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<br />CHAPTER 7 <br />HYDROLOGIC METHODS <br /> <br />PRECIPITATION <br /> <br />1. BACKGROUND THEORY. <br /> <br />Synthetic or hypothetical storms are frequently used by hydrologists and engineers <br />to generate synthetic flood events for evaluation of existing flood conditions and for study <br />of the effects of flood-mitigation components. The degree of dependency on synthetic <br />flood events during a flood evaluation depends generally on the number of gages and the <br />length of hydrologic record available. For reservoir studies there is typically at least one <br />long-record stream gage near the dam site, and thus the storage required for reservoir <br />food control is usually computed by period-of-record simulation techniques using recorded <br />data. Channels, levees, and other local protection projects are more likely to be sized by <br />synthetic storm-flood analyses since these projects often extend over many miles of the <br />stream, and it is rare that gage information would be available throughout the reach. <br /> <br />Hypothetical storms are also used to ensure that rare storm-flood events are <br />included in an overall assessment of the proposal. Since large storms that cause major <br />floods are infrequent, the available runoff record quite possibly may not reflect the <br />occurrence of these rare events. Analyses of major reservoirs, although relying mainly on <br />recorded stream data, also require the use of synthetic storms (Standard Project and <br />Probable Maximum Storms) to evaluate the safety of major dam components, such as the <br />spillway and outlet works, 'and .guardagainst overtopping of the dam. Synthetic <br />precipitation data are also used for analyses of flooding throughout a basin, testing of the <br />hydrologic effects of altemative land uses and flood plain regulations, and evaluation and <br />design of flood control components. This is usually done with a model of the watershed <br />to simulate the rainfall-runoff process, with the rainfall specified by hypothetical storms. <br /> <br />2. DATA SOURCES. <br /> <br />The primary sources of hypothetical storm information for the United States are <br />various technical pUblications (TP) of the National Weather Service (NWS) and <br />hydrometeorological reports (HMR) of the National Oceanic and Atmospheric <br />Administration (NOAA). All publications for development of hypothetical frequency storms <br />feature generalized isopluvial rainfall maps and/or regression equations. Other methods. <br />such as statistical analysis of nearby long-record rain gages to derive hypothetical storms <br />of particular frequencies in lieu of the NWS and NOAA procedures, are used extensively <br />in some parts of the United States but not discussed here. <br /> <br />I' <br /> <br />The geographical areas to which these publications apply include NWS Hydro 35, <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFI' <br /> <br />7.1 <br />