5 Full lecture 2006.pdf - Mountain Drainage Research Group Page


5 Full lecture 2006.pdf - Mountain Drainage Research Group Page

From Schumm (1963):ESS 426 5-6 Spring 2006

ESS 426 5-7 Spring 2006

A more detailed channel reach classification developed by Rosgen (1994) recognizes 7major and 42 minor channel types based on channel pattern, entrenchment, width-to-depthratio, sinuosity, slope, and bed material size. Although Rosgen (1994) demonstrated that hismajor channel types exhibit distinct roughness coefficients and hydraulic geometry relationships,the classification lacks a process basis. This shortcoming is emphasized by the lack of anyexplanation of the rationale underlying Rosgen's assessment of response potential for each minorchannel type. In contrast, Whiting and Bradley (1993) present a process-based classification forheadwater channels that associates channel morphology with the potential for debris flowimpacts, channel substrate size, and the processes and rates of fluvial sediment transport.Paustian et al. (1992) provide an excellent example of a valley-scale classification emphasizingregion-specific associations with channel morphology and processes.ESS 426 5-8 Spring 2006

Hierarchical ClassificationDifferent spatial scales are incorporated explicitly into some classification schemes. Theprinciple is common to many schemes but is made explicit here: the character of each level inthe hierarchy is dependent on the the context set by the higher level.ESS 426 5-9 Spring 2006

Other examples:From Washington State’s Timber, Fish, and Wildlife approach:Forcingfunctions:Responsevariables:ESS 426 5-10 Spring 2006

Montgomery and Buffington (1997)This paper established a process-based scheme for classifying channels, with the explicitintent of tying morphologic conditions to channel processes, particularly the role of debris flows,large woody debris, and sediment transport and deposition. It was developed explicitly formountain drainage basins, i.e. watersheds with steep bedrock headwaters that decline intoincreasingly broad alluvial valleys. Note that although this is a common pattern in watersheds itis not ubiquitous—and so, as with any other classification scheme, it is not universally applicable.Their different channel types are:• Colluvial Channels: The small channels that are wholly surrounded by colluvium (i.e.,sediment transported by hillslope processes such as creep or landsliding and not by streamtransport) that generally lie at the tips of the channel network.• Cascade Channels: The steepest of the alluvial channels, characterized by large clasts thatform the primary roughness elements and impose a strongly three-dimensional structure tothe flow. Tumbling flow around individual boulders dissipates most of the energy of the flow;bed morphology is disorganized with at most small pools that span a fraction of the totalchannel width.• Step-Pool Channels: Channels displaying full-width-spanning accumulations of coarsesediment that form a sequence of steps, typically one to four channel-widths apart, thatseparate low-gradient pools filled with finer sediment. The step-forming sediment is mobilebut only at very high discharges; in contrast, sediment in the pools can be rapidly flusheddownstream over the intervening steps. The spacing of the steps appears to maximize theflow resistance (Whittaker and Jaeggi, 1982) suggesting that this morphology is essential formaintaining a stable low-flow bed under slope and discharge conditions that would otherwisereadily transport sediment downstream. Both “free” and “forced” step-pool channels can beidentified, depending on whether alluvial (i.e., episodically transported) sediment orimmovable obstructions (e.g., bedrock or large logs) form the majority of the steps.• Plane-Bed Channels: Channels lacking well-defined bedforms and instead displaying long,and commonly channel-wide, reaches of uniform “riffles” or “glides.” In contrast to thesteeper channels any flow oscillation is generally horizontal, not vertical, but the lateralvariations are insufficient to produce pronounced meanders and associated pools.• Pool-Riffle Channels: The most common of the lowland stream channels, with laterallyoscillating flow producing a sequence of pools at the outside of bends with corresponding barson the inside of bends. In the relatively straight reach between each bend a more laterallyuniform riffle forms. Analogous to step-pool channels, the classification recognizes “free”ESS 426 5-11 Spring 2006

pool-riffle channels, where this distinctive morphology forms simply by virtue of the inertialcharacteristics of the water moving in a sinuous or meandering channel; and “forced” poolrifflechannels where the presence of pools is closely tied to obstructions, such as LWD, butwhere the removal of such obstructions could yield a morphology more closely akin to planebedchannels.• Dune-Ripple Channels: The classic lowland sand-bedded channels typical of large rivers,where the character of the predominant bedform will change in response to increasingdischarge from plane bed at low flows to ripples, sand waves, dunes, high-energy plane bed,and antidunes at highest flows.ESS 426 5-12 Spring 2006

