Evaluation of Bridge Scour Research: Geomorphic Processes and Predictions (2011)

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2.1 2. REVIEW OF CURRENT PRACTICE For this project, current practice is defined as the guidance that is contained in the three FHWA documents that address the topics of geomorphology, aggradation, degradation, lateral migration and channel widening. The three documents are HEC-20 "Stream Stability at Highway Structures: 3rd Edition" (Lagasse et al. 2001), HEC-18 "Evaluating Scour at Bridges: 4th Edition" (Richardson and Davis 2001), and HDS 6 "River Engineering for Highway Encroachments: Highways in the River Environment" (Richardson et al. 2001). HEC-20 is the primary FHWA reference that addresses these topics. HEC-18 includes discussion of these topics only to the extent that stream instability is a component of total scour at a bridge. HDS 6 addresses these topics at a higher level because it is a more comprehensive treatment of river engineering, especially regarding sediment transport, river morphology and river response. Table 2.1 shows the current practice as a matrix based on the type of analysis (geomorphology, aggradation/degradation, and lateral migration/channel widening) subdivided into three levels of analysis (assessment, analysis, and advanced methods). Chapter 3 of HEC-20 is an in-depth discussion of the three level approach and provides the following rationale for this approach, which states: The analysis of any complex problem should begin with an overview or general evaluation, including a qualitative assessment of the problem and its solution. This fundamental initial step should be directed towards providing insight and understanding of significant physical processes, without being too concerned with the specifics of any given component of the problem. The understanding generated from such analyses assures that subsequent detailed analyses are properly designed. The progression to more detailed analyses should begin with application of basic principles, followed as required, with more complex solution techniques. This solution approach, beginning with qualitative analyses, proceeding through basic quantitative principles and then utilizing, as required, more complex or state-of-the-art solution procedures assures that accurate and reasonable results are obtained while minimizing the expenditure of time and effort. In Table 2.1, each analysis type and level is further divided into topics and the pertinent sections of HEC-20, HEC-18, and HDS 6 are identified. There are a total of 33 topics (12 related to geomorphology, 13 related to aggradation and degradation, and 8 related to lateral migration and widening. As expected, the majority of the topics are addressed in HEC-20, several are addressed in HDS 6, and relatively few are addressed in HEC-18. The following sections provide brief discussions on the topics included in Table 2.1. 2.1 Geomorphology Level 1 Topics 2.1.1 Geomorphic Factors HEC-20, Section 2.3 describes the geomorphic factors that affect stream stability. These factors are presented in Figure 2.1 (Figure 2.6 from HEC-20), which was adapted from Brice and Blodgett (1978). Although these factors are not presented as a classification system, a wide variety of stream characteristics are categorized. HDS 6, Section 5.4.1 uses the same figure (Figure 2.1) and presents it as a simple classification system oriented primarily to lateral stability.

2.2 Table 2.1. Current Practice Based on Type and Level of Analysis. Level of Analysis Level 1 – Assessment (qualitative or conceptual) Level 2 – Analysis (quantitative) Level 3 – Advanced Methods (in-depth quantitative) FHWA Manual HEC-20 (HEC-18) HDS 6 HEC-20 (HEC-18) HDS 6 HEC-20 (HEC-18) HDS 6 Type of Analysis Geomorphology Geomorphic Factors Channel Response Stream Reconnaissance 2.3 5.4.1 4.4 5.5 4.2, App. C - Channel Type/Sediment Load Lane Relationship Complex Response 3.5.3 5.4.1 4.4.2 5.5.1 4.4.4 - Rapid Assessment Lane S-Q thresholds 4.5, App. D - 4.4.2 5.4.5 Channel Classification Channel Evolution 4.3 5.4.1 2.2 - Aerial Photo Review Aerial Photo Evaluation 6.2.2 8.1.4 6.2.2 - Aggradation and Degradation Bridge Inspection Records Rating Curve Shifts Sediment Transport Modeling 3.3.1, 6.3.1, (4.3.1, 11.2, 11.3.8) - 3.6.7, (4.3.2) - 6.3.3 4.9, 5.6.2 Field Evidence Sediment Continuity Physical Modeling 6.3.1 - 2.4.2, 6.3.3 - 3.7 5.6.1 Lane Relationship Equilibrium Slope Erodibility Testing 4.4.2 5.5.1 6.3.2 - (App. L & M) Base Level Change (+/-) Incipient Motion 6.3.2 - 3.6.5, 6.3.2 3.5 Headcuts and Nickpoints Armoring 6.3.2 5.2.4 3.6.6, 6.3.2 - Lateral Migration and Channel Widening Bridge Inspection Records Aerial Photo Evaluation Sediment Transport Modeling Including Geotechnical Stability 3.3.1 (11.2, 11.3.8) - 6.2.2 - Field Evidence Geotechnical Stability App. B - 2.3.9, 3.5.4 5.8.1 2.3.9, App. B - Aerial Photo Review Regime Equations 6.2.2 - - 5.4.6 Channel Evolution 2.2 -

