Civil Engineering Reference
In-Depth Information
near 0% impervious and the majority of rainwater is retained by natu-
ral vegetation long enough to be absorbed into the ground or released
very slowly. Typically, about 50% of the rainfall is absorbed, about 40%
retained long enough to be evaporated, and only about 10% runs off into
streams and rivers.
However, with development, a significant portion of natural veg-
etation is typically replaced by impervious surfaces. Impervious surfaces
are mainly constructed surfaces—rooftops, sidewalks, roads, and parking
lots—covered by impenetrable materials such as asphalt, concrete, brick,
and stone. These materials seal surfaces, repel water, and prevent pre-
cipitation and meltwater from infiltrating soils. Soils compacted by urban
development (even including most lawns) are also highly impervious.
The science clearly shows that there is a negative impact on water-
shed water quality as a result of this increased runoff. Surface pollution
due to human activity—organic waste, fertilizers and other landscape
chemicals, vehicle VOCs from oil and gasoline spills, etc.—are carried by
the runoff into the local watershed.
To reduce the negative impact of runoff, every facility should con-
sider the following:
•
As discussed above, natural areas are far more pervious than other
planted areas, especially lawns—they retain water and reduce run-
off. Lawns not only are more impervious, increasing the rate and
quantity of stormwater runoff, but they typically are fertilized and
treated with other chemicals that become part of the runoff, result-
ing in watershed pollution. Thus, replacing grassed areas with natu-
ral planting areas will reduce both the quantity of stormwater run-
off and improve the quality of what runoff that does occur.
The construction of “rain gardens” is becoming popular to cre-
ate artificial wetlands that filter runoff and slow its rate of release.
•
Rainwater or air-conditioning condensate that is captured and used
for plant irrigation reduces the need for purchased water. Also, har-
vested rainwater reduces site runoff since the use of the water is
spread out over a much longer time period.
•
Improve stormwater retention: Runoff from developed sites is in-
evitable. However, retaining stormwater and releasing it over a
longer period reduces its negative impacts. First, contaminants that
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