Channel Classification: Understanding Different Types of Rivers and Their Characteristics, Study notes of Geomorphology

Download Channel Classification: Understanding Different Types of Rivers and Their Characteristics and more Study notes Geomorphology in PDF only on Docsity! ESS 426 5-1 Spring 2006 LECTURE SECTION 5 Channel Classification Channel classifications use similarities of form and function to impose order on a continuum of natural stream types or morphologies: “The desire of geomorphologists to classify river channels can be explained as a means of reducing an extremely complex environmental feature into a series of discrete units which facilitate further study or help organize management operations. Classifications provide a weak form of explanation because all schemes involve a set of criteria which relate to an a priori expectation of the way in which researchers believe their river channels to be distinguished. As the criteria for any one classification scheme are unlikely to be generally applicable for numerous uses, designs for classification tend to be specific to the intended purpose of that scheme, and this is one of a series of fundamental attributes of classification outlined by Grigg (1967) , and summarized by Mosley (1987). Within fluvial geomorphology, the majority of river classifications have concerned natural river channel patterns, sub-dividing according to distinct morphological characteristics. These characteristics may indicate discrete physical processes for the particular channel category, thus facilitating an explanation of the resulting morphology” (Downs, 1995, in Gurnell and Petts, Managing River Channels). In plain English, this means that rivers are classified by how they look, with the expectation that appearance correlates with how they do (and will) behave. • Each of the channel classifications in common use has advantages and disadvantages in geological, engineering, and ecological applications -- no single classification can satisfy all possible purposes or likely encompass all possible channel types. • Channel classifications have been proposed a variety of purposes: o geomorphological (what the channel looks like, how it “works”) o biological (what types of organisms prefer it) o geological (relationship of channel and network form to geology) o engineering (relative stability) o aesthetic (by one standard or another….) We will primarily discuss geomorphological classifications applicable to a variety of spatial scales. ESS 426 5-2 Spring 2006 Some Thoughts on Channel Classifications (adapted from Montgomery and Buffington, 1997) Early geomorphological delineations of different types of channels focused on broad criteria (Powell 1875, Gilbert 1877), but recent classifications include more detailed consideration of channel pattern, bed material or mobility, sediment transport mechanisms, position within the channel network, and various combinations of slope and valley characteristics. Most geomorphological classifications are designed for large floodplain rivers, although Schumm's (1977) general delineation of erosion, transport, and deposition zones provides a conceptual framework within which to couple channel type and channel response potential throughout drainage basins. Degree of Degree of Sinuosity Braiding SS 1, 1 - Los 0. <5% RAI BET 2. 1.06 -1.25 1, 5 - 34% ~= se 2.35 - 659 3. >1.26 to —__- 3. SERS 3. >65% Character of Character of Sinuosity Braiding — eee a. Single Phase, Equiwidth a. Mostly Bars Channel, Deep ee Or; eo b. Singie Phase, Equiwidth b. Bars and Islands Channel, ~ os Mam = CQO t. Single Phase, Wider at c. Mostly talands, Bends, Chutes Rore Diverse Shape g 8 Se a 7 SSS d. Single Phase, Wider at Bends, Chutes Common IGA ©. Singte Phase, irrequlor Width Voriation d. Mostly Islands, Long end Narrow ESS 426 5-5 Degree of Anabranching Por Oo. <5% Vere 1. 5 - 34% PO-= 2. 35 - 65% 3. >65% . Character of Anabranching a, Sinvous Side Channels Mainly AA_A. b. Cutoff Loops Mainly =CAss c. Split Channel, Sinvous Anabranches —_—. d@. Split Channel, Sub- paraltel Ancbranches « Composite Spring 2006 From Schumm (1963): CHANNEL TYPE Suspended Load Mixed Load Bed Lood ‘ ae 3 x & ! 8 é ras 5 + 88 38 is Sa nas z [tt 338 « m 5x wets ast Fr 3S re bZi¢2 Hh 22 am|o. aw =o Woe 9433 = 23 AZ) 2% pag wa [Og tad Zwye 343 Zz /s age 5 LF = 36 uO Legend Ve a ($4 | —~— chennel Soundory a s BJe.