FLOOD11754 - Laserfiche WebLink

Home Browse Search

Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] Floodflow-frequency curves for gaging stations are routinely published by the USGS as part of regional floodflow-frequency studies. The reviewing Mapping Partner can compare these published flood discharges to the proposed flood discharges to judge their reasonableness. In cases where major flood events have occurred since the floodflow-frequency curves were published, the reviewing Mapping Partner must confirm that the impacts of these events have been reflected in the flood discharge calculations. The Mapping Partner shall compare the effective flood discharges to the confidence limits of the proposed flood discharges to determine which flood discharges are more appropriate. For regulated watersheds, floodflow-frequency curves are often developed for wlregulated conditions and then converted to regulated conditions by utilizing the current reservoir operation criteria. The reviewing Mapping Partner shall review the regulated floodflow-frequency curve to determine whether acceptable procedures were used to convert to regulated conditions. USACE Engineering Manual No.1l10-2-14l5 (USACE, 1993) provides guidance on floodflow- frequency analysis for regulated watersheds. C.2.2 Hydrologic Analysis Based on Regional Regression Equations [February 2002] The reviewing. Mapping Partner shall compare the proposed 1-percent-annual-chance flood discharges computed from regional regression equations to the effective flood discharge, to flood discharges from other (published) regression equations that are applicable to the region, and to flood discharges at gaging stations in the vicinity. In general, proposed regional regression equations should be the most recent published equations developed by the USGS for the region unless justification is provided for the use of earlitlr equations. The reviewing Mapping Partner shall assume the proposed regression equations are applicable if the watershed, climatic, and urbanization characteristics of the ungaged sites are within the range of those at the gaging stations used to develop the equations, and if flow is not regulated. If appropriate, the regional regression equations may be adjusted for urbanization using procedures in Flood Characteristics of Urban Watersheds in the United States (USGS, 1983) or, if available, urban regression equations for the applicable state or metropolitan area. The reviewing Mapping Partner shall compare the proposed regression estimates to gaging station estimates in nearby watersheds having similar characteristics to those of th(~ studied streams. The reviewing Mapping Partner may obtain estimates of I-percent-annual-chance flood discharges at nearby gaging stations from published USGS regional flood reports if the floodflow-frequency curves were published in the last 10 years and if no major floods have occurred in the intervening time. Otherwise, the reviewing Mapping Partner is to update the floodflow-frequency estimates for the gaging stations in accordance with Bulletin 17B (Interagency Advisory Committee on Water Data, 1982). The reviewing Mapping Partner shall plot the 1-percent-annual-chance flood discharge estimates from these sources against drainage area on logarithmic paper to determine whether the proposed flood discharges are reasonable. The reviewing Mapping Partner shall estimate confidence intervals of the gaging station estimates using Bulletin 17B (Interagency Advisory Committee on Water Data, 1982) or procedures given in Frequency and Risk Analysis (Kite, 1999) or in C-13 Section C2