This manual was prepared to provide Reclamation personnel with updated guidance on deploying effective bank stabilization methods. Looking back several decades, the two measures of a successful river design were feasibility and sustainability. Feasibility was measured in terms of cost effectiveness and public safety. A feasible structure did not harm the public, and fulfilled its intended function at the lowest possible construction cost. Sustainability was defined as the ability to withstand river changes with minimal or no maintenance for the life of the project. The words “bank stabilization design” were commonly synonymous with sizing riprap. In recent decades, environmental effectiveness and sustainability have gained recognition as a third fundamental measure of project success. Environmental effectiveness can be evaluated on two levels: (1) The degree to which the design is compatible with river and geomorphic processes; and (2) The amount of disruption of the riparian habitat and natural environment. Environmental and geomorphic sustainability is reduced when the design is lacking in geomorphic compatibility. A goal of these guidelines is to increase considerations of sustainability and longterm feasibility, and to increase environmental effectiveness in bank stabilization designs. Most of the bank protection methods presented in this manual can be applied in ways that meet all three measures of a successful design: feasibility, sustainability and environmental effectiveness. Although traditional methods of hardening a bank with a full riprap revetment often fail the test of environmental effectiveness, this method is still necessary under specific conditions and is included if other methods cannot be used or do not meet project objectives.
Bank Stabilization Design Guidelines Report No SRH-2015-25 Albuquerque Area Office Science and Technology Policy and Administration (Manuals and Standards) Yuma Area Office 1.1.1 1.1.2 U.S Department of the Interior Bureau of Reclamation Technical Service Center Denver, Colorado June 2015 Mission Statements The U.S Department of the Interior protects America’s natural resources and heritage, honors our cultures and tribal communities, and supplies the energy to power our future The mission of the Bureau of Reclamation is to manage, develop, and protect water and related resources in an environmentally and economically sound manner in the interest of the American public Cover Photograph: Stream vanes on the Calapooia River, Washington Courtesy of Scott Wright, Natural Resources Conservation Service BUREAU OF RECLAMATION Technical Service Center, Denver, Colorado Sedimentation and River Hydraulics Group, 86-68240 Report No.: SRH-2015-25 Bank Stabilization Design Guidelines Report Prepared by: Drew C Baird Ph.D, P.E., D.WRE, Hydraulic Engineer Sedimentation and River Hydraulics Group, Technical Service Center Lisa Fotherby Ph.D., P.E Hydraulic Engineer Sedimentation and River Hydraulics Group, Technical Service Center Cassie C Klumpp, M.S., P.E Retired Hydraulic Engineer Sedimentation and River Hydraulics Group, Technical Service Center S Michael Sculock, Ph.D., Research Scientist Department of Civil and Environmental Engineering Engineering Research Center Colorado State University Report Peer-Reviewed by: Blair Greimann Ph.D., P.E., Technical Specialist Sedimentation and River Hydraulics Group, Technical Service Center (Reviewed chapters 1-11 and 13-14) Nathan Holste, M.S., P.E., Hydraulic Engineer Sedimentation and River Hydraulics Group, Technical Service Center (Reviewed chapter 12) 1.1.5 1.1.6 U.S Department of the Interior Bureau of Reclamation Technical Service Center Denver, Colorado June 2015 BUREAU OF RECLAMATION Technical Service Center, Denver, Colorado Sedimentation and River Hydraulics Group, 86-68240 Report No.: SRH-2013-25 Bank Stabilization Design Guidelines Reclamation Authors: _ Drew C Baird, Ph.D., P.E., D.WRE, Hydraulic