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CIRIA C742 London, 2015
Manual on scour at
bridges and other hydraulic
structures, second edition
A M Kirby Mott MacDonald
M Roca HR Wallingford
A Kitchen JBA Consulting
M Escarameia HR Wallingford
0 J Chesterton Mott MacDonald
Griffin Court, 15 Long Lane, London, EC1A 9PN
Email: [email protected] Website: www.ciria.org
Tel: 020 7549 3300 Fax: 020 7549 3349
Contents
Acknowledgements
Summary vi
Glossary *viii
Abbreviations and acronyms xxvii
Notation xxix
1 Introduction 1
1.1 What is scour? 1
1.2 Types of scour 1
1.3 Aims of the manual 1
1.4 Scope of the manual 2
1.5 How to use this guide 3
1.6 Further reading 4
2 Scour processes 6
2.1 Introduction to river morphology 6
2.2 Types of scour 8
2.2.1 Natural scour 8
2.2.2 Contraction scour 10
2.2.3 Local scour H
2.2.4 Total scour 13
2.2.5 Other types of scour 14
2.3 Sediment types and behaviour 15
2.3.1 Non-cohesive sediments 16
2.3.2 Cohesive sediments 17
2.3.3 Sediment strata and layering 18
2.3.4 Rock 18
2.4 Time effects in scour processes 19
2.4.1 General considerations 19
2.4.2 Clear-water and live-bed scour 19
2.4.3 The transient nature of scour 20
2.4.4 Scour hole evolution 21
2.4.5 Tidal effects 21
2.4.6 Scour due to flash floods 22
2.5 Failure mechanisms 23
2.5.1 Introduction 23
2.5.2 Scour at foundations 24
2.5.3 Scour at edges 25
2.5.4 Bank erosion and channel migration 25
2.5.5 Exposure of buried pipelines 26
2.5.6 Scour of mortar and other materials 26
2.5.7 Hydraulic forces 27
2.5.8 Debris accumulation 27
2.5.9 Ice 28
3 Scour risk management 29
3.1 Scour risk management strategy 30
3.2 Anticipation 31
Manual on scour at bridges andother hydraulic structures, second edition vii
3.2.1 Screening az
3.2.2 Asset register 32
3.3 Assessment 33
3.3.1 Initial assessment 34
3.3.2 Survey and inspection 35
3.3.3 Detailed assessment 40
3.3.4 Re-assessment 41
3.3.5 Monitoring 41
3.3.6 Environmental assessment 42
3.4 Prevention 43
3.4.1 Debris management 43
3.4.2 Economic appraisal 44
3.5 Preparation, response and recovery 47
3.5.1 Emergency scour plan 47
3.5.2 Response arrangements 48
3.5.3 Closure and re-opening criteria 49
3.5.4 Training and exercising 50
3.5.5 Response and recovery 50
3.6 Standards and guidance 50
3.6.1 Network Rail 51
3.6.2 Rail Safety and Standards Board (RSSB) 51
3.6.3 Design Manual for Roads and Bridges (DMRB) 52
3.6.4 National Bridge Inspection Standards (NBIS) 53
3.6.5 Hydraulic Engineering Circulars (HEC) 53
3.6.6 US Forest Service 54
4 Input parameters for scour and mitigation assessment 55
4.1 Level datums 55
4.2 Design flood event 55
4.3 Discharge 57
4.3.1 Fluvial flows 57
4.3.2 Estuarine flows 61
4.4 Cross-sectional and plan geometry 61
4.5 Water levels and flow depths 62
4.6 Flow velocities 64
4.7 Bed material 67
4.8 Foundation type and depths 67
4.8.1 Types of foundation 68
4.8.2 Methods of investigation 68
5 Estimation of scour 69
5.1 Natural scour 69
5.1.1 Estimation of channel stability 70
5.1.2 Degradation of channel 77
5.1.3 Short-term scour during floods 79
5.1.4 Lateral channel migration 80
5.1.5 Bend scour 82
5.1.6 Confluence scour 83
5.2 Contraction scour 83
5.3 Local scour 85
5.3.1 Introduction 85
5.3.2 General procedure for estimating levels of local scour 88
5.3.3 Bridge piers, caissons and cofferdams 89
5.3.4 Abutments 92
5.3.5 Guide banks and revetments 95
5.3.6 Spur dikes or groynes 97
vill CIRIA, C742
5.3.7 Gates 99
5.3.8 Culverts 101
5.3.9 Weirs and drop structures 102
5.3.10 Rigid aprons 103
5.3.11 Pipeline crossings 103
5.3.12 Closures in fluvial and tidal channels 104
5.4 Local scour in tidal conditions 104
5.5 Numerical models 106
5.5.1 One-dimensional modelling 107
5.5.2 Two-dimensional modelling 107
