Creating a Watershed to Collect Surface Water

Often, the desire to grade a project so that water is directed toward other locations on-site—or to create a sheet flow from one area to another—is not entirely feasible. A reasonable alternative is to simply sheet flow or direct surface water to be collected either in a detention pond or a system of catch basins that are connected by underground piping and carried to a city storm water collection system.

One way to visualize a catchment system is to think of such a system as a watershed, a small one perhaps, serving an area of several hundred square feet or less—or a larger area, but less than one acre. The photograph in Figure 12.9-A was taken of a portion of a mountain range in Southern California. The mountains in the photograph make up three adjacent sub-watersheds, and each of the three is further subdivided into drainage courses or natural swales. A watershed is an area bounded by higher landform—for example, a series of ridges with a swale or V-shaped valley or stream—that carries the rainwater that falls within the bounded area ultimately to a single watercourse or body of water, such as a river, lake, wetlands, or ocean. The photograph in

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Figure 12.9-a Watershed in Southern California mountains and small watershed-like area within a larger paved walkway

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Figure 12.9-B Portion of a professional grading plan designed to gather surface water from amphitheater and behind building to retention pond below

Courtesy of reed hilderbrAnd