ANS:C - 4 hours

To calculate the inlet time for the overland flow from the critical point to the drain, we can use the kinematic wave equation: t=L/V ​ Where:

• t is the time of concentration (in hours),
• L is the length of flow path (in meters),
• V is the velocity of flow (in meters per second).

First, we need to convert the given length of overland flow from kilometers to meters: L=8km×1000m/km=8000m Now, we need to find the velocity of flow. The velocity of flow can be determined using the Manning's equation: V=n1​⋅R2/3⋅S1/2 Where:

• n is the Manning's roughness coefficient,
• R is the hydraulic radius,
• S is the slope of the flow path.

Given that the difference in level is 12.4 m and the length of flow path is 8000 m, we can calculate the slope �S as: S=Difference in level/Length of flow path   ​= 12.4m/8000m ​ Now, we need to determine the Manning's roughness coefficient n. This coefficient depends on the type of surface over which the flow is occurring. Once we have S and n, we can calculate the velocity V. Then, we can use the velocity and the length of flow path to find the inlet time t. Without specific values for n, we cannot directly determine the inlet time. However, once the values for n and S are provided, we can proceed with the calculations.