5.5.3 Three-dimensional modelling 107
5.5.4 Summary 108
5.6 Physical models 1°8
5.7 Uncertainties in estimation 109
6 Scour mitigation measures HI
6.1 Scour mitigation philosophy 112
6.1.1 Scour reduction measures 112
6.1.2 Structural measures 112
6.1.3 Scour protection measures 112
6.1.4 General considerations 113
6.2 General construction considerations 116
6.2.1 General H6
6.2.2 Underwater construction 116
6.2.3 Availability of labour, plant and materials 117
6.2.4 Health and safety 117
6.2.5 Access H7
6.2.6 Temporary works 118
6.2.7 Undermining 119
6.3 Scour reduction measures 119
6.3.1 Introduction 119
6.3.2 Structure alignment and channel stability 120
6.3.3 Hydraulic design 123
6.3.4 Streamlining structural elements 125
6.3.5 River training 127
6.3.6 Deflectors 13°
6.4 Structural measures 132
6.4.1 General guidance 132
6.4.2 Spread footings 133
6.4.3 Piled foundations 134
6.4.4 Structural repairs 135
6.5 Scour protection measures 139
6.5.1 Selection of scour protection measure 139
6.5.2 Design principles 142
6.5.3 Rip-rap 146
6.5.3.1 Sizing 147
6.5.3.2 Rip-rap sizing: general formula 148
6.5.3.3 Rip-rap sizing: piers 151
6.5.3.4 Rip-rap sizing: abutments 151
6.5.3.5 Grading 153
6.5.3.6 Armourstone gradings 153
6.5.3.7 Falling aprons 154
6.5.3.8 Environmental considerations 157
6.5.3.9 Construction issues 157
6.5.4 Gabion mattresses, boxes and sacks 158
6.5.4.1 Sizing 160
Malnua^ionsco^^ and other hydrauflcstructures, second edition 'x
6.5.4.2 Grading and thickness 161
6.5.4.3 Environmental considerations 162
6.5.4.4 Construction issues 162
6.5.5 Articulated concrete blocks (cable-tied and interlocking) 163
6.5.5.1 Sizing 164
6.5.5.2 Environmental considerations 165
6.5.6 Grout or concrete filled mattresses 165
6.5.6.1 Design 167
6.5.6.2 Environmental considerations 167
6.5.6.3 Construction issues 167
6.5.7 Bituminous systems 167
6.5.7.1 Sizing 168
6.5.7.2 Environmental considerations 168
6.5.8 Biotechnical solutions 169
6.5.8.1 Fascine mattresses 169
6.5.8.2 Faggots 169
6.5.8.3 2D and 3D soil reinforcement geotextiles 170
6.5.8.4 Design 170
6.5.8.5 Environmental considerations 170
6.5.9 Concrete aprons 171
6.5.9.1 Design 171
6.5.9.2 Environmental considerations 172
6.5.10 Stone pitching 172
6.5.10.1 Design 172
6.5.10.2 Environmental considerations 173
6.5.11 Sheet piling 173
6.5.11.1 Design 173
6.5.11.2 Environmental considerations 174
6.5.12 Pipeline crossings 174
6.5.13 Culverts and energy dissipation structures 175
6.6 Filter design 177
6.6.1 General issues 177
6.6.2 Granular filter design 178
6.6.3 Geotextile filter design 180
6.6.4 Filter construction 181
References 183
Statutes 197
Further reading 202
Al Legislative framework 203
Al.l Introduction 203
A1.2 Health and safety law 203
Al.2.1 Public safety and emergency planning 204
Al.2.2 Health and safety at work 204
Al.2.3 Occupiers' liability 205
Al.2.4 Corporate manslaughter 205
A1.3 Transport law 206
Al.3.1 Roads 206
Al.3.2 Railways 207
Al.3.3 Canals 207
A1.4 Land drainage and flood risk management law 207
A1.5 Environmental law 209
Al.5.1 Habitats Directive 209
Al.5.2 Water Framework Directive 210
xCIRIA, C742
Al.5.3 Bathing Water Directive 211
Al.5.4 Fish Directives 211
Al.5.5 Waste Framework Directive 211
A1.6 Common law 212
Al.6.1 Nuisance 212
A1.7 Consent requirements 212
Al.7.1 Planning permission 212
Al.7.1.1 Permitted development 213
Al.7.1.2 Environmental impact assessment 213
Al.7.1.3 Flood risk assessment 214
Al.7.2 Consent to work in watercourses 214
Al.7.2.1 England and Wales 214
Al.7.2.2 Scotland 214
Al.7.2.3 Northern Ireland 215
Al.7.2.4 Ireland 215
Al.7.2.5 Environmental appraisal or assessment 215
Al.7.2.6 WFD assessment 215
Al.7.2.7 Environmental site survey 216
