Repeat question 8.11 above using the direct step method. Assume that the required change in water level is (3.722 – 1.250) = 2.472m divided into eight steps of DD = 0.309m. (a) Compare the chainages at which the upstream depth in the two questions first becomes 1.250m. Is there a significant difference? (b) Why might the answers in part (a) be expected to differ? What factors affect the accuracy of the analysis?

Question 8.11

A weir built to supply water to an old paper mill raises the water level in the river for some distance upstream. To reduce flooding it is proposed that the weir should be removed, but the effect of this needs to be determined first. Just upstream of the weir the depth is 3.722m when the discharge is 15.333m3 /s. The river channel is rectangular and prismatic, 6.5m wide, n = 0.035s/m1/3, the bed slope is 1 in 200, and a can be taken as 1.00. (a) Estimate the normal depth in the channel. (b) Determine by how much the depth of flow is increased by the weir at the weir. (c) Using the standard step method, obtain the elevation of the water surface upstream of the weir. (d) At approximately what distance does the backwater curve regain normal depth? (e) If houses and commercial properties line both banks, comment upon the possible repercussions of removing the weir