Current through August 31, 2023
The following criteria shall be used by the Director as a basis
to evaluate the design of new embankment dams and reservoirs. These guidelines
are intended for a broad range of circumstances, and engineers should not
consider them as a restriction to the use of other sound engineering design
principles. Exclusion from these established criteria will be considered by the
Director on a case-by-case basis during design review of plans, drawings,
reports, and specifications submitted for approval prior to commencing
construction. Structures which are or will be constructed of other materials,
for example concrete, timber, steel, or combinations thereof shall comply with
these criteria as found appropriate by the Director, and with other engineering
design methods and construction standards of care approved by the Director.
(3-30-23)
01.
Embankment
Stability. Slope stability analyses shall determine the appropriate
upstream and downstream slopes. Unless a discrete slope stability analysis
determines otherwise, the embankment slopes of earthen dams shall comply with
the following:
|
Upstream slope |
3:1 or flatter |
|
Downstream slope |
2.5:1 or flatter |
(3-30-23)
a.
Embankments shall be designed, constructed, and maintained to assure stability
under static loads and prevent instability due to seepage or uplift forces,
rapid drawdown conditions, and applied seismic loads. (3-30-23)
b. The design analysis shall consider the
need for installing filters, including but not limited to chimney drains,
blanket drains, or toe drains, to avoid developing saturated conditions and to
protect against piping of the embankment fill material. Transmission of seepage
through the embankment, abutments, and foundation shall be controlled to
prevent internal erosion, the removal of material, or the creation of
instability. (3-30-23)
c. The
minimum factor of safety for a steady state loading condition shall be one
point five (1.5.) The minimum factor of safety for rapid drawdown loading shall
be one point two (1.2.) The minimum factor of safety for seismic loading shall
be one point zero (1.0.) (3-30-23)
d. Seismic Stability. (3-30-23)
i. The stability of an embankment subjected
to earthquake ground motions may be analyzed by the engineer using either a
dynamic response or pseudo-static analyses. Pseudo-static analyses are
acceptable for embankment dams and foundations composed of non-liquifiable
soils that preclude the generation of excess pore water pressures due to
shaking. Otherwise, the stability analysis shall employ a dynamic response
method. (3-30-23)
ii. Slope
deformation analyses are required for structures that are constructed of
cohesionless soils exhibiting fine grain-size gradation and/or on foundations
that may be subject to liquefaction. (3-30-23)
iii. The design analysis for regulated dams
shall include in the seismic stability analysis peak ground accelerations
obtained from Seismic Hazard Maps published by the United States Geological
Survey (USGS) using a minimum return interval of 2 percent (2%) probability of
exceedance in fifty (50) years, or greater interval, as determined by the
Director. (3-30-23)
iv. The design
analyses for large and high hazard dams shall include a report or report(s)
covering geology, geologic hazard, and seismicity. The report(s) shall identify
the location of faults, evaluate landslide potential, and include a history of
seismicity. A comparison using deterministic and probabilistic analyses to
calculate peak ground acceleration at the dam site may be required for
geographic areas of the state showing evidence of seismic faults or faulting,
as determined by the Director. (3-30-23)
e. Where in the opinion of the Director,
embankment design or conditions warrant, the owner may be required to
instrument their embankment or foundation. (3-30-23)
02.
Top Width. The minimum top
width for any embankment shall be twelve (12) feet to allow safe access by
wheeled vehicles or tracked equipment for maintenance or repair.
(3-30-23)
03.
Cutoff Trenches
or Walls. Cutoff trenches shall be excavated into competent foundation
material to bear on an approved stratum or zone, as site conditions require and
when employed. (3-30-23)
a. The cutoff trench
shall be backfilled with suitable material free from organic matter and debris
and compacted to the specified moisture and density. The cutoff trench shall
extend up the sides of both abutments to the design maximum storage elevation.
(3-30-23)
b. Cutoff trenches shall
be wide enough to allow the free movement of excavation and compaction
equipment. To provide for proper compaction side slopes shall be no steeper
than one to one (1:1) for shallow depths up to twelve (12) feet, and no steeper
than one and one half to one (1.5:1) for greater depths. Flatter slopes may be
required for safety and stability, as determined by the Director.
