Methodology for Predicting Channel Migration (2004) / Chapter Skim
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From page 90... ...
77 CHAPTER 3 INTERPRETATION, APPRAISAL, APPLICATION OVERVIEW This chapter presents interpretation, appraisal, and applications of the methodologies for predicting the rate and extent of channel meander migration presented in the Handbook. While several topics are extracted from the Handbook, much of this information was not necessary for the Handbook and appears only in this Final Report.
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From page 91... ...
78 Brice Data Set The data set collected by Brice was part of an FHWA project to develop a simple method for determining relative stability of streams based on stream type (49)
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79 Ayres Associates Data Set Ayres Associates compiled a complete data set for both the middle Sacramento River (211, 212) in California and the lower Alabama River in Alabama (217)
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80 Up-to-date discharge data was obtained, where possible, for gaging stations in close proximity to all the meander sites. Mean daily discharge and annual peak discharge data is available free from the Water Resources web site of the USGS.
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81 photos were estimated based on known or approximate distances between reference points. However, distortion of the photographs was a significant problem in some cases, especially with regard to the alignment of reference points on the photos from the two different time periods.
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82 cross-valley lines to define the valley orientation line. Cross-valley lines and the valley orientation lines were drawn to reflect a significant change in valley direction.
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83 Figure 26. Simple (A)
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84 Extension (Across-Valley Migration) Translation (Down-Valley Migration)
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85 A change in any of these four modes of movement results in a change in the location of the outer bankline. Combinations of these modes of movement would result in a wide variety of meander bend shapes through time.
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86 Figure 28. Some of the bend measurements made using the Data Logger.
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87 Channel Width Channel widths are measured from top bank to top bank at the crossings and at the widest point in the bend. The channel width of the crossing in the data set is the average of the channel width at both crossings.
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88 A classification procedure modified from the channel pattern classification originally developed by Brice (72) (Figure 10)
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89 Figure 29 suggests that the range of channels from straight through braided forms a continuum, but experimental work and field studies have indicated that within the continuum, river-pattern thresholds can be identified where the pattern changes between straight, meandering, and braided. The pattern changes take place at critical values of stream power, gradient, and sediment load (202)
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90 As compared to the bed-load channel pattern, the five-mixed load patterns (Figure 30B) are relatively narrower and deeper, and there is greater bank stability.
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91 Increasing Valley Slope Increasing Stream Power ⇒ Increasing Sediment Load Figure 30. The range of alluvial channel patterns.
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92 Figure 31. Idealized long profile from hillslopes and unchanneled hollows downslope through the channel network showing the general distribution of alluvial channel types (from (205)
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94 Figure 32. Key to classification of rivers in Rosgen's method (modified from Rosgen (206)
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95 Figure 33. Longitudinal, cross-sectional, and planform views of major stream types in Rosgen's method (modified from Rosgen (206)
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96 STEP 2 - The maps and photos must be enlarged or reduced to a common working scale. The scale of the most recent map or photo should be used since it will be the basis for making and comparing historical meander pattern changes and predicting the position of a given bend in the future.
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97 Figure 35. Aerial photo of a site on the White River in Indiana showing the four registration points common to the 1937 aerial photo in Figure 34.
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98 STEP 5 - Once the banklines for each of the historic aerial photos have been traced, circles are best-fit to the outer bank of each bend to define the average bankline arc, the radius of curvature (RC) of the bend, and the bend centroid position (Figure 38)
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99 1937 RC Figure 38. Circles that define the average outer banklines from the 1937 aerial photo of the White River site in Indiana.
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100 STEP 6 - The position of the bend at a selected date in the future can be predicted by simple extrapolation if it is assumed that the bend will continue to move at the same rate and in approximately the same direction as it has in the past. To estimate the position of a bend centroid in 1998, for example, the distance the centroid would be expected to move during the 32 years between 1966 and 1998 can be determined by multiplying the annual rate of movement for the 1937 to 1966 period by 32.
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101 Flow 1 2 3 4 5 6 7 Figure 41. Aerial photo of the White River in 1966 showing the actual 1937 banklines (white)
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102 1 2 3 4 5 6 7 Levee Figure 42. Aerial photograph of the White River site in Indiana in 1998 comparing the predicted bankline positions with the actual banklines.
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103 Although the Federal standards for accuracy may seem reasonable, the true accuracy of topographic maps may be insufficient or problematic when using the comparison techniques for defining and predicting meander migration. For example, if the potential horizontal error of the topographic map used in the comparison is a significant percentage of the actual channel width, then there can be substantial error between the mapped bankline position and the true bankline position for the same time period and between time periods.
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104 coverage are optimal. Photos flown during early spring, prior to leaf-out, may be useful, but spring floods may obscure the tops of the banks.
