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11.1 INTRODUCTION Problems associated with riverbank erosion and channel migration have plagued societies reliant on engineered river crossings for thousands of years. Engineers have dealt with these problems by attempting to site bridges in particularly stable or laterally constrained reaches, over-engineering bridges to withstand all but catastrophic erosion, or building bridges that are easy to replace. However, the risks associated with channel migration at bridge crossings have necessitated that regular inspections be conducted to identify erosion prob- lems and that repairs and maintenance be implemented to avoid bridge failures. Despite all that has been done to avoid or identify the hazards posed by channel migration, no method- ology for routine prediction of meander migration in the vicinity of bridges exists, and uncertainty concerning the level of risk remains unacceptably high. Most streams that present a hazard through lateral migration at road crossings are alluvial. In alluvial streams, the channel is formed by the action of flowing water on boundary ma- terials that have been deposited by the stream and that can be eroded and transported by the stream. In alluvial streams, it is the rule rather than the exception that the banks will migrate through erosion and accretion and that floodplains, islands, and side channels will undergo modification with time. This is particularly the case in actively meandering streams, which continually change their positions and shapes as a consequence of fluvial processes and hydraulic forces exerted on their beds and banks. These changes may be incre- mental or episodic, gradual or rapid, and systemwide or local in scale. Meanders grow and move naturally, but human activities may accelerate the rate of change or trigger new changes caused by morphological response in the stream sys- tem. The fact that highway infrastructure is stationary makes it subject to potentially serious impacts caused by bend migra- tion in actively meandering alluvial streams. Bend migration is a major consideration in designing bridge crossings and other transportation facilities in meandering channels because it causes the channel alignment and approach conditions present during construction to deteriorate as the upstream channel location and orientation change with time. Channel migration can result in damage or destruction of the bridge through the following: ⢠Excessive bridge pier and abutment scour, ⢠Flanking of bridge approaches and other highway infra- structure, ⢠Excessive scour and pressure loading caused by debris accumulation, and ⢠Loss of conveyance through bridge openings because of misalignment and point bar development. Bend migration may be characterized by lateral channel shift (expressed in terms of distance moved perpendicular to the channel centerline, per year) and down-valley migration (expressed as distance moved along the valley, per year). It is a natural phenomenon that occurs in the absence of spe- cific disturbances, although it may be exacerbated by basin- wide factors such as land use changes, gravel mining, dam construction, and removal of vegetation. Engineers must recognize that processes operating in upstream reaches and subwatersheds may affect rates of lateral bend shift and down-valley migration in the vicinity of the bridge or high- way. Therefore, any attempt at predicting bend migration must consider all factors that may affect channel migration rates, including the magnitude and frequency of formative river flows and past, present, and possible future disturbances to the upstream watershed and drainage system that might affect channel processes. Channel migration in meandering streams is driven pri- marily by the tendency of meander bends to grow in amplitude and move downstream with time. However, meander growth means that a bend eventually may become so extended and tightly curved that it is abandoned by a chute or neck cut- off. Hence, channel migration is predominantly a progres- sive process that may be punctuated periodically by abrupt changes in channel alignment and position because of cut- offs. On meandering streams, a problem at a bridge site may become apparent only two or three decades after the bridge is constructed. Channel migration is often evident along considerable reaches of river in a drainage basin; however, it can be local- ized in the vicinity of a bridge. Remedial action, such as con- structing spurs or installing bank protection along substantial reaches of a meandering channel, is expensive and difficult to justify economically except in areas with very high land values. Remedial action is also dependent on the risk to the existing structure and the cost of replacement. Given that channel migration at the reach-scale is likely to persist, a prac- tical methodology is needed to evaluate the potential for bend movement, to define the rates and direction of historic chan- nel shifts, and to predict future channel migration in order to evaluate the hazard posed by channel migration to highway infrastructure within its design life. CHAPTER 1 INTRODUCTION AND APPLICATIONS
