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<br />1935 rainfall as inferred from historical flood and paleoflood data. This modeling effort will <br />provide an approach to assess the reliability of the historic rainfall observations and estimate <br />maximum rainfall corresponding to maximum flooding in Cherry Creek and adjacent basins. <br />The rainfall-runoff (RF -RO) modeling study will focus on Palmer Divide sub-basins. <br />That includes, but is not limited to Cherry, Plum, Bijou, Kiowa and Monument Creeks. The <br />approach will be to; <br />1) obtain the best estimates of flood peaks to be used for RF - RO modeling, the final product <br />being characteristics for severe floods (e.g. mostly 1965 floods, but also other large floods), <br />via on-site hydraulic data collection and calculation (or recalculation) of flood discharge <br />2) obtain paleoflood data (recent, historic and maximum) at these sites, and <br />3) back calculate the flood producing rainfall via RF -RO modeling. <br />From all the sub-basins evaluated, those having a range of mean elevation, geographic <br />location, reliable flood peak estimates and available rainfall data will be used in the modeling <br />study. Some sub-basins have USGS gage data (E. Plum Creek at Castle Rock, West Kiowa <br />Creek, Kiowa Creek, Tributary nr Elbert, Middle Bijou Creek nr Wiggins, Middle Bijou Creek <br />Tributary nr Deer Trail, West Monument Creek) but it is not known at this time if any rainfall <br />data exists for any of those locations. <br />Peak discharge estimates will be modeled for the 1935 and 1965 storms. HEC-l parameters <br />will be obtained from field visits, NRCS Soil Surveys, regional relations for lag time, and <br />USACE Cherry Creek HEC-l model. From the 1935 and 1965 storm observed rainfall <br />isohyetals, rainfall depths for selected sub-basins in/near the area of maximum rainfall will be <br />determined and input into HEC-l. Also, a rainfall distribution curve based on the 1935 and 1965 <br />storm mass curve data will be developed and input into HEC-l. Initial values for antecedent <br />moisture conditions (AMC) will be estimated from 1935 and 1965 storm data and historical <br />information. <br />For these selected sub-basins HEC-l results will be compared against paleoflood data for the <br />1935 and 1965 storms, in addition to historical flood data for the 1965 flood and any other <br />notable storms identified. Favorable comparison indicates confidence in the observed storm <br />data. If model results do not compare favorably to historical and paleoflood data, AMC will be <br />adjusted within a realistic range, and HEC-l rerun to attempt to simulate the observed peak <br />values. Again, favorable comparison indicates confidence in the observed storm data. If that <br />cannot be accomplished, various HEC -1 model runs will be made, while varying rainfall depth <br />until measured peak discharge values are simulated. It is assumed that other model parameters <br />are reasonably accurate and thus will not be adjusted. That will allow peak discharge to be a <br />function of rainfall depth for given spatial isohyetal patterns and temporal distributions. By rot <br />varying other model parameters, complex parameter interaction is minimized. Such parameter <br />interaction would cause difficulty in focusing on rainfall depth. Such modeling will result in <br />realistic maximum depths for the region (i.e. Palmer Divide) which will include an estimate and <br />explanation of the reliability of historic rainfall observations. From the final adjusted peak <br />rainfall depths from the 1935 and other storms, judgements will be made related to the use of the <br />historic rainfall amounts for developing the new site-specific PMP values. <br />