7000 HARDESON RD 2020-06-05 - Laserfiche WebLink

The area of pavement repair around the administration building is not a good candidate for permeable pavement as there is a high ground water issue in this location. Ground water currently percolates up through the pavement to the surface which has caused the pavement to degrade and precipitated the need to replace the pavement as part of this project. o Rain Gardens/Bioretention- This is a maintenance project which consists of replacement of existing pavements with no changes to the existing established flowpath. Rain Gardens and Bioretention facilities would require significant changes to the flowpaths, conveyance and grading and are not feasible for this maintenance project. The existing stormwater collection system on the site consists of trench drains and other shallow pipes and these high connection elevations preclude connection from a properly functioning bioretention facility. There are no locations on the site where bioretention facilities could be located where they would receive tributary runoff as the original site grading and conveyance system was not designed to accommodate bioretention or flow directed towards the planters. The pavement replacement around the administration building has visible groundwater at the surface. Bioretention is infeasible in areas of high ground water. • Sheet Flow Dispersion in accordance with BMP T5.12: Sheet Flow Dispersion, or Concentrated Flow Dispersion in accordance with BMP T5.11: Concentrated Flow Dispersion. This project consists of replacement of existing pavements with no changes to the existing established flowpath. These dispersion techniques all require a redirection of the flow into a dispersion area which is thus infeasible for this project. In addition, the site is already built out and there are no available dispersion areas. • 8. Conveyance System Modifications The storm drainage work in this project includes: • Reestablishing grade so that surface flows enter into the existing trench drains in the north asphalt parking lot, • Removal of the majority of trench drains in the concrete bus staging area, • Replacement of the flow splitters with new redesigned structures that include spill control on the outfall pipes. Within the concrete bus staging area, approximately 2,400 linear feet of trench drain will be removed and replaced with a surface gutter and catch basin inlet collection/conveyance system. The trench drains will be replaced with a wide shallow surface gutter in the pavement which has catch basins frequently spaced to collect the runoff. The new catch basins and pipe convey stormwater to the eastern side of the site where they combine with a mainline heading north. At each junction connection to the mainline, there is a flow splitter which allows the water quality flowrate to continue onsite to the north and the higher flows to overtop a weir and discharge to Merrill Creek. Merrill Creek Stormwater Code Compliance—Community Transit 5