(3-30-23)
c. Concrete cutoff walls
may be used in a similar manner as cutoff trenches, with the base firmly
entrenched in the underlying foundation material. Where suitable bedrock or
suitable foundation material exists, concrete cutoff walls shall be doweled
with steel rebar a minimum depth and spacing determined by the engineer
necessary to create a structural bond with the underlying foundation. Concrete
walls shall have a minimum vertical projection above the foundation surface of
three (3) feet, oriented perpendicular to the surface, and shall have a minimum
thickness of twelve (12) inches. Reinforcement of the concrete may be required
in addition to being doweled into suitable foundation material(s).
(3-30-23)
04.
Impermeable Core Material. Soils used to construct the inner sectional
core of an embankment shall consist of relatively impermeable cohesive
materials approved by the engineer and compacted in strict accordance with the
approved plans and specifications. A minimum ninety-five percent (95%) maximum
dry density compacted in accordance with the American Society Testing Materials
(ASTM) D-698 is required. The use of other relatively impermeable however
non-cohesive material is subject to approval by the Director on a case-by-case
basis. (3-30-23)
05.
Drains. Toe, blanket, or chimney drains consisting of approved free
draining material or approved manufactured drainage geotextile shall be
installed where necessary to maintain the phreatic line at or near the design
level(s) within the embankment. (3-30-23)
a.
Filter design for toe, blanket, or chimney drains, or any combination thereof
shall be included in the design plans and specifications submitted by the
engineer for review and approval by the Director. (3-30-23)
b. Perforated and slotted drainpipes must be
four (4) inches diameter or greater and shall be surrounded by permeable
drainage material to a distance equal to or greater than the outside pipe
diameter. The maximum particle size of the drainage material shall be between
one-half (1/2) inch to three-fourths (3/4) inch, or as specified by the design
engineer based on the drainage filter analysis. Underdrains and collection
pipes must be constructed of noncorrosive material, taking care to ensure slots
and perforations are appropriately sized to avoid long-term migration of the
drain material into the pipe. (3-30-23)
06.
Freeboard. The elevation of
the top of the embankment shall be constructed and maintained above the design
flood surcharge level, including the vertical height of wind generated waves
estimated for the greatest distance of open water measured perpendicular to the
major axis of the dam. Camber estimated for post-construction settlement shall
be included in the design and incorporated in the construction of the top of
the embankment. (3-30-23)
a. The minimum
freeboard shall be two (2) feet plus wave height as calculated for the design
spillway flow capacity during passage of the one percent (1%) flood, or
greater. (3-30-23)
07.
Riprap. All embankments which are subject to erosion on either the
upstream and downstream slope(s) shall be protected using riprap or other
approved material. Pipes, cables, brush, tree growth, dead growth, logs, or
floating debris are not acceptable substitutes for approved riprap. The
engineer, with approval of the Director, shall determine the extent of slope
protection as deemed necessary for existing site, seasonal, and operating
conditions. (3-30-23)
a. Where rock riprap or
other approved material is used for erosion protection on the upstream slope,
it shall be placed on an approved thickness of well-graded and free-draining
granular bedding material. Riprap or other approved erosion protection material
shall extend up the slope a sufficient height. (3-30-23)
08.
Outlet Conduits. All
reservoirs impounding water shall have an outlet conduit of sufficient capacity
to prevent interference with natural streamflow through the reservoir to the
injury of downstream appropriators. In addition to any natural flow releases,
the outlet conduit should be of sufficient capacity to pass at the same time,
the maximum water requirement of the owner. A larger outlet conduit may be
required to provide adequate release capacity as determined by the Director.
Upon recommendation n of the design engineer, the Director may waive this
requirement for off channel reservoirs. (3-30-23)
a. Outlet conduits shall be laid on a firm
and stable foundation material to avoid the likelihood of differential
settlement or consolidation causing the separation or misalignment of the
conduit. Outlet conduits shall be encased on all sides by concrete of approved
compressive strength and having a minimum thickness of twelve (12) inches.
During construction outlet conduits shall be properly aligned on an established
grade and adequately supported to prevent movement or damage caused by
placement of concrete or by compaction equipment. (3-30-23)
b. Unless otherwise required, the outlet
conduit shall have a minimum inside diameter of twelve (12) inches. The
conduits shall consist of approved material and composition as approved by the
Director. Exceptions may be made only where conditions warrant, but in no case
shall the reasonable life expectancy of the pipe be less than the design life
of the embankment. (3-30-23)
09.
