United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-88/015 Sept. 1988
&EPA Project Summary
Freeboard Determination and
Management in Hazardous
Waste Surface Impoundments
Sidney H. Johnson and David C. Anderson
A ru le-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 fetches
are short, as is typical in hazardous
waste surface impoundments. The
procedure takes into account all of
the parameters that influence
freeboard and presents the
information in a format that can be
used on a site-specific basis.
Additional support information in the
full 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
This Project Summary was
developed by EPA's Hazardous Waste
Engineering Research Laboratory,
Cincinnati, OH, to announce key
findings of the research project that
is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Purpose and Scope
The purpose of the full document is to
present all of the information necessary
to calculate freeboard requirements on
site-specific basis. Section 3.0 presents
the procedure for calculating freeboard
and provides a discussion of the factors
that 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 two basic
methods for detecting and maintaining a
predetermined liquid level. Both
methods, active and passive level control,
are discussed along with suggested
safety margins.
Supplemental material included in the
appendices is intended to augment the
information presented in the body of the
document. The appendices include: (1)
the procedure for calculating freeboard
using a hypothetical surface
impoundment design; (2) data collected
from a field study on a liquid mass
balance at a surface impoundment; and
(3) a discussion of various types of liquid
level detection equipment that are
currently available.
Regulatory Context
Regulations listed in 40 CFR require
owners and operators to maintain surface
impoundment liquid levels in a manner
that prevents overtopping. Regulations
that address overtopping are found in
CFR 264, 265, and 270. Part 264
addresses operational standards for
hazardous waste treatment, storage, and
disposal facilities. Part 265 presents
interim standards for these facilities and
Part 270 discusses the current permitting
requirements. Regulations, current
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through July 15, 1985, are presented
below:
Section 264.221 (f) states:
"a surface impoundment must be
designed, constructed, maintained,
and operated to prevent overtopping
resulting from normal or abnormal
operations; overfilling; wind and wave
action; rainfall; run-on; malfunctions
of level controllers, alarms, and other
equipment; and human error."
Section 264.226(b) (1) 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 165.222 (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
overtopping, must be maintained at
the facility."
Section 265.226 (a) (1) 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 U.S. 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 how 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.
Section 270.17(d):
"A description of how each surface
impoundment, including the liner and
cover systems and appurtenances
control of overtopping, will be
inspected in order to meet the
requirements of 264.226(b). 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 the full 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 of the full 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 the full
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 the selected
system as well as the ability of the
system to meet the regulatory
requirements.
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 surfac
impoundment, such as weirs (
spillways which are insensitive 1
inflow should be incorporated inl
the design. In the event waste
released, these structures direi
liquid waste to an on-site holding <
treatment facility. Passive outfc
structures are intended for use on
in the event of an automated lev
control system malfunction, groj
human error, or unforesee
catastrophic natural events;
2. If outfall structures are sensitive
inflow (i e , where outfall rates mu
be increased to maintain freeboai
as inflow increases) automat
control should be provided v
signals from level sensin
instruments. In these situations th
automated system should include
high-level alarm;
3. For surface impoundments receivir
waste via inflow structures, desic
features should be incorporate
which allow for flow of waste to th
surface impoundment to be halte
immediately in the event <
overfilling or failure of any surfac
impoundment component. The flo
of waste can be controlled by <
automated level sensing syste
which, in the case of a syste
failure, can be operated manually;
4. Run-on control structures should t
designed to divert the pec
discharge from a 100 year/24 ho
storm unless it can be shown that \\
surface impoundment is engineer*
to accept the additional volurr
without sacrificing minimu
freeboard;
5. Freeboard should be defined as tl
minimum distance between U
highest liquid level in the surfa<
impoundment, where the highe
liquid level includes changes in wat
elevation due to a 100 year/24-ho
storm surge, and the liquid lev
which would result m the release
stored liquid from the surfac
impoundment by overtoppin
Freeboard allowances should t
calculated for all surfac
impoundments using the maximu
fetch (usually the diagon
measurement across the surfai
impoundment) and the maximu
historically determined sustain*
wind speed to calculate wind s<
up, wave height, and wave run-i
The minimum amount of freeboa
maintained in the impoundme
should be based on site speci
calculations but should never be le
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than two feet (60 cm) except as
provided for in 40 CFR 265.222 (b).
If the impoundment is equipped with
a passive outfall such as a weir or
spillway, freeboard should be
measured from the highest allowable
liquid Jevel to the top of the lowest
discharge level of the passive outfall
structure. When no passive outfalls
are present, the freeboard should be
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 100
year/24 hour storm should be
engineered into the normal storage
capacity of the surface
impoundment; and
6. 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.
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
!dth to determine the maximum fetch as
ell 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, calculations
for wave height, wave period, wind set-
up, and wave run-up can be used to
determine freeboard requirements.
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. Regardless
of the type of system selected, it is
advisable to identify a specific system
prior to finalizing impoundment design so
needed modifications 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 in 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.
Quality Assurance
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 structures,
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 interfere 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 verification 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.
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 and
by other investigators. Much of this
information is dated and does not take
into account some of the variables that
affect the ultimate freeboard value. To
address these short falls a new
procedure was developed which
addresses all of the factors that
determine freeboard and incorporates
them into an easy-to-follow format.
Many of the coefficients used in the
original work have been updated using
new values derived from ongoing
research. 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
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new procedure presented in the full
report represents the most up-to-date
method for determining freeboard. The
procedure takes into account all of the
climatic factors and liquid characteristics
that influence freeboard.
Sidney H. Johnson and David C. Anderson are with K. W. Brown and
Associates, Inc., College Station, TX 77840.
Paul R. de Percin is the EPA Project Officer (see below).
The complete report, entitled "FreeQQ&d Determination and Management in
Hazardous Waste Surface* jitnpoundments," (Order No. PB 88-243
7871 AS; Cost: $21.95, subject 'to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
/'POSTAGE
EPA
EEflMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-88/015
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