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- 13 - <br />SPILLWAY OONSTRUCTION <br />• We understand all reservoirs will be provided with service <br />spillways to pass normal flows sad the emergency spillway will operate <br />only during high flow. Open channel emergency spillway construction <br />will primarily utilize cut sections into the natural ground at <br />embanlrntent abutments. Due to topographic features, it will be <br />necessary to construct emergency spillways over the embankment in some <br />instances. The subsoils encountered during our investigation are <br />moderately susceptible to erosion when subjected to concentrated <br />flows. To reduce erosion and maintain a stable flow surface, rock <br />riprap will be required in the emergency spillways. Spillways located <br />at the abutments should be provided with a 6-inth layer of granular <br />material beneath the riprap for bedding. This will provide an even <br />surface for placement of the riprap and reduce undermining of the <br />riprap. The riprap layer should be at least 10 inches in thickness <br />and consist of angular rorJc fragments with at least 708 of the mass <br />6 to 10 inches in dimension and 958 larger than 1 inch. <br />Where the spillway flows over the embanlmient, 14 inches of <br />riprap is recommended with at least 70~ between 8 and 14 ind~es in <br />dimension and 958 larger than 1 inch. Placement of riprap should be <br />extended from the upstream face of the embanlanent to at least 10 feet <br />beyond the embanlanent crest centerline. Spillways over the embank- <br />ments should be provided with a riprap stilling basin to prevent <br />erosion at the downstream tce. <br />A possible alternative to riprap placement within the spillway <br />is the use of a concrete control wall installed well below the invert <br /> <br />Revised 11-80 <br />