Infiltration Trench
Technical Standards for Infiltration Trenches 
1. Infiltration devices are primarily used for 85% TSS removal water quality enhancement and volume management up to the two-year design storm. Infiltration practices that are to capture more than the one (1) inch design storm may utilize additional storage to provide sufficient reductions to the 1-year and 2-year peak discharge as required by the general storm water code of the community.
2. Infiltration devises should be located on sites with less than one (1) percent slope, and in soils with rapid infiltration capacity and near site perimeters at the lower points of a project site.
3. Infiltration devices such as French drains, dry wells and sand filters shall be constructed after all site work is finished and the drainage area easily calculated to determine the design storm holding capacity of the facility.
4. Infiltration devices must be protected until their drainage area has been adequately stabilized with turf grass to prevent clogging with construction site erosion sediments.
5. Infiltration devices shall utilize perforated drainage pipe system with outlet to ensure full draw down of infiltrated storm water. If existing soils permeability rate is less than 0.5/hour. See a standard design Table for permeability rates of local soil horizons.
6. Type of infiltration devises that shall follow these standards include French drains, dry wells, sand filters, bio-retention facilities, and porous pavement.
7. Observation wells (standpipe cleanouts) must be used on all infiltration devises as a means to monitor the reservoir depths and draw down capabilities as a part of the operations and maintenance of the storm water.
8. Runoff Pretreatment facilities must be used with all infiltration BMPs. Infiltration devices shall be preceded by a pretreatment facility to capture grease, oil, floatable organic materials, and water relocated solids. These site pollutants should be removed from the runoff before it enters the infiltration facility. Vegetated filters, storm water buffers, grass strips at least twenty (20) feet wide or sediment traps and forebays are recommended pretreatment strategies. Devices with surface inlets should be designed to capture sediment from the runoff before it enters the infiltration reservoir. All pretreatment facilities and infiltration facilities shall be designed with maintenance and inspection requirements in mind.
9. Darcy’s Law, Q = f I S A, is recommended for sizing all infiltration devices to determine the flow rate through porous material. This assumes that the drain time of the facility is controlled by flow through the bottom surface of the facility. TS
-
Q = rate of exfiltration into soil in cfs.
-
f = infiltration rate of the soil in ft/hr (Assume
-
0.5’/hr washed limestone)
-
I = hydraulic gradient (assume I = 1)
-
SA = bottom surface area of facility in ft 2
A safety factor of 2 shall be applied to the infiltration rate determined from the soil analysis to compensate for clogging of the surface area by infiltrated sediments. The design soil infiltration rate, fd, therefore, is equal to one-half of the actual rate: fd = 0.5f
10. Minimum soil exfiltration rate of 0.5 in/hr is required. The draw down time for water quality runoff volume is 0.5 days (minimum).The draw down time for all stored runoff volume is 5 days.
11. The aggregate graduation should be uniform sand, gravel, or crushed stone (uniformity coefficient of 2 or smaller).
12. A sediment forebay, grass channel, or flow diffuser or equivalent upstream pretreatment, must be provided. 
Pretreatment must be sized to hold 25 % of the water quality volume, 50% when underlying soil exfiltration rates are greater than 2 inches/hour.
13. Maximum contributing drainage area is five (5) acres, minimum size shall be as needed without clearing excess site features.
14. An observation well is required to monitor exfiltration rates. The storage area above and within the filter media (usually stone aggregate) shall be sized to hold the water quality volume.
