Question 2 A flood alleviation scheme is to be designed to divert part of a river flow to a storage area, of plan area 7650m2, during periods of exceptionally high flow. The storage area will fill to a total depth of 2m via a flow over a weir when the river exceeds the weir crest. From historical flood data it has been determined that the storage area needs to fill in 5 hours 40 mins to reduce the downstream flood risk from 1 in 2 years to 1 in 25 years (a) Two designs of weir are proposed for the scheme: A 100° V-Notch weir (Cd = 0.85) and a 1.5m wide rectangular weir (Cd = 0.76). Determine the height of the water over each weir design assuming a constant flow rate during filling (6 marks) (b) It is proposed to expand the scheme to reduce the flood risk to a 1 in 50 year event by increasing the capacity of the storage area to 27540 m3. This must stl fill in 5 hours 40 mins to achieve the required level of protection. Determine how you could modify cach of the two weir designs to enable this, if the maximum allowable depth of water above the weir crest is 0.5 m. Assume the values of Cd are as stated above (6 marks) (c) This expanded flood storage area empties via a 0.6m diameter pipe when the river level has returned to normal. Treating this as an orifice flow from a tank and assuming an initial depth of 2m, determine the time for the storage area to empty. Assume a discharge coefficient of 0.79 (3 marks)
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