The Parking Lot That Drains to the Wrong Place
A 1.8-acre asphalt parking lot drains to a grass island and then to a 24-inch storm sewer 300 feet east. The lot slopes at 0.8% toward the island, and ponding 4 to 6 inches deep has been reported after storms. Soils are 65% clay with an infiltration rate of 0.05 inches per hour. The local manual requires retaining the 2-year, 24-hour storm onsite with a 25% safety factor; the rainfall depth is 2.6 inches. The existing lot has 62 parking spaces, and the client wants to keep all of them plus add four ADA spaces. A proposed bioretention area is 6,000 square feet with 18 inches of ponding and a 12-inch engineered soil layer. The existing island is 0.15 acre. The storm sewer invert is 3.5 feet below the island surface. The geotechnical report warns of expansive clay and a seasonal high water table at 6 feet. The site is in a freeze-thaw region.
1. Calculate the required storage and determine whether the proposed bioretention area is adequate.
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Use the rational method for a conservative check. Area equals 1.8 acres, or 78,408 square feet. Rainfall equals 2.6 inches, or 0.2167 feet. Use C equals 0.9 for asphalt. Volume equals 78,408 times 0.9 times 0.2167, which is about 15,290 cubic feet. Add the 25% safety factor: about 19,115 cubic feet, or 0.439 acre-foot.
Provided storage: surface ponding is 6,000 square feet times 1.5 feet, or 9,000 cubic feet; engineered soil is 6,000 square feet times 1.0 foot, or 6,000 cubic feet; total 15,000 cubic feet, or 0.344 acre-foot. That is about 4,115 cubic feet short, roughly 21%, and it assumes all soil storage is usable. It also has no freeboard. The proposed cell does not meet the requirement. Infiltration at 0.05 inch per hour is negligible during the storm, so do not count it.
2. How should the grading and layout be revised to keep the parking spaces and meet ADA requirements?
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Set the island as a shallow basin with 6 inches of freeboard below the pavement, 3:1 side slopes, and a concrete overflow weir at the design elevation to the storm sewer. Because the lot slopes at 0.8% toward the island, the island is the low point; 4 to 6 inches of ponding means the pavement edge is too low. Raise the pavement edge or provide a curb and graded apron so water enters the basin without standing on asphalt.
Keep 62 spaces by re-striping to 9 by 18 feet and using compact stalls only where the code allows; add four ADA spaces with 8-foot access aisles and 11-foot van spaces at less than 2% slope in all directions. Check the accessible route from each ADA space to the building entrance; it cannot cross the basin or a curb without a ramp. If the accessible route conflicts with the basin, shift the basin, not the route. Confirm the overflow elevation leaves the storm sewer invert below the weir.
3. What geotechnical and design changes are needed for the expansive clay and high water table?
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Do not rely on infiltration. The clay and the 0.05 inch-per-hour rate mean the native soil acts as a liner. Design a lined bioretention cell with an underdrain to the storm sewer, or a detention basin with a controlled outlet. Over-excavate at least 12 inches below the basin and replace expansive clay with engineered soil; verify the subgrade before placement.
Because the seasonal high water table is 6 feet below the surface, keep the basin bottom at least 2 to 3 feet above it, or use an impermeable liner and underdrain. Add a pretreatment forebay and sediment sump. To meet the 0.439 acre-foot requirement, increase surface area to about 7,700 square feet at 1.5 feet of ponding, or use 2.0 feet of ponding plus 12 inches of soil. Confirm the invert, freeboard, and freeze-thaw durability of the outlet.