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the downstream limit of the system represents the origin of thethalweg profile. It follows from these facts that changes in baselevel have strong potential to trigger system instability.Base-level lowering, due to engineering interventions suchas meander cutoffs or channelization (Figure 3.12) triggersprocess-response by locally steepening the slope and increasingbed material transport capacity. As capacity exceeds supply, thebed scours to make up the deficit supply as the stream adjuststhrough degradation. This adjustment may generate only localinstability if armoring stabilizes the bed or a geological controlprevents significant bed lowering. However, if unchecked by alocal stream response or control, a wave of degradation migratesupstream through the system as a headcut or knickpoint. Ifdegradation triggers bank instability, then a wave of streamwidening may follow the headcut, generating further morphologicaladjustments and additional sediment input to the stream. As thedegradational wave moves upstream, the zone of increased slopeand additional sediment production moves with it. Sedimentsupply to the downstream reaches, coupled with local slopereduction due to upstream bed lowering, then triggers aggradation,which also migrates upstream through the system. Subsequently,sediment output and bed elevation at the downstream limit of thesystem display a damped oscillation until, following a number ofcycles of degradation/aggradation, the long profile is adjusted tothe new base level and stability is restored.Figure 3.12 – ChannelizedStream and Abandoned OldChannel52 Fundamentals of Fluvial Geomorphology and Stream Processes
3.7.4.2 Upstream FactorsThe stability of a fluvial system can also be significantlyaffected by changes to upstream reaches that alter thedownstream discharge or sediment supply. The flow regime andsediment load together, constitute the two main driving variablesresponsible for forming and maintaining the stream and it is nosurprise that the stability of an alluvial river may well be disturbedby changes in one or both of these factors. Upstream factors areoften affected by engineering and river-management projects andthe case of river regulation by a dam or diversion structure is acommon cause of downstream channel adjustment that serves toillustrate the types and complexity of system response that mayresult from such changes.Channel response downstream of a dam or diversionstructure depends on the way the works are constructed andoperated. When the structure is built, sediment supplydownstream may be elevated due to disruption of the channel andfloodplain during construction. This may increase supply overtransport capacity, inducing an initial adjustment throughaggradation. However, this response will be absent if appropriatesediment control measures are applied on site. Once the works arecomplete, process-response downstream will depend on thebalance of changes to the water and sediment regimes. Followingclosure of the dam or diversion, sediment is trapped in the poolupstream of the structure. Sediment-free water released from thestructure then scours the bed downstream, generating degradationin the first few kilometers below the dam. Initially, the flush ofsediment produced drives aggradation further downstream, but asthe stream slope immediately downstream of the dam flattens,sediment output decreases and the leading edge of the zone ofdegradation migrates downstream to re-erode recently depositedsediment and sends it further downstream, as an aggradationalwave. This conceptual model of river response to closure of a damhas been observed in many rivers, and yet this pattern ofadjustment is by no means universal. To explain why, it isnecessary to consider the other morphological responses that maydominate adjustment of the fluvial system. For example, if the beddownstream of the dam includes a widely graded, coarse-grainedfraction, bed armoring may limit degradation and stabilize the bedat a steeper slope than that prior to dam construction. The sameeffect may result from the presence of a geological control, whilewidening with limited reduction in bed level may be triggered if thestream banks downstream of the dam are close to the criticalheight for mass instability (Thorne and Osman 1988). If regulationby the dam significantly reduces the magnitude or frequency ofsediment transporting flows, degradation may be limited ornegligible, and if a reduction in the competence of the main streamis coupled with the input of a substantial sediment load fromunregulated tributaries, aggradation may occur downstream of theFundamentals of Fluvial Geomorphology and Stream Processes 53
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 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 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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