Stormwater Drainage Design

Rational Method · SA IDF rainfall data (SANRAL) · Manning's pipe sizing · SANS 677 standard sizes

1Catchment Parameters

Catchment area A (ha)

Rational Method: A ≤ 15 ha for urban drainage.

Land use / runoff coefficient C

C = 0.50

Mixed catchment — weighted C

Tag / drain reference (optional)

2Time of Concentration t_c

Flow-path length L (m)

Average catchment slope S (m/m)

t_c = 0.0195 × L^0.77 × S^−0.385

Kirpich (1940) — common for overland flow. Minimum t_c = 5 min for urban drainage.

3Design Rainfall (IDF)

City / region

Return period T (years)

Typical: 5yr local drainage · 10yr streets · 25yr major roads · 100yr critical infrastructure

4Pipe Design Parameters

Pipe invert slope S (m/m)

Min 0.003 m/m for self-cleansing. Match ground level where possible.

Pipe material

Pipe length (m) — for cost estimate

Min velocity (m/s)

Self-cleansing (sediment)

Max velocity (m/s)

Erosion limit

Guidance only. IDF values are representative of published SANRAL data. Local authority IDF data and the SANRAL Drainage Manual (5th Ed.) must be verified before use in construction drawings or formal design reports.

Enter catchment area, t_c, and rainfall data

Results update live — SA IDF embedded for 8 cities