Impermeable Surfaces

Impermeable surfaces come in two types: (1) anthropogenic ones and (2) those produced by natural processes. When most people use this term, they are especially concerned with the [Page 1549]anthropogenic types of impermeable surfaces that inhibit or prevent ready infiltration of water into the substrate, with the result that the volumes and velocities of overland flow of water increase and floods result.

Urbanization of watersheds commonly increases production of impermeable surfaces through the introduction of buildings, roads, paved parking lots, and various asphalt construction layers. Where impermeable surfaces are introduced into a watershed, especially if they are formed in later times after bridges and other structures have already been built downstream, there is then an increased propensity for even larger flash floods downstream, which the older ...