Current through Register Vol. 50, No. 9, September 20, 2024
A. The proper
design of a dam involves a complex combination of engineering applications. It
is not within the scope or intent of this document, nor will it be the practice
of the staff of the DOTD, to instruct in the detailed procedures for the design
of a dam. All dams and impoundment structures to be permitted under this
program will be designed by a professional civil engineer(s), registered by the
Louisiana State Board of Registration for Professional Engineers and Land
Surveyors. The registered civil engineer will certify the designs and plans by
professional seal. Designs must conform to nationally recognized standards,
further explained in the following Paragraphs and in the Appendices. The
completed design package will state the intended design life of the structure,
and will include the operations and maintenance procedures necessary to ensure
that the structure will function as designed for its stated design
life.
B. Failure of an impoundment
structure and the instantaneous release of large volumes of water is referred
to as a dam breach. It is the primary risk associated with dams, and is the
fundamental reason for the state to assume regulatory authority over dams
through the Louisiana Dam Safety Program. Breaching may occur during fair
weather due to the cumulative effects of erosion or seepage, or it may occur as
a result of stresses caused by excess water produced during a storm event. The
hydraulic and hydrologic (H and H) design will determine which of the two
scenarios poses the greater hazard, the volume of water which is likely to be
released, and the rate of flow.
C.
It is the H and H design which determines the volumes and flow rates with which
the impoundment structure(s) must contend. The geotechnical and structural
designs must ensure that the impoundment structure(s) can safely accommodate
the hydraulic forces imposed by the conditions predicted by the H and H design.
Following are the sequential steps which are necessary in any dam/impoundment
structure design, and each step must be documented with design calculations and
all supporting data, certified by a Registered Professional Civil Engineer:
1. Hydrology and Hydraulics (H and H) Design
a. Impact (Hazard) Classification.
b. Determination of controlling design
condition and associated storm runoff.
c. Setting of spillway and stilling basin
widths and elevations, top of embankment elevation, and normal pool
stage.
2. Structural and
Geotechnical Design of Embankment, Spillways, and Drawdown Structures
3. Development and Documentation of
Operations and Maintenance Procedures
Note: For the purpose of the Dam Safety Program, the
Emergency Spillway shall be defined as being overtopped by the
100-year storm or greater and the Principal Spillway shall be
defined as being overtopped by a storm less than the 100-year storm.
D. Hydrology and
Hydraulics (H and H) Design
1. Before the
structural design of the dam can begin, the requirements of hydraulic capacity
must be determined. The height of the dam, the amount of freeboard above normal
pool elevation, the size and capacity of the principle and emergency spillways,
must all be designed to balance the hydrological and hydraulic properties of
the location of the reservoir. A properly designed drawdown structure, capable
of reducing the stage of the reservoir at a suitable rate in the event of
emergency, must also be designed to meet the capacity requirements of the
site.
2. H and H design begins with
the Impact Classification (also referred to as Hazard Classification in some
texts) of the dam. The Impact Classification is determined by an evaluation of
the probable maximum impacts of a dam breach. Low impact structures are those
for which, because of size and/or location, little or no significant damage to
life or property is likely to result from a failure of the structure.
Significant impact structures are those which could cause appreciable damage to
property or could pose possible threat to human life in the event of failure.
High impact structures are those for which failure would cause excessive
property damage or make loss of human life likely.
Note: The inflow design flood (IDF) is determined by the
various Hydrograph Methods after the precipitation amount is developed. The
major source of precipitation data is the National Weather Service (NWS). The
DOTD has final authority for approval of the method to be utilized to determine
the IDF.
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Table 1. Impact Classification and Inflow
Design Flood
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Impact Category
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Potential Loss of Life
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Potential Economic Loss
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Minimum Inflow Design (IDF)
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Low
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Not Likely
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Minimal
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50-Yr. Freq.
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Significant
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Possible
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Appreciable
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100-Yr. Freq.
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High
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Likely
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Excessive
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1/2 PMF
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3. Further guidance in assessing the
potential hazards and associated impact classification for dams may be found in
the publication referred to in §727 It is the responsibility of the
owner/applicant to establish impact classification, and all dams will be
considered to be of High Impact potential until demonstrated to be otherwise by
a documented analysis provided by the applicant. The proposed impact
classification must be supported by sufficient analysis and documentation, and
the DOTD will have final authority for assigning Impact
Classification.
4. Having
established the Impact Classification for the structure, the next step is to
establish the magnitude of the meteorological event on which the entire design
is to be based. Dams must be designed to be able to safely withstand the
passage of a flood of design magnitude. The Inflow Design Flood (IDF) is the
largest storm event to be considered in the design of the structure, and the
magnitude of the storm event for which the IDF is computed is related to the
Impact Classification. The values shown for IDF in Table I are minimums, and
the storm event to be used as the IDF will be determined by a site specific
analysis. For low impact structures, the primary consideration is the
protection against loss of the dam and its benefits in the event of failure,
while for significant and high impact structures, adequate protection of life
and property must be assured.
