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Least Action Principle, Equilibrium States, Iterative Adjustment and the Stability of Alluvial Channels
Abstract
The energy based least action principle (LAP) has proven to be very successful for explaining natural phenomena in both classical and modern physics. This paper briefly reviews its historical development and details how, in three ways, it governs the behaviour and stability of alluvial rivers. First, the LAP embodies the special stationary equilibrium state of motion and so its incorporation with the principle of energy conservation explains why so many optimizing hypotheses have been proposed in fluvial geomorphology. Second, the variational approach underlying the LAP provides a more straightforward and simpler fuzzy-object orientated method for solving river regime problems than do the various complex Newtonian formulations. Third, it is shown that in fluvial systems with surplus energy the surplus can be expended with slope and/or channel geometry adjustments, with the degree of channel geometry adjustment quantified by the dimensionless numbers F for depth dominated adjustment and H for width/depth dominated adjustment. Different planforms are preferred at different energy levels, with H providing a quantitative measure of the flow's efficiency for moving sediment. In rivers with insufficient energy, the interactions of endogenous and exogenous factors are shown to be capable, in certain circumstances, of achieving a stationary equilibrium condition which acts as the attractor state. Importantly, this study describes how iterative changes enable systems to achieve such a stable equilibrium. Copyright © 2007 John Wiley & Sons, Ltd.
... As a result, Recking et al. (2008) concluded that no equilibrium conditions could be obtained for flows with non-uniform material. In fact, this oscillation behaviour represents a dynamic equilibrium, which is commonly found in fluvial systems (Huang et al., 2004;Nanson and Huang, 2008;Phillips, 2011). Huang et al. (2004) and Nanson and Huang (2008) provided insightful reviews regarding the concept of dynamic equilibrium. ...
... In fact, this oscillation behaviour represents a dynamic equilibrium, which is commonly found in fluvial systems (Huang et al., 2004;Nanson and Huang, 2008;Phillips, 2011). Huang et al. (2004) and Nanson and Huang (2008) provided insightful reviews regarding the concept of dynamic equilibrium. They used a swing pendulum to illustrate the behaviour of a dynamic process and emphasized the importance of a stationary equilibrium state with the pendulum at rest. ...
... In a river where flow energy is dissipated as fiction without doing any additional work, and the energy is exactly sufficient to move the water and sediment supplied, the river is 'in stationary equilibrium'. When a river still has sufficient energy to perform additional work, the river system acts like a swinging pendulum and is 'in dynamic equilibrium' (Nanson and Huang, 2008). In this study, the oscillation behaviours of the slope and sediment transport characterize the dynamic equilibrium of the flume system. ...
Channel bifurcations are common features observed in fluvial landforms from upstream gravel‐bed rivers to downstream deltas. Extensive research has been carried out to study the stability and equilibrium configurations of bifurcations in the last two decades. However, existing studies generally employed oversimplified flow resistance equations, and the influence of the development of bedforms on the stability of bifurcations is rarely explicitly considered. The morphological features of bedforms in sand‐bed rivers vary with flow conditions which in turn affect flow resistance and sediment transport processes. Such a mutual feedback process is expected to exert a significant influence on the evolution of a bifurcation. A theoretical one‐dimensional (1D) model is built based on the classical model proposed by Bolla Pittaluga et al. (2003) to evaluate the influence of the development of dunes on the stability of bifurcations. Results show that the development of dunes enhances the stability of symmetric bifurcations for low Shields stress conditions, whereas it exhibits an opposite effect under the situation of high Shields stress. The response of flow resistance to the perturbations of the Shields stress dominates over the response to the perturbations of the flow depth. In addition, based on the two‐dimensional (2D) model proposed by Redolfi et al. (2016), this study compared the results of the 1D and 2D models taking into account the development of dunes, and evident differences between the models present at small Shields stresses. Meanwhile, 2D models with or without considering the influence of dunes exhibit different results as well. The results of this study demonstrate that considering the development of dunes is of significance when analyzing the stability of bifurcations in sand‐bed rivers.
... In terms of hydrodynamic relationships governing flow in alluvial channels, Huang and collaborators developed a fully rational equilibrium theory using a channel-shape based variational approach to explain the self-adjustment of alluvial river channels (Huang, 2010;Huang & Chang, 2006;Huang & Nanson, 1997Huang et al., 2004Huang et al., , 2014Nanson & Huang, 2008, 2017. The variational approach yields a curvilinear equilibrium relationship between channel geometry and bedload transport capacity, revealing an inherent optimum equilibrium condition termed maximum flow efficiency (MFE) where the imposed bedload within the channel can be moved using the least amount of energy. ...
... At this point, E H takes a constant value of approximately 0.3. When an alluvial channel has excess energy, it can adopt a dynamic equilibrium state, not that of stationary equilibrium, and E H takes a value either larger or smaller than 0.3 by adopting an anabranching, meandering or braided pattern in order to expend the excess energy and maintain channel stability, albeit not maximally (Nanson & Huang, 2008, 2017. Although it is evident that the value of the E H number is related to the development of different channel patterns, previous studies have focused mainly on the justification of the ideal state of 0.3 E H , and few of those studies have applied this non-dimensional number to understand channel-form adjustment where different channel patterns occur. ...
... Nanson & Huang, 2008. ...
Alluvial rivers commonly are subject to the integrated effects of hydrodynamic and eco‐geomorphologic controls and there has been a lack of suitable methods to measure the effects. By taking the alluvial reach of the Yellow River over the Yinchuan Plain as a suitable example, this study evaluates the degree of hydrodynamic control in the channel‐form adjustment of different channel patterns in light of the advances in equilibrium theory governing alluvial channel flow. In response to the significant variations in flow regime and channel forms, the non‐dimensional number H that measures the equilibrium state of alluvial channel flow varies in the ranges of 0.23–0.65, 0.047–0.17 and 0.0012–0.0024 respectively in the anabranching, meandering and braided reaches during 1993–2015. The significant differences among the H‐ranges are mainly because the individual anabranches have neither very narrow nor very deep cross‐sections, while the single‐thread channels in the meandering and braided reaches take moderately and significantly wider and shallower cross‐sections, respectively. These results demonstrate that the H number is a good discriminator of river channel patterns, and the relatively small variability of H within each channel pattern implies that the channels in the study reach are resilient to significant change in flow regime but yet hydrodynamic control is only partial. While the H number is capable of embodying the outcome of the complex integrated effects of multiple localized eco‐geomorphic controls with flow dynamics, more studies are required to define its specific varying ranges for different river channel patterns and differing eco‐geomorphic controls.
... All of these hypotheses indicate that the channel attains stability at the extreme point of the relationship curve ( Figure 3). The extremal hypotheses have been explained through the energy theory [23,24]. This states that the equilibrium condition should satisfy the conditions for equilibrium water and sediment transport. ...
... The change amplitude of the aspect ratio was between 10 and 100, and this change amplitude will have caused severe adjustment of geometries of cross sections. The extremal hypothesis has already been proven [23,24]. The field study in the NFTR also verified the extremal hypothesis [10]. ...
... The extremal hypothesis has already been proven [23,24]. The field study in the NFTR also verified the extremal hypothesis [10]. ...
In this study, the impacts of massive sediment input on channel geometry adjustment were analyzed across decades based on the downstream hydraulic geometry. Massive amounts of field data and evolution models showed that the alternation of degradation and aggradation in short-to-medium-term channel adjustment is common in evolving rivers. This phenomenon has always been challenging in research; most existing studies have focused on unidirectional adjustment in short-term channel adjustment. A few studies have considered the alternation of degradation and aggradation in short-to-medium-term channel adjustment, presuming that this phenomenon is caused by water and sediment changes. However, we found that the alternations also occurred under stable water and sediment transport in the North Fork Toutle River, southwestern Washington, USA. This adjustment across decades was analyzed by downstream hydraulic geometry in this study. It was concluded that the river consumes surplus energy to reach the optimal cross section through this short-to-medium-term adjustment under stable water and sediment transport. The objective of channel adjustment is minimal energy loss.
... Steady state is thus an attractor, with evolution tending toward a state of topographic adjustment to the climate and geologic framework. Steady-state equilibrium concepts in geomorphology have been widely discussed, debated, critiqued, and defended (e.g., Renwick, 1992;Thorn and Welford, 1994;Wainwright, 2006, 2008;Nanson and Huang, 2008), but the assumption of a tendency for landscape evolution to move toward this attractor appears in the literature regularly, right up to the present. These are generally (but often implicitly) assumed to be singular equilibrium states, which puts them in line with the bowling metaphor. ...
... Others may simply lack the necessary conditions for achieving equilibrium. For instance, Nanson and Huang's (2008) concept of achieving steady state in alluvial channels by interactive adjustments is based on mechanisms associated with dissipation of excess energy. They recognize that not all streams have excess energy to be dissipated and may thus be imbalanced and nonequilibrium. ...
... Ozawa et al. (2003) showed the equivalency of optimal principles related to atmospheric heat flux, global climate, fluid convection, and turbulent dissipation. Extremal principles related to hydraulic geometry have been shown to be consistent with respect to their fundamental hydrological and geomorphological implications, and Huang and Nanson (2000) (Nanson and Huang, 2008) indicate that all can be subsumed under a more general principle of least action (i.e., geomorphic work is performed with the minimum possible energy). ...
This chapter focuses on the (not necessarily final) destination of landscape evolution—the attractors that landscapes may move toward and the goal functions that govern these trajectories. Single-outcome concepts posit that landscape systems move toward a single self-perpetuating state. These include notions of progression toward climax or mature forms, stable equilibrium conditions, or self-organized critical states. Multi-outcome models include notions of alternative stable states, nonequilibrium systems, and unstable attractors. As they evolve, landscapes have plasticity defined by their degrees of freedom, and constraints imposed by limits on energy, matter, and geographical space. These can be described using concepts of a multidimensional resource or landscape evolution space. Goal functions for landscape evolution are generally based on increasing fitness, often assessed in terms of optimality hypotheses, which systems strive toward maximizing or minimizing some aspect of energy and/or mass flux. Many of these are directly or indirectly related to the least action principle and maximum entropy production. Apparent goal functions can generally be explained on the basis of emergent phenomena. Landscape systems cannot aspire to anything in a literal sense, and there exist no laws that dictate trends toward the optimal states. However, if these optimal states are associated with advantages in the formation, survival, and replication of landscape components, then trends toward the optima will frequently be observed. Emergence and general principles of selection can tie together the majority of the concepts of attractors and goal functions in landscape evolution.
