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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Lower Big Dry Creek Hydrologic Study <br /> <br />Banks dry out at low flows, and at high flows, water moves into the pore spaces in the <br />soil, reducing pore space tension, and the soil becomes less cohesive and erosion prone <br />(Bledsoe 2001). In order to balance the sediment transport capacity of the high flow, <br />sediment is removed from the banks of the stream. As sediment is removed from the <br />bank, surface erosion, undercutting and sloughing can occur. As the flow recedes, the <br />banks dry again" and the cycle repeats itself (Knighton 1998). <br /> <br />4. Significant urbanization has occurred and will continue to occur in the central portion of <br />the Big Dry Creek watershed. The fact that urbanization significantly alters the <br />frequency, duration and peak flows of natural runoff events is a well documented <br />phenomenon that is true for Big Dry Creek. The use of regional and on-site detention can <br />help to control peak flows; however, increased frequency and duration of runoff events <br />will occur, even when detention facilities are used. <br /> <br />5. Limitations associated with the period of record for both stream gages in the watershed <br />and accuracy limitations of the lower stream gage on Big Dry Creek make it difficult to <br />conduct a rigorous statistical analysis that quantitatively defines hydrologic changes in the <br />watershed. Nonetheless, photo documentation and anecdotal accounts clearly show that <br />lower Big Dry Creek is experiencing erosion, high water and flooding. Based on photos <br />provided immediately following the July and August storms and the field visit in 2005, <br />much of the major erosion on the creek is attributable to the large storms during the <br />summer of 2004. <br /> <br />6. The precipitation associated with the types of storms that occurred in July 2004 should <br />not be considered typical or common. Even in the absence of urbanization, this type of <br />storm would have likely produced flood conditions and erosion in the lower watershed. <br />Many of the impacted areas are located within the 100-year floodplain, as mapped by <br />FEMA in 1979 and 1982, prior to the development boom of the 1990's in the urbanized <br />portion of the watershed. <br /> <br />7. The stormwater management strategies implemented in the urbanized portion of the <br />watershed are consistent with the regional and national state of the practices. The vast <br /> <br />971-179.092 <br />June 2005 <br /> <br />Wright Water Engineers, Inc. <br /> <br />Page 4 <br />