2.3 Figure 2.1. Geomorphic factors that affect stream stability (adapted from Brice and Blodgett).

2.4 The characteristics include:  Stream size  Flow habit  Bed material  Valley setting  Floodplains  Natural levees  Apparent incision  Channel boundaries  Tree cover on banks  Channel sinuosity  Channel braiding  Channel anabranching, and  Variability of channel width and development of point bars Some of these factors indicate that a channel is more susceptible to stream lateral or vertical instability. For example, a channel with well-developed natural levees tends to have lower rates of lateral migration. 2.1.2 Channel Type and Sediment Load HEC-20, Section 3.5.3 and HDS 6, Section 5.4.1 present a figure from Shen et al. (1981) (Figure 2.2), that relates channel form (straight, meandering and braided) and sediment load (suspended, mixed and bed load) to relative stability (high stability to low stability). This figure illustrates how meandering alluvial channels may range from low to high stability depending on the mode of sediment transport. Figure 2.2. Channel classification and relative stability as hydraulic factors are varied (after Shen et al.).

2.5 2.1.3 Rapid Assessment HEC-20, Section 4.5 and Appendix D present an approach that incorporates several factors (channel characteristics, hydraulic and sediment transport characteristics) to quickly rate a channel from poor (highly unstable) to excellent (highly stable). The factors are:  Bank soil texture and coherence  Average bank slope angle  Vegetative bank protection  Bank cutting  Mass wasting or bank failure  Bar development  Debris jam potential  Obstructions, flow deflectors, and sediment traps  Channel bed material consolidation and armoring  Shear stress ratio  Approach angle to bridge or culvert  Bridge or culvert distance from meander impact point  Percentage of channel constriction The factors are rated from 1 (excellent) to 12 (poor) and have assigned weights. The resulting weighted sum can be related to overall stability, lateral stability, or vertical stability. The primary reference is Johnson et al. (1999) and an in-depth review of an expanded and improved version of this methodology (Johnson 2006) was performed under Task 4 of this project. 2.1.4 Channel Classification HEC-20, Section 4.3 and HDS 6, Section 5.4.1 address channel classification. The treatment in HEC-20 is more comprehensive. It includes classifications based on channel pattern by Brice (1975), mode of sediment transport and channel pattern by Schumm (1977 and 1981), mountain channel classifications by Montgomery and Buffington (1997), and the Rosgen method, which includes measures of entrenchment, width-depth ratio, sinuosity, channel slope, and bed material (Rosgen 1994 as presented by Thorne 1997). HEC-20 indicates that channel classification systems are useful communicating tools for describing and categorizing the characteristics of a stream reach. It is the channel characteristics, however, that are important in identifying processes and the associated channel responses. As a Level 1 method, channel classification is a valuable first step in evaluating channel stability and predicting channel change. 2.1.5 Aerial Photo Review Aerial photography is mentioned 30 times in HEC-20, 7 times in HEC-18, and 33 times in HDS 6. Many of the instances where aerial photography is mentioned in HEC-20 relate to geomorphic assessment, especially as it relates to lateral channel migration. HEC-20, Section 6.2.2 includes a 2-page discussion on the use of aerial photography for evaluation and prediction of lateral migration. HEC-20 and HDS 6 note that aerial photography is useful not only for recording channel location through time, but also for evaluating current and historic channel characteristics (width, radius of curvature, meander wave length, and sinuosity), vegetation, land use, thalweg variability, sand bars, river controls, geologic formations, bank protection, relic channels, channel and overbank sediment deposits. HDS 6 also notes that headcuts can be located through time based on the channel disturbance that is easily identified on aerial photography.

