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<br />cylinders and the time and heigll1 of water in the inner cylinder are recorded. <br />Measurements are taken at 10. 20. 30. 45. 60. 90 and 120 minutes and E,ach hour for at <br />least four hours. A<tditional water is added and measured to maintain a relatively constant <br />head. It is important that the water level in the outer cylinder be kept at about the same <br />level as the inner on(l and should never b(ll'illed higher that the inner cylindm. Recorded <br />rates of infiltration are computed from ':he measurements and plotted VErsus time to define <br />the infiltration curve. From this relationship. parameters for use in the infil'tration methods <br />described previously Can be determined. <br /> <br />8. FLOOD RECONSTITUTION <br /> <br />Reconstituting historic or observecl storm events can give the most accurate <br />estimate for infiltration for a basin. :f a basin has rainfall/runoff data from historic storm <br />events, infiltration rates can be determine,j by "backtracking" from the observed runoff <br />volume, what infiltration values have to be applied to the observed rainfall hyetograph to <br />match the observed runoff volumes n,e examplE' below shows thiS concept. <br /> <br />---.- <br /> <br />'- <br /> <br />____1_ <br /> <br />Example 7.2- A USGS rainfall/stream gage located in tile Irondale Gulch Basin in <br />Montebello WaS used to determine historical inliltration raj,clS during actual flood events in <br />the basin. The gage used in the flood reconstiturons was USGS gage 06714310, titled <br />"Sand Creek Tributary at Denver, CO" whictl is actually in the Montebello development in <br />lrondale Gulch. The gage was locat"d on the left IJank of a concrete lined drainage ditch <br />in the median of Andrews Dr. Parkway, 50 feet downstream of Troy Street. The basin has <br />a 0.30 square mile drainage area. The tllree largest flood producing events were selected <br />out of the twenty-':wo storms with available rainfall/runoff data to determine historic <br />infiltration rates. An iterative process was Lsed to determine what infiltration rate for each <br />storm would result in the simulated volume of runoff that would closest matc!l the historic <br />volume of runoff. Three assumptions werE required for tile flood reconstitutions: <br /> <br />1. Surface Storaoe I_oss- Based on calibrated data. the Urban Drainage and l'lood Control <br />District recommends a surface storage loss of 0.35 inches for watershecls consisting of <br />lawns. This value WaS used in the flood volume reconstitutions. <br /> <br />2. Basin Percent Inmerviousness- The value for percent imperviousness fo' fhe existing <br />conditions is 40%. The calibrated storms were in tne early 1970s. so the value of percent <br />imperviousness was reduced to 30%, 10 reflect 19l0s conditions. <br /> <br />3. Antecedent Soil Conditions- Simulation of the infiltralion process waS IJa:sed on being <br />an exponential loss rate if the antececlent conditions were dry or uniform IOS:3 rate if there <br />was significant rain'all immediately prior to the storm being simulated. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRA1<T <br /> <br />7.27 <br />