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Engineering Company March 18, 1996 Mr. Jay Kennedy Assistant Director of Public Works City of Roseville 2660 Civic Center Drive Roseville, MN 55113 Re: Bennett Lake Outlet Study Dear Jay: 8300 Norman Center Drive 555 West 27th Street Minneapolis, MN 55437-1026 Hibbing, MN 55746 Phone: (612) 832-2600 Phone: (218) 262-3465 Fax: (612) 832-2601 Fax: (218) 262-3460 At your request we have studied the installation of a pipe outlet from Bennett Lake to Legion Pond. You have indicated that you wish the pipe to conduct the base flows and smaller duration flood events from Bennett Lake. The existing pumped outlet is to remain operational and will be used in larger duration flood events. The pipe must not increase the flood elevation on Legion Pond. The target 10-year flood elevation for Legion Pond used in this study is Elevation 888.4. This study utilized inflow hydrographs to Bennett Lake, Legion Pond and Owasso Lake that were developed by SEH, using a TR20 runoff model. A model of Bennett Lake, Legion Pond and Owasso Lake and their interconnections were developed in XP-SWM. XP-SWM is the US-EPA's Stormwater Management Model with a graphical interface (XP). Version 4 (1988) of the SWM model was used in the study. The accuracy of the inflow hydrography developed by the TR20 model was not determined. We recommended that a 24-inch pipe be installed for the piped outlet from Bennett Lake. A 24-inch pipe with an upstream invert at Elevation 887.0, downstream invert at Elevation 886.6 and a length of approximately 500 feet will result in a flood elevation at Legion Pond of 888.4 in a 10-year hood event. >a'or ail practical purposes up to a 42-inch pipe would also achieve the same result. In order to increase the minimum flow velocity and reduce installation costs a 24-inch pipe is recommended. The design flood for the pipe outlet is a 10-year event. The pipe will not increase flood levels downstream in the design and 100-year flood. A summary of the flood elevations estimated by this study is presented in Table 1. The hydi•ographs fora 24-inch pipe connecting Bennett Lake and Legion Pond in a 10-year flood are enclosed. The XP-SWM model has a slightly higher flood elevation on Owasso Lake than the 1989 information in the surface water management plan. The outlet to Owasso Lake was modified in 1991. This study simulated a simplification of the new outlet and did not include any basins downstream of Owasso Lake. For these reasons the Owasso Lake flood levels are approximate only.