CHAPTER 2 Post Construction Stormwater Management Practices
Sizing the Pretreatment Facility – The size of the pretreatment facility is based on the
infiltration rate of the soil in which the infiltration trench is built. For soils with infiltration
rates of 2.0 inches per hour or less, the pretreatment facility shall be sized to contain 2%
of the WQv. For infiltration rates greater than 2.0 inches per hour, the pretreatment facility
shall be sized to contain 0% of the WQv.
Exit Velocity from Pretreatment Facility – The velocity of runoff as it exits from the
pretreatment device must be non-erosive.
Drain Time Requirements – The practice is to be designed to infiltrate the Water Quality
Volume (WQv, see page 30 of this chapt.) through the bottom floor of the structure in 24 to
8 hours. Drain times in excess of 72 hours should be avoided to prevent mosquito-breed-
ing habitat from forming. Flows in excess of the WQv are to be diverted around the trench.
Dimensions – The dimensions of the storage reservoir (infiltration trench) are made by
fitting the length, width and depth into a configuration, which satisfies drain time and stor-
age volume requirements. The trench dimensions shall be sized by accepted engineering
methods such as those outlined below:
1. Determine Initial Storage Depth – The bottom of the infiltration trench must be
deeper than 2 feet to avoid freezing and shallow enough to leave at least feet between
the seasonal high-water table or bedrock and the trench bottom. Soil morphology also
must be considered in determining the dimensions of the storage reservoir to utilize the
optimum horizons or strata. The presence of a thin, slowly permeable soil horizon may
require a trench depth which completely penetrates it to more permeable underlying
material. Long trenches may need to be curved parallel to the topographic contour in
order to keep the trench bottom elevation within the optimum depth in the soil profile.
2. Determine Area of Trench Bottom – The