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<br />J arrett/Browning <br /> <br />Task 4A. There is a need to compile/collect available systematic, historic, and paleoflood <br />data in South Boulder Creek basin and within a regional framework of stations within the <br />nearby hydro climatic homogeneous study area. The purpose is to validate existing flood <br />data and extend streamflow-gaging station data using paleoflood techniques where <br />possible to: 1.) improve at-site flood-frequency relations and develop new regional flood- <br />frequency relations (tasks 4C and 5); 2.) provide paleoflood data at gaged and ungaged <br />sites as well as evaluate the effect of Gross Reservoir and inadvertent storage such as <br />irrigation diversions, and railroad embankments (tasks 4A and 4C-F); 3.) help validate <br />and provide realistic constraints for hydrologic and hydraulic modeling of the basin (task <br />4D-F and 5); 4.) allow for linkages and comparison with recorded rainfall and storm <br />reconstructions (as described in task 3) and; 5.) help assure the public, scientists, and <br />managers that available flood data is being utilized as best as possible (task I). All data <br />should be collected to meet requirements for input to task 2. <br /> <br />Task 4B. Paleoflood data in the form of paleo stage indicators (PSIs) are used as estimates <br />for maximum flood stage. Most commonly used PSIs include flood-deposited sediments <br />and botanical evidence preserved of past floods including systematic, historical, and <br />much older floods. The ages of paleofloods can be estimated using a variety of relative- <br />and absolute-dating techniques. Discharge for paleofloods with PSI data can be used <br />with various one-dimensional flow techniques and where necessary use of <br />multidimensional flow models (task 5). Paleoflood data also can be collected to obtain <br />peak-flow data for urbanized basins since unbanization of a basin. The Front Range is <br />susceptible to debris flows and wildfire, which can seriously affect flood estimates. <br />Thus, efforts are needed to evaluate these potential effects on flood magnitude and how <br />and if they are used in flood-frequency analyses and hydrologic modeling. Limitations <br />and assumptions used in the paleoflood analyses and evaluation of systematic and <br />historically data validation needs to be clearly stated. All data should be collected to <br />meet requirements for input to task 2. <br /> <br />Task 4C. This task seeks to improve at-site flood-frequency relations and develop <br />regional flood-frequency relations for South Boulder Creek and other streams in the <br />identified hydro climatic region. Flood-frequency relations are most commonly used to <br />meet FEMA regulations to help assess flood risk (tasks 4C and 5). Newer, and more <br />robust flood-frequency methods better incorporate historical and paleoflood data in the <br />frequency analyses for the at-site, including improved uncertainty (confidence limits) <br />estimates, and need to be considered for use in the analyses. Once at-site frequency <br />estimates are completed, regional flood-frequency relations can be developed. <br />Limitations and assumptions used in the flood-frequency analyses need to be clearly <br />summarized. Paleoflood estimates at ungaged sites can be used to validate and refine thearegional relations and compare/contrast these results with hydrologic modeling results. <br /> <br />