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<br />Time of concentration values were computed from an emperical <br />equation in Soi I Conservation Service Engineering Field <br />Manual - Chapter 2, intended for small rural watersheds. <br /> <br />Tc=[L^O.8((1000/CN)-9)^O.7J/1140(Y^O.5) <br />Tc= time of concentration in hours <br />L= flow length in feet <br />CN= runoff curve number <br />Y= average watershed slope <br /> <br />V=100CI/A <br />Y= average watershed slope in percent <br />A= drainage area in square feet <br />1= contour i nterva I in feet <br />C= total contour length in feet <br /> <br />The following Table 1 shows peak discharge - frequency <br />values developed by TR-20 for designated cross sections. An <br />incfuded map shows the locations of these cross sections. <br /> <br />It was determined in a meeting on Jan. 6, 1992 that the <br />Rational Formula (Q=CIA) would be useful to the city in <br />making site specefic peak discharge computations. Therefore <br />the TR-20 discharge values determined herein could be used <br />to develop necessary runoff coeficients (C values) for local <br />use. <br /> <br />Since the SCS 24 hour type II rainfal I distribution was used <br />in TR-20, it should also be used to determine rainfall <br />intensity values (I). Most of the appl ication of the <br />rational formula wi I I be for smal I drainage areas where the <br />time of concentration is less than 30 minutes. Therefore a <br />rainfall duration of 30 minutes was selected for determining <br />I. The greatest 30 minute intensity from the SCS 24 hour <br />type II rainfal I distribution produces 37.8 X of the total <br />24 hour rainfal I depth. This 30 minute rainfall depth was <br />determined for each storm frequency of interest and then <br />converted to inches per hour for use as I, see Table 4. <br /> <br />The rational formula was rearranged (C=Q/IA) to compute C <br />values using previously determined Q and A values from TR-20 <br />data, and I values previously discussed. A relationship <br />between C values and curVe numbers (CN) from TR-20 was <br />establ ished and is shown in included graphs. CN values for <br />the TR-20 watersheds reflect weighted average CNs for a <br />variety of land uses and soi Is within each watershed with <br />CNs that did not exceed the low 80s. Since the rational <br />equation may be used for individual land uses and soils, <br />such as a city block of commercial bui Idings, the CNs in the <br />included graphs may not be high enough. Therefore for land <br />uses with eNs greater than the low 80s, use table 14-1 to <br />reflect C values for various types of land uSeS. Table 14-1 <br />values are not supported by TR-20 evaluations. <br /> <br />z <br />