Al.7.2.8 Programme constraints 216
Al.7.3 Marine licence 216
A2 Case studies 217
A3 Hydrodynamic forces 251
A3.1 Introduction 251
A3.2 Basis of assessment 251
A3.2.1 Standards and guidance 251
A3.2.2 Applied actions 252
A3.2.3 Assessment scenarios 253
A3.2.3.1 Flows, water levels and velocities 253
A3.2.3.2 Bulk density of water 253
A3.2.3.3 Debris accumulation 253
A3.2.3.4 Buoyancy 254
A3.2.3.5 Skew 254
A3.2.4 Failure mechanisms (limit states) 254
A3.2.5 Partial factors 254
A3.3 Determination of actions 255
A3.3.1 Debris impact 255
A3.3.2 Drag 256
A3.3.3 Lift 258
A3.3.4 Overturning moment 259
A3.3.5 Masonry arch bridges 259
A3.4 Responses to risk of failure 260
A4 Monitoring equipment 261
A4.1 Introduction 261
A4.2 Choice of method 262
A4.2.1 Primary function 262
A4.2.2 Other factors 262
A4.2.3 Cost 264
A4.3 Visual inspection 265
A4.3.1 Site inspection 265
A4.3.2 Cameras 265
A4.4 Analogues for scour 265
A4.4.1 Water level gauge 266
A4.4.2 Velocity or flow meters 266
A4.5 Maximum scour depth 266
Manual on scourat bridges and other hydraulic structures, secondedition xl
A4.5.1 Physical probing zo°
A4.5.2 Sounding rods 267
A4.5.3 Magnetic sliding collars 268
A4.5.4 Float-out devices 268
A4.5.5 Tethered buried switches 269
A4.6 Scour development over time 269
A4.6.1 Sonar 269
A4.6.2 Geophysical instruments 271
A4.6.3 Buried or driven rods 271
A4.6.4 Time-domain reflectometry (TDR) 272
A4.7 Movement sensors 272
A4.7.1 Tilt sensors 272
A4.7.2 Accelerometer 272
A5 Methods of investigation 273
A5.1 Choice of method 273
A5.2 Desk-based methods 274
A5.2.1 Drawings 274
A5.2.2 Comparison with similar bridges 274
A5.2.3 Reverse engineering 275
A5.3 Intrusive methods 275
A5.3.1 Trial pits 276
A5.3.2 Rotary drilled coring 276
A5.4 Surface non-intrusive methods 276
A5.4.1 Sonic echo 277
A5.4.2 Bending waves 277
A5.4.3 Ultra-seismic 277
A5.4.4 Surface wave spectral analysis 277
A5.4.5 Ground penetrating radar 277
A5.4.6 Dynamic foundation response 278
A5.5 Sub-surface non-intrusive methods 278
A5.5.1 Parallel seismic 278
A5.5.2 Borehole sonic 279
A5.5.3 Cross hole sonic 279
A5.5.4 Borehole radar 279
A5.5.5 Induction field 279
A5.5.6 Borehole magnetic 279
A6 Alternative method for calculating local scour at bridge piers 280
Boxes
Box 2.1 Extreme rainfall event in August 2004, Boscastle, UK 22
Box 2.2 Exceptional flooding in November 2009, Cumbria, UK 23
Box 3.1 Failure of scour protection, Malahide viaduct, County Dublin, Ireland 33
Box 3.2 Databases for scour assessment 34
Box 3.3 Health and safety: site inspection 38
Box 3.4 Health and safety: structural inspection 39
Box 3.5 Health and safety: underwater survey 40
Box 3.6 Health and safety: wading survey 40
Box 3.7 Network Rail inspection regime 44
Box 3.8 Health and safety: debris management 44
Box 3.9 Example of options appraisal using benefit-cost ratio 46
Box 3.10 Example of choosing design standard using incremental benefit-cost ratio 47
Box 3.11 Review of plans following the floods in 2009 in Cumbria, UK 48
Box 3.12 Flood warning database 48
Box 4.1 Calculation of discharge or water levels in rivers and channels 59
xli CIRIA, C742
Box 4.2
Box 4.3
Box 4.4
Box 5.1
Box 5.2
Box 5.3
Box 5.4
Box 5.5
Box 5.6
Box 5.7
Box 5.8
Box 5.9
Box 5.10
Box 5.11
Box 6.1
Box 6.2
Box 6.3
Box 6.4
Box 6.5
Box 6.6
Box A4.1
Box A4.2
Box A4.3
Case studies
Case study A2.1
Case study A2.2
Case study A2.3
Case study A2.4
Case study A2.5
Case study A2.6
Case study A2.7
Case study A2.8
Case study A2.9
Case study A2.10
Case study A2.ll
Case study A2.12
Case study A2.13
Case study A2.14
Case study A2.15
Case study A2.16
Case study A2.17
Case study A2.18
Figures
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Discharge in compound channels 60
