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1 METHODOLOGY FOR PREDICTING CHANNEL MIGRATION SUMMARY This research accomplished its basic objective of developing 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 developed will 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. Based on an extensive literature review it was concluded that the only complete model of a river is the river itself. While the past behavior of a meandering reach is not necessarily indicative of its future behavior, at least the historical record integrates the effects of all the relevant variables as they operate in that location. The conclusions from the literature review are supported by an evaluation of empirical and deterministic (computer modeling) approaches to predicting meander migration. This project confirmed the conclusions of other investigations that because of limitations in data availability and model capabilities, it is extremely difficult to model the detailed time variation of stream movement; however, it is entirely feasible to analyze channel history and infer trends in the stream alignment and average migration rates. At present, empirical approaches are more likely than deterministic approaches to yield a practical methodology that will be useful to practicing engineers. Thus, the research approach for this project emphasized enhancing and using empirical data bases to develop photogrammetric comparison techniques as a basis for predicting meander migration. The principal product of this research is a stand-alone Handbook for predicting stream meander migration using aerial photographs and maps. The Handbook is published separately as NCHRP Report 533 and can be purchased through the TRB bookstore (trb.org/bookstore). The Handbook deals specifically with the problem of incremental channel shift and provides a methodology for predicting the rate and extent of lateral channel shifting and down valley migration of meanders. The methodology is based, primarily, on the analysis of bend movement using map and aerial photo comparison techniques; but frequency analysis results are provided to supplement the comparative analysis. The methodology enables practicing engineers to evaluate the potential for adverse impacts due to incremental meander migration over the design life of a bridge or highway river crossing and ascertain the need for countermeasures to protect the bridge from any associated hazards. An essential first step in applying the methodology is screening and classifying the river reach(s) under consideration. This project verified and extended the results of earlier research which indicated that meandering channels that do not vary significantly in width are relatively stable, whereas channels that are wider at bends are generally more active. As presented in the Handbook, this simple stratification of meanders is of value to the bridge engineer as a screening procedure, allowing preliminary identification of meanders that are very stable. As a result, this class of equiwidth meandering streams can be given a lower priority or eliminated from further analysis. The more actively meandering streams can be analyzed by the photogrammetric comparison techniques presented in the Handbook.
2 The key to application of the methodologies presented in the Handbook is obtaining time sequential aerial photography (or maps) of the meander site to be analyzed. Historical and contemporary aerial photos and maps can be obtained inexpensively from a number of Federal, State, and local agencies. The Internet provides numerous sites with links to data resources and sites having searchable data bases pertaining to maps and aerial photography. It is this ready availability of aerial photography resources that makes the methodologies presented in the Handbook powerful and practical tools for predicting meander migration. The comparison of sequential historical aerial photography, maps, and surveys provides an easy and relatively accurate method of determining migration rates and direction. The amount of detail available for analysis increases as the length of time between successive maps or photos decreases. However, a longer period of record for comparison will tend to "average out" anomalies in the record and provide a better basis for predicting meander migration by extrapolation. Abrupt changes in migration rate and major position shifts can often be accounted for by analyzing maps and photos for land use changes, and nearby stream gage records can be examined for extreme flow events. Predictions of migration for channels that have been extensively modified or have undergone major adjustments attributable to extensive land use changes will be much less reliable than those made for channels in relatively stable watersheds. The overlay comparison technique developed for this project consists of overlaying channel banklines or centerlines traced from successive historic maps or photos. The maps and photos are first enlarged or reduced to a common scale. Then, common reference points are identified, and the banklines of the meander bend are delineated on successive photos. The banklines are then overlain on each other by matching the common reference points. The overlain bankline positions can then be evaluated by measuring the change in radius and movement of the centroid of best-fit circles on the banklines to provide a quantitative estimate of migration distance, rate, and direction over time. Using the information and data obtained from this type of analysis, predictions can then be made on the potential position of the river at some point in the future. This process can be completed manually by tracing bends and inscribed circles on mylar overlays. However, the availability of computer photo editing software provides an alternative approach for performing the photo comparison techniques outlined above. For example, photo comparison and prediction can take advantage of the photo editing capabilities in Microsoft Word or PowerPoint. In addition, computer aided design (CAD) software, such as AutoCAD and Bentleyâs MicroStation, can be used to perform the photo comparisons with greater precision and accuracy, especially when the maps and photos are geo-referenced. The geo-referenced photos and banklines and associated data can be imported into a Geographic Information System (GIS) such as ArcView, a GIS and mapping software package developed by Environmental Systems Research Institute Inc. (ESRI). An ArcView extension, the Data Logger, a menu-driven circle template methodology was developed for this project to streamline the measurement and analysis of bend migration data and aid in predicting channel migration. The Channel Migration Predictor, another custom ArcView extension, uses the data archived by the Data Logger in predicting the probable magnitude and direction of bend migration at some specified time in the future. The ArcView extensions are included in CRP- CD-48, which comes with NCHRP Report 533. The Data Logger provides users with a quick and easy way to gather and archive river planform data. The bend delineation points for each bend and each historical record are archived
3 to provide a graphical record of the userâs interpretation of each bend. For each river bend and each historical record, the Data Logger records various river characteristics which are organized by river reach and recorded in a table identified by the reach name. Finally, the Channel Migration Predictor examines a table of river reach data for several bends and two or three historical records per bend, and then predicts rates of change in bend radius and bend center position for some time in the future. Another deliverable for this project is an archive of the database compiled on CRP-CD- 49 to include all meander site data acquired for this study. The CD-ROM archives contain the Excel workbooks, MicroStation files, 1990s and historic (where applicable) aerial photos, and the topographic maps for each site in digital file format. The database includes 141 meander sites containing 1,503 meander bends on 89 rivers in the U.S. The data for each meander site is compiled in Microsoft Excel workbooks. There are multiple spreadsheets within each of the workbooks. A General Data spreadsheet, contains the general information compiled from various sources and an aerial photo showing the site limits and the included meander bends. There are individual spreadsheets, designated by the bend number, which contain detailed historic data for each of the bends of the site. A summary spreadsheet contains all the measured data for all the bends of the site. The data base includes four spreadsheets that cross-reference each data site by the (1) source of the data, (2) stream classification, (3) river name, and (4) state in which the site is located. This format permits cross-referencing and provides a simple and useable approach to searching the data base. With this archive data set, future researchers will have a readily accessible data base in a very useable format for a variety of studies. These studies could include additional empirical analyses, more complex regressions based on the archive data, and research to develop more practical deterministic models of the meandering process. Much work remains to be done before the potential impacts of meander migration on transportation infrastructure can be predicted with certainty and ease using statistical or deterministic methods. While the results of this research, comparative analysis based on maps and aerial photography, could be viewed as an interim approach, it is not likely that this approach will be replaced by more sophisticated analytical techniques in the near future. The techniques presented in the Handbook will always be useful at the reconnaissance level or as a "reality check" on other approaches to solving the problem of predicting meander migration. The Handbook contains applications guidance and examples for the analytical products of this research, map and aerial photograph comparison techniques and guidelines to predict channel migration in proximity to transportation facilities. The Handbook provides the methodology in a stand-alone package with guidance and examples to facilitate ease of application. This methodology will be useful in reconnaissance, design, maintenance and inspection of highway facilities, and will help reduce the cost of construction, repair, rehabilitation and countermeasures for lateral channel instability. The screening procedure to identify stable meandering stream reaches will ensure that engineering and inspection resources are not allocated to locations where there is little probability of a problem developing. The end result will be a more efficient use of highway resources and a reduction in costs associated with the impacts of channel migration on highway facilities.
