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

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3.1 3. RESEARCH PRIORITIZATION AND EVALUATION 3.1 Research Prioritization The purpose of Task 3 was to critically review the research literature compiled in Task 2 (the annotated bibliography in Appendix A) and to prepare a prioritized list based on a qualitative assessment of the research. After completion of the critical review, the research team submitted a prioritized list of documents to the panel. Later, the prioritized list was updated according to the comments from the panel members. The annotated bibliography included in Appendix A includes only the publications that were included in the prioritized list. Each document in the annotated bibliography was categorized into, based on its primary topic, (1) Geomorphology, (2) Reconnaissance, (3) Aggradation/Degradation, (4) Channel Migration, (5) Channel Widening, (6) Sediment Dynamics, and (7) Numerical Modeling. Among the 186 publications reviewed, 52 were in the topic of Geomorphology, 27 in Reconnaissance, 21 in Aggradation/Degradation, 33 in Channel Migration, 14 in Channel Widening, 11 in Sediment Dynamics, and 28 in Numerical Modeling. Table 3.1 shows the total number of publications in each topic. For each publication, the research results were classified based on the suitability in providing useful information in its corresponding topic. Specifically, the publications were assigned an ID depending on whether it Definitely, May, or definitely Not addressed the topic applicable to the project. Next, even if a paper definitely addressed the topic, it may not be suitable for AASHTO adoption and inclusion in FHWA documents (e.g. HEC-20). Therefore, each document was assigned a rank, based on a qualitative assessment of what is already included in HEC-20 and what studies could be used to enhance HEC-20. The ranks are:  1 = Research or method could be incorporated into FHWA documents (or already is in FHWA documents but could be updated based on more recent work)  2 = Research or method is promising, but probably not ready to be included into FHWA documents  3 = Research or method is not suited for inclusion into FHWA documents The research team then considered only publications with a D-1 for in-depth review in Task 4. Table 3.1 shows the number of D-1 publications per topic. As provided in Appendix B, each document is identified by number in the annotated bibliography, a brief description, and the reason(s) for the D-1 rank. Although all 186 publications in the annotated bibliography were ranked, Appendix B includes only the 60 documents ranked D-1. Table 3.1. Summary of Prioritization. Topic Total Number of Publications Number of D-1 Publications Geomorphology 52 17 Reconnaissance 27 8 Aggradation/Degradation 21 6 Channel Migration 33 9 Channel Widening 14 10 Sediment Dynamics 11 4 Numerical Modeling 28 6 Total 186 60

3.2 Finally, the 60 publications from all 7 topics were prioritized for the in-depth review under Task 4. Many of the documents that have been removed were superseded by later papers by the same author(s). For example, References 3.23 and 3.24 were removed from the in-depth review because they were contained within Reference 10.1 (which also includes some newer material), References 3.5 and 3.6 were dropped from the in-depth review as these were superseded by 3.2, and References 5.37 and 5.40 were not included in the prioritized list because these were incorporated into References 5.46 and 5.48. In addition, several books made the in-depth review list more than once as they cover at least two topics. Seven chapters from ASCE’s "Sedimentation Engineering" manual, Reference 10.0, contain information on Geomorphology, Aggradation/Degradation, Channel Migration, and Channel Widening. Similarly, Melville and Coleman’s "Bridge Scour," Reference 3.18, covers Aggradation/Degradation, Channel Migration, and Channel Widening, and Maryland Office of Bridge Development’s "Chapter 14: Stream Morphology and Channel Crossings," Reference 1.2, contains information on Aggradation/Degradation and Channel Migration. A summary of the number of recommended publications for each topic is shown in Table 3.2. Table 3.2. Summary of Recommendations. Topic Number of D-1 Publications Number of Recommended Publications Geomorphology 17 8 Reconnaissance 8 4 Aggradation/Degradation 6 3 Channel Migration 9 8 Channel Widening 10 6 Sediment Dynamics 4 2 Numerical Modeling 6 5 Total 60 36 3.2 In-Depth Evaluation of Priority Research Under Task 4, the research team conducted an in-depth evaluation of the technical adequacy and limitations of the research results approved by NCHRP in Task 3 and documented strengths and limitations of the research results. The original criteria included: 1. How does the research relate to the current state of practice? 2. Is the research founded in sound scientific theory? 3. Does the research adequately describe the physical process? 4. Has the research been tested by the original researcher? 5. Has the research been validated in practice by other researchers or practitioners? 6. What conditions has the research been applied? 7. Is the research of practical use or could it be made practical? 8. What are the strengths of the research? 9. What are the limitations of the research? 10. Does the research of pertain to a single physiographic region or is it broadly applicable? 11. Does the research pertain to a limited set of channel conditions or is it broadly applicable? Upon further examination, the original criteria were modified slightly to eliminate redundancies. The modified review criteria included the following changes. Question 1 was modified by removing the word "How" to make it a yes, or no answer. We included a discussion of how the research relates to the topic and whether it is an extension or advancement of current practice. Question 7 deals with the practicality of the research. It was renumbered to be question 2 and

