1st Edition
Scour Manual Current-Related Erosion
Ever since the publication in 1997 the original Scour Manual has helped many practising hydraulic engineers to deal with scour processes near hydraulic structures. In recent years new insights, such as probabilistic calculations, offered new opportunities to design structures more economically. These new insights are included in this update of the original Scour Manual, which is focussing entirely on current-related scour. This manual provides the engineer with useful practical methods to calculate the dimensions of scour holes in the pre-feasibility and preliminary stages of a project, and gives an introduction to the most relevant literature.
This updated Scour Manual contains guidelines that can be used to solve problems related to scour in engineering practice and also reflects the main results of all research projects in the Netherlands in recent decades. The so-called Breusers equilibrium method has a central role, which can basically be applied to all situations where local scour is expected. The method allows to predict the scour depth as a function of time, provided that the available knowledge about scour at the specific structure is sufficient. For structures with insufficient knowledge available, alternative scour prediction rules are presented.
The treatment of local scour is classified according to the different types of structures. Each type of structure is necessarily schematised to a simple, basic layout. The main parameters of a structure and the main parts of the flow pattern near a structure are described briefly insofar they are relevant to the description of scour phenomena. New scour formulas for the equilibrium scour have been elucidated. Evaluating a balance of forces for a control volume, it is possible to develop scour equations for different types of flow fields and structures, i.e. jets, abutments and bridge piers.
As many scour problems are still not fully understood, attention is paid to the validity ranges and limitations of the formulas, as well as to the accuracy of the scour predictions. This information can also be used to carry out a risk assessment using a safety philosophy based on a probabilistic analysis or an approach with a safety factor. Moreover, the information on the strength of soils is extended and aspects are addressed such as scour due to shear failures or flow slides, that can progressively damage the bed protection which might lead to the failure of hydraulic structures.
This updated Scour Manual presents scour prediction methods and deals with practically related scour problems. Consultants and contractors were invited to provide case studies of realized projects, including the methods that were followed. These case studies will help with grasping the concept of scour by the flow of water. This manual provides the engineer with the latest knowledge and with case studies that show how to apply the formulas and their limitations.
Foreword xv
Acknowledgements xvii
List of main symbols xix
List of main definitions xxiv
1 Introduction 1
1.1 General 1
1.2 Scope of this manual 2
1.3 Reading guide 3
2 Design process 6
2.1 Introduction 6
2.2 Boundary conditions 8
2.2.1 Introduction 8
2.2.2 Hydraulic conditions 9
2.2.3 Morphological conditions 10
2.2.4 Geotechnical conditions 10
2.3 Risk assessment 10
2.3.1 Introduction 10
2.3.2 Fault tree analysis 12
2.3.3 Safety factor 14
2.3.4 Failure probability approach 15
2.4 Protective measures 16
2.4.1 Introduction 16
2.4.2 Bed protection 16
2.4.3 Falling apron 18
2.4.4 Other counter measures 19
2.5 Examples 19
2.5.1 Introduction 19
2.5.2 Determination of the length of a bed protection with a