| -T7A Flow 5 5 as ajoo to és @ |) 22| cee Bors 83 a é< HIGH —_— RELATIVE STABILITY Sad Low (3%>) Law=-— Bed Load -Total Load Ratia High (>11%) Small «——~ Sediment Size ———» Large ~ Small *—— Sediment Load ———* Large Low === Flow Velocity ——* High Low =~ Stream Power ——» High ESS 426 5-6 Spring 2006 ESS 426 INCREASING CHANNEL GRADIENT -——-—_—_-—______» DECREASING CHANNEL STABILITY -—-—___-—_-——_}» INCREASING SEDIMENT CALIBRE DECREASING CHANNEL STABILITY INCREASING SEDIMENT SUPPLY bed material supply dominated channels boulders, cobbles i step-pool cascade — wandering channels meandering channels anastomosed channels Wash material supply dominated channels 5-7 Spring 2006 ESS 426 5-10 Spring 2006 Other examples: From Washington State’s Timber, Fish, and Wildlife approach: Forcing functions: Response variables: ESS 426 5-11 Spring 2006 Montgomery and Buffington (1997) This paper established a process-based scheme for classifying channels, with the explicit intent 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 for mountain drainage basins, i.e. watersheds with steep bedrock headwaters that decline into increasingly broad alluvial valleys. Note that although this is a common pattern in watersheds it is 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 stream transport) that generally lie at the tips of the channel network. • Cascade Channels: The steepest of the alluvial channels, characterized by large clasts that form the primary roughness elements and impose a strongly three-dimensional structure to the 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 total channel width. • Step-Pool Channels: Channels displaying full-width-spanning accumulations of coarse sediment that form a sequence of steps, typically one to four channel-widths apart, that separate low-gradient pools filled with finer sediment. The step-forming sediment is mobile but only at very high discharges; in contrast, sediment in the pools can be rapidly flushed downstream over the intervening steps. The spacing of the steps appears to maximize the flow resistance (Whittaker and Jaeggi, 1982) suggesting that this morphology is essential for maintaining a stable low-flow bed under slope and discharge conditions that would otherwise readily transport sediment downstream. Both “free” and “forced” step-pool channels can be identified, depending on whether alluvial (i.e., episodically transported) sediment or immovable 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 the steeper channels any flow oscillation is generally horizontal, not vertical, but the lateral variations are insufficient to produce pronounced meanders and associated pools. • Pool-Riffle Channels: The most common of the lowland stream channels, with laterally oscillating flow producing a sequence of pools at the outside of bends with corresponding bars on the inside of bends. In the relatively straight reach between each bend a more laterally uniform riffle forms. Analogous to step-pool channels, the classification recognizes “free” ESS 426 5-12 Spring 2006 pool-riffle channels, where this distinctive morphology forms simply by virtue of the inertial characteristics of the water moving in a sinuous or meandering channel; and “forced” pool- riffle channels where the presence of pools is closely tied to obstructions, such as LWD, but where the removal of such obstructions could yield a morphology more closely akin to plane- bed channels. • 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 increasing discharge from plane bed at low flows to ripples, sand waves, dunes, high-energy plane bed, and antidunes at highest flows. ESS 426 5-15 Spring 2006 PLANE BED CHANNELS Flume-like No organization to bed, but not chaotic Relatively uniform grain size, typically cobble gravel Pools are infrequent, around isolated boulders or logs Transport capacity ≈ sediment supply Grain size can adjust to accommodate changing sediment supply ESS 426 5-16 Spring 2006 POOL-RIFFLE CHANNELS The “classic” fluvial channel Alternate bars, meander bends, floodplains Gravel bedded Pools typically 5-7 channel widths apart ESS 426 5-17 Spring 2006 DUNE-RIPPLE CHANNELS Multiple-scale bedforms Sediment mobile at low flows Typically sand bedded Pools occur where forced by channel pattern Transport capacity << sediment supply 100 T cumulative percent finer 0 100 200 300 grain size (mm) ESS 426 5-20 Spring 2006  valley segment colluvial alluvial bedro channel reach colluvial duoe-ripple pool-riffle plane-beu slep-poot sascade, bedrock Qc << Qs “ , Qe >> Qs (transport limited) {supply limited) cascade plane-bed dune-ripple 4 step-pool pool-riffle fa transport capacity (Qc) (sO) Ayddns yuawnpas drainage area supply limited transport limited ESS 426 5-21 Spring 2006 ESS 426 5-22 Spring 2006 FORCED MORPHOLOGIES FREE MORPHOLOGIES Removal of forcing elements (e.g. LWD) can result in complex changes to channel morphology.