Methodology for Predicting Channel Migration (2004) / Chapter Skim
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11 CHAPTER 2 FINDINGS LITERATURE REVIEW Introduction The literature review process resulted in about 1,300 citations relating to the keyword 'meander.' However, initial examination of the titles, keywords and abstracts of the cited literature revealed that a great number of these articles were not directly relevant to this study, in general, and practical prediction of meander migration, in particular. In screening the large number of initial citations, reviewers sought to retain the key articles necessary to underpin a study of meander migration, and target the literature review on acquiring the knowledge contained in those articles for use when evaluating the relative merits of different prediction approaches.
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12 • Water flowing over ice • Ocean currents • Planetary jet stream • Channels carved by molten lava on the Moon • Sub-surface water flows on Mars The propensity for flowing fluids to meander indicates that this behavior is inherent to shear flows and cannot be attributed solely to local non-uniformity of sediment transport or bank erosion, although both are necessary for meandering in alluvial rivers. It is clear that meanders can form almost spontaneously given the right conditions.
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13 1970s, direct measurements of primary and secondary velocities at bends revealed that a small, counter-rotating cell may exist next to the outer bank (19)
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14 As bends evolve through time, they tend to increase in amplitude and decrease in radius. Tightening of the bend produces significant changes in the flow pattern.
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15 Changes in bend geometry and migration rate may be largely explained by the patterns of flow at bends and the way in which curvature effects first strengthen and later modify velocity and shear stress distributions as the bend evolves.
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16 the channel half-width. There are theoretical difficulties too.
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17 Figure 4. Simple mathematical functions used to represent meander shape.
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18 In light of this, it seems wise to characterize meander bend geometry by a simple function such as a circular arc, but recognize that the true bend forms in meandering channels will inevitably deviate and scatter around this simple representation. In fact, few natural meanders display a classic or idealized planform in any case, due to non-uniformity in the bed and bank materials (58)
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19 10 W λ ≅ 4 R λ c ≅ 2 W Rc ≅ 0.510Qλ ≅ where: λ = meander wavelength W = bankfull width Rc = radius of curvature Q = bankfull discharge Chang and Toebes (67, 68) investigated the effect of discharge on meander bend radius for two areas in the Wabash Basin with contrasting glacial histories and suggested: 2/1 arcm Q24R = (older)
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20 While indicative of a geometry that is common to meanders of very different scales and on rivers of different types, these equations have no basis in theory and are at best "morphological rules of thumb." It is no surprise then that the use of simple morphological relationships outside the area for which they were developed in river engineering and restoration schemes has been criticized by Rinaldi and Johnson (70)
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21 Figure 5. Brice classification of single-thread rivers based on the degree and character of sinuosity (adapted from (72)
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22 Brice applied his planform analysis and classification techniques to many practical problems: for example those associated with shifting of the Sacramento River (80) and channel response to artificial cutoffs (81)
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23 Figure 6. Schematic diagram showing in planform features and geomorphic surfaces associated with meander bends.
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24 Nanson and Hickin (90) and Hickin and Nanson (100)
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25 A further source of variability was revealed by observations of channel evolution on the Lower Mississippi River by Larsen and Shen (108)
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26 • Bank erodibility (ability of banks to withstand fluvial shear stress) • Bank geotechnics (bank stability with respect to slip failure)
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27 The scour resistance of material generally increases with grain size, but for very fine sediment erosion resistance and scour depth may be limited by cohesion. Rhoads and Miller (130)
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28 Des Plaines River in Illinois. The response of the river was minor and Rhoads and Miller attributed this in part to the high erosion resistance of the cohesive banks.
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29 Riparian Vegetation and Land-Use The influence of bank vegetation on meander migration has been recognized since the early 1980s. Hickin (144)
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30 The results of Murgatroyd and Ternan (152) are often quoted as challenging the generality that land-use change involving deforestation accelerates meander migration.
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31 Friedkin (11) conducted laboratory experiments to demonstrate the effect of bank revetments on meander geometry, showing how attempts to stabilize the outer bank in a meander led to deeper scour that tended to undermine the revetment.
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32 a1 b3 b2 b1 c d1 d2 e2e1 f a A fault or uplift steepens valley downstream of fault or axis of uplift (dashed line)
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33 The effects of most of these variables are shown diagrammatically in Figure 8. Each example is based upon observations made on rivers, and each numbered example of Table 1 is considered as follows: 1 A fault, which crosses the river and steepens it locally, will cause a change of sinuosity (Figure 8-a1)
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34 11,12,13 Human activities both upstream and downstream can significantly impact a river. Therefore, the highway engineer must consider future work on the river and changes of land use when evaluating meander impacts.
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35 Eventually, because of flow separation at the inner bank downstream end of the point bar, which effectively establishes a minimum resistance to flow, meander activity switches from primarily driving growth, to promoting downstream migration, which is referred to as translation (24, 29, 31)
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36 Conceptual and Empirical Models of Meander Evolution Conceptual Models of Meander Migration The evolution of a bend through time should be predictable, bearing in mind that channel migration has been shown to be a discontinuous process that is highly dependent on the occurrence of morphogenetically significant hydrological events (71, 88, 90, 91, 93)
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37 period between 1981 and 1986 in which two significant floods occurred (1983, 1986)
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38 Over 60 percent of future meander migrations could have been predicted from the characteristics of each individual bend in the initial channel pattern. Martin et al.
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39 Chang (172) produced a numerical water and sediment routing model (Fluvial-11)
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40 The work of Ligeng and Schiara (176) and Levent (177)
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41 Cherry et al.
