2004-02-11_HYDROLOGY - M1977493 (3)

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Mr. Blair, P.E. February 11, 2004 Page 2 late August, the sediment from the downstream side of the cofferdam was removed using slurry pumps. Note that the cofferdam has been left in place to minimize future inundation of the outlet with sediment from the reservoir. The top of the cofferdam is about 47 feet below the level of the emergency spillway. The attached photos one and two show the completed cofferdam. A large amount of rock material was used in the construction of the cofferdam. This seemed to result in a stable structure as the downstream side of the cofferdam was dewatered and cleaned. Seepage through the structure was minimal, except for a significant seepage source on the right side of the cofferdam. This seep flowed approximately 100 gpm and appeared to be the result of a flow path through the riprap on the upstream slope of Robinson Dam. The seep did not appear to be eroding embankment fill from the dam. The process of removing the sediment and cleaning the outlet structure took several weeks and was completed in late September. The outlet structure was washed with water so that the condition of the structure could be evaluated. Sediment from inside the outlet conduit was removed using a hydro -jet pressure wash system. Video Inspection of Outlet After the inside of the outlet conduit was thoroughly cleaned, Freedom Sewer and Drain performed a video inspection of the conduit. A copy of the videotape of the inspection has been provided to you. The camera was inserted into the pipe from the upstream end. The inspection indicated that the conduit was in relatively good condition. The cement mortar lining that was installed in 1981 appeared to be sound, with no apparent spalling or deterioration of the lining. There were also no noticeable deformations, cracks, or discontinuities of the pipe that would indicate collapse, differential settlement, or other significant movement of the pipe.