Engineer Date Sedimentation and River Hydraulics Group, Technical Service Center _ Lisa Fotherby, Ph.D., P.E Hydraulic Engineer Date Sedimentation and River Hydraulics Group, Technical Service Center Report Peer-Reviewed Certification: _ Blair Greimann, Ph.D., P.E., Technical Specialist Date Sedimentation and River Hydraulics Group, Technical Service Center (Reviewed chapters 1-11, 13 and 14) _ Nathan Holste, M.S., P.E., Hydraulic Engineer Sedimentation and River Hydraulics Group, Technical Service Center (Reviewed chapter 12) Date Disclaimer This document was prepared under the sponsorship of the Bureau of Reclamation in the interest of information exchange The United States Government assumes no liability for its contents or use thereof The contents of this report represent the views of the authors and are provided as guidelines This report does not constitute a standard, specification, or regulation The United States Government does not endorse products or manufacturers Product names are included to show type and availability, and not constitute endorsement for their specific use Acknowledgements This document was prepared using funding from the Albuquerque Area Office, Reclamation’s Science and Technology Program, Technical Service Center Manuals and Standards, and the Yuma Area Office Meg Jonas of the U.S Army Corps of Engineers, and Jon Fripp of the Natural Resources Conservation Service, provided their input and review of the topics covered and practical aspects of guideline contents Robert Padilla of the Albuquerque Area Office reviewed drafts of this guide and provided valuable comments Peter Martin of the Technical Service Center formatted the document and organized figures and tables These contributions are gratefully acknowledged ii Bank Stabilization Design Guidelines Contents Page Introduction 1.1 Challenges of Modern River Design 1.2 Bank Stabilization 1.3 Project Development 1.4 Organization of Design Guidelines PART I – PRELIMINARY INVESTIGATIONS AND METHOD SELECTION Project Requirements and General Assessment 11 2.1 Design Criteria 11 2.2 Ecological Goals 12 2.3 Risk Assessment 13 2.4 Hydrologic Data for Site Analysis 14 2.5 Other Site Data 17 2.6 Permitting 18 The Role of Geomorphology in River Projects 21 3.1 Geomorphic Assessment 22 3.2 Procedural Steps 23 3.3 Geomorphic Considerations to Promote a Stable Channel 24 Hydraulic Assessment of Energy, River Form, and Shear Forces 25 4.1 Energy and Width/Depth Ratio 25 4.2 Energy and Complex Channels 26 4.3 Energy and River Form 27 4.4 Computing Erosive Force and Assessing Material and Methods Suitability 34 4.5 Traditional Riprap Revetments 36 4.6 Sediment Analysis and Modeling 39 Scour Assessment 41 5.1 Forms of Vertical Scour 41 5.2 Descriptions 41 5.3 Countermeasures 47 5.4 Scour Assessment Method 47 5.5 Scour Computation Topics 51 5.6 Basis of Scour Guidelines 58 Selecting a Bank Stabilization Method 61 6.1 A Comprehensive Selection Process 61 6.2 Methods 63 iii Bank Stabilization Design Guidelines 6.3 6.4 6.5 6.6 Preserving the Floodplain 63 Hardening Banks 66 Methods Combination 67 Methods Selection 69 PART II – DESIGN AND CONSTRUCTION 79 Preserving the Floodplain 81 7.1 Infrastructure Relocations 81 7.2 Conservation Easements 82 7.3 Design Procedure 83 7.4 Discussion and Recommendations 85 Re-establishing Floodplain 87 8.1 Island/Bank Clearing and Destabilization 88 8.2 Longitudinal Bank Lowering 96 8.3 Side Channels 102 8.4 Channel Embayments 108 Design of Vegetated, Deformable Banklines 113 9.1 Shape and Deformable Banklines 114 9.2 Vegetation Bank Protection 117 9.3 Live Staking/Pole Planting 126 9.4 Fabric Encapsulated Lifts 135 9.5 Degradable/Deformable Stone Toe 139 9.6 Bio-Engineering on a Stone Toe 147 10 Design of Wood and Boulders 155 10.1 Large Woody Debris and Rootwads 156 10.2 Native Material Revetments 158 10.3 Engineered Log Jams (ELJs) 163 10.4 Boulder Clusters 173 11 