Drainage_and_Flooding_Final_Report-dated_06-16-2008

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Mr. Ron Bottoms Tune 16, 2008 Page 2 of 6 klotz associates event conveyance path for Creekmont Subdivision Section 2. It should be noted that survey will be needed for the detailed design of the selected alternative. The Creekmont Subdivision Section 2 has experienced several complaints of flooding and ponding water due to the limitation of extreme event conveyance paths to relieve the flows out to Big Island Slough. Using LiDAR-based contours (see Exhibit 2), the area known as Creekmont Subdivision Section 2 naturally drains to Big Island Slough but Creekmont Park currently restricts the overflow path. Creekmont Subdivision Section 2 has its own storm sewer drainage system that can handle a 3 year storm event but does not have an extreme event conveyance path for the large storm events. The storm events analysed for impact are the 5-, 10-, 25-, 50 -, 100-year stones since these frequencies generate sheet flow. The areas labeled "Additional Drainage Axeas" in Exhibit 3 are the areas of concern that this report addresses. Reconstruction of the park could consist of removing soil, regarding the park area and rebuilding the parr, adding an overland flow path that would require significant improvements for it to function properly. This idea was not discussed considering the park is in place, would be significantly expensive and the park is already being used by the community. Creekmont Subdivision Section 2 storm sewer system connects into a 42-inch pipe that is part of Creekinont Subdivision Section 1 (see Exhibit 4). The drainage limitations have been addressed by the different alternatives that are briefly described below. The profiles for the existing conditions and alternatives are it Appendix B Hydraflow Storm Sewer Output. Adding flow to the current existing state may have to be mitigated and the effect will be analyzed with HEC- RAS. A conceptual estimate of the pipes sizes and costs for each option can be found in Tables 3 to 5 and the detail cost estimated are in Table 6 to 8. 'These cost estimates are based on using a contractor to construct the improvements and the cost could be cheaper if part or the entire selected alternative were built by city force and if High Density Polyethylene (HDPE) Pipe were used instead of concrete pipe. HDPE pipe are only built up to a 60-inch and greater size pipes would have to be concrete pipe. i. Alternative I Alternative 1 consist of placing inlets at the entrance of the park to collect the sheet flow from a larger storm event and directing the flow with stonn sewer pipes that will connect to the storm sewer system in North H Street (see Exhibit 5) Alternative 1 is a separate system that will affect the