Rainfall Runoff
Surface Runoff · Rational Method · Sewer Systems · Stormwater Retention
Rainfall Runoff Calculator
Formulas & Fundamentals
Q = C · i · A [m³/s]
C = Runoff coefficient [−] | i = Rainfall intensity [mm/h or m/s]
A = Catchment area [ha or km²]
C = Q / (i · A)
Surface behavior: 0 (impermeable, infiltration) to 1 (fully sealed).
i = Q / (C · A) [mm/h]
Rain per unit area per unit time; depends on duration (intensity decreases).
A = Q / (C · i)
Total sewer catchment area in ha or km² (roofs + roads + green areas).
Typical Runoff Coefficients
| Surface Type | C | Description |
|---|---|---|
| Forest land | 0.05–0.10 | Highly permeable |
| Grassland / Parks | 0.10–0.20 | Low sealing |
| Residential area | 0.25–0.40 | Medium sealing |
| Commercial / Roofs | 0.50–0.70 | Highly sealed |
| Traffic area | 0.85–0.95 | Fully sealed |
Rainfall Runoff – Rational Method for Sewer Design
What is Rainfall Runoff?
Rainfall runoff (surface runoff, stormwater) is the water volumetric flow that flows over the land surface to water bodies or sewers after precipitation. The rational method is a proven procedure for calculating rainfall runoff based on rainfall duration, catchment area, and surface characteristics.
Runoff is proportional to runoff coefficient, rainfall intensity, and catchment area.
Runoff Coefficient C
Surface character: 0 = water infiltrates completely
1 = water flows off completely
Example: Green space C ≈ 0.10
Asphalt C ≈ 0.85
Rainfall Intensity i
Frequency & Duration:
Short rain 15 min: i ≈ 100–200 mm/h
Duration 60 min: i ≈ 30–60 mm/h
Regionally different (use local rainfall
frequency tables)
Applications: Sewers, Retention, Drainage
Sewer Systems & Wastewater
Design of storm sewers, mixed sewers, or separate systems. Critical: 2–5 year rainfall (HQ2, HQ5) design storm.
Retention Basins
Rainwater storage, stormwater detention ponds, buffer storage for peak reduction. Storage volume per rainfall series.
Surface Drainage
Parking lot, road, roof drainage. Swales, ditches, gutters designed accordingly.
Example: Sewer Design for Residential Area
Task:
Residential area with 25 ha total area. Average runoff coefficient C = 0.35. Rainfall duration 15 min (HQ2): intensity i = 100 mm/h. What discharge Q must the sewer accommodate?
Solution:
- Q = C · i · A = 0.35 · 100 · 25
- Q = 875 mm/h · ha = 875 / 360 ≈ 2.43 m³/s (Conversion: 1 ha·mm/h = 1/3600 m³/s)
- Sewer cross-section for DN ~800 mm (half-circle) or parallel pipes
Frequently Asked Questions
Rule of thumb:
• Forest / Grassland: 0.05–0.15
• Residential area: 0.30–0.40
• Urban / Traffic: 0.70–0.95
For mixed surfaces: weighted average C_total = (C₁·A₁ + C₂·A₂ + ...) / A_total
Example (typical US area):
• 5 min, 2-year: i ≈ 200 mm/h
• 15 min, 2-year: i ≈ 100 mm/h
• 60 min, 2-year: i ≈ 35 mm/h
Rainfall frequency tables (e.g., NOAA / local authorities) vary by region.
C_total = (C_roof·A_roof + C_road·A_road + C_green·A_green) / (A_roof + A_road + A_green)
Example: 8 ha roof (0.95) + 10 ha road (0.85) + 7 ha green (0.15)
C_total = (0.95·8 + 0.85·10 + 0.15·7) / 25 = 20.05 / 25 ≈ 0.80
1. Sewer Relief: Smaller pipes required
2. Stream Protection: Reduced peak flows to receiving waters
3. Infiltration: In permeable soils, water can seep into the ground
Storage volume is designed based on rainfall series (precipitation – discharge).
Summary
Discharge Q
Q = C·i·A
[m³/s or l/s]
Runoff Coefficient
C
[0 = infiltration to 1 = runoff]
Rainfall Intensity
i [mm/h]
by duration & region
Typical Applications
- Sewer Design: Storm sewers, mixed sewers, separate systems for design storms (2yr, 5yr, 10yr)
- Retention Basins: Storage volume for runoff reduction and peak flow attenuation
- Parking & Roads: Surface drainage with swales, ditches, or drain pipes
- Roof Drainage: Design of roof area drains and discharge capacity
- Green Area Drainage: Infiltration or temporary storage areas for stormwater retention
|
|
|
|