reliability index 20
2.5.3 Determination of the failure probability using a FORM approach 21
2.5.4 Determination scour depth using a safety factor 24
Contents
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viii Contents
3 Design tools 26
3.1 Introduction 26
3.2 Mathematical scour and erosion models 26
3.2.1 Introduction 26
3.2.2 Types of modelling 27
3.2.3 Large-scale RANS models 28
3.2.3.1 Shallow water modelling 28
3.2.3.2 Turbulence modelling 29
3.2.4 High-resolution hydrodynamic models 29
3.2.4.1 Hydrodynamic model LES 29
3.2.4.2 Application of LES 29
3.2.4.3 Hydrodynamic model DNS 30
3.2.5 Particle-based multiphase models 31
3.2.5.1 Soil mechanics: MPM 31
3.2.5.2 Hydraulic model: SPH 31
3.3 General scour 32
3.3.1 Introduction 32
3.3.2 Overall degradation or aggradation 32
3.3.3 Constriction scour 33
3.3.4 Bend scour 34
3.3.5 Confluence scour 36
3.4 Local scour 37
3.4.1 Introduction 37
3.4.2 Time-dependent scour 39
3.4.3 Equilibrium scour 41
3.4.4 Conditions of transport 43
3.5 Geotechnical aspects 44
3.5.1 Introduction 44
3.5.2 Liquefaction 46
3.5.3 Effects of groundwater flow 46
3.5.4 Non-homogeneous subsoils 48
3.5.5 Upstream and side slopes 50
3.5.6 Critical failure length 53
3.6 Examples 55
3.6.1 Introduction 55
3.6.2 Constriction scour 55
3.6.3 Critical slope angles and critical failure lengths 55
4 Initiation of motion 57
4.1 Introduction 57
4.2 Flow and turbulence characteristics 57
4.2.1 Introduction 57
4.2.2 Sills 60
4.2.3 Bridge piers and abutments 61
4.2.4 Indicative values of flow velocity and turbulence 62
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Contents ix
4.3 Non-cohesive sediments 63
4.3.1 Introduction 63
4.3.2 Shields diagram 65
4.3.3 Design approaches 67
4.3.4 Critical flow velocity 69
4.3.5 Rock 70
4.4 Cohesive Sediments 71
4.4.1 Introduction 71
4.4.2 Critical shear stress 72
4.4.3 Critical flow velocity 73
4.4.4 Empirical shear stress formulas 77
4.4.5 Erosion rate 79
4.4.6 Peat 81
4.5 Examples 82
4.5.1 Introduction 82
4.5.2 Turbulence at bridge piers and groynes 83
4.5.2.1 Bridge Piers 83
4.5.2.2 Groynes 83
4.5.3 Critical flow velocity of peat 84
4.5.4 Critical mean flow velocity and critical bed shear stress in
an open channel with sand dunes 85
4.5.5 Critical depth-averaged flow velocity according to
Mirtskhoulava (1988) 86
4.5.6 Comparison critical strength of clay 86
5 Jets 88
5.1 Introduction 88
5.2 Flow characteristics 88
5.2.1 Introduction 88
5.2.1 Introduction 88
5.2.2 Flow velocities 88
5.2.3 Hydraulic jump 90
5.3 Time scale of jet scour 92
5.4 Plunging jets 93
5.4.1 Introduction 93
5.4.2 Calculation methods 93
5.4.2.1 2D-V jets 95
5.4.2.2 3D-V jets 96
5.4.3 Discussion 96
5.5 Two-dimensional culverts 97
5.5.1 Introduction 97
5.5.2 Calculation methods 97
5.5.3 Discussion 101
5.6 Three-dimensional culverts 101
5.6.1 Introduction 101
5.6.2 Calculation methods 102
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x Contents
5.6.3 Discussion 105
5.7 Ship-induced flow and erosion 105
5.7.1 Introduction 105
5.7.2 Scour due to the return current of a sailing vessel 106
5.7.3 Scour due to propeller and thruster jets 107
5.7.4 Discussion 109
5.8 Scour at broken pipelines 110
5.9 Scour control 110
5.10 Examples 114
5.10.1 Introduction 114
5.10.2 Two-dimensional scour downstream a broad-crested sill 114
5.10.3 Three-dimensional scour downstream a short-crested
overflow weir 115
5.10.4 Two-dimensional scour downstream an under flow gate 117
6 Sills 120
6.1 Introduction 120
6.2 Flow characteristics 120
6.3 Scour depth modelling in the Netherlands 123
6.3.1 Introduction 123
6.3.2 Scour depth formula 125
6.3.3 Characteristic time 127
6.3.4 Relative turbulence intensity 129
6.3.5 Scour coefficient 130