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42 Technical Problems Related to Meander Measurement, Characterization, and Monitoring To apply any of the empirical or numerical methods to predict meander movement requires accurate measurements of meander planform. Measuring and characterizing meanders and meander migration are by no means straightforward tasks.
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43 In spite of evidence that the prediction of meander shift using numerical models is possible in principle, many difficulties remain unresolved with this approach. Most models require field calibration that demands unrealistic lead times before predictions can be obtained.
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44 evaluation of empirical relationships. Brice's data included the following: channel width, meander wavelength, sinuosity, gradient, valley slope, drainage area, erosion rate and some hydrologic data.
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45 of channels, there has been little general testing of these models over a range of hydrologic and geologic conditions. After testing the bend flow model for 26 of the meandering sites in the Brice data set, the Johns Hopkins study concluded that both the accuracy and applicability of the bend-flow meander migration model are limited by a number of simplifying assumptions.
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46 • Geomorphic methods - relying primarily on historic data and geomorphic investigations; • Engineering methods - relying primarily on predictive equations based on engineering and geomorphic principles, and • Mathematical modeling methods - relying primarily on computer modeling of fluvial processes. A Project Working Group (PWG)
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47 Evaluation of Channel Changes The FEMA study concludes that mathematical representation of fluvial fluid mechanics is difficult due to imperfect knowledge of the complex physical phenomena involved. The many attempts to modeling of fluvial processes have shortcomings largely due to the fact that sediment transport equations commonly overpredict or underpredict sediment loads by orders of magnitude of actual measured sediment transport rates.
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48 interpreters, planners, and mapping specialists. Some of these professions require advanced degrees in their specialties.
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49 Statistical analyses, typically regression, also involve uncertainty for a variety of reasons.
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50 material properties would need to be determined for the various strata comprising the bank. These properties include not only grain size and erodibility, but also mechanical properties such as shear strength, angle of internal friction and cohesion for varying soil moisture and saturation.
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51 median fall diameter, cohesion, effective cohesion, pore fluid salt concentration in the bank material, sodium absorption ratio of the bank material, dielectric dispersion of the bank material, unit weights of the bed and bank materials, angle and effective angle of internal friction, friction factor and Manning n, bed material porosity and ratio of tension crack depth to bank height. Many of these parameters can be reasonably estimated, but the amount of data required is still much greater than could normally be justified for the purposes of a meander migration estimate.
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52 that has bed and banks composed of sediment transported by the river. That is, the channel is not confined by bedrock or terraces, but it is flanked by a floodplain.
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53 Figure 10. Channel pattern classification devised by Brice (after (72)
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54 Figure 11. Modified Brice classification.
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55 REGRESSION ANALYSIS Introduction Data from the Brice (72) sites (including data from recent aerial photographs)
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56 proportion of the B2 and C bends (40 and 20 percent) show low rates of movement (less than 3 ft/yr)
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57 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 5 10 15 20 25 30 35 40 45 50 Apex Movement (ft/yr) C um ul at iv e Pe rc en t Brice A Sites Brice B1 Sites Brice B2 Sites Brice C Sites Figure 13.
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58 Table 2. Apex Movement Statistical Characteristics.
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59 Figure 15. Migration Rate (MR/W)
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60 C Sites y = 0.3965x0.4747 R2 = 0.1193 B2 Sites y = 0.0143x0.9834 R2 = 0.375 C & B2 Sites y = 0.115x0.6669 R2 = 0.1974 0.1 1 10 100 10 100 1000 10000 Channel Width at Apex (ft)
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61 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 Rc n/ Rc i Time 1-3 (52 yr.
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62 Using a frequency analysis approach relies on identifying the channel classification and applying a rate based on the frequency occurrence. The rates for the Brice classes and different probabilities are shown in Table 3.
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63 Table 4. Years to Migrate One Channel Width.
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64 In applying the frequency analysis approach, one could plot the 50 percent migration potential. If this amount would cause a problem for the structure, some countermeasure would be warranted.
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65 • Frequency analysis • Sources of error and limitations • Illustrated examples and applications using manual overlay techniques Chapter 1 provides an introduction to the Handbook and a discussion of a range of potential applications of the techniques 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.
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66 are not georeferenced for use with the Channel Migration Predictor can be found in Appendix D A glossary of terms used in the Handbook is provided in Appendix F
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67 Table 5. Sources of Contemporary and Historical Aerial Photographs and Maps.
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68 Extensive topographic map coverage of the United States at a variety of scales can be obtained from the local or regional offices of the U.S. Geological Survey (USGS)
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69 Map and Aerial Photo Comparison Techniques 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 determine the rate and extent of historic changes and assess potential future changes.
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70 scale of the photos. Common reference points can include constructed features such as building corners, roads, fence posts and intersections, irrigation channels and canals, or natural features such as isolated rock outcrops, large boulders, trees, drainage intersections, stream confluences, and the irregular boundaries of water bodies.
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71 The photos and banklines can also be geo-referenced and the associated data can be imported into a GIS. For example, for the data collection effort for this project, the bend characteristics and meander migration patterns for more than 1,500 bends on numerous rivers distributed across the continental United States were recorded using a digitizing tablet in conjunction with Bentley's MicroStation (see discussion of Archive Data Base)
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72 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. This rate data allows the Channel Migration Predictor to estimate the location of a bend at user specified times.
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73 The Excel spreadsheet format adopted for the data base permits cross-referencing based on data source, meander class, river name, or State. This provides a simple and useable approach to searching the data base.
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74 Figure 23. Example directory for the Archive Data Base.
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75 Figure 24. General Data spreadsheet containing the existing data and aerial photograph with bend locations for a site on the Brazos River, Texas.
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76 Figure 25. Bend spreadsheet containing measured and existing data for a bend on the Brazos River, Texas.
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