3.3 the answer to this question includes a discussion of whether the research is a Level 1, 2, or 3 approach. Questions 2 and 3 were combined because adequately describing the physical process is a requirement for being founded in sound scientific theory. The modified question (3) also includes empirical evidence as part of this review criterion. Question 4 is unchanged. We found that Question 5 was difficult to evaluate systematically. Rather than eliminate the question we checked the number of times the reference is cited by other papers in the technical literature. This was done by tracking the paper in Google Scholar (http://scholar.google.com/) and noting the number of citations. A large number of citations usually indicates higher quality research. A note is included when a paper is recent (2009) and few citations are expected. Questions 6 and 10 are considered to be qualifiers that impact the strengths and weaknesses of the research (Questions 8 and 9) or would be covered within the context of the other remaining questions. Questions 8 and 9 were kept unchanged. One final item was added to the list, which is our recommendation (Task 5) on whether the research should be adopted by AASHTO and included in the next revision of HEC-20. The modified critical review questions are: 1. Does the research relate to the current state of practice? 2. Is the research practical or could it be made practical? 3. Is the research founded in sound scientific theory or substantial empirical evidence? 4. Has the research been tested by the original researcher? 5. Has the research been cited by others? 6. What are the strengths of the research? 7. What are the limitations of the research? 8. Recommendation. The following provides a brief description of the findings for each of the 7 research topics. The complete evaluation of each research item, including whether the item should be adopted by AASHTO or included in HEC-20, is in Appendix C. Tables 3.3 through 3.9 provide a summary of the strengths, weaknesses, and recommended analysis level (1, 2, or 3) of the recommended papers for each research topic. 3.2.1 Geomorphology Geomorphology represents one of the three types of analysis that are addressed under HEC- 18, HEC-20, and HDS 6. All three types of analysis are further subdivided into three levels of analysis (assessment, analysis, and advanced methods) as shown in Table 2.1. As shown in the table, there are 12 topics related to Geomorphology, 5 of which are under the Level 1 analysis, 5 under the Level 2 analysis, and 2 under the Level 3 analysis. Given the importance of geomorphic factors in evaluating stream stability at bridges, it was determined that Geomorphology should be one of the seven primary research topics to be evaluated. Of the 17 Geomorphology research references submitted, the 8 that are recommended are:  Downstream hydraulic geometry of alluvial channels  Channel avulsions on alluvial fans  Rosgen classification system and "Natural Channel Design" (see note at the end of this section)  Toolkit for fluvial system analysis  Regional risk analysis of channel stability  Fundamental concepts of fluvial geomorphology and river mechanics  Current state of practice for applying geomorphology to river engineering  Environmental performance standards for bridges

3.4 Table 3.3. Strengths and Weaknesses of Recommended Methods in Geomorphology Topic. Paper Number Authors and Title Level Strengths Weaknesses Geomorphology 3.11. Lee and Julien 2006. Downstream Hydraulic Geometry of Alluvial Channels 2 1. Presents massive database (1,485 sites). 2. Covers wide range of flow conditions for sand/gravel/cobble streams with meandering to braided planforms. 3. Database used to calibrate and validate new and improved hydraulic geometry equations. 4. 95% of the calculated hydraulic geometry parameters between 50% and 200% of field measurements. 5. Equations can be used as a template to indicate whether or not a channel is in regime. 1. Equations only apply to stable alluvial channels. 2. Range on calculated hydraulic geometry parameters (50% to 200% of observations) is large. 3. Wohl (2004) indicates regime equations have mixed results for mountain streams. 4. Relationships can be expected to be poorly suited to describe resistance to flow. 5. Relationships are strictly only applicable to the data sets from which they were derived. 5.47. Field 2001. Channel Avulsion on Alluvial Fans in Southern Arizona. 2 1. Provides guidance on identifying and predicting sites of potential avulsion on active alluvial fans upstream of a highway crossing. 2. Could be used to identify and implement countermeasures to prevent potential avulsions. 1. It is not known if methodology has been implemented specifically for the protection of transport infrastructure. 2. Methodology predicts the location of a potential avulsion, but the predicting the timing may be more subjective because it depends on occurrence of flow events. 7.1. Simon et al. 2007. Critical Evaluation of the Rosgen classi- fication and associated "Natural Channel Design" Methods (see note at the end of this section). 2 1. Explains why stream channel design and restoration should be based on physically- based analyses and process-based approaches that are currently available and which are founded on well-established scientific and engineering literature. 1. Authors have been known to be very critical of Rosgen. Consequently, those in the restoration business are likely to turn a deaf ear to these criticisms. 