Dry Well / Infiltration Trench Sizing
The size of a stone-filled dry well or infiltration trench (soakaway) that stores runoff and soaks it into the ground.
Example
You enter
- Runoff (storage) volume (ft³) 200
- Aggregate porosity n (void fraction, voids / total, ~0.35) 0.35
- Trench/pit depth (ft) 4
- Soil infiltration rate (in/hr, perc test) 0.5
You get
- Excavation volume 571.4
- Footprint (at this depth) 143 sf
- Draindown time 33.6
Checked against published worked examples from: Walton County, FL.
Details, formula, and sources
ScopeClean crushed stone holds water only in its VOIDS (~30-40%), so the excavation is larger than the stored water by 1/porosity (void fraction): storing 200 ft^3 of runoff in 0.35-void stone needs a 571 ft^3 pit, a 143 sf footprint at 4 ft deep. It empties by infiltration; a rough draindown is the void-water column (depth x void) over the soil rate, so a 4 ft pit at 0.35 void over 0.5 in/hr soil drains in ~34 hr (a good system empties between storms, ~24-72 hr). Runoff comes from the design storm/area (a rational/detention calc), the void from the actual aggregate, and the infiltration rate from a field PERC test -- not a default. An overflow/bypass is required. A sizing screen; the perc test, the stormwater code, and the AHJ/geotech govern.
Formula or tableexcavation_volume_ft3 = runoff_volume_ft3 / void_ratio; footprint_sf = excavation_volume_ft3 / trench_depth_ft; draindown_time_hr = 12 x trench_depth_ft x void_ratio / infiltration_rate_in_hr.
Source and editionDry well / infiltration trench (soakaway) sizing by void-ratio storage and soil infiltration, standard stormwater / on-site infiltration practice, by name; the field percolation test, the stormwater code, and the AHJ / geotech govern.
Free accessThe storage and draindown relations are public first-principles (voids store water; infiltration empties it); the runoff, porosity (void fraction), depth, and infiltration rate are the design, aggregate, and field-test values.
What governsEstimate. AHJ and licensed professional govern.
API field namesrunoff_volume_ft3, void_ratio, trench_depth_ft, infiltration_rate_in_hr, excavation_volume_ft3, footprint_sf, draindown_time_hr
- Void-ratio storage excavation = runoff / porosity (void fraction) (stone ~0.35); footprint = excavation / depth; stone holds water only in its voidson-site infiltration practice
- Draindown + perc test draindown ~ 12 x depth x void / infiltration rate (target < ~24-72 hr); the infiltration rate MUST come from a field perc test, not a default; an overflow is requiredfield percolation test / stormwater code