chapter 3 fundamentals of fluvial geomorphology and stream ...

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