Rainwater Runoff and Drainage Calculator
Estimate stormwater runoff volume and peak discharge using the Rational Method (Q = CiA) and the SCS Curve Number method. Useful for drainage design, flood estimation, and land development planning.
Catchment Properties
Rainfall Data
SCS Curve Number Method
Formulas Used
1. Rational Method (Peak Flow Rate)
Q = C × i × A / 360
- Q = peak flow rate (m³/s)
- C = dimensionless runoff coefficient (0–1)
- i = rainfall intensity (mm/hr)
- A = catchment area (hectares)
- 360 = unit conversion factor (1 mm/hr × 1 ha = 1/360 m³/s)
2. Time of Concentration – Kirpich Formula
Tc = 0.0195 × L0.77 × S−0.385
- Tc = time of concentration (minutes)
- L = flow path length (metres)
- S = average slope (m/m)
3. SCS Curve Number Method (Runoff Depth)
S = (25400 / CN) − 254 | Iₐ = 0.2 × S | Q = (P − Iₐ)² / (P − Iₐ + S) [if P > Iₐ]
- S = potential maximum retention (mm)
- CN = SCS Curve Number (1–100)
- Iₐ = initial abstraction (mm) — losses before runoff begins
- P = total rainfall depth (mm)
- Q = direct runoff depth (mm)
4. Manning's Equation – Circular Pipe (Full Flow)
Q = (1/n) × (π/4 × D²) × (D/4)2/3 × S1/2 → D = [Q × n / (0.3117 × √S)]3/8
- n = Manning's roughness coefficient (0.013 for concrete)
- D = pipe diameter (m)
- S = pipe slope (m/m)
5. Gumbel Frequency Factor
K = −(√6 / π) × [0.5772 + ln(ln(T / (T−1)))] | QT = Q × (1 + K × Cv)
- T = return period (years)
- Cv = coefficient of variation (assumed 0.3)
Assumptions & References
- The Rational Method is most accurate for catchments < 80 ha with uniform land use and short concentration times (AS/NZS 3500, ASCE Manual of Engineering Practice No. 36).
- Runoff coefficients C are based on standard engineering tables (ASCE, Australian Rainfall & Runoff AR&R 2019).
- The Kirpich formula was developed for small agricultural watersheds (Kirpich, 1940); results should be verified for urban catchments.
- The SCS CN method (now NRCS TR-55) uses Antecedent Moisture Condition II (average conditions). Initial abstraction ratio Iₐ = 0.2S is the standard assumption.
- Curve Numbers are from USDA NRCS National Engineering Handbook, Part 630, Chapter 9.
- Manning's n = 0.013 is standard for smooth concrete pipes (Chaudhry, Open-Channel Hydraulics).
- Pipe slope defaults to 0.5% if catchment slope is not provided — verify against site survey data.
- Standard pipe sizes follow AS 1597 / ASTM C76 nominal diameter series.
- The Gumbel frequency analysis assumes a coefficient of variation Cv = 0.3, which is typical for temperate climates. Local IFD (Intensity-Frequency-Duration) data should be used for design.
- This calculator does not account for baseflow, antecedent soil moisture beyond CN class II, or complex catchment routing — use dedicated hydrological software (DRAINS, SWMM, HEC-HMS) for detailed design.