2. Only further research and documenting of the success or failure of Rosgen’s approach will determine whether or not it will stand the test of time. 7.2 Bledsoe et al. 2007. GeoTools: A Toolkit for Fluvial System Analysis, 2 1. GeoTools has been designed to provide a wide range of useful information from a parsimonious set of inputs and to bypass the need for individual investigators to produce custom, ''homegrown'' data analysis tools. 1. Risk-based models based on metrics from Geo-Tools require regional calibration. 2. Even though GeoTools has undergone beta testing on a range of different computer types and configurations, compatibility problems may still exist. 9.13. Bledsoe 2000. Regional Risk Analysis of Channel Stability. 2 1. The mobility index has explanatory power practically equaling that of models containing slope, discharge, and D50 as separate independent variables, especially for sand bed channels. 2. The approach can be for predicting channel instability and scaling channel processes across diverse geological and climatic regions. 3. Logistic regression models that use mobility index can predict unstable channel forms. 4. Logistic models also provide a means of gauging channel sensitivity to modest changes in the controlling variables. 1. Predictions of widening in gravel bed channels are less reliable due to uncertainties associated with defining the bank characteristics. 10.6. Schumm and Harvey 2008 Engineering Geomorphology. Chapter 18, ASCE Sediment Engineering Manual 1/2 1. Chapter provides a concise review of the current state of practice. 2. Covers a number of concepts that are not included in HDS 6 and HEC-20 but which should be added. 3. Covers systems approach to evaluating channel stability at a site; consideration of geomorphologic factors that influence landforms (engineering sites) and hazards associated with them, and, development of dimensionless stability numbers for evaluating incised channel evolution. 1. The chapter does not provide any original research 2. The concepts and approaches identified by the authors only provide general guidance on how one can identify existing hazards or problems and potentially identify future hazards or problems as they relate to a particular site. 10.7. Biedenharn et 1/2 1. Chapter provides a good overview of fluvial 1. Chapter does not provide any original

3.5 Table 3.3. Strengths and Weaknesses of Recommended Methods in Geomorphology Topic. Paper Number Authors and Title Level Strengths Weaknesses Geomorphology al. 2008. Funda- mentals of Fluvial Geomorphology. Chapter 6, ASCE Sediment Engineering Manual geomorphology and river mechanics concepts that will be of use to engineers. 2. Many of these concepts are covered only generally in HEC-20 and HDS 6. research and is primarily a reference tool. 11.1. Oregon Dept. of Transportation 2005. OTIA III State Bridge Delivery Program Environ- mental Performance Standards 1/2 1. Research reported addresses an issue that is very important to DOTs, which is avoiding difficulties in permitting. 2. Research also addresses the philosophy of sound bridge design, which includes avoiding stream stability issues over the life of the bridge. 3. Each of these are goals is addressed by considering the function, continuity and connectivity of the stream and floodplain. 1. Fluvial design standard was targeted at conditions in Oregon so other standards would need to be developed for other regions. 2. Another limitation, even potentially for Oregon, pertains to definition of the ‘functional floodplain’. 3. There are no theoretical explanations given for defining the functional floodplain as 2.2 times the bankfull width. 4. No justification is given for using the 10-year recurrence interval flood as the reference discharge for zero contraction scour. Note on Reference 7.1. Reference 7.1 became part of the literature database based on the search criteria and specific journals that were included. It addresses limitations of Rosgen’s methodologies as perceived by the authors of Reference 7.1. In order to not provide an exclusively one-sided discussion of Rosgen’s approaches by this project, Reference 7.1 should be considered as a starting point for considering both limitations and benefits these approaches. Lave (2009) provides a discussion which could serve as a source for discussing both sides of this issue. From the standpoint of this project, it is important to frame the discussion in the context of HEC-20, which is not a restoration manual. Only six pages of HEC-20 are devoted to channel restoration concepts. Rosgen’s work related to Natural Channel Design (NCD) as a restoration method should be discussed in the HEC-20 restoration concepts section. NCD is not pertinent to predicting types and rates of channel instability because that is not the intent of NCD. As indicated by Lave (2009), the Rosgen NCD approach has as its stated goal the design of stable channels that do not adjust in dimension, horizontally or vertically. Although this goal may be shared by bridge engineers, in most cases it is better to recognize the potential for channel instability and allow for future channel adjustments in the design process. 3.2.2 Reconnaissance Reconnaissance of a bridge site is an important tool for collecting appropriate data and information for use in the assessing stream stability at the site and, therefore, is included as one of the seven primary research topics to be evaluated. Although not a specific type of analysis, bridge site reconnaissance provides data and information relative to the various factors identified for each of the three types of analysis (Table 2.1) as follows: Level 1 – geomorphic factors, channel type, rapid assessment, field evidence, headcuts and nickpoints Level 2 – channel evolution, armoring, geotechnical stability Level 3 – stream reconnaissance, erodibility testing

