PB88-243787 •
Freeb'ard Determination and
Harzardous
Waste Surface Impoundments
Brown (K.W.) and Associates, Inc.
College Station, TX
Prepared for
Environmental Protection Agency, Cincinnati, OH
Aug 88
1
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TECHNICAL REPORT DATA
IPItat read Imiructiont on iHe rtverse ttrforr camp/fling/
I. HEPORT NO.
EPA/60Q/2-88/Q15
RECIPIENT'S ACCESSiCN NO
PB88 24S787IAS
.TITLE AND SUBTITLE
Freeboard Determination and Management in
Hazardous Waste Surface Impoundments
5. REPORT OATE
August 1988
6. PERFORMING ORGANIZATION CCCe
>-UTMOR(SI
8. PERFORMING ORGANIZATION REPCBT NO
Sidney H. Johnson
David C. Anderson
f. PERFORMING ORGANIZATION NAME AND ADDRESS
K. W. Brown and Associates
6A Graham Road
College Station, Texas 77840
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-03-1816
12. SPONSORING AGENCY NAME ANO AOORESS
Hazardous Waste Engineering Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati , Ohio 45268
13. TYPE Of REPORT ANO PERIOD COVERED
Final Report 1/87
14. SPONS
SPONSORING A
EPA/600/1 4
AGENCY CODE
15. SUPPLEMENTARY NOTES
Paul R. de Percin, Project Officer, 513/569-7797
16. ABSTRACT
A rule-of-thumb minimum freeboard requirement of two feet (60 cm) has been used
in the past for hazardous waste surface impoundments.' In many situations, however,
this minimum value may not be sufficient to prevent overtopping. Consequently, a
procedure was developed for calculating freeboard values in surface impoundments
where the liquid depths are shallow and the fetches are short, as is typical in
hazardous waste surface impoundments. The procedure takes into account all of
the parameters which influence freeboard and presents the information in a format
which can be used on a site-specific basis. Additional support information in
the report includes an example calculation of freeboard requirement, site specific
data obtained from research using a mass liquid balance, and a listing of the
various types of liquid level detection equipment.
KEY WORDS ANO DOCUMENT ANALYSIS
DESCRIPTORS
b.lOENTIFIERS/OPIN ENO6D TERMS
c. COSATI Field/Group
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (Tint Report/
Unclassified
21. NO. Of PAGES
102
22. PRICE
EPA Farm 2220-1 (R«». 4-77) PXCVIOU* COITION IS OBJOLtTE
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EPA/600/2-88/015
August 1988
FREEBOARD DETERMINATION AND MANAGEMENT IN
HAZARDOUS WASTE SURFACE IMPOUNDMENTS
by
Sidney H. Johnson and David C. Anderson
K. W. Brown and Associates, Inc.
6A Graham Road
College Station, Texas 77840
Contract No. 68-03-1816
Project Officer
Paul R. de Percin
Land Pollution Control Division
Hazardous Waste Engineering Research Laboratory.
Cincinnati, Ohio 45268
HAZARDOUS WASTE ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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NOTICE -
This report has been reviewed in accordance with the U. S.
Environmental Protection Agency's peer and administrative review
policies and approved for presentation and publication. Mention of
trade names or commercial products does not constitute endorsement or
recommendation for use.
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FOREWORD ~
Today's rapidly developing and changing technologies and industrial
products and practices frequently carry with them the increased generation
of solid and hazardous wastes. These materials, if improperly dealt
with, can threaten both public health and the environment. Abandoned
waste sites and accidental releases of toxic and hazardous substances to
the environment have important environmental and public health implica-
tions. The Hazardous Waste Engineering Research Laboratory assists in
providing an authoritative and defensible engineering basis for assessing
and solving these problems. Its products support the policies, programs,
and regulations of the Environmental Protection Agency, the permitting
and other responsibilities of State and local governments and the needs
of both large and small businesses in handling their wastes responsibly
and economically.
This report presents all of the information necessary to calculate
freeboard requirements for hazardous waste surface impoundments. Each
factor is discussed and incorporated into the mathmatical procedure for
determining freeboard. This information will be useful to designers and
permit reviewers for surface impoundments. For further information,
please contact the Land Pollution Control Division of the Hazardous Waste
Engineering Research Laboratory.
Thomas R. Hauser, Director
Hazardous Waste Engineering
Research Laboratory
n i
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ABSTRACT -
A rule-of-thumb minimum freeboard requirement of two feet (60 cm)
has been used in the past for hazardous waste surface impoundments. In
many situations, however, this minimum value may not be sufficient to
prevent overtopping. Consequently, a procedure was developed for calcu-
lating freeboard values in surface impoundments where the liquid depths
are shallow and the fetches are short, as is typical in hazardous waste
surface impoundments. The procedure takes into account all of the
parameters which influence freeboard and presents the information in a
format which can be used on a site-specific basis. Additional support
information in the report includes an example calculation of freeboard
requirement, site specific data obtained from research using a mass
liquid balance, and a listing of the various types of liquid level
detection equipment.
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TABLE OF CONTENTS
Page
FOREWORD iii
ABSTRACT iv
LIST OF FIGURES vi
LIST OF TABLES vi
INTRODUCTION 1
Purpose and Scope 1
Regulatory Context 1
SUMMARY AND RECOMMENDATIONS 4
FREEBOARD DETRMINATIONS 5
Factors Which Affect Freeboard 6
Calculating Freeboard 12
Freeboard Allowance 14
FREEBOARD MANAGEMENT 17
Active Level Control 17
Passive Level Control 18
Quality Assurance 26
REFERENCES 27
APPENDIX A: Definition of Technical Terms 29
APPENDIX B: Calculating Freeboard Requirements
APPENDIX C: Water Balance
APPENDIX D: Liquid Level Detection Equipment
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LIST OF FIGURfS
Page
Figure 3-1. Definition sketch for freeboard 7
Figure 3-2. Definition sketch for fetch 8
Figure 3-3. Definition sketch for wind set-up(s) 10
Figure 3-4. Definition sketch for wave run-up 11
Figure 3-5. Comparison of wave run-up on smooth slopes 15
Figure 4-1. Schanatic of liquid level control 19
Figure 4-2. Schematic of liquid level management system 21
Figure 4-3. Discharge of an overflow spillway (Linsley and
Franzini, 1979) o 23
Figure 4-4. Open channel flow 25
LIST OF TABLES
Page
Table 4-1. Liquid Level Detection Systems 20
VI
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SECTION 1 *
INTRODUCTION
Limited information has been supplied to owners, operators, and agency
personnel concerning freeboard determination and liquid level control ,for
surface impoundments. The rule-of-thumb which has generally been used in
establishing freeboard, regardless of the size of the impoundment, has been
to use two feet (60 cm) as a minimum requirement. In many situations this
"minimum" value may not be sufficient to prevent overtopping during natural
climatic events. This document presents a procedure for calculating
freeboard which takes into account all of the factors which determine the
behavior of a liquid in an impoundment.
PURPOSE AND SCOPE
The purpose of this document is to present all of the information
necessary to calculate freeboard requirements on a site-specific basis.
Section 3.0 presents the procedure for calculating freeboard and provides
a discussion of the factors which influence freeboard, such as fetch,
liquid depth, wave height and period, wind set-up, and wave run-up. Each
factor is discussed and incorporated into the mathematical procedure for
determining freeboard. Section 4.0 presents information on the two basic
methods for detecting and maintaining a predetermined liquid level. Both
methods, active and passive level control, are discussed along with
suggested saftey margins.
Supplemental material included in the appendices is intended to aug-
ment the information presented in the body of the document. The appendi-
ces include: 1) a list of technical term definitions (Appendix A);
2) the procedure for calculating freeboard using a hypothetical surface
impoundment design (Appendix B); 3) data collected from a field study on
a liquid mass balance at a surface impoundment (Appendix C); and 4) a
discussion of the various types of liquid level detection equipment which
are currently available (Appendix 0).
REGULATORY CONTEXT
Regulations listed in 40 CFR require owners and operators to maintain
surface impoundment liquid levels in a manner which prevents overtopping.
Regulations which address overtopping are found in CFR 264, 265, and 270.
Part 264 addresses the operational standards for hazardous waste treatment,
storage, and disposal facilities. Part 265 presents interim standards for
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these facilities and Part 270 discusses the current permitting require-
ments. Regulations, current through July 15, 1985, are presented
below:
Section 264.221.!f) states:
"a surface impoundment must be designed, constructed, mainta-'"ed,
and operated to prevent overtopping resulting from normal or aonormal
operations; overfilling; wind and wave action; rainfall; run-on;
malfunctions of level controllers, alarms, and other equipment; and
human error."
Section 264.226(b)(l) states:
"while a surface impoundment is in operation, it must be inspected
weekly and after storms to detect evidence of deterioration,
malfunctions, and improper operation of overtopping control systems."
Section 265.222(a) and (b) state:
(a) "A surface impoundment must maintain enough freeboard to
prevent any overtopping of the dike by overfilling, wave action, or a
storm. Except as provided in paragraph (b) of this section, there
must be at least 60 centimeters (two feet) freeboard."
(b) "A freeboard level less than 60 centimeters (two feet) may
be maintained if the owner or operator obtains certification by a
qualified engineer that alternate design features or operating plans
will, to the best of his knowledge and opinion, prevent overtopping
of the dike. The certification, along with a written identification
of alternate design features or operating plans preventing overtopp-
ing, must be maintained at the facility."
Section 265.226(a)(l) states:
"The owner or operator must inspect the freeboard level at least
once each operating day to ensure compliance with 265.222."
Additional information requirements are necessary for the
Environmental Protection Agency (EPA) to determine compliance with the
Part 264 standards, including:
Section 270.17(b)(2):
"Detailed plans and an engineering report describing hew the
surface impoundment is or will be designed, constructed, operated and
maintained to meet the requirements of 264.221." This submission
addresses the prevention of overtopping.
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Sections 270.17(d):
"A description of how each surface impoundment, including the
liner and cover systems and appurtenances for control of overtopping,
will be inspected in order to meet the requirements of 264.226 (bj.
This information should be included in the inspection plan submitted
under 270.14(b)(5)."
The EPA plans to update the above regulations. These new regulations
should be consulted to determine if any changes have been made to those
given above.
One intent of this document is to present a method for calculating
freeboard which complies with regulations concerning overtopping as defined
by 40 CFR 264, 265, and 270. These regulations have been formulated with
the goal of eliminating, to the extent practical, the overtopping of
liquids from surface impoundments. No single system, however, provides for
absolute prevention of escape. Therefore, it is also the intent of this
document to present information for use in designing a containment system
which provides the maximum, practically achievable, level of freeboard
safety during the operational life of the facility.
The goal or this document is to present performance guidelines and
operating characteristics rather than specific numerical design values.
However, a minimum freeboard of two feet is recommended. Information
provided in this document is intended to offer the owner/operator
flexibility in designing a suitable overtopping prevention system without
dictating rigid design requirements.
Procedures set forth in this document for calculating minimum
freeboard are based on current technology. Several design methods exist,
however, which meet the requirements of 40 CFR 264.221(F). It is the
responsibility of the owner/ operator to document the integrity of •* "-fh-c
selected system as well as the ability of the system to meet the regulatory
requi rements.
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SECTION 2 -
SUMMARY AND RECOMMENDATIONS
Procedures used in the past for calculating freeboard at surface
impoundments were generally based on procedures and information developed
by the Waterways Experiment Station (Corps of Engineers, 1984) and by
Saville (1956). Much of this information is dated and does not take into
account some of the variables which affect the ultimate freeboard value.
To address these short-falls a new procedure was developed which addresses
all of the factors which determine freeboard and incorporates them into
an easy-to-follcw format. Many of the coefficients used in the original
work have been updated using new values derived from ongoing research
(Herbich, 1986a). Unfortunately, updated information was not available
for all parameters (e.g.,'roughness coefficient), therefore, values were
either extrapolated or out-of-date published values were used. In spite
of these limitations, the new procedure presented in this report repre-
sents the most up-to-date method for determining freeboard. The proced-
ure takes into account all of the climatic factors and liquid characteris-
tics which influence freeboard.
Based on the information gathered during preparation of the freeboard
equation, two areas where information was lacking were noted. These
included (1) the absence of roughness coefficients for smooth surfaces
such as synthetic liner materials and (2) curves to determine wave run-up
on these surfaces in impoundments where fetches are short and liquid
depths are shallow. Therefore, it is recommended that a study be con-
ducted to verify the roughness coefficient values established by Saville
(1956) and to establish new coefficients for the synthetic materials
commonly used to construct surface impoundment liners. Once these coeffi-
cients have been established it will be necessary to generate curves
specifically designed to determine wave run-up in surface impoundments.
One other area which may need attention in the future is determining the
effects random waves have on run-up values. The equation presented in
this document assumes all waves are monochromatic.
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SECTION 3_
FREEBOARD DETERMINATIONS
„ The overall design for freeboard allowance should be tailored to
surface impoundments on a case by case basis to ensure that overtopping
does not occur. To minimize the potential for overtopping, surface
impoundments should include the following:
1. Passive outfalls from the surface impoundment, such as weirs or
spillways which are insensitive to inflow should be incorporated
into the design. In the event waste is released, these
structures direct liquid waste to an on-site holding or treatment
facility. Passive outfall structures are intended for use only
in the event of an automated level control system malfunction,
gross human error, or unforeseen catastrophic natural events;
o
2. If outfall structures are sensitive to inflow (i.e., where
outfall rates must be increased to maintain freeboard as inflow
increases) automatic control should be provided via signals
from level sensing instruments. In these situations the
automated system should include a high level alarm;
3. For surface impoundments receiving waste via inflow structures,
design features should be incorporated which allow for flow of
waste to the surface impoundment to be halted immediately in
the event of overfilling or failure of any surface impoundment
component. The flow of waste can be controlled by an automated
level sensing system which, in the case of a system failure,
can be operated manually;
4. Run-on control structures should be designed to divert the peak
discharge from a 100 yaar/24-hour storm unless it can be shown
that the surface impoundment is engineered to accept the
additional volume without sacrificing minimum freeboard;
5. Freeboard should be defined as the minimum distance between the
highest liquid level in the surface impoundment, where the
highest liquid level includes changes in water elevation due to
a 100 year/24-hour storm surge, and the liquid level which
would result in the release of stored liquid from the surface
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impoundment by overtopping (Figure 3.1). Freeboard allowances
should be calculated for all surface impoundments using the
maximum fetch (usually the diagonal measurement across the
surface impoundment) and the maximum historically determined
sustained wind speed to calculate wind set-up, wave height, and
wave run-up. The minimum amount of freeboard maintained in the
impoundment should be based on site specific calculations but
should never be less than two feet (60 en) except as provided
for in 40 CFR 265.222(b). If the impoundment is equipped with a
passive outfall such as a wei> or spillway, freeboard should be
measured from the highest allowable liquid level to the lowest
discharge level of the passive outfall structure. When no
passive outfalls are present, the freeboard should b« the
distance from the highest allowable liquid level to the top of
the lowest elevation of the retaining structure. Freeboard
should not be considered as storage capacity. Changes in liquid
level due to a 1QO year/24-hour stonm should be engineered into
the normal storage capacity of the surface impoundment; and
A weekly inspection schedule of all overtopping prevention
equipment should be followed along with a daily inspection of
freeboard. Additional inspections should be conducted
following significant rainfall events to verify the integrity
of the system and that minimum allowable freeboard has been
maintained. Inspections should be made on level control
sensors, alarms, and outfall structures. A written record
should be maintained which documents the liquid levels, when
inspections were conducted, who performed the inspections, and
any observations made as to the integrity of the overtopping
control systems. It is also recommended that a routine
maintenance schedule be implemented for all overtopping control
systems.
