25 October 2004
reflect dynamic resource protection and regulatory issues. Communities and bases
often have extensive watershed management and natural resources conservation
goals; master plans identify sensitive environmental areas and preservation areas such
as wetlands, mature woods, and habitats. The LID site design should address any
potential impacts to these areas and encourage conservation of these areas within the
site. Examples of conservation include:
Preserving a forest corridor that connects with an existing stream valley
Maintaining flow volume and discharge rates to offsite wetlands
Incorporating buffers around sensitive habitat areas
Minimization of Development Impacts. Within the portion of the site
selected for the placement of roads, buildings, and other development activities,
minimal disturbance techniques (site fingerprinting) can be used to avoid soil
compaction, retain mature trees, and limit the environmental impact of staging areas.
Examples of minimal disturbance techniques include:
Delineating and flagging the smallest site disturbance area possible
Minimizing the size of construction impacts or offsite easements and
Minimizing the size of material storage areas during and after construction
Maintaining flow patterns
Control of Watershed Timing and Runoff Patterns. Maintaining the site's
natural runoff control areas and restricting building over the site's more pervious soils
will help keep the infiltration capacity of the site close to predevelopment levels.
Maintaining the watershed timing of a site is also important. The cumulative effects of
decreasing the post-development watershed times of concentration of several sites can
have a significant impact on downstream habitat. It is also desirable to maintain natural
vegetation in steeply sloped areas and to retain natural drainage divides. This will
encourage dispersed flow paths and, consequently, help reduce the development of
channels that lead to erosion and flooding problems.
Adequate drainage from buildings, walkways, and roads must be provided.
Traditional designs often create a drainage system that has the effect of increasing the
rate at which runoff moves into receiving waters during storm events. In turn, this
produces a higher volume of runoff, a higher peak rate of flow, and an earlier runoff
event than would occur under less developed conditions. The opportunity for
groundwater recharge is eliminated, because infiltration into swales and grassed areas
cannot effectively occur if runoff passes through quickly.
The overall grading objective for LID is to provide a surface landform that will
distribute flows in a shallow and slow moving pattern toward areas where the infiltration