2.6 2.2 Geomorphology Level 2 Topics 2.2.1 Channel Response HEC-20, Section 4.4 and HDS 6, Section 5.5 provide a general discussion of channel response to change. Each document indicates that there are a large number of variables that are involved in channel response and that predicting river response is a complex task. Channels can respond to changes in flow, sediment supply (quantity and size), and vegetation or can respond to channel straightening, cut-offs, or other modifications. The following sections describe Level 2 topics that address predicting the type of channel response that can be expected based on certain types of change. 2.2.2 Lane Relationship HEC-20, Section 4.4.2 and HDS 6, Section 5.5.1 discuss the Lane relationship (Lane 1955). The Lane relationship is a useful conceptual tool for relating channel response to a change in the system. The relationship is a proportionality between discharge (Q), median sediment size (D50), sediment discharge (Qs), and channel slope (S). The relationship is: QS ~ QsD50. If the relationship represents a system in equilibrium, then a change in one or more variables will initially put the system out of equilibrium. Another variable would have to respond to bring the system back into equilibrium. For example, an increase in sediment supply would require an increase in slope to bring the system back into equilibrium if discharge and sediment size are assumed to remain constant. An increase in slope would typically infer aggradation, but could also result from straightening of a meandering channel. This relationship is a useful tool for thinking about channel response but does not make any quantitative predictions. It also does not identify other possible channel responses, such as widening. 2.2.3 Lane Slope-Discharge Thresholds HEC-20, Section 4.4.2 and HDS 6, Section 5.4.5 provide a discussion of slope-discharge thresholds developed by Lane (1957) and Leopold and Wolman (1960). The thresholds discriminate channels that tend to be braided, transitional between braided and meandering, and meandering. These relations illustrate that a channel can respond to a change in discharge and/or slope by changing its form. If the channel slope increases because of an increase in sediment supply, a change from meandering to transitional or from transitional to braided may also occur. Like the Lane relationship, this is useful conceptual tool for channel response. 2.2.4 Channel Evolution HEC-20, Section 2.2 provides a brief discussion of channel evolution in the context of channel incision leading to widening, then aggradation and eventually relative stability (from Schumm et al. 1984). The section also references the topic of complex channel response that is addressed in a later section. 2.2.5 Aerial Photo Evaluation For the purpose of this discussion of current practice, aerial photo evaluation is distinguished from aerial photo review (Section 2.1.5) based on the level of effort and purpose of quantifying a change in channel morphology. HEC-20, Section 6.2.2 discusses the need to rectify aerial photography to reduce distortion, especially when using older photography or photography

2.7 that was not intended for detailed use. Once distortion has been removed from the photography, detailed measurements of bank location can be made and rates of channel migration can be determined. 2.3 Geomorphology Level 3 Topics 2.3.1 Stream Reconnaissance Conducting a site visit, reviewing aerial photos and maps, and reviewing the geomorphic factors sheet (Figure 2.1) would constitute a Level 1 stream reconnaissance. The rapid assessment would require additional information, including bank angle and other observations. In HEC-20, Section 4.2 and Appendix C, include a detailed level of reconnaissance developed by Thorne (1998). The reconnaissance includes local, reach scale and watershed scale observations. It is intended to be a comprehensive approach to documenting stream channel and watershed conditions. Therefore, it can also be simplified and tailored for use in individual regions. The sections and major parts of the reconnaissance forms are: Section 1. Scope and Purpose Section 2. Region and Valley Description Part 1. Area Around River Valley Part 2. River Valley and Valley Sides Part 3. Floodplain (Valley Floor) Part 4. Vertical Relation to Channel to Valley Part 5. Lateral Relation of Channel to Valley Section 3. Channel Description Part 6. Channel Description Part 7. Bed Sediment Description Section 4. Bank Survey Part 8. Bank Characteristics Part 9. Bank-Face Vegetation Part 10. Bank Erosion Part 11. Bank Geotechnical Failures Part 12. Bank Toe Sediment Accumulation 2.3.2 Complex Response HEC-20, Section 4.4.4 provides a brief discussion on the topic of complex response. This discussion is provided in the context of progressive versus episodic change, such as gradual meander bend growth may result in a natural cut-off that produces a period of rapid channel change. Another example of complex response that is discussed is base level lowering causing degradation to progress up through the channel and tributaries. As the tributaries produce more sediment the main channel may then respond with a period of aggradation.

