Scour Manual

G.J.C.M. Hoffmans, H.J. Verheij

• Publisher: CRC Press
• Year: 2017
• eBook ISBN: 9781351417495
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

The mechanisms and behaviour of the scour process is a challenging subject, and one which is expertly dealt with in this informative, illustrated volume. Specifically, this book addresses issues relating to computing and controlling the scour process near hydraulic structures, and pays special attention to the time-dependent character of the scour processes and the predictability of scour relations. Providing information on the latest developments in scouring, this text is intended for practising hydraulic engineers.

CHAPTER 1

Introduction

1.1 GENERAL

A hydraulic structure is generally intended to provide a practical measure to solve an identified problem. After problem identification subsequent stages are determined by a series of decisions and actions culminating in the creation of a structure or structures to resolve the problem. Aspects that may affect the eventual outcome of the design process have to be assessed. In addition to hydraulic, geotechnical and engineering characteristics, phenomena such as social conditions, economics, environmental impact and safety requirements also influence the design process.

Within the scope of the Dutch Delta works, systematic research with respect to the prediction of the formation of scour holes was conducted by the Dutch Ministry of Transport, Public Works and Water Management and Delft Hydraulics. After the catastrophic flood disaster in 1953, the Delta plan was formulated to protect the Rhine-Meuse-Scheldt delta against future disasters. Dams with large-scale sluices were planned in some estuaries. The severe scour that was expected necessitated the acquisition of a better understanding of the scour process.

To obtain detailed information about the physical processes playing a role in scour development, many experiments in which various parameters of the flow and the scoured material were varied, were carried out by Delft Hydraulics. From the results of experiments in flumes, with all the attendant difficulties of scale effects and limitations in instrumentation, some semi-empirical relations were obtained that describe the erosion process as a function of time and position (Breusers 1966, 1967; van der Meulen & Vinjé 1975). In addition, design criteria for the length of the bed protection were deduced. These were based on many hundreds of shear failures and flow slides that occurred along the coastline in the south-western part of the Netherlands.

Understanding of the physical processes and mathematical modelling of the water and sediment movement in rivers, estuaries and coastal waters have made much progress in recent years. This has led to a number of more or less ready-to-use mathematical model systems, but it has also raised many new research questions. In the early nineties a morphological model for the generation of scour holes behind hydraulic structures was developed (Hoffmans 1992). This morphological model was based on the 2-D Navier Stokes and convection-diffusion equations and used for the calibration and verification of semi-empirical relations that predict the scour process.

This manual highlights the so-called Breusers-equilibrium method which describes the maximum scour depth as function of time, including the practical equilibrium value near hydraulic structures. Scour due to three-dimensional flow can easily be predicted when this method is applied in combination with computational results of depth-averaged models or with measurements obtained from scale models. The accuracy of the scour computation depends mainly on the accuracy of the flow velocities and the turbulence intensities just above the protected bed. According to Breusers (1966), the development of the scour process depends entirely on the average flow velocity and relative turbulence intensity at the transition of the fixed to the erodible bed. By applying this concept, the scour prediction is restricted to one computation; no information is needed concerning the near bed velocities and bed turbulence in the scour hole.

