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<br />Guide For Approximate Zone A Areas <br /> <br />Developing BFEs <br /> <br />The general form of these regression equations is: <br /> <br />Q = K * AX * BY * CZ <br /> <br />where: Q = discharge (cfs) <br /> K = regression equation constant <br /> A,B, <br /> and C = watershed variables <br /> X,Y, <br /> and Z = exponents <br /> <br />Watershed variables may include parameters such as drainage <br />area (in square miles), stream slope (in feet/mile), and <br />impervious area (in percent). <br /> <br />Limitations - Care must be taken when using these publications <br />because restrictions generally apply when the watershed is <br />heavily urbanized (i.e., high percentage of impervious land), <br />or where the runoff is regulated by the use of dams, detention <br />ponds, canals and other flow diversions. other restrictions <br />based on the physical parameters of the watershed, such as <br />drainage area or stream slope, may also apply. Limi tat ions of <br />these equations are normally stated in each report and should <br />be examined closely. <br /> <br />TR-55 <br /> <br />The NRCS TR-55 "Urban Hydrology for Small Watersheds" contains <br />two methods for computing flood discharges: the Graphical <br />Peak Discharge method and the Tabular Hydrograph method. TR- <br />55 is straightforward in its approach and method of <br />computation. TR-55 takes into account the effects of <br />urbanization, rainfall distribution, soil types and <br />conditions, ground cover types, and other watershed <br />characteristics. A method for estimating the effects of <br />storage on peak flood discharges is also included in TR-55. <br /> <br />Limitations - In general, TR-55 should not be used in areas <br />where flow is divided between closed storm sewer systems and <br />overland conveyance areas, or where drainage areas exceed <br />2,000 acres. More specific limitations for using TR-55 are <br />contained in Chapters 2 through 6 of the NRCS TR-55 manual. <br /> <br />Rational Formula <br /> <br />This method estimates peak discharge rates for small watershed <br />areas not covered by regression equations and for areas where <br />the NRCS TR-55 method is not applicable. The Rational Formula <br /> <br />V-20 <br />