Simons, RR; Charles, E; (2007) Investigation into scour around the proposed Mersey gateway crossing, in: Chapter 7 Hydrodynamics and Estuarine Processes: Appendix 7.8. (GLPO 30817 ). Halton Borough Council

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Abstract

This report describes a 3-month investigation into the Upper Mersey Estuary and its interaction with scour patterns around piers associated with a proposed bridge crossing (Mersey Gateway) near Runcorn. The work has been performed in three stages: 1. Review of literature relating to scour around large structures (large relative to water depth) under reversing flows in estuaries with a significant tidal range. 2. Flume tests to measure scour around a model structure which is large relative to water depth, with the aim of simulating local tidal conditions and associated water depths in the range 0.1D to 0.5D. In the event, ten tests were completed, nine with clear water scour and one with live bed scour. Water depth to cylinder diameter was varied from 0.33 to 0.67; two cases involved flow reversal. 3. Construction, design and testing of a new small-scale shallow water tidal flow tank to simulate processes in the Mersey between Runcorn Gap and Fiddler‟s Ferry and to investigate patterns of channel evolution, migration and interaction with model bridge piers. From the many previous studies of scour around bridge piers in rivers, 18 approaches were selected for comparison with the results from flume experiments in the present study. Some of these formulae are very simple, but most involve a power law and take into account the effects of varying flow velocity and water depth to different extents. The formulae given in May (1998), May and Escarameia (2002), May, Ackers and Kirby (2002) and Breusers et al. (1977) were all found to give very good agreement for relatively large structures in shallow flows just below or close to critical velocity for initiation of sediment motion. For these models, the maximum scour depth predicted was close to the observed value or slightly conservative. The approach proposed by Chitale (1962) gave reasonable agreement but consistently under - predicted the observed scour. There are no reliable “off the shelf” formulae designed specifically for the case of scour in tidally reversing flows. The two tests performed to simulate tidally reversing flows showed significant initial reduction in scour depth as downstream deposits filled the scour hole formed before flow reversal. The extent to which scour depth is limited in tidally reversing flows has been shown by May and Escarameia (2002) to depends principally on the relationship between tidal period and the half-life of the equilibrium scour process in an equivalent unidirectional current. A previous study at UCL has confirmed that under certain conditions the reduction can be 50% or more. However, extreme flow events are likely to reduce the characteristic time scale of the scour and therefore limit any possible reduction in predicted scour. A shallow water tank has been designed and commissioned during the project to simulate tidal processes in the Upper Mersey. Driven by the computer-controlled rise and fall of external header tanks, reversing flow and tidally-varying water levels have been generated. During a preliminary test with an initially flat sand bed, channels were seen to start forming at the both ends of the test section in the tank; no movement was observed in the central region where it is proposed to site the new bridge. A further test, with narrow pre-formed ebb and flood channels around a shallow sandbank, was run for 4000 tidal cycles (44 hours) and a time-lapse sequence of photographs created. This identified dynamic areas within the model where bed forms and channel geometry shift relatively rapidly. Small channels were seen to form and develop under the action of the flood tide on the western, seaward end of the bank. No lateral movement of the main channels was observed; this may be related to the choice of sediment size and to the constant tidal amplitude adopted in this test. Small cylindrical structures were introduced into the model to simulate the effects of the proposed bridge piers; these formed scour holes consistent in scale with the flume experiments. When these model piers were located in small dynamic banks and channels in the sand bed, no visible interference was caused to the natural bed processes.

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