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23 5.1 INTRODUCTION A large number of geographic features and geomorphic planform characteristics used in the evaluation of meander migration are readily discernible on aerial photographs and topographic maps. Thus, a comparison of many of these features and characteristics over time can be made to deter- mine the rate and extent of historic changes and assess poten- tial future changes. The Handbook deals with assessments using aerial photos, but the methods described in the Hand- book can be used when making assessments or measurements from maps. Bank retreat and meander migration can be physically mea- sured in the field using repeat surveys and erosion pins. How- ever, the most common (or traditional) methods for assessing lateral bank erosion and meander migration use measurements acquired from maps and aerial photographs. Bank retreat and meander migration can be determined in three ways: â¢ Estimated by visual comparison of two aerial photos taken at different times, â¢ Measured by scaling distances directly from the bank to common reference points on sequential, historic aerial photographs, and â¢ Measured on a drawing on which historic channel banklines taken from sequential aerial photos are super- imposed at the same scale. Visual comparison of aerial photos provides a preliminary assessment of stability, especially where significant changes in bank position have occurred. Scaling distances on aerial pho- tos requires making measurements on two photos along a line described by two reference points on each side of the river. The scaling method will usually provide only a few accurate mea- surements along the bank, depending on the number of refer- ence points and the number of lines that can be drawn across the bend. Additional problems may be associated with the location of the lines because they may not be perpendicular to bank retreat or allow a measurement at the point of maximum retreat. The overlay method is relatively easy and accurateâ channel banklines or centerlines from sequential historic aerial photos at a known scale are superimposed on each other and compared. Through this comparison, the rates of bank retreat are determined. Manual overlay techniques are outlined in detail in Sections 5.2 and 7.2. In addition to the manual overlay techniques, the rate and extent of meander migration can be interpreted from sequen- tial aerial photographs (or maps) using computer-assisted and GIS-based approaches. Computer-assisted techniques are summarized in Sections 5.3 and 7.3. GIS-based measurement and extrapolation techniques are discussed in Section 5.4. The user guides for the GIS-based tools are included in Section 7.4 of the Handbook, and the supporting software is included on CRP-CD-48, which is provided on the inside back cover of the Handbook. 5.2 MANUAL OVERLAY TECHNIQUES An easy and relatively accurate method of determining migration rates and direction is through the comparison of sequential historical aerial photography, maps, and surveys. Accuracy in such an analysis is greatly dependent on the time intervals over which migration is evaluated, the amount and magnitude of internal and external perturbations forced on the system over time, and the number and quality of sequential aerial photos and maps. Geologic and structural features as well as major changes in watershed characteristics and hydro- logic conditions can have a profound effect on meander migration patterns and rates. An analysis of long-term changes can be useful on a chan- nel that has coverage consisting of data sets (aerial photos, maps, and surveys) that cover multiple time intervals over a long period of time (several decades to more than 100 years). Long-term changes can be documented, and changing migra- tion rates can be evaluated with regard to changes in watershed characteristics and hydrology over time. If only two or three data sets covering a short time period (several years to a few decades) are available, a short-term analysis may be conducted. A short-term analysis covering recent data can provide information on existing migration rates and conditions. Predictions of migration for channels that have been exten- sively modified or have undergone major adjustments attrib- utable to extensive land use changes will be much less reliable than those made for channels in relatively stable watersheds. The manual overlay technique consists of overlaying chan- nel banklines or centerlines traced from successive historic maps or photos that have been registered to one another on a base map or photo. The first requirement of conducting a sim- ple overlay technique is obtaining the necessary aerial photos and maps for the period of observation. 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 âaverage outâ anomalies CHAPTER 5 MAP AND AERIAL PHOTO COMPARISON TECHNIQUES
