Designing 🌧️ Stormwater Drainage systems is essential to ensure the efficient collection and disposal of surface runoff from rainfall. Proper pipe sizing ensures that the stormwater is carried safely without causing flooding, ponding, or structural damage.
📘 1. Key Formula: Rational Method
The Rational Method is widely used for stormwater pipe sizing in small urban drainage systems: Q=C ⋅ i ⋅ A
Where:
- Q = Peak runoff (m³/s or cfs)
- C = Runoff coefficient (depends on surface type)
- i = Rainfall intensity (mm/hr or in/hr)
- A = Catchment area (hectares or acres)
📐 2. Step-by-Step Pipe Sizing Procedure
🔹 Step 1: Define Catchment Area
Measure the area contributing runoff to the pipe. Use site drawings or GIS tools.
- Units: hectares (ha) or square meters (m²)
🔹 Step 2: Assign Runoff Coefficient (C)
Based on surface type:
| Surface | C value |
|---|---|
| Paved / Asphalt | 0.85–0.95 |
| Roof surface | 0.75–0.95 |
| Lawn / grass area | 0.10–0.35 |
| Concrete surface | 0.90–0.95 |
🔹 Step 3: Determine Rainfall Intensity (i)
Use local IDF curves or rainfall data for the required return period (e.g., 10-year, 25-year storm).
- Units: mm/hr or in/hr
🔹 Step 4: Calculate Peak Flow (Q)
Q = (C⋅i⋅A) / 360
- Result in m³/s if using mm/hr and hectares
- Adjust for multiple catchments if needed
🔹 Step 5: Select Pipe Diameter
Use Manning’s Formula for flow in pipes: Q = 1/ n ⋅ A ⋅ R ^ 2/3 ⋅ S ^1/2
Where:
- n = Manning’s roughness coefficient (typically 0.013 for concrete, 0.011 for PVC)
- A = Cross-sectional area of pipe
- R = Hydraulic radius = A / P (P = wetted perimeter)
- S = Slope of pipe
🧰 Example Calculation
- Catchment Area (A) = 0.5 ha
- Surface = Pavement ⇒ C = 0.90`
- Rainfall Intensity (i) = 75 mm/hr
- Pipe Slope (S) = 0.005 (0.5%)
- Manning’s n = 0.013
Q = 0.9 ⋅ 75 ⋅ 0.5 / 360 = 0.094 m3/s
Use Manning’s formula to back-calculate required pipe diameter that will carry 0.094 m³/s.
f = arc sin (0.4r/r) = 0.412 radians = 23.578 degrees
q = 180+2*f = 3.965 radians = 227.156 degrees
Sample Calculation for an 8″ (0.203mm) pipe. D = 0.203
FLOW AREA
Method 1
= 0.102^2*(227.156*pi/180-SIN(227.156))/2
= 0.024 m2
Method 2
= 0.102^2*(3.965/2+COS(0.412)*SIN(0.412)
= 0.024 m2
WETTED PERIMETER
= 0.102*3.965
= 0.402 m
B. CALCULATION OF DISCHARGE CAPACITY, Qc
Manning’s Formula
Assume flow depth equal to 0.7 times the pipe diameter for the max. design flow rate.
Qc = 3600 * A * R2/3 * S 1/2 / n
Where,
D = Pipe Diameter, m
A = Flow Area, m2
P = Wetted Perimeter, m
S = Bed Gradient
n = Roughness Coefficient
R = Hydraulic Radius, m
= A/P
C.CALCULATION OF VELOCITIES
Qc = V * A
Where ,
v = velocity, m/s
A = Flow Area, m2
vmax ≤ 3 m/s
vmin ≥ 0.75 m/s Qcmin = 0.2 x Qcmax
✅ Design Tips
- Always check full flow capacity (pipe not surcharged)
- Include minimum self-cleansing velocity (≥ 0.6 m/s)
- Consider future expansion (use 25–50% extra capacity)
- Provide manholes and catchpits at changes in slope or direction
📚 Codes & References
- IS 1172, IS 2065, IS 3370, and SP 35
- US EPA SWMM software
- StormCAD / Civil 3D for advanced modeling
- Local municipal design manuals
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