Three Hundred Millimetres of What? Ponding Depth Set by Soil Drawdown, Not by the Standard Detail

Rain gardens are commonly detailed with a standard ponding depth — 300 mm appears in guide after guide — as if the depth were a property of the garden rather than of the soil beneath it. This paper computes drawdown time, the hours ponded water takes to clear, across six soil classes at two common depths. A 300 mm pond clears in 6.0 hours on sand and 25.0 hours on sandy loam, but takes 50.0 hours on loam and 120.0 hours on clay loam, far beyond the 48-hour limit that plant health and mosquito control impose. The maximum ponding depth compatible with 48 hours is 288 mm on loam and only 120 mm on clay loam. The sizing rule that follows replaces the standard depth with a soil test: ponding depth must not exceed forty-eight times the measured infiltration rate, and gardens on slow soils must be made shallower and larger in area, not deeper. All results are simulated, not measured.