La. Admin. Code tit. 56 § III-713

Current through Register Vol. 50, No. 11, November 20, 2024

Section III-713 - Design

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.

Table 1. Impact Classification and Inflow Design Flood
Impact Category	Potential Loss of Life	Potential Economic Loss	Minimum Inflow Design (IDF)
Low	Not Likely	Minimal	50-Yr. Freq.
Significant	Possible	Appreciable	100-Yr. Freq.
High	Likely	Excessive	1/2 PMF

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.

Table 2. Factor of Safety for Stability Analysis
Analysis Condition	Factor of Safety
Rapid Drawdown	1.25
Partial Pool	1.40
Steady Seepage	1.40
After Construction	1.30
Earthquake	1.15

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.

La. Admin. Code tit. 56, § III-713

Promulgated by the Department of Transportation and Development, Office of Public Works, LR 22:1237 (December 1985), repromulgated by the Department of Transportation and Development, Office of Public Works, LR 31:942 (April 2005).

AUTHORITY NOTE: Promulgated in accordance with R.S. 38:24.