2011-12-16_REVISION - M1977572 (71)

Home Browse Search

III. EXISTING CONDITION HYDRAULIC ANALYSES Analysis of the existing condition 100 -year floodplain was conducted and compared with the effective FEMA regulatory floodplain in order to identify the impacts of mining activities on the Valco property. The existing condition analyses were initially conducted in Phase I of the study. However, an updated analysis is provided in Phase II in order to incorporate additional survey data collected subsequent to the Phase I study. 3.1 Effective and Duplicate Effective Condition Floodplain Analysis In order to assess the impacts of the existing gravel pits and berms on the 100 -year FEMA regulated floodplain it was necessary to develop a duplicate effective hydraulic model. Since the original FEMA model was not available, the hard copy HEC -2 model documented in the 1987 WRC letter report was utilized as the effective model. Hard copy data of the WRC HEC -2 model was entered into the HEC -2 program and re -run to create a duplicate effective HEC -2 model. This included the regulatory 100 -year discharge of 40,000 cfs published in the City of Lamar FIS. A duplicate effective model was developed in order to verify that the entered HEC -2 data reasonably reproduced the results published in both the WRC letter report and the City of Lamar FIS. A map showing the WRC duplicate effective cross section locations and 1980 topography is provided in Figure 3.1. The duplicate effective HEC -2 model matched the 100 -year water surface elevations provided in the FIS to +/- 0.9 feet. The USACE HEC -2 hydraulic modeling program has been updated and replaced by the USACE HEC -RAS program (Version 4.0). The duplicate effective HEC -2 model was converted to HEC -RAS and re- run. Due to changes between the older HEC -2 and newer HEC -RAS hydraulic programs, differences of up to 1.2 feet were noted between the duplicate effective HEC -2 and duplicate effective HEC -RAS 100 -year water surface elevations. A few inconsistencies were identified in the duplicate effective modeling conducted by WRC including: (a) the identification of ineffective flow areas; (b) conveyance of flow through gravel pits; and (c) the method for modeling the Highway 50 Bridge. Based upon the best available information including models and maps, the duplicate effective condition 100 -year water surface would not overtop the Highway 50 Bridge and its approach roads. Furthermore, the 100 -year water surface elevations would be contained between the earthen abutments of the abandoned railroad bridge. Therefore, both the Highway 50 Bridge and abandoned railroad embankment would constrict the approximately 5,000 -foot wide floodplain to a width as little as 550 and 1,500 feet, respectively. Typically, ineffective flow areas are applied upstream and downstream of bridges and /or constrictions in the floodplain. Ineffective flow areas were not included in the duplicate effective model. In addition, it was noted that the duplicate effective model was accounting for a large portion of conveyance in the bottom of the gravel pits. Significant conveyance of flow through gravel pits is not available below the top of the embankment surrounding the pit. COVALCOO1_Phase 11 Report_082211.docx 3.1 ANdERSON CONsuLTiNq ENGINEERS, INC.