... The dynamic equilibrium state of rivers is a very important concept in fluvial geomorphology and river engineering for it embodies the physical mechanism governing the complex interactions among river flow, sediment transport and channel forms. Although rivers in many circumstances can deviate from the state of dynamic equilibrium, the state determines the adjusting direction of river channel forms as it acts as an attractor in river systems [3][4][5]. In recent decades, many rivers encounter intensive human disturbances, such as riverbank reinforcement, dam construction, floodplain occupation for urbanization and agricultural development, logging practices and many more, and so it is urgent to know to what an extent these rivers deviate from dynamic equilibrium, or if they are capable of regaining dynamic equilibrium. ...
... As a result, stable equilibrium state is identifiable where sediment transport discharge reaches a maximum or energy gradient achieves a minimum. Importantly, the conditions of maximum sediment transport discharge and minimum energy gradient have been demonstrated to be the different realization forms of the general physical principle of least action in river systems [3][4][5][6][52][53][54][55]. ...
... It can be seen from Figure 2 that for each of the specific values of θ , an increase in channel width-depth ratio ζ from 10 to 1000 makes s Q increase gradually at the early stage, then reach a maximum and afterwards decline gradually. Importantly, it is seen clearly that in the situation of , flow has no excessive power or energy to expend, and so can take only a unique channel which is neither very narrower and deeper nor very wider and shallower [3][4][5][6]. In contrast, flow has excessive power or energy to expend in the situation of smaxm s Q Q < and so can take either a much narrower and deeper channel or a much wider and shallower one, because the channels of the two shapes can yield much larger boundary resistance. ...
Riverbanks vary considerably in anti-scourability and consequently take various profiles. By using an isosceles trapezoid as the generalized form of river channel cross-sections and then incorporating the effects of bank angle into the variational analytical approach developed by Huang and Nanson (2000), this study presents a detailed theoretical investigation of the self-adjustment of alluvial channel forms. It is demonstrated that when alluvial channel flow achieves stable equilibrium, a significant decrease in riverbank steepness leads to a slight decrease in maximum sediment (bedload) discharge, and yet results in a significant increase in optimal channel width and a considerable decrease in optimal channel depth. The hydraulic geometry relations, theoretically derived for bank steepness to vary across a wide range, show that among the multivariant controls, the roles of bed sediment size, channel roughness, flow discharge and sediment (bedload) discharge are independent of bank steepness. While the effects of bank steepness illustrated in the theoretically derived hydraulic geometry relations are highly consistent with the results of threshold theory and previous empirical studies, limitations on using bank angle to reflect the anti-scourability of natural riverbanks are also highlighted.
... The first step in this assessment is to determine whether the lower Yuba River fulfills the general morphologic (planform) requirements for anabranching. Nanson and Huang (2008) provide a classification scheme (Figure 40), which is a function of the equilibrium between the valley slope S and the minimum energy slope Se,min. In this context, the minimum energy slope represents the ratio between sediment load and discharge (Huang 2008). ...
... Nanson and Huang (2008) provide a classification scheme (Figure 40), which is a function of the equilibrium between the valley slope S and the minimum energy slope Se,min. In this context, the minimum energy slope represents the ratio between sediment load and discharge (Huang 2008). High sediment load and small discharge (transport capacity-limited streams) lead to high minimum energy slopes. ...
... Small sediment load and high discharge (sediment supply limited streams) lead to small values, which are in the upper part of the triangle shown in Figure 40. In addition to providing Figure 40, Nanson and Huang (2008) state the following: ...
The Yuba River Ecosystem Restoration Feasibility Study (YRERFS) was one of three ecosystem restoration project studies authorized by the U.S. Army Corps of Engineers (USACE) in 2014. Congressional authorization to initiate the study was granted through the Energy and Water Development Appropriations Act, 2014, Division D, P.L. 113-76. After completion of a reconnaissance study in 2014 the USACE, with Yuba County Water Agency (YCWA, now Yuba Water Agency – YWA) as the local non-federal project sponsor, initiated the YRERFS in 2015 to develop a contemporary, science-based assessment of the Yuba River and to identify and evaluate options to enhance habitat for anadromous salmonids, with an emphasis upon the federally-listed spring-run Chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss).
... What is an appropriate measure of fitness in Earth surface systems (ESS)? A number of hypotheses have been proposed for optimal structures and configurations of, e.g., ecosystems, channel networks, topography, etc. Most, if not all, of these principles can be reduced to (or subsumed under) the idea of efficiency or resistance selection--more efficient and resistant forms and structures are more likely to persist and grow, or to occur repeatedly, than less efficient ones (see Jorgensen, 1997;Fath et al., 2001;Ulanowicz et al., 2006 on ecological systems; Huang and Nanson, 2000;Nanson and Huang, 2008;2018;Paik and Kumar, 2010;Phillips, 2010;2011;Smith, 2010;and Huang et al., 2014 on geomorphic systems;andOzawa et al., 2003 andKleidon et al., 2013 on geophysical phenomena more generally). To the extent that "fitter" (more efficient and/or resistant) geomorphic features and structures are subject to not just preferential preservation, but also enhancement (via positive feedbacks) and/or propagation, then algorithmic evolution models may apply to geomorphology, too--implying the possibility of geomorphic creativity. ...
... Thus creativity and innovation in biological evolution was accompanied by creativity and innovation in fluvial systems. However, selection for maximum efficiency structures and configurations has been shown to operate for a number of abiotic phenomena, including atmospheric fluid flows (Ozawa, et al., 2003); development of subsurface flow networks (Hunt, 2016), and evolution of stream channel networks, planforms, and cross-sections (Huang and Nanson, 2000;Nanson and Huang, 2008;2018;Kleidon et al., 2013;Paik and Kumar, 2010;Phillips, 2010;2011;Smith, 2010;Huang et al., 2014). ...
... Straight, single-channel alluvial river channels are non-durable, low resistance featuresthey are easily transformed into more sinuous and/or multi-channel patterns. They are also inefficient with respect to energy dissipation (Nanson and Huang 2008;. They are relatively rare and short-lived, and thus subject to individual "extinction" as more efficient meandering or multi-channel patterns inevitably emerge. ...
In biological evolution, creativity occurs in the appearance of new entities by evolutionary dynamics. This is linked to mutations and genetic drift, which cannot occur in geophysical phenomena. Biota can exhibit evolutionary creativity that influences landforms, but how does creativity (defined here as the capacity for emergence of new entities that increase the adjustedness of the landscape to environmental conditions) occur in landforms and landscapes as entities independent of biota? Creativity in geomorphic evolution does not require any sort of goal functions or purposeful innovation‐‐just that geomorphic development is capable of producing novelties that may be better adapted (more efficient or durable) than predecessors. Independently of biota, evidence exists that landforms may develop to become more or less "fit" in terms of efficiency and/or durability. Thus emergence of novel features may lead to their persistence. Emergence of novel forms is illustrated for the case of karst sinkholes (dolines), which indicates increasing geomorphic diversity over Ma and Ga timescales. A case study of fluviokarst chronosequences in Kentucky demonstrates emergence and elimination of landforms as landscapes evolve. Some of these may represent generally (as opposed to locally) novel landforms. While this paper is more suggestive than demonstrative, results strongly suggest evolutionary creativity in geomorphology both tied to, and independent of, biological evolution. This occurs due to emergence of geomorphic entities that are subject to selection that tends to increase efficiency and durability.
... American Society of Civil Engineers (ASCE) Task Committee [8] has grouped regime methods into empirical (regime theory and power law), rational, or mechanistic and extremal hypotheses. Review of regime methods can be found in [9][10][11][12][13]. There is progressive development in the methods where old regime methods act as stepping stones for new methods which try to define dominant physical processes forming the regime channels. ...
... Similarly, rational methods also have many drawbacks, for example, only regime channel depth can be readily determined from Parker's model, which yields larger depth for smaller channels and smaller depth for larger channels. Moreover, Nanson and Huang [13] argued that rational regime models can only be used for stable profiles at an individual cross-section of a channel. Of late, Khodashenas [24] found that rational methods did not give satisfactory results in predicting regime channels and proposed an additional study of these methods. ...
... Although extremal methods have been called the illusion of progress [25], they have been used for stable canal design and to determine channel pattern [26], river response to land use changes, and channel changes [4], and to predict the equilibrium geometry of a river [27]. However, which extremal hypothesis should be used remains a topic of debate because there is no convincing theoretical explanation for any of the hypotheses [13,28]. ...
Regime channels are important for stable canal design and to determine river response to environmental changes, e.g., due to the construction of a dam, land use change, and climate shifts. A plethora of methods is available describing the hydraulic geometry of alluvial rivers in the regime. However, comparison of these methods using the same set of data seems lacking. In this study, we evaluate and compare four different extremal hypothesis-based regime methods, namely minimization of Froude number (MFN), maximum entropy and minimum energy dissipation rate (ME and MEDR), maximum flow efficiency (MFE), and Millar's method, by dividing regime channel data into sand and gravel beds. The results show that for sand bed channels MFN gives a very high accuracy of prediction for regime channel width and depth. For gravel bed channels we find that MFN and 'ME and MEDR' give a very high accuracy of prediction for width and depth. Therefore the notion that extremal hypotheses which do not contain bank stability criteria are inappropriate for use is shown false as both MFN and 'ME and MEDR' lack bank stability criteria. Also, we find that bank vegetation has significant influence in the prediction of hydraulic geometry by MFN and 'ME and MEDR'.
... Extremal principles related to hydraulic geometry (interrelationships between fluvial channels and the flows within them) are consistent with respect to their fundamental hydrological and geomorphological implications, and Huang and Nanson (2000;Nanson and Huang, 2008; argue that all can be subsumed under a more general principle of least action (i.e., geomorphic work is performed with the minimum possible energy). Phillips (2010) generalized this even further, contending that water flows will be more prevalent along more efficient rather than less efficient pathways, and that emergent feedbacks cause these paths to be preferentially preserved and enhanced. ...
