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<br />Guide For Approximate Zone A Areas <br /> <br />Developing BFEs <br /> <br />is based on the drainage <br />time of concentration, <br />generalized equation is: <br /> <br />area, rainfall intensity, watershed <br />and a runoff coefficient. The <br /> <br />Q = C * I * A <br /> <br />where: Q <br />C <br />I <br />A <br /> <br />= discharge (cfs) <br />= runoff coefficient <br />= rainfall intensity (inches/hour) <br />= drainage area (acres) <br /> <br />The runoff coefficient, C, varies with soil type, land use, <br />and terrain slope and can be obtained from text books on <br />hydrology. The intensity of rainfall, I, is determined based <br />on the total rainfall for a selected exceedence probability <br />and a duration equal to the time of concentration for the <br />watershed. The time of concentration for the watershed can be <br />computed using the method described in the NRCS TR-55 manual <br />or methods described in hydrology text books. For approximate <br />Zone A areas, the exceedence probability is equal to 1 percent <br />(lDO-year storm frequency). The 1 percent exceedence <br />probability total rainfall (lDD-year rainfall) for the <br />computed duration can be obtained from Technical Paper No. 4D, <br />Hydro 35, and precipitation-frequency atlases published by the <br />National Weather Service. Dividing the total rainfall by the <br />computed duration will yield the intensity of rainfall. <br /> <br />Limitations - This method must not be used where the runoff is <br />regulated by the use of dams, detention ponds, canals and <br />other flow diversions. Also, this method is not recommended <br />for drainage areas greater than 200 acres, but can be used <br />with caution for drainage areas up to 640 acres (one square <br />mile) . <br /> <br />Other Hydrograph Methods <br /> <br />There are numerous other methods that can be used to determine <br />flood discharges based on rainfall-runoff relationships. The <br />following hydrograph methods are described in detail within <br />their respective technical reports and, therefore, will not be <br />described in detail within this guide. These methodologies in <br />general are good for any size watershed, and most of the <br />methods include computations that take into consideration <br />areas where the runoff is regulated by the use of dams, <br />detention ponds, canals and other flow diversions. These <br />methods are recommended for determining BFEs for ponds or <br />lakes that are designated as approximate Zone A. Besides TR- <br />55, two of the more widely used hydrograph methods are the <br /> <br />V-21 <br />