2008-08-21_REVISION - M1978091UG

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This document provides the details of the design features which comprise the proposed Deadman Gulch channelization project. Steve Bellz/BHC LLC/Hydrologist; was contracted to study the Deadman Gulch watershed system and propose a channel design that would route flows away from the Level 6 waste rock, and maintain the hydrologic balance of the site setting. The following information summarizes the site hydrologic conditions which lead to the design features, as well as the details of the channel design itself. Site Conditions Presently, an excavation reveals a seep area from which seepage water is routed into a ditch that contours around the hillside to the south then down the hillslope into Deadman Gulch. Observations by Stan Whinnery during the week of April 25, 2008 found that portions of the ditch embankment are sloughing into the ditch and the ditch is carrying an estimated 25 gpm. Several seeps along the ditch are currently contributing water to the ditch flow. The watershed area upstream of the ditch is approximately 40 acres of which about 85% is a Vulcan gravelly loam and about 15% is a Woodhall extremely rocky loam. Both soils are classified as well drained. The most limiting permeability layer in each is classified as moderately low to moderately high (0.06 to 0.20 in/hr). Local precipitation and intensity information from NOAA includes the following: Total Precipitation Precipitation Intensity Event (inches) (in/hr) 2-year/ 6-hour 0.89 0.15 2-year/ 24-hour 1.24 0.05 100-year/ 6-hour 2.13 0.36 100-year/ 24-hour 3.14 0.13 Based on the above, the two year runoff rate could be as much as 0.09 inches per hour during a 2-year/6- hour rainfall event or 0.30 inches per hour during a 100-year/6-hour rainfall event. From the 40 acre watershed, this could yield flow rates of 3.6 cfs and 12.0 cfs for the 2-year/6-hour and 100-year/6-hour events, respectively. However, the respective flow rates could be as low as 0.0 cfs and 6.4 cfs depending on the actual permeability condition of the watershed. Design and sizing of any water routing features will consider the above possible flow rates. Channel and/or conduit capacity will depend on travel distance, gradient and materials used. Proposed Design Components In order to abate the possible saturated slope conditions which were created from the existing geotextile channel, it was determined that a `culvert pipe' connection at the approximate mid-point along the length of the channel is a more suitable amendment, rather than simply attaching an additional feature to the end of the existing geotextile channel (Figure C.1). At the geotextile channel `joint', a cement-catchment