National Academies Press: OpenBook

Methodology for Predicting Channel Migration (2004)

Chapter: Chapter 1: Introduction and Research Approach

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Suggested Citation:"Chapter 1: Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Methodology for Predicting Channel Migration. Washington, DC: The National Academies Press. doi: 10.17226/23352.
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Suggested Citation:"Chapter 1: Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Methodology for Predicting Channel Migration. Washington, DC: The National Academies Press. doi: 10.17226/23352.
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Suggested Citation:"Chapter 1: Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Methodology for Predicting Channel Migration. Washington, DC: The National Academies Press. doi: 10.17226/23352.
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Suggested Citation:"Chapter 1: Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Methodology for Predicting Channel Migration. Washington, DC: The National Academies Press. doi: 10.17226/23352.
×
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Suggested Citation:"Chapter 1: Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Methodology for Predicting Channel Migration. Washington, DC: The National Academies Press. doi: 10.17226/23352.
×
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Suggested Citation:"Chapter 1: Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Methodology for Predicting Channel Migration. Washington, DC: The National Academies Press. doi: 10.17226/23352.
×
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Suggested Citation:"Chapter 1: Introduction and Research Approach." National Academies of Sciences, Engineering, and Medicine. 2004. Methodology for Predicting Channel Migration. Washington, DC: The National Academies Press. doi: 10.17226/23352.
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4 CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH PROBLEM STATEMENT AND RESEARCH OBJECTIVES Rivers prone to channel migration may be spanned by structures and paralleled by fixed highway alignments and appurtenances. Channel migration (alluvial river meander, planform deformation) is a major consideration in designing bridge crossings and other transportation facilities in affected areas; it causes the channel alignment and approach conditions present during construction to deteriorate as the upstream channel location changes. Channel migration can result in the following: (a) excess bridge pier and abutment scour, (b) threats to bridge approaches and other highway infrastructure, (c) worsened debris problems, and (d) obstructed conveyance through bridge openings. Channel migration is typically an incremental process. On meandering streams, the problem at a bridge site may become apparent two or three decades after the bridge is constructed. Channel migration is often evident throughout large sections of a drainage basin; it is not localized in the vicinity of a bridge. It is a natural phenomenon that occurs in the absence of specific disturbances, but may be exacerbated by such basin-wide factors as land use changes, gravel mining, dam construction, and removal of vegetation. Remedial action such as constructing spurs or installing bank protection becomes increasingly expensive or difficult as the channel migrates. A methodology is needed to evaluate the potential for channel movement and predict future channel migration. Channel migration includes lateral channel shift (expressed in terms of distance moved perpendicular to the channel center line, per year) and down valley migration (expressed in distance moved along the valley, per year). Engineers are concerned with predicting channel migration as it moves through the bridge elements (piers and abutments) or endangers other highway infrastructure during its design life. The role of multiple reaches and subwatersheds in predicting factors affecting the rates of lateral channel shift and down valley migration is also important to the understanding of channel migration in the vicinity of transportation facilities, as is the impact the transportation infrastructure itself has on those rates. In addition, any methodology for predicting channel migration rates will need to consider factors that affect natural channel migration rates such as the size and frequency of formative river flows, and past, present, and possible future disturbances to that channel migration. The basic objective of this research was to develop a practical methodology to predict the rate and extent of channel migration (i.e., lateral channel shift and down valley migration) in proximity to transportation facilities. The methodology should enable practicing engineers to evaluate and determine bridge and other highway facility locations and sizes and ascertain the need for countermeasures considering the potential impacts of channel meander migration over the life of a bridge or highway river crossing. The methodology could be applied to locate and design a new bridge or highway facility to accommodate anticipated channel migration or to evaluate the risk to existing facilities, and if necessary, to determine the need for and design countermeasures against the effects of channel

