8.1.5 BMP L602a - Downspout Infiltration Trenches
8.1.5.1 Design Criteria for Downspout Infiltration Trenches
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
Use Figure 4 - 2: Typical Downspout Infiltration Trench (also called Green Stormwater Infrastructure - Figure 001) which shows a typical downspout infiltration trench system, or Figure 4 - 3: Alternative Infiltration Trench System for Coarse Sand and Gravel which shows an alternative infiltration trench system for sites with coarse sand and cobble soils for design criteria.
Minimum trench length shall be based on native soil type as shown in Table 4 - 7: Downspout Infiltration Length for USDA Soil Type. Native soil type shall be determined using methods described in Subsurface Characterization. Table 4 - 7: Downspout Infiltration Length for USDA Soil Type Total Trench Length (linear feet per 1,000 square feet of roof area) “Coarse Sand” (more than 50% of sand fraction remains on a #4 sieve) “Medium Sand” (more than 50% of the sand fraction remains on the #40 sieve)
The minimum bottom width shall be 24". Table 4 - 7: Downspout Infiltration Length for USDA Soil Type lengths are based upon a minimum bottom width of 24". If alternative width and lengths are proposed, a Washington State Licensed Professional Engineer must model the proposed infiltration trench (trenches) using an Ecology approved continuous simulation model (assuming a 15-minute timestep) and the LID Performance Standard must be used if the facility is proposed to be installed to meet Minimum Requirement 5 - Onsite Stormwater Management.
Maximum length of trench shall not exceed 100 feet from the inlet sump.
Minimum spacing between trenches shall be 6 feet measured from trench centerline.
Non-woven geotextile fabric shall be placed over the trench aggregate prior to backfilling. Geotextile fabric shall not be placed on the bottom of the trench to avoid clogging. The non-woven geotextile shall conform to A2000 - Geotextile Specifications, Geotextile for Underground Drainage - Low Survivability, Class C. Scarify the bottom of the facility 2", minimum to enhance infiltration capabilities.
Distribution pipe shall be minimum 4” PVC slotted or perforated pipe placed at 0% slope .
Install a rock layer as follows:
A minimum 12" thick layer of rock shall be placed under the top of the distribution pipe.
A minimum 2" thick layer of rock shall be placed above the top of the distribution pipe.
The rock layer shall be ¾" to 1 ½" washed rock or rock meeting the WSDOT Standard Specification 9-03.12(5) - Gravel Backfill for Drywells.
Place final cover measures over the rock layer.
All disturbed areas not covered with impervious surfaces must be fully stabilized by planting and amended per BMP L613 - Post-Construction Soil Quality and Depth.
Ensure the appropriate subgrade material is installed for paved areas.
The rock layer shall not extend to the surface unless using the alternative trench design (Figure 4 - 3: Alternative Infiltration Trench System for Coarse Sand and Gravel) which is only allowed where the native soils are coarse sand or gravel.
Cleanout are recommended at all bends in pipe and at the infiltration trench terminus for inspection and maintenance practices.
Install a catch basin or yard drain upstream from the infiltration trench.
Catch basin or yard drain shall have a minimum 6" sump depth below the tee.
The tee shall have a ½" mesh screen on top and bottom to capture large debris.
Catch basin or yard drain shall be traffic-bearing in areas subject to traffic.
A short section of non-perforated pipe shall lead from the catch basin to the slotted or perforated pipe section.
The catch basin or yard drain is not required for projects using the alternative trench design (Figure 4 - 3: Alternative Infiltration Trench System for Coarse Sand and Gravel) which is only allowed where the native soils are coarse sand or gravel.
A minimum of three feet of separation is required from the proposed final grade to the seasonal high groundwater table.
A minimum of 1 foot of separation is required from the bottom of the infiltration trench to the seasonal high groundwater table.
Infiltration trenches may be placed in fill material if the fill is placed and compacted under the direct supervision of a Washington State Licensed Professional Engineer, and if the measured infiltration rate is at least 8 inches per hour. Infiltration rates can be tested using the methods described in Appendix A - Appendix A Determining the Design Infiltration Rate.
Trenches may be located under pavement if a small yard drain or catch basin with grate cover is placed at the end of the trench pipe such that overflow would occur out of the catch basin at an elevation at least one foot below that of the pavement, and in a location which can accommodate the overflow without creating a significant adverse impact to downhill properties or stormwater systems. This is intended to prevent saturation of the pavement in the event of system failure.