Laserfiche WebLink
<br />. <br /> <br /> <br />. <br /> <br />CHAPTER 4 <br />FLOODPLAIN <br />ADMINISTRATION <br />AND <br />.ELlNEATION <br /> <br />SECTION 2.0 <br />FLOODPLAIN <br />DELINEATION <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />All drainage basin characteristics that affect the rainfall-runoff <br />relationship shall be documented, including, but not limited to, <br />delineation of basin and subbasin boundaries, size, shape, <br />length, slope, general aspect, elevation extremes, time of <br />concentration, land use, and soil types and compositions. <br /> <br />When actual precipitation records of major recorded storm <br />events are available from area weather stations, such data <br />shall be used in conjunction with rainfall data. <br /> <br />. When less than 10 years of stream-flow records are available, <br />the selected flood frequency flows shall be calculated by <br />comparison to similar watersheds, USGS regression analysis, <br />and by estimates using synthetic hydrologic models and <br />precipitation records. <br /> <br />Depending on the floodplain analysis requirements, it may be <br />necessary to develop a synthetic rainfall-runoff model even when <br />sufficient amount of gage peak flow records are available. The <br />following is a list of some of these cases: <br /> <br />. Various flood frequency hydrographs are required, but the <br />statistical analysis alone cannot generate the necessary <br />hydrographs. <br />. The subject watershed is undergoing or projected to undergo <br />a substantial amount of new development. <br />. Comparison of before and after development hydrographs. <br /> <br />Whenever possible, synthetic rainfall-runoff models should be <br />calibrated to match the statistical analysis results. The calibrated <br />synthetic model can then be used to generate the hydrographs. <br /> <br />2.6.1.2 DETENTION <br /> <br />The hydrologic analysis should include <br />detention facilities designed and <br />constructed with the purpose of impounding <br />water for flood detention that are owned, <br />operated, and maintained by a govemment <br />body. Detention structures that are <br />randomly located, privately owned, or <br />privately maintained shall not be included in <br />the hydrologic analyses unless it can be <br />shown that they exacerbate downstream <br />peak discharges. <br /> <br />If existing detention basins are not included <br />in the hydrologic analysis, discussions <br />should be provided in the report describing <br />the detention basins and reasons why they <br />were not considered in the analysis. <br /> <br />AUGUST 2002 <br /> <br />FLOODPLAIN DELINEATION <br /> <br />The hydrologic <br />analysis should <br />include detention <br />facilities <br />designed and <br />constructed with <br />the purpose of <br />impounding <br />water for flood <br />detention that <br />are owned, <br />operated, and <br />maintained by a <br />government <br />body. <br /> <br />CH4-207 <br />