08-11-11 Planning Comm. Agenda - Laserfiche WebLink

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11. Existing Conditions The drainage system for the Park and Ride site is an urban roadway corridor with storm sewer serving the right -of -way and offsite drainage. The current site is comprised of vegetated ditches, trees, steep hills, and right -of -way grasses. The major receiving waters from the project are MacCarrons Lake and an unnamed wetland to the east of the site. The general drainage is north to the T.H 36 right -of -way, south to County Road B, and east to an unnamed wetland. The newly constructed SE ramp is the north boundary of the site drainage area with only the shoulder draining south. Geotechnical report provided by American Engineering Testing, Inc., dated April 26, 2011 (see Appendix B), reveals predominantly glacially deposited till classified as clayey sand to sandy lean clay. In addition, demolition -type fill is on the site consisting of concrete, bituminous, pile, wood, and ashes and cinders, clays, and clayey sands. Because of the soils and slow draining, ground water elevations were not present within the boring depths. Infiltration rates of the various soil types are SC or CL soils which have infiltration rates of 0.2 or less. Phase II Environmental Site Assessment Report provided by American Engineering Testing, Inc., dated April 19, 2011 reveals contaminated soils in the north of the property in association with the SE on -ramp construction. Because the western two- thirds of the site may have unregulated fill and the inaccessibility of the eastern one -third of the Site, a Construction Contingency Plan is recommended (a Draft CCP is available). lll. Jurisdictional Requirements The project was designed to meet Mn/DOT and Ramsey - Washington Metro Watershed District (RWMWD) requirements for storm sewer design and storm water management. The following summarizes the design standards: • Storm sewer system designed for a 10 -year return period. • Mn /DOT Drainage Manual (August 30, 2000) and applicable technical memorandums. • Proposed basins were analyzed for the 100 -year return period event to determine high water levels (HWL). • Peak rate of stormwater runoff from the project shall not exceed the predevelopment peak runoff rates for the 2 -year, 10 -year and 100 -year, 24 -hour storm events. • Volume shall be achieved onsite in the amount equivalent to the runoff generated from one inch of rainfall over the impervious surfaces. Hwy 36 and Rice Street Park and Ride Storm Water Narrative — 50% Review Set 7/20/2011 Page 1 3