Definitions of variables in a cross-section 63
Flow velocities in channels 65
Calculation of flow velocity at threshold of bed movement 72
Formulae for regime equations 75
Recommended procedure for estimating short-term scour during floods 80
Procedure for estimating contraction scour 84
General procedure for estimating levels of local scour 89
Factors to estimate local scour at bridge piers 90
Factors to estimate local scour at abutments 94
Methods to estimate scour at guide banks and revetments 97
Method to estimate scour at the toe of a spur dike 98
Method to estimate scour at a gate 101
Procedure for estimation of tidal scour depth 105
Scour mitigation design checklist 114
Key points for channel stability 123
Key points for hydraulic considerations 125
Key points for structural design 133
Example of emergency repairs to a cutwater during flood conditions 137
Key design principles for scour protection 143
Using a scour indicator device (SID) to identify scour 268
Scanning sonar and multi-beam swathe sonar 270
The benefits of acoustic imaging 271
Puslinch Bridge, Devon, UK 219
Head Bridge, North Devon, UK 221
Banff Bridge, Aberdeenshire, UK 223
Oreti River road bridge, New Zealand 225
Tillynaught Bridge, Aberdeenshire, Scotland 226
Lower Ashenbottom viaduct, Lancashire, UK 228
Malahide, County Dublin, Ireland 230
River Crane bridge, Feltham, UK 232
Swat Valley bridges, Pakistan, South Asia 234
Ponte Hintze Riberio, Portugal 236
Glanrhyd railway bridge, Carmarthenshire, Wales 237
Bulls Road bridge, New Zealand 239
Bealey Road bridge, New Zealand 240
Scoharie Creek road bridge, USA 242
Hatchie River road bridge, USA 243
Jubilee River, UK 245
Musbury House culvert, Devon, UK 247
Pipeline exposure in a gravel bed river crossing 249
Sediment movement through the system 6
Sediment sources through a river catchment 7
Bridge failure due to scour in Calabria, Italy 8
Bend scour 10
Contraction scour at a bridge 11
Contraction of flow due to blockage of outer arches of the bridge, River Tiber, Rome, Italy 11
Variation of flow field with reducing approach flow depth 12
Flow structure around an abutment 13
Schematic illustrating total scour 13
Example of non-cohesive (a) and cohesive (b) bed material 15
Manualon scour at bridges and otherhydraulic structures, second edition xiii
Figure 2.11 Example of a gravel armoured layer over a sandy bed 18
Figure 2.12 Undermined bridge pier founded on weathered rock that led to the closure of a bridge in
2012, Devon, UK 19
Figure 2.13 Comparison of development of clear-water and live-bed scour 20
Figure 2.14 Example of scour hole evolution downstream of a bed sill measured in the laboratory 21
Figure 2.15 Bridge blocked by extreme flooding on the River Valency, Boscastle, UK 22
Figure 2.16 Example of bridge failure in Cumbria during the 2009 floods 23
Figure 2.17 Example of scour at exit from culvert endangering bank stability 24
Figure 2.18 Example of failure of a mattress bank revetment due to local scour at its edge (toe) 25
Figure 2.19 Example of bank erosion downstream of a weir 25
Figure 2.20 Failure mechanisms of buried pipelines due to scour leading to exposure 26
Figure 2.21 Example of debris accumulation at a bridge 27
Figure 3.1 Scour risk management process 29
Figure 3.2 Scour risk management cycle 31
Figure 3.3 Colour-coded risk rating of bridge elements 33
Figure 3.4 Radial survey of bed levels at piers and abutments 36
Figure 3.5 Digital terrain mode (DTM) showing local scour hole 37
Figure 3.6 Relationship between damage and probability 46
Figure 3.7 Water level marker 49
Figure 4.1 Channel cross-section 59
Figure 4.2 Compound channel cross-section 60
Figure 4.3 Channel plan and section 63
