Step 1: how much water reaches the pipe
Add up the hard area that drains to the pipe: roofs, drives, paths and car parks. Convert it to hectares (10,000 m² = 1 ha). Then use the rational method:
Q (l/s) = 2.78 × C × i × A
- A - the area in hectares.
- C - how much of the rain runs off. Use 1.0 for roofs and tarmac (the usual UK practice for impermeable areas); lawns and gardens add much less and are often left out of a small scheme.
- i - the rainfall intensity in mm/h for the storm you're designing for. It depends on where the site is, how often the storm comes (the return period) and how long it lasts. Short storms are the most intense, so small sites are designed for short ones. Take it from your rainfall data (FEH) or from the drainage engineer; 50 mm/h is only the figure used in the example here.
The rational method is meant for small areas. For a whole estate, use a network design that adds time of entry and the flow time through the pipes, as drainage design software does.
Step 2: how much a pipe can carry
A pipe's capacity depends on its diameter, its gradient and how rough it is inside. This table is for a pipe running full:
| Pipe | Gradient | Capacity, running full | Velocity, full |
|---|---|---|---|
| 100 mm | 1 in 40 | 9.6 l/s | 1.22 m/s |
| 100 mm | 1 in 100 | 6.0 l/s | 0.77 m/s |
| 150 mm | 1 in 60 | 23.0 l/s | 1.30 m/s |
| 150 mm | 1 in 100 | 17.7 l/s | 1.00 m/s |
| 150 mm | 1 in 150 | 14.4 l/s | 0.82 m/s |
| 225 mm | 1 in 100 | 51.9 l/s | 1.31 m/s |
| 225 mm | 1 in 150 | 42.3 l/s | 1.06 m/s |
| 225 mm | 1 in 200 | 36.5 l/s | 0.92 m/s |
| 300 mm | 1 in 100 | 111.0 l/s | 1.57 m/s |
| 300 mm | 1 in 150 | 90.5 l/s | 1.28 m/s |
| 300 mm | 1 in 200 | 78.2 l/s | 1.11 m/s |
Colebrook-White, roughness ks = 0.6 mm (the surface water value in Sewers for Adoption), water at 15°C.
Steeper pipes carry more. Doubling the gradient adds about 40% to the capacity, while one size up roughly triples it.
Step 3: check it keeps itself clean
A pipe that runs too slowly silts up. For surface water, Sewers for Adoption looks for at least 1.0 m/s with the pipe running full. In the table, a 150 mm pipe reaches that at 1 in 100 and a 225 mm pipe at about 1 in 150, so a bigger pipe can run flatter. A 150 mm pipe at 1 in 150 only manages 0.82 m/s.
Worked example
A small block with a 600 m² roof and a 400 m² car park, all to one outfall: A = 1,000 m² = 0.1 ha, C = 1.0, i = 50 mm/h.
Q = 2.78 × 1.0 × 50 × 0.1 = 13.9 l/s.
- 100 mm at 1 in 40 carries 9.6 l/s - too small.
- 150 mm at 1 in 150 carries 14.4 l/s but only at 0.82 m/s - too slow to self-clean.
- 150 mm at 1 in 100 carries 17.7 l/s at 1.00 m/s - this one works.
If the levels won't allow 1 in 100, go up to 225 mm, which carries the flow at 1 in 200 and still runs at 0.92 m/s - a little under 1.0 m/s, so check it with whoever adopts the sewer.
The minimums for sewers for adoption
- Surface water sewers: at least 150 mm; laterals serving one property: 100 mm.
- Self-cleansing: 1.0 m/s running full.
- Pipe roughness for the sums: ks = 0.6 mm for surface water (1.5 mm for foul).
- No surcharge in the design storm (usually 1 in 2 years), and no flooding in a 1 in 30 year storm.
These are from Water UK's Design and Construction Guidance (Sewers for Adoption). Check the current version and anything extra your sewerage company asks for.