2.8 2.4 Aggradation and Degradation Level 1 Topics 2.4.1 Bridge Inspection Records There are two instances where bridge inspection is mentioned in HEC-20 related to the topic of stream instability. The first reference is in Section 3.3.1 (Data Needs for Level 1 Analyses), where it states "Typically, a cross section of the bridge waterway at the time of each inspection will provide a chronological picture of the bridge waterway." The second reference is in Section 6.3.1 (Overview of Vertical Channel Stability) where it states ("Bridge inspection reports, which should include soundings at each bent, are a valuable tool for assessing historic channel vertical stability and can be used to predict future trends." HEC-18 provides more in-depth discussion of the use of bridge inspection records to identify lateral and vertical stream instability problems, primarily in Sections 4.3.1, 11.2, and 11.3.8. Section 4.3.1 discusses the use of bridge inspection cross sections to identify aggradation and degradation trends. Section 11.2 indicates that the office review conducted prior to a bridge inspection should include review of prior bridge inspections cross sections to identify aggradation, degradation and lateral movement. Section 11.3.8 states that a bridge inspector should compare the current cross section to previous cross sections, and may want to do so while still on site. 2.4.2 Field Evidence Only one reference to field evidence of degradation is included in the overview of vertical channel stability (Section 6.3.1 of HEC-20). The statement is "Direct evidence of channel degradation includes (1) exposed utility crossings, (2) exposed bridge foundations, (3) channel banks failing due to excessive height and (4) comparison of channel profiles and cross sections. 2.4.3 Lane Relationship The Lane Relationship, as discussed in the geomorphology section, is a Level 1 approach for qualitatively assessing the potential for aggradation and degradation. 2.4.4 Base Level Change Base level control is mentioned once in HEC-18 and more than 20 times in both HEC-20 and HDS 6, although only HEC-20 includes a specific, though brief, discussion on base level change as it relates to degradation. The discussion occurs in the context of headcuts and nickpoints in Section 6.3.2. 2.4.5 Headcuts and Nickpoints Headcuts and nickpoints are mentioned frequently in HEC-20 and in HDS 6. Each document includes specific discussion related to these topics including Section 6.3.2 (Degradation Analysis) in HEC-20 and Section 5.2.4 (Nickpoint Migration and Headcutting) in HDS 6. HDS 6 suggests that these features can be identified and tracked in aerial photography, especially in more arid climates, due to easily identified channel disturbance immediately downstream of these features. It is also suggested that the drop height can be used as an estimate of future degradation upstream of the headcut or nickpoint.

2.9 2.5 Aggradation and Degradation Level 2 Topics 2.5.1 Rating Curve Shifts HEC-20, Section 3.6.7 provides a detailed explanation of using rating curve shifts (specific gage analysis) as Step 7 of a Level 2 analysis for evaluating channel degradation. A specific gage analysis tracks the stage for a specific discharge through time as the rating curve at a stream gage is updated. A low discharge is typically selected because the difference in water surface and bed elevation can be assumed to be quite consistent for low flow rates. HEC-18, Section 4.3.2 also discusses specific gage analysis. This is a Level 2 analysis because it involves data evaluation, but it does not provide any information related to the cause of degradation (or aggradation). A prediction of future change can be in the form of a linear extrapolation or it can asymptotically approach a constant elevation if the data indicate a trend toward future stability. 2.5.2 Sediment Continuity The sediment continuity concept states that, in an alluvial channel, if the sediment supply is not in balance with the sediment transport capacity, then either aggradation or degradation will occur. If sediment supply exceeds transport capacity, aggradation is expected, and if sediment supply is less than transport capacity, degradation is expected. This concept is discussed in HEC-20, Section 2.4.1. HEC-20, Section 6.3.3 provides guidance on performing a sediment continuity analysis. A sediment continuity analysis compares sediment supply to sediment transport capacity for a period of time and uses the volumetric difference to estimate an amount of aggradation or degradation. The transport capacity is computed using applicable sediment transport formulas. Sediment supply can be estimated in several ways, one of which is to apply the sediment transport formula to a supply reach that has been identified as being stable because it is in equilibrium with its sediment supply. Several reaches can be evaluated by using the sediment transport capacity of each reach as the supply for the next downstream reach. A sediment continuity analysis is not considered sediment transport modeling because the channel geometry and hydraulic variables are not modified as a result of the computed aggradation or degradation. The time period for this analysis can be a single event hydrograph or a long-term analysis based on flow-duration curves. The live-bed and clear-water contraction scour equations in HEC-18 are based on sediment continuity analysis. Each equation determines the amount of contraction scour to match the sediment transport capacity of the constricted section to the sediment supply from the upstream, unconstructed "approach" section. For live-bed, the sediment supply is determined as the transport capacity of the approach section. For clear-water, the sediment supply is zero, so the sediment continuity analysis is reduced to an incipient motion or zero-transport condition. The contraction scour is computed for ultimate conditions and the amount of time is not determined, although there are cases when the amount of contraction scour can be limited by the duration of intense flow.