6.3.6 Non-steady flow 133
6.3.7 Upstream supply of sediment 135
6.4 Upstream scour slopes 138
6.4.1 Introduction 138
6.4.2 Hydraulic and morphological stability criterion 138
6.4.3 Undermining 139
6.5 Additional measures 140
6.6 Field experiments 141
6.6.1 Introduction 141
6.6.2 Hydraulic and geotechnical conditions 141
6.6.3 Discussion 142
6.6.3.1 Upstream scour slope 143
6.6.3.2 Undermining 143
6.6.3.3 Time scale 144
6.6.3.4 Equilibrium scour depth 145
6.6.3.5 Closing remarks 145
6.6.4 Experiences Eastern Scheldt 145
6.7 Example 147
6.7.1 Introduction 147
6.7.2 Critical upstream scour slope downstream a sill 147
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Contents xi
7 Abutments and groynes 152
7.1 Introduction 152
7.2 Geometry characteristics and flow patterns 152
7.2.1 Introduction 152
7.2.2 Wing-wall abutments 154
7.2.3 Spill-through abutments 154
7.2.4 Vertical-wall abutments 156
7.2.5 Flow pattern 156
7.3 Dutch modelling 159
7.3.1 Introduction 159
7.3.2 Breusers approach 159
7.3.3 Closure procedures 161
7.4 Equilibrium scour depth 163
7.4.1 Introduction 163
7.4.2 Calculation methods 164
7.4.3 Discussion 169
7.5 Combined scour 169
7.5.1 Introduction 169
7.5.2 Combined local scour and constriction or bend scour 170
7.6 Failure mechanism and measures to prevent local scour 170
7.6.1 Introduction 170
7.6.2 Scour slopes 171
7.6.3 Outflanking 172
7.6.4 Riprap protection 173
7.7 Examples 175
7.7.1 Introduction 175
7.7.2 Scour due to lowering of existing abutments 175
7.7.3 Influence of the permeability of an abutment on the scour 177
8 Bridges 179
8.1 Introduction 179
8.2 Characteristic flow pattern 179
8.2.1 Introduction 179
8.2.2 Submerged bridges 181
8.3 Time scale 182
8.4 Equilibrium scour depth 185
8.4.1 Introduction 185
8.4.2 Calculation methods 185
8.4.3 Pressure scour 189
8.4.4 Discussion 190
8.5 Effects of specific parameters 191
8.5.1 Introduction 191
8.5.2 Pier shape 192
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xii Contents
8.5.3 Alignment of the pier to the flow 192
8.5.4 Gradation of bed material 194
8.5.5 Group of piers 194
8.6 Scour slopes 196
8.6.1 Introduction 196
8.6.2 Single cylindrical pier 196
8.6.3 Other types of piers 198
8.6.4 Winnowing 200
8.7 Measures to prevent local scour 201
8.7.1 Introduction 201
8.7.2 Riprap protection 201
8.7.3 Mattress protection 202
8.7.4 Deflectors 203
8.8 Example 203
8.8.1 Introduction 203
8.8.2 Local scour around bridge piers 203
8.8.2.1 Slender piers 205
8.8.2.2 Wide piers 207
9 Case studies on prototype scale 209
9.1 Introduction 209
9.2 Camden motorway bypass bridge pier scour assessment (RHDHV) 209
9.2.1 Introduction 209
9.2.2 Assessment of scour 210
9.2.3 Scour assessment results 212
9.2.4 Constriction scour 212
9.2.5 Abutment scour 214
9.2.6 Pier scour 215
9.2.7 Numerical Model Verification 216
9.2.8 Scour mitigation 217
9.2.9 Conclusions 218
9.3 Project Waterdunen (Svasek) 219
9.3.1 Introduction 219
9.3.2 Bed protection 220
9.3.3 Hydraulic loads 221
9.3.4 Scour depth 222
9.3.5 Additional remarks 225
9.3.5.1 Gate control 225
9.3.5.2 Safety factors 225
9.3.5.3 Sensitivity calculations 225
9.3.5.4 Turbulence 225
9.4 Full-scale erosion test propeller jet (Deme) 225
9.4.1 Introduction 225
9.4.2 Objective of the full-scale erosion tests and estimated flow field 226
9.4.3 Scour prediction methods 226
9.4.4 Results 228
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Contents xiii
9.5 Scour due to ship thrusters in the Rotterdam port area (Port of
Rotterdam) 229
9.5.1 Introduction 229
9.5.2 Full-scale test with inland vessels at the Parkkade 231
9.5.2.1 Scope 231
9.5.2.2 Observed scour depth versus predictions with Breusers formulas 232
