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<br />ApPENDIX C <br /> <br /> <br />CONSULTING ENGINEERS I LAND SURVEYORS <br /> <br />2150 Hwy. 6 & 50, Grand Junction, CO 81505.9422' 970/242-5202. FAX 970/242.1672 <br /> <br />January 22, 2003 <br /> <br />Silt Water Conservancy District <br />P.O. Box 8 <br />Silt, CO 81652 <br /> <br />ATTN: <br /> <br />Scot Dodero, Manager <br /> <br />RE: <br /> <br />Harvey Gap Dam Seepage, Preliminary Cost Estimate <br /> <br />Scot: <br /> <br />As requested, we have reviewed the "Report on Investigations into the Harvey Gap Dam <br />Outlet Tunnel Leakage" dated October 4,2002 by Martin and Wood as well as John <br />Blair's August 13, 2002 inspection report, his October 4, 2002 review notes on the Martin <br />and Wood report and your dam inspection/monitoring data for the period from January, <br />2001 through August, 2002. The purpose of this review was to provide a rough estimate <br />of anticipated costs for a likely scenario to remediate the seepage concerns. <br /> <br />The Martin and Wood report provides a fairly comprehensive description of site geology <br />which is very helpful in evaluating the subsurface conditions which control the seepage <br />flows. We have a few observations based on our review of the above documents: <br /> <br />1. A dam embankment/foundation piping failure can be rapid and uncontrollable with <br />little advance warning. Because of these potentially catastrophic consequences of a <br />piping failure, any significant seepage which could conceivably result in removal of <br />embankmrnt or foundation material must be addressed. Unless clear, strong and <br />conclusive evidence can be provided that there is no possibility of such piping, it must <br />be assumed that tlle potrntial exists. Therefore, the State Engineer's concerns seem <br />legitimate. The following considerations and obseIVations might suggest a minimal <br />potential for piping removal of embankment/foundation material: <br /> <br />a. The Martin and Wood report indicated that the existence of continuous joints <br />through the geologic horizon that most likely provides the seepage conduit is <br />un1ikely. Piping of soil through this zone would therefore be improbable. <br /> <br />b. The Martin and Wood report also suggested, based on their observations of the <br />exposed rock in the outlet tunne~ that internal erosion of the sandstone bedrock <br />seemed un1ikely. <br /> <br />c. Over the relatively long period of operation of the dam and reservoir, there have <br />apparently been no surface expressions of embankment/foundation removal <br />, reported. <br />