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Mar 25 03 10:4oa Civil Resources LLC 3038332850 p.5 <br />Ms. Kate Pickford <br />March Z5, 2003 <br />Page Z <br />The strip of land running north-south between Pods 1 and 3 is proposed to have a finished <br />geometry including a 110-foot top width, 3 to 1 finished slopes, a total vertical height of <br />approximately 30 feet and contains a gas line within it. A geotechnical stability analysis for this <br />embankment was completed by Tetra Tech RMC and was found to be stable under static <br />conditions. The DMG's concern is more focused on the embankments ability to withstand <br />erosional and hydrodynamic forces during a significant flood event (10-year to 100-year return <br />frequencies) due to flow from Pod 1 into Pod 3 (i.e. over the proposed embankment). Front <br />Range rivers (and most continental United States rivers that are not mountain streams) operate <br />within the sub-critical regime of flows under normal conditions and therefore are downstream <br />controlled flows. Sub-critical flows "build" the resultant water surface elevation profile from a <br />downstream control or normal depth and the upstream water surface subsequently responds. <br />Therefore, the floodwater should fill Pods 1 and 3 as a side overflow weir simultaneously from <br />the Big Thompson River into each respective pit, not from upstream to downstream. Civil <br />Resources recommends that the side overtlow weir action be enhanced by providing a river <br />bank profile that is graded to have a consistent offset from the BFE's along Pods 1 and 3. <br />Further, Civil Resources recommends that the west end of Pod 3 be finished at a 5 to 1 slope <br />or flatter. <br />Please call with any questions or further Comments. <br />Sincerely, <br />CIVIL RE~SOU~RC~E~[SJ, LLC~~~ <br />~~'°~o""'mac. G/ !~> !/ " <br />Brad L. Hagen, P.E. <br />Civil/Water Resources Engineer <br />Encls: <br />C:\Civi I Resou roes\proJ ects\Hall-Irwin\Bemhardfldmg•responsel,doc <br />