Importance of Hydraulic-Model Uncertainty in Flood-Stage Estimation

by Satvinder Singh, New Jersey Dept of Envir. Protection, and Energy, Trenton, United States,
Charles S. Melching, New Jersey Dept of Envir. Protection, and Energy, Trenton, United States,

Abstract: The delineation of the regulatory floodplain and determination of the respective flood stage is subject to hydraulic-model uncertainty in transforming the design discharge into a flood stage, in addition to the hydrologic uncertainty in the design discharge. The hydraulic-model uncertainty included uncertainties in the model formulation used to approximate the dynamic movement of the flood wave, the channel-geometry definition, and the model parameters. An HEC-2 model analysis was performed for the South Branch of the Raritan River near Stanton, New Jersey. The advanced first-order, second moment method of reliability analysis was used to determine the exceedance probability for flood stage, subject to uncertainties in (1) model formulation and channel geometry, (2) Manning's n of the main channel and overbank, and (3) design discharge. The model formulation was assumed to have a coefficient of variation of prediction of 10 percent, including uncertainty in channel-geometry definition. The coefficients of variation for Manning's n for the overbank and main channel conditions were 30 to 27.5 percent, respectively. The 100-year flood stage estimated by traditional methods, that is, with only the design discharge uncertain, has an exceedance probability of once in 57 years, considering all significant uncertainties . The combined model-formulation and channel-geometry uncertainties contributed much more to the prediction uncertainty than did Manning's n. Accurate estimation of the flood stage with an exceedance probability of once in 100 years is aided by the use of reliability analysis considering all significant sources of uncertainty. A more theoretically correct model aids in the reliable delineation of a regulatory floodplain when traditional frequency analysis procedure are used.

Subject Headings: Uncertainty principles | Hydraulic models | Hydrologic models | Flood frequency | Floods | Model analysis | Water discharge | Hydraulic design | New Jersey | North America | United States

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