... showed the equivalency of optimal principles related to atmospheric heat flux, global climate, fluid convection, and turbulent dissipation. Extremal principles related to hydraulic geometry (interrelationships between fluvial channels and the flows within them) have been shown to be consistent with respect to their fundamental hydrological and geomorphological implications, and Huang and Nanson (2000;Nanson and Huang, 2008) indicate that all can be subsumed under a more general principle of least action (i.e., geomorphic work is performed with the minimum possible energy). ...
... Many extremal principles are based on optimal outcomes as tendencies or probabilities and make no claims of determinism or inevitability (e.g., Smith 1986;Nanson and Huang, 2008;Dewar 2010;2013;Lin 2015). The mechanisms, however, are generally based on feedbacks that reinforce some outcomes and/or inhibit others. ...
This document is a collection of Jonathan Phillips’ Geoscience Blog posts from its inception (29 May 2014) through 2 July 2017. The major sections include (1) How it's Done; (2) Earth Surface System Theory 1: Equilibrium & Otherwise; (3)Earth Surface System Theory 2: Nonlinear Dynamics, Complexity, Self-Organization, Power Laws; (4) Earth Surface System Theory 3: Optimality & Selection; (5) Forest Biogeomorphology; (6) Climate & Sea-Level Rise; (7) Coevolution; (8) Rivers & Streams; (9) Environmental Management; (10) Geomorphology; (11) Soil, Regolith & Karst
... Ozawa et al. (2003) showed the equivalency of optimal principles related to atmospheric heat flux, global climate, fluid convection, and turbulent dissipation. Extremal principles related to hydraulic geometry (interrelationships between fluvial channels and the flows within them) have been shown to be consistent with respect to their fundamental hydrological and geomorphological implications, and Huang and Nanson (2000;Nanson and Huang, 2008) indicate that all can be subsumed under a more general principle of least action (i.e., geomorphic work is performed with the minimum possible energy). ...
... Many extremal principles are based on optimal outcomes as tendencies or probabilities and make no claims of determinism or inevitability (e.g., Smith 1986;Lapenis 2002;Nanson and Huang, 2008;Dewar 2010;Lin 2015). The mechanisms, however, are generally based on feedbacks that reinforce some outcomes and/or inhibit others. ...
... With respect to morphologies resulting from turbulent flows, Nanson and Huang (2008) considered feedbacks of slope in river channels. These feedback effects, through a series of iterative adjustments, nudge the fluvial system toward a steady state defined by transport capacity ≈ imposed water and sediment load. ...
Metanarratives are critiqued and even rejected by many geographers and geoscientists. Yet, despite the inescapable role of geographical and historical contingency in physical geography, metanarratives are helpful, perhaps even necessary, in part because equifinality is common in Earth surface systems (ESS). Similarity of forms and patterns implies a possible single underlying cause. However, by definition the similar outcomes of equifinality are not the result of the same underlying processes, indicating that any encompassing construct must be in the form of a metanarrative. An effective metanarrative need not be strictly true, but should be useful in explanation, and its implications subject to empirical verification. Metanarratives should also be simplifying rather than complexifying. An example proposed here is the principle of efficiency selection: the most efficient pathways and modes of mass and energy flux are preferentially preserved and enhanced. This explains and unifies optimality principles proposed for a variety of ESS. Efficiency selection is testable based on observations and simplifying in that it encompasses a number of situations with a single concise proposition. According to the principle of efficiency selection, apparent optimality in ESS is neither teleological nor deterministically inevitable, but rather an emergent property.
... As a result, Recking et al. (2008) concluded that no equilibrium conditions could be obtained for flows with non-uniform material. In fact, this oscillation behaviour represents a dynamic equilibrium, which is commonly found in fluvial systems (Huang et al., 2004;Nanson and Huang, 2008;Phillips, 2011). Huang et al. (2004) and Nanson and Huang (2008) provided insightful reviews regarding the concept of dynamic equilibrium. ...
... In fact, this oscillation behaviour represents a dynamic equilibrium, which is commonly found in fluvial systems (Huang et al., 2004;Nanson and Huang, 2008;Phillips, 2011). Huang et al. (2004) and Nanson and Huang (2008) provided insightful reviews regarding the concept of dynamic equilibrium. They used a swing pendulum to illustrate the behaviour of a dynamic process and emphasized the importance of a stationary equilibrium state with the pendulum at rest. ...
... In a river where flow energy is dissipated as fiction without doing any additional work, and the energy is exactly sufficient to move the water and sediment supplied, the river is 'in stationary equilibrium'. When a river still has sufficient energy to perform additional work, the river system acts like a swinging pendulum and is 'in dynamic equilibrium' (Nanson and Huang, 2008). In this study, the oscillation behaviours of the slope and sediment transport characterize the dynamic equilibrium of the flume system. ...
The assumption of the equilibrium state of gravel surfaces in flume experiments under feeding or recirculating conditions is generally justified by three equilibrium criteria based on sediment transport, slopes, and bed features. When these parametersbecome stable, an experiment is expected to reach equilibrium. This equilibrium state, however, is based on a one dimensional (1D) model, the Exner equation, which may not truly reflect the equilibrium state of the system considering the complex flow and sediment processes. In this paper, the evolutionary process of a gravel surface is investigated based on a large-scale recirculating flume experiment. The performances of the three equilibrium criteria are evaluated first, and then, the evolution of the bed morphology is studied. The key findings include the following: (1) the sediment transport rate, slopes of water and bed surfaces, and 1D morphological features reach equilibrium roughly simultaneously; (2) two-dimensional (2D) morphology continually evolves after these characteristics reach equilibrium, which is confirmed by the characteristics of the sediment transport process; and (3) the results from a numerical simulation suggest that a much longer time is required to reach an equilibrium state. Our results suggest that sufficient experimental time is required to investigate the equilibrium morphological characteristics of gravel surfaces, which is much longer than the equilibrium time reflected by the 1D equilibrium criteria.
... That is, ∕ ≤ 0 , where is the entropy production rate, " ∕ = 0 " corresponds to the steady nonequilibrium state, and " ∕ 0 " corresponds to a deviation from the steady nonequilibrium state. This principle has been very successfully used to explain natural phenomena in classical and modern physics (Huang et al., 2004;Jansen et al., 2004;Nanson & Huang, 2008). According to Nanson and Huang (2008), rivers are natural "machines" that consume energy to perform work. ...
... This principle has been very successfully used to explain natural phenomena in classical and modern physics (Huang et al., 2004;Jansen et al., 2004;Nanson & Huang, 2008). According to Nanson and Huang (2008), rivers are natural "machines" that consume energy to perform work. In river systems with surplus energy, the surplus energy can be consumed by adjusting the flow wave characteristics and area. ...
During two‐phase fluid displacement in a porous medium, self‐regulation mechanisms provide negative feedback to approach a state of minimum operating power. The versatility of these self‐regulation mechanisms results in a large diversity of fluid displacement patterns. For example, the introduction of a highly tortuous displacement channel, the appearance of the retention and development regions of the displaced phase, and the expansion of the displacement path area of the invading phase are the macroscopic responses of the two‐phase flow system to the self‐regulation mechanisms under different constraints. These new findings provide compelling physical explanations for the underlying mechanisms governing fluid displacement pattern transitions and the influence of inertial effects on the displacement patterns.
... Changes are numerous like geology, structure and composition of rocks, variations in climate, vegetation, altitudes, and most variable and dynamic force -discharge. All rivers try to achieve equilibrium in its course and in this process, they erode, eroded materials are transported to its fullest capacity, depending upon available sediment and when the sediment is more than the carrying capacity, they are deposited (Nanson and Huang, 2008) [19] . Rivers are continuously changing due to different factors like tectonic movement, climate change, human interference in the catchment and varying discharges. ...
... Changes are numerous like geology, structure and composition of rocks, variations in climate, vegetation, altitudes, and most variable and dynamic force -discharge. All rivers try to achieve equilibrium in its course and in this process, they erode, eroded materials are transported to its fullest capacity, depending upon available sediment and when the sediment is more than the carrying capacity, they are deposited (Nanson and Huang, 2008) [19] . Rivers are continuously changing due to different factors like tectonic movement, climate change, human interference in the catchment and varying discharges. ...
... Here, I develop a new approach to modeling river profiles that overcomes many of these limitations by leveraging an assumption of river channel optimality, which has been used to explain slope and width variations across a range of river systems [Turowski et al., 2007[Turowski et al., , 2009Nanson and Huang, 2008;. The approach presented here provides a computationally efficient method that only introduces one new variable, a channel width adjustment efficiency factor (kw). ...
... The finite-difference river profile model described in sections 2-2.2 incorporates an algorithm for dynamic channel width based on the assumption of river channel optimization [Turowski et al., 2007;Nanson and Huang, 2008;. ...
The evolution of rivers in eroding landscapes plays a key role in determining landscape relief and modulating climate-tectonic interactions. A common approach to quantifying river system evolution uses a one-dimensional, detachment-limited stream power equation. One potential drawback of this model is that it does not incorporate the effects of changes in channel width or the role of sediment transport dynamics. Here I present a new method for modeling the influence of channel width on river dynamics to explore how variable width and sediment transport impact river profile evolution. With this approach, vertical river erosion can operate based on any number of river erosion models, such as a simple shear stress model (e.g., detachment limited), sediment cover-shear stress hybrid models, or mechanistic saltation-abrasion models. I explore the sensitivity of these three models to increases in rock-uplift rate (i.e., 2, 3, 5, 10, and 20× increase). Generally, the results show that incorporating channel width adjustment or sediment transport dynamics lowers the sensitivity of a river profile to rock-uplift rate. For the sediment transport-dependent models, the degree of sensitivity depends on whether the system is limited by bedrock exposure or erosion potential (i.e., detachment potential). The approach produces transient responses that reveal distinct patterns of width and slope, which may provide valuable insight into the limiting physical mechanisms of bedrock erosion in a region. The implications of the work are broad and include the potential to distinguish underlying erosion controls from field observations of width and slope as well as understanding climate-tectonic interactions.
... Extremal/optimal hypotheses rely on principles of energy and power and are based on arguments in which some parameters are minimized or maximized through the adjustment of the basic hydraulic variables., These optimal or extremal principles have been shown to be mostly equivalent to each other (e.g., Davies and Sutherland, 1983;Phillips, 1991;Huang and Nanson, 2000;Nanson and Huang, 2008;Paik and Kumar, 2010). Nanson and Huang (2008) showed that the extremal hypotheses for fluvial hydraulic geometry are special cases of the LAP. ...
