Water Runoff Volume Calculator
Water Runoff Volume Calculator: Calculates runoff from a given area during a storm event.
Formulas Used
The runoff volume is calculated using the Rational Method, suitable for small watersheds.
- Runoff Flow Rate (Q):
\\[ Q = \frac{C \cdot I \cdot A}{360} \\]
Where:
- \\( Q \\): Runoff flow rate (m³/s)
- \\( C \\): Runoff coefficient (0–1, depends on surface type)
- \\( I \\): Rainfall intensity (mm/h)
- \\( A \\): Catchment area (ha)
- 360: Conversion factor (ha·mm/h to m³/s)
- Runoff Volume (V):
\\[ V = Q \cdot T \cdot 3600 \\]
Where:
- \\( V \\): Runoff volume (m³)
- \\( T \\): Rainfall duration (hours)
- 3600: Converts seconds to hours
- Runoff Intensity (per hectare):
\\[ R_{\text{int}} = \frac{V}{A} \\]
Runoff volume per hectare (m³/ha) for impact assessment.
- Impact Level:
Based on \\( R_{\text{int}} \\):
- Low: \\( R_{\text{int}} \leq 50 \\) m³/ha
- Moderate: \\( 50 < R_{\text{int}} \leq 200 \\) m³/ha
- High: \\( 200 < R_{\text{int}} \leq 500 \\) m³/ha
- Severe: \\( R_{\text{int}} > 500 \\) m³/ha
Example Calculations
Example 1: Urban Parking Lot
Inputs: Rainfall Intensity = 50 mm/h, Area = 2 ha, Surface Type = Impervious (C = 0.9), Duration = 1 h
Calculations:
- Runoff Flow Rate: \\[ Q = \frac{0.9 \cdot 50 \cdot 2}{360} = \frac{90}{360} = 0.25 \, \text{m}^3/\text{s} \\]
- Runoff Volume: \\[ V = 0.25 \cdot 1 \cdot 3600 = 900 \, \text{m}^3 \\]
- Runoff Intensity: \\[ \frac{900}{2} = 450 \, \text{m}^3/\text{ha} \\]
- Impact Level: High (200–500 m³/ha)
Result: Runoff Volume: 900 m³ (High)
Example 2: Grassy Field
Inputs: Rainfall Intensity = 30 mm/h, Area = 10 ha, Surface Type = Partially Pervious (C = 0.6), Duration = 2 h
Calculations:
- Runoff Flow Rate: \\[ Q = \frac{0.6 \cdot 30 \cdot 10}{360} = \frac{180}{360} = 0.5 \, \text{m}^3/\text{s} \\]
- Runoff Volume: \\[ V = 0.5 \cdot 2 \cdot 3600 = 3600 \, \text{m}^3 \\]
- Runoff Intensity: \\[ \frac{3600}{10} = 360 \, \text{m}^3/\text{ha} \\]
- Impact Level: High (200–500 m³/ha)
Result: Runoff Volume: 3600 m³ (High)
Example 3: Forested Area
Inputs: Rainfall Intensity = 20 mm/h, Area = 5 ha, Surface Type = Highly Vegetated (C = 0.15), Duration = 0.5 h
Calculations:
- Runoff Flow Rate: \\[ Q = \frac{0.15 \cdot 20 \cdot 5}{360} = \frac{15}{360} \approx 0.04167 \, \text{m}^3/\text{s} \\]
- Runoff Volume: \\[ V = 0.04167 \cdot 0.5 \cdot 3600 \approx 75 \, \text{m}^3 \\]
- Runoff Intensity: \\[ \frac{75}{5} = 15 \, \text{m}^3/\text{ha} \\]
- Impact Level: Low (≤50 m³/ha)
Result: Runoff Volume: 75 m³ (Low)
How to Use the Calculator
Follow these steps to estimate the stormwater runoff volume:
- Enter Rainfall Intensity: Input the rainfall rate in mm/h (1–200, e.g., 50). Use the decimal button (.) for precision.
- Enter Catchment Area: Input the area in hectares (0.01–1000, e.g., 5). Use the decimal button for precision.
- Select Surface Type: Choose impervious (C=0.9), partially pervious (C=0.6), pervious (C=0.3), or highly vegetated (C=0.15).
- Enter Rainfall Duration: Input the duration in hours (0.1–24, e.g., 1). Use the decimal button for precision.
- Calculate: Click “Calculate Runoff Volume” to see the result.
- Interpret Result: The result shows the runoff volume in m³ with an impact level (Low: ≤50, Moderate: 50–200, High: 200–500, Severe: >500 m³/ha). If you see “Please fill in all fields,” ensure all inputs are valid.
- Share or Embed: Use the share buttons to post results on social media, copy the result, or get an embed code.
Note: This is a simplified model based on the Rational Method. Actual runoff depends on factors like soil saturation, topography, and drainage systems.