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 considera- tions 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 and incremental channel migration; (2) aggradation or degradation of the streambed; (3) short- term fluctuations in streambed elevation (scour and fill); and (4) avulsion. NCHRP Report 533: Handbook for Predict- ing Stream Meander Migration (Handbook) is concerned specifically with lateral channel instability (including bend radius expansion, across-valley extension, and down-valley migration) resulting from incremental meander migration. The Handbook covers the following topics: ⢠Screening and classification of meander sites, ⢠Sources of mapping and aerial photographic data, ⢠Basic principles and theory of aerial photograph com- parison, ⢠Manual overlay techniques, ⢠Computer-assisted techniques, ⢠Measurement and extrapolation techniques based on geographic information systems (GISs), ⢠Frequency analysis, ⢠Sources of error and limitations, and ⢠Illustrated examples using manual overlay techniques. Chapter 1 provides an introduction to the Handbook and a discussion of a range of potential applications of the tech- niques described in the Handbook. Chapter 2 describes the basic principles and processes of stream channel meander migration and discusses the potential hazards caused by meander migration as well as by avulsions and cutoffs. Chapter 3 presents a geomorphic classification scheme, modified from the channel pattern classification originally developed by Brice (1975), as an approach for both screening and classification. The most common river types (or meander modes) likely to be encountered by hydraulic engineers in the field are addressed by this classification. The screening procedure to identify stable meandering stream reaches ensures that engineering and inspection resources are not allocated to locations where there is little probability of a problem developing. The basic principles of photogrammetry, the types and sources of aerial photography, and the application of aerial photography to meander migration analysis are discussed in Chapter 4. Chapter 5 describes a manual overlay technique that uses historic bankline positions acquired from sequential historic maps and aerial photos to assess historic channel position. By inscribing and tracking the movement of circles of known radius on a bend over time, a prediction can be made on the probable position of the bend at some point in the future. Chapter 5 provides information on three ways to apply the overlay technique: (1) using a manual method, (2) using computer-assisted methods, and (3) using the ArcView- based Data Logger and Channel Migration Predictor tools developed for use with the Handbook. 2 The potential sources of error and limitations associated with the use of historic aerial photographs and maps in con- ducting a meander migration assessment and prediction are described in Chapter 6. A detailed description is provided in Chapter 7 of manual, computer-assisted, and GIS-based methodologies using map and aerial photo comparison techniques to conduct the over- lay and prediction of meander migration over time. The GIS- based measurement and extrapolation tools are included on CRP-CD-48, which is provided on the back inside cover of the Handbook. The use of the frequency analysis results developed under NCHRP Project 24-16 to assist in accurately predicting meander migration is described as well. Chapter 8 provides detailed, step-by-step examples of assessing historic meander migration and predicting future meander development using the methodologies described in the previous chapters. Appendix A describes how to download TerraServer images from the Internet for use in the analysis and prediction of meander migration. Methods for delineating the bankline of a channel and determining the radius of a meander bend are provided in Appendix B. Instructions on installing the ArcViewâbased Data Logger and Channel Migration Pre- dictor tools are provided in Appendix C. Tips for delineating banklines from historic aerial photos that are not georefer- enced for use with the Channel Migration Predictor can be found in Appendix D. Appendix E supplements the basic prediction techniques by providing a method to consider bend rotation. A glossary of terms used in the Handbook is provided in Appendix F. 1.2 APPLICATIONS Although the methodologies provided in the Handbook were developed to assist in identifying potential hazards to highway facilities from active channel migration, there are a number of other applications in which these methodologies could be used. When used properly, the Handbook will allow the user to identify potential problems associated with channel migration and determine the need for revetments, structural solutions, or biotechnical solutions to counter the threat posed by an actively migrating channel. 1.2.1 Transportation Facilities The techniques described in the Handbook could be used to assess the potential threat to an existing or proposed trans- portation facility or to evaluate the need for structural solutions or countermeasures to inhibit or halt active channel migration in the proximity of an existing or proposed transportation facility. Active channel migration poses a significant threat to the stability of existing bridges and other highway facilities. Bend migration, as it moves through a bridge reach, may pose a major hazard to a bridge, especially those bridges with bents located in the floodplain. In many locations where bridges
3span rivers and their floodways, only the bents and pilings within the channelized section have deep foundations, whereas those located on the floodplain or marginal to the channel may have shallow foundations. When channel migration removes a significant amount of bankline underneath a bridge, floodplain bents and pilings with shallow foundations may become exposed and undermined and may potentially fail. In some cases, the failure of only one bent may be sufficient to cause a significant portion of the entire bridge to collapse, as was the case of a major highway bridge over the Hatchie River in Tennessee in 1989 (see HEC-23, Design Guideline 1 [Lagasse et al., 2001]). In addition, significant bankline loss associated with either incremental (see Figure 1.1) or catastrophic (see Figure 1.2) channel migration can threaten to undermine or flank the bridge abutments, especially where they rest on, or are shal- lowly founded in, floodplain soils. In contrast, the components of a bridge and its approach embankments may exacerbate or hinder the processes associated with meander migration. The techniques described in this Handbook can also be used to evaluate the potential threat from channel migration to existing highway embankments, especially where the embank- ment is located on the active floodplain and runs parallel to the flow direction of a meandering stream. 