from a stiff piece of metal cut in the  shape of, but
somewhat larger than, the channel cross  section. A carefully cut weir notch is located along the top
edge of the metal sheet.  The metal sheet is forced into the channel bottom and sides, perpendicular to
the direction of flow.  The crest is adjusted until level. Portable long-throated flumes can be used at
sites where insufficient head exists for sharp-crested weirs  (see Section 3.3). In larger channels, weir
plates are installed in bulkheads that have been sealed and  sandbagged into  place to prevent shifting as
water pressure builds up behind the weir plate.

      The stream of water leaving the weir crest is called the nappe.  Proper measurement conditions
occur when the nappe flows 'free' over the weir crest. Free flow, or critical flow, occurs when the
nappe is thrown clear of the weir face and air flows freely under the nappe, and between the nappe and
the weir face. Weirs provide accurate discharge measurements only within flow ranges specified by  the
size and geometry of the weir notch or crest. When the downstream water  level  rises to a point where
air no longer flows freely beneath the nappe, the nappe is not ventilated and accuracy of the discharge
measurement suffers because of low pressure beneath the nappe. Weir measurements are not usable
when the downstream water level submerges the weir crest (Grant and Dawson, 1997).

      The actual measuring point is located upstream of the weir plate in the weir basin.  A staff gage is
commonly used to measure the head (height of water above the crest) at a point in the weir basin
upstream from the point where drawdown begins. Drawdown, or surface contraction, is the slight


Quality Flow Measurements at Mine Sites
lowering of the water surface as the water approaches the weir. Drawdown typically begins at a
distance of about twice the elevation head on the crest upstream of the weir. The gage should be
situated a distance upstream of the weir equal to four times the maximum head expected over the weir
(Grant and Barnes, 1997). Measurement accuracy can be enhanced by using a recording gage situated
in a stilling well, instead of a staff gage in the weir basin (BOR, 1997).

3.2.1 Siting Criteria for Weirs

      Weirs should be sited in a straight reach of the channel, normal to the line(s) and direction of
flow. The weir crest must be level and the bulkhead plumb. Adequate cut-off walls are tamped in
place to prevent water from undermining the weir structure. The stream reach or channel selected must
allow positioning of the weir so that all stream flow is channeled over the weir crest. Flow undermining
the weir structure can cause relatively severe errors in the discharge measurement, especially during low
flow conditions.  The average width of the approach channel should approximate the width of the weir
box for a distance of 10 to 20 feet upstream  for small weirs and greater than 50 feet for the larger
structures. This insures that flow entering the weir structure is uniform.

      Weirs are relatively easy to construct and maintain, and measurements are accurate as long as the
proper flow conditions and channel dimensions are maintained. Weirs are not suitable for flat sloped
channels  that will not generate enough head loss for a free flowing nappe.  Weirs are also not suitable
for water carrying significant loads of silt or  other suspended solids, unless the weir is designed such
that solids can be periodically flushed out through the bottom of the weir.  Otherwise, deposition of
suspended solids in the weir basin will alter flow conditions and a loss in measurement accuracy will be
realized.  (Grant and Dawson, 1997).  Other specific limitations are discussed under the specific weir

3.2.2 Sharp-Crested Weirs

      A  sharp-crested weir has a blade with a sharp upstream edge (Brooks et al., 1994).  Passing
water only touches the thin upstream edge of the blade; the nappe clears the remainder of the crest
(Brooks et al., 1994). The traditional sharp-crested weirs used for measuring discharge are:
rectangular weirs, V-notch weirs, and Cipolletti weirs (Figure 3-1) (BOR, 1997).

Quality Flow Measurements at Mine Sites
                                       Contracted Rectangular
                                         Cipoletti Contracted
                                   Contracted Triangular or V-Notch
                            Figure 3-1. Sharp-crested Weirs (BOR, 1997)

      Sharp-crested weirs are designed such that the minimum distance of the sides of the weir from
the channel banks is at least twice the expected head. The crest of the weir should be at least twice the
expected head above the bottom of the weir basin (Grant and Dawson, 1997). The downstream water
surface should always remain at least 0.2 feet below the V-notch or crest. Discharge readings should
be discarded if the contraction is not springing underneath for the entire length of the nappe (Roberson
etal., 1988).

Quality Flow Measurements at Mine Sites
      BOR (1997) identifies 12 necessary conditions for accurate flow measurement using sharp-
crested weirs.

      •     The upstream face of the weir plates and bulkhead must be plumb, smooth, and
           perpendicular to channel flow.

      •     The weir crest needs to be level for rectangular and trapezoidal shapes. The bisector of
           V-notch angles must be plumb.

           The edges of the weir opening must be located in one plane.  The corners of the weir
           opening must have proper specified angles.

           The top thickness of the crest and side plates should measure between 0.03 and 0.08 inch.

      •     All weir plates need to have the same thickness over the entire overflow crest boundary.
           Plates thicker than 0.08 inch should be beveled to an angle of at least 45 degrees on the
           downstream edge of the crest and sides. The downstream edge of V-notches should have
           a 60 degree angle to prevent water from clinging to the downstream face of the weir.

      •     Upstream edges must be straight and sharp. Burrs and scratches should be removed by
           machining or perpendicular filing; abrasive cloth or paper should not be used.

      •     The bottom edge plates and upstream fastener projection should be located at least the
           distance of two measuring heads from the crest. All upstream faces must be free of oil and

      ••    The overflow sheet or nappe touches only the upstream faces of the crest and side plates.

      •     The weir head measurement is the difference in elevation between the crest and the water
           surface at a point located upstream.  The upstream point is a distance  equal to four times
           the maximum expected head on the weir.

      •     The weir head measurement should be at least 0.2 feet to prevent the  overflow sheet from
           clinging to the downstream weir face.

      • •    The weir approach should be kept clear of sediment deposits and other debris.

      The following sections describe common sharp-crested weirs and include an equation for
calculating discharge from each.  A range of measurable flow is presented for the smallest


Quality Flow Measurements at Mine Sites
recommended design for each of the sharp-crested weirs, as well as a maximum flow for the largest
structure of each type. The minimum range is presented under the assumption that discharges from
mine sites are typically small and diffuse.  The reader is referred to Grant and Dawson (1997) for more
specific design factors, measurement ranges and values for the discharge equation constants for the
three types of sharp-crested weirs presented.

Rectangular Weirs

      Rectangular weirs have vertical sides and a horizontal crest (Brooks et al, 1994).  A crest length
of one foot is the minimum recommended for a rectangular weir since V-notch weirs can more
accurately measure the same flow rates as rectangular weirs smaller than one foot.  The minimum
recommended crest length of one foot generally corresponds to a minimum flow of 0.286 cfs (128
gpm), assuming an elevation head of 0.2 feet, and a maximum flow of 1.06 cfs (476 gpm) with a head
of 0.5 feet. Rectangular weirs can be designed to measure flows up to 335 cfs (150,000 gpm),
assuming full end contractions, a crest length of 10 feet and head of 5.0 feet (Grant and Dawson,

      The equation commonly used for obtaining actual discharge is (Grant and Dawson, 1997):

                                     Q" K(L* 0.2H)Hl 5


       Q   =  flow rate
       K   =  constant depending on units
       L    =  crest length of weir
       H   =  head on the weir

V-Notch Weirs

      Sharp-crested V-notch or triangular weirs are used in situations where precise measurements of
low flows are required.  V-notch weirs are effective for low flow measurement because this type of
weir has no crest length and requires less elevation head for the nappe to spring free of the crest. The
minimum elevation head on a weir of this type is 0.2 feet with a maximum of 2.0 feet. A small V-notch
weir with a notch angle of 22 1A degrees can measure flows ranging from 0.009 cfs (3.99 gpm) at a
head of 0.2 feet, up to 2.81 cfs (1,260 gpm) at a head of 2.0 feet. A notch angle of 120 degrees can
measure flows up to 24.5 cfs (11,000 gpm) at a head of 2.0 feet (Grant and Dawson, 1997).

Quality Flow Measurements at Mine Sites
      The discharge for a free flowing V-notch weir is given by (Grant and Dawson, 1997):

                                         O- KH2-5

       Q  =  flow rate
       K  =  a constant defined by the angle of the notch and units of measurement
       H  =  head on the weir

Compound Weirs

      Compound weirs are often used in situations where a V-notch weir could handle the normal
range of flow but measurement of occasional larger flows would require a rectangular weir.  The two
profiles, rectangular and V-notch, can be combined to form a compound weir. Compound weirs
accurately measure flow whether the weir is functioning as a V-notch or a rectangular weir.  The
problem, however, with compound weirs is accurate measurement in the transition between V-notch
and rectangular weir behavior. Thin sheets of water will begin to flow over the rectangular weir crest in
an unpredictable manner when discharge begins to exceed the capacity of the V-notch. This problem
can be minimized if the sizes of the V-notch and rectangular sections are selected such that discharge
measurements in the transition zone are of minimal importance. Discharge over a compound weir is
calculated by applying the standard discharge equation for each segment of the weir to the head on that
segment of the weir (Grant and Dawson, 1997).  Compound weirs can be useful at mine sites simply
because they accommodate a wide range of flows; the V-notch portion can accurately measure fall and
winter low flows while the rectangular portion, plus the V-notch, provides accurate measurement of
spring peak flows.

Trapezoidal (Cipolletti) Weirs

      Trapezoidal weirs are similar to rectangular weirs except that the sides incline outwardly, not
vertically. When the sides have an outward inclination of 1 horizontal to 4 vertical, the weir is known as
a Cipolletti weir. Compared to a rectangular weir, the discharge equation is simpler because no
correction factor is included for the crest width; discharge occurs as if the end contractions are
suppressed.  Trapezoidal weirs have a slightly greater measurement range, although measurement
accuracy is less than would be obtained from a V-notch or rectangular weir.  The accuracy of the
Cipolletti equation is reported by BOR (1997) to be within ±5 percent of actual discharge.  The
smallest recommended trapezoidal weirs with a crest length of one foot has a measurement range

Quality Flow Measurements at Mine Sites
between 0.301 cfs (135 gpm) at a head of 0.2 feet and 1.19 cfs (534 gpm) at a maximum head of 0.5
feet. A large trapezoidal weir with a crest length of 10 feet can measure flows up to 376 cfs (169,000
gpm) at a maximum head of 5.0 feet (Grant and Dawson, 1997).

      The equation for calculating discharge using a free flowing Cipolletti weir takes the form of (Grant
and Dawson, 1997):


       Q  =  flow rate
       K  =  constant depending on units
       L   =  length of the weir crest
       H  =  head on the weir

3.2.3 Broad-Crested Weirs

      A broad-crested weir has a flat or broad surface over which the stream discharge flows. Broad-
crested weirs are used much less frequently than sharp-crested weirs and are often pre-existing
structures: dams, levees, and diversion structures. Broad-crested weirs are commonly used when
sensitivity to low flows is not critical and where sharp crests could be dulled or damaged by sediment
or flowing debris (Brooks et al, 1994).  Flumes, however, are generally preferable for measuring
debris and sediment-laden flows because broad-crested weir accuracy will be diminished by sediment
accumulation upstream of the weir face.

      True broad-crested weir flow occurs when the upstream vertical head above the crest is between
the limits of one-twentieth to one-half of the crest length in the direction of flow (BOR, 1997). Sharp-
crested weirs are preferable to broad-crested weirs for low flow measurement; however, under
moderate to high discharges, the accuracy of a broad-crested weir approaches that of a sharp-crested
weir, while maintaining several advantages (BOR, 1997):

           Broad-crested weirs can be computer calibrated.

      •     Broad-crested weirs should be considered when rust, abrasion, or impact might cause
           maintenance problems for sharp-crested weirs.

           Specially shaped broad-crested weirs can be designed to fit complicated channel cross-


Quality Flow Measurements at Mine Sites
           Broad-crested weirs pass sediment and debris better than sharp-crested weirs, but can
           accumulate sediment upstream of the weir face.

           Broad-crested weirs can be submerged between 80 and 90 percent without affecting the
           measurement, depending on the shape of the downstream transition in the channel.

      Broad-crested weirs are hydraulically similar to long-throated flumes.  Computer calibrations of
broad-crested weirs use the same principles and theories developed for long-throated flumes (see
Section 3.3).

3.3   FLUMES

      A flume is an artificial open channel constructed to contain flow within a designed cross section
and length (Brooks et al, 1994).  Flumes do not impound water like weirs, rather they restrict the
channel area or change the channel slope to increase flow velocity and change the level of water flowing
through the flume (Grant and Dawson, 1997). Flumes are typically constructed in streams with channel
characteristics such that the natural stage-discharge relationship is subject to changing channel
morphology, or is insensitive (Kilpatrick and Schneider, 1983). Flumes are well-suited to small flashy
streams where current-meter discharge measurements are impractical due to rapid changes in stage.
Flumes are also used in situations where existing channel head loss is too small to permit use of a weir
or when significant quantities of sediment or solids must pass through the measurement device. The
high velocity of flow passing through the flume keeps solids in suspension and functions as a self-
cleaning mechanism (Grant and Dawson, 1997).

      Flumes are commonly designed with a contraction in channel width  and/or a drop or steepening
of bed slope to produce critical or super-critical flow in the throat of the flume (Figures 3-2 through 3-
4).  The throat of the flume is the region where contraction occurs. The increase in slope, narrowing of
the channel, or a combination of the two increases flow velocity to a value  greater than the critical
velocity for the discharge(s) of interest. To satisfy the continuity equation,  depth of flow must decrease
when a given quantity of water is discharged at a higher velocity.  Critical flow and depth can only
occur in a previous sub-critical channel by introduction of external processes (i.e., steepening of slope
or constriction) that force the flow to pass through the  critical region (Grant  and Dawson, 1997).  A
hydraulic jump will typically occur at the point where flow passes from critical back into the sub-critical
region and is visually observed as a wave preceding a return to sub-critical depth and velocity. The
hydraulic jump may occur at the end of the constriction or at the point when  channel slope becomes
shallower. The hydraulic jump is the release of specific energy generated by  inducing critical and super-
critical flow.

Quality Flow Measurements at Mine Sites
      The relation between the depth of water measured at a point upstream of the water surface
drawdown and discharge is a function only of the configuration of the flume (Kilpatrick and Schneider,
1983), and this relation can be determined prior to installation.  In situations where critical depth cannot
be achieved, head must be measured in both the approach section and in the throat in order to
determine the discharge rate (Grant and Dawson, 1997).

3.3.1 Siting Criteria for Flumes

      Once the decision has been made that a flume is the appropriate measuring device for a site, the
decision must be made as to whether to use a critical flow flume or a super-critical flow flume.  Either
type of flume will transport debris of considerable size without deposition in the structure. Excessively
large rocks may become deposited at, or upstream from, the critical depth section of either critical or
super-critical flow flumes. If this occurs in a critical flow flume, the discharge rating will change since
head is measured upstream of the critical flow section and a large, solid object in the flow path may
affect the depth of water.  Super-critical flow flumes, such as the San Dimas flume, should be selected
for sites where this is likely to happen.  This type of flume measures head downstream of the critical
depth section and less likely to be affected by flow disruptions above the critical depth section.

      A critical-flow flume should be selected if the flume can pass the transported sediment load. The
discharge rating for a critical-flow flume is more sensitive than the discharge rating for a super-critical-
flow flume. The HS, H, and HL flumes have the smallest capacities of the critical flow flumes and have
relatively high precision of measurement. These flumes are used extensively in small watershed research
studies.  The Parshall flume is generally selected for all other situations where the use of a critical-flow
flume is desired.  These flume types will be described later.