FACTORS WHICH AFFECT FREEBOARD
Determination of freeboard requires that several specific parameters
be accurately measured. They includa:
1. fetch;
2. Liquid depth; and
3. Embankment slope.
Fetch, as used in this document, is defined as the maximum unobstructed
distance across a free liquid surface over which wind can act. Typically,
the longest fetch will be the diagonal measurement across the surface of
the impoundment (Figure 3.2). Liquid depth, for the purpose of
calculating freeboard, is the maximum possible depth of liquid (dm) in
the surface impoundment (including storm surge). Embankment slopes of
the surface impoundment should be clearly expressed for each sidewall of
the impoundment (i.e., 3 horizontal to 1 vertical). If the embankment
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PASSIVE OUTFLOW
FREEBOARD
*
V ]
i
V
MAX. UQUID LEVEL dm
1
1
t
i
t
i
25% SAFETY FACTOR
Re RUN -UP (CORRECTED)
1
S SET-UP
ACCUMULATED RAINFALL
d NORMAL LIQUID LEVE1
FIGURE 3-1. DEFINITION SKETCH FOR FREEBOARD
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FIGURE 3-2. DEFINITION SKETCH FOR FETCH.
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slopes vary, calculations should be performed using each slope value to
determine which slope results in the greatest calculated freeboard.
Other parameters which affect freeboard include:
1. Wind speed;
2. Slope roughness;
3. Wind set-up;
4. Wave height and wave period; and
5. Wave run-up.
Wind speeds used in freeboard calculations should be based on local
historical data for maximum sustained winds. In the absence of such
data, it is suggested that a value of 75 mph be used for areas which are
not subject to hurricanes and a value of 100 mph for areas which are
hurricane prone. For the purpose of this document, "hurricane prone
areas" are considered to be any area which is within 50 statute miles of
a coastline subject to hurricanes.
A roughness factor for the interior face of the berm should be
determined to establish freeboard requirements. Roughness factors will
vary from a value of 1.0 for smooth synthetic liners to 0.45 for coarse
.surfaces such as rip-rap (Corps of Engineers, 1984). A rough surface
will dissipate wave energy more quickly than a smooth slope, thereby
resulting in a smaller value for wa«e run-up. Curves which relate slope
roughness to other freeboard parameters are an acceptable means for
determining roughness effects (assuming a roughness coefficient of 1.0 is
appropriate in most situations.) Using a roughness coefficient of 1.0
means that no correction is required for any of the wave forecasting
equations included in this document.
Wind set-up is an important parameter to be considered when
determining freeboard requirements since it has the net effect of raising
the water level on the downwind side of the impoundment (Figure 3.3). As
wind acts on the liquid surface, the liquid is "pushed" to the far side
of the impoundment, causing a slight gradient on the water surface.
Wave height and wave length will determine the type of waves
generated within a given impoundment. The relationship between these
two parameters will define whether shallow or deep waves are generated
and if the wave will break. Under most circumstances waves will be
shallow and non-breaking, therefore, all of the equations included in
this document are for use in situations where waves do not break.
The final and most important parameter, wave run-up, is dependent on
many of the preceding parameters. Run-up is defined as the vertical
height to which water rises above the still water level on a sloped
embankment (Figure 3.4). Final freeboard determinations must be based on
this value.
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<^ WIND
FIGURE. 3-3. DEFINITION SKETCH FOR WIND SET-UP(S).
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-POINT OF MAXIMUM WAVE RUN-UP
T
FIGURE 3-4. DEFINITION SKETCH FOR WAVE
RUN-UP
11
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Values for these variables must be accurately defined to determine
freeboard requirements for surface impoundments. Consequently,
owners/operators should use site-specific data in calculating all
parameters. In instances where site-specific data is unavailable,
conservative estimates of these values should be incorporated into
calculations of minimum freeboard. -
CALCULATING FREEBOARD
The following procedure was developed to allow estimation of
freeboard in conditions where short fetches and shallow liquid depths
predominate. For the purpose of this document, short fetches will be
considered any distance less than 5,280 feet (1,600 meters) and shallow
depths will be defined as values less than 30 feet (9 meters). These
values were selected because more than 90% of hazardous waste surface
impoundments fit into these definitions.
The first step in calculating freeboard is to accurately measure the
physical dimensions of the surface impoundment. These measurements
include length and width to determine the maximum fetch as well as the
maximum liquid depth possible in the surface impoundment. Sidewall
slopes should be measured or determined from as-built engineering
drawings. Having defined these values it will be necessary to collect
local information on such weather conditions as rainfall and wind speed.
Based on this information, the following calculations for wave
height, wave period, wind set-up, and wave run-up can be used to
determine freeboard requiranents. In the following equations, U is wind
speed (ft/sec) and Ua is a wind stress factor (ft/sec). The relationship
hat-wean thaea tun ualtiac ie- II = fl C8Q ll'*^^
between these two values is: Ua
Wave Height:
0.589
gH
0.283 tanh
0.53
0.75-1
tanh
0.00555
-
Kl
tanh
/9
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Wave Period:
0.333
7.54 tanh
0.833
0.375 ~\
0.0379
tanh-,
tanh
0.833
Where:
Ua = Wind Stress Factor (ft/sec)
T = Wave Period (sec)
F = Fetch (ft)
dm = Maximum Depth of Impounded Liquid (ft)
g - Acceleration of Gravity (32.16 ft/sec2)
Wind Set-Up:
S
f
o.s
given:
21.0
0.33
Where:
A
B
Ua
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Wave Run-Up:
Unlike the previous parameter determinations, wave run-up is an
involved procedure of calculating and adjusting formula variables. The
procedure which will be followed is adapted from Herbich, 1985b. Much of
the data which support the various types of freeboard calculations can be
found in the Shore Protection Manual (U. S. Army Corps of Engineers,
1984) and Saville (1956). Steps in determining wave run-up are as
follows:
- Calculate deep water wave length (L0).
Lo 3 (lJ T2 = 5-12 T2 (in feet)
2
- Calculate dm/L0°.
- Locate the value of d/L0 (dm/L0) in Table 4-1 and read
corresponding values for d/L and H/H0. If d/L0 (dm/L0) is
greater than or equal to 1.000, then H = H/H0.
- Calculate the value of Ho' from H/H0.
u' H'
"0 0
- Calculate and
- Obtain R using H*/gT2 from Figure 3-5.
Where:
dm = Maximum Liquid Depth (ft)
H0 = Deep Water Wave Height (ft)
H = Shallow Water Wave Height (ft)
T = Wave Period (sec)
L0 = Deep Water Wave Length (ft)
L = Shallow Water Wave Length (ft)
R = Run-up (ft)
g = Acceleration of Gravity (32.16 ft/sec2)
Once a value for run-up (R) has been established it will be necessary
to correct this value using a coefficient of 1.67 (Rc = 1.67 R; where Rc
is the corrected run-up value). The preceding wave forecasting calcula-
tions are based on averages for the highest wave which would occur out of
three waves generated (H]/3). It is advisable to use a more conservative
approach in handling hazardous wastes. One such conservative approach
would be to use R values which account for the highest wave that would
occur out of 100 waves generated (H-j/igg). The coefficient used to find
Rc is based on work by Longuet-Higgins and Stewart (1964) and can also be
found in Kirrsman (1965).
14
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I Reproduced from
[ t«5l available copy.
0.2
=cz
r-
ai
•~r
O.OO03
0.001
0.0
O03
FIGURE 3-5. COMPARISON OF WAVE RUN-UP ON SMOOTH SLOPES (DATA
FOR d/Ho>3.0; SAVILLE, 1956).
» VALUES GREATER THAN 0.013 HAVE BEEN EXTRAPOLATED.
15
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FREEBOARD ALLOWANCE
The minimum freeboard which should be observed is the sum of the
calculated values for set-ui (S) and corrected wave run-up (Rc) from the
preceding equations. Changes in liquid elevations due to the 100 year/
24-hour storm surge should never be included in the freeboard allowance.
Wave forecasting is not an exact science and these equations assume
ideal conditions using monochromatic waves and in some cases extrapolated
values for short fetches and shallow water depths. For surface impound-
ments that handle hazardous wastes, it is advisable to add a safety
factor of 25% to the freeboard allowance and to never allow the freeboard
to be less than two feet, except as provided for in 40 CFR 265.222(b).
The 25% correction should apply to the sum of the corrected run-up (Rc)
and set-up (S). Therefore, the total freeboard allowance (f) should be
expressed as follows:
f = 1.25 (Rc + S) if it is less than 2 feet, use the recommended
2 foot minimum.
Where:
f = Freeboard (ft)
Rc= Wave Run-up, corrected (ft)
S = Wind Set-up (ft)
16
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SECTION 4
FREEBOARD MANAGEMENT
Maintaining liquid level can be viewed as a two phase problem.
First, there is the need to monitor and control the liquid level at or
below the established value. To accomplish this, passive and active
(electrically operated) level control systems may be employed. .Regard-
less of the type of system selected, it is advisable to identify a
specific system prior to finalizing impoundment design so needed modifi-
cations can be incorporated into the design and construction plan
for the impoundment.
The second phase in the design of a SI should include a passive
level control device such as a weir, spillway, or outflow pipe. The
purpose of the passive structure is-to prevent catastrophic failure of
the impoundment dike in the event that the active level control system
fails or an unforeseen natural event occurs. Passive level control
devices should be designed for use only i-n emergency situations, not as
part of normal facility operations unless the passive structure is part
of a flow through treatment process. In an emergency, the passive level
control should direct the liquid to a tank or another surface
impoundment.
ACTIVE LEVEL CONTROL
Once freeboard allowances have been determined it will be nr:essary
to implement some type of system which monitors liquid level. Numerous
systems have been used in the past, most of «uich require human input to
function properly (e.g., staff gauges). By and large, these types of
systems are incapable of sensing the liquid level ,ind therefore can not
be used to control freeboard directly. Bearing this in mind, it is
advisable to consider the use of an automated (active) liquid level
sensing system. Active level sensing systems offer several advantages
including continuous level monitoring which can be used to automatically
maintain freeboard, the ability to translate level readings into a volume
for waste inventory when interfaced with a controlling unit, and the
inherent fail-safe feature of many systems which virtually eliminates
human error.
Active control of the liquid level in a surface impoundment is
typically comprised of four phases. The overall objective of the
integrated four phase system is to control the liquid level in such a
manner that a predetermined maximum level (and minimum level if necessary)
is not exceeded. To this end, each of the four phases must be linked
17
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together in a logical controlling sequence. The four phases include:
1) the primary element, ?.} the measuring element, 3) the controlling
element, and 4) the final control element (Figure 4-1).
The primary element, Phase I (e.g., float, prone, ultrasonic beam,
etc.) detects or senses changes in the liquid level. The measuring
element, Phase II, receives the signal from the primary element and
measures th>- amount the liquid°level has deviated from a set point or
from the last measurement. The controlling element, Phase III, uses data
from the measuring element to activate or alter power to the final
control element. The controlling element is generally a computer, such
as a digital or analog type, where the liquid level limits are sat and
the decision is made as to the amount of liquid to be allowed to inflow
or outflow. The final control element, Phase IV, is typically a pump or
valve which actually influences the amount of liquid flowing into or out
of the impoundment.
The primary and measuring elements are generally combined into a
single unit referred to as the level detection system. A wide variety of
level detection equipment is available, and generally is the most
variable of the elements in total level control system (Table 4-1). The
level detection, monitoring and control system described above may be
designed to operate as a totally automated system. That is, the system
can detect, monitor and control the liquid level with little contact from
technicians other than routine maintenance and calibration.
As stated, controlling liquid level in a surface impoundment, is a
process which includes several phases. Figure 4-2 gives a visual
display of the level monitoring and control procedure, including inflow
to the impoundment and outflow from the impoundment. To aid in system
selection a discussion of the various types of level detecting devices is
offered in Appendix D and can also be found in greater detail in Shiver
et al. (1985).
PASSIVE LEVEL CONTROL
The easiest types of systems which can be used to control liquid
level are passive outfall structures such as a spillways, weirs, or
pipes. Many of these types of structures can be engineered to npter the
rate of flow, and as a whole are typically insensitive to inflow, meaning
that as the rate of inflow to the impoundment increases the rate of flow
through the outfall structure increases without recalibration or manual
adjustment. The use of these types of outfall or outlet works are common
in flow- through processes (e.g., settling basins or polishing ponds).
In these situations the liquid outfall is directed from the impoundment
to the next step in the treatment process via the outlet structure, and
therefore does not constitute an uncontrolled release.
-------
INFLOW
IMPOUNDMENT
^-PRIMARY ELEMENT'
- FLOAT
- PROBE
- BEAM
\
MEASURING
ELEMENT
OUTFLOW
sj
e-1
/
>>-
/
EMERGENCY
LEVEL
REDUCTION
FINAI r_ONTF?ni ...' . rONTROL I IMfi
ELEMENT ELEMENT
.-PUMP
- VALVE
-ate.
TT-
DISPLAY
DATA
i
i
SIGNAL
ALARM
FIGURE
SCHEMATIC OF LIQUID LEVEL CONTROL.
-------
Table 4-1. Liquid Level Detection Systems.
S,
Apo • I cat Ion
PrInciple of
Advantages
1.1 ml fat (on*
e ' Non-corrosive wastes; Measurement of stored Fall safe operation,
corrosive w/mod electrical charge easy operation
Based on I(quid dial*
•ctrlc constant,
affected by coating
Conductivity Non-corrosIv«,
conduct Iv« mat'Is
Completion of circuit
through the liquid
Casy Installation.
Inexpensive
Primarily point lev*I
only, requires con-
duct I v« liquids
Non-corrosl»• and
corrosive wastes
Differential Non-corrosive end
Pressure corrosive
Measurement or pres-
sure Increase dua to
liquid heed
Measurement of pres-
sure difference be-
tween liquid bottom
and atmospheric
Olapnragn does not
contact liquid,
unaffected by minor
surface agitation
Sensor does not
contact «oste
Poor sensitivity to
small level change,
based on 11 quid
density
Based on I (quid
density
Olsplacers
Non-corrosIv« wastes,
corrosive w/mbd,
noo-coatlng wastes
Measurement of frrca
of displaced float,
converted to depth
Used f
-------
INFLOW
LEVEL
DETECTION
CONTROLLER
ALARM FINAL CONTROL
ts>
OUTFLOW
FIGURE 4-2. SCHEMATIC OF LIQUID LEVEL MANAGEMENT SYSTEM.
-------
Surface impoundments used for liquid storage or disposal are notably
different than the flow-through process discussed above. In these types
of surface impoundments, passive outfall structures should only be used
to control liquid level in order to prevent overtopping in the event
active level control systems fail. Therefore, in these situations the
passive outfall structure should be viewed as a backup "safety valve" to
the active level control .system with the s«le purpose of preventing
overtopping which could cause catastrophic dike failure. Since most of
these types of impoundments are not designed to return liquid to a
treatment process, any flow of liquid through the passive structure would
constitute an uncontrolled release.