Channel Relocations/Construction 181 11.1 Design Procedure 182 11.2 Pilot Channels 191 11.3 Discussion and Recommendations 192 12 Transverse or Indirect Methods 195 12.1 Bendway Weirs 197 12.2 Vanes or Barbs 210 12.3 Spur Dikes (Groins, Hard Points, L Dikes, and T Dikes) 225 12.4 J-Hooks 238 13 Hardened Banks 243 13.1 Upstream and Downstream Limits of the Work 244 13.2 Sizing Riprap 245 13.3 Riprap Revetment 252 13.4 Riprap Windrow and Trench Filled Riprap 264 13.5 Longitudinal Peak Stone Toe 269 14 Future Directions 275 iv Contents References 277 15 APPENDIX A – Scour Computation Methods 297 A.1 Long Term Aggradation or Degradation 297 A.2 Bend Scour Equations 301 A.3 Confluence Scour Equation 306 A.4 Scour Equations for Near-Structure Locations 306 Figures Page Figure 4–1 Relationship of width/depth ratio to weighted mean percentage of silt-clay by Schumm (1960) 26 Figure 4–2 Channel cross section with low flow channel, bankfull channel, floodplain, and high flow side channel (NRCS 2007a) 27 Figure 5–1 Example of channel degradation, looking upstream 43 Figure 5–2 Flow structure including macro-turbulence generated by flow around abutments in a narrow main channel, causing contraction scour 45 Figure 5–3 Looking downstream past scour at old railroad embankment and former bridge crossing, and towards a more recently constructed railroad embankment and timber bridge 45 Figure 5–4 Horseshoe scour pattern in a laboratory flume at a circular pier 46 Figure 5–5 Depth of pier scour hole is partially limited by the pile cap/footing that interrupts the downward diving flow of the secondary flow pattern 48 Figure 5–6 Generalized relationships for scour in cohesive materials 53 Figure 5–7 Debris caught on pile-piers of the Old Bridge from spring flows at the Little Colorado River, April 2013 55 Figure 5–8 Arkansas River at Rocky Ford bridge crossing 57 Figure 8–1 Figure 8–2 Figure 8–3 Figure 8–4 Figure 8–5 Island and bank clearing schematic 90 Discs pulled by tractors on Platte River to remove vegetation 94 Longitudinal bank lowering or compound channels schematic 97 Side channel plan view 103 Side channels 103 Figure 9–1 Bank zones 116 Figure 9–2 Limiting velocities for erosional resistance of vegetation 121 Figure 9–3 Constructing fabric encapsulated lifts (FES) 136 Figure 9–4 Deformable stone toe with bioengineering and bank line features 140 Figure 9–5 Deformable stone toe with bioengineering and bank lowering after bank line deformation 141 v Bank Stabilization Design Guidelines Figure 9–6 Examples of a degradable bank toe 142 Figure 9–7 Longitudinal stone toe with bioengineering 148 Figure 10–1 Typical rootwad installation 157 Figure 10–2 Rootwad and native material revetment, sectional view 159 Figure 10–3 Rootwad revetment, plan view 160 Figure 10–4 Large Woody Debris design details 166 Figure 10–5 Engineered log jam 167 Figure 10–7 Drag and lift force analysis of boulders/particles 175 Figure 10–8 Plan view showing boulder placement between riffles 177 Figure 10–9 Boulder clusters 179 Figure 12–1 Bendway weirs 198 Figure 12–2 Bendway weir key length 203 Figure 12–3 Graph to Estimate Blench Zero Bed Factor 206 Figure 12–4 Erosion of bank face at a meander bend with bendway weirs, Big Creek, Clark County, Ill 209 Figure 12–5 Plan schematic of transverse feature parameters 217 Figure 12–6 Cross section and structure profile view schematic of evaluated structures in trapezoidal model 218 Figure 12–7 Vane velocity equation parameter response 220 Figure 12–8 Spur-dike velocity equation parameter response 234 Figure 12–9 Vane with J-hooks typical drawing 239 Figure 13–1 Extent of protection required at a channel bend 245 Figure 13–2 Riprap design velocities for a natural channel 248 Figure 13–3 Correction for vertical velocity distribution in bend and riprap thickness 249 Figure 13–4 Correction for side-slope angle 250 Figure 13–5 Riprap size versus velocity for a bend of Rc/W=10 