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<br />SECTIO.TWO <br /> <br />Existing Conditions Of Stormwater Management <br /> <br />This section discusses the current operational conditions and financial mechanisms and includes <br />a general discussion of the hydrology and drainage systems in the Grand Valley, as well as the <br />problems and needs that have led to this report. The problems and needs include the significant <br />gaps and overlaps of services provided by the five entities. Alternative methods to address these <br />gaps and overlaps were a primary focus of the Steering Committee. <br /> <br />The concept of "Level of Service" (discussed in detail in Section 4.4) for stormwater <br />infrastructure, defmed as the rainfall event that the system can accommodate without significant <br />hazards or problems, is discussed in detail in Section 4. In general, the Level of Service <br />provided in the valley was determined to be inadequate. There is a need to provide protection of <br />property, the safety of residents, and to address federal regulations. <br /> <br />2.1 HYDROLOGY AND DRAINAGE SYSTEM <br /> <br />The Grand Valley contains many basins that are tributaries to the Colorado River, and 28 of <br />these basins effect the urban areas of the valley. Although the area only gets approximately eight <br />inches of precipitation per year, flash flooding is common due to the meteorological and <br />topographical conditions. The basins on the south side of the river experience the greatest <br />flooding because they have relatively short basins with headwaters in the Colorado National <br />Monument with its steep cliffs and impervious slick rock. In addition, 95 percent of the storms <br />in the Grand Valley come from the southwest, and the storms start at the tops of the basins and <br />continue down into the valley. Therefore, when the flows from rainfall at the headwaters reach <br />the lower ends of the basins, the storm is over the lower end and peak flows are significantly <br />increased. <br /> <br />Urban areas on the north side of the river see less <br />flooding than the south side due to the distance of <br />the BookCliffs and the storm pattern mentioned <br />above; however, when they do flood, they affect a <br />larger population base than south of the river. <br /> <br />Because the Grand Valley's land use was <br />primarily agricultural and progressed to urban <br />development, it relies on a system of agricultural <br />drains and natural washes for a significant <br />percentage of its stormwater conveyance. The <br />agricultural drains were constructed by the Bureau <br />of Reclamation to lower water tables and are now <br />owned, managed and operated by the Grand <br />Junction Drainage District (the District). When the <br />Grand Valley's land use changed from <br />agricultural land to urban land, the typical urban <br />drainage systems of collection channels, <br />conveyance pipes and defined outfalls to existing <br />water bodies were either not constructed at all or <br />were constructed in a piece-meal fashion without <br />an overall plan. Irrigation ditches were relied <br />upon for a significant percentage of the <br /> <br /> <br />Sediment Laden Flow from Colorado National <br />Monument <br /> <br />URS <br /> <br />T:\PROJECTS\22236022_GRAND_ VALLEY\5UB_OO\6.0]ROJ_DElIV\FINAL REPORT\FINAl REPT REV 4.DOC\9-JUL-03\\ 2-1 <br />