Regardless of the situation where the passive outfall is to be used,
there are general design specifications which should be observed. First,
the vype of outlet to be used irust be identified. Spillways or weirs
will represent the most desirable option for most situations. These
types of structures can be easily sized to meet the outflow needs of the
impoundment, are relatively inexpensive, and when used with a stage
recorder can be used to meter the fl ow of liquid. Pipe or conduit outlet
works are not generally recommended because they require placing a hole
through the liner and berm of the impoundment. If a good seal is not
realized around the conduit, seepage of liquid may occur which ultimately
could lead to piping of the soil, thereby resulting in failure of the
benm. If pipes are to be used it is recommended that*metal pipes be
avoided. Since many types of waste are corrosive and most soils are
inherently co rosive to some extent, metal pipes will have a finite life
expectancy. If metal pipes are used, consideration should be given to
coating the pipes with a resistant material and/or providing cathodic
protection.
Designing an outlet for a surface impoundment will require defining
the relationship between inflow and outflow volumes. Unlike the more
conventional applications for passive outlet structures such as dams at
lakes, the inflow to a surface impoundment can easily be determined from
operational processes and climatic data which define the maximum
operating level. Having identified the maximum volume which can be added
to the impoundment, the outflow structure can be designed to handle this
volume. The following weir equation (Linsley and Franzini, 1979) defines
the discharge volume over a spillway:
Q - Cw
where: Q = Discharge
Cw= discharge coefficient
L = Length of the crest
h = head on the spillway (vertical distance from the
crest of the spillway to the reservoir level).
The discharge coefficient will vary depending on the design of the
spillway and amount of head. Figure 4-3 illustrates coefficients for a
standard crest shaped spillway.
22
-------
+-X
WHERE h' IS THE DESIGN HEAD
TYPICAL CREST SHAPE
o
z
UJ
*
u
u
u.
u.
Ul
o
o
g:
UJ
tJt
4.1
4.0
3.9
3.8
3.7
3.6
3J
3.4
3.3
3.2
3.1
TO
/
/
/
1
1
h1/
A
Ha
//,
V
//
'S
h
h'
0
n
= GI
= oe
/
*$
VE)
3IC
'
2X3
4 HE
Ml-
X
^
:AO
EAI
•"^
^
,^"
—
.
-
"
s s 5 B e B E
WEIR COEFFICIENT, C^tSI METRIC UNITS!
0.2
0.4
Q6 0.8
h/h'
1.0 1.2
1.4
FIGURE 4-3.
DISCHARGE OF AN OVERFLOW SPILLWAY
( LINSLEY AND FRANZIN1, 1979).
23
-------
If the use of a weir is proposed, several different designs can be
selected including the V-notch, rectangular, and the Cipolletti types
(Figure 4-4). A related structure which may be considered is a flume.
The specific goal of these structures is to provide a passive outfall
which allows the flow of liquid through the structure to be metered.
Flow calculations for each are listed below.
From Considine (1974):
V-Notch Weir: 60° Notch Q = 1.46 H5/2
(see Figure 4-4a)
90° Notch Q = 2.52 H2-47
Rectangular Weir: Q = 3.33(L - 0.2H)3/2 (see Figure 4-4b)
Cipolletti Weir: Q = 3.367 LH3/2 (see Figure 4-4c)
From Linsley and Franzini (1979):
1.522B°-02<5
Parshall Flume: Q = 4Bha (see Figure 4-4d).
The final category of passive outfalls include, pipes, nozzles, and
conduits. As previously mentioned, these types of outfalls are not
generally recommended for use at surface impoundments. Problems which
may arise when these structures are used include liner leakage or metal
pipe corrosion which may lead to soil erosion and dike failure, seepage
of waste through the liner, and the limited ability of the structure to
adjust to increases in liquid flow volume. Determination of flow through
pipes can be calculated as follows:
Darcy-Weisbach Formula for fairly smooth pipe (Hicks, 1972):
nF = 0-38 L V1'86 solve for V and use Q = VA
1000 D1-25
where: hp = friction head (ft) D = diameter of pipe (ft)
L = length of pipe (ft) Q-= flow volume (ft)
V = velocity of liquid (ft/sec) A = area of pipe (ft2)
Manning Formula for full flow through round pipes (Hicks, 1972):
2.159 Q n 3/8
D =
where: D = diameter of pipe (ft) Q - flow volume (cfs)
n = roughness coefficient S = gradient of pipe (hL/L)
24
-------
(A) V-NOTCH WEIR
,4,
0 —
"WAX
(B) RECTANGULAR WEIR
(L>3HMAXI E
(C ) C1POLLETTI WEIR
nMAX
STILLING WELL
(D) PftflSHALL FLUME
WATER SURFACE
FOR FREE FLOW
1:4 LEVEL
FLOOR
FIGURE 4-4. OPEN CHANNEL FLOW.
-------
e.g.: n = 0.009 for smooth brass'of glass
n = 0.013 for concrete
n = 0.014 for average drain tile
n = 0.021 for corrugated iron
The above procedures are only a brief overview of the techniques
available to passively control liquid levels. If further information is
required or there is a specific need for an unusual situation it is
advised that the listed references be consulted.
QUALITY ASSURANCE
o
It is advisable to implement a quality assurance program to ensure
that the freeboard control system selected operates according to the
manufacturer's design specifications. Since a specific freeboard control
system (or any components of the system) are not recommended, no specific
quality assurance program will be recommended. Rather, the approach
taken is to present general procedures which should be observed to ensure
that all level control devices are installed correctly and operate
properly.
All surface impoundments should use accurately calibrated equipment
to measure both inflow and outflow. Automated inflow and outflow struc-
tures, when used, should have the capacity to be operated manually in
the event automatic controls fail to regulate the flow of liquid. All
surface impoundments should be equipped with fail safe high level alarms.
It is also advisable to install level sensing probes which interface with
inflow and outflow structures.
Regardless of the type of overtopping system selected, the owner/
operator should maintain a written record documenting the procedures used
to install and calibrate all equipment and structures associated with
liquid level control. In addition, documentation should include verifi-
cation that the type of system selected is° compatible with the type
of waste impounded. Once installed, the system should be tested to
verify that it is fail safe. These tests should be designed to test the
integrity of the entire system, including deliberate actions to verify
operation of all fail safe aspects of the system.
After the system has been verified as operating properly, the
calibration and testing procedures should be incorporated into a program
for routine maintenance of all liquid level control system components.
Personnel assigned the responsibility for daily inspection and routine
maintenance of the liquid level control system should be familiar with
operation of all system components and should have a written protocol
detailing the lines of authority, the procedures and schedule for
testing the equipment (including calibration specifications), reporting
requirements, and all associated contingency plans.
26
-------
REFERENCES -
1. Considine, 0. M. (ed.) 1974. pp. 6-162 to 6-175.. Process Instruments
and Control Handbook, Second Edition. McGraw Hill, NY.
2. Corps of Engineers. 1984. Shore Protection Manual. Vol. I and II,
Fourth Ed. Waterways Experiment Station. Department of the Army. U.S.
Government Printing Office.
3. Herbich, J. B. 1986a. Personal Communication.
4. Herbich, J. B. 1986b. Guidelines for Surface Impoundment Freeboard
Control. Consulting and Research Services, Inc. Report No. JBK-867.
5. Hicks, T. G. 1972. Standard Handbook of Engineering Calculations.
McGraw-H-'ll, NY.
6. Kinsman, B. 1965. Wind Waves. Prentice-Hall, Englewood Cliffs, NO.
7. Linsley, R. K. and 0. 8. Franzini. 1979. Water-Resources Engineering.
McGraw-Hill, NY.
8. Longuet-Higgins, M. S. and R. W. Stewart. 1964. Radiation Stress in
Water Waves; A Physical Discussion with Application. Deep-Sea Research,
Vol. 11:529-562.
9. Saville, T. 1956. Wave Run-Up on Shore Structures. Journal Waterways
and Harbors Division, ASCE. Vol. 82:925-1 - 925-14.
10. Shiver, R., S. Johnson and J. 0. Zabcik. 1985. Methods for Detecting
Liquid Level and Maintaining Minimum Freeboard at Hazardous Waste Sur-
face Impoundments. Draft Technical Report Prepared by K. W. Brown
and Associates, Inc. Contract No. 68-03-1816, Assignment 6.
27
-------
APPENDIX A -
DEFINITION OF TECHNICAL TERMS
28
-------
DEFINITION OF TECHNICAL TERMS
Fetch - The maximum unobstructed distance across a free liquid surface
over which wind can act (typically the diagonal measurement
across an impoundment).
Freeboard - The distance between the highest calculated liquid level
in a surface impoundment and the liquid level which would
result in the release of stored liquid from the impoundment.
Monochromatic - Waves which all have the same wave length (as used in
this document).
Overtopping - Flow of stored liquid over the top of the dike or levee.
Outfall - A structure, e.g., weir or spillway, that diverts and controls
the liquid flow.
Outrun - Flow of liquid out of the impoundment other than by
overtopping.
Roughness Factor - A measure of smoothness of the berm inside wall which
varies from 1.0 for synthetic plastic liners to 0.45
for stone rip-rap.
Run-On - A flow of liquid into the surface impoundment due to rain
and the local topography.
Wind Set-Up - The extent to which a gradient on the water surface is
caused by wind pushing the liquid to the far side of an
impoundment.
Run-Up - The vertical height to which liquid rises above a still
liquid level on a sloped embankment due to waves.
29
-------
APPENDIX B
CALCULATING FREEBOARD REQUIREMENTS
-------
TABLE OF CONTENTS
Page
B.I INTRODUCTION - B-l
B.2 Wave Height B-l
B.3 Wave Period 8-1
B.4 Wind Set-Up B-2
B.5 Wave Run-Up .- B-2
B.6 Freeboard Allowance B-5
B.6.1 Wave Height B-7
5.6.2 Wave Period B-7
5.6.3 Wave Set-Up B-8
5.6.4 Wave Run-Up 8-3
5.6.5 Freeboard Allowance. B-9
B.7 References 8-10
-------
8.1 INTRODUCTION
The following procedure was designed to estimate freeboard in
impoundments where short fetches and shallow liquid depth* predominate.
For the purpose of this document, short fetches will b-> considered any
distance less than 5,280 feet (1,600 meters) and shallow depths will be
defined as values less than 30 feet (9 meters).
In the following equations, 1) is wind speed (ft/see) and Ua is a wind
stress factor (ft/sec). The relationship between these two values is: U, =
Q
0.589 U1'23.
B.2 WAVE HEIGHT
gH
?"
a
- 0.283 tanh
0.75-1
tanh
1
/9M *
. 0.00565 _
u /
\ a/
tanh
0.53 |_J!]
K
L \ a / -1
j
Where:
Ua = Wind Stress Factor (ft/sec)
H. j=_Wave Height for Shallow Water (ft)
F = Fetch (ft)
dm = Maximum Depth of Impounded Liquid (ft)
g = Acceleration of Gravity (32.16 ft/sec^)
8.3 WAVE PERIOD
= 7.54 tanh
0.833
tann
Where:
Ua = Wind Stress Factor (ft/sec)
T = Wave Period (sec)
F = Fetch (ft)
dm = Maximum Depth of Impounded Liquid (ftl
g = Acceleration of Gravity (32.16 ft/sec^)
0.333
0.0379
tanh
0.833
0.376-!
i1 J
B-l
-------
B.4 WIND SET-UP
0.5
I 9
-------
- Calculate dm/L0.
- Locate the value of d/L0 (dm/L0) in the Appendix and read
corresponding values for d/L and H/H0. If d/L0 (dm/L0) is
greater than or equal to 1.000, then H =
- Calculate the value of H0 fromH/H0.
HO'
-------
b«sl
from
copy.
0.0003
0.001
0.0)
FIGURE B -1, COMPARISON OF WAVE RUN- UP ON SMOOTH SLOPES (DATA
FOR d/Ho>3.0; SAVILLE, 1956).
GREATER THAN 0.013 HAVE SEEN EXTRAPOLATED.
B-4
-------
B.6 FREEBOARD ALLOWANCE
The minimum freeboard which should be observed is the sum of the
calculated values for set-up (S) and corrected wave run-up (Rc), from the
preceding equations. Changes in liquid "elevations due to the 100 year/24
hour storm surge are not included in this freeboard allowance calculation
(Figure 8-2).
Wave forecasting is not an exact science since it assumes ideal
conditions using monochromatic waves, and in some cases extrapolated values
for short fetches and shallow water depths. It may be advisable to include
a margin of safety (e.g. 25%) in the final freeboard allowance. If a
correction factor is used, it should apply to the sum of the corrected run-
up (Rc) and set-up (S).
Example:
Given a SI has a maximum fetch of 300 feet, a liquid depth of 10 feet,
sidewall slopes of 3 horizontal to 1 vertical, and is located in an area
not subject to hurricanes, calculate the minimum freeboard requirement.
F = 300 feet
dm = 10 feet
Slope = 3:1
U = 75 mph (wind speed)
Firsc it is necessary to convert U and Ua into units of ft/sec.
75 miles 1 hour 5,280 ft
U = x x ~ 110 ft/sec
hour 3.GOO sec 1 mile
Ua = 0.589 (110 ft/sec)1-23 = 191 ft/sec
B-5
-------
PASSIVE OUTFLOW
1
FREEBOARD
V ]
j
MAX. LIQUID DEPTH dm
\
i
1
•
25% SAFETY FACTOR
Re RUN -UP (CORRECTED)
S SET-UP
ACCUMULATED RAINFALL
d NORMAL LIQUID DEFT)
FIGURE B- 2. DEFINITION SKETCH FOR FREEBOARD.
8-6
-------
8.6.1 WAVE HETGHT
Substituting given values into the wave height equation yields:
(32.16 ft/sec2) H 0.283 tanh
(191 ft/sec)2
— I
0.53/ (32.16 ft/sec2)(10 ft)
V0.75
(191 ft/sec)2J
0.00565
(32.16 ft/sec2)(300 ft)*
J.5
tanh
(191 ft/seep
tanh
0.53
(32.16 ft/sec2)(10 ft)
(191 ft/sec)2
0.75
H = 0.92 ft
B.6.2 WAVE PERIOD
Solving the wave period equation gives:
(32.16 ft/sec2) T 7.54 tanh
(191 ft/sec)
0.833 ( (32.16 ft/sec2)(10 ft)
(191 ft/sec)2
\0.375
0.0379
tanh
(32.16 ft/sec2)(300 ft)
0.333
(191 ft/sec)2
tanh
0.8331
(32.16 ft/sec2)(10 ft)
(191 ft/sec)2
0.375
T = 1.08 sec.
B-7
-------
8.6.3 WAVE SET-UP
Using the wave set-up equation solve for S given the liquid has a
kinematic viscosity of 0.22 H2/sec a density of 53.06 lbs/ft3 and the
density of air is 0.075 lbs/ft3.
