A 25.5 m wide rectangular navigation channel is shown in Figure Q6 [Page 11 of 23]. The channel has been constructed using reinforced concrete walls in a deep natural granular layer. The slope of the channel is 1:225 with a maximum flow of 253 m3/s. The riverbed is a deep layer of sandy Gravel with a d50 of 1.75 mm to a depth of 1.5 m. This is underlain by 4 m of impermeable clay on top of bedrock. The invert of the wall is as shown in Figure Q6. One of the supporting walls has shown signs of distress, and there is a likelihood of failure. As a short-term measure, engineers are considering supporting the wall using steel sheet piling backfilled with stone material as shown in Figure Q6. In the interests of providing a safe working area, it is proposed to reduce the channel width of 7.5m. Assume all sediment has a specific gravity of 2.65
. (a) Evaluate the constriction scour (for n = 0.025) for a flow depth just upstream of the constriction of 2.75 m, where a normal flow depth is experienced within the area of the constriction, prior to works being carried out. Consider both live bed and clear water scour equations in your answer and explain the relevance of each alternative.
(b) Appraise the depth of scour in relation to risk for the remaining exposed abutment. Clearly state all other assumptions.
(c) Estimate the limiting grain diameter where sediment transport would begin to cease within the constriction. Consider the limitations to the applied theory in your answer.
d) Critically appraise the difference between clearwater and livebed scour and their impact on current bridge stocks in Northern Ireland.