3.6 Therefore, per Task 2 of this project, a bibliography was completed that included research literature conducted since 1990 covering Reconnaissance and related topics. Of the 8 Reconnaissance research references submitted, the 4 that are recommended for inclusion are:  Assessment of channel stability at bridges in physiographic regions  Geomorphic analysis of large alluvial rivers based on widely accepted classification and analysis techniques  Digital mapping at bridge sites for detailed, advanced reconnaissance and monitoring  Diagnostic approach to assessing and monitoring stream channels Table 3.4. Strengths and Weaknesses of Recommended Methods in Reconnaissance Topic. Paper Number Authors and Title Level Strengths Weaknesses Reconnaissance  1.3. Johnson 2006, Assessing Stream Channel Stability at Bridges in Physio- graphic Regions 2 1. This method avoids averaging out problematic conditions by rating vertical and lateral stability separately from overall stability. 2. Several rating factors in the prior "Rapid Assess- ment" technique have been modified or replaced. 3. Data sheets have been revised to make the method more systematic. 4. Physiographic regions have influenced selection of stability factors to make the method broadly applicable. 5. The method is targeted at identifying problems that could be of concern in a relatively short period of time (2-year inspection interval). 1. Because the method is simplified, there is risk of incorrect characterization. However, this limitation is countered by the recommendation that an indication of instability should lead to additional site investigation. 5.44. Thorne 2002 Geomorphic Analysis of Large Alluvial Rivers, 3 1. Research includes a systematic and flexible approach to dealing with catchment, reach and project scales. 2. For each scale, a specific item or deliverable is identified, data requirements are identified, and a relative level of effort is identified. 1. Its use may be limited as it is a Level 3 analysis of geomorphological assessment, though for complex problems or large river crossings this would be a valuable resource. 6.1. Hauet et al. 2009. Digital Map- ping of Riverine Waterway Hydro- dynamic and Geo- morphic Features 3 1. Research provides an approach for detailed monitoring how river features near a bridge change through time using oblique (distorted) digital photography. 2. Method may also be used to measure map flow velocities and pattern of water currents. 1. Specialized equipment, software, and training are required. 2. Method proposed would only be applicable to limited conditions. 7.10. Montgomery and MacDonald 2002. Diagnostic Approach to Stream Channel Assessment and Monitoring, 2/3 1. Recognizes complexities of fluvial systems and range of responses that can occur. 2. Identifies processes rather than forms. 3. Requires investigation of the stream channel within the context of the watershed and geomorphic system. 4. Does not try to oversimplify, but ties channel assessment with potential responses. 5. Indicates which channel types are more susceptible to instability from specific changes in sediment and discharge. 6. Method is flexible and adaptable. 1. Any diagnosis system is susceptible to bias or misinterpretation. 2. System requires more comprehensive information than is typically collected or available. 3. System requires experienced field staff with knowledge beyond that gained from training workshops and short courses. 4. Authors acknowledge a bias towards mountainous western streams. 5. These limitations make widespread adoption of the method unlikely. 3.2.3 Aggradation / Degradation Degradation is the long-term lowering of bed elevation. It can be a significant component of total scour, but is not caused by the bridge or highway constriction. Rather than occurring only in the vicinity of the bridge, degradation extends well up- and downstream of the bridge. There are two primary causes of degradation; sediment deficiency and headcuts. Sediment

3.7 deficiency results when there is an imbalance between the sediment supply and the sediment transport capacity in a river reach. The reasons for this imbalance include reservoirs, urbanization, and other land use changes. Headcuts (and nickpoints) progress from downstream and result from base level lowering. Aggradation also results from a sediment imbalance when the sediment supply exceeds sediment transport capacity. Although not a scour component, aggradation impacts bridge hydraulic capacity and should be considered in design. Of the 6 Aggradation/Degradation research items considered, the 3 that are recommended are:  MDSHA cumulative degradation method, pool base-level method and degraded stream profile method  Methods included in Melville and Coleman "Bridge Scour" manual  Stream gage regression method Table 3.5. Strengths and Weaknesses of Recommended Methods in Aggradation/Degradation Topic. Paper Number Authors and Title Level Strengths Weaknesses Aggradation/Degradation 1.2. MDSHA 2007 Guidance on evaluation of long- term channel degradation, Chapter 14: Manual for Hydrologic and Hydraulic Design. 2 1. Cumulative Degradation Method and Pool Base-Level Method are reasonable and uncomplicated ways to estimate long-term channel degradation. 2. Pool Base-Level Method does not require a downstream control point. 3. Estimation of Degraded Stream Profile uses the riffle-crest line to calculate the degraded stream profile, which is a good first-order approximation. 4. These methods are useful alternatives to using detailed sediment transport models. 