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HANSEN PARK 7 <br /> <br />2. Excavation of 67,631 CY (bank yards) of sediment <br />The average depth of dredging is approximately 4 feet. Deeper dredging is planned upstream (south) of <br />the island to create a forebay to capture the majority of the sediment load. This location will provide easier <br />access for future maintenance. The island will be reshaped to create a stagnation point for water entering <br />the pond from RCD 2 and increase the flow path length and residence time. <br /> <br />5 HYDRAULIC ANALYSIS <br />5.1 EXISTING HYDRAULICS <br />RCD 2 conveys stormwater from a 4,675 acre watershed through Hansen Park. Due to the size of the <br />watershed compared to the surface area of the pond, the pond in Hansen Park is subject to substantial <br />increase in elevation from the normal pool (i.e., bounce) from large storm events, in spite of the width of <br />the existing dam crest. The 2-, 10-, and 100-year, 24-hour rainfall events at Hansen Park were simulated <br />using the RCD 2, 3, & 5 Storm Water Management Model (SWMM) developed by the RCWD through the <br />District Wide Modeling Program in 2011. The RCD 2, 3, & 5 SWMM model was developed through the <br />propriety software “InfoSWMM” and uses the SCS-method for determining hydrologic flows. The 10-year, <br />24-hour rainfall event (4.2-inches) at Hansen Park is contained within the secondary berm at an elevation <br />of 878.2. However, for larger rainfall events the secondary berm is overtopped, at which point the three <br />42-inch by 52-inch arch pipe culverts and embankment at 7th Street NW becomes the hydraulic control. <br /> <br />The 100-year, 24-hour rainfall event (7.5 inches)2 results in a peak water surface elevation of 881.01, and <br />is considered the critical-duration flood event per RCWD hydraulic modeling standards. Two existing <br />residences are inundated by the floodplain along the west side of the park. One residence immediately <br />east (and downstream) of the existing dam is well below the 100-year, 24-hour rainfall event peak flood <br />elevation. <br /> <br /> <br />2 Based on the National Oceanic and Atmospheric Association (NOAA) Atlas 14 publication.