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Rainwater Flow Calculator

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Input Data
Enter your project data
Total surface area contributing to runoff. Auto-detects scale: ≤1000 m2 (Building), 1000-10000 m2 (Site), >10000 m2 (Infrastructure).
Select preset regional values or enter custom rainfall intensity
Select based on local climate conditions. Standard: 300 L/s/ha for typical urban design.
Custom intensity in L/s/ha. Typical range: 100-800 for buildings, up to 5000 for extreme infrastructure events. IDF curves from local meteorological data.
AUTO-CALCULATED: Slope optimized based on pipe diameter and scale. Building: 1.0%, Site: 0.5%, Infrastructure: 0.15-0.3%.
Auto-selected: PVC for Building/Site (≤10000 m2), Concrete for Infrastructure (>10000 m2). User can override. Manning's roughness coefficient affects flow capacity.
Leave 0 or blank for automatic pipe sizing. Enter value to check capacity of specific pipe. Infrastructure range: 300-2000mm.
Total pipe length for excavation depth calculation. Used for infrastructure cost estimation.
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Calculation Results
Calculated instantly from your inputs
Rainwater
Rainwater Flow Calculator

This calculator determines stormwater runoff flow using the Rational Method and sizes drainage pipes using Manning's Equation.

SCALE AUTO-DETECTION:
• Building: ≤1000 m2 (residential/commercial buildings)
• Site: 1000-10000 m2 (parking lots, small developments)
• Infrastructure: >10000 m2 (municipal collectors, highways)

Rational Method: Q = i x A
- Q = Peak flow (L/s or m3/h)
- i = Rainfall intensity (L/s/ha)
- A = Drainage area (m2)

Manning's Equation: V = (1/n) x R^(2/3) x S^(1/2)
- V = Flow velocity (m/s)
- n = Roughness coefficient
- R = Hydraulic radius (D/4 for full pipe)
- S = Slope (m/m)

AUTO-OPTIMIZED SLOPES:
• Building (≤1000 m2): 1.0% fixed
• Site (1000-10000 m2): 0.5% fixed, 0.3% if D>400mm
• Infrastructure (>10000 m2):
- D ≤400mm: 0.5%
- D ≤800mm: 0.3%
- D ≤1200mm: 0.2%
- D ≤2000mm: 0.15%

AUTO-MATERIAL SELECTION:
• ≤10000 m2: PVC (smooth, economical)
• >10000 m2: Concrete (infrastructure grade)

Design Guidelines:
- Velocity: 0.6-3.0 m/s (min: self-cleansing, max: erosion prevention)
- Infrastructure velocity: max 2.5 m/s for concrete pipes
- Fill ratio: < 85% for free surface flow
- Excavation depth: H = L x S (pipe length x slope)

Standards:
- Rational Method: Q = CiA (Q in m3/s, i in m/s, A in m2)
- Manning's Equation: SI units with n from standard tables
- Pipe sizing: 25% safety margin on capacity
- Infrastructure: EN 476, ASTM C76 for RCP

Key Points:
  • Automatic scale detection (Building/Site/Infrastructure)
  • Auto-optimized slopes for excavation cost minimization
  • Auto-material selection based on scale
  • Rational Method for peak runoff calculation
  • Preset intensities: 100-800 L/s/ha for different climates
  • Manual intensity up to 5000 L/s/ha for extreme events
  • Manning's equation for pipe flow capacity
  • Automatic pipe sizing with 25% safety margin
  • Limited alternatives display (9 pipes: selected ±4)
  • Excavation depth calculation for cost estimation
  • Overflow warning when capacity exceeded
Calculation Steps:
  1. Enter all required parameters
  2. Click "Update Calculation"
  3. Review results and analysis
Formula Variables:
Q Flow Rate (m3/h)
i Rainfall Intensity (L/s/ha)
A Drainage Area (m2)
n Manning's n (-)
D Pipe Diameter (mm)
V Flow Velocity (m/s)
S Optimized Slope (%)
H Excavation Depth (m)