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105 require at least 4 common registration points) and should not change significantly in size over time.
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106 The second method requires the use of the edge of water at the outer bankline of the channel on the photo or map. This should provide a relatively close approximation of the outer bankline radius of curvature.
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107 point along the bend. These four parameters represent different types of error in the prediction.
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108 1937 1969 1996 – Actual 1996 – Predicted 100 m 300 ft Figure 43. Channel migration comparison for the Tombigbee River near Amory, MS.
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109 measurable amount)
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110 10 100 1000 10 100 1000 Actual Bend Radius or Maximum Bank Migration (m)
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111 Figures 46 and 47 also show the maximum difference in bankline at any location along the bend. This is the most extreme measure of error for the methodology.
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112 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100 >100 Percentage of Channel Width Cu m ul at iv e Pe rc en t Error in Migration Maximum Error In Migration Change in Channel Width Figure 47. Error in bank migration relative to channel width (cumulative percent)
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113 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Rci/Wi R cn /R ci Time 1-3 (52 yr.
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114 greater change for longer time periods. The R2 for this data is 0.96, although this is really only a measure of how well the equation fits the mean trend of the data.
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115 C Sites y = 0.40x0.47 R2 = 0.12 0.1 1 10 100 10 100 1000 10000 Channel Width at Apex (ft)
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117 Table 8. Task 8 State Beta Test Status Participants.
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118 were made available to the evaluators. An enlarged black and white print (optimal scale = 1:27,000)
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119 YEAR 1 YEAR 2 YEAR 3 Figure 53. Banklines and circles inscribed on outer bankline positions for a hypothetical channel at 3 different years.
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120 Once the circles defining each bend for all years had been delineated, the evaluator used the changes in the radius of curvature and bend centroid position to predict the position and radius of the bend for some year in the future. In the case of the Sacramento River site, the position of the centroids and the radii of the three bends were predicted for the year 2028.
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121 The banklines were overlain in a separate file from the bend circles for later comparison and assessment. Once the bend circles were overlain and the bend centroids delineated, a scale was placed on the overlays based on the georeferenced Terraserver photo, which has a known scale.
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122 Computer Assisted – Minnesota River Site Table C3 provides a comparison of the results of this technique for each evaluator. Again, there are difference among the evaluators in measuring migration distance and radii for each bend, which can be explained by differences in judgement in delineating the banklines and fitting the outer bank circles.
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123 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 ARCVIEW Radius (ft)
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124 Figures 56 and 57 indicate that generally consistent results (within 20 percent) are to be expected between the three techniques for measuring and predicting bend radius and movement.
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125 As shown in the tables, the ArcView measurements produced slightly lower standard deviations than the manual techniques, so ArcView produces more consistent results. Also, measurements of bend radius had much lower standard deviations than migration distance, so measurements of radius are more consistent than migration distance.
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126 • PowerPoint and ArcView (were) a little frustrating at times.
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127 Chapter 5 – The data that needs to be measured with the data logger is presented without much rationale for why or how the data is used. Chapter 6 – The description of how to fit a circle to an odd shaped bend could be amplified more.
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128 • Measuring the wavelength and Amplitude was confusing. The start and end points for the wavelength and amplitude should be clearly defined.
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129 resulting prediction of the future bend location. During this procedure, only two steps are involved and no change of scale takes place.
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130 Manual Overlay Technique • It would be helpful to have more detailed instructions on justifying the three maps. It took a good deal of trial and error to produce maps that were of comparable scale.
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131 • The one exception was that there was some confusion when using the rate of change of migration angle equation. Computer Assisted Technique • MicroStation (CADD)
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132 • Rewrite section on Frequency Analysis and provide more explanation. • Clarify where and how to make critical measurements (e.g., amplitude, bend apex, channel width, wave length)
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133 Rhodes and Trent (227) document that $1.2 billion was expended for the restoration of flood damaged highway facilities during the 1980s.
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134 Implementation The Audience The target audience for the results of this research are hydraulic engineers and maintenance and inspection personnel in state, federal, and local agencies with a river-related responsibility. These would include in rough order or priority: • State Highway Agencies • Federal Highway Administration • City/County Bridge Engineers • State Departments of Natural Resources • U.S.
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135 audience. The numerous committees of the American Association of State Highway and Transportation Officials (AASHTO)
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136 Criteria for Success The best criteria for judging the success of this implementation plan will be acceptance of the methodology and techniques that resulted from this research by state highway agency engineers and others with responsibility for design, maintenance, rehabilitation, or inspection of highway facilities. Progress can be gaged by peer reviews of technical presentations and publications and by the reaction of state DOT personnel during presentation of results at NHI courses.
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