Question

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Question 5 (a) A 200-acre watershed is currently being used to grow and harvest hay (meadow). The watershed contains B soil with an average land slope of 7%. However, next year the land use is scheduled to change to row crops (contoured). The hydraulic length of the watershed is 3,200ft. Determine the time of concentration before and after the land use change occurs. A drainage structure will be designed for a 10-year design storm Determine the design discharge. Assume C 0.30 (b) Determine the flow rate in a 6.5 feet by 6.5 feet concrete culvert if the headwater is limited to a depth of 13.3 feet above the invert (culvert bottom). The culvert has a square edged entrance and is 50 feet long with a slope of 1%. Assume that the outlet is not submerged (i.e, hydraulic operation category (c)), but once the flow rate is found, verify that it is not category (b) by checking normal depth

Answer 1

Ans. (a) Time of concentration, T_c

T_c= (l ^0.8 (S+1) ^0.7) / (1140Y^0.5)

l= 3200 ft

S=(1000/cn) - 10

S= (1000/63)-10 =5.87

Y= 7 %

Hence T_c=(3200 ^0.8 (5.87 +1) ^0.7 ) / (1140 * 7 ^0.5)

T_c= 0.83 hours

Ans. (b) Flow rate Q= (1.486/n)* A * R^2/3 * S^1/2

= (1.486/0.012)* (6.5*6.5) * (6.5/4)^2/3 * .01^1/2

=2286.826 cubic feet per second