A bioretention basin cross-section is excavated into the existing subgrade and backfilled with layered media for stormwater management. The bottom layer consists of a twelve-inch thickness of three-quarter-inch washed gravel containing a six-inch perforated PVC underdrain pipe. Overlying the gravel bed is an eighteen-inch depth of engineering soil mix composed of fifty percent sand, twenty-five percent topsoil, and twenty-five percent compost. A three-inch topsoil layer topped with sod finishes the top surface within the sloped basin geometry.
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