THE LEGAL BASIS FOR LID: REGULATORY STANDARDS AND LID DESIGN CRITERIA - Low Impact Development and Sustainable Stormwater Management - page 104

The BMPs described in this topic place emphasis on infiltration of precipitation
as an important solution, but this is only one of the three basic methods that
reduce the volume of runoff from land development. These three methods can
be summarized as follows:
1. Infiltration
2. Capture and reuse
3. Vegetation systems that provide evapotranspiration
All of the stormwater management systems described in this topic include one
or more of these methods, depending on specific site conditions that constrain
stormwater management opportunities.
Volume Control Criteria
Volume control is essential to mitigate the impacts of increased runoff. To do
this, volume-reduction BMPs must meet the following criteria:
1. Protect stream channel morphology . The increased volume of runoff
results in an increase in the frequency of bankfull or near-bankfull flow
conditions in stream channels. The increased occurrence of high flow conditions
in riparian sections has a detrimental effect on stream ecology, including stream
channel impacts and overall stream morphology, with stream bank erosion
greatly accelerated (Figure 5-3). As banks are eroded and undercut and as
stream channels are gouged and straightened, meanders, pools, riffles, and other
essential elements of habitat are lost or diminished. Research has demonstrated
[12] that bankfull stream flow in a natural channel typically occurs between the
1- and 2-year-frequency storm event (often around the 1.5-year-frequency storm).
Land development will cause bankfull stream flows to occur more often than the
1-year-frequency storm event, and in highly urbanized watersheds (over 25%
impervious cover), bankfull flow can occur as frequently as every few months
(Figure 5-4).
2. Maintain groundwater recharge . Over 80% of the annual precipitation infil-
trates into the soil mantle in most watersheds under natural conditions, with
more than half of this rainfall taken up by vegetation and transpired. The rest of
this infiltrated water moves deeper into the unsaturated soil and bedrock until it
encounters the water table, where it moves down gradient to emerge as springs
and seeps, feeding local wetlands and surface streams or enters the aquifers that
supply drinking water wells. Without the constant recharge of groundwater, sur-
face stream flows and groundwater in wells would diminish or disappear during
drought periods. Certain undeveloped land areas recharge more than others, and
protecting these critical recharge areas is important in maintaining the water
cycle balance, so understanding how the water balance works on a specific site
is critical.