7000 HARDESON RD 2020-06-05 - Laserfiche WebLink

2. Description of Work The project consists of spot repaving within the north and south employee parking lots, concrete panel replacement within the bus parking area, and replacement of the existing trench drains within the concrete bus parking area with a gutter and catch basin collection system.The conveyance system from the bus staging area to the onsite water quality treatment vault will be replaced including five flow splitters and their high-flow outfalls to Merrill Creek. Downturned elbows will be added to the outfall pipes to provide spill control.The work associated with this project consists of repaving existing pavements with no increase in impervious surfacing and no change in storm drainage runoff characteristics. There will be approximately 7.4 acres of pavement replacement total. Within the north parking lot, areas of asphalt paving will be replaced adjacent to the existing concrete trench drains since the pavement has settled in relation to the top of the trench drain. In addition, asphalt will be removed and replaced in areas where tree roots have heaved the pavement. Cracked and decayed asphalt will be crack sealed or replaced. The entire north parking lot will be seal coated and restriped. The project also includes miscellaneous sidewalk, curb and landscape replacement. Within the south parking lot, areas of degraded asphalt will be removed and repaved with a thicker asphalt section. Much of the existing pavement in this area is very thin and there is year round water percolating up through the thin pavement section. The asphalt that has settled adjacent to the trench drain will also be removed and replaced and the grade corrected to allow drainage to freely enter the trench drain.The entire south parking lot will be seal coated and restriped. The ADA ramps in front of the Administration building will be reconstructed. There will be extensive concrete panel replacement to the bus staging area in the middle of the site.The project will replace existing concrete panels with a thicker pavement section.The bus staging area sheet flows into three trench drains which extend across the site in the east-west direction.The concrete pavement adjacent to the trench drains is spalling and is in need of repair. The project proposes to replace the trench drains with a concrete gutter instead.The gutter flow line will include several catch basin inlets to collect the storm drainage in lieu of the trench drains. 3. Existing Conditions Summary Onsite stormwater is collected by approximately 4,500 linear feet of 7-inch and 11-inch wide trench drains and 1,500 linear feet of 12-inch and 15-inch diameter CMP pipes. The trench drains generally drain to the east side of the site where the CMP pipes then route drainage to the north. The main conveyance system on the east side of the site contains a series of five flow splitter catch basins which were designed with an orifice restricted pipe going to the water quality system to the north and a high flow weir wall which when overtopped goes to an overflow pipe which discharges into Merrill Creek.The water quality flows are conveyed north onsite to the wet vault which then discharges into a Contech Stormfilter Vault before discharging into Merrill Creek at the north boundary of the site. The five overflow outfalls into Merrill Creek have a valve at the end of the pipe on the creek side. In recent years, all the valves have been left closed due to owner concerns over the lack of spill control in the flow splitters as well as deterioration of the flow splitters causing them to allow low flows to discharge out the bypass pipe. Offsite flows consist of storm drainage runoff from the vegetated hillside on the west side of the site is collected in an interceptor ditch and/or french drain. The northern half of the interceptor ditch drains northerly to a catch basin Merrill Creek Stormwater Code Compliance—Community Transit 2