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The Division feels that an Antecedent Moisture Condition of II is <br />applicable to worse case climatic conditions at Colowyo. <br />A sedcad run, Pond 7a, was run by the Division substituting the <br />Division's curve number values given on the first page of this letter. <br />This sedcad run does not take into consideration the changes in time <br />of concentration that the Division requests that Colowyo make to its <br />submittal (see below). <br />Time of Concentrations: <br />In deriving times of concentrations, Colowyo used overland flow <br />conditions for long distances in routing water to the South Prospect <br />Ditch. The Soil Conservation Service's Urban Hydrology for Small <br />Watersheds (TR-55) generally assumes overland flow does not occur over <br />distances greater than 300 feet. The Division considers this an <br />appropriate assumption in surface water modelling systems. <br />The Division will accept the use of overland flow conditions where <br />contour furrows are present (i.e. reclaimed lands) that could serve <br />to retard the development of small upland gullies. Such conditions <br />would not often be found under other conditions. <br />The values for time of concentration on the enclosure accompanying <br />this letter were derived by the Division for Colowyo's model. These <br />values could not be substituted into Colowyo's submittal because K and <br />X values were not determined at that time and time restraints did not <br />allow the Division to rerun the time of concentrations. Therefore, <br />the Sedcad run provided with this letter does not incorporate the <br />Division's values for time of concentration. <br />Resolution of the Division's Concerns: <br />Based on the factors discussed above, the Division feels that a more <br />appropriate rationale for modelling the Prospect Pond watershed would <br />include the curve numbers on page one of this letter and the time of <br />concentration values on the enclosure. A final review of the Prospect <br />Pond and South Collection Ditch based upon these revised factors can <br />be completed if Colowyo will submit Sedcad data derived from the <br />assumptions outlined above. We would be happy to review any <br />documentation justifying the curve numbers or time of concentration <br />values in Colowyo's latest submittal if that is available. <br />