5 migration. A prediction of channel migration could also be used to alert bridge inspection personnel to the potential for channel change that could affect the safety of a bridge. SCOPE OF RESEARCH Predicting channel migration requires consideration of both local and system-wide factors. The morphology and behavior of a given river reach is strongly determined by the sediment and water discharge from upstream. Therefore, any significant modification of sediment load and water discharge, as a result of human or natural events, will impact local rates of channel change. Even without changes to the supply of water or sediment, lateral migration can occur and adversely impact highway structures. Locally, the distribution of velocity and shear stress and the characteristics of bed and bank sediment will control channel behavior. Therefore, local channel morphology such as dimensions (width, depth, meander wavelength and amplitude), pattern (sinuosity, bend radius of curvature), shape (width-depth ratio), and gradient will not only reflect upstream controls, but will also provide information on the direction and rate of channel migration. For example, highly sinuous, equal-width channels are relatively stable, whereas less sinuous channels of variable width may migrate rapidly. While geomorphologists may view channel stability from the perspective of hundreds or thousands of years, for highway engineering purposes, a stream channel can be considered unstable if the rate or magnitude of change is such that the planning, location, design, or maintenance considerations for a highway crossing are significantly affected during the life of the facility. The kinds of changes that are of concern are: (1) lateral bank erosion, including the erosion that occurs from meander migration; (2) aggradation or degradation of the stream bed that progresses with time; and (3) short-term fluctuations in streambed elevation that are usually associated with the passage of a flood (scour and fill). This research is concerned specifically with lateral channel instability (including down valley migration) resulting from meander migration. The original scope of this research envisioned the following approach: • Enhance existing data sets by acquiring recent aerial photography at selected study sites and conduct field work, if necessary, to obtain hydraulic and geotechnical/soils characteristics of selected sites. • Analyze the enhanced data sets with photogrammetric comparisons which in general encompass the time periods of the 1930s, 1960s, and 1990s. Determine and analyze the geotechnical characteristics of bed and bank materials at selected sites, emphasizing descriptors of bankline erodibility and floodplain characteristics. • Develop a quantitative screening procedure to identify stable meandering reaches. This information would be significant to both bridge design engineers and bridge inspectors and would provide a basis for concentrating design and inspection resources on less stable problem reaches.

6 • Develop quantitative multiple regression or other statistical relationships for predicting direction, location, and rate of meander migration in unstable meandering stream reaches. Support these empirical relationships with an overall applications methodology, perhaps in spread sheet and graphical (CAD) formats. Following a literature review, documentation of case histories on meander migration and assembly and evaluation of existing data, an Interim Report was submitted for Panel review in September 1999. At a Panel meeting in November 1999, the Panel concurred with the Research Team's recommendation to eliminate the field work originally proposed. Direct costs originally allocated to field work were reallocated to other activities. The Interim Report suggested that the Panel consider increasing the project scope to include two additional activities: • Develop a map/aerial photograph comparison Handbook for predicting meander migration as a stand-alone deliverable. • Extend the scope to include a methodology to predict avulsive or catastrophic channel change in addition to incremental shift. In October 2000, NCHRP informed the Research Team that the Panel had requested continuation funds at the conclusion of the November 1999 interim meeting. The AASHTO Standing Committee on Research had reviewed the Panel's request and during the March 2000 meeting approved the request. The Panel's objectives for the continuation funds were as follows: • Develop a map/aerial photograph comparison Handbook • Analyze an additional 500 river meander sites • Compile a data base and archive on a CD-ROM data from all sites analyzed The Panel did not support the suggestion to include a methodology to predict channel avulsion in the scope of work. A proposal for continuation funding was submitted to NCHRP and the Panel in October 2000. The PI met with the Panel at TRB in December 2000 and comments from the Panel were received in February 2001. A revised Research Work Plan in March 2001 incorporated the additions to the scope and budget and revised the schedule from 30 months to 48 months for project completion. RESEARCH APPROACH The fluvial processes involved in predicting meander migration are very complicated and the variables of importance are difficult to isolate. The major factors affecting alluvial stream channel forms are: (1) stream discharge (magnitude and duration), temperature, viscosity; (2) sediment load, including types and caliber of sediments; (3) longitudinal valley slope; (4) bank and bed resistance to erosion; (5) vegetation; (6) geology, including bedrock outcrops, clay plugs, changes of valley slope; and (7) human activity. In an analysis of flow in alluvial rivers, the flow field is further complicated by the constantly changing discharge. Significant variables are, therefore, quite difficult to relate mathematically. It is often necessary to list measurable or computable variables, which effectively describe the processes occurring, and then to reduce the