      When the flume type and size are selected for the expected flow conditions, the flume must be
fitted for optimum compatibility with the natural channel. Four factors must be considered in the precise
fitting and placement of flumes:

           Channel characteristics.
      • •    Range of discharge to be gaged.
      • •    Desired precision of measurement.
      ••    Maximum allowable backwater.

      Flumes should not be installed too close to reaches with turbulent, surging or unbalanced flow, or
in a stream reach with a poorly distributed velocity pattern (i.e., unequal and non-parallel lines of flow).
Any of these flow conditions in the reach upstream of the flume may cause large errors in discharge
measurements. Flumes should be placed in reaches with tranquil flow, defined as reaches with fully
developed flow in long straight, mildly sloped channels that are free of curves, projections, and waves


Quality Flow Measurements at Mine Sites
(BOR, 1997). As a general rule, the approach channel should be a distance equal to 40 times the
hydraulic radius, or 10 times the channel width at the water surface (BOR, 1997). Hydraulic radius is
the cross-sectional flow area divided by the wetted perimeter.

      The velocity of the approach channel should exceed 1 foot per second to discourage aquatic
pests, insects, and sediment deposition (BOR, 1997). Approach channel flow with a Froude number
less than or equal to 0.5 (sub-critical flow) over the entire range of expected discharges will prevent
waves from forming and interfering with head measurement (BOR, 1997). Additionally, the channel
reach selected for flume placement should have a stable and consistent bottom elevation.

      A common failing in siting a flume is incorrect vertical placement. Excessive downstream channel
scour and erosion can occur if the flume is placed too high in the channel. Furthermore, if the flume is
placed too low, excessive backwater may cause submergence at higher flows.  If the user is unsure
about the proper size flume for a particular channel, it is generally better to select a flume that may be
too big rather than one that is too small. An undersized flume can result in excessive backwater with
frequent overtopping and possible scour around the edge of the flume. However, all flumes represent a
compromise between measurement sensitivity (i.e., precision) and accuracy over the entire flow range.

3.3.2 Long-Throated Flumes

      Long-throated flumes are common water measuring devices because they are easily fitted into a
wide variety of channel shapes, ranging from simple to complex.  Long-throated flumes have numerous
advantages over other measuring devices, including long-term accuracy, technical performance, design,
and calibration.

      Long-throated flumes constrict the channel to cause critical flow; channel steepening is not
necessary. A simple type of long-throated flume consists of a flat raised sill or crest across a
trapezoidal channel.  The approach ramp transitions from the approach channel invert.  The crest drops
vertically back to the downstream channel invert. Figure 3-2 is an illustration of a flat-crested, long-
throated flume.

Quality Flow Measurements at Mine Sites
                           Milling wed
Quality Flow Measurements at Mine Sites
           Long-throated flumes tend to be more economical than other measurement devices.

      ••    Long-throated flumes can be designed and calibrated using computer applications.

3.3.3 Short-Throated Flumes

      Short-throated flumes are considered "short" because the flume is designed to control flow in a
region that produces curvilinear flow.  The overall specified length of the flume structure may be
relatively long, however, the area of critical flow depth (i.e., the throat) is short. Accurate calibration of
these types of flumes is difficult because calibration changes by site and level of discharge.  For this
reason, rating curves for short-throated flumes are usually determined empirically by comparing water
depth at the measuring point with other more precise and accurate methods of determining discharge
(e.g., current meters or other means  of direct measurement).  Parshall and Venturi flumes are two
common examples of short-throated flumes.

Parshall Flume

      The Parshall flume is a variation of the Venturi flume. A Parshall flume is characterized by a
contracting inlet, a parallel-sided throat, and an expanding outlet,  all with vertical walls (Brooks et al,
1994). Parshall flumes have a sharp drop in the slope of the floor through the throat in the flume.  The
break in floor slope causes the critical depth and location for measurement of vertical head to occur at
the entrance to the throat. This feature creates a control that commonly requires that water depth
(head) only be measured at a single point located near the approach to the throat. (Kilpatrick and
Schneider, 1983). Parshall flumes have been developed and calibrated with throat widths ranging from
1 inch to 50 feet (Kilpatrick and Schneider, 1983). Figure 3-3 presents a plan and side view of a
typical Parshall flume.

      Parshall flumes can measure flows under submerged conditions, an advantage over the Venturi
flume (Brooks et al., 1994). Submerged conditions occur when the water surface downstream of the
flume is high enough to reduce the discharge.  Parshall flumes typically contain two water-level
recorders to measure discharge under submerged conditions, one located in the sidewall of the
contracting inlet and one located slightly upstream from the lowest point of flow in the throat (Brooks et
al., 1994). Both water-level recorders are used to determine the  difference in vertical head between
the two measuring points. This difference is then applied when calculating discharge under submerged
conditions. Only the upper measuring point is used when calculating discharge under non-submerged
conditions (Brooks et al., 1994).

Quality Flow Measurements at Mine Sites
                            'Note: Three-inch to eight-foot flumes
                                   have rounded approach wing walls.
                                         Upper Head, Hc
                                         Gage point
                                               PLAN VIEW


Zero Datum^

Nf !^^y
b a-
•T 	 — — •"
                                               SIDE VIEW
                Figure 3-3. Configuration and Proportions for Parshall Flumes
                              (Kilpatrick and Schneider, 1983)
      The Parshall flume was developed primarily for use in irrigation canals, stream diversions or small
routing channels. Recently, these flumes have been successfully used as gaging-station controls in
natural streams. An advantage of the Parshall flume is that it will pass small- to medium-sized sediment
without affecting the discharge rating curve and the accuracy of the measurement. The discharge rating
will be affected by installing the flume with poor channel alignment and when there is an uneven
distribution of stream flow entering the approach. The rectangular cross section makes the Parshall
flume insensitive to low flows (Kilpatrick and Schneider, 1983).  Under low-flow conditions, temporary
V-notch weirs can be installed at the entrance to the flume throat to more accurately measure discharge
(Kilpatrick and Schneider, 1983).

      Parshall flumes have been constructed with throats ranging in width from 1-inch to 50-feet. Very
small Parshall flumes have throat widths of 1, 2, or 3 inches and have minimum - maximum flow rates of
0.010 - 0.194 cfs (4.28 - 87.3 gpm), 0.019 - 0.478 cfs (8.55 - 215 gpm), and 0.028 -  1.15 cfs (12.6
- 516 gpm), respectively.  Small Parshall flumes have throat widths of 6-inches through 8-feet, and
large Parshall flumes have throat widths greater than 10-feet (Grant and Dawson, 1997).

Quality Flow Measurements at Mine Sites
Venturi Flume

      A Venturi flume is any rectangular, trapezoidal, triangular, or other regular shape with a gradually
contracting section that leads to a constricted throat (Brooks et al., 1994).  The flume expands
immediately downstream of the throat (Brooks et al., 1994).  The floor of a Venturi flume is the same
grade as the channel (Brooks et al., 1994).  Stilling wells are located at the entrance and at the throat to
measure head.  The difference in head between the two wells is related to stream discharge.

      Trapezoidal flumes are increasingly used to measure flows associated with hardrock mine
discharges. The trapezoidal flume is designed to measure super-critical flows.  The trapezoidal shape
conforms to the natural shape of a channel, minimizing the required length of the transition section, as
compared with a rectangular flume. Trapezoidal flumes require flow to transition between the
trapezoidal channel cross-section and the rectangular flume cross-section.  The walls of a trapezoidal
flume slope outward to provide increased sensitivity to low flows while maintaining measurement of
higher flows. Thus, trapezoidal flumes can measure a larger range of discharges than other types of
flumes.  Trapezoidal flumes also have a flat bottom. The flat bottom allows the flume to be placed
directly on the channel bottom and permits the flume to pass sediment and other debris (Grant and
Dawson, 1997).

Special Short-Throated Flumes

      Many flumes have been designed to meet specific organizational needs and for special use
situations, and are often termed Special Flumes. These flumes are commonly referred to as short-form
or short-throated flumes.  Two special flumes are discussed below, the H-type flumes, and the San
Dimas flume. These flumes are extremely difficult to calibrate because of the sharp connection of the
convergence and divergence of flows.

      H.HS. and HL Flumes

      The U.S. Natural Resource Conservation Service (NRCS), formerly Soil Conservation Service
(SCS); developed HS, H, and HL flumes, or generally H-type flumes, for measuring intermittent run-off
from small watersheds (Kilpatrick and  Schneider, 1983).  H-type flumes are commonly used to
measure run-off from feedlots, infiltration areas, and low flows of streams in pollution abatement
projects.  H-type flumes can measure a wide range of flows with reasonably good accuracy and are
simple to construct and install.  The wide measurement range makes these flumes particularly useful for
measuring drainage water.

      H, HS, and HL type flumes have converging vertical sidewalls cut back on a slope at the outlet to
provide a trapezoidal projection.  These sidewalls promote self-cleaning of the flume floor. The flumes


Quality Flow Measurements at Mine Sites
have a level floor that becomes extremely narrow at the downstream end.  This narrow area was
designed to increase the sensitivity of the discharge rating curve, and thus, increasing the precision of the
discharge measurement (Kilpatrick and Schneider, 1983).  A free fall is used to establish critical flow at
the downstream end of the flume. While the flume is designed for use under free fall conditions,
submergences of up to 50 percent do not significantly affect the head versus discharge relationship
(Kilpatrick and Schneider, 1983). Vertical head is measured upstream from the end of the flume in the
converging approach section (Kilpatrick and Schneider, 1983).

      The three flumes are similar in general configuration but have different proportional dimensions.
The HL flume has the greatest capacity, where the letter L is an indication of a large capacity.
Conversely, the HS flume has the smallest capacity, where the letter S is an indication of small capacity
(Kilpatrick and Schneider, 1983). HS flumes were designed to measure small discharges with
maximum flow rates between 0.085 and 0.821 cfs (38.1 to 368 gpm). H flumes were designed to
measure medium discharges with maximum flow rates between 0.347 and 84.5 cfs (156 to 37,900
gpm). HL flumes were designed to measure larger flows, up to 117 cfs (52,600 gpm) (Grant and
Dawson, 1997). Typical HS, H, and HL flumes are presented in Figure 3-4.

      San Dimas Flume

      A San Dimas flume has the same function as a broad-crested weir with the exception that flow is
constricted from the side, rather than the bottom (Brooks et al., 1994). This flume has a converging
approach reach with a flat floor.  A hump is located at the downstream end of the approach reach; this
is the critical depth cross section. The super-critical reach, downstream from the hump, has a
rectangular cross section with a 3 percent slope.  Head is measured in the super-critical-flow reach of
the flume, three feet downstream from the critical depth cross section. San Dimas flumes are very
insensitive to low flow conditions and exhibit very poor accuracy.  This is because of the rectangular
shape and because the measurements are made from a section of the flume with super-critical flow.
San Dimas flumes can be operated in conjunction with sharp-crested weirs to measure low flow
conditions. Provisions, however, must be made to bypass high flows around the sharp-crested weir
because sediment loads can damage the crest blade.

      Small  San Dimas flumes can be designed to measure flows with a minimum head of around 0.1
feet and minimum discharges of 0.16 cfs (71.8 gpm). Large  San Dimas flumes can be designed to
measure flows up to 300 cfs (134,625 gpm) (Kilpatrick and Schneider, 1983). The San Dimas flume
is best used to measure debris-laden flows in mountain streams under relatively high stream flow
regimes. The side constriction prevents sediment deposition and relative high flow velocities keep the
flume clean of debris (Brooks et al., 1994).

Quality Flow Measurements at Mine Sites
                 SIDE SHEET

            ,0 383D    , . 0
           HS FLUME
                                             SIDE SHEET
    H  FLUME
                                                                     3 2D
                                                                             \	 GAGE
                                                                PLAN VIEW   |
                                                                         I  Z 12 ID  J
                                                                          SIDE SHEET'
                                                                                 0 50
              FRONT VIEW
                 HL  FLUME
    Figure 3-4.  Configuration and Proportions of Type Hs, H, and HI Flumes (BOR, 1997)
Cutthroat Flumes

      The cutthroat flume gets its name from the absence of a parallel-wall throat section.  This flume is
a simple, flat-bottom device that can be placed directly on an existing channel without further
excavation. Cutthroat flumes work well in flat-gradient channels where measurement conditions occur
under both free and submerged flow conditions. Free flow conditions are preferable as only one head
measurement needs to be taken, upstream of the constriction. Submerged flow requires measurements
of both upstream and downstream head. The cutthroat flume is dimensionally defined by a
characteristic length and throat width.  All other flume dimensions can be derived from length and width
(Grant and Dawson, 1997).  Grant and Dawson (1997) present common sizes of cutthroat flumes;
other sizes can be interpolated from these.

Quality Flow Measurements at Mine Sites
      Cutthroat flumes have the advantage of ease of design, construction, calibration, and installation.
However, the rectangular cross-section negatively affects measurement accuracy when trying to
measure flows lower than those for which the flume was designed.


      Current meters measure flow velocity at specific horizontal and vertical points within a cross
section of a channel or stream and therefore are less accurate than flume or weir measurement methods
when measuring uneven flow channels. Each measured velocity point is assigned to a small portion of
the cross-sectional flow area, where the computed discharge for a given point is the measured velocity
multiplied by the cross-sectional area represented by that point measurement. This method results in
several partial discharges being computed  across a single cross section of the channel being measured.
The total discharge for the cross section is the sum of the partial discharges.  Velocity data is typically
collected over the expected range of total discharges.

3.4.1 Siting Criteria for Current Meters

      Current meter gaging stations should be situated in straight, uniform channel reaches with smooth
banks and stable beds. The gaging station should be sited as far as possible from areas of disturbance
in the flow pattern. Flow disturbance diminishes the reliability of the relationship between gage height
and discharge. If current meters are used to develop a stage-discharge relationship, this rating curve
should be frequently recalculated for channels with unstable beds, changing bed and bank conditions, or
with large amounts of aquatic vegetation. These factors all change the relationship between the stage
height of water and the cross sectional area of the channel.

3.4.2 Types  of Current Meters

      Several different types of current meters are used in water measurement:  anemometer and
propeller velocity meters; electromagnetic  velocity meters; Doppler velocity meters; and optical strobe
velocity meters. These meters are described in the following sections. The anemometer and propeller
type meters are the most commonly used devices and are the most readily available.

Anemometer and Propeller Velocity Meters

      Anemometer and propeller velocity meters measure velocity with anemometer cup wheels or
propellers. The Price current meter and the smaller pygmy meter modification are the most common
examples of this type of meter. These meters provide a small electronic pulse that is transmitted to a
small head-set worn by the user. The meter can be set to produce a countable  pulse for each complete
revolution or for every 10 complete revolutions, depending on the range of velocity of the flow being


Quality Flow Measurements at Mine Sites
measured. These types of velocity meters do not measure the direction of velocity, restricting their use
to sites with relatively laminar, and critical or sub-critical flow regimes (BOR, 1997).

Electromagnetic Velocity Meters

      Electromagnetic current meters produce and log voltage that is proportional to the velocity of the
flow.  In this manner, the meter provides a direct reading of velocity.  The user is not required to count
revolutions of the meter, as is required with anemometer and propeller type velocity meters.
Electromagnetic velocity meters are able to measure cross sectional and directional flows (BOR,
1997). These meters cannot be used near metallic objects.

