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TR-55 <br /> The NRCS(NRCS 1986;http://www.cpesc.org/reference/tr55.pdf) approach relies on using <br /> physical characteristics for a given watershed to estimate the runoff response for a 24 hour <br /> rainfall event. Using ground cover characteristics, a curve-number is developed that is used to <br /> estimate the runoff response to rainfall. Channel characteristics such as length and slope are <br /> used to determine the travel time and time of concentration. These estimates are then used with <br /> a graphical method to develop a flood hydrograph over a determined time range for a given <br /> storm. The graphical method is based on the following equation: <br /> qp = 9uAmQFp <br /> where <br /> q,is the peak discharge, <br /> q.is the unit peak discharge, <br /> A„ is the drainage area of the watershed, <br /> Q is the runoff depth, and <br /> F,is a factor used to adjust for the presence of the pond on the property. <br /> The runoff depth is estimated using a rainfall depth and the curve-number. The unit discharge <br /> is read from an NRCS table based on rainfall depth and soil characteristics. The pond factor is <br /> pulled from an NRCS table based on the percentage of the watershed area taken up by the <br /> pond. For 24 hour rainfall depths,we turned to the NOAA Climatic Atlas to get rainfall depths <br /> for the 1.5,2, 5, 10, 25,50 and 100 year storms. The technique was performed using <br /> the HydroCAD (http://www.hydrocad.net/) software package that uses the graphical method <br /> with our input to generate the flows. <br /> The Rational Method <br /> Similar to the TR-55 approach, the Rational Method (Kuichling 1889)estimates the response of a <br /> watershed to a given rainfall intensity based on travel time and ground cover <br /> characteristics. The equation used for estimation of the flows is: <br /> Q = CiA <br /> where <br /> Q is the peak discharge that occurs at the time of concentration t�, <br /> C is the runoff coefficient for the area based on ground cover characteristics, <br /> i is the rainfall intensity over that same time period, and <br /> A is the area of the watershed. <br /> C values are selected from tables for given return periods and i values were taken from the <br /> NOAA Climatic Atlas. <br /> 7 <br />