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 bridges will not handle. This increased “flashiness,” as the result of construction of new impermeable surfaces upstream, often goes unrecognized in flood-control construction because the surfaces are spatially and temporally scattered. A further problem resulting from construction of impermeable surfaces is the increased pollution of nearby rivers, lakes, bays, and estuaries. The newly accelerated or channelized overland flow carries with it much of the animal and human wastes that would normally be spread more slowly into the soil and thereby degraded in the processes of infiltration. In some places, the introduction of concrete paving stones or brick and cobblestone pavers makes it somewhat more permeable and allows greater infiltration because of the common introduction of sand in the joints between the blocks, although they still provide the desired firm surface for bearing traffic.

Natural varieties of impermeable surfaces are certain forms of eroded or bare bedrock that is relatively unfractured, which thereby prevents water infiltration. While many kinds of lava flows are fairly impermeable when they are first extruded, later cooling can result in fractures that do allow some water to infiltrate into them. An unusual group of impermeable surfaces that can be entirely natural but that are commonly exacerbated or accelerated by humans, are the various kinds of duricrusts (hard, thin layers on the surface of the soil) that form in different climatic regimes. In the humid tropics, natural subsoil accumulations of iron and aluminum sequioxides form in the subsoil from long-term weathering of bedrock into soft, clay-rich saprolites that can be subsequently exposed by natural or human-caused soil erosion and hardened into rigid fersialitic (iron, silica, aluminum) crusts. Those lateritic duricrusts that become exposed on the surface can be quite impermeable and commonly resemble a thick and even shiny layer of iron (iron shield or cuirasse) on the ground. In more arid regions, the cementation can be rich in calcium carbonate so that impermeable calcrete or caliche crusts and caprocks result. Where precipitated from a lake that has dried up through climate change, as has happened at the Great Salt Lake and Bonneville Salt Flats in Utah or Death Valley in California, the high concentration of salt in the evaporating waters can result in an impermeable layer of rock salt (salcrete) or gypsum (gypcrete) on the surface. In arid parts of Australia, silcretes bearing opal are relatively impermeable concentrations of long-term weathering and siliceous cementation.

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