1. These methods are primarily relevant to wadeable gravel-bed streams with a pool-riffle morphology. 2. The range of channel slope covered extends only from 0.2% to 4%. 3.18. Melville & Coleman 2000. Quantitative assessment of aggradation and degradation, Section 4.3 in Bridge Scour. 2 1. Regime Formulations are easy to follow. 2. Tractive Force and Competent Velocity Methods are physically based, so that they have the potential to produce reliable results. 3. Methods described have simple equations that are not difficult to apply. 1. Graphical redistribution of the average scour depth to obtain the maximum scour depth is subjective. 2. Regime Formulations are not generally applicable: for example that of Lacey (1930) was designed for uncontracted sandy alluvial channel and that of Blench (1969) is valid only in well-maintained sand-bed irrigation canal systems. 3. The limitations of the Tractive Force Method are not discussed. 4. The Competent Velocity Methods of Neill (1973), Alvarez and Alfaro (1973) and Holmes (1974) all have limitations. 4.14. James, 1997. Channel Incision on the Lower American River, California, from Stream-flow Gage Records. 2 1. Regression approach is simple to apply (spreadsheet) and can be used for any long- term gage. 2. The paper illustrates the gage analysis approach and shows how bridge inspection records can be used for verification. 1. Only valid for locations with a nearby, long- term gage. 2. Use of extrapolation for predicting future degradation is a significant limitation. However, residual plots indicate time trends of reduced degradation if these are present. 3.2.4 Channel Migration This topic refers specifically to changes through time in the location of the channel of a watercourse that occur due to retreat of one bank at an erosion rate that is approximately matched by advance of the opposite bank through accretion. Channel migration results in lateral movement of the channel across the floodplain either through incremental shifting at a rate related to channel width or more rapid relocation of the channel through an avulsion.

3.8 Lateral migration in a bridge reach can pose a geomorphic hazard through altering the alignment of the channel relative to the bridge, generating scour adjacent to one of the abutments and in severe cases threatening to flank the bridge entirely. It also generates additional sediment load and recruits large woody debris that may increase the risk of partial or complete blockage. Of the 9 Channel Migration research references submitted, the 8 that are recommended are:  Channel lateral movement zone  Aerial photo comparison method  Methods presented by Melville and Coleman  Vegetation influence on migration  Multiple bend cutoffs risks  Wood and logjam risks  Channel realignment to reduce hazard  Theory and modeling related to channel migration Table 3.6. Strengths and Weaknesses of Recommended Methods in Channel Migration Topic. Paper Number Authors and Title Level Strengths Weaknesses Channel Migration 1.2. MDSHA 2007. Guidance on evalu- ation of lateral channel movement, Chapter 14 in Man- ual for Hydrologic & Hydraulic Design. 2 1. Delineation of the Channel Lateral Movement Zone and the frequency analysis of lateral channel movement are an improvement from Aerial Photo Review in HEC-20. 2. Approach presented is straightforward. 1. The procedure for delineating the Channel Lateral Movement Zone is still coarse. However, the manual states that a more detailed explanation of this procedure is under development. 1.6. NCHRP 24-16 2004. Method ology for predicting channel migration. 2 1. History of lateral migration at actual site in question provides a sound basis for prediction of future behavior. 2. Attributes like soil strength and vegetation are implicitly accounted for in observed and predicted migration rates. 3. Widely proven performance of R/W as a reasonable predictor of bend evolution. 4. Extensive empirical database. 5. Capability to adapt method to available data/ expertise. 1. The main limitation is that because analysis is based on past history at the site, predictions may be unreliable if watershed or climate changes impact the hydrological or sediment regimes. 2. Application of the more sophisticated versions of the model use GIS software that is now out of date. 3.18. Melville and Coleman 2000. Bridge Scour (especially, Section 4.8). 1/2 1. The research reported has been selected by the authors as being suitable for assessing the likelihood, rate and hazard associated with channel migration in both dynamically stable and unstable streams. 1. Main limitations stem from limited research, development & testing of methods presented. 2. Some methods presented have been superseded by later versions developed since this book was published. 4.8. Perucca et al. 2007. Significance of the riparian vegetation dynamics on meandering river morphodynamics 2/3 1. Research establishes vegetation growth and decay interact with fluvial processes in meandering rivers, influencing rates, spatial & temporal distributions of channel migration. 2. It demonstrates reliable channel migration predictions are only possible when vegeta- tion dynamics are taken into account. 3. It shows vegetation cannot be treated as a passive attribute of riparian zone when assessing channel migration hazards at bridges. 1. The complexity of the models, heavy data requirements and the need for advanced modeling expertise currently preclude practical application of the method. 2. The fluvial model uses a linear theory, which is known to be an inadequate representation of meander behavior. 3. Thirdly, models fail to account for changes in river width, variation of flow resistance with vegetation density, and influence of woody debris entering stream due to bank retreat.

