Hydrologic Issues for Dams - Association of State Dam Safety Officials

approach. With the average parameter approach, some parameters are assigned average valuesbased upon regional analyses, such as the log-space skew or standard deviation. Other parametersare estimated using at-site data, or regression on physiographic basin characteristics, perhaps the realor log-space mean. The index flood method is a special case of the average parameter approach.The specific frequency approach employs regression relationships between drainage basincharacteristics and particular quantiles of a flood frequency distribution.Index Flood Method. The index flood procedure is a simple regionalization technique witha long history in hydrology and flood frequency analysis (Dalrymple, 1960). It uses data sets fromseveral sites in an effort to construct more reliable flood-quantile estimators. A similarregionalization approach in precipitation frequency analysis is the station-year method, whichcombines precipitation data from several sites without adjustment to obtain a large composite recordto support frequency analyses. The concept underlying the index flood method is that thedistributions of floods at different sites in a "region" are the same except for a scale or index-floodparameter which reflects the size, rainfall and runoff characteristics of each watershed. Generallythe mean is employed as the index flood (Hosking and Wallis, 1997).Average Shape Parameter. As at-site records increase in length, procedures that estimatetwo parameters, with at-site data to be used with a regional shape parameter, have been shown toperform better that index flood methods in many cases (Stedinger and Lu, 1995). For record lengthsof even 100 years, 2-parameter estimators with a good estimate of the third shape parameter, aregenerally more accurate than are 3-parameter estimators (Lu and Stedinger, 1992; Stedinger and Lu,1995). However, whether or not it is better to also regionalize the coefficient of variation dependsupon the heterogeneity of the regions and the coefficients of variability of the flows. In regions withhigh coefficients of variation (and high coefficients of skewness) index flood methods are moreattractive.Regional Regression. Regional analysis can be use d to derive equation s to predict thevalues of various hydrologic statistics (including means, standard deviation s, quantiles, andnormalized regional flood quantiles) as a function of physiographic characteristics and otherparameters. Stedinger and Tasker (1985, 1986a, 1986b) developed a specialized Generalized LeastSquares (GLS) regression methodology to address the region alization of hydrologic statistics.Advantages of the GLS procedure include more efficient parame ter estimates when some sites haves hort records, an unbiased model-error estimator, and a better description of the relationship betweenhydrologic data and information for hydrologic network analysis and design.Design Event-Based Precipitation-Runoff ModelingPrecipitation-runoff modeling is typically used as an event-based method for determiningextreme floods. A single set of hydrometeorological parameters and watershed characteristics areused to simulate a design flood event. The major inputs to a design event-based precipitation-runoffmodel are: (1) climate data (rainfall, snowfall, and other variables needed to predict snowmelt); (2)losses (infiltration/interception); (3) physical watershed characteristics for runoff and routingsimulations (drainage areas, watershed and channel slopes, lag times, antecedent moisture, etc.); (4)precipitation-runoff transformation function; and (5) runoff conveyance/routing mechanisms. Modeloutput includes runoff hydrographs at user-specified locations, maximum peak discharges, and totalPaper 18 – Swain, Bowles, Ostenaa 132