Calculate the discharge (in cubic meters per second) for a 20 km long stream with velocity 2 m/s and a rectangular channel 10 m wide and 4 m deep.

• A. 40 m3/s
• B. 80 m3/s
• C. 160 m3/s
• D. 320 m3/s

Answer: (B)

Discharge is the volume of water passing a cross-section each second. It equals the cross-sectional area of the flow times the flow velocity. For a rectangular channel, that area is width times depth. Here the width is 10 m and the depth is 4 m, so the area is 10 × 4 = 40 m^2. Multiply by the velocity: 2 m/s × 40 m^2 = 80 m^3/s. So the discharge is 80 cubic meters per second. If the area or velocity changed, the discharge would change in direct proportion—doubling the area or doubling the velocity would double the discharge, for example.