... These optimal or extremal principles have been shown to be mostly equivalent to each other (e.g., Davies and Sutherland, 1983;Phillips, 1991;Huang and Nanson, 2000;Nanson and Huang, 2008;Paik and Kumar, 2010). Nanson and Huang (2008) showed that the extremal hypotheses for fluvial hydraulic geometry are special cases of the LAP. The LAP applied to fluvial systems is, in turn, equivalent to a tendency to favor configurations of maximum efficiency Nanson, 2000, 2007;Huang, 2008, 2016;Phillips, 2010Phillips, , 2011Huang et al., 2014). ...
... However, this criterion is only valid if the conditions used in the analysis are not altered and if the channel is not subject to restrictions that prevent morphological and planform adjustments to occur. In a subsequent study, Nanson and Huang (2008) demonstrated that the principle of least action governs the behaviour and equilibrium stability of alluvial rivers. ...
... The main criticism of analytical modelling tools based on the use of extremal hypotheses is that they simply present a method of calculating steady-state channel dimensions while not suggesting a mechanism by which this is achieved, a fact that is also true for the statistical modelling approaches (Bettess et al. 1988) discussed previously. Hence these hypotheses involve an essentially metaphysical method of predicting steady-state channel dimensions which offers no explanatory power (Ferguson 1986;Nanson and Huang 2008). In addition, extremal hypotheses do not consider the dynamics of environmental systems (e.g. ...
... River morphodynamics depends on both natural and anthropogenic factors such as floods, river transport, hydraulic works (Nanson and Huang, 2008;Yang et al., 2015;Boudet et al., 2017;Hagstrom et al., 2018;. Changes in river morphodynamics can cause erosion or sediment deposition and they can have also consequences on coastal dynamics and longshore transport, causing erosions and advances even at considerable distances from river mouths (Komar, 2000;Perez-Arlucea et al., 2005;Tomasicchio et al., 2013;Barbaro et al., 2014;Li et al., 2014;Tomasicchio et al., 2015;Tomasicchio et al., 2018;Foti et al., 2019;Zainescu et al., 2019). ...
Sediment deposition is an issue that affecting many rivers. To oppose it, sediment withdrawals are often used. However, this practice can trigger further problems in river and coastal sections but, on the other hand, periodic removal of deposited sediments can reduce flooding risk. Also, these sediments can be used for beach nourishment. Therefore, before carrying out a sediment withdrawal, it is necessary to analyze the whole basin to evaluate the consequences for the sediment balance. The paper describes a methodology to evaluate the effects of river sediment withdrawals through a case study on the Amendolea River, a torrential river in southern Italy. In particular, the methodology is divided into three main phases. The first phase was developed with GIS software in order to perimeter and to morphometrically characterize the river basin and its hydraulically and sedimentologically homogeneous sub-basins. The second phase was developed using the HEC-HMS software in order to evaluate the hydrological balance of the basin and its sub-basins. The last phase was developed using the HEC-RAS software (through the SIAM model) in order to identify the areas in erosion, in deposition and in equilibrium. Moreover, through this model is possible to identify the sections where sediments can be withdrawn, and to estimate the possible morphological river changes due to withdrawals. In this case study, the sediments will be taken from the sections where the bottom has elevations near or above those of adjacent lands, such as to increase the flooding risk. Sediment withdrawals will be taken without eroding the bottom and at a distance from the river banks that does not stabilize them.
Finally, the methodology described in this paper is based on the joint use of various open source software and can be used in any other river with hydrological regime similar to the Amendolea River.
... I T is still something of a mystery why fluviatile sediments are more fully preserved in some river valleys than in others. One attempt to solve this problem is that of Nanson and Huang (2008) who used ideas derived from physics such as the principle of least action to help explain why certain types of alluvial channel are more stable than others and why alluvial sediments are better preserved in some sectors of a river than in others. It is therefore worth emphasising at the outset that any attempts to reconstruct prehistoric environments using alluvial sediments suffer from one huge disadvantage-the innately fragmentary nature of the fluviatile record (Williams, 2012a). ...
Pioneering archaeological surveys in the Son and Belan valleys of north–central India in the 1970s revealed that these valleys had been occupied at least intermittently during the Lower Palaeolithic, Middle Palaeolithic, Upper Palaeolithic and Neolithic. Later work in the early 1980s provided a reliable stratigraphic framework for the prehistoric sites and also resulted in the chance discovery of volcanic ash erupted from Toba volcano in Sumatra, Indonesia, 74,000 years ago. The discovery of the first Quaternary volcanic ash ever found in India prompted a search for the ash across the Indian sub–continent. By the early 1990s it was apparent that the entire sub–continent had been covered in a layer of ash 10–15 cm thick. Later work showed that some of this ash had been reworked by runoff and soil creep soon after deposition and accumulated in topographic depressions and valley bottoms where it remained in a relatively pure state. However, some of the ash has been reworked more than once since first deposited. Use of the ash as an isochronous marker depends upon establishing whether it is still in primary context and if not the degree to which it has been mixed with younger sediment. A key and as yet unresolved issue is whether or not the eruption had a major or minor regional and global environmental (including climatic) impact. High resolution records from speleothems, pollen spectra, varved lake deposits and ice cores will be needed to answer this question. The presence of similar stone artefact assemblages above and beneath the ash tells us nothing useful about the actual environmental impact of the eruption.
... Second, they highlight the strong influence of vegetation on many aspects of channel process and form, and so contribute to a growing body of hydroecological and ecogeomorphological research (e.g., Bennett and Simon, 2004). Third, they provide key lines of evidence for the energy-based, least-action principle that helps to explain the iterative adjustment and relative stability of different alluvial channel styles (Nanson and Huang, 2008). In these and other ways, study of dryland rivers is once again contributing to the building of general theories of river behavior, just as it did in the early development of fluvial geomorphological science. ...
The idea that dryland fluvial environments are somehow distinctive or even unique compared to fluvial environments in wetter and/or cooler climatic settings has arisen in much of the literature, despite the fact that drylands are themselves inherently diverse, covering a wide range of local climatic, tectonic, structural, lithological, and phytogeographical settings. Focusing on rivers in warm drylands, this chapter outlines the main characteristics of dryland rivers, including flow and sediment transport conditions, channel forms and dynamics, channel and floodplain sedimentology, and equilibrium and nonequilibrium behavior. Although dryland rivers are commonly ascribed a set of restricted characteristics that have been derived mainly from studies in short, steep, tectonically active catchments, investigations in larger, lower-gradient, tectonically stable catchments have revealed much greater diversity in river character both within and between drylands, and have demonstrated overlap with river characteristics in other climatic settings. With this global perspective, I argue that many previous statements regarding the distinctiveness of dryland river characteristics either can no longer be sustained as generalizations, or, at the very least, the geographical and geomorphological contexts for those generalizations need to be clarified. One of the key priorities for future research will be to identify and explain the spatial extent and frequency of occurrence of river styles across different drylands. In particular, research into the controls of meandering, straight, and anabranching rivers characteristic of some moderate-to low-energy Southern Hemisphere drylands has not only contributed to greater appreciation of the global diversity of dryland rivers but is also leading to a better understanding of rivers in general. Technological and methodological developments (especially in geochronology) are enhancing understanding of dryland river process, form, and change, and this will continue to facilitate interdisciplinary work in diverse scientific and applied contexts.
... In geomorphic and hydrologic systems, for instance, principles of gradient, resistance, and efficiency selection operate so as to make certain flux patterns more probable, and to reinforce them once developed. Examples include development of fluvial channels and channel networks, karst conduits, and soil water preferential flow paths (Hunt, 2016;Kaufmann, 2009;Leopold, 1994;Nanson and Huang, 2008;2017;Phillips, 2011a;Smith, 2010). ...
History matters. Global (independent of place and time) principles are necessary to explain landscape evolution, as are place factors (geographical and environmental context). But, by themselves, they are not sufficient. To explain landscape evolution—which by definition has important temporal dimensions—history must also be incorporated. Landscape evolution is historically contingent. This chapter outlines several different types of historical contingency, discusses multiple pathways and outcomes in landscape evolution, and evaluates why some imaginable pathways are rare and others common, and some trends are divergent and others convergent. Methods for evaluating path stability of historical trajectories are introduced and applied to several different Earth surface systems. The chapter concludes with a consideration of landscape evolution in the context of convergence, divergence, and equifinality.
... River and coastal dynamics depend on both natural and anthropogenic factors [9,10]. Regarding natural factors, the most important are wave motion [11,12] and river and coastal sediment transport [13][14][15][16][17]. Amongst the anthropogenic factors [18,19], the increase in anthropogenic pressure, observed in coastal areas over the last 50 years, has increased the vulnerability of the territory under the action of natural events such as floods [20][21][22], storms and coastal flooding [23,24], or a combination of these [25][26][27]. ...
Over recent decades, Soil Erosion by Water (WSE) has become a severe and extended issue affecting all European countries. The European Mediterranean countries are particularly prone to erosion because they are subject to prolonged dry periods followed by heavy erosive rains falling on steep slopes characterized by fragile soils. In particular, natural conditions and the human impact have made Calabria region, in southern Italy, particularly prone to intense WSE. The paper describes the methodology adopted to quantify solid transport in the Allaro river basin, located on the Ionian coast of Calabria. The methodology is divided into three phases, as follows: morphometric characterization of the river basin, identification of the available gauges and analysis of rainfall and temperature data and influence area of each gauge, and estimate of solid transport using the Gavrilovic model. This model is particularly valid for rivers such as the Calabrian ones where most of the solid transport is linked to the WSE. The methodology described in this paper is suitable to all basins with morphometric and hydrological characteristics like those Allaro and Calabrian basins. Furthermore, an improvement in the river transport estimate is important not only for the planning and management of the areas near the rivers but also for the coastal areas near the mouths.
... The robustness and universality of these scalings have long intrigued scientists, who have tried to discover the channel self-adjustment mechanisms hidden behind them using rational arguments (e.g. shear stress redistribution, see Parker, 1978) or the principle of maximization/minimization of an ad hoc functional (Davies & Sutherland, 1983;Nanson & Huang, 2008), a principle often criticized for being unphysical (Gray, Ghidaoui, & Karney, 2018). Combining various scaling relationships, Parker (1979) proposed the following bedload transport equation for a straight gravel-bed river: ...