in the record and provide a better basis for predicting mean- der migration by extrapolation. Abrupt changes in migration rate and major position shifts can be accounted for by ana- lyzing maps and photos for land use changes, and nearby stream gauge records can be examined for extreme flow events. Distortion of the image on aerial photos is a common prob- lem and becomes greater with distance away from the center of the photo. Expensive equipment that is needed to rectify and eliminate aerial photo distortion is often unavailable to users of aerial photos, so distortion and scale differences must be accounted for by some other means. Although most photos come with an optimal scale (e.g., 1:20,000), the scale is not always accurate for the purposes of analysis. The scale prob- lem can be overcome through the use of multiple distance measurements taken between reference points on both a pho- tograph and a related base map. Measurements of distance for several reference-point pairs common to both the photo and the map are then averaged to define a relatively accurate approximation of the scale of the photos. Common reference points can include constructed features (such as building cor- ners, roads, fence posts, intersections, and irrigation channels and canals) or natural features (such as isolated rock outcrops, large boulders, trees, drainage intersections, stream conflu- ences, and the irregular boundaries of water bodies). The fol- lowing relationship is used to determine the scale of the aerial photo relative to the base scale of the base map or photo: Once the scale of each historic aerial photo is estimated, the photo will need to be enlarged or reduced to the scale of the base, whether it is another photo or a map. This can be done using a copier with a reduction or enlargement mode or using a flatbed scanner. Copied images are generally difficult to use because of poor tonal quality and resolution, and copiers may stretch the image slightly in the direction of scan. It is often better to trace the bankline or centerline and as many geo- graphic features as possible onto a transparent medium and enlarge or reduce the tracing as necessary. The use of a scan- ner (many scanners are relatively inexpensive) is advanta- geous because the resolution of the original photo can be retained, and the photo can easily be used in conjunction with computer-aided design (CAD) or GIS software. Accurate delineation of a bankline on an aerial photo is dependent primarily on vegetation density at the top of the bank. The bank top is most easily defined if stereopairs of photos are available, but individual photos can also be used when evaluated by experienced personnel. A detailed discus- sion of the methods for delineating a bankline is provided in Appendix B. For banks with little or no vegetation, the bank top is eas- ily identified in a photo. The abrupt change between the water and the top of the bank along the outer bank in a bend or an Length Between Aerial Photo Reference Points Length Between Same Reference Points on Base Base Scale Aerial Photo Scale (5.1)Ã = 24 eroding cutbank is defined by a sharp change in the contrast and color (color photo) or gray tone (black and white photo). Usually, the water is significantly darker than the bank top. Along the inner bank of a bend, exposed bar sediment is a lighter color or tone than the river or the bank top. The bank top along a point bar is usually defined by well-established vegetation, such as mature trees and shrubs. Where vegetation is sporadic along a bank, sections of the bank top may be visible so that the bankline can be interpo- lated between the visible sections. As vegetation becomes more continuous, the bankline may be completely obscured, and the user may have to locate it by approximation. In this case, it can be assumed that the trunks of the largest trees grow- ing along the edge of the stream are nearly vertical and are located just landward of the bank top. Therefore, a line that approximates the bank top may be drawn just riverward of the center of the crown of the tree. The error involved with this method increases with decreasing stream size because of the relationship of stream size to the crown size of the trees. If the density of vegetation along a stream is such that an accurate delineation of the bankline cannot be made, then the use of the channel centerline may be required. The centerline is drawn with reference to bankfull conditions. Therefore, the channel centerline will frequently cross exposed point bars. Usually, the channel centerline can be delineated more easily than a bankline masked by vegetation because the centerline can be drawn equidistant from the edge of mature vegetation on either side of the channel. However, the outer bank may still erode relatively rapidly while the position of the channel centerline remains unchanged. This can occur following a flood or where significant widening of the channel occurs. Once the maps and photos have been enlarged or reduced to a common scale, the common reference points have been iden- tified, the banklines or centerlines have been delineated, and these elements (reference points, banklines, or centerlines) have been traced onto a transparent medium, the banklines or centerlines are superimposed on each other. Once the banklines or centerlines have been superimposed on each other, their positions can then be evaluated with regard to migration dis- tance, rate, and direction over time. Using the information and data obtained from this type of analysis, predictions can be made on the potential position of the river at some point in the future. 5.3 COMPUTER-SUPPORTED TECHNIQUES Thanks to the availability of powerful computers and photo- editing software, most users of the Handbook can perform the photo comparison techniques discussed in the previous section relatively quickly and easily. For example, photo comparison and prediction can take advantage of the photo-editing capa- bilities in Microsoft Word or PowerPoint. These features were used to develop the illustrative examples provided in Chapters 7 and 8. In addition, computer-aided design and drawing (CADD) software (such as AutoCAD and Bentleyâs MicroStation) and
