Hydrologic Issues for Dams - Association of State Dam Safety Officials

Use of Atmospheric Models in Rainfall-Runoff ModelingAn atmospheric model (Danard 6,7 ) was used in stochastic modeling of extreme floods forMica Dam on the Upper Columbia River 26 in British Columbia. The model provideddistributed daily precipitation and temperature information for a historical 96-year periodfor the Upper Columbia watershed that was calibrated to precipitation and temperaturegage measurements. This information was used to provide antecedent watershedconditions for WATFLOOD 11,12 a distributed hydrologic model. The temporal and spatialdistributions of extreme storms in the historical period were also used in a resamplingprocedure for stochastic modeling of extreme floods.Resampling Spatial and Temporal Patterns from Historical StormsWe are presently working with the Army Corps of Engineers in the analysis of the spatialand temporal patterns of historical storms on the American River in central California 21 .These patterns will be used in a resampling procedure for the stochastic modeling of floodsfor Folsom Dam on the Amer ican River. A procedure is currently under development thatwould allow combinations of spatial and temporal patterns from separate storms to be usedin rainfall-runoff modeling. This would significantly increase the dataset of storms andinclude combinations both more severe and less severe than observed in the original sampleof storms. The resampling approach replaces the conventional practice of using a syntheticdesign storm and can be used with either the conventional deterministic, mixeddeterministic-probabilistic, or stochastic modeling approaches.Spatial Mapping of Regional Precipitation-Frequency InformationRegional precipitation-frequency analysis 9 has been conducted at numerous locationsaround the globe since the late-1980’s. Recent improvements to the spatial mapping ofprecipitation pioneered by Daly 4,5 and Oregon Climate Service 15 now allow high-resolutionmapping of precipitation-frequency information. The spatial mapping of 100-year and1000-year precipitation has recently been completed 27 for the 6-hour, 24-hour, and 72-hourdurations for southwest British Columbia for use by BChydro in their dam safety program.RESEARCH AND DEVELOPMENT NEEDSResearch about the hydrologic processes has been conducted for well over 70-years.Through past research, a sound scientific foundation now exists for describing nearly all ofthe hydrologic and hydraulic processes that are important for estimation of extreme floods.However, there continues to be a large chasm between the tools developed by the researchcommunity and the tools in common use by hydrologic engineers.This chasm is primarily due to the practical problem of applying advanced models insituations where the necessary input data are either, unavailable, inadequate, or overlyexpensive to obtain on a watershed basis. Because of these constraints, it is difficult to takeadvantage of the capabilities of the advanced models. As a result, hydrologists continue touse simplified conceptual and empirical models in conjunction with numerous policies,procedures, guidelines, and standards-of-practice. In many cases, existing policies andprocedures were developed decades ago based on information and experiences of past eras.Paper 16 - Schaefer 114