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3.7 STREAM STABILITY ANDINSTABILITYIn designing river enhancement and stream-rehabilitationprojects, the design engineer must recognize that rivers aredynamic systems, and must consider both, the existing andpossible future stream morphologies, in the design. The problem iscompounded when engineering interventions are planned becausethe future morphology of the stream depends not only on thenatural, or autonomous evolution of the system, but also streamresponse to construction, operation, and maintenance of theproject. For this reason, it is important for the design engineer toacquire a broad understanding of the current stability status of theproject reach and the extended stream network, and to use thisunderstanding to predict the type and extent of adjustments to thefluvial system likely to be triggered by the project. The capability topredict system response to the proposed works is vital in order toensure that the selected enhancement or rehabilitation measureswill work in harmony with both existing and future river conditions.The concept of stream stability status (which incorporatesinstability) builds on the basic geomorphic principles introducedpreviously and may be applied to the river at system and localscales.3.7.1 System StabilityThe geomorphic concept underpinning stability assessmentin rivers is that over time the cross-sectional dimensions andlongitudinal slope of the stream of an alluvial stream adjust so thatthe stream is able to convey the discharges of water and sedimentsupplied from upstream with no net change to hydraulic geometryor planform. On this basis, a stream may be classified as eitherstable or unstable depending on whether the stream has adjustedor is still adjusting to the flow and sediment regimes. Mackin(1948) expressed the stability concept in his definition of thegraded stream:A graded stream is one in which, over a period ofyears, slope is delicately adjusted to provide, withavailable discharge and with prevailing channelcharacteristics, just the velocity required for thetransportation of the load supplied from the drainagebasin. The graded stream is a system in equilibrium.By definition, a graded stream does not have to have astream that is static or fixed and it may exhibit temporarymorphological changes in response to the impacts of extremeevents. Alluvial stream morphology is certain to be affected bymajor floods or protracted periods of low water, but provided thetime taken for moderate events to restore the graded morphology(termed the recovery time) is shorter than the return period for theextreme event (recurrence interval) then the stream may be44 Fundamentals of Fluvial Geomorphology and Stream Processes
considered to be dynamically stable. The key attribute of a gradedstream is that fluvial processes operating under formative flowstend to restore stream morphology to the graded conditionfollowing disturbance, rather than perpetuating or amplifying thechanges imposed by the extreme event. A commonly used term forthis type of stability is dynamic equilibrium.The concept of dynamic equilibrium is inherent to a widelyapplied (and often mis-applied), qualitative relationship foradjustment in alluvial streams proposed by Lane (1955):QS ~ Q s D 50where: Q = water discharge,S = slope,Q s = bed material load, andD 50 = median size of the bed material.This relationship is commonly visualized as Lane's balance(Figure 3.10). Mackin's explanation of how a graded streamresponds to changes in the controlling variables is easily illustratedby Lane's balance, which shows how a change in any of the fourdriving variables will tend to produce a response in the others suchthat equilibrium is restored. When a stream is in dynamicequilibrium, it has adjusted these four variables such that thesediment transported into the reach is also transported out, withoutaggradation or degradation.Figure 3.10 – Lane’s Balance(after E. W. Lane, from Chorleyet al. (1985))Fundamentals of Fluvial Geomorphology and Stream Processes 45
Page 6 and 7: materials. Also, the threshold mobi
Page 8 and 9: ank of a large river. From an engin
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 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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