Question

4. Fill in the blanks

a. As per the USACE, what radius of curvature would you suggest for large ditches with top width = 11 m if the longitudinal slope is 0.0004? Please report the result in meter.

b. Consider a channel with bend angle 1200. If the average velocity in a channel is 6 ft/sec and velocity at 20% slope is 7.2 ft/sec, what centerline radius do you recommend if the water surface width is 15ft? Consider the bottom width and depth ratio as 6.7.

Answer 1

Ans a) According to USACE , for large ditches with top width 11 m (>10.7 m) and longitudinal slope 0.0004 (<0.00057) , suggested radius of curvature is 180 m .

Ans b) Since,the top width and bottom width are different, channel is trapezoidal and for trapezoidal channel,

Vs = Vavg [1.71 - 0.78 Log (R/W)]

where, Vs = Velocity at 20% slope = 7.2 ft/s

Vavg = Average velocity = 6 ft/s

R = Centerline radius

   W = width of water surface = 15 ft

Putting values,

=> 7.2 = 6 [1.71 - 0.78 Log(R/15)]

=> 0.51 = 0.78 Log (R/15)

=> Log (R/15) = 0.653

=> R/15 = 10^0.653

=> R = 67.50 ft

Hence, recommended centerline radius is 67.50 ft