By the late nineteenth century, the scientific study of bedload transport had emerged as an offshoot of hydraulics and geomorphology. Since then, computing bedload transport rates has attracted considerable attention, but whereas other environmental sciences have seen their predictive capacities grow over time, particularly thanks to increased computing power, engineers and scientists are unable to predict bedload transport rates to within better than one order of magnitude. Why have we failed to improve our predictive capacity to any significant degree? A commonly shared view is that the study of bedload transport has more in common with the earth sciences than hydraulics: bedload transport rates depend on many processes that vary nonlinearly, involve various time and space scales, and are interrelated to each other. All this makes it difficult to view bedload as merely particle transport in a turbulent flow – something which can be studied in the laboratory in isolation from the natural environment. Over the last two decades, more emphasis has been put on the noisy dynamics characterizing bedload transport. This Vision Paper makes a strong case for recognizing noise (e.g. bedload transport rate fluctuations) as an intrinsic feature of bedload transport. Improving our predictive capacities requires a better understanding of the origins and nature of noise in bedload transport. This paper also reviews some of the challenges that need to be addressed in current research and teaching.
... Huang et al. 2004 reviewed them in light of the "principle of minimum energy" and proposed ways to compute a theoretical optimal value for the width/depth ratio that would act as an attractor for a given discharge. They subsequently enlarged this approach, notably in Nanson and Huang 2008;Nanson and Huang 2016 with the "least action principle," attempting to propose a somewhat universal principle governing very long-term channel evolution based on a "mass-balance equilibrium". Although their working hypothesis was that the long-term form of a river channel mostly adjusts to convey the sediment load supplied to it, they agreed that flows having excessive energy, e.g., during a particularly high flood event, will adjust the channel geometry to dissipate it, usually by friction. ...
Steep gravel‐bed rivers sometimes experience floods that dramatically rework river bed structure and topography. Hazard assessments and paleo‐event reconstructions require better knowledge of such phenomena. This paper explores morphodynamic evolution of steep channels carrying bedload‐laden flows, using a generic Froude‐scaled model. Bedload‐laden floods were introduced in a narrow flume and spread into a five‐times wider unconfined area with a 0.1‐steep slope (m/m). Image analysis enabled measurements taken at an unprecedented level of accuracy on unconfined flows laden with bedload. A flow reconstruction procedure was used to compute depth, Froude (Fr) and Shields (τ*) numbers on millions of pixels based on a friction law and measurements of surface velocity, slope, and roughness. Despite the steep slope, Froude numbers proved to be mostly subcritical in all but the regions experiencing the most active sediment transport. Competent flows, identified by the transport stage higher than unity τ*/τ*cr>1, were near‐critical and seldom had Fr>1.3‐1.5. This demonstrates that, providing that bed width and structure can adjust, hydraulic features such as standing waves, hydraulic jumps and lateral shock waves dissipate energy very efficiently in addition to adjusting channel features. These competent flows also tend to rework channels to approach the energy minimum of near‐critical flows. This hypothesis was postulated by Gordon Grant (1997, Wat. Resour. Res. 33(2):349‐358), but demonstrated here for the first time at this scale. Considering near‐critical flows during discharges high enough to be clearly competent in laterally unconfined reaches seems reasonable as a first approximation in steep channels.
... Extremal principles related to hydraulic geometry (interrelationships between fluvial channels and the flows within them) have commonalities with respect to their fundamental hydrological and geomorphological implications, and Huang and Nanson (2000;Nanson and Huang, 2008; argue that all can be subsumed under a more general principle of least action (i.e., geomorphic work is performed with the minimum possible energy). Phillips (2010a) generalized this even further, contending that water flow will be more prevalent along more efficient rather than less efficient pathways and that emergent feedbacks cause these paths to be preferentially preserved and enhanced. ...
Understanding evolution of soils and landforms (and other Earth surface systems) has itself evolved from concepts of single-path, single-outcome development to those that recognize multiple possible developmental trajectories and different maturely developed states. Soil geomorphology and pedology should now move beyond showing that multiple trajectories are possible to investigating why some evolutionary pathways (EPs) are common and persistent, whereas others are rare and transient. A typology of EPs is developed and applied to soil formation in the North Carolina coastal plain. Some EP are impossible because of violation of generally applicable laws or absence of necessary conditions; others are currently impossible, having occurred in the past but requiring conditions that no longer exist. Improbable paths are possible but rare, because necessary circumstances involve rare events or boundary conditions. Inhibited EPs are also possible but rare because of resistance factors or feedbacks that prevent or inhibit them. Transient paths may be common but are not long-lived or well preserved and are thus rarely observed. Recurring but nonrepeating EPs occur in different locations but are irreversible in any given location and cannot recur except in the case of system-resetting disturbance or new inputs. Recurring EPs are not inhibited or self-limited, occur in different locations, and may be repeated because of ongoing or recurrent processes or conditions. Selected path types occur in multiple situations, but with increased probability due to feedbacks or responses that encourage or enhance recurrence and/or persistence. The case study shows examples of all possible EP types.
... In this paper, we analyze how basin area scales with estimated Bankfull Channel Depth (eBCD, herein defined as floodplain height above a low flow channel; see the following section), Active Channel Width (ACW), and Low Flow water Channel Width (LFCW). The existence of power relationships among discharge, channel depth and width has been extensively debated in the literature, part of which is referred to as hydraulic geometry (Gob et al., 2014;Leopold and Wolman, 1957;Nanson and Huang, 2008;Parker et al., 2007;Pistocchi and Pennington, 2006). The exponents of these Hydraulic Scale Laws (hereafter referred to in the text as HSLs) are influenced by geological and climate context and ultimately affect channel geometry and typology. ...
Remote Sensing (RS) technology has recently offered new and promising opportunities to analyze river systems. In this paper, we present a calibration of characteristic Hydraulic Scaling Law (HSL) using a regional database of river geomorphic features. We consistently linked discharge with channel geometry features for estimated Bankfull Channel Depth (eBCD), Active Channel Width (ACW), and Low Flow water Channel Width (LFCW), which are continuously available from RS data along the river course. We then used historical information and external sources of information on channel reaches that were relatively unaffected by human pressure over periods ranging from a few decades to a century (measured in comparable geographical areas) to infer relatively Unaltered HSLs (rUHSLs). Adopting rUHSL validated with available local historical evidence on channel geometry, we were able to assess historical changes in channel geometry consistently over the entire region and within the studied temporal window.
The case study was conducted for the Po basin in the Piedmont Region, northwest of Italy. From our analysis, it emerges that regionally 74% of the river network has riverbed incisions exceeding one meter, while 66% of channels have halved their historical widths with a total of 617 ha of land subtracted from the active channel. LFCW is, on average, wider in Alpine rivers compared to those located in the North Apennines.
Although it is currently not possible to measure the accuracy of these estimates, the evidence generated is coherent with available historical information, characteristic hydraulic scaling laws, evidence from relatively unaltered reaches and the available literature on local fluvial systems. This methodology provides robust, novel and quantitative information regarding decadal to secular channel changes that have occurred on a regional scale. This new layer of information enriches our ability to rationally address assessments of large‐scale past and future channel trajectories.
... In the latter cases, selection is generally based on the preferential preservation of phenomena that are more resistant, resilient (dynamically stable), or efficient (e.g. Leopold, 1994;Nanson and Huang, 2008;2017;Phillips, 2011;Twidale, 2004). 6 Pred's place model "Place as historically contingent process" is the title of a highly influential article by Pred (1984). ...
Nine axioms for interpreting landscapes from a geoscience perspective are presented, and illustrated via a case study. The axioms are the self-evident portions of several key theoretical frameworks: multiple causality; the law–place–history triad; individualism; evolution space; selection principles; and place as historically contingent process. Reading of natural landscapes is approached from a perspective of place formation. Six of the axioms relate to processes or phenomena: (1) spatial structuring and differentiation processes occur due to fluxes of mass, energy, and information; (2) some structures and patterns associated with those fluxes are preferentially preserved and enhanced; (3) coalescence occurs as structuring and selection solidify portions of space into zones (places) that are internally defined or linked by mass or energy fluxes or other functional relationships, and/or characterized by distinctive internal similarity of traits; (4) landscapes have unique, individualistic aspects, but development is bounded by an evolution space defined by applicable laws and available energy, matter, and space resources; (5) mutual adjustments occur between process and form (pattern, structure), and among environmental archetypes, historical imprinting, and environmental transformations; and (6) place formation is canalized (constrained) between clock-resetting events. The other three axioms recognize that Earth surface systems are always changing or subject to change; that some place formation processes are reversible; and that all the relevant phenomena may manifest across a range of spatial and temporal scales. The axioms are applied to a study of soil landscape evolution in central Kentucky, USA.
... Such measures have traditionally been used to understand or characterise the regional setting (Park 1977;Rhodes 1977), and the sediment transport dynamics or bank strength of a fluvial system, or to investigate more complex theoretical considerations of equilibrium fluvial forms (e.g. Yang 1971;Pickup 1976;Nanson and Huang 2008). Nanson et al. (2010) measured at-a-station and bankfull hydraulic geometry at 19 temporary gaging stations along three of the Barrington peatland channels. ...
... The dominant drivers of this balance (discharge, slope, sediment supply and size) were made conventional wisdom by Lane (1955), whose conceptual model has been a foundational reference for river researchers and educators for over 60 years. More recently, researchers have expanded the model to explain the qualitative responses of other variables such as width-to-depth ratio (e.g., Nanson and Huang 2008), sinuosity, and bedform amplitude (Dust and Wohl 2012), among other uses. Simply from the perspective of assessing the collective energy of a setting relative to its resistance, the framework can be amended to accommodate any alluvial setting by considering whether a variable contributes to or resists erosion (figure 2). ...
Despite large advances in the state of the science of stream ecology and river mechanics, the practitioner-driven field of stream restoration remains plagued by narrowly focused projects that sometimes even fail to improve aquatic habitat or geomorphic stability—two nearly universal project goals. The intent of this article is to provide an accessible framework that bridges that gap between the current state of practice and a more geomorphically robust and ecologically holistic foundation that also provides better accounting of socioeconomic factors in support of more sustainable stream restoration outcomes. It points to several more comprehensive design references and presents some simple strategies that could be used to protect against common failure mechanisms of ubiquitous design approaches (i.e., regional curves, Rosgen planform, and grade control). From the simple structure design to the watershed-scale restoration program, this may be a first step toward a more geomorphically principled, ecologically holistic, and socioeconomically sustainable field.
