Y--PISTOL CREEK LAKE SEDIMENT REMOVAL

Greenbelt Lake is located in Bicentennial Park, a popular municipal park in the center of the City. This is a 5.4 acre lake that is located adjacent to the public library and serves as a centerpiece for Bicentennial Park. The existing control structure at the lake consists of a concrete gravity weir. The weir is about 100 feet wide. Elevation drops about 14 feet from the lake impoundment to the Pistol Creek channel below the dam. The dam is equipped with two low-level screw gate outlets to allow drawdown of the lake. The lake has suffered detrimental impacts due to severe sedimentation and a large population of nuisance wildlife (Canada geese and various duck species). These waterfowl add large quantities of nutrients and bacteria to the lake, negatively impacting the quality of the aquatic environment. The proposed project would excavate sediment from Greenbelt Lake and relocate most of the waterfowl. Prior to sediment removal activities, the lake will be drained by siphoning the water around the dam. The inflows from Pistol and Brown Creek will be diverted through temporary piping to the spillway south of the dam to prevent an increase in turbidity downstream of the lake. Approximately 2,230 feet of piping will be required to divert the water. Cofferdams, approximately 7 feet high, equipped with outlet pipes at the base, will be placed at the outfalls of Pistol and Brown Creeks as a detainment area for the inflows prior to discharge through the temporary piping system. The exact height and location of the cofferdams will be determined through site specific measurements. Cofferdams will be placed in such a way as to maximize the lakebed area available for excavation and facilitate connectivity with the shoreline. The cofferdam system will incorporate the use of nylon or polyester reinforced fabric to create watertight seals with the existing shoreline. A flume boot will be installed to create a watertight seal around the discharge pipe. Any necessary reinforcement within the substrate support system will be evaluated and remedied by a certified installation technician. Conventional mechanical equipment, such as excavators and bulldozers, will be used to remove sediment from Greenbelt Lake. The sediment will be de-watered at designated locations within the project site. The sediment will then be trucked to the local landfill for disposal. Temporary bridges will be utilized throughout the lake as crossing areas for the excavation equipment. A sediment weir structure will be constructed at the small island near the upstream end of the impoundment. The purpose of the weir is to facilitate sediment deposition in the upstream end of the lake. This will trap and remove sediment during high flows before it can be transported downstream. The weir will consist of large-sized boulders (3ft x 2 ft) to create a natural boulder wall. The top elevation of the weir will be approximately 1 foot above the water surface. To provide additional erosion protection and enhanced fisheries habitat, large cobblestones will be placed on top of geotextile fabric at the lake bed. The upstream impoundment area will be used as a sediment collection area, which will require periodic maintenance at approximately IS-year intervals. The Sponsor is aware of this requirement. Greenbelt Lake, Alternative GL - 3. Excavate to 8-foot depth in all areas, with steep