Hydrologic Issues for Dams - Association of State Dam Safety Officials

pro cedures are used to allow the hydrometeorological input parameters to vary inaccordance with that observed in nature while preserving the natural dependencies thatexist between many of the climatic and hydrologic parameters.Multi-thousand computer simulations are conducted where each simulation containsa set of input parameters that were selected based on the historical record andcollectively preserves the dependencies between parameters. The simulated floodsconstitute elements of an annual maxima flood series that can be analyzed bystandard flood-frequency methods. The resultant flood magnitude-frequencyestimates reflect the likelihood of occurrence of the various combinations ofhydrometeorological factors that affect flood magnitude. The use of the stochasticapproach allows the development of separate magnitude-frequency curves for floodpeak discharge, flood runoff volume, and maximum reservoir level. Frequencyinformation about maximum reservoir level is particularly important for use inhydrologic risk assessments because it accounts for flood peak discharge, runoffvolume, hydrograph shape, initial reservoir level, and reservoir operations.PROBLEMS FACEDEnormous sums of money have been spent in the past, and will be spent in the future, forupgrading of dams due to perceived spillway inadequacies. The decision to upgrade is oftenbased largely on the findings of rainfall-runoff modeling for extreme floods. Currently,hydrologists use simplified conceptual and empirical models in conjunction with numerouspolicies, procedures, guidelines, and standards-of-practice in modeling extreme floods. Thisheavy reliance on institutionalized policies and procedures provides consistency ofapplication, but does not necessarily provide accurate estimates of extreme floods.If accuracy is defined as nearness to the truth, there are several factors that currently limit theability of the engineering community to consistently achieve accurate results. First, in mostapplications, there is no benchmark for measuring accuracy. By the very nature of theproblem, we are estimating extreme floods, events that are beyond, oftentimes well beyond,the historical data. Without a benchmark for accuracy, it is often difficult to make acompelling case that a given model, method, or procedure is superior to another. Whilemodels can be calibrated to past floods, there is almost always a need to extrapolate to obtainsolutions for the extreme conditions of interest.Second, while there may be a large set of models to choose amongst for modeling thehydrologic processes, we are typically woefully short of obtaining the field data necessaryfor properly applying any of those models on a watershed basis. This leads to the use ofsimplified physically-based models and conceptual models of the hydrologic processes andexcessive lumping of inputs. Some of these models may not perform well beyond the limitsfor which they were originally developed or calibrated.Given the life-safety and economic considerations involved in the decision to upgradespillways, it is important that the rainfall-runoff modeling be conducted in a manner thattruly emulates watershed flood response for extreme floods. Consideration of the discussionabove indicates that greater research emphasis should be placed on analyses of extremehistorical floods. The goal would be to improve the knowledge base and identify howPaper 16 - Schaefer 112