Boriss Gjunsburgs, Jelena Govsha, Oskars Lauva

Scour at layered river bed: reason of the structures failure

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Abstract

The prediction of damages and/or failure of the bridge structures in flow, because of the scour near foundations in plain rivers, is one of the main tasks for engineers. In spite of the problem importance – the scour development in time at stratified bed conditions is not studied well. The aim of the study is to propose new method and to estimate the influence of the river bed stratification on depth of scour near bridge structures in flow. The differential equation of equilibrium for bed sediment movement in clear water was used and a new calculation method for the scour development in time at vertical wall abutments in plain rivers at clear water and at stratified bed conditions with uniform grain-size sands layers was elaborated and confirmed by test results. This method allows one to calculate the scour depth in layers with different mean grain size, thickness and sequence combination. Relative depth of scour dependence on flow and stratified bed data is presented in figures. The calculation of scour depth near bridge structures in flow by using only the data on grain size on the top of the river bed and neglecting the stratification of the river bed can lead to wrong results and finally to considerable damages and losses. The most critical conditions for structures are when fine-sand layer is under a coarse-sand layer. Test results and method presented confirm that the depth of scour is always greater when a fine-sand layer is under a coarse-sand layer(s). Using the mean grain size on the top of the river bed for the depth of scour calculation as it is accepted now and neglecting river bed stratification can lead to wrong results and possible structures failure.

Keywords:scour depth; river bed; layering; grain size.