Surface Water Drainage Calculator — Runoff, Pipe Capacity & Gullies

The BS EN 752 flat-rate method gives runoff Q (L/s) = effective area (m²) × rainfall intensity (mm/hr) ÷ 3600. The effective area is the plan area multiplied by a runoff coefficient (1.0 for roofs, 0.9 for concrete/asphalt, 0.7 for block paving, 0.4 for gravel). The standard UK flat rate is 50 mm/hr for drains up to 200 m; 75 mm/hr is used for commercial, heavy-rainfall or ponding-sensitive sites where the authority requires enhanced protection. Use our Soakaway Crate Calculator to size infiltration systems for the runoff.

DMRB CD 526 (highway drainage) sets a maximum of 200 m² of impervious catchment per road gully, so divide the area by 200 and round up — for example a 600 m² yard needs at least three gullies. For linear/channel drainage on roads and car parks, gullies are also spaced by distance (typically up to 50 m on cambered roads, closer where gradients are steep or the channel superelevated); use the tighter of the two limits. Each gully connects to the underground pipe via a trapped outlet to prevent odour.

Manning's equation calculates pipe flow velocity: V = (1/n) × R^(2/3) × S^(1/2), where n is the roughness coefficient (0.011 for uPVC), R is the hydraulic radius (pipe diameter/4 for full flow), and S is the pipe gradient. Flow rate Q = V × pipe cross-sectional area.

Where separate public sewers exist, surface water must not discharge to the foul sewer except with the sewerage undertaker’s approval (Water Industry Act 1991 s.106(2)(b); s.109 makes an unauthorised connection an offence). On new developments the surface-water disposal hierarchy in Approved Document H is followed — soakaway first, then watercourse, then a surface-water sewer, with a combined sewer only as a last resort. Where a combined sewer serves the area, surface water may go to it subject to the undertaker’s consent. SuDS (soakaways, permeable paving) are preferred — see our Swale & Bioretention Calculator for sustainable drainage options.

Approved Document H gives minimum gradients of 1:100 for 100mm pipe and 1:150 for 150mm pipe, with a self-cleansing velocity of about 0.75 m/s at low flow. Capacity rises with the square root of the gradient, so a 110mm pipe at 1:40 carries roughly 1.6 times as much as at 1:100 (√(2.5) ≈ 1.58), not double.