7 list by making simplifying assumptions and examining relative magnitudes of variables. This means that it is necessary to strive toward an acceptable balance between accuracy and limitations posed by data needs and analytical complexity. Many laboratory and field studies have been carried out in an attempt to determine the variables controlling river response. To the present time, the problem has been more amenable to an empirical solution than an analytical one. Computer solutions to complex hydraulic problems have extended the range of fluvial process problems that can be solved analytically; but simplifying assumptions are still required. While the mathematical complexity of the analytical solution may be justified for research purposes, empirical approaches may produce results of greater utility to practicing engineers. In addition to channel and bank characteristics, floodplain characteristics must also be incorporated into an analysis procedure. The floodplain characteristics that should affect meander migration include geologic controls, alluvial deposits and topographic variability. Geologic controls include bedrock outcrops and erosion resistant features along the valley sides. Alluvial deposits frequently include oxbows, meander scrolls and scars, and clay plugs, each with different erodibility characteristics. Topographic variability that should be considered include the cross valley slope of the adjacent floodplain and whether channels are migrating into alluvial deposits or into adjacent hillslopes. After careful review of empirical and deterministic (physical process mathematical modeling) approaches to predicting meander migration it was concluded that empirical approaches are more likely than deterministic approaches to yield a practical methodology that will be useful to practicing engineers. Thus, the research approach emphasized enhancing and using empirical data bases to develop photogrammetric comparison techniques and predictive multiple regression or other statistical relationships which include descriptors of, or surrogates for, bankline erodibility and floodplain characteristics. The approach was essentially empirical. The basis for suggesting this approach was two- fold: 1. The limited success, to date, achieved in using bend-flow models to predict the direction, location, and rate of bank erosion and meander migration, and 2. The inherent complexity of bend-flow modeling. For State DOT’s, the primary users of the results of this research, empirical approaches are much more likely to provide a methodology that can and will be used in the field by practicing highway hydraulic engineers. In outline, the approach consisted of: • Conduct a complete and thorough literature review using standard reference sources, and update and critically review literature searches on meander migration previously completed by members of the Research Team. • Access and evaluate a number of existing data sets that contain time-sequential aerial photography, stream gaging data, and field measurements of hydraulic and geomorphic variables.

8 • Contact state, federal, other agencies, and researchers who have assembled similar data bases to collect data to supplement and expand the existing data sets. • Choose a broad distribution of sites geographically so that regional climate, geology, and geomorphology are represented. • Enhance existing data sets by acquiring recent aerial photography at selected study sites and obtain data on hydraulic and geotechnical/soils characteristics. • Analyze the enhanced data sets with photogrammetric comparisons which in general encompass the time periods of the 1930s, 1960s, and 1990s. Determine and analyze the geotechnical characteristics of bed and bank materials at selected sites, emphasizing descriptors of bankline erodibility and floodplain characteristics. • Develop a quantitative screening procedure to identify stable meandering reaches. This information would be significant to both bridge design engineers and bridge inspectors and would provide a basis for concentrating design and inspection resources on less stable problem reaches. • Develop a classification system for river/meander types to support stratification of the data base. • Develop a stand-alone Handbook for map/aerial photograph comparison techniques for measuring and predicting meander migration. Support the comparison techniques with an overall applications methodology in GIS format. • Develop a frequency (probability) analysis for all meander classes with sufficient data to test the reasonableness of results obtained by comparison techniques. • Compile and archive a data base on CD-ROM which includes all meander site data acquired for this study. • Conduct the necessary testing and evaluation, both internally and with State DOT’s, and revise the methodology as necessary. • Develop a detailed plan and recommendations for incorporating the results of this research in ongoing FHWA/National Highway Institute technology transfer programs. Considering the research approach outlined above, the following specific tasks will accomplish the objectives of NCHRP Project 24-16. These tasks parallel those suggested in the Research Project Statement. Task 1 - Literature Review Conduct a critical review of published and unpublished literature to determine the existing state of knowledge and to identify sources of data pertaining to channel migration. Conduct a complete and thorough literature review on meander migration using such standard sources as GeoRef for the geomorphic-geologic literature and Water Resources Abstracts for the engineering literature. Critically review the literature on deterministic modeling approaches to validate the proposed methodology. Task 2 - Document Case Histories

