2011-12-16_REVISION - M1977572 (71)

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W e 1660 SOUTH ALBION STREET • SUITE 500 • DENVER, COLORADO 80222 • (303) 757 -8513 Mr. William Eisenman General Manager VALCO, Inc. P.O. Box 591 Lamar, CO 81052 Dear *Bill: W�C ENGINEEfING, 1Nc. June 30 1987 WRC File: 1624/10 WRC Engineering, Inc. has performed a HEC -2 backwater analysis of the proposed Arkansas River realignment through the VALCO, Inc. property in Lamar, Colorado. The purpose of the analysis was to determine the impact of the realignment on the 100 -year floodplain and flood profile of the Arkansas River and was in response to the request from the USACE for such an analysis . . The HEC -2 analysis was performed for two conditions: (1) Condition 1, based on the mining activity at the time of the original . FEMA floodplain designation which used aerial mapping by Landmark Mapping Ltd. dated 1980 and (2) Condition 2, based on the proposed river realignment and mining plan. The 100 -year flood profile was computed for both conditions using a peak flow value of 40,000 cfs. Nineteen cross - sections were defined (see Figure 1) through an area 2,100 feet downstream and 1,900 feet upstream of the VALCO property. US Highwa 50 bridge was modeled using the normal bridge routine, while the railroad bridge was modeled as a normal channel section, due to the relatively limited stream flow restriction of the railroad trestle. The pits created by the mining were modeled by assuming the water surface elevation in the pond was the ground elevation, but with a lower Manning's N roughness coefficient. The cross- sections were.determined 'from the original floodplain mapping, supplemented by USGS, quadrangle elevations and field inspections. The channel inverts were obtained from the FEMA flood profile drawings. Condition 1, called the baseline run, was compared to the flood profile prepared by FEMA and good agreement was found (see Table 1). The differences in the flood profile elevations are due to the more detailed analysis by WRC Engineering, Inc. and the interpolation between the cross- sections used by the FEMA study. Next, condition 2 was evaluated and compared to the elevations of Cqndition 1 (see Table 1). For Condition 2, the cross - sections reflect the proposed spillway (L = 400'), the new channel cross - section, and the dike at elevation 3610. Upstream and downstream of the US 50 bridge, a maximum increase of 0.11 feet was calculated between Condition 1 and 2, which is considered within the accuracy of the analysis. In the vicinity of the US CONSULTING ENGINEERS