Rainwater Flow Calculator

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.
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)