Greened Acre Calculation Methods Clause Samples

Greened Acre Calculation Methods. Acre is a metric to describe the volume of stormwater, in acre-inches, managed by GSI. It is equivalent to the product of the acres of directly connected impervious drainage area and the inches of runoff captured over that area. Where: As stated in the COA, GSI designs aim to control at least 1.0 inch of runoff, and up to 1.5 inches of runoff, unless otherwise deemed feasible by engineering design. Based on Philadelphia’s hydrology and infiltration performance, a maximum runoff depth of 2.0 inches is deemed appropriate for Greened Acres. Because rainfall events exceeding that depth are infrequent in a typical year, PWD has capped runoff depth at 2.0 inches. To determine the depth of runoff captured, the available static storage volume is calculated by analyzing post-construction stormwater management plans. Available static storage is the volume of void space between the top of storage elevation and bottom of storage elevation. The volume of void space is dependent on the porosity of the storage media present. Present assumptions for typical storage media are as follows: 40% for gravel, 30% for sand, 20% for soil, 92% for perforated pipes, and 100% for tanks and solid storage pipes. For proprietary structures, the porosity is as defined by the manufacturer. Static storage volume calculation methods can vary slightly, depending on the design of the system. The volume is converted from cubic feet to inches of runoff, which is then used in the Greened Acre equation. Storage volume can be managed in one of three ways: infiltration, slow release, or in the case of green roofs, evapotranspiration. Infiltration volume is the static storage below the lowest outlet point to the combined sewer. The top of storage is the elevation of an orifice invert, weir, grate, or riser, below which all water will infiltrate into the surrounding soils. Slow release volume is the static storage volume above an orifice, with the top of storage equal to the head that produces the peak allowable release rate, in cubic feet per second per acre of impervious drainage area, or the elevation of an overflow outlet such as a ▇▇▇▇ or larger orifice. Public GSI systems are designed with enough storage for the water quality volume only, between