Gates and Valves. (3-30-23)
a. Conduits shall be gated on the upstream
end to avoid pressurizing the conduit inside the embankment. Designed
pressurized conduits shall be fitted with both a guard gate and a control gate
or valve. (3-30-23)
b. All conduits
shall be vented directly behind the gate. (3-30-23)
c. All gate stem pedestals shall be securely
founded to prevent future movement. (3-30-23)
d. At least one (1) of the sides of the inlet
structure shall be open to allow water to flow into the outlet conduit. The
opening shall be covered with a trash rack. (3-30-23)
e. Trash racks should be designed to
facilitate cleaning of trash and debris. If fish screens are used, they shall
be placed over the trash rack and shall be removable for cleaning or be
self-cleaning. (3-30-23)
10.
Outlet Controls. Outlet
controls shall be installed at a stable location, on the crest or on an
elevated platform, or within an enclosure when required, but secured to prevent
unauthorized operation. Reservoirs storing water during the winter and subject
to severe freezing conditions shall have inclined gate stems or other
controlling mechanical or hydraulic features enclosed in a protective sleeve
which is buried beneath the upstream slope to suitable depth, to prevent damage
or movement caused by ice. (3-30-23)
11.
Release Capacity. Based on
the size of the dam and the downstream hazard classification assigned by the
Director, the release capacity shall equal or exceed the inflow design flood as
set forth in the following table. Where the table specifies an inflow design
flood range, the governing inflow design flood shall be determined by the
professional engineer in responsible charge of design and IDWR based on a
site-specific review of the proposed dam, watershed conditions, and downstream
hazard potential. The minimum flow capacity of the emergency spillway(s) shall
be sized using the one-percent (1%) rate of flow (i.e., Q100 cfs) calculated
for the contributing watershed upstream from the dam, plus two (2) feet of
freeboard, plus wave height.
|
Hazard
Classification |
Dam Size
Classification |
Inflow Design Flood
(IDF) |
|
Low |
All Sizes |
Q100 |
|
Significant |
Small |
Q100 |
|
Intermediate |
Q100 to Q500 |
|
Large |
Q500 |
|
High |
Small |
Q100 to Q500 |
|
Intermediate |
Q500 |
|
Large |
Q500 to PMF |
(3-30-23)
a. All
spillways shall be stabilized for the discharge of flow using concrete,
masonry, riprap, or sod, if not constructed in resistant rock.
(3-30-23)
b. For embankment dams,
where site conditions allow, the spillway shall be constructed independent of
the embankment. The spillway(s) shall guide the discharge of water away from
the embankment. (3-30-23)
c. The
minimum base width of an open-channel spillway shall be ten (10) feet, or
greater to allow access by mechanical equipment. Siphon pipes or pumps are not
acceptable substitutes for an open-channel spillway. (3-30-23)
d. The effective flow capacity of spillways
shall be undiminished by bridges, fences, pipelines, or other obstructions.
(3-30-23)
e. The installation of
stop logs or flashboards in the spillway is prohibited unless they are part of
an approved design and included as an integral part of an approved operation
plan. (3-30-23)
12.
Reservoir Site. Prior to filling the reservoir, the site shall be
cleared of all woody material, growth or debris that is large enough to lodge
in the spillway, or outlet works. (3-30-23)
13.
Inspection and Completion
Reports. As construction proceeds, it is the responsibility of the
engineer to submit test reports (e.g., soil material analyses, density tests,
concrete strength tests, etc.) along with periodic inspection and progress
reports to the Director. (3-30-23)
a. Upon
completion of construction the owner or their engineer shall provide the
Director a written narrative account of all items of construction. Record
drawings (i.e., as-builts or as-constructed drawings) and revised
specifications shall be submitted to the Director to accurately reflect the
completed project works. (3-30-23)
b. The engineer, acting on behalf of and
representing the owner, shall certify that the construction, reconstruction,
enlargement, replacement, or repair of the embankment and appurtenances was
completed in accordance with the record drawings and specifications.
(3-30-23)
