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i <br /> I <br /> In a situation where the roadway at one of the culvert installations <br /> is overtopped, even if the road embankment and culvert should wash <br /> out, such failure will not materially add to the destructiveness of <br /> the flood that caused the initial failure. The risk to life and pro- <br /> perty in and downstream of the mine area is not substantially affected <br /> by the adequacy of the existing drainage system. <br /> I : <br /> An alternative to- the existing Road Systems Area drainage system <br /> described above would consist of construction of retention and/or <br /> detention ponds in the drainage sub-basins above the roads and cul- <br /> verts throughout the area. This alternative was rejected for the <br /> following reasons: <br /> a) The existing culverts have been handling the <br /> historic flows and are more than adequate <br /> for the specified design storm event. <br /> i . <br /> b) If a larger precipitation event occurs the <br /> roadway embankment will serve as a dam for a <br /> temporary detention pond when the inflow <br /> t exceeds the culvert capacity. <br /> c) Construction of retention or detention ponds <br /> r in areas above the roads and culverts would <br /> ' 46 <br /> l : ' <br />