Bioretention

National baseline

Among Tier A manuals that mention bioretention, facility-oriented numeric guidance is partial but recurring. Minimum bioretention media depth is the best-populated practice-specific parameter (n=21): mode and median both sit at 24 inches, with reported minima spanning 12–48 inches. Surface ponding depth is less frequently specified (n=15); where stated, mode and median are 12 inches, with a reported span of 3–24 inches. Design infiltration rates for sizing appear in 20 manuals, clustering at 0.5 in/hr (mode and median), though extracted values range from 0.01 to 2.41 in/hr. Separately, many jurisdictions publish site-wide water-quality volume depth (n=30, mode 1 inch, median 1 inch, range 0.37–1.61 inches), maximum drawdown time (n=36, mode 72 hours, median 48 hours, range 4–120 hours), and seasonal high water table separation (n=36, mode 24 inches, median 36 inches, range 12–120 inches); these figures describe broader stormwater sizing and siting context rather than bioretention cell cross-sections alone. For temperate, Type II–ish design discussions, a workable synthesis anchor is 24 inches minimum filter media, about 12 inches allowable ponding where manuals give a ponding cap, and 0.5 in/hr as the most common stated design infiltration rate—always overlaid on local WQv, drawdown, and SHWT requirements where the matrix provides them.

Matrix summary

For bioretention in Tier A manuals (60 of 76 mention the practice), practice-specific numerics are led by minimum media depth (n=21, mode 24 in, median 24 in, range 12–48 in) and design infiltration rate (n=20, mode 0.5 in/hr, median 0.5 in/hr, range 0.01–2.41 in/hr), while ponding depth remains thinly reported (n=15, mode 12 in, median 12 in, range 3–24 in). Site-wide WQv depth (n=30, mode 1 in), max drawdown hours (n=36, mode 72 h, median 48 h, range 4–120 h), and SHWT separation (n=36, mode 24 in, median 36 in, range 12–120 in) describe broader jurisdictional stormwater context; modes and ranges reflect extracted manual values, not an adopted national standard.

Cross-state parameters

Regional modifiers

Open issues

Bioretention ponding depth (n=15, 45 missing among mentioning manuals) and minimum media depth (n=21, 38 missing) lack numeric consensus strong enough to call a national facility standard; site-wide WQv depth, drawdown, and SHWT are better populated but are jurisdiction sizing context, not uniform cell specs. Design infiltration rate extractions span three orders of magnitude at the low end (0.01–0.0208 in/hr vs. 0.5 in/hr mode), suggesting mixed definitions (native soil, amended media, conservative defaults) that the matrix does not disaggregate. Florida FSESCI Tier 1 reports 4-hour max drawdown as an outlier within the 4–120 hour site-wide range. San Diego’s 120-inch SHWT separation and Philadelphia’s 3-inch ponding depth illustrate tail values that resist a single temperate default. No Tier A numeric field in this extract set defines underdrain, overflow, or pretreatment as universal bioretention requirements.