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 Page 20 All three scenarios result in similar peak flood elevations at Hansen Park for each of the modeled rainfall events, and result in no change to the 100-year, 24-hour rainfall event compared to existing conditions. (Note – the model results do show increases of 0.04 feet or less in the 100-year, 24-hour simulation, which are less than the expected model error {+/- 0.1’}.) The peak flood elevation is not affected by the project since the culvert at 7th St. NW (and not the dam structure) is the hydraulic control for this large of a rainfall event. In contrast, the peak water surface elevations for the 2 - and 10-year, 24-hour rainfall event will be increased by the three scenarios, by approximately 1.3’ and 0.8’, respectively. This will not affect any existing structures, but may increase the inundation period for portions of the existing trail system encircling the pond at their present elevation, of the trail is not raised or relocated. Currently, the lowest portion of the trail system is inundated for a period of 32 hours following a 2-year rainfall event (estimated at 2.8-inches of rainfall in a 24-hour period). Figure 7 shows the flood elevations for seven days following the start of a 2-year rainfall event for the proposed concept plan (Scenario 1). Reducing the flood duration to 24-hours following the 2-year rainfall event will require raising the trail to a minimum elevation of 877.7. Table 5 indicates the inundation durations of the trail at varies elevations for the 2-year rainfall. Table 5. Flood Inundation Duration for 2-Year, 24-Hour Rainfall Event (hours) Trail Elevation (feet Inundation Duration (hours) 876 39.9 876.5 36.4 877 32.3 877.5 26.9 878 20.1 878.5 5.0 4.1.2 10TH STREET NW (GARDEN VIEW APARTMENTS) 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 0.15’. 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 6. Table 6. Peak Water Surface Elevations Upstream of 10th St. NW (feet) Rainfall Event* Existing Conditions Scenario 1 - Proposed Concept Scenario 2 - Max Grading Extents Scenario 3 – Max Grading Depth 10-year 873.82 872.71 872.69 872.43 100-year 877.74 877.59 877.54 877.24 * 24-hour duration