Infiltration Trenches

5.4.16 Infiltration Trenches

5.4.16.1 BMP Description

Infiltration trenches (BMP T7.20 in Volume V, Chapter 5 of the Ecology Manual) are trenches backfilled with a coarse aggregate. Stormwater runoff can enter the trench as overland surface flow through a grate or exposed aggregate surface, or as concentrated flow delivered to the aggregate-filled trench using a perforated or slotted distribution pipe.

Infiltration trenches that include perforated pipe are UICs and shall follow the applicable requirements in Volume I, Chapter 4 in the Ecology Manual.

If this BMP is proposed to be used for Runoff Treatment, the design shall show that the criteria for Runoff Treatment in Volume V, Section 5.6 in the Ecology Manual are met.

5.4.16.2 Performance Mechanism

Flow control occurs through temporary storage of stormwater runoff in the spatial voids of the aggregate material and subsequent infiltration of stormwater into the underlying soils. Pollutant removal mechanisms include infiltration, filtration, adsorption, and biodegradation.

5.4.16.3 Application and Limitations

An infiltration trench can be designed to provide on-site stormwater management, flow control and/or runoff treatment (see Table II-5.19).

Table II-5.19. Infiltration Trenches Applicability.^a
BMP	MR #5: On-site Stormwater Management		MR #6: Runoff Treatment				MR #7: Flow Control
BMP	List	LID Performance Standard	Basic	Enhanced	Oil Control	Phosphorus	MR #7: Flow Control
Infiltration Trenches		X	X^a	X^a			X
Notes: Site suitability criteria apply (see Volume V, Section 5.6 of the Ecology Manual).

5.4.16.4 Site Considerations

Site considerations for the applicability of infiltration BMPs including trenches are provided in Vol II–5.3.2 Determine Infiltration Feasibility.

5.4.16.5 Design Information

Refer to BMP T7.20 in Volume V, Chapter 5 of the Ecology Manual for detailed design guidance and criteria on infiltration trenches.

5.4.16.6 Minimum Construction Requirements

During construction, it is critical to prevent clogging and over-compaction of the subgrade. Minimum requirements associated with infiltration trench construction include the following:

Aggregate Placement and Compaction – Place stone aggregate in lifts and compact using plate compactors. In general, a maximum loose lift thickness of 12 inches is recommended. The compaction process ensures geotextile conformity to the excavation sides, thereby reducing potential piping and geotextile clogging, and settlement problems.
Potential Contamination – Prevent natural or fill soils from intermixing with the aggregate. Remove all contaminated aggregate and replace with uncontaminated aggregate.
Overlap – Following the stone aggregate placement, fold the geotextile over the stone aggregate to form a 12‑inch minimum longitudinal overlap. When geotextile overlaps are required between rolls, overlap the upstream roll a minimum of 2 feet over the downstream roll in order to provide a shingled effect.

5.4.16.7 Operations and Maintenance

Monitor sediment buildup in the top foot of stone aggregate or the surface inlet on the same schedule as the observation well.