Two Gates for a Soakaway: Storage Scanned Over Storm Durations, and a Recovery Test That Size Cannot Pass

Soakaways are still frequently sized by folklore — a cubic metre per so many square metres of roof — without reference to the storm durations that actually govern storage. This paper sizes a soakaway for a 120 square metre catchment by the storage method, maximising required volume over eight storm durations for two soils. On sandy loam at an infiltration rate of 1×10⁻⁵ m/s (halved by a stated safety factor), the required storage is 3.48 cubic metres, set by the 6-hour duration, with a half-emptying time of 9.7 hours. On clay loam at 1×10⁻⁶ m/s the requirement rises to 6.17 cubic metres, set by the 24-hour storm, and the half-emptying time stretches to 171.3 hours — beyond the twenty-four-hour recovery a soakaway needs between storms, so on that soil the device fails its recovery test no matter how large it is built. The paper states the sizing procedure and the recovery criterion together, since a soakaway that passes the first and fails the second is a buried tank that happens to leak slowly. All results are simulated, not measured.