bend 252 Figure 13–6 Riprap size versus velocity for a bend of Rc/W=3 253 Figure 13–7 Riprap revetment with buried toe 254 Figure 13–8 Riprap revetment with mounded toe section for launching 256 Figure 13–9 Launched stone schematic 256 Figure 13–10 Riprap windrow and trench filled riprap 265 Figure 13–11 Longitudinal peak stone toe 270 Figure A–1 Shields’ relation from beginning of motion (adapted from Gessler 1971) 299 Figure A–2 Tractive force versus transportable sediment size, after Lane (Pemberton and Lara, 1984; figure 4) 300 Figure A–3 Illustration of terminology for bend scour (Simons, Li & Associates, 1985, figure 5.25) 302 Figure A–4 Definition sketch for bend scour (Maynord, 1996) 303 Figure A–5 Scour depth in bends (USACE, 1994; plate B-41) 305 vi APPENDIX A – Scour Computation Methods Figure A–5 Scour depth in bends (USACE, 1994; plate B-41) 305 Bank Stabilization Design Guidelines A.3 Confluence Scour Equation At the confluence of a tributary channel, the Ashmore and Parker (1983) method can be used for both sand and gravel sizes: ymcf / yu = 2.24 + 0.031 θ Where: ycfs = ymcf - yu ymcf yu ycfs θ = = = = (15.13) (15.14) Maximum water depth at the confluence (ft) Mean anabranch flow depth for converging channels (ft) Depth of confluence scour below thalweg (ft) Angle formed by two converging anabranches (degrees) Notes: Function is specified for θ of 30 to 90 degrees, and sand and gravel beds Less scour results in fine sand or cohesive bed material A.4 Scour Equations for Near-Structure Locations A.4.1 Contraction Scour – Modified Laursen’s Live-Bed Equation Shown below is the modified version of Laursen’s live-bed equation (Laursen,1960), as presented in HEC-18 (FHWA, 2012): Where: 𝑦𝑦1 = �𝑄𝑄2 � 𝑄𝑄 ys y1 y2 y0 = = = = Q1 = Q2 = W1 = W2 = k1 = 306 6⁄7 𝑊𝑊 𝑘𝑘1 𝑦𝑦2 �𝑊𝑊 � (15.15) y2 –y0 Average depth in the upstream main channel (ft) Average depth in the contracted section (ft) Existing depth in the contracted section before scour (ft) (see note on the following page) Flow in the upstream channel transporting sediment (ft3/s) Flow in the contracted channel (ft3/s) Bottom width of the upstream main channel that is transporting bed material (ft) Bottom width of main channel in contracted section less pier width(s) (ft) Correction factor for the mode of bed material transport from table A-1 APPENDIX A – Scour Computation Methods Table A–1 Correction Factor, k1, for Mode of Bed Material Transport Where: U*/ ω k1 < 0.50 0.59 Mostly contact bed material discharge 0.50 to 2.0 0.64 Some suspended bed material discharge >2.0 0.69 Mostly suspended bed material discharge U* ω g S1 = = = = τo = 𝜌𝜌 = 𝝉𝝉𝟎𝟎 ⁄𝝆𝝆 = �𝑔𝑔𝑦𝑦1 𝑆𝑆1 Mode of bed material transport , shear velocity in the upstream section (ft/s) Fall velocity of bed material based on the D50 (figure A-6) Acceleration of gravity (32.2 ft2/s) Slope of energy grade line of main channel (ft/ft) Shear stress on the bed (lb./ft2) Density of water (1.94 slugs/ft3) Figure A–6 Fall velocity of sand-sized particles with specific gravity of 2.65 in metric units 307 Bank Stabilization Design Guidelines Notes: When there is overbank flow on a flood plain being forced back to the main channel by the approaches to the bridge There are three potential cases: a The river channel width becomes narrower, either due to the bridge abutments projecting into the channel or the bridge being located at a narrowing reach of the river; b No contraction of the main channel, but the overbank flow area is completely obstructed by an embankment; or c Abutments are set back from the stream channel See HEC-18 (FHWA, 2012) for more information on Case and Cases 2-4 Q2 may be the total flow going through the bridge opening, as in case a and b It is not the total flow for case c For case 1c, contraction scour must be computed separately for the main channel and the left and/or right overbank areas Q1 is the flow in the main channel upstream of the bridge, not