9.5.2.3 Observed versus predicted scour for thrusters with
PIANC formulas 234
9.5.2.4 Conclusions 236
9.5.3 Scour due to operational use of Maasvlakte quay wall for
large seagoing container vessels 237
9.5.3.1 Observed scour 237
9.5.3.2 Computed scour 239
9.5.3.3 Conclusions 239
9.6 Crossing of high voltage power line (Witteveen & Bos) 240
9.6.1 Introduction 240
9.6.2 Scour for a single pier 240
9.6.3 Scour for multiple piers 242
9.6.4 Results and discussion 244
9.7 Scour development in front of culvert (van Oord) 244
9.7.1 Introduction 244
9.7.2 Initial bottom protection and scouring 246
9.7.3 New design bottom protection 248
9.7.4 Result redesign 250
9.8 Bed protection at railway bridge in a bypass of the river Waal (Deltares) 250
9.8.1 Introduction 250
9.8.2 Flow condition 251
9.8.3 Scouring 252
9.8.4 Designed bed protection 252
9.8.5 Final remarks 252
9.9 Pressure scour around bridge piers (Arcadis) 253
9.9.1 Introduction 253
9.9.2 Flow conditions 254
9.9.3 Scour computation 256
9.9.4 Results 257
9.10 Bed protection at the weir at Grave in the river Meuse (Rijkswaterstaat) 259
9.10.1 Introduction 259
9.10.2 Scope 259
9.10.3 Flow condition 261
9.10.4 Scour and bed protection 262
9.10.5 Condition after the flood 263
9.10.6 Hindcast 264
References 267
Index 279
Biography
Mr. Hoffmans has more than 30 years of professional experience in the field of scour, internal erosion, river engineering, dike and dam engineering and flood protection projects and has acted several times as Expert as well as Project Manager. He started his career as a research engineer regarding scour and erosion and the corresponding geotechnical processes. In addition, he examined the erodibility of both bed protections around hydraulic structures in non-uniform flow conditions, and revetments at dikes, for example grass covers. With this up-to-date knowledge he advised several clients all over the world with designing and assessing hydraulic structures. His design experience has been gained in various countries and covers the river training works and its geotechnical engineering and stability aspects. More specifically, this experience also focused on hydraulic structures such as estimating the dimensions of scour holes in relation to the magnitude of bed protection and the risks of shear failures and/or flow slides, considering several types of scour. The combination of research and project-related advice on site has increased his insight in the mathematical modelling of internal erosion phenomena, especially piping. He is author of several manuals and of more than 50 publications.
Mr. Henk Verheij graduated in 1978 at Delft University of Technology in Civil Engineering (M.Sc.) and, subsequently, joined Delft Hydraulics (now Deltares) until his retirement in 2014. From 2006 till mid 2020 he was lecturer Ports & Waterways at the Delft University. Mr. Henk Verheij is a senior hydraulic engineer in the fields of dikes, embankments, hydraulic structures, ship-induced water motions, revetments and scour and its effects on the stability of bed protections, embankments and dikes along canals and rivers. He has experience with small-scale models as well as full-scale prototype investigations. He participated in research on dike breaches in cohesive materials, environmental-friendly protection methods, strength of grass dikes by overtopping tests, and reed as protection material. The expertise of Mr. Henk Verheij is being used for preparing guidelines and manuals for filter design, river dike revetments, scour, and the safety against flooding. He published over 50 papers, and is co-author of PIANC report 180 Guidelines for protecting berthing structures from scour caused by ships.