(32.16 ft/sec2)(53.06 Ibs/ft3j(0.22 ft2/sec) \°'33
U0 • 21.0
(0.075 lbs/ft3)
U0 - 359 ft/sec
S = (3.3 x 10-6)(191 ft/sec)2 + 2.08 x 10'4 (191 ft/sec - 359 ft/sec)2
(300 ft) (32.16 ft./sec2)(10 ft) (.32.16 ft/sec2)(10 ft)
10 ft N °'5
300 ft
/
S - 1.11 ft
B.6.4 WAVE RUN-UP
To calculate wave run-up, use the following procedure:
Find the deep water wave length
L0 = 5.12 ft/sec2 T2
= (5.12 ft/sec2)(1.08 sec)2
= 5.97 ft
Find the ratio of dm/L0 = 10 ft/5.97 ft = 1.68
Since the ratio of dm/L0 > 1.0, there is no need to correct the H value
derived earlier (H^ = 0.92 ft).
Find HQ/LO and H0/gT2
H0/L0 = 0.92 ft/5.97 ft = 0.15
H0/gT2 = 0.92 ft/(32.16 ft/sec2)(1.08 sec)2 = 0.025
B-8
-------
From Figure B-l find R/H^, using Hg/gT2 for a 1:3 slope.
R/HQ = 0.75
R = 0.75 (0.92 ft) = 0.69 ft
Solve for the corrected run-up value:
Rc = 1.67 (0.69 ft) = 1.15 ft
8.6.5 FREEBOARu ALLOWANCE
3
Using the preceding calculations, find the freeboard allowance (f).
f =• 1.25 (Rc + S)
= 1.25 (1.15 ft .-i- 1.11 ft)
= 2.83 ft'
For this impoundment, 2.83 feet represents the minimum amount of freeboard
which should be maintained.
B-9
-------
B.7 REFERENCES
Harbich, J. 8. 1986. Guidelines for Surface Impoundment Freeboard
Control. Consulting and Research Services, Inc. Report No. JBH-867.
Kinsman, B. 1965. Wind Waves. Prentice-Hall, Englewood Cliffs, N.J.
Longuet-Kiggins M. S. and R. W. Stewart.- 1964. Radiation Stress in Water
Waves; A Physical Discussion with Application. Deep-Sea Research. 11:529-
562.
Saville, T. 1956. Wave Run-up on Shore Structures. J. Waterways and
Harvors Div., ASCE. 82:925-1-14.
U.S. Army Corps of Engineers. 1984. Shore Protection Manua1.,Vo1 s. I and
II, 4th Ed. Waterways Experiment Station. Dept. of the Army. U.S. Gov't.
Printing Office.
8-10
-------
APPENDIX C
WATER BALANCE
C-'
-------
TABLE OF CONTENTS
Page
C.I. INTRODUCTION C-l
C.2. CONCLUSIONS _. C-2
C.3. RECOMMENDATIONS C-3
C.4 MATERIALS AND METHODS C-4
C.4.1 Site Description C-4
C.4.2 Study Instruments C-14
C.4.3 Data Collection C-16
C.5 DATA INTERPRETATION C-17
C.5.1 Climatic Conditions C-17
C.5.2 Evaporation C-17
C.5.3 Calculations and Error C-20
C.6. REFERENCES C-30
-------
C.I INTRODUCTION
The primary purpose of this study was to investigate the use of the
water balance method to quantify seepage from surface impoundments. Water
balance parameters tnat were examined included surface inflow and outflow,
precipitation, evaporation, change in storage, and seepage. A field study
was conducted usingoselected methods and~instruments. A discussion of what
is known about surface impoundment! nd what would be gained by conducting
water balances on these facilities follows.
A recent study by the EPA (1983) determined that there were over
180,000 surface impoundments in the United States. These impoundments are
used to treat, store, and dispose industrial, municipal, agricultural,
mining, and oil production w\stes. Approximately 28,000 of the impound-
ments contained industrial wastes such as those covered under EPA hazardous
waste regulations.
Over 50 percent o'f the industrial impoundments are locaterf over un-
saturated zones that are either very thin or very permeable. This would
provide very little potential for attenuating hazardous waste constituents
before these pollutants reach groundwater. Over 80 percent of the indus-
trial impoundments are located over thicfc and very permeable aquifers.
This would allow rapid movement of contaminant plumes once these pollutants
reach groundwater. Over 40 percent of the industrial sites are located in
areas with thin or permeable unsaturated zones and overlie high flow rate
aquifers containing water that is currently in use or of high quality.
Only two percent of the sites are located more than one mile from high
quality groundwater. Finally, only seven percent of the sites are located
in hydrogeologic settings that provide a high degree of groundwater protec-
tion (EPA, 1983). Only 30 percent of the industrial sites are lined.
Groundwater contamination has already been documented at hundreds of
the 28,000 industrial surface impoundments. Over 30 percent of these
documented cases of contamination are from impoundments associated with the
industrial category of "Chemical and Allied Products". The cause of
groundwater contamination in the vast majority of cases (86.3 percent) was
listed as seepage or liner failure (EPA, 1983). For the failed sites that
were lined, the primary reason for failure was loss of liner integrity.
Contamination was detected most often (45 percent) by the discovery of
adversely affected water supplies. About 30 percent of the cases were
detected by monitoring wells, but many of the impoundments had monitoring
wells installed only after seepage was noted. In less than five percent of
the cases, a water balance was used to detect seepage prior to obvious
groundwater contamination. It is highly probable that had a water balance
been maintained on the other surface impoundments, excessive seepage would
have been detected before groundwater contamination had occurred.
Water balances have long been used to quantify leakage trom large
lakes and drinking water reservoirs. The degree of sophistication used to
determine the water balance of a body ot water can vary from extremely
C-l
-------
simple to extremely complex (Mather, 1978). While the most complex methods
may require highly skilled personnel, the simplest methods may be extremely
inaccurate. The approach taken in this water balance study was to aim tor
the highest level of accuracy possible using methods that could be rou-
tinely used by unskilled personnel.
Conclusions and recommendations derived from the water balance study
are given in Sections C.2 and C.3, respectively. Materials and methods are
presented in Section C.4, and Section C.5 offers interpretations of the
data.
C.2 CONCLUSIONS
There is a need for a method to quantify seepage from surface impound-
ments. Such a method should be easy to operate, accurate in field condi-
tions, and sensitive enough to detect small rates of seepage. In addition,
the method should both be relatively inexpensive and generate unambiguous
data to facilitate its interpretation by the user and regulatory communi-
ties. The water balance method described in this report can satisfy all of
the above objectives.
When performing a water balance to quantify seepage from a surface
impoundment, the primary concerns are accuracy and sensitivity. There
should be a nigh degree of accuracy in the measurement of the individual
parameters. The water balance should also sense the smallest practical
rate seepage in order to serve as an early warning of impending groundwater
contamination. In most cases, the one water balance parameter that cannot
be measured is seepage. The water balance equation is, therefore, used to
solve for this unknown parameter. The sensitivity and level of accuracy
for the measurements of the other water balance parameters will determine
the magnitude of seepage that can be detected. Methods and instruments used
to perform the water balance will define the sensitivity and accuracy of
the system.
Two of the methods that can be used to solve the water balance equa-
tion are calculating volume changes and calculating level changes. By
calculating volume of the impoundment and monitoring all volumetric inputs
and outputs, it is possible to define volumetric seepage. This procedure
has neither the accuracy nor sensitivity necessary because it uses calcula-
tions based on an impoundment volume equation which can introduce large
errors. Therefore, in most situations, this approach should not be used.
The second method is based on using a level recorder to directly measure
level changes to within 1 mm. Both short term and continuous inputs and
outputs can be documented extremely accurately as water level changes.
This procedure eliminates many of the sources of error inherent in the
volumetric water balance calculations. The largest source of error in
calculating level changes results from the need to distinguish between
seepage and evaporation losses. Since both are output parameters which are
not short-term events, the accuracy with which evaporation is measured will
define what rate of seepage can be detected. Therefore, tne method used to
monitor evaporation losses becomes the largest remaining source of error.
C-2
-------
Many methods :xist for monitoring evaporation losses tron. surface
impoundments. Thesj range from extremely complicated energy budget methods
to the simple evaporation pan method. It is generally felt that the more
complicated methods offer a higher degree of accuracy and that accuracy is
compromised by using the simplest approach. The intensity with which
evaporation must be monitored for an energy budget method and the skill
level necessary to use the required instruments renders the method too
complex for routine use. In contrast, the use of evaporation pans requires
little skill and the devices can readily be used for routine monitoring.
Many types of evaporation pans are available and each has advantages and
disadvantages. Generally, evaporation from pans that are buried and con-
tain larger volumes of water will more accurately approximate the evapor-
ation from the impoundment.
Relating the evaporation rate of the pan to that of the impoundment is
the most critical step in the water balance method. Correlation of the
evaporation values is done by means of a pan coefficient. Pan coefficients
vary depending on the type and size of pan used. The value of pan coeffi-
cients are usually ba'sed on annual data and caution should be used when
evaluating evaporation for a shorter period of time.
It appears that the level monitoring method is the most accurate and
most sensitive method for this type of study as long as the following
conditions exist:
1} The level recording device is as accurate as possible and
has a sensitivity of 1 mm.
2) Inputs and outputs are short-term events of known duration.
3) The pan coefficient used is based on annual data.
If these conditions are met, the water level of an impoundment can be
monitored with the degree of accuracy necessary to quantify very low seep-
age rates.
C.3 RECOMMENDATIONS
A standard method should be established for the routine monitoring of
the water balance in hazardous waste surface impoundments. During this
study, a method was developed that can adequately monitor the water balance
of most surface impoundment configurations. The level monitoring method
can be a very accurate and easy to use technique which is capable of
quantifying small seepage rates. Two areas where uncertainty exists in-
clude 1) the correlation of evaporation values between the pan and impound-
ment and 2) the accuracy ot volume calculations in a water balance equa-
tion. Further research should be conducted to:
1) establish evaporation pan coefficients as they apply to
various wastes over long time periods;
C-3
-------
2) derive volume formulas within0 known error values;
3) determine the limitations ot the level method in areas wnere
freezing temperatures and solid precipitation are prevalent;
4) develop a system of monitoring for solids accumulation; and
5) reevaluate this method when used for clay lined Impoundment.
Additional research is needed to develop long term data on several
actual impoundments using the level monitoring method of evaluating water
balances. This would provide the EPA with the information necessary to
establish guidelines for future surface impoundment monitoring plans. If
the level monitoring water balance technique is required, consideration
should be given to requiring a detailed documentation of impoundment dimen-
sions following the completion of impoundment construction. This would
allow a more accurate calculation of storage capacity for the impoundment
and, consequently, the seepage value calculated would be more accurate.
C.4 MATERIALS AND METHODS
This section presents information about how the monitoring system for
the water balance system was set up. Special equipment needed to conduct
the study, such as the PVC pier, are discussed. A discussion of•the
operation of the instruments is included along with their associated error
term and/or sensitivity setting. Also presented is the schedule used to
monitor the system and alternatives to this schedule which might be con-
sidered.
C.4.1 Site Description
Research for.this project was conducted on the campus of the Southwest
Research Institute which is located on the southwest side of San Antonio,
Texas. The site is characterized by gently rolling terrain and native
vegetation. Climatic conditions for the area are summarized in Table C-l
which lists normal values for temperature, precipitation, and relative
humidity from National Weather Service records for the period, 1941-1970,
The impoundment where the study was conducted is lined with high
density polyethylene plastic and measures 61 x 61 m (200 x 200 feet) and is
approximately 2.1 m (7 feet) deep (Figure C-l). It is situated on the side
of a gently sloping hill with three sides, constructed from elevated berms.
Extending from the bottom of the impoundment is a discharge pioe to which
an in-line flow meter was connected (Figure C-2) This type of flow meter
should be positioned to ensure continuous, non-turbulent, full pipe flow.
This was accomplished by raising the pipe that extended past the meter to a.
height wnich would maintain a constant head sufficient to ensure full pipe
flow.
C-4
-------
Table C-l. National Weather Service Climatic Normals for San Antonio, Texas
for the Period from 1941 to 1970.
o
I
en
Temperature
Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Mov
Dec
Year
t Fastest
* ?4-hnnr
Min
39.8
43.4
49.1
50.8
65.7
72.0
73.8
73.4
68.6
59.2
48.2
41.8
57.8
Max
61.6
65.6
72.5
80.3
86.2
92.4
95.6
95.9
89.8
81.8
71.1
64.6
79.8
sustained wind
time .
Ave
9.2
9.9
10.6
10.6
10.2
10.1
9.2
8.6
8.5
8.5
8.9
8.7
9.4
speed
Wind Speed
Oir
N
NE
SE
SE
SE
SE
SE
SE
SE
N
N
N
SE
for one
Fast Mile1
35
35
35
39
40
35
48
35
42
31
32
30
48
minute.
Relative
00
76
75
72
77
81
80
75
74
78
77
76
76
76
6
81
80
79
83
88
88
87
86
86
84
81
80
84
0
Humidity*
12
60
57
53
57
56
56
51
51
55
53
55
57
55
ia
58
53
47
52
51
51
45
46
52
53
56
57
52
Precipi-
tation
1.66
2.06
1.54
2.54
3.07
2.79
1.69
2.41
3.71
2.84
1.77
1.46
27.54
-------
o
I
MONITORING
STATION
PIER
DRAIN AND
FLOW METER
FIGURE C-l. SITE PERSPECTIVE OF STUDY AREA WITH MONITORING EQUIPMENT.
-------
o
I
DRAIN PIPE
HOPE LINER
FIGURE C-2. IN LINE FLOW METER TO MEASURE DISCHARGE.
-------
On the eastern side of the impoundment, a PVC pier was constructed to
support tne level recording device (Figure C-3). PVC was selected because
of its resistance to corrosion and tne light weight construction offered
(by four-inch schedule 40) without sacrificing strength. Attached to the
pier is a stilling well constructed from (10-inch inside diameter) PVC pipe
(Figure C-4). The stilling well was mounted to the pier so that the upper
lip was flush with the pier deck and the lower lip was several inches from
the bottom of the impoundment. A baffle was installed in the lower portion
of the stilling well. The baffle consfsted of a 10-inch diameter plexi-
glass plate with a 1/4 inch hole in the center to which a flexible hose was
connected. The plexiglass plate was mounted in position at the bottom of
the stilling we?i and sealed by silicone caul* with the 1/4 inch flexible
hose extending from the center hole. The hose was then coiled and placed
on the bottom of tne impoundment where it acted as a damping chamber to
eliminate the effects of wind and wave action.
On the northern side, a monitoring station was constructed consisting
of two evaporation pans with level recording devices, a precipitation
gauge, and a data logger (Figure C-5). Both of the evaporation pans were
buried in the ground with four inches of "freeboard" exposed. Each pan was
then filled with water to the point that the water level was equal to the
ground level. A stilling well was placed in both evaporation pans to
improve the accuracy of the float recorder by removing unnecessary movement
caused by wind and wave action. Each pan was covered by wire screen (2-
inch mesh) to prevent access to the water surface by wildlife and to reduce
the amount of debris that might blow into the pan. The level recorder for
each evaporation pan was mounted on four-inch channel iron which was then
placed across the edge of the pan. By placing the iron bar on the edge of
the pan, errors that might result from settling of the pan or bar can be
avoided (Figure C-6).
Precipitation at the site is measured by a tilting bucket rain gauge
which is recessed in the ground so that the orifice of the ga.uge is flush
with the ground surface. A splash guard, consisting of plastic garden
edging, was placed around the orifice. The edging was pushed into the
ground so that approximately one inch remained exposed. It was arranged in
a spiral extending from the orifice outward to a distance of fourteen
inches with each successive spiral being an inch and a half from the
preceding spiral (Figure C-7). Grass was then planted between the spirals
to stabilize the soil and to help prevent splash-in during a rainfall
event.