including overbank flows The Manning n ratio can be eliminated in Laursen live-bed equation as explained here The ratio can be significant for a condition of dune bed in the upstream channel and a corresponding plane bed, washed out dunes, or antidunes in the contracted channel However, Laursen's equation does not correctly account for the increase in transport that will occur as the result of the bed planning out (which decreases resistance to flow, increases the velocity, and increases the transport of bed material at the bridge) That is, Laursen's equation indicates a decrease in scour for this case; whereas in reality, there would be an increase in scour depth In addition, at floodflows, a plane bedform will usually exist upstream and through the bridge waterway, and the values of Manning n will be equal Consequently, the n value ratio is not recommended or presented in the equation W1 and W2 are not always easily defined In some cases, it is acceptable to use the top width of the main channel to define these widths Whether top width or bottom width is used, it is important to be consistent, so that W1 and W2 refer to either bottom widths or top widths The average width of the bridge opening (W2) is normally taken as the bottom width, with the width of the piers subtracted Laursen's equation will overestimate the depth of scour at the bridge if the bridge is located at the upstream end of a natural contraction, or if the contraction is the result of the bridge abutments and piers At this time, however, it is the best equation available In sand channel streams where the contraction scour hole is filled in on the falling stage, the y0 depth may be approximated by y1 Sketches or surveys through the bridge can help in determining the existing bed elevation 308 APPENDIX A – Scour Computation Methods Scour depths with live-bed contraction scour may be limited by coarse sediments in the bed material armoring the bed Where coarse sediments are present, it is recommended that scour depths be calculated for live-bed scour conditions using the clear-water scour equation (given in HEC-18 [FHWA, 2012]), in addition to the live-bed equation, and that the smaller calculated scour depth be used 10 See FHWA, 2012 for example problems and for information on adjusted approaches for cohesive soils, erodible rock, open bottom culverts, and pressure flow at bridges A.4.2 Local Scour at a Pier – CSU Equation The CSU local pier scour equation developed by Richardson et al., (1990), as reported by FHWA, (2012) is: The HEC-18 approach, based on the Colorado State University (CSU) equation, predicts a maximum scour depth for alluvial sand bed streams and is used for both live-bed and clear water conditions The HEC-18 equation is: 𝑦𝑦𝑠𝑠 𝑦𝑦1 𝑎𝑎 0.65 = 𝐾𝐾1 𝐾𝐾2 𝐾𝐾3 �𝑦𝑦 � 𝐹𝐹𝐹𝐹1 0.43 (15.16) As a rule of thumb, the maximum scour depth for round nose piers aligned with the flow is: ys ≤ 2.4 times the pier width (a) for Fr1 ≤ 0.8 ys ≤ 3.0 times the pier width (a) for Fr1 > 0.8 (15.17) In terms of ys/a, Equation 15-16 is: 𝑦𝑦𝑠𝑠 𝑎𝑎 Where: ys = y1 = K1 = K2 = K3 = A = L = Fr1 = V1 = g = 𝑦𝑦 0.35 = 𝐾𝐾1 𝐾𝐾2 𝐾𝐾3 � 𝑎𝑎1 � 𝐹𝐹𝐹𝐹1 0.43 (15.18) Scour depth (ft) flow depth directly upstream of the pier (approach flow depth) (ft) Correction factor for pier nose shape from table A-2 Correction factor for angle of attack of flow from table A-3 or equation 15-19 Correction factor for bed condition from table A-4 Pier width (ft) Length of pier (ft) Froude number directly upstream of the pier = V1/(gy)1/2 Mean velocity of flow directly upstream of the pier (ft/s) Acceleration of gravity (32.2 ft/s2) 309 Bank Stabilization Design Guidelines Table A–2 Correction Factor, K1, for Pier Nose Shape Shape of pier nose K1 Square nose 1.1 Round nose 1.0 Circular cylinder 1.0 Group of cylinders 1.0 Sharp nose 0.9 The correction factor, K2, for angle of attack of the flow, a, is calculated using the following equation: Where: 𝐾𝐾2 = (𝐶𝐶𝐶𝐶𝐶𝐶 