Figure 4.4 Contours of velocity and vertical velocity profile 65
Figure 5.1 Flow chart for calculation of scour 69
Figure 5.2 Potential for scour by river type 70
Figure 5.3 Hjulstrom curve definingthresholds for sediment deposition, erosion and transport 71
Figure 5.4 Shields diagram for the threshold of movement of sediment particles 72
Figure 5.5 Geometry of river meanders 81
Figure 5.6 Sketch of a long contraction: planform (a) and profile (b) 83
Figure 5.7 Main variables to estimate contraction scour 84
Figure 5.8 Typical flow pattern around a structure 85
Figure 5.9 Variation of scour depth with flow velocity for bridge pier and non-cohesive material
(qualitative scale) 87
Figure 5.10 Examples of pier structures 90
Figure 5.11 Abutments in rectangular channels (a) and compound channels (b) 95
Figure 5.12 Chashma Barrage in the Indus River, Pakistan 96
Figure 5.13 Typical plan layout of spur dikes (a) and spur dikes at both banks in the Rhine River (b)...97
Figure 5.14 Scour produced by 2D jet 99
Figure 5.15 Scour produced by 3D jet from culvert 101
Figure 5.16 Scour produced by plunging jet from drop structure 102
Figure 5.17 Scour produced by residual turbulence downstream of rigid apron 103
Figure 5.18 Example of flow velocities obtained with a 2D model 107
Figure 5.19 Testing a complex bridge pier in a flume 109
Figure 5.20 Example of a physical model that includes four sets of piers: from left to right, the
construction phase where two sets of a road bridge are present, an existing railway
bridge and a disused railway bridge 109
Figure 6.1 Detailed scour assessment process Ill
Figure 6.2 Scour mitigation philosophy 112
Figure 6.3 Scour protection works being undertaken in good conditions 118
Figure 6.4 Placing rip-rap in restricted headroom 118
Figure 6.5 Road undermined by lateral erosion from an upland wadi 120
Figure 6.6 Change in channel alignment at bend 121
Figure 6.7 Lateral channel migration exposing foundations on floodplain 121
Figure 6.8 Scour at bridge abutment caused by meander development 122
Figure 6.9 Degradation and lateral erosion caused by deforestation 123
xlvCIRIA, C742
Figure 6.10 Rubbish dumped at the right abutment has re-directed flow and undermined the left
abutment 125
Figure 6.11 Bed degradation due to gravel extraction 125
Figure 6.12 Streamlining of pier base and piled foundation 125
Figure 6.13 Debris build-up against a pier 126
Figure 6.14 Examples of transitions between vertical and sloping banks 127
Figure 6.15 River training works 128
Figure 6.16 Stub groynes used for river training 128
Figure 6.17 Bed control using a downstream sill 129
Figure 6.18 Typical guide bank layout 130
Figure 6.19 Debris/ice deflector upstream of the Charles Bridge, Prague 131
Figure 6.20 Example of the inappropriate use of spread footings for a bridge crossing a steep highly
mobile river 133
Figure 6.21 Recommended footing locations 134
Figure 6.22 Principles of pile and pile cap location 135
Figure 6.23 Typical structural repairs 136
Figure 6.24 Concrete repairs to a bridge abutment 136
Figure 6.25 Factors influencing scour protection type 140
Figure 6.26 Alternative recommendations for extent of scour protection 143
Figure 6.27 Recommended scour protection extent at piers 144
Figure 6.28 Scour protection at bridge abutments and piers 145
Figure 6.29 Toe and falling apron details 156
Figure 6.30 Examples of gabion scour protection 158
Figure 6.31 Interlocking concrete block protection to bridge 163
Figure 6.32 Grout filled mattress edge detail 166
Figure 6.33 Examples of grout-filled mattress protection 167
Figure 6.34 Construction of rock armour and willow faggoting repair to eroded river bank 169