A fence, of two-inch mesh chicken wire, was constructed to enclose the
evaporation pans and rain gauge to prevent disturbance by wildlife. Out-
side the fence, an instrument shelter was constructed to house the data
logger which was linked to the evaporation pan level recorders and the
precioitation gauge. By placing the data logger in the shelter it was
possible to shade the unit as well as ground it, to reduce any interference
which might result from excessive heat or static electricity.
C-8
-------
SIDE VIEW
END VIEW
CHART RECORDER
FIGURE C-3. PVC PllIR WITH STILLING WELL AND FLOAT RECORDER.
-------
LEVEL RECORDER
BAFFLE
FIGURE C-4. CLOSEUP OF STILLING WELL WITH RECORDER.
C-10
-------
-INSTRUMENT SHELTER
t: ;;rii::::;;;;;;::/-r--
i:i-".::"":""/-"•••••""••: -:
PUUSE GENERATOR
•RAIN GAUGE
o
I
jfimm^y
*':; •••••••.••/••v,-/X
'•i:'6:\>','.Q:
-------
PULSE GENERATOR
o
I
IS)
4"CHANNEL IRON
TO RECORDER
K—BURLED,. EVAPORATION
I PAN i
FIGURE C-6. , BURIED EVAPORATION PAN WITH SCREEN COVER AND PULSE GENERATOR.
-------
o
I
CJ
JILTING BUCKET
RAIN GAUGE
SPLASH GUARD
BURfED RAIN
GAUGE
>TO RECORDER
FIGURE C-7. TILTING BUCKET RAIN GAUGE BURIED AT GROUND LEVEL
WITH SPLASH GUARD.
-------
C.4.2 Study Instruments
The thrust of this section is to design a standard system which can be
installed at any surface impoundment. A brief overview of each component
of the system 'will be discussed as to how it tits into the system and why
it was selected. In choosing the various components, the following cri-
teria were used; accuracy, ease of operation, compatibility within the
system, and cost.
For the initial study, the on site parameters which must be measured
include precipitation, evaporation rates of the pan and the impoundment,
and the values for the amount of waste added and discharged. For a more
complete understanding of the climatic conditions affecting the study site,
it will be necessary to measure and record several temperature values such
as air temperature, and the temperatures of the waste in the impoundment
and in the evaporation pans.
To maintain complete records with the required level of accuracy, it
is necessary to use an automated system. The heart of this automated
system will be the recorder. Advantages offered by a recorder include
continuous monitoring, reduced maintenance, and removal of human errors or
bias associated with taking measurements.
Precipitation, as mentioned before, is a difficult parameter to mea-
sure accurately. The accuracy of the measured value will depend on the
type of gauge selected, the location of the gauge, and the type r.f precipi-
tation measured. The type of gauge which should be used is a tipping
bucket gauge. This type of gauge offers an unlimited capacity with
accuracy and resolution values of 0.5 percent and 0.01 ir.ches, respec-
tively. By placing the gauge orifice at ground level, the recorded pre-
cipitation rata should express the "true" precipitation rate. It will be
necessary to protect the gauge from splash in by placing a splash guard
around the gauge. This type of system will be most effective if the
predominate form of precipitation is rainfall.
Evaporation measurements are critical in that any error in measurement
would have the tendency to mask or exaggerate the presence of a leak. The
proposed system would include an evaporation pan which should be as large
(and deep) as practical since the larger volume would be more representa-
tive of the impoundment. The pan should be buried near the impoundment
with only four inches of the rim exposed above ground. By burying the
system, daily fluctuations in temperature should be more representative of
the impoundment. The evaporation rate should be measured by a float re-
corder which responds to small changes in the level of the waste. A float
system is preferred over other systems because a float gives a direct
measurement of the waste level regardless of the type of waste being con-
sidered. The accuracy of float systems varies widely but instruments are
available that are sensitive to changes of less than 1 mm. Two level
gauges will be necessary, one for the impoundment and one for the
evaporation pan.
C-14
-------
Waste addition and discharge values can accurately be measured by a
simple flow meter which records total flow volume. The recorded value
should be very accurate provided the metering system functions properly.
In selecting each of the instruments a conscious effort was made to
select the best instrument based on accuracy, ease of operation, minimum
maintenance, and reasonable cost. Instruments were selected to measure
precipitation, waste inflow and outflow, and level changes of the impound-
ment and evaporation pan. The remainder of this section will deal with the
individual instruments selected stating their specifications and mode of
operations.
Waste inflow and outflow volumes are to be monitored by a Model MLFT-
SGH-1 Main Line Flanged Tube Meter (3-inch) produced by Badger Meter Inc.
of Milwaukee, Wisconsin. The tube is constructed of cast iron and is
partially lined with stainless steel. Moving parts of the unit are sealed
to prevent corrosion and are constructed from bronze, Graphitar, and high
density plastic. Flow values are recorded by a sealed six digit totalizer
which is accurate to within +2 percent of actual flow. Restrictions which
govern the use of this unit are: 1) that 10 pipe diameters, 76 cm (30
inches), of unobstructed straight piping be placed before and after the
tube meter to reduce turbulent flow; and 2) the pipe must be filled during
flow to ensure accuracy.
For the initial study it was decided that two evaporation pans would
be needed; one to measure the evaporation of water and the other to measure
the evaporation rate of the waste. The evaporation pans will consist of
two galvanized circular stock tanks which measure 2.4 m (8 feet) in
diameter by 0.6 m (2 feet) deep. To record level changes of the two
evaporation pans, a Data Acquisition System (DAS) will be employed which is
produced by Leupold and Stevens, Inc. of Beaverton, Oregon. The DAS con-
sists of a Hydromark data logger, a hand-held data programmer, a two stand
alone pulse generators to measure level changes, as well as a tilting
bucket precipitation transmitter. Accuracy of the pulse generators is 1 mm
(0.003 foot) with a 1:1 gear ratio and a 7-.iay chart capacity. Power for
the system will be provided by a 12 volt battery. Changes in the surface
level of the impoundment will be measured by Type F Water Level Recorder
which is also manufactured by Leupold and Stevens. The Type F Recorder
is a free standing instrument sensitive to a level change of 1 mm (0.003
foot) with a 7-day chart capacity at a gear ratio of 1:1. Floats used on
all three recorders measure 20.3 cm (8 inches) in diameter and were
selected because of the increased sensitivity offered by the larger size.
In addition to the larger size it was necessary to sue floats constructed
from stainless steel to eliminate any complications associated with corro-
sive waste.
Precipitation will be recorded by Leupold and Stevens Tilting Bucket
transmitter which is connected to the DAS. The tilting bucket system is
accurate to 0.5 percent with a resolution 0.25 mm (0.01 inch) and has an
unlimited capacity.
C-15
-------
All of the above systems are designed to operate on DC voltage oro-
vided by a standard 12 volt battery, or.they are fr.e standing requiring no
external power source. The overall system is designed to require main-
tenance only once every 7 days. Each component of the system is weather
proof which eliminates the need to provide an instrument shelter.
C.4.3 Data Collection
Data were collected for a two-month*period from mid-March to mid-May.
During that time, the site was visited an average of once every seven days.
Each visit consisted of acquiring data from three sources: the data acquis-
ition system (DAS) which monitored level changes in the evaporation pans
and the precipitation gauge; the float recorder used to measure level
changes of the impoundment; and the in-line flow meter which recorded
discharge volume through the drain line. All data collected during the
study are presented in Section C-5.
The DAS is designed to accumulate and transmit data in an encoded
digital format. Input'to the data logger was from the two pulse generators
and the tilting bucket rain gauge. The pulse generators were designed to
respond to a level change of I mm. When this threshold was exceeded a
signal was sent to the data logger where it was recorded. It is possible
to either set the. threshold of the data logger to an increment that is the
same as the pulse generator in which case every signal is recorded, or set
the threshold at some higher limit that would require several signals from
the pulse generator before a value is recorded. Precipitation was measured
in increments of 0.25 mm (0.01 inch). During a rainfall event, each tip of
the rain gauge (0.25 mm) would se-^d a signal to the data logger where the
time of each pulse and a running total of the amount of precipitation would
be recorded.
Information once stored can be obtained by interrogating the data
logger by means of the hand-held programmer which presents the data one
event at a time or by connecting the data logger to a compatible printer
which will present a "hard" copy of the stored information. For this study
the data were obtained by using the hand-held programmer.
Level changes of the impoundment were recorded by a mechanical float
recorder which operates by means of a float pulley. The changes in water
level were recorded by a pen which traversed the chart at a given speed as
the float pulley rotated the drum chart. This type of system allows losses
and gains in water volume to be recorded as well as the time that the event
occurred.
Although this system was monitored once every seven days it. would have
been possible to monitor the instrument less frequently. The longest time
frame that the impoundment float recorder can operate autonomously is
limited by the maximum chart capacity of 32 days. The other instruments
generally have the capacity to operate autonomously for greater than the
32-day period.
C-16
-------
One limiting factor present in both the pulse generators and the
impoundment float recorder is that of adequate float cable. These instru-
ments operate by a float pulley which is driven by looping a cable around a
pulley wnicn is in turn connected to a tsoat and counter weight. The cable
used to operate the system must be sufficiently long to allow for the
change in water level expected. For example, if the impoundment level is
to be lowered by one meter (3.3 feet) there must be a minimum of one meter
of free cable between the counter weight and the float pulley to accom-
modate the level change.
0.5 DATA INTERPRETATION
As previously described, the parameters measured included pan
evaporation (in duplicate), impoundment level fluctuations, precipitation,
and impoundment discharge. Meteorological observations (e.g., temperature,
wind, and relative humidity) were obtained from the local San Antonio
office of the National Weather Service. Data for the test period are
presented in Table C-2.
C.5.1 Climatic Conditions
The climate during the study was typical of early spring in this
region, characterized by periodic passage of maritime polar air masses on a
three to four day cycle. Thus, a look at the record of temperature, wind,
and relative humidity shows the expected wide fluctuations. Such changes
in turn greatly affect the evaporative loss of water, as can be seen by
comparing pan evaporation over time with a graph of temperature variacions
during tne same period (Figure C-3).
Alternating between polar and tropical air masses influences
evaporative losses in a number of ways. As just mentioned, air
temperatures brought about by advection of air masses influenced the
evaporative demand by altering the saturation vapor pressure of the air
(i.e., the concentration of water vapor that can potentially be supported).
These differences are slightly counteracted by the much lower vapor
pressure usually associated with the polar air masses compared to humid
tropical air. When evaporative demand was high due to waru to hot
temperatures in the tropical air, the associated winds were relatively
light. Since the rate of water vapor transported away from a surface also
controls evaporative losses, light winds that limit advection and turbulent
mtxing decrease the evaporation losses, Topographic effects that are
manifested by changes in wind direction may also have significant effects,
as will be discussed later.
Evaporation
ince precipv
pan versus i mpoundtneri'. evapura uiun (.an ue arrived at quite easily. urapn:
of evaporation for both pans (averaye) ai.d the impoundment are shown ii
Since precipitation was only slight during the study, comparisons of
pan versus impoundment evaporation can be arrived at quite easily. Graphs
C-17
-------
Table C-2. Meteorological Data from the National Weather Service,
San Antonio, TX, for the Study Period.
Te«per«tur«
0«t«
Htrch 2U
21
22
23
24
2S
2b
27
28
29
30
31
Aprl 1 1
2
3
4
S
6
7
8
9
10
11
12
13
14
15
16
17
18
ly
20
21
22
2J
24
25
26
27
28
29
30
««y 1
2
3
4
5
M(n
40
43
56
62
48
43
62
64
47
38
S3
56
56
S3
46
SO
40
47
62
54
48
54
52
54
51
52
48
46
41
49
57
70
6b
59
Si,
55
54
70
6«
66
60
59
60
70
69
69
73
*.
76
78
76
tt
78
80
So
96
70
• 71
72
78
72
84
83
76
73
72
73
64
91
88
88
82
89
81
78
79
86
92
98
94
9U
01
85
90
82
96
81*
87
93
78
as
88
95
100
100
m
Avt
59
58
61
66
73
43
62
74
8U
59
55
63
64
69
65
63
57
60
68
69
70
71
70
70
70
67
63
63
64
71
78
82
79
70
7U
73
Ob
83
78
77
81
69
73
79
82
82
87
Wind Speed (»ph)
AM
7.9
10.7
13.0
10.3
8.1
9.8
8.0
15.7
19.3
5.8
11.8
6.6
11.9
9.4
11.1
11. 0
4.8
10.4
10.S
10.1
9.0
10.0
7.7
8.7
7.2
13.9
12.9
10.9
7.5
7.2
8.1
12.9
12. tt
4.8
7.6
12.8
12.6
10.7
13.1
8.3
13.9
12.1
8.4
8.2
11.7
10.9
12.7
-Gust
D
23
26
26
21
22
16
40
3V
16
21
18
2'j
20
26
30
13
24
22
2S
21
29
17
27
16
26
29
29
19
20
2U
31
3u
29
23
35
31
25
31
20
31
26
21
21
31
35
29
Olr
N
SC
S£
N
N
SC
SI
KU
NU
N
e
SC
SC
NU
Ml
N
S
SC
SE
KU
SC
SE
S
X
S
NU
NU
NU
S
S
S
S
»'W
N
rf
S
St.
S
N
SE
NU
M
SE
SE
NU
S
S
Ftlt
M1t«
16
18
2U
34
32
17
12
30
3U
13
16
12
18
16
la
17
13
16
16
21
IS
20
14
20
10
21
22
21
IS
12
14
17
la
18
14
20
17
14
.21
13
21
17
IS
13
18
17
17
Relative Hualdlty ID
00
56
63
67
81
33
54
6b
81
37
33
37
77
84
84
61
27
41
49
78
87
48
51
46
51
42
76
35
40
3V
45
6
73
73
81
84
63
73
78
87
b'2
62
52
90
8i
90
37
41
65
69
84
75
74
77
69
77
43
24
35
44
49
66
12
20
27
48
51
27
37
11
12
21
22
33
57
65
SS
16
22
31
59
71
34
23
12
22
31
39
IS
2U
16
16
37
18
25
31.
55
34
21
43
37
14
18
21
29
SO
59
24
17
22
26
68
84
26
20
19
24'
31
31
IS
17
13
12
37
•
C-18
-------
10
9
I 8
i7
H
?3 6-
0. ,
< 5
liJ .
2-
I-
TEMPERATURE (AVERAGE)
8 10 12 14
18 20 22 24 26 28 3O 32 34 36 38 40 42 4.4
DAY
FIGURE C-8. DAILY EVAPORATION VALUES FOR THE EVAPORATION PANS
RELATIVE TO THE AVERAGE DAILY TEMPERATURE.
-------
Figures C-9 and C-1G. Air mass effects are readily apparent and result in
wide diurnal fluctuations. Thus, data should be expressed on a weekly or
monthly basis to damp out these air mass effects and indicate broader
trends. However, the daily data do allow a more detailed comparison of the
pan versus impoundment evaporation rates. .
