Design a combined sewer section for a 45 hector residential area having runoff coefficient `0.40, 0 70. 0.25, 0.80, 0.10` for area of `15,20,25,10` and `30%` respectively with altogether `1500` population? Average rainfall duration is `21` min. Self-cleansing velocity is `0.88` m/s. Assume water supply rate = 100 lpcd and time of concentration =20 min.

Solution:

Assuming `80%` of the supplied water reaches the sewer, the average sanitary discharge is given by

`Q_(DWF)=(0.8**1500**0.10)/(86400)`

`=1.388**10^-3\ m^3/s`

The peak sanitary discharge is, `=2**1.388**10^-3=2.78**10^-3`

Now,

Time of concentration, `T_c=20\ m i n`

And the storm duration is 21 min.

Since the storm duration is greater than the time of concentration, whole of the area (45 hectares) will contribute for the runoff.

Now, the intensity of rainfall is,

`i=1020/(t+20)`

`=1020/(21+20)`

`=24.88\ (m m)/(hr)`

Now, average impermeability coefficient is given by,

`C=(C_1A_1+C_2A_2+C_3A_3+C_4A_4+C_5A_5)/(A_1+A_2+A_3+A_4+A_5)`

`=0.3725`

Now, The storm discharge is,

`Q_( W W F)=(CiA)/360`

`=(0.375**24.87**45)/360`

`=1.1583\ m^3/s`

Thus, the combined discharge is,

`Q=2.77**10^-3+1.1583=1.16115\ m^3/s`

Given, Self cleansing velo....

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