1.2 SCOPE OF THIS MANUAL

The purpose of this scour manual is to provide the civil engineer with practical methods to calculate the dimensions of scour holes and to furnish an introduction to the most relevant literature. The manual contains guidelines which can be used to solve problems related to scour in engineering practice and also reflects the main results of all the Dutch research projects directed to the phenomena of scour which have been realised in the Netherlands during recent decades. A complete review of all the available references on scour is beyond the scope of this manual. A more general introduction to scour is given by Breusers & Raudkivi (1991). The scour depth as function of time can be predicted by the so-called Breusers-equilibrium method. Basically, this method can be applied for all situations where local scour is expected. However, the available knowledge about scour is not sufficient, to apply the method for the prediction of scour at each type of structure. Structure specific scour prediction rules are presented then. The treatment of local scour is classified according to different types of structures. Each type of structure is necessarily schematised to a simple, basic layout. There is a brief description of the main parameters of a structure and of the main parts of the flow pattern near a structure, in so far as they are relevant to the description of scour phenomena. Detailed and theoretical descriptions of the flow phenomena are not included because at this stage the consequences of such descriptions are minimal in relation to engineering practice. As many scour problems are still not fully understood attention is paid to the validity ranges and the limitations of the formulas, and to the information about the accuracy of the calculated parameters, i.e. the maximum scour depth expected during the lifetime, the upstream scour slope and the expected failure length. Due to shear failures or flow slides, the scour process can progressively damage the bed protection; this will lead to the failure of hydraulic structures.

Summarising, the presented Breusers-equilibrium method can be applied directly in the engineering practice for nearly all types of structures. One may expect that in the near future accurate local flow velocities and turbulence intensities as a result of three-dimensional flow models will act as input for the Breusers-equilibrium method, which can be considered as a constination and an expansion of the work of Breusers (1966). In other words, one may speak of a revitalisation of the Breusers formula, with which a lot of experience has been gained, mainly in the Netherlands but also abroad.

1.3 SUMMARY

The manual is divided into seven parts, the first of which is a general introduction to the subject. The next five parts deal with calculation methods for predicting scour near hydraulic structures and, in the final part, some examples of scour at prototype scale are described. A brief summary of each chapter follows.

Chapter 2 – Basic concepts

The most relevant characteristics that influence the scour process, such as hydraulic, morphological and geotechnical conditions are discussed. The functional design of hydraulic structures is also introduced. If safe long term functioning of hydraulic structures is to be ensured, access to information relating to failure mechanisms and boundary conditions is indispensable. The scour which may occur at the site of a structure may be considered as a combination of bed scour resulting from different processes (general scour and local scour). In addition, time phases can be distinguished in the scour process. These phenomena and relations for the critical velocity (based on the Shields diagram) are presented for both current and waves. The influence of nonuniform flow is usually expressed by either a turbulence coefficient or a dominating flow velocity or by both. Scouring is a more or less continuous process which may suddenly be disturbed by the occurrence of geotechnical instabilities along the upstream scour slope. Shear failures and flow slides influence the stability of hydraulic structures. In the extreme case these instabilities involve large masses of sediment and cause a major change of the shape of the upstream side of the scour hole in a relatively short period of time. Some design criteria which are based on storage models are presented. In the subsequent chapters the basic scour concept for a number of hydraulic structures and applications is discussed.

Chapter 3 – Sills

Calculation methods with respect to sills are summarised. A distinction is drawn between sills with a broad or a sharp crest and between sills with and without bed protection. Usually, the flow above a sill is subcritical but, depending on the downstream water level, the flow may become supercritical. The time-dependent and equilibrium behaviour of scour holes in sandy beds related to closure works (broad-crested sills) in tidal channels is discussed. Special attention is paid to the effects of turbulence and flow pattern on the scour process. An approximate method (reduction method) for calculation of the maximum scour depth is described. This takes into account the influence of upstream sediment supply. In addition, a method to adjust this calculation method for unsteady flow, especially tidal flow, is given. These methods were successfully applied during the design of the Eastern Scheldt Storm Surge barrier. The upstream scour slope determines the stability of the upstream part of the scour hole and the adjacent bed protection. A relation for the upstream scour slope, based on a probabilistic bed load model for bed load transport, is presented. Relations derived from the so-called systematic scour investigation were verified by two field experiments.

