Laserfiche WebLink
<br />cylinders and the time and height of water in the inner cylinder are recorded. <br />Measurements are taken at 10, 20, 30, 45, 60, 90 and 120 minutes and each hour for at <br />least four hours. Additional water is addod and measured to maintain a relat~lely 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 one and should nevor be filled higher that the inner cylinder. Recorded <br />rates of infiitration are computed from the measurements and plotted versus time to define <br />the infiitration curve. From this relationship, parameters for use in the infiitration methods <br />described previously can be determined. <br /> <br />8. FLOOD RECONSTITUTION <br /> <br />Reconstituting historic or observed storm events can give the most accurate <br />estimate for infiitration for a basin. If a basin has rainfalVrunoff data from historic storm <br />events, infiltration rates can be determined by "backtracking" from the observed runoff <br />volume, what infiitration values have to be applied to the observed rainfall hyetograph to <br />match the observed runoff volumes. The example below shows this concept. <br /> <br />- <br /> <br />Example 7.2- A USGS rainfall/stream gage located in the Irondale Gulc;h Basin in <br />Montebello was used to determine historical infiltration rates during actual flood events in <br />the basin. The gage used in the flood reconstitutions was USGS gage 067'14310, titled <br />"Sand Creek Tributary at Denver, CO" which is actually in the Montebello development in <br />Irondale Gulch. The gage was located on the left bank 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 three largest flood producing events were selected <br />out of the twenty-two storms with available rainfall/runoff data to determine historic <br />infiitration rates. An iterative process was used to determine what infiitration rate for each <br />storm would resuit in the simulated volume of runoff that would closest match the historic <br />volume of runoff. Three assumptions were required for the flood reconstitutions: <br /> <br />1. Surface Storaoe Loss- Based on calibrated data, the Urban Drainage and Flood Control <br />District recommends a surface storage loss of 0.35 inches for watersheds consisting of <br />lawns. This value was used in the flood volume reconstitutions. <br /> <br />2. Basin Percent Imoerviousness- The valuo for percent imperviousness for the existing <br />conditions is 40%. The calibrated storms were in the early 1970s, so the value of percent <br />imperviousness was reduced to 30% to reflect 1970s conditions. <br /> <br />3. Antecedent Soil Conditions- Simulation of the infiitration process was basEld on being <br />an exponential loss rate if the antecedent conditions were dry or uniform loss rate it there <br />was significant rainfall immediately prior to the storm being simulated. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.27 <br />