Drawn to Fit the Site, Sized by the Flow: The Length a Grass Swale Needs to Treat Runoff

Grass swales are drawn on drainage plans as green lines of whatever length the site happens to offer, and credited with pollutant removal as if the credit were automatic. This paper derives the length a swale actually needs from its hydraulics. For a trapezoidal swale with 0.6 m bottom width flowing 0.15 m deep through dense grass, the residence time required for eighty percent removal of suspended solids at a stated first-order rate is 14.6 minutes, which converts to a required length of 27.9 m at a treatment flow of 5 L/s, 55.7 m at 10 L/s, and 111.5 m at 20 L/s. Velocity, and therefore length, rises in direct proportion to flow while residence time is fixed by the removal target, so a swale that treats a small catchment's runoff adequately fails a catchment twice the size unless it is twice as long or the flow is split. The paper states the sizing rule — length from target residence time times design velocity, checked against the slope that keeps treatment flow shallow — and identifies the common shortfall: swales drawn to fit the site rather than the flow. All results are simulated, not measured.