Soakaways
Soakaway pit depth (BRE Digest 365)
- where
- inflow volume (m³)
- impermeable area drained (m²)
- rainfall depth for the storm (mm)
- effective depth needed (m)
- free volume (porosity) of the fill
- pit length (m)
- pit width (m)
- soil infiltration rate (m/s)
- storm duration (min)
- storage volume (m³)
- time to half empty (h)
Explain this equation
This sizes the effective depth of a rectangular soakaway for one storm duration, using the BRE Digest 365 balance of inflow, outflow and storage, and checks it half-empties in time.
A is the roof or paved area drained and R the rainfall depth for the storm, so I is the volume of water arriving. n is the free void space in the fill, L and W the pit plan size, f the soil’s infiltration rate and Dm the storm length in minutes. S is the water stored and ts50 the time to drain half of it, in hours.
Storage needed equals inflow minus what soaks out during the storm. Only the side area up to half depth, (L + W)·d, counts, because the base silts up. Setting the stored volume n·L·W·d equal to I minus outflow and solving gives d. The ×60 and ×3600 convert minutes and hours to seconds.
Sense check: 25 m² of roof in a 10 mm, 5-minute storm sends 0.25 m³; in stone fill with 30% voids that needs about 0.8 m³ of pit.
Watch out: try a range of storm durations and design for the deepest answer; longer storms often govern in slow soils.
Effective depth of a rectangular soakaway of chosen plan size so that storage equals inflow minus outflow for a storm, with the BRE 365 half-emptying check.
Enter your values to see the result.
Good to know
BRE 365: storage S = A·R − as50·f·D, where as50 is the side area up to half the effective depth (the base is ignored because it silts up). For a rectangular pit as50 = (L + W)·d, so the depth can be solved directly. Repeat for storm durations from 5–10 minutes up to 24 hours using the 10-year rainfall depths for the site (M5-60 and r from BRE 365 or the Flood Estimation Handbook) and design for the deepest result. AD H para 3.27: design to a 1 in 10-year return period; for 25 m² or less, 10 mm in 5 minutes may be assumed. Local authorities often ask for a climate change allowance (for example +30% on rainfall) and for bigger sites a 1 in 30 or 100-year check.
Common mistake
Running only a short, intense storm. For slow-draining soils the longer storms (one to several hours) usually govern.
Limits
Single storm duration per run; rectangular pit with vertical sides; fill voids treated as uniform. Over 100 m² drained, AD H points to BRE 365 or BS EN 752 for full design. Keep soakaways at least 5 m from buildings and away from boundaries as building control requires.
Sources
- BRE Digest 365 Soakaway design (2016): f = Vp75–25 / (ap50 × tp75–25); S = A·R − as50·f·D; ts50 = 0.5·S / (as50·f) ≤ 24 h
- BRE Digest 365 soakaway design worked example (125 m², M10 rainfall, f = 3.3×10⁻⁵ m/s, 2.4 m × 1.5 m pit, 30% voids)
- Runnymede Borough Council, Surface water drainage guidance for new development (BRE 365 storage and half-emptying formulas, ts50 ≤ 24 h)
- Approved Document H: Drainage and waste disposal, 2015 edition (Table 6 minimum gradients for foul drains; Tables 11 and 12 access fittings, inspection chambers and manholes; para 3.15 rainwater drain gradients; paras 3.27–3.30 soakaways)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.