Impervious Surfaces
What are impervious surfaces?
Impervious surfaces are land surfaces that repel rainwater and do not permit it to infiltrate (soak into) the ground. Some natural surfaces can be relatively impervious, for example compacted clay in some desert environments. However, for the most part this term refers to surfaces found in urban and suburban landscapes such as roads, parking lots, driveways, sidewalks and roofs. Adding these surfaces to the landscape can alter the flow of rain water and streams.
How do impervious surfaces change the flow of water?
In natural landscapes such as forests and meadows, spongy soil and plant roots enable water to infiltrate the soil. After water seeps into the ground, it may:
- Nourish plants, and cycle back into the air through plant processes
- Continue to flow just beneath the surface to nearby streams (as subsurface flow)
- Continue into deeper groundwater
Physical and chemical processes accomplished by microorganisms and plant roots help to filter and purify this water. Large volumes of water are stored in the soil and in wetlands. Sudden rainstorms in natural areas thus cause only a gradual change in the water level of streams. Evaporation of stored water also helps to cool the air in natural areas.
In urban landscapes dominated by impervious surfaces, instead of infiltrating,
rainwater flows across the impervious surfaces. A brief rainstorm over
a large area urban area can cause a great amount of water to suddenly
flow into the storm drains and streams. Since no water is stored in
impervious surfaces, they quickly dry after a rainstorm. Warming from
the sun therefore has a much greater effect; this is one reason why
cities often several degrees warmer than the countryside.
(See a more detailed explanation in Water Cycles.)
What effects do impervious surfaces have on ecosystems?
- Replenishment of groundwater through infiltration is important for maintaining a baseflow in streams throughout the summer. Impervious surfaces prevent much of the rainfall from replenishing the groundwater and this can cause the water table (the level of groundwater) to drop. During dry to normal conditions, a low water table may cause streams to dry up so they can no longer support fish and other aquatic species.
- During storm events, the high velocity and high volume of runoff from impervious surfaces can overcome the capacity of streams. This can cause stream banks to "blow out" and erode the sides and bottom of the channel. Aquatic habitat is lost and the resulting erosion carries sediment (sand, silt, mud) downstream.
- Excessive sediment from erosion can smother salmon spawning beds and bury the habitat of bottom-dwelling plants and animals.
- Toxic chemicals are often present on impervious surfaces, and are carried directly into streams, wetlands and the ocean. For example: oils and gasoline are leaked from vehicles; heavy metals are deposited from the atmosphere in industrial areas; pesticides and fertilizers are washed out onto streets and sidewalks.
What effects do impervious surfaces have on people?
- Impervious surfaces can prevent the natural replenishment of groundwater, an important drinking water source in many areas. Unlike surface water, groundwater moves very slowly and requires decades to centuries or more to replenish depleted reserves.
- The high velocity and high volume of water flowing off impervious surfaces causes greater and more frequent flooding. The high flows also cause destruction of stream channel characteristics (streamside vegetation, pools, meanders) that in a healthy system help to reduce the energy of the water. The loss of these features in turn results in greater flood damage, in a vicious cycle of destruction.
- Erosion from increased stream flows can cause property loss and damage that can amount to millions or even billions of dollars. Deposition of sediment downstream of eroded areas also damages property.
- Pollution and alteration of stream flows and channel characteristics can destroy the habitat of commercially important species such as salmon.
- Wetlands filter and purify drinking water. When they are filled in for suburban development or degraded by pollution from impervious surfaces, millions of dollars may be required to install a water treatment plant.
- Installing and upgrading water infrastructure in order to accommodate the large volumes of water that flow off impervious surfaces costs huge sums of taxpayers' money.
Additional References & Links
Websites
- Water Balance Model for BC
- Stormwater Planning: A Guidebook for BC: BC Ministry of Water, Land and Air Protection
- Impervious Surfaces: Duluth Streams
- Reducing Runoff: Nonpoint Education for Municipal Officers, University of Connecticut
- Impervious Surfaces: Lealanau Government and Community Center
- Smart Growth BC
- Smart Growth Online
- Green Roofs for Healthy Cities
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