9 Contact state, federal, and other appropriate agencies to develop a list of documented case histories and to collect information on channel migration throughout North America. Task 3 - Assemble and Evaluate Existing Data Access and evaluate the existing data bases and develop additional data on meander migration as described in Task 2. By correlating these data sets, determine the extent to which the full range of empirical data necessary to the research approach already exists. Task 4 - First Interim Report Prepare and submit an Interim Report. The Interim Report will include a detailed outline of the proposed methodology, document the results of Tasks 1 through 3, and update the Work Plan for completing the project. Task 5 - Acquire Data and Develop Methodologies Obtain the necessary data and aerial photography, integrate that data with existing data sets, and develop a methodology which meets project objectives. Task 6 - Test and Evaluate Methodology Test the methodology using independent data sets and recalibrate as appropriate. Evaluate the accuracy of the methodology and discuss the implications for application. Task 7 - Second Interim Report Prepare and submit a second interim report which will include analysis of the data, a draft of the map/aerial photograph comparison Handbook and draft documentation of the GIS based measurement and extrapolation techniques. Task 8 - State Evaluation Provide the methodology to five states for their independent assessment and report the results. Modify the methodology, as appropriate. Task 9 - Compile and Archive Data Base Compile and archive the data base on a CD-ROM (or several CD-ROMS) which will include all meander site data acquired for this study. Task 10 - Prepare an Aerial Photo Comparison Handbook Prepare the Handbook following National Highway Institute (NHI) standards. As a minimum, the Handbook will cover the following topics: • Screening and classification of meander sites

10 • Sources of mapping and aerial photographic data (e.g., the MSN TerraServer Web site or the USGS EROS Data Center) • Basic principles and theory of aerial photograph comparison (e.g., scale, distortion, etc.) • Simple overlay techniques • GIS or computer supported techniques (e.g., software such as ArcView) • GIS based measurement and extrapolation techniques • Sources of error and limitations • Illustrated examples and applications • Supplementing photo/map comparison techniques with regression results Task 11 - Submit Final Report Submit a final report documenting the entire research effort. The map/aerial photograph comparison Handbook will be a stand-alone document. The final report will contain a detailed plan and recommendation for incorporating the results of this research in ongoing FHWA National Highway Institute technology transfer programs and courses.

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TRB’s National Cooperative Highway Research Program (NCHRP) Web Document 67: Methodology for Predicting Channel Migration documents and presents the results of a study to develop a practical methodology to predict the rate and extent of channel migration in proximity to transportation facilities. The principal product of this research was NCHRP Report 533: Handbook for Predicting Stream Meander Migration, a stand-alone handbook for predicting stream meander migration using aerial photographs and maps. A companion product to NCHRP Web Document 67 is NCHRP CD 49: Archived River Meander Bend Database, a four-CD-ROM set that contains a database of 141 meander sites containing 1,503 meander bends on 89 rivers in the United States.

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