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<br />e <br />I <br /> <br />,e <br /> <br />I' <br /> <br />e <br /> <br />(2) Design Analysis of Selected Components -- A design storm is used to <br />generate the runoff that is selected as the standard against which <br />the performance of a facility can be evaluated, The design flODd is <br />simply the runoff from the design storm. Hypothetical frequency <br />storms are used to design channels, storm sewers, agricultural <br />levees, detention areas, and other features. <br /> <br />The Standard Project Storm is the design storm to be used where some <br />small degree of risk can be accepted but an unusually high degree of <br />protection is required (due to loss of 11 fe, high property values, <br />etc,). It is typically used for studies for main river levees in <br />urban areas. <br /> <br />The Probable Maximum Storm is used as a design storm where virtually <br />no risk of failure can be tolerated, such as for dam spillways. <br /> <br />(3) Flood Plain Management (FPM) Studies -- Hypothetical storms are <br />frequently used to develop flood hazard information. USually, <br />hypothetical frequency storms are used to generate the flood events <br />of various return periods, such as the lOO-yr event for flood <br />insurance studies. The Standard Project Storm is often included for <br />flood hazard information. Flood hazard information based on the <br />Probable Maximum Storm is not used in FPM studies. <br /> <br />4.4 construction of Hypothetical Storms <br /> <br />When constructing a hypothetical storm, it is necessary to establish the <br />appropriate storm duration and the time interval for subdividing the storm <br />rainfall. One can then take rainfall values from the appropriate NWS <br />publication, make adjustments to compensate for size of drainage area, adjust <br />from partial to annual series (i f recp.lired), and finally increment and <br />arrange the storm rainfall for later use with the hydrologic model of the <br />watershed. Each of these steps is described in detail with examples in the <br />Appendix. <br /> <br />The storm duration and the time increment for calculations are functions <br />of the type and size of the drainage area. The appropriate duration of the <br /> <br />21 <br />