Question

A real estate project on a 12 hectare site contains 6 buildings, each with ground coverage of 3 percent of the site area. The landscaped area is 40 percent of the site and rest of the area are roads. Assume coefficient of runoff for landscaped area and road area to be 0.15 and 0.6 respectively. Ignore the rainwater from the roof of the buildings and additional water from outside areas. Considering average rainfall intensity of 70 mm per hour, the estimated peak surface runoff rate from the site is ________ \(m^{3}\)/s. (rounded off to two decimal places)

Hint:

In estimating surface runoff, always account for the different types of land cover (landscaped, roads, etc.) and their respective coefficients of runoff.

Answer 1

Step 1: The total site area is 12 hectares, equivalent to \( 12 \times 10^4 \, \text{m}^2 \), or 120,000 m².

Step 2: Building ground coverage is 3% of the site area. Thus, the area occupied by buildings is: \[ \text{Ground coverage area} = 0.03 \times 120,000 = 3,600 \, \text{m}^2 \]

Step 3: The landscaped area constitutes 40% of the site area: \[ \text{Landscaped area} = 0.4 \times 120,000 = 48,000 \, \text{m}^2 \]

Step 4: The remaining area, designated for roads, is calculated as \( 120,000 - 3,600 - 48,000 = 68,400 \, \text{m}^2 \).

Step 5: Runoff coefficients are 0.15 for landscaped areas and 0.6 for roads. Surface runoff from each area is determined as follows: \[ \text{Runoff from landscaped area} = 0.15 \times 48,000 \times 0.07 = 504 \, \text{m}^3/\text{hr} \] \[ \text{Runoff from roads} = 0.6 \times 68,400 \times 0.07 = 2,868 \, \text{m}^3/\text{hr} \]

Step 6: The aggregate runoff per hour is: \[ \text{Total runoff} = 504 + 2,868 = 3,372 \, \text{m}^3/\text{hr} \]

Step 7: Runoff is converted to cubic meters per second: \[ \text{Runoff in m}^3/\text{s} = \frac{3,372}{3600} \approx 0.70 \, \text{m}^3/\text{s} \]

Conclusion: The estimated peak surface runoff rate from the site is 0.70 m³/s.