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ank of a large river. From an engineering perspective, it isparticularly important to recognize that analyses, techniques, andsolutions designed for one scale of stream may not be directlytransferable to another. Deciding whether an analytical tool,stabilization technique, or stream enhancement solution developedfor streams of a particular size is transferable across streams atother scales demands a thorough understanding of theunderpinning science and engineering principles involved. It is notenough to have demonstrated repeatedly that a given approachworks when applied to streams of a particular scale. Before tools,techniques, or solutions developed in one system scale arepromulgated for wider application, it must be established how andwhy they work. Principles, such as stabilizing a retreating banklineby increasing bank erosion resistance and mass stability orretarding near bank velocities, are transferable across differentscales of river; however, the hydraulic models, bank stabilityanalyses, and structural measures appropriate to control bankretreat successfully may not be scale-independent.3.3 STREAM MORPHOLOGYAlluvial rivers and streams are dynamic and continuouslychange position, shape, and other morphological characteristics inresponse to variations in discharge, sediment load, and boundaryconditions. It is, therefore, important to study, not only the existingmorphology of the river, but also the possible variations during thelifetime of the project. The morphology of the river is determined bythe water discharge, quantity and character of the sediment load,characteristics of the bed and bank materials (including vegetationeffects), geologic controls, and valley topography. Morphologicalchanges and adjustments take place in response to variations inany of these parameters through time or human activities. Topredict the behavior of a river in a natural state or as affected byhuman activities, we must understand how fluvial and geotechnicalprocesses operate on the boundary materials to form and adjustthe morphological features of the stream through time.A schematic diagram defining the morphological featuresassociated with straight and meandering streams is shown inFigure 3.3. The thalweg is the trace of the deepest point of thestream. The thalweg and associated line of maximum velocitycross from side to side within the stream, and this pattern of flowaffects the overall cross-sectional geometry of the stream. At abend, there is a concentration of flow in the outer half of the streamdue to secondary flow. This causes the scour depth to increase atthe outside of the bend, to produce a pool. As the thalwegcrosses the stream downstream of a bend, the velocity distributionand cross-sectional shape become more symmetrical and scour24 Fundamentals of Fluvial Geomorphology and Stream Processes
depths decrease due to deposition of sediment eroded from thepool upstream. This area is known as the riffle or crossing.Figure 3.3 – FeaturesAssociated with Straight andSinuous Rivers (FederalInteragency Stream RestorationWorking Group (FISRWG)1998)Pool-riffle sequences are characteristic of cobble, gravel,and mixed load rivers of moderate gradient (S < 5%) (Sear 1996).Riffles are topographic high points in an undulating bed profile andpools are low points. Typically, sediment grain size is coarser onriffles than in pools. A sorting mechanism was proposed by Keller(1971) to explain this variation. According to Keller (1971), finesediment is removed from riffles during low flows and deposited inpools because velocities and bed shear stresses are higher atriffles. As discharge rises, velocity and shear stress in the poolincrease quickly, with little, if any increase over the riffle.Consequently at the formative flow, velocities and shear stressesin pools are higher than that at riffles, resulting in scour of largesediment from the pools and deposition on the next riffledownstream. However, field evidence for this conceptualexplanation is equivocal. Ashworth (1987), Petit (1987), andClifford (1990) have measured the shear stress reversalhypothesized by Keller, but other studies have suggested that pooland riffle velocities equalize at bankfull flow, but do not reverse(Lisle 1979; Carling 1991).Yalin (1971) suggests that pools and riffles may beexplained by macro-turbulent eddies generated at the boundariesof a straight, uniform stream that produce alternate accelerationand deceleration of flow. Yalin showed theoretically that thelongitudinal spacing of faster and slower zones would average πw(w = stream width) for macro-turbulent eddies with diameterssimilar to the stream width. This is about half the riffle spacing of 5to 7 times the stream width observed in nature (Keller and Melhorn1973). Hey (1976) proposed a resolution to this differencebetween theory and observation by proposing that the largestFundamentals of Fluvial Geomorphology and Stream Processes 25
Page 6 and 7: materials. Also, the threshold mobi
Page 10 and 11: eddies in a stream do not scale on
Page 12 and 13: aiding. Braid bars are sediment fea
Page 14 and 15: Sinuosity (P) is a commonly used pa
Page 16 and 17: outcrops, other non-erodible materi
Page 18 and 19: continent (Inglis 1941, 1947, 1949)
Page 20 and 21: instances where the bankfull discha
Page 22 and 23: uses readily available mean daily f
Page 24 and 25: of flows. The results are plotted a
Page 26 and 27: Similarly, in regime theory the con
Page 28 and 29: 3.7 STREAM STABILITY ANDINSTABILITY
Page 30 and 31: It should be noted that the map coo
Page 32 and 33: and slope of the reach are not sign
Page 34 and 35: (a) Bed and Bank InstabilityFigure
Page 36 and 37: the downstream limit of the system
Page 38 and 39: dam, where degradation was expected
Page 40 and 41: Figure 3.13 - Channel PatternClassi
Page 42 and 43: definition of a graded stream in wh
Page 44 and 45: midwestern experience, while Rosgen
Page 46 and 47: Schumm et al. (1981, 1984) and thei
Page 48 and 49: ut at a much reduced rate. The sedi
Page 50 and 51: • integration of results into eng
Page 52: 68 Fundamentals of Fluvial Geomorph

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