Structure-Induced Seafloor Scour
Authors: Sheppard D.M. , Niedoroda A.W. , Karanumuni A. 
Published: March 2009

The problem of predicting the depth of erosion in a scour hole formed by asteady current passing a vertical cylinder has been examined in order toprovide a synthesis of existing data and a method that can be applied incoastal, ocean, and river engineering contexts. Nearly a hundred experimentaldata points have been obtained from the literature and found to be of goodquality. These data were fit with a cubic equation in four dimensions andcomparisons of the predictive capabilities of this, and other equations weremade. The range of input parameters for which this relationship can be used wasdetermined and is illustrated in this paper. The results are of a quality whichallow extension of the method to less restricted field conditions.

Introduction

The formation of scour holes has been the subject of considerable study duringthe last three decades. Although substantial progress has occurred, there arestill many problems in making reliable predictions. These problems include:making the proper evaluation of the type of scour that will occur and selectingan appropriate method of analysis. The study herein reported was undertaken tomake a full evaluation of the presently available data and predictive equationsand to synthesize a reliable method of scour hole prediction.

Although all sea floor scour develops as the result of local flowintensification caused by the structure there are many possible outcomesdepending on the nature of the flow, the shape, size, and orientation of thestructure, and the properties of the sea floor sediments. Primary divisions canbe made between horizontal members (e.g. beams, slabs, and pipelines) andvertical members (piles and sup- port columns). Whether the flow is steady(currents only), oscillating (waves), or combined (waves and currents) is alsoan important distinction. Finally, the obstructing structure may be small orlarge as well as have a variety of cross sectional shapes.

An intense literature search was carried out and hundred sof references wereobtained. No comprehensive and generally accepted predictive relationship wasdiscovered for any of the general types of bottom scour. Consequently, asynthesis of available data and relationships was conducted.

The overall study covered vertical and horizontal structures in steady, oscillating, and combined flows. Individual and grouped structures were alsoincluded. These results are too voluminous for this paper so only the casescharacterized by individual vertical cylinders in a steady current aredescribed. As the vast majority of the available data are concerned with thissubject, it is an appropriate subject for concentration. Furthermore, oncethese Processes are well described by a mathematical relations Lp, the resultscan subsequently be extended to wave, and combined wave and currentconditions.

The purpose of this paper is to describe a new and comprehensive analysis ofexisting data concerning bottom scour about a vertical circular clipper in asteady current. The analysis adopts commonly used dimensionless parameters andresults in a cubic equation in four dimensions that is shown to have superiorpredictive capabilities over a large range of realistic nerd conditions. No newexperiments or measurements were undertaken in this study.