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 $${{\text{P}}_{\text{s}}} = \frac{{900}}{{{\text{t}} + 60}}$$ the maximum run off is A. 0.10 cumec B. 0.12 cumec C. 0.15 cumec D. 0.20 cumec

The correct answer is $\boxed{\text{B}. 0.12 \text{ cumec}}$.

The peak discharge $Q_p$ is given by the formula:

$$Q_p = \frac{K A}{\tau}$$

where:

• $K$ is the runoff coefficient
• $A$ is the drainage area
• $\tau$ is the time of concentration

The runoff coefficient $K$ is a dimensionless number that represents the fraction of precipitation that will runoff. It depends on the land use and surface characteristics. For hard pavement, $K = 0.85$. For unpaved streets, $K = 0.20$. For wooded areas, $K = 0.15$.

The drainage area $A$ is given to be 12 hectares. The time of concentration $\tau$ is given to be 30 minutes.

Substituting these values into the formula, we get:

$$Q_p = \frac{0.85 \times 12 \text{ ha}}{{30 \text{ min}} \times \frac{60 \text{ min}}{1 \text{ hour}}} = 0.12 \text{ cumec}$$

Therefore, the maximum runoff is $\boxed{\text{B}. 0.12 \text{ cumec}}$.

The other options are incorrect because they do not represent the correct value of the peak discharge.