Computer simulations are used to examine interacting pollutant sources in the watershed. By modeling the runoff process, a planner can predict the effects of proposed plans, and the ability of controls to solve potential problems. Several models are described in Section 10.9. Water quality criteria standards can be recommended after investigating pollution sources and the ability of receiving water to absorb loadings.
When watershed goals are set, the planning agency has two choices for achieving water quality standards. Individual sites can be forced to comply with the practices and performance standards set forth in the master plan, or the basin system must be designed and maintained as a public utility. Isolated development tracts can be controlled by requiring developers to follow specific source control practices, or a simple set of performance standards can be applied and the choice of practices can be left up to the developer. For example, the agency can require that runoff from developed sites must not exceed predevelop-ment intensity. The developer will have to minimize runoff-producing areas and provide detention facilities at the site.
Planners must also consider the effects of their actions on areas outside the watershed. For example, a system where storm flow is detained in a downstream watershed while it remains unregulated upstream can cause higher flood levels in a river than a completely unregulated system.
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