About Stormwater Drainage Calculator
The stormwater drainage calculator finds the peak runoff from a catchment using the Rational Method and sizes the drainage pipe with Manning equation per ASCE, EN 752, and AS/NZS 3500.3. It is used by civil and drainage engineers to design storm sewers, size pipes, and estimate detention storage.
Define one or more catchment areas with surface types, set the return period, storm duration, pipe material, and gradient, and the tool returns the weighted runoff coefficient, rainfall intensity, peak flow, the selected pipe diameter, velocity, capacity, utilization, and a detention volume estimate in real time.
How It Works
- Compute the weighted runoff coefficient C from the area and runoff coefficient of each catchment.
- Find the rainfall intensity from the IDF relation i = a / (tc + b) for the chosen return period, or use a manual override.
- Calculate the peak flow with the Rational Method Q = C * i * A / 3600 in L/s.
- Select the smallest standard pipe whose Manning full-flow capacity exceeds the peak flow, and check the velocity against the self-cleansing range.
Worked Example
Three catchments: roof 500 m2 (C = 0.95), paved 300 m2 (C = 0.90), and lawn 200 m2 (C = 0.25). Total area = 1000 m2. Weighted C = (0.95*500 + 0.90*300 + 0.25*200) / 1000 = 795/1000 = 0.795. For a 10-year storm (a = 2400, b = 10) over 15 min: i = 2400 / (15 + 10) = 96 mm/hr. Peak flow Q = 0.795 * 96 * 1000 / 3600 = 21.2 L/s.
Formulas
- Weighted runoff coefficient
C = sum(Ci * Ai) / sum(Ai)- Rainfall intensity (IDF)
i = a / (tc + b)- Rational Method peak flow
Q = (C * i * A) / 3600- Manning pipe capacity (full flow)
Q = (1 / n) * A * R^(2/3) * S^(1/2)- Time of concentration (Kirpich)
tc = 0.0195 * L^0.77 * S^(-0.385)
Standards & References
- ASCE Rational Method
- EN 752
- AS/NZS 3500.3
Frequently Asked Questions
What is the Rational Method?
The Rational Method estimates peak runoff as Q = C * i * A, where C is the runoff coefficient, i is the rainfall intensity for the design return period, and A is the catchment area. It is the standard approach for small urban catchments.
How is the runoff coefficient determined?
Each catchment has a surface type (roof, paved, gravel, lawn, woodland, or bare soil) with a default runoff coefficient. The tool area-weights these to give a single weighted C for the combined catchment.
How is the pipe sized?
The tool computes full-flow capacity with the Manning equation for each standard pipe size and the chosen material roughness, then selects the smallest diameter whose capacity exceeds the peak flow.
What is the self-cleansing velocity check?
Drainage pipes need a minimum velocity (around 0.6 m/s) to keep solids in suspension and a maximum (around 6.0 m/s) to avoid scour. The tool flags whether the computed velocity falls within this self-cleansing range.