... Equilibrium and self-adjustment are the two basic influential theories of channel silting and scouring (Kennedy, 1895;Lacey, 1930;Nanson et al., 2016a;Huang et al., 2014). The ultimate purpose of self-adjustment of an alluvial river is to convey the water and transport the sediment downstream while maintaining relative equilibrium of the river, i.e., neither aggradation nor degradation of the river bed over time (Mackin, 1948;Nanson et al., 2008). ...
physical principles underlying HG behavior. One impediment is the complexity of the natural
... Afin de proposer des justifications physiques à ces relations géométriques, plusieurs travaux invoquent des hypothèses dites 'extrémales' ou 'd'optimisation' tels que la maximisation du transport sédimentaire, la minimisation de la friction ou de la pente Nanson and Huang, 2008]. Cette théorie est basée sur le 'principe de moindre action', dictant qu'un système soumis à plusieurs scénarii d'adaptation choisira toujours celui qui lui coûtera le moins en énergie. ...
Dans les chaînes de montagnes, les séismes de magnitudes intermédiaires à fortes (Mw>6) déclenchent systématiquement un grand nombre de glissements de terrain responsables de l'introduction de volumes massifs de sédiments dans le réseau fluviatile. L'évacuation progressive de ces sédiments hors de la zone épicentrale affecte la dynamique des rivières et provoque des aléas hydro-sédimentaires dans les plaines alluviales (avulsion des rivières, crues...). La quantification des transferts sédimentaires est essentielle pour mieux comprendre l'évolution des paysages à court et moyen terme (de l'heure au siècle) et permettre une gestion efficace des risques dans les zones d'accumulation. Cependant, les flux de sédiments grossiers étant difficiles à mesurer, les facteurs contrôlant l'évacuation des glissements de terrain restent à ce jour mal compris. Cette thèse a donc porté sur l'étude, via la modélisation, des paramètres influençant la mobilisation des glissements de terrain, la préservation de la capacité de transport la transition entre gorge et plaine alluviale et la dynamique court terme des cônes alluviaux soumis à de forts apports sédimentaires. Les approches développées sont appliquées au contexte de la côte Ouest de la Nouvelle Zélande où la probabilité d'occurrence d'un séisme de magnitude 8 est de 50% dans les 50 ans à venir. Cette problématique à été abordée analytiquement et via une approche numérique avec le modèle 2D d'évolution des paysages et des rivières, Eros. Avec l'approche analytique, nous démontrons que la conservation de la capacité de transport long terme à la transition entre gorges et plaines alluviales est généralement réalisée par le passage à un système en tresse. Nous identifions aussi la variabilité des débits comme facteur dominant de la capacité de transport long terme comparé à l'effet de la végétation riparienne. Avec l'approche numérique, nous utilisons Eros qui est composé 1. d'un modèle hydrodynamique 2D, 2. d'un modèle de transport/dépôt de sédiments et 3. de modèles gérant les flux latéraux d'érosion et de dépôt. La combinaison de ces éléments permet l'émergence de diverses géométries de rivières alluviales (droites/sinueuses ou en tresses) en fonction des forçages externes qu'elles subissent (débit d'eau, flux sédimentaires). L'application d'Eros à des cas naturels a nécessité la validation et la calibration de ses paramètres principaux à l'aide: 1. de solutions analytiques et 2. de la reproduction morphodynamique de systèmes naturels, tel que l'évolution de la rivière Poerua en Nouvelle Zélande suite au glissement de terrain du Mont Adams. Dans la partie aval du bassin, les simulations numériques démontrent les capacités du modèle 1) à prédire efficacement l'évolution de plaines alluviales soumises à plusieurs scénario d'apports sédimentaires massifs et 2) à générer des cartes de risques probabilistes. Dans la partie amont du bassin, les résultats mettent en évidence le rôle clef de la réduction dynamique de largeur des rivières par rapport à la largeur de la gorge fluviatile, sur l'accélération de l'évacuation des sédiments issus des glissements de terrain. Une loi unique caractérisant les temps d'export d'une distribution de glissements de terrain peut être définie en fonction du rapport entre volume de sédiment et capacité de transport initiale de la rivière, permettant ainsi d'estimer leur temps de résidence moyen à 5-30 ans pour un scénario de séisme de Mw=8 beaucoup plus faibles que ceux estimés précédemment (~100 ans). L'approche numérique développée dans ce travail suggère que l'étude de la réponse des chaînes de montagnes à un forçage sismique fort ne peut être effectuée efficacement qu'avec un modèle 2D capable de prendre en compte les non-linéarités entre écoulements des rivières, leurs géométries et le transport sédimentaire. Les résultats obtenus permettent une meilleure caractérisation de la dynamique des paysages à l'échelle du cycle sismique et des aléas à court terme.
... As noted earlier, a stable channel can be defined as one with no net change over the time interval being considered. Channel stability thus refers to the ability of a channel to resist changes in cross-sectional geometry, planform, or gradient during a specified time interval or to return to pre-disturbance conditions (Mackin 1948;Nanson and Huang 2008). A stable channel with a resistant boundary experiences relatively little net erosion or deposition during a large flood. ...
For more than 100 years, the ordinary high water mark (OHWM) has been used to define water boundaries in a number of contexts in the United States. This Special Report summarizes the scientific literature pertaining to the indicators used to identify the OHWM in fluvial systems, building on more than a decade of research and publications related to the OHWM in the ongoing process to implement the Clean Water Act and the Rivers and Harbors Act of 1899. This report does not change or redefine the indicators used to identify the OHWM, nor is it a manual for how to delineate the OHWM. This report first reviews established concepts in river science that relate to the OHWM then reviews various sources of information that can be used to delineate the OHWM, discusses geographic variations in OHWM indicators among river segments, reviews human activities that can affect the OHWM, and finally presents examples of the OHWM in diverse channel types and regions.
... Several scientists have proposed various relationships for predicting stable channel dimensions. In general, there are three methods of determining stable channel dimensions: (1) empirical relationships and the regime method (Leopold and Maddock, 1953; Farias et al., 1998), (2) the mechanical method of tensile force (Glover and Florey, 1951; Lane, 1955), and (3) extremal or limiting methods (Huang and Nanson, 2000; Singh et al., 2003; Nanson and Huang, 2008). Some of these theories are independent while some follow completely different principles than others. ...
Predicting the geometry of channels and alluvial rivers is of primary importance in river engineering science. Appropriately designing channels and predicting stable river cross sections can decrease costs and prevent the destruction of installations and agricultural land by rivers. Consequently, researchers have applied different empirical and regression methods to achieve relations for predicting stable channel and river geometry. In this study, Group Method of Data Handling (GMDH) models are used to predict three geometric variables of stable channels viz. width (w), depth (h) and slope (s). The effect of different input parameters, such discharge (Q), median grain size (d50) and the Shields parameter (τ*) on the GMDH models is assessed with regard to predicting stable channel geometry. The results indicate that the GMDH model with mean absolute percentage error (MAPE) of 5.53%, 4.05% and 4.89% for channel width, depth and slope prediction respectively, exhibits good accuracy. Moreover, a comparison of the GMDH models with previous theoretical equations (based on regression analysis) indicates the superiority of GMDH model performance, with error reductions of one-fifth, one-eighth and one-sixth compared to the regression equations for channel width, depth and slope prediction respectively. This article is protected by copyright. All rights reserved.
... 第 7 期 解哲辉 等：游荡性河流演变规律研究进展及其河型归属探讨 中国的黄河下游上部花园口-高村河段， 国内 大多数研究认为其应属于一种独特的河型， 即游荡 型， 英语分别称之为 "wandering" (Huang et al, 2004;王光谦等, 2005; 王卫红等, 2013)、 "wandering braided" 等 (Xu, 1990;陆中臣等, 2000)， 但也有一些研究 将其归类为辫状河型(braided) (钱宁等, 1961;王随 继, 2003;吴保生等, 2003;Xia et al, 2008 Schumm, 1963, 1977王随继等, 1999Leopold et al, 1957方宗岱, 1964Rust, 1978Lane, 1957Chang, 1979aCarling et al, 2014钱宁, 1985Schumm, 1985Huang et al, 2004Nanson et al, 2008 (Lane, 1957;Leopold et al, 1957) (Rust, 1978;Brice, 1984;Schumm, 1985;Miall, 1996;Nanson et al, 1996)。有研究注意到在平滩等大流量条件下， 辫状河道心滩被淹没， 河道呈现出单流路河道特征 (Chitale, 1970;Kellerhals et al, 1976;Thorne et al, 1993)。Ashmore (1991) (Leopold et al, 1957;Parker, 1976;Chang, 1979b;Wang et al, 1989;Knighton, 1998;Millar, 2000;Surian, 2015 G Q, Zhang X H, Li Yong, et al. 2000 Huanghe xiayou shuisha bianhua ji hedao yanbian fenxi [J]. Yellow River, 22 (9) [ Wang G Q. 2007. ...
... This hybrid model combines the physics-based thinking of regime theory with the empirical basis of hydraulic geometry, and further emphasizes the utility of both approaches. Work by Huang and Nanson (2008) uses the least action principle to develop an alternative system for predicting channel dimensions. ...
Rivers respond to environmental changes such as climate shifts, land use changes and the construction of hydro-power dams in a variety of ways. Often there are multiple potential responses to any given change. Traditionally, potential stream channel response has been assessed using simple, qualitative frameworks based largely on professional judgement and field experience, or using some form of regime theory. Regime theory represents an attempt to use a physically based approach to predict the configuration of stable channels that can transport the imposed sediment supply with the available discharge. We review the development of regime theory, and then present a specific regime model that we have created as a stand-alone computer program, calledthe UBC Regime Model (UBCRM). UBCRM differs from other regime models in that it constrains its predictions using a bank stability criterion, as well as a pattern stability criterion; it predicts both the stable channel cross sectional dimensions, as well as the number of anabranches that the stream must have in order to establish a stable channel pattern. UBCRM also differs from other models in that it can be used in a stochastic modelling mode that translates uncertainty in the input variables into uncertainty in the predicted channel characteristics. However, since regime models are fundamentally based on the concept of grade, there are circumstances in which the model does not perform well. We explore the strengths and weaknesses of the UBCRM in this paper, and we attempt to illustrate how the UBCRM can be used to augment the existing qualitative frameworks, and to help guide professionals in their assessments. This article is protected by copyright. All rights reserved.
