Infiltration Trench

National baseline

Among Tier A manuals that mention infiltration trenches, numeric guidance is sparse for practice-specific facility dimensions; the extracted numbers cluster around jurisdiction-wide water-quality and drawdown context rather than a uniform trench cross-section standard. Where manuals state a design infiltration rate for sizing, 17 sources report values with a mode and median of 0.5 in/hr and a range from 0.01 to 2.41 in/hr—useful as a screening assumption in temperate settings with moderate soils, not as a national design constant. Site-wide water-quality volume depth appears in 30 manuals with mode 1 inch, median 1 inch, and range 0.37–1.61 inches; maximum ponding or drawdown time appears in 36 manuals with mode 72 hours, median 48 hours, and range 4–120 hours; separation from seasonal high water table appears in 36 manuals with mode 24 inches, median 36 inches, and range 12–120 inches. Committee discussion should treat these as overlapping jurisdictional sizing and siting guardrails when trenches are used to meet volume or infiltration performance, while recognizing that 45 of 76 scanned manuals lack extracted WQv depth, 38 lack drawdown hours, 38 lack SHWT separation, and 23 of 41 mentioning manuals lack a stated design infiltration rate—so a single numeric national baseline for trench geometry is not yet established from this matrix alone.

Matrix summary

For infiltration_trench in Tier A manuals (41 of 76 mention the practice): site-wide wqv_depth_inches n=30, mode=1, median=1, range 0.37–1.61; max_drawdown_hours n=36, mode=72, median=48, range 4–120; shwt_separation_inches n=36, mode=24, median=36, range 12–120; design_infiltration_rate_in_per_hr n=17, mode=0.5, median=0.5, range 0.01–2.41—with substantial missing data for each field.

Cross-state parameters

Regional modifiers

Open issues

Practice-specific trench geometry (length, width, rock reservoir depth, underdrain details) is largely absent from the numeric matrix; only jurisdiction site-wide WQv depth, drawdown, SHWT separation, and sporadic design infililation rates were extracted. Design infiltration rate has n=17 with high spread (0.01–2.41 in/hr), so the 0.5 in/hr mode is a weak national anchor. Max drawdown ranges from 4 hours (FL fl-fsesci-tier1) to 120 hours (MS ms-state-vol2), indicating inconsistent definitions or storm/climate framing across manuals. SHWT separation mode is 24 inches but median 36 inches, with an upper tail to 120 inches (CA san-diego-ca), so siting rules dominate but are not harmonized. Nearly half of scanned Tier A manuals lack extracted values for the three site-wide fields, and a majority of manuals mentioning the practice lack design infiltration rate—limiting committee confidence in a single numeric synthesis for infiltration trench alone versus general infiltration BMP context.