Constructed Wetland

National baseline

Among Tier A manuals that mention constructed wetlands, explicit facility sizing is sparse compared with site-wide water-quality and drainage criteria that jurisdictions apply across BMP types. Where permanent pool depth is stated (n=10), the modal value is 36 inches with a reported range of 18–96 inches; the median is 42 inches, so a shallow permanent zone near 3 ft is the most frequently cited anchor, though several manuals specify deeper pools. Length-to-width guidance appears in 17 manuals with mode and median of 2:1 and a range from 0.3:1 to 3:1, suggesting elongated basins are commonly recommended but not uniform. Wetland-specific detention time is rarely numerified (n=3); where given, values cluster around 24–48 hours with mode 24 hours—too few sources to treat as a national facility standard. Site-wide context in the same manual set commonly references water-quality volume depths near 1 inch (n=30, mode 1, range 0.37–1.61), maximum drawdown times often centered on 48–72 hours (n=36, mode 72, median 48, range 4–120), and separation from seasonal high water table frequently at 24 inches mode with median 36 inches (n=36, range 12–120)—these describe jurisdiction sizing assumptions rather than constructed-wetland geometry alone.

Matrix summary

For constructed wetland in Tier A manuals (46 of 76 mention the practice): site-wide WQv depth n=30, mode 1 in, range 0.37–1.61; site-wide max drawdown n=36, mode 72 h, median 48 h, range 4–120; site-wide SHWT separation n=36, mode 24 in, median 36 in, range 12–120. Practice-specific fields: permanent pool depth n=10, mode 36 in, range 18–96; length-to-width n=17, mode 2, range 0.3–3; wetland detention hours n=3, mode 24 h, range 24–48.

Cross-state parameters

Regional modifiers

Open issues

Permanent pool depth, length-to-width ratio, and wetland detention hours have low extraction counts (n=10, 17, and 3 respectively among manuals mentioning the practice), so numeric facility baselines are not yet established nationally from Tier A text alone. Wetland_detention_hours at n=3 cannot support committee-level numeric adoption. Site-wide WQv depth, max drawdown, and SHWT separation are widely documented but are jurisdiction-wide sizing context and may not map one-to-one to constructed wetland cells; 45–42 manuals lack WQv or wetland-specific numerics in the scanned set. Extreme drawdown values (4 hours Florida FSESCI Tier 1; 120 hours Mississippi) and SHWT span (12–120 inches) indicate unresolved regional policy spread without a single consensus facility standard. Chapter proxies and state drafts in the matrix add uncertainty for cross-manual comparison.