25 GIS-based software (such as ArcView and ArcInfo) can be used in conjunction with a flatbed scanner and digitizing tablet to perform the photo comparisons with greater precision and accuracy, especially when the maps and photos are georefer- enced. When digital files of aerial photos are unavailable, a flatbed scanner can be used to scan aerial photos to a relatively high resolution. Software that can manipulate a photographic image, such as MicroStation Descartes, can be used to warp a scanned aerial photo to fit a map or another resolved aerial photo. A digitizing tablet can be used to record the registration points and bankline positions, as well as other features from historical aerial photos, directly onto a georeferenced drawing, map, or photo. The photos and banklines can also be georeferenced and the associated data can be saved in a GIS. For example, the bend characteristics and meander migration patterns for more than 1,500 bends on numerous rivers distributed across the conti- nental United States were recorded using a digitizing tablet in conjunction with Bentleyâs MicroStation as part of NCHRP Project 24-16. The acquired data were used to develop the photogrammetric comparison methods to predict the rate and direction of bend migration presented in Chapters 7 and 8. 5.4 GIS-BASED MEASUREMENT AND EXTRAPOLATION TECHNIQUES ArcView is a GIS and mapping software package developed by Environmental Systems Research Institute Inc. (ESRI). An ArcView extension, the Data Logger, is a GIS-based, menu- driven circle template methodology that was developed for NCHRP Project 24-16 to streamline the measurement and analysis of bend migration data and aid in predicting channel migration. The Channel Migration Predictor is another ArcView extension that was developed under NCHRP Project 24-16 using the extrapolation techniques described in Chap- ters 7 and 8. Both extensions were developed using ArcView Version 3.2. The predictor tool uses the data archived by the Data Log- ger in predicting the probable magnitude and direction of bend migration at some specified time in the future. The Data Logger and the Channel Migration Predictor are ArcView extensions that were created using Avenue, a programming language and development environment that is part of the ArcView software package. Avenue is used to create special- ized graphical user interfaces and to run scripts that customize the functionality of ArcView. An ArcView project is a file used to store the work done with ArcView on a particular application, such as recording river bankline data. An ArcView project file contains all the views, tables, charts, themes, and scripts associated with an application. Both the Data Logger and the Channel Migration Predictor consist of a set of ArcView scripts. A script is the component of an ArcView project that contains Avenue code. Just like macros, procedures, or scripts in other programming or script- ing languages, ArcView scripts are used to automate tasks and add new capabilities to ArcView. The Data Logger provides users with a quick and easy way to gather and archive river planform data. The physical banklines are represented by one or more ArcView themes. A theme is a set of geographic features in a view. A view is an interactive map that allows the user to display, explore, query, and analyze geographic data in ArcView. The bend delin- eation points for each bend and each historical record are archived in individual themes 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 (e.g., bend radius, bend centroid, river widths, bend wavelength, etc.). These data are organized by river reach and recorded in a table identified by the reach name. These tables provide a permanent record of several river plan- form characteristics that can be further studied using the Channel Migration Predictor or various statistical procedures. The Data Logger requires the following steps to be per- formed at each bend for each historical record: 1. Locate the bankline delineation points on the outside of a river bend, 2. Fit a circle to the bankline demarcation points to describe the radius and orientation of the bend, 3. Use consecutive historical records and the data col- lected in Steps 1 through 3 to estimate the extension and translation rates for a bend, and 4. Use the migration and extension rates to extrapolate and estimate the future bankline locations. The Channel Migration Predictor examines a table of river reach data for several bends and two or three historical records per bend and then calculates rates of change in bend radius and bend center position. These rate data allow the Channel Migration Predictor to estimate the location of a bend at user-specified times. As discussed above, river reach data tables can be created using the Data Logger. Users can also create tables for input to the Channel Migration Predic- tor in the form of properly formatted database files using other means, such as Excel or Avenue. Instructions on installing the ArcView-based Data Logger and Channel Migration Predictor are provided in Appendix C. The file requirements and userâs guides for the Data Logger and the Channel Migration Predictor are provided in Section 7.4. Examples using these tools to conduct the planform mea- surements and meander migration prediction are provided in Section 8.4.