2007-08-10_PERMIT FILE - C1980007 (3)

Home Browse Search

The capacity of the pre-existing 12-inch cast-iron pipe portions of the modified low-level outlet works was calculated using the Darcy-Weisbach Equation for friction loss and coefficients for the minor losses. Calculations are included in Appendix D. The 300-foot length of the existing pipe is the controlling factor in the capacity of the outlet works. The new 24-inch reinforced concrete pipe (RCP) downstream of the new drop structure was sized to avoid backwatering the outlet of the cast- >,ron pipe. The friction factor for the existing cast-iron pipe was increased to reflect its age. The approximate decrease in capacity was also verified by comparison with the Hazen-Williams coefficient CH used for new (130) and old (100) cast-iron pipes. The friction loss coefficient of 12 was combined with the minor losses of 1.85. Available head between the pond level at the spillway outlet (Elev. 7330.2) and the end of the 12-inch cast-iron pipe (Elev. 7272.6) is 57.6 feet. The resulting velocity is 16.5 feet per second (fps) for a discharge of 12.9 cubic feet per second (cfs). Section 5.9.6.2.1 of the Rules and Regulations for Dam Safety and Construction of the Colorado Department of Natural Resources requires that "the outlets for High Hazard dams shall be capable of releasing the top five feet of the reservoir capacity in five days, ..." The outlet capacity ranges from 12.9 cfs (25.6 acre-feet/day) at the spillway elevation to 12.3 cfs (24.5 acre-feet/day) when the reservoir water level is five feet lower. Since the surface area of the pond at the spillway level is approximately 21.5 acres, the modified outlet meets the requirement. The new drop structure consists of an 8-foot-diameter concrete manhole. The 12-inch cast-iron pipe enters the side of the manhole at Elevation 7272.6 and the 24-inch RCP leaves the base of the manhole at Elevation 7,266. Because the 12-inch cast-iron pipe discharge jet would hit the far wall of the manhole at maximum discharge, the portion of the manhole wall subject to such impact will be covered with a steel plate. The velocity through the trash rack at the design discharge of 12.9 cfs was conservatively estimated to be 0.6 fps, which results in negligible head loss. As akeady noted, the new 24-inch RCP downstream of the drop structure was sized to carry the discharge of the cast-iron pipe. For the design discharge of 12.9 cfs with the reservoir at the spillway invert, the RCP will flow approximately one-half full with a velocity of 7.4 fps._ The required head in the drop structure to create this flow was checked for both barrel control and inlet control. Inlet control was found to be critical with a head in the manhole of 0.4 feet above the RCP pipe crown. The final portion of the low-level outlet system is the plunge pool (see Drawing C-1.16). It was designed using the method presented in the Maine Erosion and Sediment Control BMP-3/2003 P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 9