Ga. Comp. R. & Regs. R. 391-3-8-.09 - Standards for the Design and Evaluation of Dams

(1) The design and/or evaluation of new and existing dams shall conform to accepted practices of the engineering profession and dam safety industry. Design manuals, evaluation guidelines, and procedures used by the following agencies can be considered as acceptable design or evaluation references, except as those references differ from Georgia Law and these regulations:

(a) U.S. Army Corps of Engineers;

(b) Natural Resources Conservation Service;

(c) U.S. Department of Interior, Bureau of Reclamation;

(d) Federal Energy Regulatory Commission.

(2) Other design and evaluation methods may be used to demonstrate compliance with the objectives of these rules, but are subject to the approval of the Director.

(3) Design and Evaluation of Dams under paragraph (1) and (2) above shall, as a minimum, consider the following basic principles:

(a) All dams must be stable under all conditions of construction and/or operation of the impoundment. Details of stability evaluation shall be submitted to the Director for approval. Analyses using the methods, guidelines and procedures of the agencies listed in paragraph (1) yielding the following Minimum Safety Factors can be considered as acceptable stability:

1. Earthen Embankments

(i) End of Construction: 1.3

(ii) Steady State Seepage: 1.5

(iii) Steady State Seepage with Seismic Loading: 1.1

(iv) Rapid Drawdown (Upstream): 1.3

(v) Submerged Toe with Rapid Drawdown: 1.3

2. Concrete Structures (cohesion included)

(i) Normal Reservoir: 3.0

(ii) Normal Reservoir with Seismic Loading: 1.0

(iii) Design Flood: 2.0

(b) Details of the engineering evaluation of material properties in the dam or appurtenant structures shall be submitted to the Director for review and approval. Conservative selections for soil strength values shall be used for analyses or evaluations. Details of any foundation investigation and laboratory testing supporting assumed design or evaluation parameters shall be included for review.

(c) All dams and appurtenant structures shall be capable of withstanding seismic accelerations defined in the most current "Map for Peak Acceleration with a 2% exceedance in 50 years" for the contiguous United States published by the United States Geological Survey (NEHRP maps). The minimum seismic acceleration shall be .050 g. The seismic accelerations may be reduced or seismic evaluation eliminated if the applicant's engineer can successfully demonstrate to the Director by engineering analyses or judgment that smaller seismic accelerations are appropriate or no seismic evaluation is needed.

(d) All dams shall have a means of draining the reservoir to a safe level as demonstrated by the applicant's engineer. The submittal by the applicant's engineer shall include the computation of the maximum time required to drain the reservoir. Exceptions to this rule may be given by the Director based on an engineering evaluation demonstrating the lack of this capability would not endanger the public.

(e) All earthen embankments shall be protected from surface erosion by appropriate vegetation, or some other type of protective surface such as riprap or paving, and shall be maintained in a safe condition. Examples of appropriate vegetation include, but are not limited to, Bermuda, Tall Fescue, Centipede grasses and Lespedeza sericea. Inappropriate vegetation on existing dams such as trees shall be removed only after consultation with the Division or other qualified persons on the proper procedures for removal. Hedges and small shrubs may be allowed on existing dams if they do not obscure inspection or interfere with the operation and maintenance of the dam.

(f) Design Storm. Each dam shall be capable of safely passing the fraction of the flood developed from the PMP hydrograph depending on the subclassification of the dam. The design storm for each subclassification of a dam is as follows:

1. Small Dam: 25 percent PMP

2. Medium Dam: 33.3 percent PMP

3. Large Dam: 50 percent PMP

4. Very Large Dam: 100 percent PMP

5. Based on visual inspection and detailed hydrologic and hydraulic evaluation, including documentation of completed design and construction procedures, up to a 10 percent lower design storm requirement (22.5, 30, 45, or 90 percent) may be accepted on existing Public Law 566(PL-566), including Resource Conservation & Development structures, and Public Law 534(PL-534) Project Dams at the discretion of the Director, provided the project is in an acceptable state of maintenance. The design storm requirement may be reduced on existing dams if the applicant's engineer can successfully demonstrate to the Director, by engineering analysis, that the dam is sufficient to protect against probable loss of human life downstream at a lesser design storm. Earthen emergency spillways shall not function until the 50 year storm.