1.2.2 Urban and Commercial Development Urban and commercial developments are increasingly encroaching into the floodplain. Although the law requires that these developments are to be built so that they are above or outside the limits of the 100-year floodplain for flood insur- ance purposes, the potential threat posed by active channel migration is not a consideration in many floodplain regula- tions. In addition, developers and city planners rarely recog- nize the potential hazards posed by active channel migration until it is too late. The Handbook can be used to identify potential threats to existing structures from channel migration and to identify and delineate areas where active channel migration may pose a serious hazard to new developments. 1.2.3 Flood Control Facilities In many areas along larger river systems and in close proximity to large urban areas, flood control facilities such as levees, dikes, and flood relief structures are used to protect the public from major flood events. However, as shown in the Chapter 8 examples on the Sacramento River (Sections 8.4 and 8.5), these flood control facilities are often located in, or in close proximity to, the active meanderbelt and can be threat- ened by active channel migration. On the Sacramento River as well as along many other major rivers and streams across the country, the U.S. Army Corps of Engineers regularly conducts studies to determine the potential threat to the levee system posed by channel migration. Although most threats are iden- Figure 1.1. Aerial photo of a bridge over the Wapsipinicon River near De Witt, Iowa, showing the incremental shift of the outer bank of the bend as a result of meander migration over time. The bridge was installed after 1969. 1937 1969 1994 Figure 1.2. The US-95A bridge over the Carson River near Weeks, Nevada, in 1975 and 1986. Significant meander migration during the 1986 flood caused severe erosion of the south embankment and flanking of both bridge abutments, which ultimately resulted in failure of the bridge. 1975 1986
tified early, major floods can cause significant channel migra- tion over the duration of the flood event. In these cases, the U.S. Army Corps of Engineers may be required to perform emer- gency repairs or install emergency countermeasures to halt the degradation of a levee or flood control structure. Because this type of analysis is not a common practice among private levee districts, smaller municipalities, and private owners, the Hand- book can provide these groups with a simple procedure neces- sary to successfully counter possible threats posed by active channel migration within their districts and river reaches. 1.2.4 Riparian Corridor In numerous locations throughout the United States, flood- plain encroachment by urbanization and development have significantly reduced the amount of riparian land along river and stream corridors. In many places, efforts are being made to acquire, maintain, and increase remaining riparian lands for ecological and biological purposes. Because riparian corridors are often fairly narrow, the erosion and removal of mature riparian vegetation on one side of the channel by active chan- nel migration can be sufficiently rapid to reset or significantly alter the natural establishment and succession of new vegeta- tion on the opposite side of the river or stream. Because active meander migration is generally necessary to maintain a healthy riparian community, a meanderbelt that is sufficiently wide to allow for active channel migration should be main- tained. The Handbook can be used to define the width require- ments of a healthy riparian corridor taking into account an actively migrating channel. 1.2.5 Agriculture Many areas along a migrating stream that are not urban- ized or are not riparian are agricultural. In many locations where there is a minimal riparian corridor and no urban 4 development, farm fields or pastures border streams and rivers, often with cropland extending to the very edge of the channel. In these areas, active channel migration not only results in a loss of private property but also in a loss of pro- ductive land and a reduction in annual crop yields. Active channel migration can also threaten irrigation diversion, intake, and return structures; irrigation canals; and other agricultural facilities. Thus, the methodologies described in the Handbook can be used to assess the potential long-term impacts from the loss of productive land associated with active channel migration. The procedures in the Handbook can also be used to aid in stabilizing existing irrigation struc- tures and in siting new irrigation structures with regard to potential channel migration. 1.2.6 Channel Restoration/ Rehabilitation Works There are many examples of streambank stabilization and rehabilitation measures that failed because they were placed on actively migrating streams and rivers without evaluating the characteristics of channel migration at those locations. The analysis of radius of curvature and migration rate for numerous bends on the Sacramento River conducted by Water Engineering and Technology, Inc. (WET) (WET, 1988) and described in Chapter 2 is a good example of using meander migration analysis to evaluate the potential effec- tiveness and survivability of bank protection works. The method of assessing meander migration with regard to radius of curvature to width ratio versus migration rate (WET, 1988) can be used on straightened or channelized streams where meanders are to be restored or incorporated into chan- nel rehabilitation and restoration efforts. This can be done by evaluating active channel migration on nearby streams and then designing the new or restored meanders of the rehabili- tated channel based on the stable meander patterns identified from nearby streams.