2.10 2.5.3 Equilibrium Slope A channel that has a sediment transport capacity equal to the sediment supply is in a state of equilibrium. If the sediment supply, flow rate or some other variable is altered, then the channel will respond. If an adjustment in slope is assumed, then an equilibrium slope analysis can be used to estimate the new channel equilibrium slope for the altered condition. HEC-20, Section 6.3.2 includes a subsection on equilibrium slope analysis. Two methods are provided to determine the equilibrium slope for zero sediment supply. One uses Shields incipient motion concept and the other uses the Meyer-Peter Muller equation for the beginning of sediment transport. The equilibrium slope section also provides two relationships for an altered sediment supply. One is derived for a known sediment supply and the other is derived for a ratio of existing to future sediment supply. Once the future equilibrium slope is determined, then the amount of degradation can be estimated by projecting this slope from a downstream base-level control up to the location of interest. This approach does not predict the amount of time it will take for the channel to reach the new condition, nor does it consider other types of channel response, such as channel widening. 2.5.4 Incipient Motion The concept of incipient motion deals with the hydraulic condition when a sediment particle begins to move. HDS 6, Section 3.5 includes considerable discussion and background on this topic. HEC-20, Section 3.6.5 provides a discussion on incipient motion as part of a Level 2 analysis and Section 6.3.2 provides two (of many possible) methods for calculating the shear stress and the associated particle size that would at the beginning-of-motion condition. An incipient motion analysis does not, by itself, predict degradation. It is, however, an important part of many other types of degradation analyses, including sediment transport, equilibrium slope and armoring. 2.5.5 Armoring An armor layer forms on a channel bed when the hydraulic forces are sufficient to transport the smaller fraction of the bed material, leaving the coarser fraction as a layer at the bed surface. HEC-20, Section 3.6.6 provides a discussion of armoring and Section 6.3.2 provides a method for estimating the amount of degradation that would be required to produce an armor layer. This amount of degradation may be used to limit to the amount of degradation that is computed by other approaches. Some sediment transport models include armoring in their formulation. The results of an armoring calculation must be evaluated for the potential of a greater flow disturbing the armor layer and restarting the degradation process. 2.6 Aggradation and Degradation Level 3 Topics 2.6.1 Sediment Transport Modeling Sediment transport modeling involves hydraulic modeling coupled with sediment transport calculations. The cross section geometry is updated as the channel bed aggrades or degrades. In some models there is also the ability to include channel widening. Sediment transport models incorporate the concepts of sediment supply, sediment transport capacity, sediment continuity, incipient motion, and armoring to determine channel response over individual hydrographs or many years and decades. The strength of sediment transport models is the fact they are not as limited by the assumptions built into simpler methods. They are limited by the complexity and effort required for their application, making them a clear