(g) Seepage Control. All dams shall be able to prevent the development of instability due to excessive seepage forces, uplift forces, or loss of materials in the embankment, abutments, spillway areas, or foundation. For new dams, seepage analysis for design, and inspection during construction, shall be in sufficient detail to prevent the occurrence of critical seepage gradients.

1. For new dams, the design shall include a seepage control method that meets the minimum acceptable safety standards, as determined by the Division. All internal drainage systems with pipe collection systems shall have cleanouts.

2. In existing dams, seepage shall be investigated by an engineer and appropriate control measures shall be taken as necessary.

(h) Monitoring Devices.

1. Monitoring devices, including but not limited to piezometers, settlement plates, telltale stakes, seepage outlets and weirs, and permanent bench marks may be required by the Director for use in the inspection and monitoring of the safety of a dam during operation.

2. Where appropriate for new or existing dams, a reservoir filling monitoring and surveillance plan to be implemented during reservoir filling or re-filling shall be submitted to the Director for approval prior to start of filling or re-filling.

(i) Design Life. The design life for new dams and reservoirs shall be adequate for the dams and reservoirs to perform effectively as planned, as determined by the following criteria:

1. The time required to fill the reservoir with sediment from the contributing watershed; and

2. The durability of appurtenances and materials used to construct the dams.

(j) Freeboard. Appropriate freeboard for wave action shall be considered by an engineer through engineering analysis. The required freeboard shall be provided above the maximum reservoir surface elevation that would result from the inflow from the design storm for the structure. The resulting maximum reservoir surface elevation plus freeboard shall determine the elevation of the top of the dam. In lieu of determining the appropriate amount of freeboard by engineering analysis, a minimum of three (3) feet of freeboard shall be provided on earthen dams.

(k) Existing concrete and/or masonry dams and appurtenant structures shall be structurally sound and shall have joints free of trees and other vegetation and shall show no signs of significant structural deterioration such as excessive cracks, spalling, efflorescence and exposed reinforcing steel.

(4) Other design standards may be imposed as deemed appropriate by the Director after review of design of new structures or through a visual inspection of an existing structure conducted pursuant to Rule 391-3-8-.08(2)(b) of these regulations, or based on a review of the detailed engineering study prepared by an engineer.

Notes

Ga. Comp. R. & Regs. R. 391-3-8-.09

O.C.G.A. §§ 12-5-370 through 12-5-385.

Original Rule entitled "Permits for the Construction and Operation of New Dams" was filed as Emergency Rule 391-3-8-0.5-.09 on August 28, 1978, effective July 28, 1978, the date of adoption, to remain in effect for a period of 120 days or until the effective date of a permanent Rule covering the same subject matter superseding said Emergency Rule, as specified by the Agency. Amended: Permanent Rule of the same title adopted superseding Emergency Rule 391-3-8-0.5-.09. Filed August 31, 1978; effective September 20, 1978. Amended: Rule renumbered as Rule 391-3-8-.10 and Rule 391-3-8-.08 entitled "Design Standards for Existing Dams" amended and renumbered as Rule 391-3-8-.09. Filed October 29, 1985; effective November 18, 1985. Amended: F. Aug. 31, 1990; eff. Sept. 20, 1990. Amended: F. Oct. 5, 1998; eff. Oct. 25, 1998. Amended: Title changed from "Standards for the Design and Evaluation of Dams. Amended" (as recorded in the Official Compilation, eff. Oct. 25, 1998) to "Standards for the Design and Evaluation of Dams." F. Sep. 13, 2016; eff. Oct. 3, 2016.