Role and Impact of Hydrograph Shape on Tidal Current-Induced Scour in Physical-Modelling Environments

Alexander Schendel,Mario Welzel,Tai-Wen Hsu,Arndt Hildebrandt,Torsten Schlurmann

doi:10.3390/w11122636

Alexander Schendel, Mario Welzel + Show 3 more

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https://doi.org/10.3390/w11122636

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Abstract

For physical model tests, the time-varying characteristics of tidal currents are often simplified by a hydrograph following a shape of a unidirectional current or by resolving the tidal velocity signal into discrete steps of constant flow velocity. The influence of this generalization of the hydrograph’s shape on the scouring process in tidal currents has not yet been investigated systematically, further increasing the uncertainty in the prediction of scour depth and rate. Therefore, hydraulic model tests were carried out to investigate and quantify the influence of the hydrograph shape on the scouring processes under tidal currents. Several different hydrographs including those with continuously changing velocities, constant unidirectional currents, square-tide velocities and stepped velocity time series were analyzed. Results show that the scouring process in tidal currents is characterized by concurrent sediment backfilling and displacement which can only be reproduced by hydrographs that incorporate a varying flow direction. However, if only a correct representation of final scour depths is of interest, similar scour depths as in tidal currents might be achieved by a constant, unidirectional current, provided that a suitable flow velocity is selected. The effective flow work approach was found capable to identify such suitable hydraulic loads with reasonable practical accuracy.

Highlights

Introduction and MotivationThe reliable prediction of scour depth is of crucial interest for the economic and secure design of structures in coastal and marine waters
Since modelling deficiency is understood to effect the scour development in tidal and unidirectional currents to the same degree, it can be expected that the additional imbalance of scour depth due to the change in flow direction leads to slower scour rates in tidal currents compared to unidirectional currents
Addition, reducing uncertainties in the prediction of scour depth around structures placed in tidal current the findings of this study might be used as reference for the planning of physical model tests as well affected the findings of this study might be used as reference for the planning of as for thewaters

Summary

Introduction

Introduction and MotivationThe reliable prediction of scour depth is of crucial interest for the economic and secure design of structures in coastal and marine waters. Unresolved questions remain regarding the process of scouring induced by complex flow conditions that prevail in offshore or estuarine situations, such as time-varying, asymmetrical and directional tidal currents or the influence of additional vorticity introduced by wind-induced sheared current. Water 2019, 11, 2636 laboratory facilities or due to the often profound inconsistencies of results coming from laboratory tests, complex flow situations are still seldomly taken into account for the prediction of scour development around offshore structures, despite the fact that they depict important aspects of offshore sea states. The influence of time-varying but unidirectional flows on the scour development around bridge piers has been addressed in several studies, most of which focused on the detrimental effects of flood events for design purposes

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