𝛼𝛼 + a = 𝐿𝐿 𝑎𝑎 𝑆𝑆𝑆𝑆𝑆𝑆 𝛼𝛼)0.65 (15.19) Skew angle of flow with respect to the pier If L/a is larger than 12, use L/a = 12 as a maximum in equation 15-19 and table A–3 Table A–3 illustrates the magnitude of the effect of the angle of attack, a, on local pier scour Table A–3 Correction Factor, K2, for Angle of Attack, a, of the Flow Angle, α L/a = L/a =8 L/a = 12 1 15 1.5 2.5 30 2.75 3.5 45 2.3 3.3 4.3 90 2.5 3.9 The correction factor, K3, accounts for the effects of bedforms and bedform troughs where H is defined as the bedform height Table A–4 Correction Factor, K3, for Bed Condition Bed condition (bedforms) Dune height, H, ft K3 Clear water scour NA 1.1 Plane bed and anti-dune flow NA 1.1 10 > H > 1.1 30 > H >or= 10 1.2 to 1.1 H > or = 30 1.3 Small dunes Medium dunes Large dunes 310 APPENDIX A – Scour Computation Methods Notes from HEC-18 (FHWA, 2012): The correction factor K1 for pier nose shape should be determined using table A–2 for angles of attack up to degrees For greater angles, K2 dominates and K1 should be considered as 1.0 If L/a is larger than 12, use the values for L/a = 12 as a maximum in table A–3 and Equation 15-18 The values of the correction factor K2 should be applied only when the field conditions are such that the entire length of the pier is subjected to the angle of attack of the flow Using this factor will significantly over predict scour if: (1) a portion of the pier is shielded from the direct impingement of the flow by an abutment or another pier; or (2) an abutment or another pier redirects the flow in a direction parallel to the pier For such cases, judgment must be exercised to reduce the value of the K2 factor by selecting the effective length of the pier actually subjected to the angle of attack of the flow Equation 15-18 should be used for evaluation and design Table A–3 is intended to illustrate the importance of angle of attack in pier scour computations and to establish a cutoff point for K2 (i.e., a maximum value of 5.0) The correction factor K3 results from the fact that for plane-bed conditions, which are typical of most bridge sites for the flood frequencies employed in scour design, the maximum scour may be 10 percent greater than computed with equation 15-17 In the unusual situation where a dune bed configuration with large dunes exists at a site during flood flow, the maximum pier scour may be 30 percent greater than the predicted equation value This may occur on very large rivers, such as the Mississippi For smaller streams that have a dune bed configuration at flood flow, the dunes will be smaller, and the maximum scour may be only 10 to 20 percent larger than equilibrium scour For anti-dune bed configuration, the maximum scour depth may be 10 percent greater than the computed equilibrium pier scour depth Piers set close to abutments (for example, at the toe of a spill through abutment) must be carefully evaluated for the angle of attack and velocity of the flow coming around the abutment See HEC-18 (FHWA, 2012) for information on the treatment of pier groups, wide piers, complex pier foundations, multiple skewed columns, scour debris, hole top widths, coarse bed materials, cohesive bed materials, and erodible rock 311 Bank Stabilization Design Guidelines A.4.3 Abutment Scour Froehlich Equation Froehlich (Transportation Research Board, 1989; FHWA, 2012) analyzed 170 live-bed scour measurements in laboratory flumes by regression analysis to obtain the following equation: 𝑦𝑦𝑠𝑠 Where: 𝑦𝑦𝑎𝑎 𝐿𝐿′ 0.43 = 2.27 𝐾𝐾𝑎𝑎1 𝐾𝐾𝑎𝑎2 �𝑦𝑦 � ys = ya = Ae = La = Ka1 = Ka2 = L’ = Fr1 = Ve = Qe = g = 𝑎𝑎 𝐹𝐹𝐹𝐹1 0.61 + (15.20) Scour depth (ft) Average depth of flow on the flood plain (Ae/L) (ft) Flow area of the approach cross section obstructed by the embankment (ft2) Length of embankment projected normal to the flow (ft) Coefficient for abutment shape (table A–5 and figure A–7) Coefficient for angle of embankment to flow, θ Length of active flow