Agenda - Council - 07/27/2021 - Laserfiche WebLink

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CC Regular Session Meeting Date: 07/27/2021 Submitted For: Len Linton, Engineering/Public Works By: Len Linton, Engineering/Public Works Information Title Consider Approving Modification for COR Infiltration Basin Pumping Station Inlet 5.3. Purpose/Background: The COR Infiltration Basin (Improvement Project 18-09) was designed to capture the first inch of runoff after a rainfall from properties in the COR. The design anticipated the normal water level in the pond would be at the top of the lift station inlet structure installed in the pond and water would flow over the top of the structure into the pumping chamber so the pumps could pump the water into the infiltration basins. The pumps were activated in July 2020 and tested for several weeks to ensure they were working properly. Review of the operation logs indicated the pumps were cycling on multiple times an hour. Review of the level meter for the input well indicated the level in the wet well would drop below the pump cutoff level so the pumps would stop until the level increased. This was traced to insufficient flow into the pumping station if the water level was below the rim of the structure, then the only water that entered the structure was from seepage through the joints of the structure. Staff had based the design of the inlet structure on several stormwater lift stations that were designed to provide relief for stormwater ponds without gravity outlets. Water reaching those ponds would build up until it was pumped out. The COR pond has an outlet that allows stormwater to flow out once stormwater enters the pond. Staff reviewed the design and determined that an inlet opening is required below the water surface in the pond to allow water to reach the pump chamber regardless of the level of the pond. It was also determined that the level of the pond outlet should be raised to retain the 1 inch storm, which will allow the pumps to remove the required volume of water before it leaves the pond and flows to the river. The City utilized the services of Bolton and Menk, Inc for designing the pumping system and controls. Staff consulted with Bolton & Menk, Inc. to verify that the proposed changes will work. Observations/Alternatives: Staff prepared an estimate for the required work which will entail lowering the water level of the pond while work is proceeding, drilling an 18" diameter hole in the wall of the inlet structure, and constructing a wall on top of the current outlet structure wall. The new hole will allow water to reach the pump chamber regardless of the level of the pond. The new wall will retain the 1 inch storm in the pond while allowing the pumps to move the water to the infiltration basins constructed north of the pond. The new wall will have a small opening that will allow the pond level to return to normal over time. The work will also include installing a grate over the underwater opening to keep muskrats out of the structure. Staff submitted the estimate to two contractors for quotes. The quotes are attached and are summarized below: Reshetar Systems , Inc. $ 22,500.00 Dave Perkins Contracting, Inc. $ 18,700.00 Funding Source: Funding for this project will come from the Stormwater Management Fund.