Hydrologic Issues for Dams - Association of State Dam Safety Officials

Hydrology for Dam Safety – Private SectorByCat Cecilio, P.E., P.H. 1INTRODUCTIONAlthough dams have been designed and built by private engineering consultantsthroughout the world, the concept of hydrology for dam safety for owners of existingdams have never really been addressed until perhaps in the late 1960s. Although the stateof California has an existing dam safety program implemented by the Legislature in1929, its function was more of issuance of certificate of operations to dam owners toguarantee the safe operations of the reservoirs. However, in 1966 after the failure ofBaldwin Hills Dam, the law was revised to include off-stream dams to be under thejurisdiction of the Division of Safety of Dams. At the same time a program of daminspection and review of design criteria was implemented.Pacific Gas and Electric Company (PG&E) which owned some 200 dams at that time forthe largest private dam owner in the country began discussing with the CaliforniaDivision of Safety of Dams in 1967 to implement the new dam safety program.Inspect ion programs and criteria for dam safety were evaluated and investigated.Hydrologic criteria were one of them.At tha t same time also, the Federal Power Commission (now known as the FederalEnergy Regulatory Commission) Order No. 315 was passed in 1969. The Order requiresthe inspection and review of safety of non-federal dams by an Independent Consultant.FPC Order 315 was later superseded by FERC Order 122 which expanded the safetyevaluation of dams under FERC jurisdiction.This paper will give an overview of existing methods in developing the flood potential ata dam, especially the methods employed in PG&E from 1968 to comply with therequirements for dam safety by the Federal Energy Regulatory Commission and theCalifornia Division of Safety of Dams. Advantages and disadvantages of each proceduretogether with their inherent problems or limitations will be mentioned. Research needsfor some items are discussed throughout the paper. A comparison and a set of guidelinesto determine reasonable values are also described.HYDROLOGIC CONSIDERATIONSThe design and evaluation of dams against hypothetical floods requires the determinationof the hydrologic response (infiltration, base, flow, routing and runoff) of watersheds tointense rainfall or snowmelt, verification based on historical storm and runoff data (floodhydrograph analysis). It also requires the determination of the hypothetical storms,infiltration, base flow, channel and reservoir routing. Routing involves the investigationof the adequacy of existing structures above and below the dam under investigation tosafely pass the hypothetical floods.1 PG&E-Retired, Consulting Engineer57Paper 12 - Cecilio