Unit Hydrograph:

A unit hydrograph is a hydrograph of a rainstorm of a specified duration resulting from a run-off. It is a graphical representation of the runoff response of a watershed to a unit amount of rainfall. The unit amount of rainfall is usually taken as 1 cm or 1 inch of rainfall. The unit hydrograph is a useful tool in the estimation of flood runoff from a given rainfall event.

Duration of Rainstorm:

The duration of the rainstorm is the time period during which the rainfall occurs. It is an important factor in the determination of the unit hydrograph. The duration of the rainstorm is usually expressed in hours.

Calculation of Unit Hydrograph:

The unit hydrograph is calculated by dividing the runoff from a given rainstorm by the amount of rainfall. For example, if the runoff from a 20 mm rainstorm is 10 mm, then the unit hydrograph for a 20 mm rainstorm would be 10/20 = 0.5.

Importance of Unit Hydrograph:

The unit hydrograph is an important tool in the design of hydraulic structures such as culverts, bridges, and dams. It is also used in the estimation of flood flows and the design of stormwater management systems.

Conclusion:

In conclusion, the correct answer is option B, i.e., 20 mm. The unit hydrograph is a graphical representation of the runoff response of a watershed to a unit amount of rainfall. The duration of the rainstorm is an important factor in the determination of the unit hydrograph. The unit hydrograph is an important tool in the design of hydraulic structures and stormwater management systems.