Doppler Type  Velocity Meters

      This type of current meter determines velocity from measurements of changing source light or
sound frequency from the frequency of reflections from moving particles (e.g., sediment, or air
bubbles).  Lasers are used as the source light with laser Doppler velocimeters (LDVs); acoustic
Doppler velocimeters (ADVs) use sound. Vertical current profiles can be measured using acoustic
Doppler current profilers (ADCPs). ADCPs measure average velocities of selected size cells in a
vertical series. ADCPs are typically used to measure deep flow and currents in reservoirs, oceans, and
large rivers (BOR, 1997). Doppler type velocity meters can measure velocity components from
multiple directions.

Optical Strobe Velocity Meters

      Optical strobe velocity meters are comprised of mirrors mounted around a polygon drum.  The
drum can be rotated at precisely controlled speeds. Light from the water surface is reflected into the
meter's lens system by the mirrors. The rate of drum rotation is controlled by the user who is looking  at
the reflected images through an eyepiece. The images become steady and the water surface appears to
be still when the drum is rotated at the proper speed.  Surface velocity is calculated from the rotational
speed of the drum and the distance from the mirrors to the water surface. Velocity is translated into
discharge by applying a coefficient and multiplying by the cross-sectional area of that particular reach
(BOR, 1997).

      The optical strobe velocity meter can be used to measure flood flows, high velocity flows, and
debris laden flows since the gage  does not require any parts to be immersed in the current.  The
accuracy of this meter is affected by the proper selection of the coefficient, available from standard
tables, because the meter only measures the velocity of the water surface (BOR, 1997).

Quality Flow Measurements at Mine Sites
3.4.3 Methods to Determine Flow Velocity

      Several different methods can be used to measure mean velocities with a current meter. The
methods differ in the specific depths and number of depths that velocity measurements are taken at a
measuring point along a channel cross section. The choice of the method used is dependent on the
objectives of the measurement, the relative depth of water in the cross section, and the type of channel
being measured.  These methods are summarized below:

Two-point method — The two-point method relies on velocity measurements taken at 0.2 and 0.8 of
the total depth from water surface.  Flow velocity for the measurement point is taken as the average of
the two measurements.  The use of this approach is encouraged because it increases the accuracy of
the results and is based on known hydraulic properties that typically exist in open channels. This
method should not be used at sites with water depths less than two feet (BOR, 1997).

Six-tenths-depth method — The six-tenths-depth method is generally used when flow depth is less
than two feet. This method provides satisfactory results by measuring velocity at 0.6 of the total depth
from the water surface (BOR,  1997).

Vertical velocity-curve method — The velocity profile is defined by taking velocity measurements
along a vertical profile. The accuracy of the computed mean velocity is determined by the number of
velocity measurements obtained.  This approach is highly accurate, but time consuming and expensive
(BOR, 1997).

Subsurface method — The subsurface method requires that velocity be measured near the water
surface. The measurement is multiplied by a coefficient ranging between 0.85 and 0.95, depending on
factors such as water depth, velocity range, and streambed characteristics. The accuracy of this
method depends upon the selection of the proper coefficient (BOR, 1997).

Depth integration method - The depth or traveling integration method measures velocity at various
points along a vertical line. Measurements  are taken as the meter is slowly and uniformly raised and
lowered two to three times throughout the range of water depth.  Flow velocity is the average of all
observations. The depth integration method is not accurate and should only be used for comparisons or
screening-level estimates (BOR,  1997).

Quality Flow Measurements at Mine Sites
3.4.4 Computing Stream Discharge

      Discharge is calculated from current meter data using the velocity-area principle, "total discharge
is the summation of all computed partial discharges".  A partial discharge is defined by:
and total discharge is expressed as:

       q   =  discharge for a partial cross-sectional area in cubic feet per second
       Q  =  total discharge, cubic feet per second
       Vn  =  the mean velocity of the partial cross-sectional area, feet per second
       an  =  area of the partial cross section, feet squared

      Partial discharge can also be calculated using the simple average method, midsection average
method, or Simpson's parabolic rule. These approaches are discussed extensively by BOR (1997),
and other hydrology texts.


      An acoustic velocity meter (AVM) measures the velocity of flowing water by means of a sonic
signal. AVMs work on the principle that a high frequency acoustic signal sent upstream travels slower
than a signal sent downstream. Average path velocity is calculated by accurately measuring the transit
times of signals sent in both directions along a diagonal path (BOR, 1997). Average axial velocity is
calculated from information on the angle of the acoustic path relative to the direction of flow (BOR,
1997). Meters of this type are useful for measuring discharge at streamflow sites where the relation
between discharge and stream stage varies with time (e.g., variable backwater conditions) and when
stream gradients are too flat to permit accurate measurements for slope computations.

      The AVM is a non-mechanical, non-intrusive device capable of measuring lower velocities than a
current meter.  AVMs provide a continuous and reliable record of water velocities over a wide range of
conditions, subject to four constraints.

      1.    Accuracy is reduced and performance degraded if the acoustic path is not a continuous
           straight line.  The path can be bent by reflection if it passes too close to a  stream boundary

Quality Flow Measurements at Mine Sites
            or by refraction if the path passes through density gradients resulting from changes in
            salinity or temperature.

      2.     Signal strength is attenuated by particles or bubbles that absorb, spread, or scatter sound.

      3.     Changes in streamline orientation can affect system accuracy if the variability is random.

      4.     Errors relating to signal resolution are much larger for a single threshold detection scheme
            than for multiple threshold schemes.

      AVM systems range from a simple velocity meter to complex computerized systems that collect
and transmit real-time discharges.  Site factors determine whether a single path is adequate or whether
multiple paths are required.

3.5.1 Siting Criteria for Acoustic Velocity Meters

      AVM systems can be used over a wide range of flows from low flow situations to sections where
velocity is extremely fast. BOR (1997) describes a good AVM site as:

      • •     A reach with a uniform velocity distribution and confined channel.

      •     The channel should be straight for 5  to 10 channel widths upstream and 1 to 2 channel
            widths downstream.

            The channel bottom should be relatively stable.

      •     The cross-sectional area and profile should be relatively consistent through the gaged

      Other site selection criteria include considerations of the limiting acoustic criteria, equipment
requirements, and potential installation problems.

      Data must be collected on stream cross section, water temperature, and salinity profiles. Other
conditions that may affect AVM performance include: air entrainment; algae; moss; weed growth; and
suspended sediment (Laenen, 1985).  These data may need to be collected over time so that maximum
temperature, salinity differentials, maximum suspended sediment concentrations and particle-size
distribution; and maximum and minimum stages can be reliably estimated (Laenen, 1985).  Temperature
and salinity gradients are minor in many  streamflow situations and data collection will be unnecessary.

Quality Flow Measurements at Mine Sites
Gradients would be expected in streams with slow moving or ponded water, downstream from
tributary inflows, downstream from thermal discharges, and tidal reaches (Laenen, 1985).

      AVMs cannot be used to compute velocity where large eddies persist in the stream, nor can they
be used in reaches with extreme turbulence or other poor hydraulic measuring conditions (e.g., air and
gas entrainment) (Laenen, 1985). Sources of air and gas entrainment include dam spillways, natural
stream riffles, and decaying vegetation (Laenen, 1985).

3.5.2 Types of Acoustic Velocity Meters

Single-Path Acoustic Velocity Meters

      Single-path AVMs function as flowmeters by calibrating the acoustic path velocity against mean
channel velocity, estimated using standard stream gaging techniques (BOR, 1997).  The angle between
the acoustic path and the average direction of streamflow is normally between 30 and 60 degrees.  The
discharge rating procedure involves collecting data on stage-area relationships, acoustic path velocities,
and mean discharge velocities for the expected range of flows and stages. The velocity rating is derived
using a linear regression with instantaneous mean channel velocity as the dependent variable and
acoustic path velocity as the independent variable  (BOR, 1997).  Discharge is estimated by multiplying
the predicted mean channel  velocity by the cross-sectional area at the gage site.  Flow measurement
accuracy is limited by the quality of data collected for the calibration ratings.  Single-path AVMs can
attain accuracies within ±3 percent of the actual discharge (Laenen, 1985).

Multipath Flowmeter

      Multipath flowmeters use several acoustic paths which are mounted at various elevations
throughout the measurement section. The velocity profile is established from the average axial velocity
for each acoustic path.  The velocity profile is numerically integrated over the channel's cross-sectional
area to determine the volumetric flow rate. The accuracy of a multipath flowmeter is relatively
independent of the velocity  profile (BOR, 1997).  Integration errors often occur because velocities near
the channel bottom and water surface cannot be measured because of acoustic interference. However,
if properly used, multipath flowmeters can attain accuracies within 1 percent of actual discharge
(Laenen, 1985).

Quality Flow Measurements at Mine Sites

      Tracers injected into a stream behave in a similar manner as water particles traveling with the
flow.  The measurement of the movement of a tracer is essentially the same as measuring the movement
of an element of fluid in the stream, taking into account the dispersion characteristics of the fluid. Most
tracers used for measurement of stream flow are very conservative with respect to water (i.e., they flow
and behave similarly to water molecules under conditions of flow, and do not have significant
attenuation properties).  After injection of a tracer to a stream, dispersion and mixing occurs as the
tracer moves downstream (Kilpatrick and Cobb, 1985). Mixing and dispersion occur in three
dimensions, vertical, lateral, longitudinal. Under most open channel flow conditions, an equal mixing of
a conservative tracer is usually achieved first in the vertical  direction, followed by equal mixing in the
lateral (cross-stream direction) at a point further downstream. Longitudinal mixing in the downstream
direction continues indefinitely because no boundaries are encountered in this dimension.

      Figure 3-5 shows the typical response to the injection of a tracer into a flowing channel with
downstream distance. After injection of a tracer, either as a slug or at a continuous rate, a response
curve can be plotted at any downstream point by plotting tracer concentration against time.  These
response curves form the basis for determining stream characteristics including time-of-travel,
dispersion, and discharge.

3.6.1  Siting Criteria and Sources of Error

      The accuracy of open channel discharge measurements using tracers can be affected by the
choice of the reach where measurements are taken.  Specifically, backflow eddies can delay the dye
and impede mixing. An ideal reach for tracer measurement will not contain large backflow eddies or
stagnant pools.  As with other methods, measurements should be taken in stream reaches that have
steady uniform flow, no large eddies or deep pools, and measurable cross sections.

      Accuracy is also sensitive to how well the tracer cloud's center of mass is determined with
respect to time. The first and last observations of a cloud may be difficult to  detect, and the center of
mass may not be located in the time center of the cloud. Discharge measurement accuracy with tracers
can approach ±1 percent with the use of expensive equipment  such as multiport pop valves,
turbulators, complex electrodes, and fluorometers (BOR,  1997).  Discharge estimates can also be
obtained with tracer methods.  The least accurate tracer method would be to break a bottle of dye at
an upstream station and estimate how long the center mass takes to pass by the downstream measuring

Quality Flow Measurements at Mine Sites
            Vertical and lateral mixing
             longitudinal dispersion
              (vertical not shown)
                                                                            LONG DISTANCE
       Figure 3-5. Lateral Mixing and Longitudinal Dispersion Patterns and Changes in
             Concentration Distributions from a Single, Slug Injection of a Tracer
                                  (Kilpatrick and Wilson, 1989)
3.6.2 Types of Tracers

      A tracer is anything that mixes with water or travels with the flow of water.  As previously
indicated, an ideal tracer moves conservatively with respect to the water and exhibits no significant
properties of attenuation or retardation with respect to flow. A tracer has to be detectable and
measurable at downstream points. Tracers used in hydrologic studies include:

      ••    Dyes of various colors.
      ••    Chemicals such as fertilizer, salt, or gases.
      ••    Traveling turbulent eddy pressure sequences.
      • •    Neutrally buoyant beads.

Quality Flow Measurements at Mine Sites
      Dyes and salts are the most convenient tracers for measuring discharge at mine sites.  Salt tracer
concentrations are determined by measuring the evaporated dry weight, through chemical titration, or
by electrical conductivity. When salt solutions are used as tracers, chemical or conductivity methods
are used for detection and concentration measurements.  The salt-dilution method works well in small
to medium sized turbulent streams. Large streams require excessive quantities of salt to significantly
change electrical conductivity above background values.

      Dye tracing involves introduction of dye into a water body followed by collection of water
samples over time and space to measure the response.  Response is a function of the dye concentration
in the samples, as measured using a fluorometer (Wilson et al. 1986). Fluorometric procedures for dye
tracing can be used to measure time-of-travel, dispersion, reaeration, and stream discharge.  Dyes used
as tracers include Fluorescein, Rhodamine B, Rhodamine WT, and Pontacyl Pink B. These dyes are
all easily visible and/or detectable using fluorimetry in dilute concentrations. The Rhodamine dyes are
considered nontoxic by the U.S.  Food and Drug Administration.  Rhodamine and Pontacyl Pink B are
considered relatively stable and are resistant to fading and changes in their fluorimetric properties. They
are also resistant to chemical changes by other waterborne constituents, and resist deposition onto flow
surfaces, weeds, and sediments (BOR, 1997). Prior to using any dye for flow measurement, however,
it is recommended that the selected dye(s) be tested with water and earth embankment samples for
adsorption, chemical reactions, and fading before conducting the discharge measurement program
(BOR, 1997).  For example, these dyes can be affected under strongly acidic conditions caused by
acid generating ores or waste rock. In addition, the precipitation of iron oxyhydroxides in streams that
had been dissolved as a result of acid mine drainage can also affect fluorimeter readings. Under these
conditions, the use of salt tracers would be recommended.

      Dye tracing has two characteristics that make this technique favorable for measuring stream
discharge.  First, dye tracing has low detection and measurement limits. Fluorometers can measure dye
concentrations down to one part per million (ppm) and detect dye down to one part per billion (ppb)
(BOR, 1997).  Second, measuring dye tracer concentrations is simple, relatively easy to apply in the
field, and accurate with fluorometric techniques.

      Kilpatrick and Cobb (1985) suggest that tracer methods can be particularly useful under the
following flow conditions:

            Where it is difficult or impossible to use a current meter due to high velocities, turbulence,
            or debris.

            Where, for physical reasons, the flow is inaccessible to a current meter or other measuring

Quality Flow Measurements at Mine Sites
           Where, for some conditions, the rate of change of flow is such that the time to make a
           current-meter measurement is excessive.

      •     Where, the cross-sectional area cannot be accurately measured as part of the discharge
           measurement or is changing during the measurement.

3.6.3 Methods to Determine Stream Discharge

      Two approaches can be used to measure discharge using dye or salt tracers (BOR, 1997;
Kilpatrick and Cobb, 1985).  The velocity-area method uses the time required for a tracer to travel
down a known channel distance and then uses the average cross-sectional area for the reach to
determine stream discharge.  The dilution method measures stream discharge using the measured
downstream concentration of a fully mixed tracer that is being added upstream at a constant rate (BOR,

Velocity-Area Method

      Stream discharge is calculated using the velocity-area method as follows:

       Q  =  stream discharge, in cubic feet per second
       A  =  average cross-sectional area of the reach length, in square feet
       L  =  the length of the stream reach between detection stations, in feet
       T  =  tracer travel time between the detection stations, in seconds.

      Both salts and dyes can be measured using the velocity-area method; however, each requires
different detection equipment. Dyes have the advantage of being visible, allowing for simpler
measurements when fluorometers are not used.