The data arc in agreement in that measurement of evaporation in both
the pans and the impoundment changed at nearly the same time in response to
changes in evaporative demand. The slight lags in the evaporation rate of
the impoundment during periods of rapid heating (e.g.-, days 8 and 9, and
days 15 and 16) are expected due to the larger heat storage capacity of
the impoundment. In the early part of the study, the evaporation rate of
the pan was less than or approximately equal to the evaporation rate of the
impoundment. This is the expected relationship. What happened later in
the study, was not expected. During the last three weeks of the study the
evaporation rate of the Impoundment exceeded that of the evaporation pans.
The reason for this is that the level in the impoundment was several feet
(approximately 5 feet) below the berm. At that point, the water in the
impoundment was less than 0.6 meters (2 feet) deep. This low level coupled
with the heat absorption capacity of the black liner allowed the water
temperature to fluctuate more quickly. This resulted in more rapid in-
creases and decreases in the evaporation rate of the impoundment.
During the-course of the study, the level of water In the evaporation
pans was allowed to drop. At no time during the study was any water added
to replace that which was lost due to evaporation. The reasoning behind
this was to mimic the low water level present in the impoundment. It 1s
felt, however, that the lower water level in the evaporation pan set up a
saturated air layer over the pan which reduced the evaporation rate dispro-
portionally to that of the impoundment.
The pan coefficient for this study was 1.09. This value represents
the average obtained over the 45-day period. It is felt that this value is
too high and the actual value should lie between 0.90 and 1.00. Daily
values for pan coefficients varied from 0.25 and 3.50 (Table C-3) while
extremes for weekly pan coefficients vary from 0.80 to L19 (Table C-4 and
Figure C-ll). Variability of the daily and weekly pan coefficients differ
in the magnitude of the range covered and it is felt that the longer the
data are collected, the more accurate the pan coefficient will be.
C.5.3 Calculations and Error
In acquiring data, there is always an associated error due to the
manner in which the data are collected or because of the inherent in-
accuracy in the instruments used. The following discussion will present
each term of the water balance equation and the associated error. Also
included is an examination of several of the terms which can be disregarded
in some situations.
The basic water balance equation for evaluating seepage consists of
seven components and is written as:
C-20
-------
o
I
ro
II
10
e91
° 7
g 6
0.
Ll
4
3-
2
I-I
FttN (AVERAGE)
6 8 l6 12 1416
18
20 22 24
DAY
26 28- 30 32 34 36 38 40 42 44
FIGURE C-9.
DAILY EVAPORATION VALUES FOR THE SURFACE IMPOUNDMENT
AND EVAPORATION PANS (AVERAGE).
-------
50-
\
J
2
O
tr
o
QL
< 20
ui
10-
\
IMPOUNDMENT
=— RAN i
2
\
\
3 4
WEEK
FIGURE C-IO. WEEKLY EVAPORATION VALUES FOR
BOTH EVAPORATION PANS AND THE
SURFACE IMPOUNDMENT.
C-22
-------
Table C-3. Daily Evaporation Data for Both Evaporation Pans and the Impound-
ment*
Date
Mar 21
22
23
24
25
26
27
28
29
30
31
Apr 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
May 1
2
3
4
Day
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
- 18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
Pan 1
(mm)
8
3
5
5
6
3
10
7
5
7
2
3
2
9
6
4
3
1
4
7
8
5
6
4
10
7
6
6
3
5
6
9
6
6
8
2
5
7
3
6
6
3
2
7
7
Pan 2
(mm)
7 -
4
5
4
6
3
9
8
4
7
2
3
• 2
8
7
4
3
1
3
7
7
6
6
4
9
8
6
6
3
5
6
10
6
7
6
2
5
7
3
6
6
3
2
7
7
Kl
.56
1.00
.90
1.30
1.08
.33
.60
1.57
.90
1.00
1.50
1.33
1.00
.89
1.42
2.13
1.33
3.50
.25
.57
.88
1.50
1.17
1.25.
1.00
1.21
1.25
1.08
1.50
.80
1.17
1.11
1.G8
1.17
1.13
1.50
.90
1.14
1.33
1.08
1.33
1.33
.75
1.07
1.29
*2
.71
.75
.90
1.63
1.08
.33
.67
1.38
1.13
1.00
1.50
1.33
1.00
1.00
1.21
2.13
1.33
3.50
.33
.57
1.00
1.25
1.17
1.25
1.11
1.06
1.25
1.08
1.50
.80
1.17
1.00
1.08
1.00
1.50
1.50
.90
1.14
1.33
1.08
1.33
1.33
.75
1.07
1.29
Impoundment
(mm)
5.0
"3.0
4.5
6.5
6.5
1.0
6.0
11.0
4.5
7.0
3.0
4.0
2.C
8.0
8.5
8.5
4.0
3.5
1.0
4.0
7.0
7.5
7.0
5.0
10.0
8.5
7.5
6.5
4.5
4.0
7.0
10.0
6.5
7.0
9.0
3.0
4.5
8.0
4.0
6.5
8.0
4.0
1.5
7.5
9.0
Pan coefficients (K) are given which relate the pan values to the im-
poundment values.
-------
Table C-4. Weekly Evaporation Data for Both Evaporation Pans and the
Impoundment*
Week
1
2
3
4
5
6
Total
Pan 1 (mm)
40
35
33
44
43
32
227
Pan 2 (mm) Kj
38
34
32
45
43
32
224
.80
1.13
1.08
.95
1.12
1.19
1.04
<2 Impoundment (mm)
.84
1.16
1.11
.93
1.12
1.19
1.05
32
39.5
35.5
42
48
38
235
* Pan coefficients (K) are given which relate the pan values to the im-
poundment value.
C-24
-------
f-
Z
LJ
O
1.20-
1.10-
I.OO-
0.90^
0.80-
UNITY
PAN I
PAN 2
WEEK
FIGURE C-ll.
WEEKLY BXN COEFFICIENTS FOR THE
STUDY PERIOD FROM MARCH 21 - MAY
5. THE AVERAGE K VALUE FOR THIS
PERIOD IS 1.09.
C-25
-------
°u " rs + Ip + Iu - °s • °e • S
where:
Qu = underground outputs or seepage
Is = surface inputs or run-on and waste received
Ip = precipitation intercepted by the impoundment
Iu = underground inputs
03 = surface outputs or discharges
CL = evaporation losses
5 = change in storage
Each component has an associated error term (n). It is necessary to define
the limits of each error term as either a percent error or as a level of
resolution or sensitivity.
Since Ou (underground output) represents the unknown parameter, the
equation is solved in terms of the six known parameters. The sum of the
error factors associated with the known parameters will equal the error
factor for Ou.
In most cases underground input (Iu) will not be a factor since most
surface impoundments are above the water table and, in situations where the
impoundment might be in contact with the water table, the hydraulic head of
impoundment would usually prevent water inflow. In this study, Iu is
assumed to be zero and there would, therefore, be no associated error term
for Iu.
The accuracy of measurements made of precipitation intercepted by the
impoundment (Ip) is dependent on both the size of the impoundment and the
accuracy of. the rain gauge. For this study, a tilting bucket rain gauge
with an accuracy of 0.5 percent was used. In addition to the error term
associated with the instrument chosen, there is an error associated with
the positioning of the rain gauge. By placing the rain gauge in a pit with
its orifice at ground level, the expected error value due to placement was
limited to to 1.5 percent (Bleasdale, 1958). Therefore, the total expected
error value associated with measuring rainfall in this study is 2 percent.
Surface inputs (Is) may be in the form of surface run-on or input of
waste. In the case of surface run-on the magnitude of the resulting error
will depend on the intensity and duration of the run-on event. Since, in
most cases, run-on events are not metered, the magnitude of the resulting
error is unknown. Ideally it is best to eliminate all possibility of
surface run-on. If this is accomplished, the associated error value for
run-on will be zero. When liquid is added to the impoundment, it should be
directed through a metering device. During this study, a in-line flow
meter was used which is accurate to +2 percent.
Surface output (Os) is very similar to surface input in the sense that
unmetered surface output should not occur. If unmetered output were to
occur, the amount is likely not to be known. Consequently, the error value
C-26
-------
would also be unknown. If no surface output occurs, then there would be no
associated error term. In this study, discharge of liquid was measured by
the same meter that measured surface input, therefore, the error value of
+2 percent also applies to the Os term.
Output due to evaporation (Oe) in most situations is the hardest of
the parameters to measure and usually has the largest associated error
term. Evaporation during this study was measured by a float recorder which
is sensitive to a level change of 1 mm. This degree of level measurement
sensitivity yields an error of less than one percent. The main source of
error in evaporation measurement, therefore, will result from using evapor-
ation pans to estimate the evaporation rate of impoundments. This error
term will vary from site to site and depend on the type of pan used. In
addition, evaporation error will depend on whether it is calculated on the
basis of daily, weekly, monthly, or yearly average evaporation data. The
pan used in the study "is very similar to pans which have pan coefficients
of 0.92 to near unity (Brutsart, 1982). These values represent the conver-
sion factor used to calculate evaporation from large bodies of water. For
example, if the pan coefficient is 0.9 and 0.2 meter of water is lost from
the pan, 0.18 meters would be the expected loss from the impoundments.
Values for the pan coefficient obtained from this study vary from 0.25 to
3.5 on a daily basis to 0.80 to 1.19 on a weekly basis. The overall pan
coefficient for the study was 1.09. This suggests that the longer the time
period used to calculate the pan coefficient, the more accurate, the value
will be. It is felt that in this case 1.09 represents the upper'limit on
the pan coefficient. In most cases, the upper limit rarely exceeds unity
(1.00) when the coefficient represents an annual average. The lowest linit
seen on a weekly basis was 0.08. When two weeks of data were averaged the
lowest value was 0.98. Consequently, a conservative estimate for the
lower limit can be established at 0.90. In this case, the maximum error
expected would be 10.0 percent.
Monitoring for changes in storage will be done in terms of changes in
water level. The level change will be recorded by a level gauge which is
sensitive to 1 mm. This means that the recorded value is always within
1 mm of the actual level of the impoundment. Therefore, the error term
will be a constant which equals 1 mm.
By pulling all of these terms together the total error value can be
calculated. The total calculated error value may change depending on
events which occur during the observation period. For example, if no
precipitation occurs, then the error value due to precipitation is zero.
The water balance equaiton can now be stated as:
Ou = (Is *n) + (Ip +n) + (Iu ±n) - (Os +n) - (Oe +n) - ( S +n) The
value of n either represents the value of the paramter multiplied by the
percent error for that parameter or is given as a constant error. Where a
percent error is used, the magnitude of error will depend on the magnitude
of the parameter being measured. Once a value for each parameter has been
established and an error value determined for each, these factors are added
to express the total error range for the underground output. If there is a
decrease in storage that significantly exceeds the summation of the error
C-27
-------
terms, then there is a quantifiable volume of seepage. If the decrease in
storage does not exceed the total error, then the leakage rate is too small
to quantify by this method.
There are two ways to approach the water balance problem. One is to
account for all the water in terms of volume. To do this every cubic meter
in and every cubic meter out plus every cubic meter in storage is calcu-
lated. The second approach would be to represent a volume in terms of the
water level.
The second approach of monitoring level changes was adopted for this
study. This method has the advantage of eliminating error associated with
calculating volume changes using the volume formula. An additional advan-
tage is that the level recorders (with floats in stilling wells) can mea-
sure level to within 1 mm of the actual level. The main disadvantage is
that the level recorders cannot distinguish between sources of outputs or
inputs. For example, surface output (discharge) and evaporation are both
losses and both are registered as a drop in level. It is possible to
eliminate this problem if one recognizes the following:
1) evaporation is a long term event; and
2) surface output is normally a shcrt term event which occurs
over a known time period.
In a similar sense, increases in level due to precipitation and surface
input can be measured since they, are associated with known short term
events.
In using the level monitoring method for calculating the water
balance, it was necessary to establish a base line water depth in the
impoundment. This value was measured to be 1.030 meters. Cumulative
values for the water balance parameters measured during the study period
are as follows:
Is - 0 .
In = 0.0185 m
'u = °
0, ='0.4300
0, = 0.2255
5 = -0.6775 m
The water balance equation with these values inserted is:
Ou = 0 + (0.0185 m +n) + 0 - (0.4300 m +n) - (0.2255 m +n) - (-0.6755 m +ji)
The error value of 1 mm is the error term used in this calculation for Ip.
Q,, and S. This is because each of these parameters is measured by a
float recorder which is sensitive to changes in level equal to 1 mm. The
error term for Oe will include, the 1 mm value and the error value asso-
C-28
-------
elated with converting the evaporation value of the evaporation pan to that
of the impoundment. The conversion factor (K) established for this study
was 1.09. It would be premature to define the error value associated with
the pan coefficient. This is because the study period was too short to
establish a reasonable average pan coefficient. For the sake of completing
the calculation, an error value of _+10 percent will be assumed. (This
value should be reduced significantly with the collection of longer term
data.) Using that error, the pan coefficient will lie between 0.90 and
1.10, with the observed value of 1.09 Tying near the upper liir.it of the
error term. The n value for Oe will be (0.2255 m) 0.10 •*• 0.001 m. There-
fore, n for Oe will equal 0.0235. Putting these values into the water
balance equation gives:
Ou = (0.0185 +_ 0.0010 m) - (0.4300 m ^ 0.0010 m) - (0.2255 m j^ 0.0235 m) -
(-0.6755 m +_ 0.0010 m)
Ou = 0.0385 m +; 0.0265 m
From these data, a leak is suggested which may be as large as 0.0650 m or
as small as 0.0120 m. Additional data would be needed to substantiate this
finding by statistical analysis.
C-29
-------
C.6 REFERENCES
Bl'easdale, A. 1958. A compound rain-gauge for assessing some possible
errors in point rainfall measurements. Meteorological Offices,
Hydrological Memoires No. 3. As cited in Bochkov and Struzer (1972).
Brutsart, W. 1982. Evaporation into the-atmosphere. D. Reidel Publishing
Co., London, England,
EPA. 1983. Surface impoundment assessment national report. Office of
Drinking Water. U. S. Environmental Protection Agency. Washington, O.C.
EPA 570/9-84-002. 73 p.
Mather, J. R. 1978. The climatic water budget in environmental analysis.
0. C. Heath and Company. Lexington, Massachusetts. 239 p.
C-30
-------
APPENDIX D
METHODS FOR DETECTING LIQUID LEVEL
-------
TABLE CF CONTENTS
Page
D.I INTRODUCTION D-l
D.2 CAPACITANCE PROBES * D-l
0.3 CONDUCTIVITY PROBES D-3
.0.4 DIAPHRAGM DETECTORS 0-4
0.5 DIFFERENTIAL PRESSURE DETECTORS D-5
0.6 DISPLACERS 0-6
0.7 FLOATS • 0-7
0.8 IMPEDENCE PRU3ES D-8
D.9 LEVEL GAUGES D-10
0.10 INFRARED SENSORS • 0-11
0.11 RESISTANCE TAPES. -. D-12
0.12 THERMAL SENSORS 0-15
D. 13 ULTRASONIC DETECTORS D-16
0.14 OTHER METHODS OF LEVEL DETECTION D-17
0.15 REFERENCES D-19
-------
U.I INTRODUCTION
As discussed in Section 4.0, liquid level control consists of a four
phase system, with level detection consisting of the primary and measuring
elements in the sequence. The following sections discuss vtrivjs types or
level detection systems and offer information on their operation, the
advantages and disadvantages of these systems, and such specifications as
accuracy and generalized price. A quick~reference to the types of systems
discussed is provided in Table 0-1.