Chapter 4 – Jets

Scour due to several jet forms, such as plunging jets, submerged jets, horizontal and vertical jets, and two and three dimensional jets is discussed. In addition, the complex flow pattern of jets is treated. Semi-empirical relations for the scour process behind a short-crested sill are presented. These relations are often used in grade-control structures, where the flow above the sill is supercritical and for the time dependent development of the maximum scour depth downstream of a hydraulic jump. The structure of these relations show a good similarity with the Breusers approach. Since there is no universal scour relation that gives a good prediction of the practical equilibrium scour depth in all cases, some semi-empirical relations are presented. These relations must be clearly understood prior to any attempt to use them for design purposes.

Chapter 5 – Abutments and spur dikes

Relations for predicting local scour at the head of abutments, for which several names are used in the literature are presented. The flow characteristics around blunt and streamlined abutments are briefly discussed. Attention is also paid to the time scale of the scour process and types of scour (e.g. overall degradation, bend scour, constriction scour). Since the literature contains many scour relations, a number of generally acceptable predictors have been selected for this report. Finally, attention is paid to failure mechanisms and measures to mitigate scour near abutments.

Chapter 6 – Bridge piers

Relations for estimating scour around bridge piers are summarised. To date it has not been possible to determine the scour depth around bridge piers from a theoretical analysis of the water movement around the pier, so empirical relations with correction factors and design graphs for the equilibrium scour depth are discussed. Attention is paid to both the equilibrium scour depth and to the time scale of the scour process and some methods used to predict bridge piers from scour are cited.

Chapter 7 – Coastal and offshore structures

This chapter deals with the scour induced by wave and current action near pipelines, piles, vertical breakwaters, coastal defences and other submerged structures. For accurate prediction of the scour process it is necessary to have detailed descriptions of the nearshore hydrodynamics. However, these processes fall outside the scope of this manual, so the required information should be obtained from site-specific studies. Here, simple prediction methods which relate the scour depth to the incident wave conditions, the flow depth, the structure geometry and/or reflection coefficient are described. No effects of angled wave attack or of tidal or wave induced current have yet been accounted for. It must be stressed that our knowledge of scour due to waves or due to the combined effect of current and waves is still in a rather rudimentary stage and that more research and practical experience is still needed.

Chapter 8 – Prototype situations

Five cases on prototype scale, based on feasibility studies or design studies, are evaluated in order to determine the practical use of the scour relations already discussed. These cases are:

–  Lengthening of abutments in the Waal River, the Netherlands,

–  Submerged vanes in the Waal River, the Netherlands,

–  Improvement of the access channel to the port of Barranquilla, Colombia,

–  Damming of the Chao Phraya River, Thailand, for navigation improvement,

–  Gravity based structure, North Sea.

Acknowledgement

The discussions with colleagues at Delft Hydraulics were much appreciated, especially the discussions with H.N.C. Breusers, G.J. Klaassen, T. van der Meulen, L.C. van Rijn, N. Struiksma and A.M. Talmon.

The support and useful comments of A. Franken, R.E. Jorissen, A. Kraak, H.E. Klatter, J. Lindenberg and F.M. Stroeve of the Ministry of Transport Public Works and Water Management was highly appreciated.

Finally, R. van der Laag was gratefully acknowledged for the drawing.

CHAPTER 2

Basic concepts

2.1 INTRODUCTION

Hydraulic structures cause disturbances in uniform flow and sediment transport. Downstream of these structures flow velocities increase due to constriction of the channel. When the flow velocities decrease (i.e. in the deceleration zone), a higher degree of turbulence is generated and therefore a stronger erosion capacity is present. In most cases this leads to scouring and, depending on the specific hydraulic conditions, there are sometimes steep upstream slopes. Bed protection is often constructed in order to decrease the maximum scour hole and to shift the scour holes that involve a potential risk to structural stability to a greater distance from the hydraulic structure. The main d...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Foreword
• List of main symbols
• 1. Introduction
• 2. Basic concepts
• 3. Sills
• 4. Jets
• 5. Abutments and spur dikes
• 6. Bridge piers
• 7. Coastal and offshore structures
• 8. Prototype situations
• References