Salt-Velocity-Area Method

      The salt-velocity-area method is based on the fact that salt in solution increases the electrical
conductivity of water (Kilpatrick and Cobb, 1985).  This approach has the potential for higher
accuracy and precision and has been successfully used in open channels and conduits.  Sodium chloride


Quality Flow Measurements at Mine Sites
(NaCl) is typically the salt used in the tracer injection solution.  Other halide compounds, such as lithium
bromide (LiBr) or sodium bromide (NaBr), have been used in situations where high background
concentrations of chloride in stream waters interfere with the conductivity measurement (Kimball,
1999). The salt concentration in the tracer solution must be high enough to significantly increase the
electrical conductivity of the receiving water.

      The method is employed by injecting the tracer solution into the receiving water upstream from
the measurement reach. The measurement reach should be located a sufficient distance downstream
from the injection point to allow for complete mixing of the tracer in the receiving water. This reach
should be selected such that it is uniform and the channel-flow geometry can be defined exactly. A pair
of electrodes is installed near the sides of the channel at both the upstream and downstream ends of the
measurement reach.  The two pairs of electrodes should be sited far enough apart, upstream and
downstream, to allow accurate measurement of travel time between them.  A central instrument records
and graphs electrical conductivity at the electrode sites with respect to time. Electrical conductivity
increases as the tracer cloud passes through the electrodes. Data are recorded and graphed by the
central instrument.  The time of travel is equal to the length of time required for the peak mass of the
tracer cloud to pass through both sets of electrodes.

      The salt-velocity-area method is highly accurate; however, special equipment (i.e., an electrode
system) and experienced personnel are required to apply the method.  Accuracy can be enhanced by
selecting reaches where cross sections and reach length can be measured with relative ease.

Dye-Velocity-Area Method

      Maximum accuracy from dye tracer solutions can be achieved using a fluorometer.  The
procedures are similar to the salt-velocity-area method except that a fluorometer, instead of the
electrode system, is used to measure dye concentrations at a downstream cross-section. The
American Society of Mechanical Engineers Performance Test Codes (1992) can be followed to
achieve very accurate discharge measurements.

Dilution Method

      Measurement of stream discharge by dilution methods depends on the determination of the
degree of dilution of an added tracer solution by the flowing water.   Stream discharge is calculated
using the dilution method as follows:

Quality Flow Measurements at Mine Sites
and stream discharge (Q) is solved using the following equation:

       C0  =  background concentration of the tracer in the stream
       C}  =  concentration of the strong injected tracer solution
       C2  =  the concentration of tracer at the sampling station, after full mixing
       q   =  discharge of the strong solution injected into the flow

      When using salt tracers, dry weight can be substituted for concentration values and the weight of
water per second can be substituted for discharges (BOR, 1997).  The dilution method is appealing
because it does not require measurements of stream cross sections; however, possible tracer (salt or
dye) losses may be a problem. Dye losses can  generally occur when excessively long stream reaches
are used, and when there is relatively high concentrations of suspended clays or organic particles that
can absorb the dye (Kilpatrick and Cobb, 1985). Losses can also occur when significant
concentrations of chemicals like chlorine are present that oxidize or quench the dye (Kilpatrick and
Cobb, 1985).

      The dilution method does require a sufficiently long flow length to ensure that there is complete
mixing prior above the chosen sampling location. The length of stream required to provide complete
mixing can be reduced by setting up a system to simultaneous inject the tracer at several points laterally
across the stream or channel.

Quality Flow Measurements at Mine Sites

      As discussed in Section  1.0, EPA's responsibilities include National Pollutant Discharge
Elimination System (NPDES) permitting and monitoring of discharges at mine sites under jurisdiction of
the CWA, evaluation of environmental impacts associated with NEPA analyses, conducting remedial
investigations/feasibility studies (RI/FS) studies under CERCLA, or conducting baseline research.
Measurements of stream or facility discharges at mine sites are typically associated with these types of
studies or are often incorporated as part of a larger study designed to characterize surface and ground
water hydrology, characterize water quality, or identify loading sources of contamination.  The choice of
an appropriate or applicable method to measure discharges at mine sites, therefore, is highly dependent
on the objectives of the study being conducted. This section outlines and discusses a range of specific
factors which should be considered before choosing a method to conduct  discharge measurements at
mine sites.


      Selection of the proper measurement technique to measure stream discharge at a mine site is
dependent on a variety of factors. Site-specific factors, the type of mine facility or waste facility from
which discharges are occurring, and the intended use of the data must be considered and weighed by
the user. As presented by BOR (1997), the main factors to be considered  when choosing a discharge
measuring device or a measurement method are:

      ••    accuracy requirements;
      ••    cost;
           legal constraints;
           range of expected flow rates;
      ••    head loss;
      ••    adaptability to site conditions;
           adaptability to variable operating conditions;
           type of measurements and records needed;
      ••    operating requirements;
      • •    ability to pass sediment and debris;
           life expectancy of measuring device;
           maintenance requirements;
      • •    construction and installation requirements;
      ••    device standardization and calibration;
           field verification, troubleshooting, and repair;
           user acceptance of new methods;
      ••    vandalism potential; and


Quality Flow Measurements at Mine Sites
      • •    environmental impact of the method.

These factors are discussed in detail below:


      The target accuracy, or a desired accuracy required for a discharge measurement, is perhaps the
most important consideration. Most methods or measurement devices can produce accuracies within
±5 percent of actual discharge. Some specific devices, such as current meters, can attain higher
accuracies under laboratory conditions, but maintaining these accuracies in the field is often not possible
or requires increased effort and expense.  It is important to note that the accuracy which can actually be
achieved in a discharge measurement will often be lower (i.e., greater than ±10 percent error) if a
method is selected that is not appropriate for the specific site conditions (BOR,  1997).

      Expected accuracy is commonly reported for most measuring devices, such as current meters,
acoustic velocity meters, weirs, or flumes. Most of the methods outlined in Section 3.0 also rely upon
some secondary measurement device. It should be noted that any error associated with the secondary
device will reduce the accuracy reported for the primary measurement method.


      The total cost of the measurement method includes the cost of the device or devices used for the
measurement or monitoring, installation, operation and maintenance, and any associated equipment.
Cost estimates should include the potential cost of manual measurements using non-recording devices,
versus the use of automatic recording type methods.  A discharge measurement device or method
should be selected based on lowest expected cost over the serviceable life of the device, and the
appropriate accuracy for the study or monitoring program being conducted at the mine.

Legal Constraints

      Governmental or administrative water board requirements may dictate which types of water
measurement devices can be used on particular streams or geographic areas. In addition, EPA or
another agency may require the use of a particular device or measurement methods; based on
conditions of NPDES permits, consent decrees, or other requirements of a particular permit.

Quality Flow Measurements at Mine Sites
Flow Range

      Many measurement devices and methods only function within a limited range of flows.  Large
errors occur when stream discharge is outside of the prescribed range of the measurement device. For
example, sharp-crested weirs do not yield good measurements under high flow conditions, and broad-
crested weirs are inaccurate for low flow conditions.  The project objectives and the characteristics of
the stream or discharge outfall will determine what types of flows and range of flows will need to be
measured. Mine sites are particularly challenging with respect to the potential range of flow regimes
that can occur at a discharge point. This primarily occurs because mine sites often contain a variety of
storm water containment devices, channel diversions,  or discharge structures which are designed to
only discharge under low-frequency design flows  (i.e., such as for a 10- or 100-year storm event).

Head loss

      Some water measurement devices require a minimum drop in vertical elevational head (i.e., depth
of the water) between two measurement points in  order to operate properly. This is the case with
flumes and weirs.  The appropriate, minimum difference in elevational head may not be available on
some sites, particularly in areas with flat topography.  Furthermore, a trade-off can exist between head
loss and measurement cost.  Sharp-crested weirs, for example, are relatively inexpensive but require a
large head loss. Acoustic velocity meters are more expensive but require relatively little head loss.

Adaptability to Site Conditions

      The selection of a measuring device hinges on the actual site of the proposed measurement. The
shape of the flow section will favor some devices over others and the chosen device should not
interfere, or impede with site hydraulic conditions and  normal flow regimes.

Adaptability to Variable Operating Conditions

      Streamflow varies over spatial and temporal dimensions; the selected measuring device should
accommodate the expected range of flows as well  as variations in operating conditions.  Changes in
upstream or downstream head would be examples of variable operating conditions.  Weirs, for
example, should be avoided if downstream flows can  submerge the measuring device. Most
importantly, the measuring device should be useful to the water measurer.  In some cases, devices and
methods that are easily applied are more likely to be used, and used properly, than complicated
measuring devices.

Quality Flow Measurements at Mine Sites
Type of Measurements and Records Needed

      The user needs to decide if water must be measured on a continuous basis, occasionally, or
episodically. The device chosen will vary depending upon the type of data collected.  Continual
measurements require construction of a permanent structure with a recording gage.  Periodic
measurements can be accomplished with flow meters, tracers, or other non-structural measurement

Operating Requirements

      Operating requirements include labor, and operation and maintenance expenses.  The monitoring
project should determine available personnel before selecting a device. A remote site might call for a
flume or weir with a digital gage recorder.  A set-up of this type has few moving or electronic parts and
can be checked every couple of months.  At other sites it may be convenient to have measurements
taken manually, foregoing the expense of a recording gage.

Ability to Pass Sediment and Debris

      Sediment is deposited anywhere stream velocity is reduced, such as  near flow measurement
devices. Rating curves often have to be continuously recalibrated since sediment deposition changes
the cross-sectional area of flow. This affects the calculation of discharge. The measurement site should
be analyzed to determine the level of sediment expected, and whether it will be possible to clean out the
measuring device. San Dimas flumes were designed for small, turbulent mountain streams with a high
debris load.  Discharge is measured upstream from the deposition area and, theoretically,
measurements are not affected by debris.

Device Environment

      Measurement devices should be compatible with the site environment. Proper operation and
longevity are a cost consideration, and any device with moving parts and sensors is susceptible to
environmental damage. For example,  devices can be harmed by rapid temperature changes, sediment,
ice, acidity or alkalinity in water, biological growths, mineral encrustation, and other environmental
influences. Failure to take these site factors into consideration may lead to premature failure or loss of
measurement accuracy.

Maintenance Requirements

      Measuring devices have unique maintenance schedules.  Current meters, for example, require
periodic maintenance and the site must be maintained to ensure no change in the cross-sectional flow


Quality Flow Measurements at Mine Sites
area.  Regular maintenance programs are recommended for all measurement devices; lack of care will
reduce accuracy and shorten the life of the equipment.

Construction and Installation Requirements

      Installation costs include the difficulty of installing a particular measuring device at the site and any
channel work that may be required to ensure accurate measurements.  The configuration of the site, as
well as other site conditions, will often dictate the appropriate measurement device.

Device Standardization and Calibration

      A standard water measurement device implies a documented history of performance based on
theory, controlled calibration, and use.  A standard device should be fully described, accurately
calibrated, constructed correctly, installed properly, and adequately maintained according to the original
specifications and flow limitations. Many standard devices will have discharge equations and tables for
calibrating the device.  Small deviations from the specified dimensions can occur when measuring
devices are fabricated  on-site or incorrectly installed.  These deviations may affect the calibration and
accuracy of discharge  measurements. The use of pre-fabricated measuring devices avoids the
calibration problems that may occur with custom devices.

Acceptance of New Methods

      Water measurement devices should build on and integrate previously proven measurement
methods at the site, as much as possible. Proposed changes that add to the established practice are
often easier to implement and more likely to be accepted.

Vandalism Potential

      Measuring devices located near public access are prime candidates for vandalism. If vandalism
is a possibility, one should choose measuring devices or methods with minimal instrumentation or
instrumentation that is easily protected. Measuring devices are often located in sheds or placed in
buried vaults to discourage vandals.

Impact to the Environment

      Consideration should be given to potential environmental impacts associated with the various
measuring devices.  These devices vary greatly in the amount of disruption to existing conditions needed
for installation, operation, and maintenance. Weirs and flumes, for example, constrict the channel, slow

Quality Flow Measurements at Mine Sites
upstream flow, and then accelerate flow through the measurement device. These hydraulic changes can
change local sedimentation patterns, affect aquatic habitat, and impede fish movement.


      The objectives of conducting flow measurements and the specific site conditions, usually restrict
or narrow the possible choices for water measurement devices and methods. This document has
presented the various water measurement devices appropriate for natural or man-made open channels.
Tables 4-1, 4-2, and 4-3 provide a comparison of methods and devices for the major selection criteria
discussed above. Table 4-1 provides general guidelines for all discussed measurement methods.
Tables 4-2 and 4-3 do not provide non-structural approaches (e.g., dye tracing) because these
methods are portable and do not require regular site maintenance or concern about debris flows.  Table
4-2 presents minimum and maximum ranges for head and discharge for commonly used flumes and
weirs.  The minimum values correspond to the smallest measurement device of that type commonly
available and the maximum values are for the largest size of that measurement device commonly
available.  The ranges are not for a single flume or weir. The symbols used in Table 4-3 provide a
relative ranking of devices and methods.  The "+" symbol indicates that the device has positive
attributes, "0" equates to neutral, and a "-" suggests that the device may have negative attributes with
respect to that criteria.

      Each water measurement situation is unique and, as such, no universal selection method exists.
However, Tables 4-1, 4-2, and 4-3 outline the strengths and weaknesses of the various flow
measurement methods and devices discussed in this guidebook.  Personnel responsible for conducting
flow measurements at mine sites should weight the various criteria to select the optimal device or
method for the specific objectives to be met.

Quality Flow Measurements at Mine Sites
                Table 4-1. Water Measurement Device Selection Guidelines*
5.  Low cost.
6.  Good for sites where only
   periodic measurements are
7.  Can be used to back-up a
   recording gage.
8.  Can be used without a
   primary measurement device
   (i.e., weir, flume) if channel
   geometry is understood and
1.   Site must be easily accessible for
    regular observation and data collection.
2.   May miss storm driven peak flows or
    other unique events.
3.   Requires a stable, known cross-
    section, that has been surveyed.
 Gages (recording)
1.  Recording gages
   automatically record stage at
   user defined intervals.
2.  With digital recorders, the
   data log can be downloaded
   to a personal computer.
3.  Digital recorders can be
   accessed remotely.
4.  Bubblers and pressure
   transducers are not affected
   by wind, turbulence, or
5.  Ultrasonic sensors are good
   for sites that periodically
   flood or where water quality
   concerns might damage
   other types of gages.
6.  Pressure transducers and
   bubblers are highly accurate.
1.   Bubble gages require periodic
    maintenance when used in streams
    with high concentrations of grease,
    suspended solids, or silt.
2.   Recording gages are more expensive
    than non-recording gages.
3.   Bubbler and pressure transducer
    accuracy can be affected by changes
    in barometric pressure.
4.   Pressure transducers generally must
    be replaced if damaged.
5.   Requires a stable, known cross-
    section, that has been surveyed.

Quality Flow Measurements at Mine Sites
                                                         SAIC Draft Final
                Table 4-1. Water Measurement Device Selection Guidelines*
 Sharp-Crested Weirs
1.  Low cost.
2.  Ease of installation.
3.  V-notch weirs are used for
   precise measurement of low
4.  Compound weirs can
   accurately measure a wide
   range of flows.
5.  Trapezoidal weirs have a
   greater measurement range
   and a simpler discharge
   equation than V-notch or
   compound weirs.
1.  Require a head loss of at least 0.2 feet.
2.  The approach channel must be kept
   clear of sediment and debris.
3.  Cannot measure super-critical flow.
4.  Cannot measure flow when
5.  Trapezoidal weirs are less accurate
   than V-notch and compound weirs.
 Broad-Crested Weirs
1.  Useful at sites where a
   sharp-crested weir could be
   damaged by sediment or
2.  Can be computer calibrated.
3.  Can be designed to fit
   complicated channel cross-
4.  Can be submerged up to
   90% without affecting
   measurement accuracy.
1.  Require a head loss of at least 0.2 feet.
2.  Not sensitive enough for low flow
3.  Accuracy can be diminished by
   sediment accumulation upstream of the
   weir face.
4.  Not as accurate as sharp-crested
5.  Cannot measure super-critical flow.

