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Schumm et al. (1981, 1984) and their observations of incisedstreams in western Tennessee.The CEM (Figure 3.17) consists of five channel-reach types,which describe the evolutionary phases typically encountered in anincised channel. These evolutionary phases range from strongdisequilibrium to a new state of quasi-equilibrium. Quasiequilibriumimplies that the system is not static and changesthrough time, but over a period of years the average condition isone of stability. The model is based on the assumption thatmoving downstream through the system is equivalent to remainingin place and monitoring changes due to the passage of time. Theresponse at any given location in the channel can then bepredicted from the morphology of downstream channel locations.The channel reach types in the CEM are labeled Types Ithrough V, and are assumed to occur consecutively in thedownstream direction. The CEM assumes each channel type willoccur in turn at a given location as the channel evolves. The CEMchannel types are illustrated in Figure 3.17. Type I reaches arelocated upstream of the actively degrading reach and have not yetexperienced significant bed or bank instabilities. These reachesare generally characterized by U-shaped cross sections with littleor no recently deposited sediment stored in the channel bed.Type II reaches are encountered immediately downstreamof Type I reaches. Bed degradation is the dominant process inthe Type II reach. Type II channels are over-steepened reacheswhere the sediment transport capacity exceeds the sedimentsupply. Although the channel is actively degrading in a Type IIreach, the bank heights (h) do not exceed the critical bank height(h c ), and therefore, reach-scale geotechnical bank instability is notencountered.62 Fundamentals of Fluvial Geomorphology and Stream Processes
Figure 3.17 – Incised ChannelEvolution Sequence (afterSchumm et al. (1984))As bed degradation continues, the bank heights and anglescontinue to increase. When the bank heights exceed the criticalbank height for stability in Type III reaches, mass failures(geotechnical instability) begin. The dominant process in the TypeIII reach is channel widening. In places, the Type III reach maycontinue to be slightly degradational. However, the reducedsediment transport capacity resulting from the longitudinal channelslope decreasing combined with increased sediment supply fromupstream due to instability and from bank failures within the reachoften results in the initiation of sediment deposition on the channelbed.Type IV reaches are downstream of Type III reaches andrepresent the first manifestation of the incising channel returning toa new state of dynamic equilibrium. In the Type IV reach,geotechnical bank instabilities and channel widening may continue,Fundamentals of Fluvial Geomorphology and Stream Processes 63
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 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 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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