Figure 6.35 Erosion protection of geotextile-reinforced grass 170
Figure 6.36 Scour downstream of a small structure, highlighting the problem with an inflexible
concrete apron 171
Figure 6.37 Grouted stone pitching to bridge invert 172
Figure 6.38 Typical pipeline protection 174
Figure 6.39 Erosion protection downstream of a weir structure 176
Figure 6.40 Scour downstream of an offtake structure, caused by the lack of scour protection
coupled with inadequate energy dissipation 177
Figure 6.41 Principles of geometrically tight filters 178
Figure 6.42 Laying fascine mattress (tidal river) 181
Figure A2.1 Downstream elevation 219
Figure A2.2 Damage to the pier 219
Figure A2.3 Concrete bag repair (during construction of scour apron with arch dewatered) 220
Figure A2.4 Permanent repair and concrete invert 220
Figure A2.5 Upstream elevation showing new concrete apron, Head Bridge, North Devon 221
Figure A2.6 Typical scour of bedrock under pier foundation 221
Figure A2.7 Concrete being placed in scour hole 222
Figure A2.8 Scour to abutments and piers in main river channel 222
Figure A2.9 Simulated depth-averaged velocity distribution at Banff Bridge (100-year event) 223
Figure A2.10 Banff Bridge during the 2009 floods 224
Figure A2.ll Oreti River road bridge 225
Figure A2.12 Tillynaught Bridge collapse before (a) and after (b) 226
Figure A2.13 The new Tillynaught Bridge 227
Figure A2.14 Collapsed central pier 229
Figure A2.15 Following collapse 231
Figure A2.16 Repairs underway 231
Figure A2.17 Following scour failure 232
Figure A2.18 Before bridge failure 232
Manual on scour at bridges and other hydraulic structures, secondedition xv
Figure A2.19 Scour downstream of the bridge pier has caused a rotational failure of the pier 234
Figure A2.20 Bulls Road bridge failure 239
Figure A2.21 Bealey Road bridge failure 240
Figure A2.22 Schoharie Creek road bridge after collapse of second pier 242
Figure A2.23 The aftermath of the Hatchie River bridge failure 243
Figure A2.24 Severe scour was experienced on the bed and banks following a flood, Taplow Intake,
Jubilee River, UK 245
Figure A2.25 Physical model tests, Taplow Intake, Jubilee River, UK 246
Figure A2.26 Depression in road that alerted inspector 247
Figure A2.27 Scour hole 247
Figure A2.28 The two safe working zones and hoses and Venturi being used to remove loose material 248
Figure A2.29 Temporary shuttering 248
Figure A2.30 Completion of concrete placement 248
Figure A2.31 Overview of pipeline location 249
Figure A3.1 Drag coefficient for flat bridge decks 257
Figure A3.2 Lift and drag coefficient at plate type pier 257
Figure A3.3 Lift coefficient for girder and streamlined decks 258
Figure A3.4 Moment coefficient for girder and streamlined decks 259
Figure A4.1 Scour indicator device protruding from upstream cutwater 268
Figure A4.2 Plan view showing additional cores to protect against collapse 268
Figure A4.3 2D image of bridge scour captured with sector scanning sonar 270
Figure A4.4 3D image of bridge scour captured with multi-beam sonar 270
Figure A4.5 Sonar survey showing pier and riverbed 271
Figure A5.1 Choice of method for investigating foundation type and depth 273
Figure A5.2 Core drilling from a floating pontoon 276
Figure A5.3 Core recovered from drilling 276
Figure A6.1 Main geometric parameters for a non-uniform pier 281
Figure A6.2 Main geometric parameters for a skewed pier 281
Tables
Table 1.1 Chapter content 3
Table 1.2 Guide to area of interest 4
Table 2.1 Influence of engineering work on channel process/morphology 7
Table 2.2 Approximate values of angle of repose for various granular materials 17
Table 3.1 Checklist for screening 32
Table 3.2 Checklist for initial assessment 35
