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<br />. <br /> <br />All drainage basin characteristics that <br />affect the rainfall-runoff relationship <br />shall be documented, including, but not <br />limited to, delineation of basin and <br />subbasin boundaries, size, shape, length, <br />slope, general aspect, elevation extremes, <br />time of concentration, land use, and soil <br />types and compositions. <br /> <br />When actual precipitation records of major <br />recorded storm events' are available from <br />area weather stations, such data shall be <br />used in conjunction with rainfall data <br />published in "NOAA Atlas 2, Precipitation- <br />Frequency Atlas of the Western United <br />States, Volume II I -Colorado, " 1973, or <br />other sources. <br /> <br />d. When less than 10 years of streamflow <br />records are available, the selected flood <br />frequency flows shall be calculated by <br />comparison to similar watersheds and by <br />estimates using hydrologic models and <br />precipitation records. <br /> <br />. <br /> <br />3. Drainaqe Basin Conditions. The hydrologic <br />analysis shall be based on drainage basin <br />conditions that will exist in the community at <br />least 12 months following completion of the <br />report, except if future urbanization of a <br />basin is likely to occur, the hydrologic <br />analysis shall be based on foreseeable <br />development in the basin. <br /> <br />4. Flood Control Structures. Where construc- <br />tion of publicly owned, operated and <br />maintained flood control structure will not be <br />completed within 12 months following <br />completion of the report, but adequate <br />progress has been made, the impact of the <br />project shall be included in the hydrologic <br />analysis. <br /> <br />5. Detention. The hydrologic analysis shall <br />consider the effects of on-site detention for <br />rooftops, parking lots, highways, road fills, <br />railroad embankments, diversion dams, refuse <br />embankments (e.g., solid waste disposal <br />facilities), mill tailings impoundments, <br />siltation ponds, livestock water tanks, <br />erosion control dams, or other structures <br />only if they have been designed and <br /> <br />. <br /> <br />-13- <br />