3.9 Table 3.6. Strengths and Weaknesses of Recommended Methods in Channel Migration Topic. Paper Number Authors and Title Level Strengths Weaknesses Channel Migration 5.6. Hooke 2004. Cutoffs galore!: occurrence and causes of multiple cutoffs on a meandering river. 2 1. Research identifies that the probability of occurrence of a cluster of meander cutoffs (resulting in lateral channel migration and/or realignment that may pose a bridge hazard) might be predictable based on preexisting sinuosity relative to a critical value for planform instability. 2. Research is based on a theory that is increasingly accepted in fluvial geomor- phology, coupled with well documented evidence obtained from the River Bollin, UK. 3. Long-term study of actual meandering stream 1. The critical value for planform instability is poorly defined. A maximum value of 3.14 is suggested for unconstrained rivers, but this decreases with the degree of meander confinement due to limited width of the channel migration zone. 2. To be generally applicable, the relationship between critical sinuosity and degree of confinement needs to be better defined based on further research at well documented sites on meandering rivers in a range of physiographic regions. 5.20. Brummer et al. 2006. Influence of vertical channel change associated with wood accumu- lations on delinea- ting channel migration zones, Washington, USA 1/2/3 1. The research demonstrates that the addition or removal of large wood has marked impacts on avulsive channel migration. 2. The paper presents Level 1 & 2 rules of thumb to estimate channel response. 3. Numerical analyses presented in the paper could be used at Level 3 where risks justify this. 1. The geographical scope of the study is limited to the Pacific Northwest and the findings may not be simply transferable to other physiographic regions of the USA. 2. The models used are quasi-steady and do not account for the geomorphic impacts of rapidly varying flow in flashy streams. 9.2. Odgaard 2008. Stability Analysis in Stream Restoration 2/3 1. This research provides a scientifically-based alternative to use of a 'reference' reach when realigning a problematic channel to reduce hazards associated with channel migration. 1. The design method has not yet been tested or applied by practitioners. 2. The approach is too new to have been proven to reducing risks associated with channel migration in bridge reaches. 10.2. Odgaard and Abed 2007. River Meandering and Channel Stability. 3 1. This chapter presents a concise review of theory and modeling practice in the analysis of river meandering and channel migration. 1. Coverage focuses mainly on theories, analyses and stabilization measures with which the authors are particularly associated. 3.2.5 Channel Widening / Narrowing This topic refers specifically to changes in the top bank width of a channel that occur through time due to net retreat or advance of the banklines. Changes in width trigger further adjustments to the hydraulic geometry of the channel involving the wetted perimeter, mean depth, hydraulic radius, roughness, energy slope, and flow velocity. Extreme widening or narrowing are also associated with planform metamorphosis, which is the relatively rapid transformation of, for example, a meandering planform into a braided channel (with extreme widening), or vice versa (with extreme narrowing). Widening in a bridge reach can pose a geomorphic hazard through increasing the degree of constriction scour at the bridge, generating scour adjacent to the abutments and, in severe cases, threatening to flank the bridge entirely on one or both sides. It also generates additional sediment load and recruits large woody debris that may increase the risk of partial or complete blockage. Narrowing may increase velocities and general scour depths within the narrower channel. Of the 10 Width Adjustment research items submitted, the 6 that are recommended are:  Regional bankfull width relationships  Error estimation from aerial photos  Logistic analysis of channel pattern  Predicting channel pattern change  Bank erosion and vegetation effects  Methods presented in ASCE "Sedimentation Engineering"

