Environmental Engineering Reference
In-Depth Information
4. Replicate the natural hydrology on site before development . The objective
for stormwater management is to develop a program that replicates the natu-
ral hydrologic conditions of watersheds to the extent practicable. However, the
very process of clearing the existing vegetation from the site removes the single
largest component of the hydrologic regime, evapotranspiration (ET). Unless we
replace the ET component, the runoff increase will be substantial and can actually
increase the groundwater recharge with infiltration systems. Several of the BMPs
described in subsequent sections, such as infiltration, tree planting, vegetated roof
systems, and rain gardens, can help replace a portion of the ET function.
Volume Control Guideline
Whatever BMP technologies are found to be suitable for a given development
site, the following control guideline is recommended:
Do not increase the post-development total runoff volume for all storms
equal to or less than the 2-year/24-hour event.
The scientific basis for this control guideline is as follows:
• The 2-year event encompasses 95% or more of the annual runoff volume
across the state.
• Volume-reduction BMPs based on this standard will provide a storage capac-
ity to help reduce (or eliminate) the increase in peak flow rates for larger
runoff events.
• In a natural stream system in mid-Atlantic states, the bankfull stream flow
occurs with a period of approximately 1.5 years. If the runoff volume from
storms less than the 2-year event are not increased, the fluvial impacts on
streams will be significantly lessened.
• The 2-year-frequency rainfall is well defined and weather data are readily
accessible for use in stormwater management calculations.
Peak-Rate Control Guideline
Peak-rate control for large storms, up to the 100-year event, is essential to protect
against immediate downstream erosion and flooding. Most designs have achieved
peak-rate control through the use of detention structures, but peak-rate control
can also be integrated into volume-control BMPs in ways that eliminate the need
for additional peak-rate control detention systems. Nonstructural BMPs can also
contribute to rate control, as discussed later. The control guideline recommended
for peak-rate control is
Do not increase the peak rate of discharge for the 1-year through 100-year
events.
Search WWH ::




Custom Search