Question

A farmers drainage ditch has a width of 2 m and a depth of 50 cm. It is lined with concrete with a roughness of 0.011 and slopes at 0.0009. Calculate the ditch's discharge rate.

Answer 1

Using the Manning's Equation, the discharge rate can be calculated as follows:

Q = (1.49/n) x A x R^(2/3) x S^(1/2)

Where:

Q = discharge rate (m^3/sec)

n = Manning's roughness coefficient (0.011)

A = cross sectional area of the ditch (2m x 0.5m = 1 m^2)

R = hydraulic radius (half the width of the ditch, or 1 m)

S = slope of the ditch (0.0009)

Q = (1.49/0.011) x 1m^2 x 1m^(2/3) x 0.0009^(1/2)

Q = 13,636.36 m^3/sec