ANS:D - 8 hours

To calculate the inlet time for the overland flow from the critical point to the mouth of the drain, we can use the Manning's equation, which relates flow velocity, cross-sectional area, hydraulic radius, Manning's roughness coefficient, and slope of the channel: Q=1/n​⋅A⋅R^2/3⋅S1/2 Where:

• Q = flow rate (volume per unit time)
• n = Manning's roughness coefficient
• A = cross-sectional area of flow
• R = hydraulic radius
• S = slope of the channel

Given that the length of overland flow (L) is 13.58 km and the difference in level between the critical point and drain mouth (H) is 10 m, we can calculate the slope (S) using the formula: S=LH​ First, we need to convert the length of overland flow from kilometers to meters: L=13.58km×1000m/km=13580m Then, we calculate the slope (S): S=13580m10m​≈0.000736 Now, we need to know the Manning's roughness coefficient (n) and the cross-sectional area of flow (A) to proceed with the calculation. Since these values are not provided, we can't directly calculate the inlet time. The inlet time can be obtained using the relationship between flow rate (Q) and length of overland flow (L): Inlet Time=Inlet Time=QL​ To calculate Q, we need to know both n and A, which are not provided in the given information. Therefore, without these values, we can't determine the inlet time. Thus, the answer cannot be determined with the given information.