2.11 Level 3 procedure. As with simpler methods, sediment transport modeling is also limited by the uncertainties inherent in any sediment analysis. HEC-20, Section 6.3.3 discusses the data needs for sediment transport modeling and mentions two sediment transport models. Most sediment transport models include many transport equations as options. HDS 6, Section 4.9 discusses the applicability of 20 different sediment transport equations for various sediment size classes and lists the sediment transport formulas that are included in a number of models. HDS 6, Section 5.6.2 discusses sediment transport models in general and provides specific discussions on a number of individual models. Neither HEC-20 nor HDS 6 provides any guidance on the application of any sediment transport model nor do these documents recommend the use of one model over another. Models are updated frequently and new models are always being developed. Therefore, any specific information in HEC-20 or HDS 6 is likely to be outdated shortly after publication. Because all sediment transport formulas and sediment transport models have strengths, weaknesses and limitations on their applicability, the guidance documents must be general and leave it to the users to select appropriate technology for any application. 2.6.2 Physical Modeling Physical modeling of hydraulic and sediment transport is discussed briefly in HEC-20, Section 3.7 and HDS 6, Section 5.6.1, although the discussion relates primarily to localized issues rather than the large-scale issues surrounding aggradation and degradation. HDS 6 also includes the topic of similitude, distortion and limitations that result from scaling sediment. As a method for estimating aggradation or degradation, physical modeling appears to have extremely limited applicability. 2.6.3 Erodibility Testing Except for the process of headcutting, aggradation and degradation are generally analyzed in the context of alluvial channels. If a bedrock layer is present, it is usually treated as a limit to degradation. One may still need to consider the potential for degradation in erosion resistant, but erodible, clay, and rock materials. These topics are not discussed in HEC-20 and only briefly mentioned in HDS 6. The most detailed discussion of erosion of clay and rock are presented in HEC-18, Appendices L and M. Appendix L discusses the erosion of clay based on laboratory testing to determine critical shear stress and the erosion rate as a function of excess shear stress. The discussion is in the context of pier scour, but could be applicable to other types of scour and degradation. Appendix M indicates that similar approaches can be used for erodible rock material and provides a description of the Erodibility Index Method (Annandale 1995, 1999), where the erodibility index is related to available stream power. Some sediment transport models incorporate erosion of resistant layers based on excess stream power, so these types of formulations can be incorporated into degradation analyses. 2.7 Lateral Migration and Channel Widening Level 1 Topics 2.7.1 Bridge Inspection Records The discussion of bridge inspection records under the topic of aggradation and degradation is also applicable to the topic of lateral migration and channel widening.

2.12 2.7.2 Field Evidence Field evidence of channel migration and channel widening are discussed in HEC-20, Sections 2.3.9 and 3.5.4, and in HDS 6, Section 5.8.1. Field evidence includes bank condition, bank erosion and mass wasting, and also the presence of large point bars that often indicate channel migration. 2.7.3 Aerial Photo Review The discussion of aerial photo review under the geomorphology topic is also applicable to the topic of lateral migration and channel widening. 2.8 Lateral Migration and Channel Widening Level 2 Topics 2.8.1 Aerial Photo Evaluation The discussion of aerial photo evaluation under the geomorphology topic is also applicable to the topic of lateral migration and channel widening. Careful examination of aerial photos provides information that can be used to predict future channel conditions. 2.8.2 Geotechnical Stability Channel bank retreat may be the direct result of erosion but is often the result of geotechnical mass-failure of the bank. Erosion of the bank toe or channel deepening may cause an unstable bank condition, especially if the bank is weakened due to saturation. HEC-20, Section 2.3.9 and Appendix B include significant discussion on this topic, including the type of bank failures and the processes controlling these failures. General guidance is provided on the analysis of bank failure and retreat, but no specific equations or relationships are presented. 2.8.3 Regime Equations A regime channel is an alluvial channel that has attained, more or less, a state of equilibrium with respect to erosion and deposition. Regime equations relate stable alluvial channel dimensions or slope to discharge and sediment characteristics. HDS 6, Section 5.4.6 includes discussion of regime equations on the topic of hydraulic geometry of alluvial channels. Channel widening can be estimated as a function of the change in channel forming discharge using the "downstream" regime relations. In the context of changing the channel forming discharge, regime equations bear a conceptual similarity to the Lane qualitative relationship for channel response. 2.8.4 Channel Evolution The discussion of channel evolution under the geomorphology topic is also applicable to the topic of lateral migration and channel widening.

2.13 2.9 Lateral Migration and Channel Widening Level 3 Topics 2.9.1 Sediment Transport Modeling Including Geotechnical Stability Sediment transport models have primarily focused on aggradation and degradation of the channel because this is the part of the channel that is directly involved in the transport of sediment. Channel banks are sources of sediment and bank stability is impacted by channel degradation and through the fluvial entrainment of bank toe materials. Therefore, channel bank geotechnical stability has been included in sediment transport models. This can be done by assigning a critical bank height as an input to the model or by performing geotechnical stability calculations within the sediment transport model. The volume of bank material from a mass failure then becomes a sediment source. HEC-20, Appendix B includes discussion of a sediment transport model that includes geotechnical stability analysis of bank retreat (Osman and Thorne 1988, Thorne and Osman 1988).