... The abundance and coarse texture of the sediment requires high energy to transport the bedload: the channel geometry makes the best use of the limited stream power. In fluvial process studies, low-sinuosity rivers are associated with low slope, low stream power, and low capacity for sediment transport (Tooth 2000a, Nanson andHuang 2008). In the past, sand bed rivers were considered to be typical of deserts (e.g. ...
In this study, the landforms of Australia's desert country are assessed against National Heritage criteria. The landforms are grouped and discussed according to their dominant geomorphic theme:
• astroblemes (impact structures);
• sand deserts (derived from aeolian sediment transport) (sub-themes: basins; ridge-valley);
• karst (created by the dissolution of soluble rocks);
• arid coasts;
• tectonic landforms (sub-themes: flexure; faulting; diapirism);
• uplands;
• regolith (sub-themes: vertisol plains and slopes; saprolite; silcrete and gibber plains; other duricrusts);
• watercourses (sub-themes: sand-bed rivers; discontinuous ephemeral streams; anabranching rivers; mud-aggregate floodplains; freshwater basins; mound springs; banded vegetation sheetflow plains; floodouts;
palaeodrainages; megaflood landforms; playa lakes and megalakes; post- European drainage incision).
Twenty eight sites or indicative areas were identified which have either met the NHL criteria thresholds, or have been judged in this study to have potential to do so (Table 1, Fig. 1). Site clusters occur in the Amadeus Basin and the Lake Eyre Basin. Site or area NHL value is variously assessed as clear, likely, or probable (pending further investigation). The identification of these places does not, in itself, constitute a nomination, it is a recommendation that they be considered. The study area is generally under-researched, and particular knowledge gaps are identified in the Davenport-Murchison Ranges and the Great Sandy and Great Victoria Desert dunefields. Aridity, which is widely regarded as a defining characteristic of the Australian inland,
is only the latest imprint of a long series of processes. It is now the most important condition modifying the landscape, but is not the reason that this landscape exists.
The key drivers which have created the unique Australian desert landscapes are the length of time that the stable landscape has existed, the previous climates which have operated on that landscape, the development of aridity in geologically modern times, and the high degree to which these landscapes display features inherited from the past. These drivers are the context within which operate the agents that work upon
the landscape: water, wind, gravity, plate tectonics, chemical reactions, and living things.
... The supply of bed material sediment to an alluvial channel accelerates the growth of longitudinal, transverse, and point bars, thereby enhancing thalweg development and locally increasing flow velocity . Non-equilibrium between sediment supply and transport induces local channel instability, accentuating either bed erosion or deposition (Jansen and Nanson, 2010; Nanson and Huang, 2008 ). In this study, a channel stability gradient accords with both sediment movement and vegetation cover, wherein bedload transport capacity (a function of bed slope, hydraulic geometry, and sediment particle size) is related to the influence of riparian vegetation upon channel geometry/planform . ...
The influence of vegetation upon bedload transport and channel
morphodynamics is examined along a channel stability gradient ranging from
meandering to anabranching to anabranching–braided to fully
braided planform conditions along trunk and tributary reaches of the Upper
Yellow River in western China. Although the regional geology and climate are
relatively consistent across the study area, there is a distinct gradient in
the presence and abundance of riparian vegetation for these reaches atop the
Qinghai–Tibet Plateau (elevations in the study area range from 2800 to 3400 m a.s.l.).
To date, the influence of vegetative impacts upon channel planform
and bedload transport capacity of alluvial reaches of the Upper Yellow River
remains unclear because of a lack of hydrological and field data. In this
region, the types and pattern of riparian vegetation vary with planform type
as follows: trees exert the strongest influence in the anabranching reach,
the meandering reach flows through meadow vegetation, the
anabranching–braided reach has a grass, herb, and sparse shrub cover, and
the braided reach has no riparian vegetation. A non-linear relation between
vegetative cover on the valley floor and bedload transport capacity is
evident, wherein bedload transport capacity is the highest for the anabranching
reach, roughly followed by the anabranching–braided, braided, and meandering
reaches. The relationship between the bedload transport
capacity of a reach and sediment supply from upstream exerts a significant
influence upon channel stability. Bedload transport capacity during the
flood season (June–September) in the braided reach is much less than the
rate of sediment supply, inducing bed aggradation and dynamic channel
adjustments. Rates of channel adjustment are less pronounced for the
anabranching–braided and anabranching reaches, while the meandering reach is
relatively stable (i.e., this is a passive meandering reach).
Dams are built on rivers for diverse purposes, where the most are associated with Multi-purpose river valley projects. The river water is blocked to use it for domestic purposes, irrigation, industries, navigation, and hydroelectricity. Above all, most of these projects set a common “welfare” target of managing soil erosion. However, the effort of stagnating the natural flow of alluvial channels sets ample examples where adjustment of upstream channels and consequent lateral shifting exacerbates massive threats of riverbank erosion. This chapter discusses this particular phenomenon experienced in the different parts of Indian landmasses in its first part. The latter part is specific instead. The study has been carried out to examine the channel adjustment of river Ganga upstream of Farakka Barrage up to Sahebganj with the spatial extension between 24° 45′ N and 25° 30′ N latitudes and 87° 30′ E—88° 30′ E longitudes. River Ganga, the principal stream of the great Ganga basin at the Himalayan foreland, carries a substantial sedimentary load to the lower deltaic plain in Bengal. Near Rajmahal Hills, after crossing Sahebganj, the natural downstream adjustment in terms of channel slope, channel planform, and cross-section morphology of the river exhibits a drastic change in response to the modification of the channel gradient as a consequence of the human-induced changes in hydrology and sedimentary modification as well as a degree of base-level modification with the construction of the Farakka Barrage in Malda. The plateau basalt province of lower Cretaceous to the Jurassic age at the western margin and the old alluvium Barind tract in the eastern margin of the Ganga valley has a typical role. The tectonic tilting and geological settings of Rajmahal traps, which have long been a potential force of a rapid eastbound shift of the channel, are further stimulated by the Farakka Barrage. The geostrategic decision to select sites for a large Dam overrides the geomorphic and geologic realities that crave out the left bank erosion of the river Ganga in the Malda District, bringing a massive threat to human life and resources.KeywordsLateral shiftingChannel adjustmentRajmahalGeologyMeandering
No single approach effectively synthesizes analysis of river channel form. The diversity of river channels renders research aims and site specificity critical to any investigation, framed within an open-ended approach to enquiry. This inevitably entails a trade-off between resolution, scale, and time: information collected must reflect the specific needs of the task at hand at a given site. In this article, investigative techniques used to assess river channel form are appraised in relation to channel planform, cross section, long profile, and three-dimensional morphology. Analysis of three-dimensional forms provides the most rigorous approach to link form adjustment to channel processes, enabling predictive modeling of river behavior.
Anastomosed rivers include a diverse group of fluvial forms with multiple and interconnected alluvial channels. They occur in a wide range of climates and reliefs and constitute the typical pattern of the mega rivers (Qm> 17,000 m3s-1). Several authors have assumed that the development of an anabranching formation is the mechanism by which a channel maintains its maximum efficiency. However, this has only been tested by mathematical studies. In this study, we confirm this theory in a 58-km anabranching reach of the Upper Paraná River, through a hydraulic and bathymetric survey of 14 cross-sections. We conclude that where the slope can'tincrease, the channel reduces its w:d ratio, thereby maintaining its efficiency in the wider sections, according to the specific stream power equation (ω=ρgQs/w). The channel width and alluvial plain width are controlled by allochthonous factors (tectonic lineaments and bank composition). Although the nodal sections have the greatestefficiency, we confirmed that flow efficiency is maintained at a constant level of bedload transport and specific stream power along the anabranching reach.
Darwinian natural selection acting on individuals is one of only several types of selection influencing landscape evolution. Ecological filtering and abiotic selection (including the least action principle and preferential flows) apply. The overarching principle is one of efficiency selection, whereby more efficient, stable, and durable forms, structures, patterns, networks, and flux pathways are more likely to occur, grow, and persist than less efficient ones. Particularly important forms are gradient selection, favoring steeper and faster flow paths; resistance selection, whereby more resistant features are preferentially preserved; biogeochemical selection, which favors more rapid elemental cycling; network selection, which makes more efficient flux and interaction networks more likely; and thermodynamic selection, reflecting the advantages of energy use efficiency. Efficiency selection is highly local, however, one of several reasons that landscapes and environmental systems are not always inevitably becoming more efficient overall. A case study illustrating selection principles is given.
In dryland river catchments, palaeochannels provide some of the key records for understanding the changes in river character and behaviour that can occur in response to different extrinsic and intrinsic controls. Here, we combine modern hydroclimatic datasets, satellite imagery, field surveys, and optically stimulated luminescence (OSL) dating of palaeochannels to investigate the response of the ~850 km long Warrego River, located in central eastern Australia, to Holocene hydroclimatic change. The modern Warrego is an intermittent to ephemeral, low energy river (unit stream power, ω, ≤4 W m⁻²), and is characterised by low sinuosity (‘straight’), narrow (<50 m), single-thread, anabranching, and distributary channels that transport fine-grained (dominantly mud and sand) sediment. Marked downstream declines in discharge and stream power mean that many channels are discontinuous, terminating downstream at floodouts or in unchannelled floodplain wetlands before reaching the Darling River. By contrast, OSL ages and palaeohydrological reconstructions show that between ~8 and 5 ka, higher energy (~8–10 W m⁻²), wider (~160 m), meandering channels were the characteristic river style. In the Burrenbilla and Coongoola palaeochannels, prevailing La Niña conditions were likely responsible for bankfull discharges between 3 and 59 times those of the modern river, which probably enabled the palaeochannels to maintain perennial flow in continuous courses to the Darling River. After ~5 ka, El Niño events became stronger and more frequent, leading to greater catchment aridity and lower, more variable, river discharges. These extrinsic changes resulted in intrinsic adjustments to river character and behaviour, with key geomorphic thresholds being crossed in the cessation of meandering and establishment of the modern single-thread, anabranching, and distributary channels. The Warrego River's dramatic pattern transformation in response to mid- to late Holocene hydroclimatic change has similarities with some other Australian dryland river responses over equivalent timeframes. The pattern transformation demonstrates the Warrego River's sensitivity to extrinsic forcing and represents one of the most profound dryland river changes in the late Quaternary.
