2011-12-16_REVISION - M1977572 (71)

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Conservative water surface elevation modeling procedures limit the effective pit bottom elevation to the elevation of the surrounding embankment while reflecting relatively efficient conveyance (i.e., low Manning's roughness value) of flow over the impounded water in the pit. 3.2 Corrected Effective Floodplain Analysis A corrected effective HEC -RAS model was developed by modifying the duplicate effective HEC - RAS model. Ineffective flows upstream and downstream of the Highway 50 Bridge and abandoned railroad embankment were incorporated. In addition, conveyance through the bottom of the gravel pits was removed and limited to the elevation of the surrounding berms. The HEC -2 bridge modeling methods utilized at the Highway 50 Bridge were converted to bring them up to current HEC -RAS modeling standards. 3.3 Existing Condition Floodplain Analysis An existing condition HEC -RAS model was developed using the 1 -foot contour interval topography collected by KSI. Topography utilized in the HEC -RAS model outside the KSI detailed survey limits included: (a) 2 -foot contour interval topography utilized in the WRC study, and (b) 10 -foot contours obtained from the USGS Quadrangle mapping. An existing condition cross section location map is provided in Figure 3.2. 3.4 Results and Conclusions A comparison of the 100 -year water surface elevations for the effective, duplicate effective, corrected effective and existing conditions are provided in Table 3.1. A comparison of the corrected effective and existing condition results indicates that the existing configuration of the gravel pits is causing an adverse impact to the regulatory 100 -year water surface elevations through the study reach. The existing water surface elevation on the upstream side of Highway 50 is a maximum of 3.1 feet higher than the corrected effective condition, resulting in overtopping of Highway 50 to the north of the bridge opening. The 3.1 -foot increase in the 100 -year water surface elevation upstream of the Highway 50 Bridge is primarily attributed to the configuration and elevation of Berm A and to a lesser extent Berms B through E. The capacity of the Highway 50 Bridge during the 100 -year flood event has decreased from 40,000 cfs in the effective condition to approximately 32,000 cfs in the existing condition. Concurrently, the average 100 -year flow velocity through the bridge opening decreased by approximately 1 foot per second between the effective condition and the existing condition. COVALCOO1_Phase 11 Report_082211.docx 3.3 ANdERSON CONSULTING ENGINEERS, INC.