1. Infiltration Trench
An infiltration trench is a linear stormwater management BMP consisting of a continuously
perforated pipe at a minimum slope in a stone-filled trench. During small storm events,
infiltration trenches can significantly reduce volume and serve in the removal of fine sediments
and pollutants. Runoff is stored in the pipe and between the stones and infiltrates through the
bottom ofthe facility and into the surrounding soil matrix. Runoff should be pretreated using
vegetative buffer strips or swales to limit the amount of coarse sediment entering the trench
which can clog and render the trench ineffective. In all cases, an infiltration trench should be
designed with a positive overflow to a stable outlet point. For more information refer to the PA
DEP BMP Manual 6.4.4.
Design Considerations:
- Continuously perforated pipe (min 4" diameter) set at a minimum slope (l %) in a stone
filled, nearly level-bottomed trench on un-compacted soils. - The trench width and depth can vary, but it is recommended that infiltration trenches be
no wider than four (4) feet, and a minimum of thirty (30) inches and maximum six (6)
feet in depth. - Stone fill should be clean, angular stone, separated from soil layers by four (4) inches of
straw (top and bottom) or a nonwoven geotextile (top, sides, and bottom). - A minimum of 6" of topsoil can be placed over trench and vegetated.
- Cleanouts or inlets should be installed at both ends and at intersections of the infiltration
trench and at appropriate intervals to allow access to the perforated pipe. - The discharge or outlet from the infiltration trench should be safely conveyed to a stable
vegetated area, natural watercourse, the curb or gutter line of roadway or existing storm
collection/conveyance/control system as applicable. - Volume of facility = Depth x Width x Length x Void Space of the gravel bed (assume
40%).
Maintenance:
- Cleanouts, catch basins and inlets should be inspected at least two times a year and
cleaned out as necessary to maintain function of the system. - The vegetation along the surface of the infiltration trench should be maintained in good
condition and any bare spots should be re-vegetated as soon as possible. - Vehicles should not be parked or driven on the trench and care should be taken to avoidsoil compaction by lawn movers.
2. Rain Garden
A rain garden is a landscaped shallow depression that uses mulch, soil mix, and deep rooted
plants to capture, adsorb and infiltrate stormwater runoff from roofs, and pavement. For more
information refer to the PA DEP BMP Manual 6.4.5.
Design considerations:
- A rain garden should be located on nearly level to gently sloping ground and no closer
than 10 feet to a building foundation and 25 feet from septic field or wellhead. - A rain garden can vary in length, width and depth, but should have a ponding depth of 6
to 12 inches, and a total surface depth of no greater than 18 inches. - Side slopes within the garden should not exceed 3: 1 horizontal to vertical.
- The rain garden should be constructed in layers with a (min 4") perforated underdrain in
a clean angular stone envelope, separated from soil layers by four (4) inches of straw (top
and bottom) or a nonwoven geotextile (top, sides, and bottom), covered with 12 inches to
36 inches of 50-30-20 topsoil-sand-compost mix or as approved by the municipality, and
3 inches of shredded bark mulch or vegetated cover. Soil depth should be determined by
plant choices and control volume requirements. - Vegetation should be deep rooted and tolerant of wet and dry conditions, salts and
environmental stress. - An emergency overflow should be set in the rain garden such as a vertical pipe or inlet
box, with basket type grate set even with the ponding depth, below the surrounding
ground elevation and connected to the perforated underdrain and an outlet pipe. - The outlet from the rain garden should be safely conveyed to a stable vegetated area,
natural watercourse, the curb or gutter line of roadway or existing storm
collection/conveyance/control system as applicable.
Maintenance:
- Cleanouts, catch basins and inlets should be inspected at least two times a year and
cleaned out as necessary to maintain function of the system. Detritus should be removed
from the rain garden as necessary to prevent clogging of the overflow outlet. - The vegetation should be maintained in good condition and replaced as necessary. Rain
garden plants may need to be watered during dry spells. - Rain garden should be weeded and shredded bark mulch should be amended as necessary
to prevent volunteer weeds.