Spillway Replacement Plans
The existing Landmark Dam is located in Raleigh, North Carolina on an unnamed tributary to House Creek. The dam was constructed prior to 1965. This project is to bring into compliance an existing high-hazard dam per the NC Dam Safety Law. The lake formed by this dam is approximately two acres and is split between two properties where future redevelopment is desired. The existing dam is smaller than most jurisdictional dams within the state but is under the NC Dam Safety purview, listed as WAKE-333 and listed as a high-hazard potential structure, due to its risk to downstream structures, roads and properties.
For this project McAdams will complete the rehabilitation design and permitting of the dam through the State. The existing bituminous-lined Corrugated Metal Pipe (CMP)from the existing riser to the connection with the concrete pipe was beginning to degrade and there was no longer any need to maintain a permanent pool in the dam. Since the original construction of the dam pre-1965 dam safety regulations had changed and the current riser structure and primary spillway pipe could not handle the necessary design storm. McAdams coordination with the adjacent development is critical to intertwining the spillway, dam and development designs. Due to the proximity to the adjacent development, retaining walls were used by the southern adjacent development to maximize the available space. The above-ground CMP riser structure was removed and replaced with a concrete riser structure sunk into the lake bottom to eliminate the normal pool and to keep the upstream primary spillway pipe at a similar slope to existing conditions. McAdams proposed upgrading the run of CMP pipe to RCP to improve flow conditions such that the spillway could meet the requirement of passing the 1/3 PMP (Probable Maximum Precipitation). McAdams designed an articulated cradle to guard against pipe separation and minimize future maintenance efforts as well as a spillway filter to catch any seepage which may occur during attenuating storm events. The existing connection between the CMP pipe and downstream RCP was upgraded from just a grouted-up joint, to a manhole structure connecting the new and existing RCP, which can be utilized as and access point for any inspections or maintenance.