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HANSEN PARK 9 Stage (NAVD88) 10-year, 24-hour 531 495 -36 100-year, 24-hour 1,023 1,054 31 The proposed project will result in a 0.09’ decrease in the 100-yr, 24-hour rainfall event compared to existing conditions, to an elevation of 880.92. This is due to the lowered starting water elevation, increase in live storage and the change in weir shape. In contrast, the peak water surface elevations for the 2- and 10-year, 24-hour rainfall events will be increased, by approximately 0.99 feet and 0.52 feet respectively. This will not affect any existing structures. 5.2.3 HANSEN PARK POND – BELOW DAM Table 4 shows the peak water surface elevations downstream of the dam (and upstream of 7th Street NW) resulting from the 2-, 10-, and 100-year, 24-hour rainfall events. Table 4: Hydraulic Data and Critical Structure Elevations downstream of Hansen Dam Stage (NAVD88) Rainfall Event Existing Condition Proposed Condition Difference 2-year, 24-hour 873.94 873.53 -0.41 10-year, 24-hour 876.59 874.81 -1.78 100-year, 24-hour 881.00 880.73 -0.27 Discharge (cfs) Rainfall Event Existing Condition Proposed Condition Difference 2-year, 24-hour 173 147 -26 10-year, 24-hour 278 225 -53 100-year, 24-hour 359 348 -11 One residential structure is located along RCD 2 between the Hansen Park dam and 7th Street NW. This structure is inundated by the 10-year, 24-hour flood under existing conditions. The proposed project will lower the 10-year flood elevation such that the structure is no longer inundated by this flood event. However, the 100-year flood will continue to inundate the structure by several feet. 5.2.4 10TH STREET NW Garden View Apartments, located just upstream (south) of 10th Street NW along RCD 2, was one of the many properties that experienced flood damage during the July 2011 flood event (roughly equivalent to the 100-year 24-hour rainfall event computed by the RCWD). By attenuating the early runoff volume at Hansen Park, the proposed concept plan will reduce the peak flood elevation at this location by approximately 0.20 feet for the 100-year 24-hour rainfall event. This project will have a more impact for smaller, more frequent events, reducing the peak flood elevation for the 10-year, 24-hour rainfall event by over a foot. The peak flood elevations upstream of 10th St. NW are tabulated in Table 5. Table 5: Hydraulic Data and Critical Structure Elevations upstream of 10th St. NW Stage (NAVD88) Rainfall Event Existing Condition Proposed Condition Difference 2-year, 24-hour 871.17 870.79 -0.38 10-year, 24-hour 873.87 872.73 -1.14 100-year, 24-hour 877.84 877.64 -0.20