Why do we need drainage requirements?

Almost any form of development increases the amount of rainfall that tends to run off property after a heavy rainstorm. Removing trees and other vegetation reduces the amount of water absorbed by roots and the subsequent evaporation through the leaves. Regarding the land eliminates natural pockets and depressions in the soil which hold water until it is absorbed into the soil. Covering ground with roofs, pavements, and similar impervious covers prevents the underlying soil from accepting surface water. Covering porous soil with comparatively impervious top soil and lawn decreases the rate the soil can absorb water. Driving over the ground (both during and after construction) compacts the surface, decreasing the small voids in the soil which let the water filter into the ground. Placing plastic films under landscaped areas prevents the underlying soil from absorbing water. These and other factors combine to decrease the ability of the native soil to absorb rainfall as well as it would have prior to development. To minimize the impacts of stormwater runoff associated with development, Clallam County requires appropriate management of that runoff.

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1. Why do we need drainage requirements?
2. When do I need a drainage plan?