Quality Flow Measurements at Mine Sites
                                                          SAIC Draft Final
                Table 4-1.  Water Measurement Device Selection Guidelines*
1.  Good for small, flashy
2.  Require less head loss than
3.  Self cleaning.
4.  Easily fitted to a variety of
   channel shapes.
5.  Accurate to ±2% of actual
6.  Can be  computer calibrated.
7.  Long throated flumes can be
1.   Flumes should not be installed in
    reaches with turbulent, surging, or
    unbalanced flow, nor reaches with
    poorly distributed velocity patterns.
2.   Typically costs more to install,
    compared to a weir.
3.   Sites with high upstream velocity are
    generally not suitable for long-throated
4.   Upstream banks should be high enough
    to contain the increased depth caused
    by flume installation.
1.  Self cleaning.
2.  Require less head loss than
3.  Accuracy less affected by
   approach velocity.
4.  Well suited to small, flashy
5.  Parshall flumes can measure
   discharge under submerged
6.  Trapezoidal flumes can
   measure super-critical flow.
7.  Trapezoidal flumes measure
   a wider range of flow than
   other flume types.
8.  H-type flumes are simple to
   construct,  install, and
9.  San Dimas flumes  are good
   for measuring debris laden
   streams with high flows.
1.   Short-throated flumes can be difficult
    to calibrate, particularly H-type and
    San Dimas flumes.
2.   Short-throated flumes can be
    expensive and difficult to install.
3.   Flumes should not be installed in
    reaches with turbulent, surging, or
    unbalanced flow, nor reaches with
    poorly distributed velocity patterns.
4.   Parshall flumes are insensitive to low
5.   San Dimas flumes are insensitive to
    low flows and not very accurate.
6.   Upstream banks should be high enough
    to contain the increased depth caused
    by flume installation.

Quality Flow Measurements at Mine Sites
                                                          SAIC Draft Final
                Table 4-1.  Water Measurement Device Selection Guidelines*
 Current meter
 Acoustic Velocity
   Can be used at high
   discharge rates, > 150 cfs.
2.  Do not require much head
3.  Good for calibrating
   recording gages.
1.  High cost and requires manual
2.  Not accurate for measuring flows less
   than 10 cfs.
3.  Accurate measurements require
   uniform flow, smooth banks, and stable
1.  Multipath AVMs can attain
   accuracies within 1% of
   actual discharge, ±3% for
   single path acoustic
   flowmeters; under proper
   conditions and careful
2.  Able to measure bidirectional
   flow and backwater
3.  AVMs can take continuous
   measurements over a long
   period of time.
4.  Good for flat gradient
5.  Often used when section
   rating and stream gaging
   costs are high.
1.  AVMs cannot be used in reaches with
   eddies or a high degree of turbulence.
2.  Entrained gases and/or suspended
   sediment affect the acoustic signal
   strength and measurement accuracy.
3.  AVMs are generally only practical
   when channel widths are large.
4.  High initial cost.
5.  Accuracy is highly dependent on the
   quality of data collected for calibration.

Quality Flow Measurements at Mine Sites
                                                          SAIC Draft Final
                Table 4-1. Water Measurement Device Selection Guidelines*
 Tracer and dye
 dilution methods
1.  Salt-dilution method works
   well in small to medium
   sized, turbulent streams.
2.  The dye-dilution method has
   low detection and
   measurement limits.
3.  Measuring dye tracer
   concentrations is simple,
   relatively easy to apply in the
   field, and accurate with
   fluorometric techniques.
4.  Measurement accuracy can
   be ±1% of actual  discharge.
5.  Often used when flow is
   inaccessible or conditions are
   unsuitable for other
   measuring devices. For
   example, channels where a
   significant amount of the
   discharge occurs through the
   stream substrate and
6.  Suitable for measuring flow
   where the channel cross-
   section cannot be accurately
   measured or changes.
7.  Salt-velocity-area method is
   highly accurate.
1.  Measurement accuracy can be as low
   as 30% of actual discharge if the
   technique is not properly applied.
2.  Closed conduit measurements are
   typically more accurate than open
   channel measurements.
3.  Complete mixing can be a problem at
   low velocities.
4.  Dyes can be affected by strongly
   acidic conditions.
5.  Dissolved iron oxyhydroxides can
   affect fluorometer readings.
6.  Tracer methods require special
   equipment and experienced personnel.

Quality Flow Measurements at Mine Sites
                 Table 4-2. Water Measurement Device Selection Criteria
Head Loss (feet)


























Design Flow Range (cfs)









24 5







Other Considerations
Able to measure higher flows than V-
notch weirs in channels suitable for
weirs (i.e., appropriate head loss).
Accurate device particularly suited to
low flow measurement. Best weir

profile for discharges less than 1 cfs
or up to 10 cfs.
Similar to a rectangular weir except
that the inclined edges simplify the
discharge equation. Less accurate
than both the V-notch and rectangular
The compound weir is a rectangular
weir with a V-notch cut into the crest.
Compound weirs lose accuracy in the
transition between when the weir is
functioning as a V-notch and a
Most widely used flume for permanent
installations. Installation is fairly
Can measure super-critical flows, the
trapezoidal cross-section permits a
wider measurement range than other
flumes, and the flat bottom passes
sediment and other debris.
The principal advantage to H-type
flumes is the wide range of measurable
flows with reasonable accuracy.
Flume construction and installation
are relatively simple.
Passes solids better than a Parshall

due to flat bottom. Functions well

with a high degree of submergence.
 : Grant and Dawson, 1997

                           Table 4-3.  Water Measurement Device Selection; Ranking of Factors
velocity meters










t Pass








































SOURCE: BOR (1997)
  **  Symbols (+, 0, -) are relative indicators for comparing devices to the stated criteria; (+) is a positive attribute, (0) is a neutral attribute, and (-) is
      a negative attribute.
  1.   Field Verify - After construction or installation, some verification of the calibration is generally recommended.  Verification simply serves as a
      check against gross errors in construction or calibration.
  2.   Standardization - Device standardization and periodic calibration may be required.

Quality Flow Measurements at Mine Sites

      As previously discussed, the choice of an appropriate or applicable method to measure
discharges at mine sites is extremely dependent on the objectives of the study being conducted.
Selection of the proper measurement technique to measure stream discharge at a mine site is dependent
on a variety of factors which include, type of waste facility or mine feature, existing site conditions, the
need for recording or non-recording methods, site availability, and desired precision and accuracy.
With most methods of flow measurement, some experience is required to both determine the best
specific measurement method for a given purpose and site, and to accurately install the measurement
device or conduct the measurement.

      There are several  common features associated with many mine sites that offer unique difficulties in
obtaining flow measurements.  Common situations that occur at mine sites which often make it difficult
to measure flow or choose a measurement method are:

      • •   Measuring groundwater discharge from abandoned adits or ventilation shafts.

           Measuring  discharge from tailings ponds and below tailings dams.

      •    Measuring  stream flows in high mountain streams with unstable channel conditions and
           turbulent flows.

Methods which are commonly employed for these mining situations are discussed below.

4.3.1  Measuring Discharge From Adits

      Abandoned adits  commonly discharge groundwater, either ephemerally during periods of high
runoff and groundwater elevation, or perennially when the adit is located below the elevation of the
normal groundwater table.  Discharge from adits is often highly acidic (i.e., acid mine drainage)
containing high concentrations of dissolved metals and sulfate. Accurately measuring discharge from
adits can be difficult because they are often perched on hillsides. Flow often occurs down hillsides via
unstable channels or gullies that continuously change direction, size and geometry.  This prevents the use
of any recording or non-recording measurement method that relies on the development of a stage
discharge relationship or a stable channel to develop a cross-sectional profile.

      Flumes are often utilized to measure discharge from adits where conditions directly below the adit
outfall provide a relatively level surface for flume installation and sufficient room to have an adequate
approach channel  (see Section 3.3.3).  H or HS Flumes can be particularly useful for measuring flow
from adits because their design promotes self-cleaning of debris and sediment in the flume floor.


Quality Flow Measurements at Mine Sites
Installation must be accomplished to insure that all flow is routed through the flume. Specific
advantages to using an H, HS, or other type of critical flow flume are:

      • •     Use in unstable or irregular channels.
            Low maintenance and self cleaning (i.e., passes sediments and debris).
            Accurate measurements under low-flow regimes such as those that occur from adits.
      • •     Precision and consistency over time.
      ••     Low-cost.

Disadvantages may include:

      ••     Difficult to install, depending on specific site conditions and experience of user.
      • •     The method is usually non-recording.

      In general, weirs are not conducive to making flow measurements from adits because of unstable
channel conditions.  Small V-Notch weirs could be constructed to measure adit discharge, if the
discharge occurs to a relatively stable channel that is straight with parallel lines of flow for at least 10 to
20 feet upstream.  In addition, the width of the stream must also be approximately the width of the weir
box for this distance. These conditions do not generally occur near adits.  A further disadvantage of
weirs is that they are not suited to measuring flows containing high amounts of sediment.  Adit discharge
is often high in suspended solids and precipitated iron oxyhydroxides that result in acid mine drainage.
High suspended solids can damage the weir blade, causing inaccurate measurements. Likewise, the use
of current meters also require that a stable channel and stable flow conditions exist. Flow meters
require velocity measurements at several locations along a channel cross section at appropriate depths.
Flows from adits are often less than the minimum depth required to use the meter, and channel cross
sections can not be properly segregated to obtain measurements of cross sectional area (A).

      In many cases, adits are located on very steep slopes and are not conducive to direct
measurement of discharge. Study objectives for measuring flows from adits, however, are usually tied
to determining impacts to water quality in receiving streams or watersheds. In these cases,  it is  often
possible to indirectly measure impacts to water quality and quantify loading from a particular adit or
other mine feature. If there is a receiving stream below a particular adit, the loading contribution and
affect to water quality from this source can be quantified by taking synoptic samples of stream
discharge and water quality at locations  above and directly below the adit. In conducting this type of
study, appropriate measurement methods must be chosen and sites must be located both above and
below the influence of the adit or mine feature.  Measurements for stream  discharge, and corresponding
water quality samples must be taken at approximately the same time (i.e.,  as close together as possible)
without influence from recent rainfall-runoff events.  Calculations to determine relative loading and
contributions from sources are discussed in detail in Appendix A. Instantaneous load at each location is


Quality Flow Measurements at Mine Sites
calculated by multiplying stream discharge by constituent concentration. Loading is commonly
expressed in pounds per day or kilograms per day for each constituent of concern. The change in
instantaneous load between the two locations can be used to quantify the effect to water quality from
the source (EPA, 1996).

4.3.2 Measuring Discharge From Tailings Ponds

      Most mines dispose of tailings in engineered impoundments that cover areas ranging from a few
acres to more than a thousand acres. These facilities can discharge tailings water either directly from
the impounded pool or as seepage of pore fluids.  Seepage discharge typically occurs from one or
more points from raised embankments to stream channels or designed diversion channels. The
(typically engineered) foundation slope determines the location and number of seepage discharge

      A variety of methods to measure rates of seepage and/or storm water discharge from tailings
impoundments can often be employed, depending on specific site and channel conditions, below the
discharge point. Discharge usually occurs into natural or designed drainage channels with relatively
stable channel conditions. For this reason, channel geometry and channel cross-sections can be
measured or are known.  Current meters, flumes and weirs could all be applicable to measure
discharge from tailings impoundments under these conditions. However, rectangular or V-Notch weirs
are commonly employed at these sites because they offer the following advantages:

      •     Precise measurement of low flows. Small V-Notch weirs can be designed to measure
           discharges ranging between 0.01 cfs (3.99 gpm) and 24.5 cfs (11,000 gpm). These
           relatively low flows often occur from tailings discharges especially during drier seasons.
           Rectangular weirs can be  designed to measure flows up to 335 cfs (150,000 gpm).

      •     Weirs can be used to establish permanent measuring points with a high degree of precision
           between sampling dates.

      •     Weirs offer easily obtainable and consistent sampling locations for concurrent water quality

      •     Automatic recording can be employed with some weir systems by measuring stage in the
           weir basin with a pressure transducer or float mechanism.

Quality Flow Measurements at Mine Sites
4.3.3 Measuring Discharge in Remote Areas with Unstable Channel Conditions

      Flow measurements are extremely important for studies evaluating changes in water quality,
pollutant loading, and surface and ground water interactions. The identification of influent and effluent
stream reaches, and study of the impacts that specific mine facilities and features have on hydrology and
water quality within a watershed is dependent on accurate stream flow measurements taken synoptically
(e.g., samples collected at approximately the same time) at key locations within a stream.
Unfortunately, many mine facilities and many mine features occur in remote locations within a watershed
that are not conducive to many methods used to measure stream discharge. Optimum locations
immediately below specific mine features or mine discharges commonly do not have conducive stream
channel and flow conditions to measure stream discharge.  This can occur because of the remoteness of
the site, the existence of steep channels with turbulent flow conditions, or at sites where a significant
proportion of stream flow occurs within alluvial gravels and cobbles in the streambed. Under these
conditions, the use of dye or salt tracers offers  several advantages and is often the only method
available to determine stream discharge  at several locations within a remote watershed. Carefully
applied, tracer techniques combined with sampling at multiple locations can be used to:

            Determine the relative pollutant load occurring from specific mine features, waste dumps,
            tailings facilities, and adits to watersheds.

            Identify losing and gaining  stream reaches and quantify the exchange of streamwater and

      •     Evaluate loadings  of metals and other constituents in gaining stream reaches caused by
            influent ground water.

      •     Evaluate precipitation and dissolution reactions of metals in specific stream reaches or
            below tributaries of sub-watersheds.

            Determine natural  geochemistry and background water quality conditions in upper
            watersheds above the influence of mining.

Quality Flow Measurements at Mine Sites


      Quality assurance (QA) is a system of defined activities, the purpose of which is to provide
confidence that specified standards of quality are achieved.  A QA/QC program, as defined by a
Quality Assurance Project Plan (QAPP), should be included as a part of each project as a means of
integrating the quality planning, quality assessment, quality control (QC), and quality improvement
efforts needed in order to meet user requirements.  These QA/QC components include management;
QA Objectives; measurement and data acquisition; assessment and oversight; and data validation,
usability, and reporting. Included are "guidelines" for QA/QC specifications as defined for mine flow
measurement projects. Specific measurements required to achieve project objectives are also

      A QAPP is developed to define project specific QA/QC requirements.  Specific QA activities,
including QC checks, are noted in the project QAPP.  The purpose of specified quality assurance
activities is to reduce measurement errors to agreed upon limits and to produce results of acceptable
and known quality. Therefore, specific project requirements are documented as a part of the QAPP.
This will help to ensure that the measurement system is in control and that it provides the detailed
information necessary for assessment of the collected data.