3.10 Table 3.7. Strengths and Weaknesses of Recommended Methods in Change in Channel Width Topic. Paper Number Authors and Title Level Strengths Weaknesses Change in Channel Width 5.2. Faustini et al. 2009. Downstream variation in bankfull width of wadeable streams across the conterminous United States 2/3 1. This research uses advanced statistical analyses of a very large, national database. 2. Relationships are easy to apply and allow estimates of the expected width to be made on the basis of only the drainage area and bed material type (gravel or sand) at the study site. 1. Utility is limited by weak regression relationships and high uncertainties in expected widths for some ecoregions. 2. Relationships are inapplicable to large rivers (wider than 75 meters or with drainage areas > 10,000 km2). 3. Impacts of human activities in the watershed are poorly explained in several ecoregions. 5.38. Mount et al. 2003. Estimation of error in bankfull width comparisons from temporally sequenced raw and corrected aerial photographs 2 1. Provides a simple method to assess errors involved in estimating widening rates from historical sequences of aerial photographs, which are often ignored by practitioners using historical aerial photographs. 1. Application of the error estimation method requires some practitioner training in photogrammetry which may limit widespread uptake and validation of the method in the USA. 5.46. Bledsoe and Watson 2001. Logistic analysis of channel pattern thresholds: mean- dering, braiding, and incising. 2 1. Relatively high predictive capacities of the statistical models presented for stability versus instability in sand and gravel-bed rivers. 2. The fact that application of these models requires only basic data on discharge, slope and bed material size. 1. Statistical treatment has no causal basis. It cannot explain why a stream is stable or unstable. 2. Influences of sediment supply and bank erosion resistance are not accounted for. 3. Streams that plot as stable on the diagrams may still exhibit instability. 5.48. Lewin and Brewer 2001. Predicting Channel Patterns. Includes discussion by van den Berg and Bledsoe (5.40) and Reply by Lewin and Brewer (5.37). 2 1. Shows that practitioners must not put too much faith in simple predictors of channel planform type, stability and vulnerability to change. 2. Points out that simple predictors may mis- classify 10 to 15% of channels. 3. States that predictors should not be used in isolation or where risk of mis-classifying a channel is severe. 1. This research does not provide any improvement in the capability for simple prediction of planform pattern type, stability or vulnerability to change. 2. It merely points out problems with existing methods. 7.17. Beeson and Doyle, 1995. Comparison of bank erosion at vegetated and non-vegetated channel bends. 1 1. Presents case study of directly observed bank retreat that occurred during high flow events on four Canadian rivers in 1990. 2. Erosion was five times more likely at un- vegetated vs. vegetated bends. 3. 34 of 35 bends that experienced severe bank retreat (greater than 45 meters) were unvegetated. 1. Research is based on just four, medium sized rivers in British Columbia. 2. Findings may not be representative of other rivers of different sizes, with different types of vegetation or in different ecoregions of North America. 3. Further research is needed to generalize the findings 10.1. ASCE TC 2006. Streambank erosion and river width adjustment. Chapter 7: Sedimen- tation Engineering. 2 1. Older Channel Evolution Models are updated. 2. Bank and fluvial stability factors are used to identify channel evolution stages. 3. Numerical Width Adjustment Models are improved to make acceptable predictions of width adjustment. 4. A procedure to handle width adjustment problems is proposed. 1. Numerical Width Adjustment Models remain unproven because very few appropriate laboratory and field data sets are found to be suitable for testing them. 2. No universal Width Adjustment Model exists that is applicable to all the situations encountered by practitioners. 3.2.6 Sediment Dynamics Sediment plays an essential role in fluvial processes. Most geomorphic processes and forms of a river system are related to changes in sediment conditions. Channels adjust their vertical and horizontal dimensions in response to the imbalance between upstream sediment supply and the reach’s sediment transport capacity. For example, sediment trapping in reservoirs can result in severe riverbed lowering downstream, while excessive sediment resulting from landslides and bank erosion can lead to significant channel aggradation. When sediment supply from upstream reaches reduces, a braided river reach may alter itself to be a single-

3.11 thread channel. Even when sediment supply matches sediment transport capacity, channels migrate by eroding banks and depositing sediment at point bars. Therefore, the identification of sediment source areas in the river system will benefit the understanding of ongoing geomorphic processes. In addition, sediment movement at the channel bottom has an influence on the reach’s bed configuration, hydraulic resistance, and flood conveyance capacity. Of the 4 Sediment Dynamics research items, the 2 recommended for inclusion in HEC-20 are:  Channel forming discharge expanded discussion  Rosgen's WARSSS (Watershed Assessment of River Stability and Sediment Supply) concepts Table 3.8. Strengths and Weaknesses of Recommended Methods in Sediment Dynamics Topic. Paper Number Authors and Title Level Strengths Weaknesses Sediment Dynamics 3.9. Doyle et al. 2007. Channel- forming discharge 2 1. The article emphasizes that the use of a recurrence interval or bankfull discharge may only be applicable for generally stable channels. 