CASCADE CHANNELS• Disorganized bed• Continuous supercritical flow (Fr ≥ 1)• Typically boulder bedded• Thalweg hard to identify• “Pool” frequency < 1 channel width• Transport capacity >>sediment supplyESS 426 5-13 Spring 2006

STEP-POOL CHANNELS• Sediment organized into channel-spanning steps• Flow alternates between supercritical andsubcritical flow• Typically gravel to boulder bedded• Pools 1-4 channel widths apart• Gravel in pool is annually mobile; steps mobileduring multi-decade flows• Transport capacity > sediment supplyESS 426 5-14 Spring 2006

PLANE BED CHANNELS• Flume-like• No organization to bed, but not chaotic• Relatively uniform grain size, typically cobblegravel• Pools are infrequent, around isolatedboulders or logs• Transport capacity ≈ sediment supply• Grain size can adjust to accommodatechanging sediment supplyESS 426 5-15 Spring 2006

POOL-RIFFLE CHANNELS• The “classic” fluvial channel• Alternate bars, meander bends, floodplains• Gravel bedded• Pools typically 5-7 channel widths apartESS 426 5-16 Spring 2006

DUNE-RIPPLE CHANNELS• Multiple-scale bedforms• Sediment mobile at lowflows• Typically sand bedded• Pools occur where forced bychannel pattern• Transport capacity

Relationships between M&B channel types, topographic gradient, debris flow initiation anddeposition, and role of large woody debris:ESS 426 5-18 Spring 2006

BedrockchannelsStep pool/cascadePool-riffle,plane bedESS 426 5-19 Spring 2006

ESS 426 5-20 Spring 2006

ESS 426 5-21 Spring 2006

FORCED MORPHOLOGIESFREE MORPHOLOGIESRemoval of forcing elements (e.g. LWD) can result in complex changes to channel morphology.ESS 426 5-22 Spring 2006

References on Channel ClassificationChurch, M. 1992. Channel morphology and typology. Pages 126-143 in P. Carlow and G. E. Petts,editors. The Rivers Handbook. Blackwell, Oxford, United Kingdom.Church, M., and D. Jones. 1982. Channel bars in gravel-bed rivers. Pages 291-338 in R. D. Hey, J. C.Bathurst, and C. R. Thorne, editors. Gravel-Bed Rivers: Fluvial Processes, Engineering andManagement. Wiley, Chichester, United Kingdom.Gilbert, G. K. 1877. Report on the Geology of the Henry Mountains. U.S. Geological and GeographicalSurvey, Rocky Mountain Region, Government Printing Office, Washington, D. C., USA.Gilbert, G.K. 1914. The transportation of débris by running water. U.S. Geological Survey ProfessionalPaper 86, Government Printing Office, Washington, D. C., USA.Henderson, F.M. 1963. Stability of alluvial channels. Transactions of the American Society of CivilEngineers 128:657-686.Horton, R. E. 1945. Erosional development of streams and their drainage basins; Hydrophysicalapproach to quantitative morphology. Geological Society of America Bulletin 56:275-370.Kellerhals, R., M. Church, and D. I. Bray. 1976. Classification and analysis of river processes. Journal ofthe Hydraulics Division, American Soceity of Civil Engineers 102:813-829.Leopold, L. B., and M. G. Wolman. 1957. River channel patterns: braided, meandering and straight.U.S. Geological Survey Professional Paper 282-B. U.S. Government Printing Office, Washington,D. C., USA.Mollard, J. D. 1973. Air photo interpretation of fluvial features. Proceedings of the Canadian HydrologySymposium. National Research Council, Ottawa, Ontario, Canada.Morisawa, M. E. 1957. Accuracy of determination of stream lengths from topographic maps.Transactions, American Geophysical Union 38:86-88.Montgomery, D. R., and Buffington, J. M. (1997) Channel Reach Morphology in Mountain DrainageBasins, Geological Society of America Bulletin, 109, 596-611.Paustian S. J., and 13 others. 1992. A channel type users guide for the Tongass National Forest,Southeast Alaska. Technical Paper R10-TP-26, U.S. Department of Agriculture, Forest Service,Alaska Region R10.Powell, J. W. 1875. Exploration of the Colorado River of the West and Its Tributaries. GovernmentPrinting Office, Washington, D. C., USA.Rosgen, D. L. 1994. A classification of natural rivers. Catena 22:169-199.Schumm, S. A. 1977. The Fluvial System. Wiley, New York, New York, USA.Strahler, A. N. 1957. Quantitative analysis of watershed geomorphology. Transactions, AmericanGeophysical Union 38:913-920.Whiting, P. J., and J. B. Bradley. 1993. A process-based classification for headwater streams. EarthSurface Processes and Landforms 18:603-612.ESS 426 5-23 Spring 2006

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