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Waste Water Engineering - Engineering

Q1:

The drainage area of a town is 12 hectares. Its 40% area is hard pavement (K = 0.85), the 40% area is unpaved streets (K = 0.20) and the remaining is wooded areas (K = 0.15). Assuming the time of concentration for the areas as 30 minutes and using the formula the maximum run off is

A 0.10 cumec

B 0.12 cumec

C 0.15 cumec

D 0.20 cumec.

ANS:C - 0.15 cumec

Quantity of Run-off, Q = (K*Pc*A)/36.

Where,
K = Run-off Coefficient.
Pc = Intensity of rainfall (cm/hr).
A = Drainage area (hec).
Formula for finding K :-
K = (A1K1 + A2K2 + A3K3) / A.

A1 = 40% of area = 4.8 hec, K1 = 0.85.
A2 = 40% of area = 4.8 hec, K2 = 0.20.
A3 = 20% of area = 2.4 hec, K3 = 0.15.

Therefore, K = (4.8*0.85 + 4.8*0.2 + 2.4 *0.15) / 12 = 0.45.

Formula for finding Pc :-
Pc = 900 /(t+60).

Given, time of concentration (t) =30 min and also use the given formula for finding Intensity of Rainfall (Pc).

Pc = 900 / (30+60) = 10 mm/hr = 1 cm/hr.

Now, Use the rational formula for finding Quantity of storm run-off (Q).

Q =28 A*pc*k
= 28*12*1*.45
= 151 lit/sec.

Or 0.151 cumecs.