2011-12-16_REVISION - M1977572 (71)

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I. INTRODUCTION 1.1 Background Since the mid 1970s Valco, Inc. ( Valco) has successfully conducted gravel mining activities in the Arkansas River just north of the City of Lamar, Colorado. The subject property, owned and mined by Valco, is located in Prowers County, Colorado extending east approximately one -mile from the downstream side of the US Highway 50 Bridge. A vicinity map of the subject property is provided in Figure 1.1. Currently, all gravel mining activities on the property have ceased and Valco is looking to fulfill floodplain and mining reclamation obligations. The Valco property includes a one -mile section of the Arkansas River, four open gravel pits located south of the river's right bank, and an un -mined section on the north side of the river. The property is also bisected by an abandoned section of the BNSF Railroad. The BNSF Railroad right -of -way was relinquished to Valco at the time of its abandonment. Figure 1.2 provides a site map of the subject property and the study reach along with naming conventions of each pit and their surrounding berms. The subject property is located within a Federal Emergency Management Agency (FEMA) regulatory flood pla in. 1.2 Purpose of Study In August of 2008, Anderson Consulting Engineers, Inc. (ACE) of Fort Collins, Colorado was contracted by Valco, Inc. to conduct an initial floodplain evaluation of the property in order to identify potential regulatory issues associated with past mining activities. A Phase I study was completed and documented in a letter report provided to Valco entitled "Hydraulic Analysis and Stability Evaluation for the Lamar Pit on the Arkansas River" dated May 4, 2009. The Phase I study contained: • Pertinent information from previous studies; • Documentation of data collected including: (a) 2008 survey of the study area; (b) aerial photography; (c) USGS quadrangle mapping; and (d) as -built Highway 50 Bridge plans; • Evaluation of impacts to the regulatory 100 -year floodplain; and • Evaluation of erosive conditions during a 100 -year event at the Highway 50 Bridge. Conclusions of the Phase I study included: 1. The 100 -year water surface elevations have increased on upstream of and through the subject property. 2. Increases in water surface elevations are associated with the embankments (Berms A, B, C, D, and E as shown on Figure 1.2) on the upstream and downstream sides of the gravel pits. COVALCOO1_Phase 11 Report_082211.docx 1.1 ANdERSON CONSUITiNq ENg1NEERS, INC.