Laboratory investigation of scour around large structures in tidal waters

Escarameia, M. (1998) Laboratory investigation of scour around large structures in tidal waters. 3rd Int. Conference on Hydroscience and Engineering, ICHE ’98, Cottbus/Berlin, Germany, 1998.

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Large numbers of temporary structures (e.g. cofferdams) and permanent structures (e.g. caissons and bridge piers) are built in estuaries and tidal river reaches. Tidal flows can produce large depths of local scour around these structures. This can jeopardise their stability or otherwise require deeper and therefore more expensive sheet piling or local armouring of the bed. Considerable effort has in the past been directed towards the study of scour in unidirectional flows, but the important effects produced by tidal conditions have not previously been quantified. This paper describes a laboratory study carried out at HR Wallingford to investigate scour development around large obstructions under the influence of the following parameters: reversal of the flow direction, tidal cycle duration, water depth, shape of the obstruction and sediment size. The experimental facility used in the study was a 24m long flume fitted with an axial pump that allowed reversal of the flow direction. The test section was formed by a 4m long granular mobile bed where the model structures were placed. Preliminary tests were performed to measure scour development in unidirectional flows in order to allow later comparisons with tidal flow conditions. During the test programme the above flow parameters were varied to provide a wide range of conditions, and several typical shapes of structure were investigated: square, circular, rectangular and square with transverse sill. Tests were also carried out to determine the minimum requirements for the extent of protection (such as riprap) around structures. The paper presents the analysis carried out to determine the influence of the various factors on the equilibrium scour depths measured. Simple equations are given in the paper for the design of the extent of rock protection required around a structure. An outline is also given here of a method to relate the scaling law for the temporal movement of sediment with that of the water movement. By allowing the scaling of laboratory data to prototype values, the procedure will be applied to the development of design equations for predicting scour around large obstructions both in tidal and unidirectional flow conditions.

Item Type: Article
Subjects: Floods > River management
Coasts > Sediment transport and scour
Coasts > General
Divisions: Water
Depositing User: Unnamed user with email
Date Deposited: 02 Apr 2020 09:45
Last Modified: 02 Apr 2020 09:45

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