In most of the following sections, information concerning the type of
electrical output is absent. This is because the type of output signal for
most liquid level sensors consists of a 4 to 20 mA signal, where the lower
power setting represents the off signal and the higher power setting
represents the on signal. This type of output is standard within the
industry and is easily adaptable to most types of electrical controlling
systems. In situations where a different output signal is required,
engineering changes are possible to allow the level sensing device to
interface with the level controlling elements. Therefore, the type of
output signal omitted by the liquid detection devices should be adaptable
to the system selected.
0.2 CAPACITANCE PROBES
Principle of Operation
Tnis type of system operates on the ability of the unit to measure the
ratio of a stored electrical charge for a given potential difference
between two terminals separated by an insulator with a Known dielectric
constant. Typically, the probe forms one terminal or plate, and the
sidewall of a metal pipe of stilling well forms the other. As a
microelectric current is passed through the system, changes in the
capacitance are measured as a function of the differences in the dielectric
constants (K), which register a signal proportional to the length of the
probe covered by the vapor phase (K^J and the liquid phase (l^).
Considerations for sizing a probe include The diameter of the pipe or
stilling well, the dielectric constant of the liquid, and special coating
used on the probe, if any. To use a capacitance probe in an impoundment.
it will be necessary to mount the probe in a conductive shield wnich will
serve the same function as the sidewall of a tank.
Capacitance probes have the ability to continuously measure liquid
level and provide an analog output of 4-20 mA. They are equally capable of
operating point relays to be used as high level or low level indicators.
These point relays could t>e engineered to activate secondary liquid level
control systems such as pumps, valves, or alarms.
D-l
-------
Table 0-1. Liquid Level Detection Systems.
System
Icarion
Principle of
Operation
limitations
Capacitance
Non-corrosive wastes;
corrc-.Ive •/nod
Measurement of stored
electrical charge
Fall safe operation,
essy operation
Based on IIquid
-------
Advantages and 01sadvantages
Advantages include ease or installation and operation, versatility, a
relatively inexpensive price, and availability. Tney may he coated with
materials which are resistant to nostile environments, and are engineered
to function safely near explosive or ignitible materials.
Disadvantages include trie probe's dependence on the dielectric
constants of the waste. The dielectric constant 1s different for most
wastes and is sensitive to changes in temperature. The changing
dielectric constant in impoundments which receive wastes that vary slightly
in composition and consistency will compromise the accuracy of the probe.
The probe is also subject to error due to coating, which may cause a false
signal.
Range of Operation
Capacitance probes can be engineered to virtually any length for
continuous liquid level measurement. Also available are high level and low
level probes which sense critical liquid levels and have the capability to
activate secondary level control systems. Normally, capacitance probes are
designed to operate in storage tanks, but they can be modified for
operation In open environments. Protective coatings are available for
probes which will come in contact with corrosive or "sticky" materials.
o
Specifications
Costs: $250 - $450 for high level or low level switches
$350 - $700 (and up) for continuous level probes; cost in-
creases as the length of the probe increases; special coatings
would be an additional cost
Accuracy: up to +1S
References: Bailey, 1976
Liptak and Venczel, 1982
Reason, 1984
Siegwarth, 1981
0.3 CONDUCTIVITY PROBES
Principle of Operation
Conductivity probes work on the princiole ot passing an electrical
current through a conductive material. The probe(s) is situated at a
critical point above a liauid and "actively monitors" the absence of a
conductive liquid by maintaining an open circuit. When the level of the
liquid comes in contact with the probe, the circuit is closed, thus
causing a relay to activate a secondary ^vel control system.
D-3
-------
Advantages and Disadvantages
Conductivity probes nave no moving parts and are extremely easy to
Install and operate. They operate on simple electrical current and require
no encoding of an output signal. Typically, they are among tne most
inexpensive level detection system available.
Conductivity probes have limited use in liquids which are highly
conductive. They are also influenced by the wave action and presence of
foam on top of the liquid. Inherent difficulties arise by passing an
electrical current through a liquid which is 1gn1tib1e or explosive.
Electrolytical corrosion and. build-up of material on the probe are
possible, and will cause the probe to fail.
Range of Operation
Conductivity probes are incapable of continuous level measurement, but
are commonly used as high or low level indicators. The probes are limited
in their use in highly-conductive liquids or bulk materials.
Specifications
Cost: typically under $400
Accuracy: approximately _+ 1/8 inch for on-off actuation
References: Bel sterling, 1981
Hall, 1978
Liptak and Venczel, 1982
Scott, 1972
0.4 DIAPHRAGM DETECTORS
Principle of Operation
Diaphragm detector? operate on the simple principle of detecting the
pressure exerted on a sensitive membrane. The pressure sensitive switch is
usually mounted on the top of a riser pipe wnich extends into the liquid to
be monitored. The riser pipe is sealed at the top by the pressure switch
and is open at the other end, wnicn is submersed in the liquid. As the
liquid level rises, the air isolated in the riser pipe is compressed. The
increased air pressure is recorded as a positive level change. As the
liquid level drops, the air pressure in the riser pipe is reduced, which
results in a decreased reading by the pressure switch.
Advantages and Disadvantages
Tne advantages of diaphragm devices include low cost and ease of
operation. They are adaptable to use with many types of liquids, including
corrosive wastes, since the diaphragm does not come in contact with the
material. Reports by industry indicate these types of switches are
reliable.
0-4
-------
Disadvantages include poor sensitivity to small level changes, which
limits the accuracy of these-devices. Since the actuating mechanism is
dependent on pressure, and since pressure is a function ot the density of
tne liquid, variations in liquid composition may lead to error in level
calculations. Additionally, temperature variations may be sufficient to
alter liquid density, which could affect the accuracy of the instrument.
Range of Operation
Diaphragm devices provide continuous level monitoring with the
capability of sensing high and low critical levels. Continuous level
monitoring is virtually unlimited. Some devices are limited to riser pipes
which do noTexceed 60 feet and are designed for use in atmospheric tanks.
Electronic sensors operate over ranges of 0 to 1.5 meters and pressures of
0 to 300 psig (gauge pressure in pounds/inch').
Specifications
Costs: $100 to $500 for mechanical diaphragm devices, $750 to $1500
for electronic devices
Accuracy: Poor; _+ 1-6 inches for mechanical; + 0.32 full scale tor
electronic devices ~
References: Hall, 1978
Lawford, 1981
' Liptak and Yenczel. 1982
Cheremisinoff. 1981
0.5 DIFFERENTIAL PRESSURE DETECTORS
Principle of Operation
^MMMW^BBOI^^^BBBM^^nBaiBWM^V^^B^^B 0
Liquid level detection by differential pressure is an indirect method
based upon measurement of the difference in pressure between the bottom of
the liquid column and atmospheric pressure. Simply, two pressures can be
sensed, one atmospheric and one at the bottom of the liquid column, and the
difference taken. Knowing the density of the liquid, the depth can then be
found. In practical applications, it is common to use a single pressure
difference sensor to ensure the intrinsic balance of the static pressure
levels. This arrangement will eliminate the inherent inaccuracies and
problems associated with two independent pressure sensors.
Several types of differential pressure devices are available,
including dry force balance and dry motion balance designs and manometers.
Most devices employ a flexible diaphragm, bellows, liquid column or other
insulating mechanism between the high and low pressure sides. As the level
of the liquid increases, pressure on the high side of the sensor increases
and movement of the insulator occurs in response to the pressure increase.
Insulator movement is mechanically or electronically interpreted as
pressure change, and tne differential is calculated and converted to liquid
level. The signal may also be used to trigger controlling devices and
alarms.
D-5
-------
Advantages and Disadvantages
Advantages of tne differential pressure device include its
adaptability to a wide range of wastes and situations, as well as easy
calibration and operation and ability tor continuous level monitoring. The
mechanism is generally sealed and does not come in direct contact with the
waste. It is also unaffected by material conduct! vi'ty. foam and minor
surface agitation.
Disadvantages include its dependence upon the density of the liquid.
The unit must be recalibrated for each new waste according to the density
of the waste. Furthermore, the system is typically not applicable for
detection of a liquid-liquid Interface.
Range of Operation
Differential pressure devices are used for continuous level monitoring
of a wide variety of wastes. The devices are not, however, recommended for
corrosive or hard to handle wastes. The system is applicable for storage
and disposal impoundments containing non-corrosive wastes.
Specifications
Costs: moderate to nigh; $200 and up for local indicators, $600 and
up for transmitters
Accuracy: up to _+ 112%, dependent upon the system and the depth of
liquid
References: Duncan, 1979
Lawford, 1970
Lawford, 1981
Liptak and Venczel, 1982
D.6 DISPLACERS
Principle of Operation
Displacer level detectors operate on the Archimedes' principle, which
states that an object, when partially or completely submerged, will
displace a volume of liquid whose weight is equal to the buoyant force
exerted on the object. If the object (displacer) is connected to a weignt
sensitive device, any change in the displacers weignt due to contact with a
liquid can be detected. Depending on the shape and density of the
displacer and the density of the liquid, specific measurements for either a
continuous or a high/low level can be made.
For continuous level measurements, an elongate displacer (torque tube)
with a known weight and shape is used. As the displacer is submerged, the
weight ot the displacer is reduced and the liquid level can be calculated
proportional to the submerged depth.
D-6
-------
High/low level measurements can be made using a flattened disc shaped
displacer which registers a buoyant force when it comes in contact with a
liquid surface. This type of displacer will detect discrete level changes
as well as registering boundaries between layers which nave different
densities. When operated with a pulley system this type of displacer can
be lowered to the liquid surface at regular intervals to take measurements
and then be retracted.
Advantages and Disadvantages
The main advantage of this type of system (disc displacer) is its
ability to detect varying layers of different densities.
Disadvantages include the need to carefully calibrate the displacer to
the situation. This calibration is dependent on the density of the liquid.
therefore, a variation in density will result in error when measuring the
liquid level. Also, since the displacer has a known critical weight, any-
thing which would change this weight, such as coating it with a viscous
material, will alter the displacer's accuracy.
Range of Operation
Torque tube displacers can operate up to ranges of ten feet. Disc
displacers are virtually unlimited in the range they can measure.
Displacers can be designed to operate in open or closed environments.
Specifications
Costs: $500 - $1,500 depending on engineering specifications.
Accuracy: +_ I/22
References: Bailey, 1976
Hall, 1978
Liptak and Venczel, 1982
Young et al., 1975
D.7 FLOATS
Principle of Operation
The operation of float actuated level detection devices is based upon
the movement of a float with the rise and fall of the liquid level it is in
contact with. The float movement is translated by various means into a
number of control actions. Float devices are traditionally applied to
liquid surface detection, but can be modified tor liquid-liquid interface
detection. Many float actuated designs are available and most operate
using a buoyant sphere or disk.
Float and lever designs employ a float which is attached to a lever
arm or mechanism. When the float moves up or down, the level arm activates
a controlling element or output device. This type of device is generally
fixed and is used as a point-level indicator.
0-7
-------
Chain and tape level monitoring devices consist ot a tloat connected
with a flexible tape or chain to an uptake reel or wheel. The reel is
connected to a level recording or detection mechanism. The tloat remains
in constant contact with t*e liquid surface, thus enabling the level
recorder to give continuous level monitoring as opposed to point-level
detection. The float may also travel along vertical guide wires. Switches
can be located at desired elevations along the guide wires to trigger pumps
or valves as well as alarms or other warning devices and for the purpose of
volume control. ~
Advantages and Disadvantages
Advantages of the float level devices include simple and easy opera-
tion and adaptability to many situations. They can be relatively
inexpensive, and measure the liquid level directly.
Disadvantages include constant contact with the liquid, which limits
its apolicability with corrosive wastes. Floats are also affected more by
waves or surface turbulence than some other methods unless stilling wells
are used.
Range of Operation
Float level devices may be used for point-level or continuous level
detection and monitoring. Tne°y are generally used for tracking the liquid
surface but can be modified for monitoring a liquid-liquid interface. They
can also be constructed of materials suitable for use in hostile
environments.
Specifications
Cost: generally low to moderate; $200 and up
Accuracy: in the range of IS, with greater accuracy in the more
expensive systems
References: Cheremisinoff, 1981
Liptak and Venczel, 1982
Sandford, 1978
Scott, 1972
D.8 IMPEDANCE PROBES
Principle of Operation
Impedance probes operate on the same principle as the capacitance
level detection systems. Capacitance probes, unlike impedance probes,
incur problems with wastes which are sticky and tend to build up. To
overcome problems caused by waste build up, impedance probes employ a
capacitance probe which is surrounded by, and insulated from, a secondary
probe. The secondary probe is in turn insulated from the liquid (Figure D-
1). In the capacitance probe, build up from the liquid completes the
D-8
-------
TO CONTROLS
INSULATION
SECONDARY PROSE
INSULATION
CAPACITANCE PROBE
FIGURE 0-1. IMPEDANCE PROBE
0-9
-------
cfrcuit between the probe and containment wall by a low resistance oatn
through the waste. Since a.low resistance oath results in a "positive"
reading, waste build up will cause a normal capacitance probe to tail.
The impedance probe was designed to "short circuit" the low resistance
path caused by waste building up. The low resistance oath is isolated trom
the capacitance probe by passing an electrical current through the
secondary probe. The integrity of the capacitive path is then preserved.
allowing the probe to function normally regardless of waste build up. When
the liquid level rises and ccmes in contact with the probe, the capacitance
path is completed by the internal capacitance probe. This output can be
used to engage the system design (pump, alarm, etc.). The added features
of the impedance probe make it applicable to a wider range ofo wastes than
the standard capacitance probe.
Advantages and Disadvantages
Advantages of the impedance probe include its ability to handle sticky
build up of wastes on the probe. The Impedance probe also has the
advantage of no moving parts, therefore, it requires less maintenance than
mechanical systems.
Disadvantages of the impedance probe include its dependence on the
knowledge of the dielectric constant and conductivity of the waste.
Changes in these waste characteristics will affect the accuracy of the
device. The thickness of the coating which may build up on the probe can
also have an effect on the accuracy of the unit. Impedance probes are also
not well suited for non-conducting materials.
Range of Operation
„ Impedance probes are well suited for sticky, conductive wastes which
tend to build up on the probe. The probe insulators can be designed to
withstand many corrosive wastes and are used for point-level and continuous
level monitoring. The probes are applicable in storage and disposal
impoundments containing corrosive and some non-corrosive wastes.
Specifications
Cost: generally high; $700 and up for tixtd point-level units; $1100
and up for continuous level probes
Accuracy: dependent upon probe sensitivity and waste characteristics
References: Liptak and Venczel, 1982
D.9 LEVEL GAUGES
Principle of Operation
Visual level gauges have been used for many years and are an
inexpensive and reliable method of level indication. However, this method
D-10
-------
usually requires the efforts of a technician to correctly read and record
tne liquid level and to recognize levels which require action. Visual
techniques include staff gauges from which the liquid level is read
directly. The staff gauge may be permanently set within the impoundment or
be a gauge which is placed in the liquid when a level reading is desired.
Another level gauge uses a tape and plumb bob to measure the depth from
liquid surface to a known elevation. A more sophisticated visual technique
is a magnetically coupled gauge in which a float travels along with the
liquid surface. A magnet, attached to the float, moves up and down a
guide, triggering a column of indicators which relate to liquid level. All
of these techniques are relatively inexpensive and easy to install and
operate, but lack the automation which is at times desired or required.
