Infiltration Basin

National baseline

Among Tier A manuals that mention infiltration basins (56 of 76 scanned), numeric guidance is sparse for facility-specific design infiltration rates but more common for jurisdiction-level water-quality volume (WQv) depth, drawdown time, and seasonal high water table (SHWT) separation used when sizing and siting infiltration. For temperate / Type II–ish contexts, extracted site-wide WQv depth centers on 1 inch (n=30, mode=1, median=1, range 0.37–1.61 in.). Site-wide maximum drawdown time most often appears as 72 hours (n=36, mode=72, median=48, range 4–120 hr), with many manuals also specifying 48 or 24 hours. Site-wide SHWT separation commonly lists 24 inches (n=36, mode=24, median=36, range 12–120 in.), with 36 inches also frequent. Where manuals state a design infiltration rate for the practice, the mode and median are both 0.5 in/hr (n=15, range 0.01–2.41 in/hr); this baseline is not yet established numerically across the full mention set because 40 of 56 mentioning manuals lack an extracted rate. Committee discussion should treat WQv depth, drawdown, and SHWT values as jurisdiction sizing and siting context rather than dimensional facility specs for the basin alone.

Matrix summary

For infiltration basin (Tier A, 56/76 manuals mention the practice): site-wide WQv depth n=30, mode 1 in., median 1 in., range 0.37–1.61 in.; max drawdown n=36, mode 72 hr, median 48 hr, range 4–120 hr; SHWT separation n=36, mode 24 in., median 36 in., range 12–120 in.; design infiltration rate n=15, mode and median 0.5 in/hr, range 0.01–2.41 in/hr. Modes summarize extracted values, not an adopted national standard.

Cross-state parameters

Regional modifiers

Open issues

Nearly half of Tier A manuals lack extracted site-wide WQv depth (45 missing), drawdown (38 missing), or SHWT separation (38 missing), so national numeric consensus is incomplete. Design infiltration rate for infiltration basins is reported in only 15 manuals (40 missing among the 56 that mention the practice), with extreme outliers (0.01–2.41 in/hr) that may reflect chapter proxies, conservative defaults, or regional soil assumptions rather than a single design value. Site-wide WQv, drawdown, and SHWT fields are jurisdiction sizing context—not confirmed as infiltration basin facility dimensions—so synthesis cannot yet tie these numbers to internal ponding depth, freeboard, or bottom area without manual-specific interpretation. Florida (fl-fsesci-tier1) lists 4-hour drawdown versus a 72-hour mode elsewhere, highlighting protocol differences (e.g., wet detention vs infiltration framing) that Tier A extraction may not fully disaggregate. Committee discussion should flag where chapter_proxy slugs apply and where practice-specific criteria need re-scoping in source manuals.