obstructed by the embankment (ft) Length of blockage of ineffective flow is subtracted from total length of embankment If the flow in a significant portion of the cross section has low velocity and/or is shallow, then the length of embankment blocking this flow should not be used One-dimensional flow models including SRH-1D (Huang, J., and B Greimann, 2013) and HEC-RAS (USACE, 2010b) can easily compute conveyance versus distance across a cross section See HEC-18 (FHWA, 2012) for additional guidance on estimating L′ Froude number of approach flow upstream of the abutment = Ve/(gya)1/2 Qe/Ae (ft/s) Flow obstructed by the abutment and approach embankment (ft3/s) Acceleration of gravity (32.2 ft/s2) Table A–5 Abutment Correction Factor K1 for Shape of Opening Description Vertical wall abutment Vertical wall with wing walls Spill-through abutment 312 Ka1 1.00 0.82 0.55 APPENDIX A – Scour Computation Methods Figure A–7 Categories of abutment shape (FHWA, 2012) Figure A–8 Orientation of abutment embankment angle to the flow (FHWA, 2012) 𝜃𝜃 0.13 Where: Ka2 = �90� θ θ (15.21) < 90 degrees if the embankment points downstream (figure A-8), > 90 degrees if the embankment points upstream It should be noted that equation 15.20 is not consistent with the fact that as L′ tends to 0, ys also tends to The was added to the equation to envelope 98 percent of the data 313 Bank Stabilization Design Guidelines HIRE Abutment Scour Equation As presented in FHWA (2012) and FHWA (2001) the HIRE equation for abutment scour is: 𝑦𝑦𝑠𝑠 Where: 𝑦𝑦1 ys y1 = 𝐹𝐹𝐹𝐹1 0.33 = = Ka1 = Ka2 = Fr1 = V1 = g = 𝐾𝐾𝑎𝑎1 𝐾𝐾𝑎𝑎2 55 (15.22) Scour depth (ft) Flow depth directly upstream of the abutment (approach flow depth) on the overbank or in the main channel (ft) Correction factor for abutment shape from table A–5 Correction factor for skew angle of abutment to flow calculated as shown in figure A-7 Froude number directly upstream of the abutment = V1/(gy)1/2 Mean velocity of flow directly upstream of the abutment (ft/s) Acceleration of gravity (32.2 ft2/s) Based on USACE field data from spurs in the Mississippi River, this equation is applicable when: Where: L/y1 > 25 La = y1 = (15.23) Abutment length (ft) Flow depth upstream of the abutment (ft) NCHRP 24-20 Abutment Scour Approach The NCHRP 24-20 equation (FHWA, 2012; and NCHRP, 2010b) is based on a contraction scour estimate Contraction scour is multiplied by a factor to account for large-scale turbulence adjacent to the abutment Flow is more concentrated in the vicinity of the abutment, and the contraction scour component is larger than average conditions in the constricted opening (FHWA, 2012) The three scour conditions (figure A-9) are: Scour occurring when the abutment is in (or close to) the main channel Scour occurring when the abutment is set back from the main channel Scour occurring when the embankment breaches and the abutment foundation acts as a pier 314 APPENDIX A – Scour Computation Methods Figure A–9 Abutment scour conditions (NCHRP 2010) 315 Bank Stabilization Design Guidelines The NCHRP 20-24 approach assumes that there is a limiting depth of abutment scour when the geotechnical stability of the embankment or channel bank is reached The equation gives a total scour depth that includes contraction scour effects Contraction scour should not be added separately when using this equation Three advantages to using this equation are noted in HEC-18: Effective embankment length, L’, which can be difficult to determine, is not used in these computations Equations are more physically representative of the abutment scour process Contraction scour is included and does not need to be computed separately Scour equations for conditions a and b are: or ymax= αa yc (15.24) y max= 𝛼𝛼𝑏𝑏 𝑦𝑦𝑐𝑐 ys = ymax - yo (15.25) Where: ymax = yc = αa αb ys yo = = = = Maximum flow depth resulting from abutment scour (ft) Flow depth including live-bed or clear-water contraction scour (ft) Amplification factor for live-bed conditions Amplification factor for clear water conditions Abutment scour depth (ft) Flow depth prior to scour (ft) Condition A If La ≥ 0.75B1 , then Condition A and the contraction scour calculation are performed using a live-bed scour calculation Where: La = Abutment length (ft) B1 = Width of the flood plain (ft) The contraction scour equation is: 𝑞𝑞 6⁄7 𝑦𝑦𝑐𝑐 = 𝑦𝑦1 � 𝑞𝑞2𝑐𝑐� Where: yc = Flow depth including live-bed contraction scour (ft) y1 = Upstream flow depth (ft) 316 (15.26) APPENDIX A – Scour Computation Methods q1 = Upstream unit discharge (ft2/s) q2c = Unit discharge in the constricted opening accounting for non-uniform flow distribution (ft2/s) Unit discharge can be estimated either by discharge, Q, divided by width, w, or by the product of velocity and depth, v × y Condition B If La < 0.75B1 , then Condition B and the contraction scour calculation are performed using a clear water scour calculation Two clear water contraction scour equations can be used The first equation is the standard equation based on grain size: 𝑦𝑦𝑐𝑐 = � 𝑞𝑞2𝑓𝑓 𝐾𝐾𝑢𝑢 𝐷𝐷50 1⁄3 � 6⁄7 (15.27) Where: yc = Flow depth including clear water contraction scour (ft) q2f = Unit discharge in the constricted opening accounting for non-uniform flow distribution (ft2/s) Ku = 11.17, English units Ku = 6.19, International System of Units (SI units) D50 = Particle size with 50 percent finer (ft) A lower limit of particle size of 0.2 mm is reasonable because cohesive properties limit the critical velocity and shear stress for cohesive soils If the critical shear stress is known for a flood plain soil, then an alternative clear water scour equation can be used: 𝛾𝛾 3⁄7 𝑛𝑛𝑛𝑛2𝑓𝑓 6⁄7 𝑦𝑦𝑐𝑐 = �𝜏𝜏 � Where: n = 𝜏𝜏𝑐𝑐 = 𝛾𝛾 = Ku = Ku = 𝑐𝑐 � 𝐾𝐾𝑢𝑢 � (15.28) Manning n of the flood plain material under the bridge (ft) Critical shear stress for the flood plain material (lb/ft2) Unit weight of water (lb/ft3) 1.486, English units 1.0, SI units Notes: The recommended procedure for selecting the velocity and unit discharge for abutment scour calculation is to use two-dimensional modeling If onedimensional modeling is used, velocity and unit discharge are estimated as presented in FHWA (2012) 317 Bank Stabilization Design Guidelines The value of αa is selected from figure A-10 for spill through abutments and αb figure A-11 for wing-wall abutments The solid curves should be used for design The dashed curves represent theoretical conditions that have yet to be proven experimentally Figure A–10 Scour amplification factor for spill-through abutments and live-bed conditions (NCHRP 2010) 318 APPENDIX A – Scour Computation Methods Figure A–11 Scour amplification factor for wing-wall abutments and livebed conditions (NCHRP 2010) Design curve for short-contraction, scour-amplification factor, αb, for wing-wall abutments subject to Scour Condition B (abutment set back on a wide floodplain) For scour estimates determined for either condition (a) or (b), the geotechnical stability of the channel bank or embankment should be considered If the channel bank or embankment is likely to fail, then the limiting scour depth is the geotechnically stable depth, and erosion will progress laterally This may cause the embankment to breach, and another scour estimate can be performed treating the abutment foundation as pier 319 ... new design methods and specifications into bank stabilization projects are challenging initially, but can produce benefits in the improved performance of bank stabilization projects Bank Stabilization. .. the increased challenges of modern bank stabilization design It is not an all-inclusive Guide, but should help steer the designer towards a well-designed bank stabilization project that is feasible,... successful river design and a 17 Bank Stabilization Design Guidelines sustainable project Integration of geomorphic site conditions and sediment transport conditions with design of bank stabilization