      QA/QC guidelines should be incorporated by project personnel in conjunction with project
requirements. Precision, accuracy, representiveness, comparability, completeness, and sensitivity are
defined in the QAPP and are different for different projects; however, guidance is provided in this
document as to specific measurement objectives that can be achieved. Along with this guidance,
additional QA/QC considerations are included to provide elements needed for the development of an
appropriate  QAPP.  This section therefore, includes a general discussion of appropriate QA/QC
elements as would be required for flow measurements at mining sites.


5.2.1  QA Management

      QA oversight is needed for all  projects to provide an independent assessment of quality
assurance activities. It is important that QAPPs for all projects (including mine flow measurements) be
independently reviewed to determine if project objectives can be achieved per the QA/QC
requirements specified in the subject QAPP. In addition, periodic assessments of QAPP
implementation is recommended. These assessments should be performed by QA personnel who are


Quality Flow Measurements at Mine Sites
independent of management activities. QA oversight is needed in order to provide initial QAPP review
and periodic project assessment. The amount of QA oversight required will vary depending upon
project scope and the intended use of project results.

5.2.2 Quality Assurance Objectives

      Precision is the ability of a measurement system to generate reproducible data. For most
parameters, precision is determined from the results of duplicate determinations, and is reported as
relative percent difference (RPD). Relative standard deviation (RSD) is used when triplicate or
additional like measurements are made.

        RPD=  JD^_D_ xlOO
      where:     Dj and D2 are the two observed values, and Dj is > T)2.

      RSD = standard deviation x 100

      The frequency and acceptance criteria for replicate sample measurements are based upon the
objectives of the project, and are specified in the QAPP. Precision estimates are best obtained by
comparison of duplicate calibration determinations (e.g., pressure transducer check using staff gage) or
duplicate field measurements, if applicable (e.g., reading of staff gage).

      Duplicate flow measurements are best obtained by field observation. Because flow
measurements are dependent upon reading or siting of specific instrumentation, variability of two or
more measurements are not expected to differ greatly when these measurements are taken within
approximately the same time period.  Siting of a gage for example, used to measure stage height for
input into the equation to calculate discharge from a weir or flume, may vary only slightly due to surface
water disturbance. This measurement variability could be calculated and a precision estimate
determined by making two or more consecutive measurements. Current meters or acoustic flow meters
may also experience slight variability when measurements are taken in close time proximity, one to
another.  When taking two or more replicate measurements for flow meters, it is important to

      •      Time between measurements should be minimal to determine instrument precision without
            adding variation in flow.


Quality Flow Measurements at Mine Sites
           If overall flow measurement and instrument precision are desired in order to obtain average
           flow over a period of time, several measurements could be made during a specified time
           period and precision calculated using several measurements.

      •     Precision estimates are used to determine variability and not bias, therefore efforts to
           eliminate measurement bias (e.g., two separate observers) should be used when possible.

Precision estimates for most flow measurement instrumentation should probably not vary by more than
2 to 5 %.

      Accuracy is defined as the nearness of the reported result to the "true" value.  Accuracy is
assessed by comparison of flow results to engineering calculations or to manufacturer specifications.

      Accuracy is dependent upon instrument siting criteria, alignment of the measurement device in the
stream (i.e., flumes), number of different measurements obtained within a stream (i.e.,  current meters),
overall construction and placement of the instrument. In this  sense, accuracy and representativeness are
related.  Each of the previous sections in this document has discussed criteria required to obtain more
accurate results and the expected accuracy of particular instrumentation has been provided (e.g., 2 -
5%).  Accuracy of a particular instrument, however, is dependent upon manufacturer specifications and
is not included for all measurement devices due to varying specifications.  Flow measurements,
however, have no "true" results for comparison. Some instrumentation (e.g., weirs or flumes) may be
inherently more accurate than others (e.g., current meters). Choice of instrumentation will determine the
accuracy of the final result. Because stream variation and type of flow measurement are
interdependent, engineering judgment is required to determine the most accurate instrument for the
measurement being made.  If very accurate measurements were required, two different instruments
could be used for comparison. This comparison should most  likely be used only as comparative
numbers and assumptions as to which device is most accurate would be an engineering judgment.

      As noted, in most cases a particular measurement device may have  no "true" or calculated value
for comparison. Manufacturer specifications may be used or  in special situations it may be best to
express  an accuracy determination by repeat measurements of the result until the "true" value (e.g.,
average value) has been determined.  This would require enough determinations to represent a
statistical population.  For example, three separate measurements of the same flow could be made using
the same device. A percent RSD could be calculated and then a "t" table used to produce a
confidence interval. The width of the confidence interval could than be used to express the expected
accuracy of the particular device. Narrower confidence intervals could be determined by more

Quality Flow Measurements at Mine Sites
replicate measurements.  (Note that this is useful methodology for an accuracy determination when no
independent measurement can be made.) The average result, which is expressed as a confidence
interval, could be used as a standard or "true" value by which subsequent measurements could be
compared.  Accuracy for flow measurements could then be expressed as a percent bias. This is the
difference between the measured value and the calculated or "true" value as noted above.  The
equation below expresses accuracy as a percent bias.

      %Bias = (measured value-true value)/true value x 100

Examples are provided in Table 5-1 which provides issues or considerations associated with accuracy
and precision for several  types of mine flow measurements. While only a summary, more complete
information can be found in Section 3.0 which describes each of these measurements. This table is not
intended to provide comprehensive information but to stimulate additional thought when considering
"measurement quality objectives" required to satisfy project objectives.

Accuracy of measurement devices can be within +/- 5% or better depending upon conditions noted
above. In some cases, it is possible to check the accuracy and/or precision of a measurement by using
another measurement. For example, it may be possible to make occasional checks of discharge using a
graduated cylinder and stopwatch, provided the total discharge can be collected in such a manner (e.g.
discharge at a pipe). This requires small enough flows such as may occur from mine adits.  In many
cases, the accuracy or precision of the measurement will only be able to be determined based upon
criteria and conditions required for set-up and then relying upon the theoretical calculations noted for
the different measurements.  Physical inspection on a periodic basis is critical to maintaining accurate
and precise measurements. Table 5-1 also provides more comprehensive guidelines critical to
achieving accurate and precise measurements.

      Data completeness is a measure of the extent to which the data base resulting from a
measurement effort fulfills the objective for the amount of data required.  Completeness is defined as the
percentage  of valid data  obtained compared to the number of tests required to achieve a statistical level
of confidence in the results.

Quality Flow Measurements at Mine Sites
  Table 5-1.  Accuracy and Precision Considerations for Mine Flow Measurement Methods
  Measurement Method
 Gages (non-recording)
      Accuracy Considerations
1.  ability to read gage height (viewing
2.  ability to record unusual events
   (changes which occur between
   measurement periods)
variation of different observers
 Gages (recording)
1.  ability to measure small
2.  inaccuracy caused by corrosive
   conditions such as weathering
1.  free flow of water over the weir
2.  water carrying significant loads of
   sediment or debris
3.  construction, shape, placement,
   etc. (See criteria noted in section
   3.2.2 for sharp crested weirs)
occasional check of measurement
device in field (visual observation
and collection of duplicate
1.  siting criteria, requirements of
   straight channels above and below
2.  decision as to flow measured being
   critical or super-critical
3.  channel characteristics
4.  approach velocity
occasional check of measurement
device in field (visual observation
and collection of duplicate
 Current Meters
1.  channel conditions, siting criteria,
   streamline orientation
2.  laminar, critical or sub-critical flow
3.  methods used to determine flow
comparison of different methods
used for determining flow velocity
(Section 3.4.3)
 Tracers or dye
   mixing conditions including time of
   travel, dispersion, and discharge
   backflow and eddies
   calibration of measuring device
precision check of measurement

Quality Flow Measurements at Mine Sites
      Representativeness is ensured by a well-defined sampling strategy designed to collect
measurements which exhibit average properties of the site at that stage of the technology. Examples of
how to collect appropriate samples have been previously noted in this document. Representativeness is
enured by collecting sufficient samples to characterize the site or collecting measurements that
appropriately define the problem. Means of collection and standard methods used to collect flow
measurements are defined in the body of this document.  It should be remembered that representative
data are defined by the method of collection and the manner in which the method is implemented.
Collecting representative data is therefore dependent upon individual or site specific factors including
the instrument chosen for measuring a particular flow, survey methods used for instrument placement,
determining if flow is turbulent or laminar, placement of the flow instrument in the output stream, etc.

      Comparability is generally achieved by the use of standard methods. This makes collected data
comparable to other sites or projects that have similarly defined situations.  Standardizing methods for
collection of flow data is one of the objectives of this document.  Reporting the data in standard units of
measure and adhering to the specified calibration and set-up procedures all contribute to comparability
of the data.

      Unlike chemical analyses,  however, flow measurements have no specifically  recommended
procedures that are universal for  similar situations. The choosing of a flow measurement method is
dependent upon many variables  (e.g., expected flow rate, changes in seasonal flow, stream debris, etc.)
and is not specifically defined  for every situation. Therefore comparability of data from two separate
sites may be less tangible than comparison of analytical data from two  different areas.  When data from
two different sites are compared  the engineer must factor these previously mentioned variables into their
evaluation to determine if the collected data can be compared. In this manner comparability refers to
comparability of measurements from two different sites or perhaps comparability of data collected by
two separate measurement methods.

      Method detection limits (MDLs^ or Sensitivity are determined based on instrument detection
limits.  Resolution of the particular instrument will determine the MDL.  A particular instrument or
device can only measure to an absolute lower  limit.  Some typical sensitivities are given in Table 5-2
and have been discussed in the previous sections. In the planning stage of a project, required detection
limits need to be specified so that an appropriate measurement method can be selected.

Quality Flow Measurements at Mine Sites
      Table 5-2.  Sensitivity Limitations for Different Mine Flow Measurement Methods
Measurement Method
Rectangular weir
V-notch weir
Trapezoidal weir
Compound weir
Parshall flume
Trapezoidal flume
H/HS flume
San Dimas flume
Typical Sensitivity
(see appropriate section)
0.286 cfs
0.009 cfs
0.301 cfs
0.009 cfs
0.010 cfs
0.0001 cfs
0.347/0.085 cfs
0.1 6 cfs
Each of the above detection limits is given as an example and is dependent upon assumptions as
described in previous sections for each of these instrument measurement devices.

The following tables (Table 5-3 and 5-4) present typical objectives and QC checks that may be used in
a QAPP for evaluation criteria required with specific measurement devices. These tables are only
examples and are not intended to cover specific project objectives.

5.2.3  Measurement and Data Acquisition

      Several types of mine flow measurements have been previously defined in this document (see
Section 3.0). Specifically, mine flow measurements require considerations as to the type of mine, the
amount of flow expected, and the site conditions. QA considerations (precision, accuracy, and
sensitivity) associated with these measurements are summarized in previous sections and therefore are
not repeated here. QA/QC requirements and activities to ensure quality measurements are specified in
noted subsections.

Quality Flow Measurements at Mine Sites
               Table 5-3.  Example QC Objectives for Specific Measurements


stage height





+/- 5%

+/- 5%
(factory certification
or compared to
manual measure-

(compared by
duplicate height
(duplicate readings)

0.009 cfs

0.010 feet



            Table 5-4. Typical QC Objectives for Specific Measurement Device
Transducer used
to measure stage
QC Check
obstruction of
determine if there
is debris
calibration check
with staff gauge
determine if flow
is obstructed
significant build-
up such that flow
is obstructed
within 5%
Corrective Action
clean and
consider alternate
clean and
consider alternate
5.2.4 Assessment and Oversight

     Audits are an independent means of confirming the operation or capability of a measurement
system, and of documenting the use of QC measures designed to generate valid data of known and
acceptable quality. An audit is performed by a technically qualified person who is not directly involved
with the measurement system being evaluated. A performance evaluation is generally an objective audit
of a quantitative nature, and a systems audit is a qualitative evaluation of the capability of a
measurement system to produce data of known and acceptable quality.

Quality Flow Measurements at Mine Sites
      A systems audit (the type of audit anticipated for mine flow measurements) is a qualitative
determination of the overall ability of a measurement system to produce data of known and acceptable
quality, by an evaluation of all procedures, personnel, equipment, etc. utilized to generate the data. It is
an evaluation of whether adequate QC measures, policies, protocols, safeguards, and instructions are
inherent in the measurement system to enable valid data generation, and/or the immediate identification
of outlier data and subsequent actions.  It must be performed in the field and is performed by someone
independent of measurement activities.

      A systems audit should review the project organization and technical personnel involved.  Field
activities are evaluated, including: use of proper sampling equipment, procedures for equipment
maintenance, acceptable sampling protocol, calibration procedures for field measurements, and
adequate field documentation and record-keeping procedures. The purpose of the audit is to provide
recommendations as to how field measurements may better meet project requirements.  If procedures
are being followed as specified in the QAPP then the audit will document that appropriate procedures
are being followed.  Due to the nature of the measurements being evaluated, this audit must be
performed by appropriately qualified personnel.

An example of some checklist items  are provided below:

      • •    Are method specifications as provided in the QAPP being followed?

      •    Was instrument siting criteria appropriately recorded and were specifications noted in the
           field log?

      •    Are duplicate measurements obtained and how are they obtained? Does instrument
           precision appear to meet QAPP specifications?

      • •    How was flume alignment within the stream determined and was this properly recorded?

      • •    If current meters are used, what method is being performed to determine flow velocity?

           Are stream discharge data appropriately calculated?

           How often are stream flow measurement data collected and are they recorded
           appropriately in the field log?

Quality Flow Measurements at Mine Sites
            What engineering oversight was performed to ensure proper installation of the specified
            flow device?

            Have personnel who are making field measurements been appropriately trained?

5.2.5 Data Validation, Usability, and Reporting

      For analytical data to be scientifically valid, defensible, and comparable, the correct equations
and procedures must be used to prepare the data. Measurement evaluation is a systematic process of
reviewing a body of data to provide assurance that the data are adequate for their intended use. The
process includes the following activities:

            Auditing measurement system calibration and calibration verification.
      ••     Auditing QC activities.
            Screening data sets for outliers.
      ••     Reviewing data for technical credibility vs. the sample site setting.
      • •     Auditing field sample data records.
            Checking intermediate calculations.
      • •     Certifying the above process.

      Prior to data collection, determinations are made regarding the data to be gathered in the field
and the methodology to be used. Once the data are obtained, they must be reviewed and assessed as
to their adequacy. If it is determined that the initial data collection concept did not provide adequate
data, the entire process may need to be repeated to identify and correct data inadequacies. The
following sections describe the data reduction, validation, and reporting procedures that should be
incorporated into the QA program.

Data Reduction

      All measurement system outputs (e.g., flow data) must be reduced into units which are consistent
with the methods and which meet the comparability objective. In general, all raw  data are recorded in
field notebooks or on worksheets in standardized format, by the technician performing the test.  Each
method contains detailed instructions and equations for calculating the respective flow measurements.
Typical data outputs are in cfs or gpm as noted in previous sections.

Quality Flow Measurements at Mine Sites
Data Validation

      Data validation is a systematic process of reviewing data against a set of criteria to identify
outliers or errors and to delete suspect values or to flag them for the user. Data reviews starts with the
field quality control procedures specified in the QAPP. The quality control data produced are
reviewed throughout data generation using the criteria and procedures described in this section to
validate data integrity during collection and reporting of field data. Validation requirements are
described in the QAPP. Some guidance regarding validation is presented in the following discussion.

      Review of field and quality control data should initially be performed by the responsible
technician. The data are checked for errors in transcription and calculations, and for compliance with
quality control requirements. Failure to meet method performance quality control criteria results in the
re-collection of appropriate data. After the initial review is completed, the data should be collected
from summary sheets,  workbooks, or computer files and assembled into a data package.

