Eg:
A 5-hour rainfall produces a rainfall depth of 4cm. for the 1st 2 hour period & 5cm. depth for the remaining 3 hour period. The Φ-index for the stormwater is 1.4cm/hour. Calculate the total volume of runoff, if the area of the catchment is 124km².
Calculation:
To find:
The total volume of runoff =?
The runoff depth/hour
= [rainfall / hour - Φ-index/hour]
Note:
1. Φ-index is the constant infiltration of rainwater that yields the actual runoff volume of the total rainfall water.
2. Rainfall rates equal to or lesser than Φ-index does not provide any volume for the runoff. Hence they are not taken while calculating the runoff.
Therefore,
Total runoff depth
= [{2hr. rainfall depth - (2hr. × Φ-index/hr.)} + {3hr. rainfall depth - (3hr. × Φ-index /hr.)}]
= [{4cm. - (2 × 1.4cm.)} + { 5cm. - (3 × 1.4cm.)}]
= [1.2cm. + 0.8cm.]
= 2cm. = 0.02m.
Note:
As the given value of Φ-index is per hour of rainfall, we have to multiply the value as per the total hour of the rainfall.
The total volume of runoff
= [total depth of runoff х catchment area]
First, let us convert the catchment area in m²
i.e. 124 km². = 124 х 10⁶ m²
The total volume of runoff
= [0.02m. х 124 х 10⁶ m² ]
= 2.48 х 10⁶ m³
or
The total volume of runoff = 2.48 MCM.
MCM 👉 Million cubic meters.
Thank you for going through these calculation steps❤. Have a good day 😄.
