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released as runoff will be infiltrated into the soil, taken up by plants, or evaporated back
into the atmosphere. Natural land cover often provides depression storage in small
undulations in the topography. Greater storage capacity is provided in ponds or lakes.
LID Storage Concepts. LID employs site planning and grading techniques
to direct or maintain the flow of runoff to naturally occurring storage areas such as
wetlands. Keeping the storage area volume stable helps to maintain the existing
hydrologic and biological function of the storage area.
An LID design may also include small-scale retention components (retention
is defined as the volume of runoff that never reaches the drainage area outlet).
Retention can be provided in a variety of ways that not only support the management of
runoff, but also supply water for on-site use. For example, a cistern may be used to
store and release water for peak flow control as well as to store water for domestic
purposes. Additionally, some industrial buildings can provide roof storage and release
water for use in cooling systems. Another example, shown in Figure 4-4, is a green wall
within a building. The green wall is used to modify temperature and improve air quality
by having stored roof water flow across the vegetation.
Capturing runoff in small volumes helps to prevent erosion, because the
runoff is less likely to reach damaging flow rates. The distribution of storage
components also tends to result in a more robust stormwater management system,
because the failure of one component will not cause the entire system to fail. Care
must be taken when ponding or storing water to make sure there is adequate flow,
infiltration, evaporation, or discharge, and that unwanted carriers of disease such as
mosquitoes are adequately controlled.
Figure 4-4. Greenwall
Source: Greenland International Consulting, Inc., Ontario, Canada.