Extended Detention

National baseline

Among Tier A manuals that mention extended detention, site-wide water-quality volume depth is most often cited at 1 inch (n=30, median 1, range 0.37–1.61 inches), reflecting jurisdiction-level sizing rather than a single facility storage depth. Site-wide maximum drawdown time clusters at 72 hours as the mode with a median of 48 hours (n=36, range 4–120 hours). Seasonal high water table separation for pond-type BMPs is frequently 24 inches at the mode but 36 inches at the median (n=36, range 12–120 inches), again as site-wide context. Where manuals specify extended-detention drain or drawdown time for the practice itself, the mode is 24 hours with median 40 hours (n=25, range 24–72 hours). Basin geometry guidance is thin but when present favors a length-to-width ratio of 2 (n=19, median 2, range 0.3–3). These figures describe extracted manual language, not an adopted national standard.

Matrix summary

For extended detention in Tier A manuals (60 of 76 mention the practice): site-wide WQv depth mode 1 inch (n=30, 0.37–1.61); site-wide max drawdown mode 72 h, median 48 h (n=36, 4–120); site-wide SHWT separation mode 24 in., median 36 in. (n=36, 12–120); practice ed_drain_time mode 24 h, median 40 h (n=25, 24–72); length-to-width ratio mode and median 2 (n=19, 0.3–3).

Cross-state parameters

Regional modifiers

Open issues

Roughly half of Tier A manuals lack extractable numerics for key site-wide fields (WQv depth missing in 45 of 76; max drawdown and SHWT each missing in 38 of 76). Extended-detention-specific ed_drain_time_hours is documented in only 25 of 60 mentioning manuals (missing 32), and length-to-width ratio in only 19 (missing 40), so geometry and facility drain-time baselines are not yet established numerically at national scale. Site-wide drawdown ranges from 4 hours (fl-fsesci-tier1) to 120 hours (ms-state-vol2), and SHWT from 12 to 120 inches (san-diego-ca), indicating strong regional divergence not fully captured in a single temperate default. Several values rely on chapter_proxy extractions (ia-state-ch1, de-wet-ponds-draft, nj-state, va-state). Whether site-wide WQv and drawdown limits map one-to-one to extended detention basin design versus broader stormwater programs remains an interpretive gap for synthesis.