Advantages and Disadvantages
Level gauges are easy to operate, are usually inexpensive, require
very little maintenance, and many measure the liquid level directly. They
are also available for many corrosive wastes.
Disadvantages include the need for personnel to read the liquid level
and the lack of automation for full time level monitoring. Most of the
techniques are also not readily applicable to any form of secondary
function, such as triggeri- . .ips or alarms. Due to these problems, it is
not recommended that thes*: -.^es of level gauges be considered for use at
hazardous waste surface impoundments.
Range of Operation
Level gauges are applicable to storage and disposal impoundments
containing non-corrosive and some corrosive wastes. Continuous level
measurement is the general rule, but as previously mentioned, any level
measurement requires routine monitoring by on-site per'sonnel.
Specifications
Cost: low to moderate; $100 and up dependent upon the system
Accuracy: dependent upon the system and personnel making measurement
References: Belsterling, 1981
Liptak and Venczel, 1982
Young et al., 1975
0.10 INFRARED SENSORS
Principle of Operation
Infrared sensors detect liquid level using the principle of refracted
light, as defined by Snell's law. Snell's law defines the behavior of
light when it contacts a boundary between two substances when each
substance exhibits a different index of refraction. When light passes from
one substance to the other, the light is refracted at a predictable angle
based on the indices of refraction of the two substances.
D-ll
-------
This principle is used in infrared sensors where a light beam is
passed through a conical tipped conductor (quartz crystal) in which the
conical tip is desi gned to reflect the 1ignt to a phototransi stor within
the conductor (Figure 0-2). As long as the conductor is in contact with
air or vapor, the light beam is reflected within the crystal. However,
when the conical tip comes in contact with a liquid, the 1i"ht is refracted
at the crystal face into the liquid, thereby preventing internal reflection
which would normally be received by the phototransistor. When the light
beam is interrupted, the phototransistor will emit a signal. This signal
can be used to activate a secondary level control device.
Advantages and Disadvantages
Infrared sensors are very sensitive. They will also function in
virtually any type of liquid, and are safe for use in explosive or flamable
environments.
Disadvantages include fouling of the system by coatings which may
collect on the tip of-the conductor causing a false reading. Infrared
sensors are also relatively expensive and ire not considered to be very
durable.
Range of Operation
When mounted in a fixed position, these devices are limited to
high/low level sensing, but can be adapted to a movable level gauge for
continuous level detection. Infrared sensors will operate in various types
of liquids regardless of color, viscosity, density, dielectrics, or
conductivity.
Specifications
Costs: $450 - $600
Accuracy: very good with sensitivities of +lmm
References: Cheremisinoff, 1981
Liptak and Venczel, 198.'.
0.11 RESISTANCE TAPES
Principle of Operation
Resistance tapes have a helical resistance element wound in close
proximity to a contact strip. Both are protected by a flexible, corrosion
resistant sheath whicn acts as a pressure diaphragm (Figure D-3). The
unit operates by detecting hydraulic pressure which causes the two elements
(the helical coil and the contact strip) to come into contact. As the
liquid level changes, the increased hydraulic head compresses the
protective sheath, forcing the elements together. A pressure change of 0.2
psi is required to activate the tape. When the two elements touch, that
portion of the tape shorts-out. The point of tne short registers the
liquid level.
D-12
-------
•M^OB
"
i
/"""S>"V~'SL
f 1 f 1
T~ 1 1 1
1
1
1
I
1
1
1
1
1
1
1
1
i i
T t
i i
i
i
i
i
i
i
J
S 1 /
-*-^.
^^"^•4
.
REFLECTED
LIGHT
/ — '
---^_
Qf~^\
I i
1 1
1
1
v 2.
m^m
a
V
V
LIQUID
REFRACTED
LIGHT
FIGURE D-2. INFRARED LIQUID LEVEL SENSOR
0-13
-------
2nd Protective Layer Moisture Barrier Layer
Corrosion Resistant
Outer Sheath
TotalJacket System
Acts as a
Pressure-receiving
Diaphragm
Electrical Insulation
Layer
Protective
Channel
Keystrip
Stable Electrical J
Side Insulation
Steel Base Strip with
Gold Contact Strip
Resistance
Element with
Conti oiled
Tension and
Placement,
and Low
Temperature
Coefficient
To Breather Assembly
JU
Basestrip
Sealed
Outer Jacket
Resistance
Helix
Unshorted
Helix Shorted
Below Surface
FIGURE D-3. RESISTANCE TAPE LEVEL SENSOR
(METRITAPE, INC.).
D-14
-------
Since a minimum pressure is required to activate the system, the upper
most contact between tne elements is some distance below the actual liquid
surface. This distance is proportional to the density ot the liquid, and
the tape must be calibrated accordingly.
Advantages and Disadvantages
Advantages include the absence of moving parts and ease of
installation and maintenance. Most applications are inexpensive and can be
installed in hostile environments. They require only a micro-electrical
current to operate and are safe to install in areas near explosive or
ignitible materials. Since pressure is required to activate the unit,
coating the tape will not result in fouling any contacts. Tapes can be
manufactured to virtually any length to continually monitor liquid level.
Disadvantages include tne need to calibrate the tape to the density of
the liquid being measured and the need to maintain internal sheath pressure
at the same pressure as that of the external 'environment.
Range of Operation
Resistance tapes can be manufactured to "irtually any length up to 100
feet, and have the ability to measure levels of solids, slurries, and
numerous liquids fn clean and hostile environments. Some tapes also have
the capability of continuously monitoring temperature (-15* C to +82° C) in
addition to monitoring liquid level. Resistance tapes are suitable for use
in closed or atmospheric environments.
Specifications
Costs: dependent on length and level only or level/temperature
specifications; $450 - $1,300
Accuracy: resolution of +1/8 inch for compensating tapes
References: Cheremisinoff, 1981
Hall, 1978
Liptak and Venczel, 1982
0.12 THERMAL SENSORS
Principle of Operation
Thermal sensors operate by detecting temperature and thermal
conductivity differences between a liquid and vapor phase. The thermal
probe consists of a resistant heating element which is monitored for temp-
erature changes. As the probe is lowered into a liquid, the thermal
conductivity of the liquid "cools" the probe by absorbing heat. When the
thermal sensor is exposed to a vapor phase, the heating element warms the
probe since the vapor's thermal conductivity is incapable of conducting a
significant amount of heat away from the sensor. The heating and cooling
ot the probe is detected by a temperature sensitive relay which is capable
of activating a secondary level control system.
D-15
-------
Advantages and Disadvantages
Advantages include the ability to detect small changes in liquid
level, ease of operation, and no moving parts.
A major disadvantages is that thermal sensors can not be employed in
situations where .process liquids can coat the probe.
Range of Operation -
Typically, thermal sensors are used for point-level monitoring of
critical liquid levels in materials where the temperature characteristics
of the liquid do not vary widely. With the proper protective coating,
these types of sensors can be employed in virtually any situation.
Specifications
Cost: $200 to $750
Accuracy: +1/4 inch
References: Britz et al., 1982
Liptak and Venczel, 1982
Reason, 1984
D.13 ULTRASONIC DETECTORS
Principle of Operation
Ultrasonic level devices use sound waves, generally 20 kHz or above,
to detect the liquid surface.- An oscillating circuit within the device
transmits the ultrasonic waves. The ultrasonic waves will either be
dampened by the liquid or reflected off the liquid surface and used for the
determination of the liquid level. The unit may be positioned at a desired
level above the impoundment bottom for point-level detection. It may also
be mounted above the impoundment for continuous level detection and
monitoring. The unit is usually best situated for use with clean liquids,
and may be employed for the detection of liquid-liquid interface.
Several types of ultrasonic devices are available. Basically, the
units may be divided into dampened sensors, on-off transmitters and
echometer types. Dampened sensors vibrate at their resonance frequency as
long as the sensor face is not in contact with the liquid. When the liquid
rises and comes in contact with the sensor, the vibration is dampened out
and the level control action is activated. On-off transmitters contain a
transmitter and receiver which generate and receive ultrasonic pulses. As
tne pulse beam is interrupted with the rise of the.liquid, the level
control action is carried out. The echometer type level detector operates
using a burst of ultrasonic energy emitted by the unit. The sound waves
are reflected off the top of the liquid surface and the return echo is
detected by a receiver in the unit. The time required for sound waves to
travel from .transmitter to receiver is converted into liquid level based
upon the velocity of sound.
0-16
-------
Advantages and Disadvantages
Advantages ot ultrasonic level detection devices include the absence
of moving parts. The components of the device are sealed within the unit
and require much less maintenance than a mechanical system. Some ot the
ultrasonic devices (echo) are also capable of continuous level monitoring
without contacting the liquid. Also, ultrasonic systems are not dependent
upon such properties of the liquid as density or specific gravity. The
systems are easy to install and maintainT and are generally accurate.
Disadvantages of the system include problems with build-up on those
units (dampened and on-off) which contact the liquid. This system should
be used with clean liquids. If the unit contacts a waste which is viscous
and accumulates on the probe, the .device may respond with an inaccurate
level reading. Units which do not contact the waste, such as the echometer
type, will not have this tendency. Ultrasonic units are also affected by
surface turbulence and foam.. Many of the units require special filters or
circuits to prevent false readings from random noise.
Range of Operation
Ultrasonic level detectors are used for point-level and continuous
level monitoring of a wide variety of wastes. Those units which do not
contact the waste are not affected by waste characteristics. Those units
which do contact the waste are not recommended for corrosive wastes or
wastes which may coat the sensor. Generally, all types of ultrasonics are
applicable in storage and disposal impoundments containing corrosive or
non-corrosi.ve wastes.
Specifications
Cost: moderate to high; $200 and up for point-level indicators;
$1800 and up for continuous indicators
Accuracy: up to +0.1%
References: Gillespie et al., 1982
Liptak and Venczel, 1982
Reason, 1984
Smith and Nagel, 1985
Sublett, 1976
Yearous, 1979
D.14 OTHER METHODS OF LEVEL DETECTION
This section addresses level detection systems which are not easily
adaptable for use at hazardous waste surface impoundments. These systems
are offered to illustrate the wide variety of other level detection systems
available.
0-17
-------
Antenna Level Sensors
These level sensors employ an antenna which is placed vertically or
horizontally in the impoundment liquid. As the liquid rises and the
antenna is covered, the signal transmitted from the antenna changes due to
the changing dielectric constant in the transmitting medium (i.e., air to
liquid). The signal received from the antenna is then related to liquid
level depth.
Bubblers
Bubbler level monitoring devices use an air supply which is fed
through a tube to the bottom of the impoundment. A gauge measures the
pressure required to force air from the tube. Knowing the liquid density,
the recorded pressure can then be converted to liquid depth. The system
may be used for continuous level monitoring and may be operated manually or
automatically.
Microwave Level Detectors
Microwave level detectors use an oscillating source which emits
microwaves. These microwaves are then received by a detector. As the
liquid level rises, the microwaves are attenuated within the liquid. The
receiver determines liquid level from the microwave levels detected. These
systems are usually expensive and difficult to install, and are not
usually used for continuous level monitoring.
Optical Level Sensors
Optical level sensors use a beam of light to detect liquid level. A
light source emits a beam of light, which is diffused into the liquid or
reflected off the liquid surface. A receiver then detects the presence of
liquid, and by judging the intensity of the light received determines the
depth to liquid surface. Several systems are available including
noncontact systems which may be used with corrosive wastes and those which
are placed within the liquid.
Radiation Level Sensors
Radiation level sensors use a radiation source to transmit gamma rays
which are received by a detector. Since the attenuation of gamma rays is
dependent upon the density of the medium, the liquid level can be
determined by the amount of radiation detected.
References: Barnik et al., 1978
Cheremisinoff, 1981
Gaeke and Smalley, 1975
Liptak and Venczel, 1982
Reason, 1984
Sandford, 1978
Williams, 1979
Yearous, 1979
0-18
-------
0.15 REFERENCES
Bailey. S. J. 1976. Level Sensors '76: A Case ot Contact or Non-Contact.
Control Engineering. 7:25-30.
Barnik, M. I.. A. A. Zborovskii, B. B. Ivanov and S. G. Yudin. 1978.
Investigation of the Controlling Elements of Optical Automatic Level
Control Systems Using Liquid Crystals. Telecommunications and Radio
Engineering. 5:138-139.
Belsterling. C. A. 1981. A Look at Level Measurement Methods.
Instruments & Control Systems. 54:37-45.
Britz, T. J., B. de Witt, A. B. Hugo and L. C. Meyer. 1982. A Liquid
Level Control for Use in Anaerobic Digesters. Laboratory Practice.
31:1174.
Cheremisinoff, N. P. (ed.). 1981. Process Level Instrumentation and
Control. Marcel Oekker, Inc., New York.
Duncan, W. J. 1979. D/P Transmitters Handle Variety of Level
Measurements. Instruments & Control Systems. 52:43-45.
Geake, J. E. and C. Smalley. 1975. Optical Depth Gauges and Level
Controllers for Liquids. The Chemical Engineer. 297:301-304
Gillespie, A. B., M. 0. Deighton, R. B. Pike and R-. D. Watkins. 1982. A
New Ultrasonic Technique for the Measurement of Liquid Level. Ultrasonics.
20:13-17.
Hall, John. 1978. Guide to Level Monitoring. Instruments & Co.ntrol
Systems. 10:25-33. •— —— ° ~^~~
Lawford, V. N. 1981. An Answer to Some Tough Liquid Level Measurement
Problems. Instruments & Control Systems. 54:61-63.
Lawford, V. N. 1970. Hydrostatic Liquid Level Measurement. Instruments A
Control Systems. 43:79-81.
Liptak, B. G. and Kriszta Venczel (eds.). 1982. Instrument Engineers'
Handbook, Revised Edition. Chi 1 ton Book Company, Radnor, PA.
Reason, John (ed.). 1984. Level" Measurement: Wide Variety ot Sensors Fit
Many Control Needs. Power. 128:97-99.
Sandford, Jim. 1978. A Guide to Mechanical Level Control Devices.
Instruments & Control Systems. 51:33-36.
Scctt, Anthony. 1972. Liquid Level Control. Engineering. 212:700-705.
Siegwarth, J. D. 1981. Gage Installation Can Trim Level-Measurement Errors
Caused by LNG Tank Environments. Oil & Gas Journal. 79:142-152.
0-19
-------
Smith, Richard and William Nagel. 1985. Tracking Digestion Cover
Position. Water/Engineering .and Management. March, pp. 34-35.
Sublett, Ken. 1976. Non-Contact Liquid Level Control. Industrial Pollu-
tion Control Measurement & Instrumental!on. pp. 257-262. In:
Proceedings of a Specialty Conference Given at New Jersey Institute or
Technology, Newark, NJ - March 22-23, 1976.
Williams, Jerry. 1979. Tips on Nuclear Gaging. Instruments and Control
Systems. 52:47-51.
Yearous, Harlan B. 1979. Sensors for Liquid/Solid Level Controls. Plant
Engineering. 33:109-112.
Young, R. A., N. P. Cheremisinoff and E. J. Turek (eds.). 1975. Liquid
Level Control Devices. Pollution Engineering. 7:18-25.
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