      The next level of data review is the responsibility of the project manager.  The project manager
may also responsible for development and implementation of data review checklists, as needed.  The
areas addressed in the checklists typically include the following:

            Data collected according to specified methods.
            Instrumentation calibrated according to specified methods.
      ••    Calculations performed correctly and verified.
            Transcription of raw and final data done correctly.
      • •    Detection limits determined correctly and within required limits.

      Typically , field personnel will review 100% of the data generated, including any calibrations and
calculations, and will sign and date all field notebook entries. The field manager often reviews at least
10% of the generated data and all  of the data quality indicators (e.g., triplicate groundwater level
measurements for precision criteria). Any suspect data should be investigated.  All outliers are
identified in the report narrative. Outlier data should only be rejected if they can be explained by some
physical phenomenon.

Data Reporting

      All original data should be recorded in a permanent manner, and be readily traceable through all
steps of the data generation/reduction/ validation/review process. Field measurements should be

Quality Flow Measurements at Mine Sites
recorded in appropriate field notebooks and results reported in tabulated summary form, or as
otherwise specified in the QAPP. Some guidance in terms of data reporting is presented in the
following discussion.

      Field data is typically reported by the technician in an appropriate log book. The field report
should include all associated raw data, non-conformance records, memos and communication records
and any other documentation for the reported results.  The reported data is then validated, as discussed
previously, and approved for reporting.

      The reporting requirements should include:  measurement summary and cross-reference, narrative
discussion of QC sample results, field measurement methods and detection limits, and calculated data
(tabulated summary, duplicate summary, calibration summary, and all associated raw data).

      The validated field data should be used to prepare reports that evaluate the technology or project
and assess its potential applications.  The exact format and detail of the report to be prepared is not
included in this document and is project dependent. However, the report should include, at a minimum,
the following information:

      •     A thorough discussion of the procedures used to collect the measurements and to define
            data quality and usability should be included.  The discussion will focus on the data quality
            indicators such as precision, accuracy, completeness, comparability and

            The results of any assessments performed during the course of the project should be
            documented, including corrective actions initiated as a result of these assessments and any
            possible  impact on the associated data.

      •     All changes to the proposed procedures should be documented regardless of when they
            were made. The rationale for the changes will be discussed along with any consequences
            of these changes.

      •     The identification and resolution of significant QA/QC problems should be discussed.
            Where it was possible to take corrective action, the  action taken and the result of that
            action will be documented. If it were not possible to take corrective action, this, too,
            should be documented.

Quality Flow Measurements at Mine Sites
      Reliable measurements of flow data require continuous monitoring and evaluation of the
processes involved, i.e., quality assurance. To ensure optimum valid data generation, a scientifically
sound and strictly adhered to quality control program must be incorporated into the data  or
measurement collection program.  Such a QC program employs a prescribed sequence of routine
procedures to control and measure the quality of the data generated.  These procedures should be
documented for verification of the collected measurements and for purposes of repeating the program
or project as necessary. A data report is the final output that verifies appropriate data collection.

Quality Flow Measurements at Mine Sites

Barfield, B.J., R.C. Warner, and C.T. Haan. 1981. Applied Hydrology and Sedimentology for
     Disturbed Areas. Oklahoma Technical Press (Stillwater, OK).

Bloom, A.L. 1991.  Geomorphology: A Systematic Analysis of Late Cenozoic Landforms. Second
     Edition. Prentice Hall (Englewood Cliffs, New Jersey).

British Columbia Ministry of Environment, Lands and Parks, Resources Inventory Branch for the
     Aquatic Inventory Task Force, Resources Inventory Committee.  1998. Manual of Standard
     Operating Procedures for Hydrometric Surveys in B.C.  Version 1.1.
     www.for.gov.bc.ca/ric/Pubs/Aquatic/hydro/index.htm (October 20, 2000).

Brooks, K.N., P.P. Ffolliott, H.M. Gregerson, and J.L. Thames.  1991.  Hydrology and the
     Management of Watersheds.  Iowa State University Press (Ames, Iowa).

Buchanan, T.J., and W.P. Somers.  1982.  Stage Measurement at Gaging Stations.  Tech. Water-
     Resources Inv., Bk. 3, Chap. A7. United States Government Printing Office.

Carter, R.W., and J. Davidian.  1968. General Procedure for Gaging Streams. Tech. Water-
     Resources Inv., Bk. 3, Chap. A6. United States Government Printing Office.

Environment Australia. 1997.  Managing Sulphidic Mine Wastes and Acid Drainage, Best Practice
     Environmental Management in Mining, Commonwealth of Australia, May 1997, 81 pp.

Grant, D.M. and B.D. Dawson.  1997. Isco Open Channel Flow Measurement Handbook, Fifth
     Edition. Isco, Inc. (Lincoln, NE).

Hutchinson, I.P.G. and Ellison, R.D. 1991. Mine Waste Management, California Mining Association,

Kilpatrick, F.A., and E.D. Cobb.  1985.  Measurement of Discharge using Tracers. Tech. Water-
     Resources Inv., Bk. 3, Chap. A16. United States Government Printing Office.

Kilpatrick, F.A., and V.R. Schneider.  1983. Use of Flumes in Measuring Discharge. Tech. Water-
     Resources Inv., Bk. 3, Chap. A14. United States Government Printing Office.


Quality Flow Measurements at Mine Sites
Kilpatrick, F.A., and J.F. Wilson, Jr. 1989. Measurement of Time Travel in Streams by Dye Tracing.
     Tech. Water-Resources Inv., Bk. 3, Chap. A9. United States Government Printing Office.

Kulin, G. And P.R. Compton.  1975. A Guide to Methods and Standards for the Measurement of
     Water Flow. Special Publication 421, National Bureau of Standards.

Linsley, R.K., Kohler, M.A., and Paulhus, J.L.H.  1975. Hydrology for Engineers, 2nd edition,
     McGraw-Hill Series in Water Resources and Environmental Engineering, McGraw-FIill, Inc.,
     New York, NY, 482 pp.

Lacy, W.  1999. An Introduction to Geology and Hard Rock Mining. Science and Technology
     Series, Rocky Mountain Mineral Law Foundation, Denver, CO 80220.

MEND.  1995.  Hydrology of Waste Rock Dumps, Mine Environment Neutral Drainage Program
     Associate Project PA-1, Natural Resources Canada, Ottawa, ON, July 1995.

MEND.  1996.  Guide for Predicting Water Chemistry from Waste Rock Piles, Mine Environment
     Neutral Drainage Program Report 1.27. la, Natural Resources Canada, Ottawa, ON, July 1996.

O'Hearn, J.  1997. Surface Water Quantity. In: Marcus, J.J., ed., Mining Environmental
     Handbook, Effects of Mining on the Environment and American Environmental Controls
     on Mining, Imperial College Press,  London, pp. 221-225.

Roberson, J.A., J.J. Cassidy,  and M.H. Chuadhry.  1988. Hydraulic Engineering. Houghton Mifflin
     Company (Boston, MA).

Steffen Robertson and Kirsten (SRK). 1992. Mine Rock Guidelines: Design and Control of
     Drainage Quality, Saskatchewan Environment and Public Safely, Mines Pollution Control
     Branch Report #93301.

Steffen, Robertson and Kirsten (B.C.), Inc., Norelco Environmental Consultants, and Gormely Process
     Engineering, (SRK et al). 1989. Draft Acid Rock Drainage Technical Guide, British
     Columbia Acid Mine Drainage Task Force Report, BiTech Publishers, Ltd., Vancouver, B.C.

Society of Mining Metallurgy and Exploration (SME).  1992. Mining Engineering Handbook.  H.L.
     Hartman (editor), Society for Mining, Metallurgy and Exploration, Inc., Littleton, CO.


Quality Flow Measurements at Mine Sites
U.S. Department of the Interior, Bureau of Reclamation (BOR). 1997.  Water Measurement Manual:
     A Guide to Effective Water Measurement Practices for Better Water Management. Third
     Edition. U.S. Government Printing Office (Denver, CO).

U.S. Environmental Protection Agency (EPA).  1994. EIA Guidelines for Mining: Environmental
     Impact Assessment for New Source NPDES Permits, Ore Mining and Dressing and Coal
     Mining and Preparation Plants.  U.S. EPA, Office of Federal Activities, Washington, D.C.

U.S. Environmental Protection Agency (EPA).  1996. U.S. EPA NPDES Permit Writers'Manual,
     EPA-833-B-96-003, December 1996.

Vick, S.G.  1990. Planning, Design, and Analysis of Tailings Dams, BiTech Publishers,
     Vancouver, B.C., 369 pp.

                      APPENDIX A
               Description of Common Mine
Potential Impacts to Surface Water Hydrology and Water Quality,
                         and the
             Importance of Flow Measurements

Quality Flow Measurements at Mine Sites

      Mining operations produce a variety of solid and liquid wastes that often require permanent
management. Many mining operations predate environmental consciousness or laws, and at many sites,
waste management (to minimize environmental consequences) was not practiced. However, new
mining operations are required by various state and federal laws to incorporate sound design and
management of waste disposal facilities, as well as monitoring programs incorporating measurements of
both discharge and water quality. Abandoned mine sites continue to impact watersheds through
rerouting or impoundment of surface and ground water flow, and pollutant loading.

      Large mining operations can generate over a billion tons of solid wastes that cover areas
exceeding a thousand acres. Even smaller operations must handle and dispose of formidable quantities
of materials that can affect large areas. In order to prevent or minimize environmental impacts, waste
facilities must be designed and reclaimed in a manner that minimizes or prevents environmental impacts.

      The mining method employed and the type of mineral extracted are large factors in determining
the volume and types of solid waste facilities developed. The methods employed to beneficiate and
process ore are also factors associated with the types  of waste produced and the characteristics of
waste streams. Flow measurements are often required to monitor direct discharges or storm water
runoff from different types of mine facilities, such as waste rock dumps, tailings impoundments, leach
pads and process facilities.  The remainder of this section provides a summary discussion describing
different types of mining and common waste facilities that may exist at mine sites. A more detailed
discussion of mining and processing methods  can be obtained from SME (1992) and Lacy (1999), and
more detailed discussions of the design and management of waste facilities is provided by MEND
(1995), SRK (1992), and Vick (1990).

Types of Mines

      There two major classes of mining methods, surface and underground. Common surface mining
methods include open pit mining, quarrying, glory holing, strip mining, and placer mining.

      1.     Open pit mining is a surface mining method in which nearly all of the deposit and ore is
            removed in terrace-like working areas on the side of a pit. Grade and tonnage of materials
            available determine the size and limits of the pit developed as well as the size and
            configuration of waste rock dumps.  In this type of mining, it is often necessary to blend
            different ore types to maintain character and grade of the mill feed, or different types of ore
            (i.e., oxide verses sulfide and low-grade ore) may need to be managed and processed

Quality Flow Measurements at Mine Sites
            differently (Lacy, 1999). Oxide and sulfide ores require different types of beneficiation
            and processing, and some low-grade oxide ores may be processed in a leach pad facility.
            Open pits are often closed or reclaimed by simply allowing the pit to develop into a pit
            lake and waste rock dumps are usually closed and reclaimed in place.

      2.     Quarrying is similar to open-pit mining, however, it is usually restricted to mining dimension
            stone or prismatic blocks of marble, granite, limestone, sandstone, etc.

      3.     Surface Glory Hole method is generally performed on hillsides and often used to define
            irregular deposits of the surface. The method has a mine opening at the surface and ore is
            removed by gravity through raises connected to adit haulage ways. Ore is transported to
            the surface or side-hill using tramways (Lacy, 1999). This method generally results in
            smaller volumes of waste rock removed and deposited in surface dumps.

      4.     Strip mining is surface mining where reclamation is contemporaneous with extraction.  It is
            applicable to shallow, flat-lying deposits of coal, oil-shale, clay, sand, gravel, and some
            uranium, phosphate and placer deposits.  As the overburden is removed from one portion
            of a mineral deposit, it is used to fill in the trench left by the previous removal. In this
            manner, the overburden is continuously refilled to the adjacent previously mined area and

      5.     Placer mining is a method for the recovery of heavy minerals using water to excavate,
            transport, and or concentrate the mineral being mined (Lacy,  1999). Placer methods vary
            greatly depending on the size and characteristics of the deposit being mined. However,
            placer mining in general, usually affects large areas because the ore bodies are in large
            alluvial deposits with low-grade, but a high volumes of mineral. The method can be highly
            visible and  create large areas of disturbance. Many historic placer operations created
            serious impacts to stream channels, hydrologic systems and aquatic habitats.

      6.     Underground mining methods are generally similar.  Ore is extracted from underground
            slopes, rooms,  or panels located at depth. In comparison to surface mining little waste
            rock  is produced and deposited at the surface in surface waste piles on tailings material
            dumps. Depending on the mineral being extracted, however, tailings and gangue may still
            be deposited above the surface. Without reclamation, abandoned adits and slopes from
            underground mining can become sources of acid mine drainage to surface waters which
            may require treatment and measurements of flow and water quality.  This occurs when
            sulfide bearing minerals (usually pyrite) are exposed to air (oxygen) and ground water
            discharges to the surface.


Quality Flow Measurements at Mine Sites
Waste Rock Dumps

      Waste rock is removed material from above or within the ore body and consists of non-
mineralized and low-grade mineralized rock.  Waste rock includes granular, broken rock and soils
ranging in size from fine sand to large boulders, with the fines content dependent upon the nature of the
geologic formation and methods employed during mining.  Materials are designated as waste because
they contain the target minerals in concentrations that are too low to process, because they contain
additional minerals that interfere with processing and metals recovery, or because they contain the
target metal in a form that cannot be processed with technology that  existed at the time of mining.
Waste rock may be acid generating and may contain metals that can be mobilized and transported into
the environment. These materials generally require extensive geochemical testing to determine if they
can create impacts to the environment over the short or long term. Special engineering designs, waste
handling, or closure and reclamation plans may be required for those materials whose characteristics
may pose significant risks.

      In modern mining operations, waste rock and overburden that cannot be put to beneficial use or
that contain compounds that may be detrimental to the environment, generally are placed in a location
where they can be physically stabilized.  Placement of waste rock is  accomplished using a variety of
techniques that may include end-, side-hill-, or random-dumping, and dozing.  Historic operations
generally did not engineer waste rock dumps to be either geotechnically or environmentally stable.
Dump designs may vary markedly depending on the nature of the mining operation, the terrain in which
materials are being placed, and the era in which mining took place.  In steep, mountainous areas,
dumps may have faces of a few hundred meters height. Dumps placed as valley-fill deposits often
require the construction of rock underdrains to permit the flow of water through the drainage. Dump
underdrains are often tied into the mine drainage or storm water drainage systems that convey seepage
to treatment facilities and require measurements of flow and water quality. In newly planned mining
operations, the materials used to construct these drains should be thoroughly tested to ensure that they
will not contribute metals, acid, or other constituents to surface waters.

      Dumps that would contain waste rock capable of releasing significant quantities of metals, acidity,
or other constituents may require special design features or waste handling practices to minimize the
potential for environmental impacts (SRK, 1992; Environment Australia, 1997). Dumps can be
designed with features to control or reduce acid generation, control the migration of poor-quality
drainage, or collect and treat poor-quality drainage (SRK et al., 1989).

