12/19/2001 Env Bd Packet - Laserfiche WebLink

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On -Lot Infiltration Reduced Lot Grading Description Development standards often require minimum lot grades of 2 percent for adequate drainage of stormwater away from a building. Some authorities, however, have proposed reducing minimum lot grades from 2 to 0.5 percent to promote infiltration. This option is mainly intended to promote infiltration by slowing stormwater runoff from the roofs and yards and allowing it to soak into the lawn. A reduction in the lot grading is generally a viable option if the land is naturally flat. In hilly areas, alterations to the natural topography should be minimized. Developers and homeowners should check the acceptability of this practice with the local municipality, because some municipalities may not permit its use. Similarly, shallow depressions can be graded into lawns. Depressions need not be very deep to make a significant contribution to overall surface storage capacity and stormwater quality. For example, a square lawn area 50 feet on a side, sloping 2 percent toward the center, will create a low point 6 inches below the outside rim. This 6 -inch slope over 25 feet of distance is barely noticeable, and is similar to standard grading practice for lawn areas. This 50 -foot by 50 -foot by 6 -inch -deep lawn area creates a storage capacity of 413 cubic feet. If adjacent impervious surfaces, such as sidewalks, rooftops, and roads are designed to sheet flow into this concave lawn, their runoff can gradually infiltrate into the soil as well. Catch basins located at the upper edge of the concave vegetated surfaces can collect runoff from larger storms. Figure 1 illustrates these lot grading changes on a residential lot. Design Guidelines • In order to ensure that foundation drainage problems do not occur, the grading within 6 to 12 feet of a build- ing should be maintained at 2 percent or higher (local municipal standards should be reviewed to ensure that the grading around a building is in compliance). Areas outside of this boundary may be graded at less than 2 percent to create greater depression storage and promote natural infiltration. Figure 1: Examples of Lot Grading Changes Source: Ontario Ministry of the Environment, 1999 Metropolitan Council / Barr Engineering Co. 3 -143