5.
For dams classified as high impact, the IDF is defined as the flood event above
which a breach of the dam does not increase hazard to downstream interests. The
upper limit of the IDF for high impact structures is the Probable Maximum Flood
(PMF), which is the flood which may be expected from the most severe
combination of critical meteorological and hydrological conditions which are
reasonably possible. While the PMF is the upper limit for the IDF, the IDF for
high impact dams may be an event of smaller magnitude, depending upon an
incremental hazard assessment. The incremental assessment is a routing of
floods of increasingly larger magnitude through the structure and downstream
channel reaches, comparing conditions with and without a dam failure, until a
flood magnitude is reached for which the dam failure condition does not
appreciably increase the hazard potential.
6. Dams classified as having significant
impacts may or may not require a formal incremental hazard evaluation,
depending upon the extent of existing and potential downstream development, the
size of the reservoir, and the type and use of the dam. The upper limit of the
IDF for significant impact structures is the PMF.
7. For dams with low impact classification,
the incremental hazard evaluation is not required, and the IDF can be based
upon factors related to loss of service of the dam, potential maintenance
costs, etc., but with the 50-year frequency storm being the minimum design
event.
8. The Water Resources
Design and Development Section should be a partner in establishing the IDF, and
designs should not proceed until agreement has been reached between the DOTD
and the owner's engineer on the choice of the IDF. Establishing the IDF is the
foundation for the entire design process, since the dam must be designed to
safely pass and/or contain the IDF. A guideline for performing the incremental
hazard evaluation necessary to establish the IDF is provided in the publication
referred to in Subsection N.
9. How
the IDF is to be safely passed by the dam structure and the stability of the
dam against the long-term effects of hydrostatic forces is the subject of the
balance of the design effort, including the general configuration of the dam;
length, elevation, and composition of principal and emergency spillways;
storage capacity above normal pool elevation; erosion protection; and stability
design. The most practical way of assuring the integrity of the dam during an
IDF is to provide a concrete spillway which is capable of carrying the peak
flow of the storm. Principal spillways are normally sized to carry flows from
all but the largest of storms, with emergency spillways, which are not normally
armored, functioning only during major storm events. If the peak flow from the
IDF can be contained within the principal and emergency spillways, the
stability of the dam is not likely to be threatened by the erosive action of
water flowing over the embankment. The designer may wish to balance the
relative economy of providing spillway capacity versus storage capacity above
normal pool stage. But, if design calculations indicate that the embankment
will be overtopped by the IDF, provisions must be included in the design to
prevent the embankment from failing under the erosive forces of the overtopping
flows.
E. Geotechnical
Design
1. It is essential to the stability of
the structure that the material used in the impoundment structure, as well as
the foundation and adjoining earth have the necessary structural properties to
withstand the hydrostatic forces required by the design, that potential for
destructive seepage is identified and appropriately dealt with, and that the
surfaces of the structure are adequately protected from surface
erosion.
2. Field investigations
shall be adequate to define the soils and ground water conditions with respect
to stability and seepage control. Stability analysis should consider
after-construction conditions, based on the undrained shear strength parameters
determined by laboratory tests. Long-term steady seepage, partial pool, and
rapid drawdown analyses should also be performed, using shear properties
appropriate to the subject materials and minimum safety factors shown in the
following Table.
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Table 2. Factor of Safety for Stability
Analysis
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Analysis Condition
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Factor of Safety
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Rapid Drawdown
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1.25
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Partial Pool
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1.40
|
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Steady Seepage
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1.40
|
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After Construction
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1.30
|
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Earthquake
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1.15
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6. Structural Design. Structural Designs are
to be prepared in accordance with generally accepted structural engineering
practices such as those of the American Concrete Institute, the American
Institute of Steel Construction and the American Institute of Timber
Construction. Components of the spillway or other appurtenant structures shall
be designed to resist the most critical loading combination of dead loads plus
live loads that may occur during its construction or design life. Some of the
loads which must be considered in the design are: buoyancy forces, sliding
forces, hydrostatic uplift forces, bearing forces, overturning forces, water
drag forces, wing drag forces, gate-lifting and closing forces, soil and water
pressure forces, impact forces, uniform and point live load forces, etc. The
minimum factors of safety for buoyancy and sliding shall be 1.5 and 2.0,
respectively. The overturning analysis must indicate that the resultant force
falls within the center 1/3 of the base. The minimum factor of safety for pile
design shall be 2.0.
AUTHORITY NOTE:
Promulgated in accordance with
R.S.
38:24.