Tailings Impoundments

Quality Flow Measurements at Mine Sites
      Most mines dispose of tailings (sand- to silt-sized rock particles from which target minerals have
been separated) in engineered impoundments that cover areas ranging from a few acres to more than a
thousand acres. These facilities can discharge tailings water either directly from the impounded pool or
as seepage of pore fluids. At many mines, clarified decant water, formed as solid particles settle out of
suspension, is recycled to the process facility and reused. However, in wet environments, mines may
find it necessary to release tailings water to surface streams through permitted discharge points. This is
particularly true of impoundments that are used as emergency containment for excess storm runoff from
other  areas of the mine site.  Because tailings may contain acid-generating minerals and a variety of
metals, accurate knowledge of pool and seepage discharges under normal operations and during storm
events is required to ensure that permit and environmental requirements are met.

      Whether tailings impoundments are discharging facilities depends on the environmental and
physical setting of the site, the site water balance, the type of embankment, impoundment design, and
operational considerations. Tailings embankments can be designed as either water-retention dams and
raised embankments (Vick, 1990).  In modern impoundments, water-retention dams, which are
intended to prohibit horizontal fluid flow, are constructed with impervious cores of earthen materials or
concrete to their full height prior to tailings placement.  In contrast, raised embankments begin with
starter dikes designed to contain the amount of tailings expected during the first few years of
production; the embankment is raised periodically as dictated by mine operations. Starter dikes
typically permit horizontal flow because they are constructed using materials ranging from natural
borrow soils to waste rock to tailings. Seepage discharge typically occurs from one or more points
from raised embankments.

      Important design features that affect surface discharge and seepage flow include  the use of liner
systems, seepage control and collection systems, and stream and surface run on diversions. Liner
systems, intended to prevent vertical infiltration to ground water, may be installed  at sites where mill
effluent contains toxic or hazardous constituents.  Depending on the permeability of the  impoundment
foundation, seepage may occur from unlined tailings facilities. The (typically engineered) foundation
slope  determines the location and number of seepage discharge points. Seepage  control is used to
protect the structures associated with a tailings facility and to provide barriers that partly or completely
contain or direct the lateral subsurface flow of tailings water.  Types of commonly used seepage
barriers include cutoff trenches, grout curtains, and slurry walls (Vick, 1990). Seepage collection
devices include collection wells, ditches, and ponds (Vick, 1990).  Stream and run-on  diversions may
be incorporated into an impoundment if the embankment is constructed in the bottom of a valley having
significant drainage from storm runoff or in a valley that produces substantial continual runoff.
Diversions can be constructed either as conduits located below the impoundment or as ditches that skirt
the perimeter of the impoundment.

Quality Flow Measurements at Mine Sites
Leach Facilities

      Some primary ores, notably those of copper and gold, may be processed by heap or dump
leaching techniques.  Dump leaching is the process of applying a leaching agent (usually water, acid, or
cyanide) to piles of ore directly on the ground. Valuable metal(s) are extracted by leaching over a
period of months or years. Heap leaching is similar to dump leaching except the ore is placed on lined
pads or impoundments in engineered lifts or piles. Ores may be coarsely crushed prior to leaching or
may be leached as run-of-mine materials.  Spent materials contain lower concentrations of the target
mineral, and they may contain other metals, chemical complexes of the target metal, acid-generating
minerals, and small quantities of the leach solution. After leaching, the spent ore may be treated by
rinsing with fresh water or chemical additives that dilute, neutralize, or chemically decompose leach
solutions and metal complexes.

      Although the purpose of leach pads and dumps is to recover metals, these facilities cross  into the
realm of waste management upon closure (Hutchinson and Ellison, 1991). Process solutions have the
ability to degrade surface and ground waters should they escape from leach pads and solution storage
and conveyance systems. For most facilities, solution containment is achieved through the use of
impermeable liners beneath leach pads, sumps, and pregnant and barren solution ponds,  and dual-wall
piping. Hutchinson and Ellison (1991) describe the types  of natural and synthetic liners that are
commonly used for these purposes.  Regardless of the type of system that would be used, leach pads,
solution storage ponds, and solution conveyance systems need to designed to accommodate the added
volume of water that occurs during low probability storm events. This makes performing a rigorous
analysis of the predicted water balance crucial to project design, and the monitoring of discharge or
stream flow and water quality below these facilities critical.

Process Facilities

      Mining conducted to  extract and recover metals generally require beneficiation processes in
which ore is cleaned, concentrated or otherwise processed prior to shipping to the consumer, refiner,
smelter, or manufacturer who will extract or use the metal contained in the ore (EPA, 1994).
Beneficiation is the separation of valuable minerals from less valuable rock called gangue. The
processes and procedures for dressing and beneficiation of the mineral or metal ore are very similar for
many metal extraction operations.  Ore can be prepared by using operations such as crushing, grinding,
washing, drying,  sintering, briquetting, pelletizing,  or leaching, and concentrated using gravity separation,
magnetic separation, flotation or other means (EPA, 1994).  A mill includes all ancillary operations and
structures necessary for the cleaning and concentrating of the mineral or metal ore.  Unless the mined
ore is of very high grade, a mill will be located close to the mine to reduce costs of transporting the raw
ore for beneficiation.


Quality Flow Measurements at Mine Sites
      Leaching is commonly practiced to concentrate gold, copper or other metals. Leaching is the
process of extracting a metallic compound from an ore by selectively dissolving the metal in a suitable
solvent such as water,  sulfuric hydrologic acid, or sodium cyanide solution.  The desired metal is then
removed from the leach solution by a chemical process, such as precipitation, or by an electrochemical
process, such as electrowhinning and solvent extraction (EW/SX).

      In modern mining operations, process solutions are not discharged directly from facilities to
receiving waters without water treatment. Consequently, the list of chemicals used at a mine site can be
extensive and may include flotation reagents, frothing and collection agents,  scale inhibitors, flocculents,
thickeners, leach solutions, and leachate neutralizing solutions.

      Processing activities can release contaminants to surface waters in a variety of ways that include
spills of reagent materials or processing fluids (e.g., pipeline ruptures), leaks at processing facilities (e.g.,
liner tears), storage pond overflows (e.g., during storm events), and facility failures (e.g., slope failure of
a leach dump). Contaminant pathways can be direct (release directly to surface waters) or indirect.
Examples of indirect contaminant pathways include infiltration to ground water that exchanges with
surface water, seepage to soil or bedrock which discharges to surface water,  and seepage through or
below impoundment dams and berms.


      Many surface water hydrological impacts are related to mine construction and the location of
facilities.  Road construction, logging, and clearing of areas for buildings, mills, and process facilities can
reduce infiltration and increase the amount  of surface runoff to streams and other surface water bodies.
This can increase the peak flow and the total stream discharge associated with a given storm event.
Unusually high peak flows can erode stream banks, widen primary flow channels, erode bed materials,
deepen and straighten  stream channels, and alter channel grade (slope). In turn, these changes in
stream morphology can degrade aquatic habitats. Channelization (i.e., straightening) can increase flow
velocities in a stream reach, potentially affecting fish passage  to upstream reaches during moderate to
high stream flows. Increased erosion upstream and the resulting sedimentation downstream can impact
spawning gravels, egg survival and emergence of fry, as well as degrade benthic food sources.

      Reduced stream flow, caused by withdrawals of surface or ground water for mine operations,
can potentially affect aquatic habitats and requirements of aquatic resources. Fish have different flow
requirements at different times of the year and these requirements vary for different species.  Specific
flows are required for spawning, maintenance offish beds, fry emergence, juvenile rearing habitat, and

Quality Flow Measurements at Mine Sites
adult passage.  For these reasons, water withdrawals are often mitigated by establishing instream
(minimum) flow requirements at critical times of the year.  This requires adequate baseline
characterization of hydrologic flow conditions throughout the year and characterization of the available
habitat(s) associated with the fishery. Withdrawals of surface water can also reduce naturally occurring
high flows that occur during high runoff periods. High flow events are often periodically required within
a stream to entrain and transport sediments that were deposited during low flow periods when low
peak velocities caused sediment deposition.  These are known as channel maintenance flows. Channel
maintenance flows are periodically required for a channel to maintain sediment transport capacity
without aggrading, filling pools, and changing channel morphology, all of which can also affect aquatic

      Water quality issues associated with mine exploration, operation and abandonment activities
involve the potential discharge of mine water and process solutions, increased loads of metals and other
toxic pollutants, and the generation of acid generation from waste rock, spent ore, and mine workings.
If these pollutants reach surface waters, toxic conditions could degrade water quality and affect
important aquatic  species.  Potential analytes of concern for mining projects typically include pH,
cyanide, and heavy metals.  Stream flow effects caused by mining operations relate directly to potential
impacts in water quality. It is common for many water quality constituents to correlate inversely with
stream flow (i.e., chemical  concentration increases with decreasing stream flow). This is usually true for
the concentrations of most chemical constituents that occur in higher concentrations in subsurface
formations than in surface soils.  Some chemical constituents, however, correlate positively with stream
flow (increasing concentrations with increasing stream flow). This condition is typical of natural
constituents that are associated with surface soils, land applied pollutants, such as pesticides,
herbicides, and nitrates, or constituents that are transported as suspended particles.


      The accurate measurement and monitoring of stream flow and discharge from mine facilities is
extremely important for monitoring programs and programs designed to detect potential impacts to
hydrologic, water quality, or aquatic resources. Flow measurements are extremely important for
studies evaluating changes in water quality, pollutant loading, and surface and ground water interactions.

      Flow measurements are also necessary to estimate flood frequency, both at the mine site and the
watershed level. Peak flow data are used to plan or evaluate flood control and engineering structures.
Low-flow data are required to estimate water supply  dependability and drive water quality standards,
NPDES permitting, development of Total Maximum Daily Loads (TMDL), and used in investigations
conducted under the Comprehensive Environmental Response, Compensation, and Liability Act
(CERCLA). Relationships and the importance of flow measurements to each these activities or


Quality Flow Measurements at Mine Sites
programs is summarized below.  Flow measurements are also important activities that support both
basic and applied research being conducted by the MWTP.

Relationships to Hydrogeology

      Dewatering of surface and underground mines can deplete aquifers, impact ground water
recharge and discharge, and locally change the direction of ground water flow. Drawdown of an
aquifer potentially can lead to reduced spring and seep flows and reduced surface water flows in
streams that are gaining with respect to ground water. These effects can impact wetlands associated
with springs and riparian zones associated with streams. Adequate characterization of ground water
and hydrogeology and its relation to surface water flows is often difficult.  However, sufficient
characterization of surface water flow regimes and interactions with local ground water flows requires
accurate analysis of watershed conditions and measurements  of stream flow. The identification of
influent and effluent stream reaches, and impacts that mining could have on hydrology within a
watershed is dependent on accurate stream flow measurements taken synoptically, (e.g., samples
collected at approximately the  same time) and taken at key locations within a stream.

Water Quality and Pollutant Loading

      Many studies designed to assess impacts and pollutant  discharges from mine facilities and
monitoring programs require water quality sampling taken at appropriate locations and at appropriate
time intervals.  However, stream flow or discharge measurements are often not required or not
conducted in parallel to water quality samples.  It should be noted, that constituent concentrations,
which are subject to dilution in downstream surface water flows, provide limited information about the
behavior of constituents, and specifically, metals in streams.  EPA (1996) suggests that this shortcoming
can be overcome by considering constituent loads, in which the instantaneous load equals concentration
multiplied by stream discharge (e.g., stream flow) as follows:

                                          L = C*Q

where L is the instantaneous load, C is the constituent concentration, and Q is the measured stream
discharge.  The constituent load downstream of a tributary inflow (LD) is equal the sum of the upstream
loads (Lu) and contributing tributary (Lp) loads:

                                        M3 = MJ ~*~ LT

Quality Flow Measurements at Mine Sites
(EPA, 1996). An increase or decrease in load reflects an increase or decrease in the mass of the
constituent being transported per unit time. Increases in load along a stream reach can point to sources
of contamination that may be recognized (i.e., tributary inflow) or unrecognized (i.e., ground water
inflow) during conventional sampling.  In contrast, decreases in load suggest that a constituent is being
removed by one or more physical, chemical, or biological processes. Physical processes such as
sedimentation and sediment transport, chemical processes such as adsorption and colloidal
precipitation, and biological processes  such as uptake can cause changes in constituent loads.
Accurate evaluations of constituent loading, identification of pollutant sources, and analysis pollutant
retardation and attenuation factors are dependent on accurate measurements of stream flow (i.e.,

Water Quality Standards and NPDES Permitting

      EPA's involvement under the CWA primarily relates to NPDES permitting under Section 402,
and to a lesser extent, Section 404 (wetlands/dredge and fill).  Under CWA Section 402, all point
source discharges (see below) of pollutants to navigable waters of the United States must be permitted
under NPDES.  Effluent limits in NPDES permits may be technology- or water quality-based. In
evaluating mine sources, EPA typically assesses the potential for exceedances of anticipated permit
limits.  These assessments often require characterization of both discharge quantity and quality and
relationships to stream flow.

      NPDES permits require the application of technology-based or water-quality-based limits to
point source discharges, whichever are more stringent.  Each State has water quality standards that are
applied to individual streams based on its designated uses. In determining water quality based permit
limits for effluent discharges, states generally have provisions for mixing zones. The size of mixing zones
is typically determined based on dilution available during low flow conditions. The characteristic low
flow condition used for most EPA water quality compliance programs in concert with chronic aquatic
life criteria is the lowest 7-day average daily stream flow that occurs with a 10-year return period (i.e.,
7Q10L).  However,  many mine sites discharge to streams with negligible low flows (i.e.,  available
dilution is minimal).

Development of TMDLs

      Finally, Section 303(d) of the CWA requires States to identify water bodies that are not meeting
their promulgated designated uses.  These lists may include hundreds of stream segments, many of
which occur in historic or active mining districts and the source of degradation is discharges and
increased sedimentation from abandoned mine wastes.  Streams or stream segments which do not
consistently meet designated uses could require development of a TMDL. A TMDL is a technical plan


Quality Flow Measurements at Mine Sites
designed to attain water quality standards.  The development of a TMDL requires accurate
assessments of stream flow regimes, identification of sources, constituent loading for each source (see
Water Quality and Pollutant Loading), identification of low flow regimes, such as the 7Q10L, and the
loading capacity, defined as the water quality standard multiplied by a given stream flow. A TMDL is
accomplished by establishing loading allocations for the constituents) of concern for each identified
point source and non-point source occurring in a stream segment. TMDLs are implemented and
accomplished by site remediation programs, modification of point source discharges, and other means
necessary to achieve specified criteria.

CERCLA Investigations

      Mining activities have often caused releases of contaminants or hazardous constituents to the
environment, particularly surface and ground water resources. Examples include structural failures of
impoundments or waste embankments, contamination by acid mine drainage, seepage from tailings,
waste rock dumps, and heap and dump leach facilities, and spills of hazardous chemicals to soils or
receiving waters. Remedial investigations (RIs) are often required to identify the nature and extent of
contamination and to identify the fate and transport of contaminants in both surface and ground water
systems.  Feasibility studies (FSs) are then utilized to evaluate and select alternatives for remedial
action. Both RI and FS investigations require the identification  of sources, the quantification of loading
from these sources, and assessments of health risk to both human and ecological receptors. Accurate
assessments of stream flow, the characterization of flow regimes, as well as an assessment of the quality
of discharges from sources are generally required in support  of RI/FS investigations at mine sites. Flow
and discharge measurements, therefore, often play an important role in identifying sources, quantifying
contaminant loading, and providing assessments of risk.