1. Definition of effective discharge is subjective. Further justification is required for the use of ‘75% of sediment moved’. 2. Multiple discharges might be used for different purposes in stream restoration. 11.1. Rosgen. Watershed Assess- ment of River Stability and Sediment Supply 2 1. The procedure involves most factors that control watershed processes, and is easy to follow. 1. Final stage of WARSSS might recommend a sediment transport model due to the complex channel response. 2. Care must be taken when using a reference condition. 3.2.7 Numerical Modeling Numerical modeling is a valuable tool for simulating a river system’s geomorphic changes as most geomorphic processes are difficult to reproduce in terms of time scale. Numerous flow and sediment transport models have been developed or updated since 1990, partly because the computing power has been significantly growing. These models are quite different from each other at least in the following ways. First, flow and sediment movement are described in a one-dimensional, two-dimensional, or three-dimensional domain. Second, flow is modeled to be steady, quasi-steady, or unsteady. Last, the complexity of models indicates how many hydraulic and geomorphic processes are included in them and how they couple processes with different temporal and spatial scales. Note that not a single model can fit modeling needs in all circumstances, as each model has its strengths and limitations. In addition, not all models are well calibrated or validated: some are still theoretical, some have been calibrated with laboratory data, and only a few of them have been calibrated with both laboratory and field data. Numerical modeling, as a Level 3 procedure, requires a huge amount of effort, such as field data collection, parameterization, and calibration. Therefore, this topic does not include all available numerical models, but contain a few well-calibrated models and papers that provide reviews and instructions for flow and sediment transport modeling. Of the 6 Numerical Modeling research references, the 5 that are recommended are:  CONCEPTS model discussion  Sediment transport modeling review  CCHE2D model discussion  Discussion of 1-D sediment transport modeling  Discussion of 2- and 3-D sediment transport modeling

3.12 Table 3.9. Strengths and Weaknesses of Recommended Methods in Numerical Modeling Topic. Paper Number Authors and Title Level Strengths Weaknesses Numerical Modeling 3.2. Langendoen et al. 2009. Model incision and widening with calibration 3 1. The model presented (CONCEPTS) is able to simulate channel width adjustment based on the fundamental physical processes responsible for bank retreat. 1. CONCEPTS assumes one-dimensional, gradually-varying flow, does not simulate secondary flow. 2. The model is truly valid only for straight channels or channels of very low sinuosity. 3.7 Papanicolaou et al. 2008. Sediment transport modeling review 2/3 1. The article provides review comments for most available sediment transport models, and some insights about model application, strengths, and limitations. 1. To engineering practitioners, the review may too be abstract and focused on the modeling and numerical computation aspects of sediment transport prediction. 3.20. Jia and Wang 2000. 2D hydro- dynamic and sediment transport model 3 1. The model presented (CCHE2D) can predict channel migration including the effects of secondary flows, which are simulated in the model. 1. Though CCHE2D uses near bank shear stress to compute bank toe and surface erosion including secondary flow effects, it does not consider most bank failure mechanisms. 10.4. Thomas & Chang 2006. 1D model of sedimen- tation processes, Chapter 14: Sedimen- tation Engineering 2/3 1. In this summary chapter the authors provide useful insights and lessons about 1D computational sedimentation models for engineering practitioners based on long experience. 1. Coverage of how to apply 1D computational sedimentation models is descriptive and wordy. It could be improved by including flowcharts and tables. 10.5. Spasojevic & Holly 2006. 2D and 3D models of obile- bed hydrodynamics and sedimentation, Chapter 15: Sedimentation Engineering 3 1. Gives a clear introduction to the complicated procedure for numerical modeling of hydrodynamics and sedimentation that is useful background knowledge for practitioners, 2. Provides three examples that engineering practitioners can easily refer to. 1. Approaches reviewed do not include bank mechanics, which is an important part of changes in channel morphology.