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Storm water from a 45 ha catchment

Short answer

Weighted runoff coefficient is 0.5025. With tc=30 min, intensity is 19 mm/hr, and A=45 ha, the storm water quantity by Q=CiA/360 is 1.193 m3/s (1193 L/s).

The catchment area of a city is 45 hectares. Assuming that the surface on which rain falls is classified as follows:

Type of Surface% AreaRunoff Coefficient
1. Forest and Wooden Area100.15
2. Open ground + Unpaved street100.20
3. Parks + Lawns + Gardens150.15
4. Gravel Road200.25
5. Asphalt Pavements200.85
6. Water tight Roof Surfaces250.90

Calculate the quantity of storm water if time of entry is 20 minutes and time of flow is 10 minutes.

Storm water quantity by the Rational method: Q=CiA360Q=\dfrac{CiA}{360}.

Step 1: Weighted runoff coefficient

Surface% areaC% x C / 100
Forest and wooded100.150.0150
Open ground + unpaved street100.200.0200
Parks, lawns, gardens150.150.0225
Gravel road200.250.0500
Asphalt pavement200.850.1700
Watertight roof250.900.2250
Total1000.5025
C=∑CjAj∑Aj=0.5025C=\frac{\sum C_jA_j}{\sum A_j}=0.5025

Step 2: Time of concentration and intensity

tc=te+tf=20+10=30 mint_c=t_e+t_f=20+10=30\ \text{min}

No intensity formula is given, so the common formula i=760t+10i=\dfrac{760}{t+10} mm/hr is assumed:

i=76030+10=19.0 mm/hri=\frac{760}{30+10}=19.0\ \text{mm/hr}

Step 3: Discharge

Q=0.5025×19.0×45360=1.193 m3/sQ=\frac{0.5025\times19.0\times45}{360}=1.193\ \text{m}^3/\text{s}

Answer: Storm water quantity = 1.193 m³/s (1193 l/s).

✓ Answer checked by Claude Opus 5.5 (high effort)

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