Table 3.3 Checklist for site inspection 38
Table 3.4 Checklist for structural inspection of bridges 39
Table 3.5 Checklist for underwater inspection 40
Table 3.6 Detailed assessment process 41
Table 3.7 Event damages 46
Table 3 8 Comparison of options using benefit-cost ratio 46
Table 3.9 Choosing design standard using incremental benefit-cost ratio 47
Table 3.10 Overview of scour risk management procedure 50
Table 4.1 Percentage chance of a particular return period event occurring during the design life
of a structure 56
Table 4.2 Recommended intervals for survey cross-sections 62
Table 4.3 Turbulence intensities 65
Table 5.1 Features indicative ofgeneral erosion, deposition or stability for temperate river systems 71
Table 5.2 Critical velocities to initiate erosion of cohesive materials in rivers 74
Table 5.3 Main flow parameters 75
Table 5.4 Coefficients in equations 76
Table 5.5 Values of shape factor, Osliape, for structures 90
Table 5.6 Values of factor of safety, SF 91
Table 5.7 Values of shape factor Ks* 94
xviCIRIA, C742
Table 5.8 Values of alignment factor K9* 94
Table 5.9 Value of factor Ojto estimate local scour at guide banks and revetments 97
Table 5.10 Maximum depth relation between permeable and impermeable spur dikes 99
Table 5.11 Characteristics and limitations of ID, 2D and 3D models 108
Table 6.1 Issues to consider and measures to reduce scour 120
Table 6.2 Footing design considerations 134
Table 6.3 Piled foundation design considerations 134
Table 6.4 Structural repair design considerations 138
Table 6.5 Comparison of structural repair types 138
Table 6.6 Scour protection measures: selection checklist 141
Table 6.7 Rip-rap sizing formulae 147
Table 6.8 Values of turbulence intensity (Tl) under different scour conditions 149
Table 6.9 Gabion revetment sizing - thickness vs. size and mean velocity 160
Table 6.10 Choice of filter type 178
Table Al.l Summary of scour-related activities and relevant sections 203
Table A1.2 Summary of health and safety law 204
Table A1.3 Summary of transport law and authorities 206
Table A1.4 Summary of land drainage and flood risk management law 207
Table A1.5 Summary of environmental law 209
Table A1.6 Statutes and policies for planning permission 213
Table A1.7 Summary of consent to work in watercourses law 214
Table A1.8 Summary of marine licensing law 216
Table A2.1 Summary of case studies 218
Table A3.1 Standards and guidance for assessment of hydrodynamic forces 252
Table A3.2 Recommended design floods 253
Table A3.3 Recommended extent of debris accumulation 254
Table A3.4 Partial factors after Eurocodes 254
Table A3.5 Partial safety factors 255
Table A3.6 Recommended stopping distances for three-tonne log striking bridge piers 256
Table A3.7 Variables for assessment of drag 256
Table A3.8 Drag coefficient for debris on bridge decks and piers 258
Table A3.9 Variables for assessment of lift 258
Table A4.1 Performance criteria for monitoring equipment 262
Table A4.2 Factors affecting choice of scour monitoring method 263
Table A4.3 Relative costs of selected scour monitoring systems 264
Table A4.4 Summary of visual inspection methods 265
Table A4.5 Summary of analogues for scour 265
Table A4.6 Summary of methods for monitoring maximum scour depth 266
Table A4.7 Summary of methods for monitoring scour development over time 269
Table A4.8 Summary of movement sensors 272
Table A5.1 Indicative cost of methods of investigating foundation depth 274
Table A5.2 Summary of intrusive methods 275
Table A5.3 Summary of surface non-intrusive methods 277
Table A5.4 Summary of sub-surface non-intrusive methods 278
Table A6.1 Shape factor 281
Manual on scourat bridges and other hydraulic structures, second edition xvll