In natural open-channel flows over complex surfaces, a wide range of superimposed roughness elements may contribute to flow resistance. Gravel-bed rivers present a particularly interesting example of this kind of multiscalar flow resistance problem, as both individual grains and bedforms can potentially be important roughness elements. In this paper, we propose a novel method of estimating the relative contribution of different physical scales of river bed topography to the total drag, using a transform-roughness correlation (TRC) approach. The technique, which requires only a single longitudinal profile, consists of (1) a wavelet transform which decomposes the surface into roughness elements occurring at different wavelengths, and (2) a `roughness correlation' that estimates the drag associated with each wavelength based on its geometry alone, expressed as ks. We apply the TRC approach to original and published laboratory experiments and show that the multiscalar drag decomposition yields estimates of grain- and form-drag that are consistent with estimates in channels with similar morphologies. Also, we demonstrate that the roughness correlation may be used to estimate total flow resistance via a conventional equation, suggesting that it could replace representative roughness values such as median grain size or the standard deviation of elevations. An improved understanding of how various scales contribute to total flow resistance may lead to advances in hydraulics as well as channel morphodynamics.
The indeterminate channel problem arises from uncertainty in finding a closure relation for alluvial channels created by self-organizing erosional and depositional processes. Extremal hypotheses have been proposed as one potential approach to closing the system of governing equations for alluvial channels. Many different extremal hypotheses have been presented, but no substantive evidence has been developed to select which hypothesis may be most appropriate for natural alluvial river systems. This paper evaluates the ability of ten extremal hypotheses to identify dynamic equilibrium across a geomorphic gradient in the remote and undeveloped mid-latitude watershed of Rio Murta, Chile. This study (a) introduces extremal hypotheses, (b) describes the field site and geomorphic conditions, and (c) examines which extremal hypotheses are supported by the field data in identifying the evolutionary trend toward dynamic-equilibrium. The extremal hypotheses that identified dynamic equilibrium within the geomorphic gradient in the field are: (1) minimum kinetic energy, (2) minimum specific stream power, (3) maximum friction factor, and (4) maximum total friction factor, which collectively support minimizing kinetic energy of the system.
Cambridge Core - Geomorphology and Physical Geography - River Dynamics - by Bruce L. Rhoads
Understanding morphological changes of a river system is a necessary part of river management plans. Accurate assessment of the stability of alluvial channels is a significant issue in the design and retrofit of urban streams adversely impacted by rapid urbanization and climate change. In the present study, an evolutionary polynomial regression (EPR) model was designed to estimate the width (W), depth (D), and slope (S) of a stable channel based on a wide range of field and experimental datasets in sand, gravel and mountain rivers. Furthermore, the effect of 4 influential input parameters of mean sediment size (d50), flow discharge (Q), the Shields parameter (τ) and free water surface slope (S) of channel on the dimensions of stable alluvial channels is investigated using EPR model and a novel designed Gene Expression Programming (GEP) model. In addition, in the present study, the performance of 15 previous popular traditional methods in estimating stable channel dimensions is reviewed and compared with the results of the proposed models. The results show that the EPR model (Root Mean Square Error (RMSE) = 0.4, Bias = -0.04 and determination coefficient (R²) = 0.925) has less error than GEP model (RMSE = 0.45, Bias = -0.08, R² = 0.918) and by providing a simpler polynomial relationship than the GEP model, it is more accurate. The EPR and GEP models in addition to suggesting robust and straightforward relationships have higher accuracy than traditional models, especially in predicting the width and depth of the flow. The parametric analysis shows that Shields parameter (τ) is an effective parameter in predicting the channel slope. The developed GEP and EPR models with similar physical behavior to the best traditional methods have nonlinear variations respect to τ. The EPR relationships presented in this study can be used as an alternative to traditional relationships with higher accuracy and easier usability in the design and implementation of stable channels.
Modelling stable alluvial river profile is one of the most important and challenging issues in river engineering that several studies have been dedicated to it. The main objective of this study is to evaluate the back propagation-based multilayer neural network (BP-MLNN) performance in predicting stable alluvial river profile. We used eighty-five observational datasets to train and test, three separate models to predict each of the channel width (w), flow depth (h) and longitudinal slope (s) of stable channels. The network input parameters are the flow discharge (Q), mean sediment size (d50) and affecting Shields parameter (τ*) and w, h and s parameters are the output. It is concluded from the results that the proposed models to predict the width, depth, and slope of stable channels, with a correlation coefficient (R) of 0.96, 0.886, and 0.870 respectively, perform well. The mean absolute relative error (MARE) value of 0.063 related to the width estimation model in comparison with the depth and slope estimation model with MARE value of 0.077 and 0.518 shows the superior accuracy of the BP-MLNN model. The presented BP-MLNN models in this study are therefore recommended in river engineering projects to estimate the cross-sectional dimensions of stable alluvial channels as simple and robust design tools.
In this article we present a unique, detailed, basin-wide, morphological analysis of the Rhine, one of the world's larger rivers. The objectives of this study were to characterize the basin-scale morphodynamics of the Rhine River in the period 1991–2010 by quantifying the downstream fluxes of clay, silt, sand, gravel and cobbles from source to mouth and identifying the sources and sinks of these sediments using sediment budget analyses. From source to mouth, the Rhine traverses four sections with fundamentally different morphodynamic behaviour: the Alpine, impounded, free-flowing and delta section. Sediment fluxes are discontinuous, primarily because of the presence of a glacial lake trapping all sediments from upstream. The lake caused sediment fluxes to be discontinuous already before the Anthropocene. Today's sediment fluxes are strongly influenced by dredging and nourishment operations. From a global perspective, sediment fluxes in the river's headwaters are large, whereas sediment output from the Rhine towards the sea is small. A special feature of the Rhine is the fact that more sediment is transported in upstream direction from the sea into the delta than vice versa. On a basin-scale, nourishment represents the biggest source of gravel and cobbles, and tributaries the biggest source of clay, silt and sand. In the lower Rhine delta, additional large amounts of clay, silt and sand are supplied by the sea. Dredging represents a main sediment sink for all size fractions. For silt and clay, also floodplain deposition and deposition in ports represent major sinks. The Rhine is a typical example of a river in disequilibrium: large parts of the river are subject to erosion or sedimentation. In contrast to other rivers with erosion upstream and deposition downstream, the Rhine has net deposition upstream and net erosion downstream. Although human interventions contributed to the disequilibrium, the pristine Rhine already was in a state of disequilibrium. Even today, natural factors determine the location of the main sedimentation areas. The budget analysis shows that the behaviour of the clay/silt, sand and gravel/cobble fractions strongly differ from each other. A particularity is that in many reaches gravel is deposited, whereas simultaneously sand is being eroded. The budget analysis also shows that sediment dynamics in rivers are much higher than is suggested by echosoundings or transport measurements. This study provides valuable insights into the basin-scale morphodynamics of the Rhine, helps to see smaller-scale studies of parts of the Rhine Basin in a correct perspective, provides a good data basis to improve numerical prediction models, and helps to optimize nourishment, dredging and monitoring strategies in the Rhine. Furthermore, the Rhine represents an excellent reference case for other large river systems for which less data are available. Knowledge gaps of supra-regional importance identified in this study relate to (A) the fundamental disequilibrium of large river systems, (B) the effect of natural and human factors on the future morphodynamic development of the large river systems, (C) the morphodynamic role of sand in gravel-bed rivers and (D) the long-term effects of sediment nourishment.
Vegetation-free fluvial meanders are exceptional on modern Earth, and their abundance in pre-Silurian rock records is debated. Whether or not landscapes devoid of plant life could sustain sinuous planforms is nonetheless a fundamental concept that could disclose parallels with extraterrestrial sedimentary realms. The sedimentology and morphometry of an extensive (∼200 km2) unvegetated meander plain in the lower Amargosa River (Death Valley, California, USA) is investigated through remote sensing and ground checking. Results reveal sinuous bends generated by channels <2 m deep and <35 m wide that migrated laterally for as much as 200 m at rates of <1.5 m/year. The Amargosa River’s ephemeral regime excludes the proposal that discharge modulation directly aids meandering. Likewise, 97.7% of ground-checked meanders are devoid of plant life, ruling out direct control by vegetation on bank stability. Cohesion offered by mud-rich banks is predominant and aided by salt cementation, which also hinders wind erosion of fines. Overall, the meanders’ morphometry is in overlap with that of other sinuous (vegetated) channels, pointing to shared scalar properties. Meanders span over a continuum of a few large channels with lower aspect (width:depth) ratios and abundant smaller channels with higher aspect ratios. The migration of larger meandering channels generates distinctive scrolls and inclined accretionary packages; the migration of smaller widespread meanders only generate sub-tabular sets, producing an undistinctive depositional record. These results make the Amargosa River topical for the analogue modeling of pre-vegetation rivers, and suggest that unvegetated meanders might have been overlooked in the rock record.
Equilibrium is a single word that embraces multiple concepts. The largely qualitative use of equilibrium within geomorphology has fostered imprecision and even outright error; as a result the term, for many, has degenerated to the status of a non-corrigible regulative principle. Although a few geomorphologists make precise use of equilibria terminology, their precision is invariably lost on the larger audience.Equilibrium is associated with force in dynamics, with energy in thermodynamics (and probability by extension), and with pure numerical behavior in mathematics. In General System Theory, equilibrium is derived from thermodynamics but applied, by analogy, almost exclusively to mass. In addition, a purely geomorphic version of equilibrium (dynamic equilibrium) stems from the work of G. K. Gilbert (1876; 1877) and is based on sediment flux at the basin scale. Unfortunately, Gilbert's concept/term has been distorted by some geomorphologists in their attempt to establish linkage between equilibria concepts, especially between those relating to energy and mass.This paper reviews and reconsiders the various equilibria concepts. An updated version of Gilbert's dynamic equilibrium based on sediment transfer proposed by Ahnert (n.d.), and termed by us “mass flux equilibrium,” holds promise. In addition, the purely mathematical concept of equilibrium outlined by Howard (1988) has considerable utility, provided union with geomorphological theory is achieved. Given the difficulty of measuring thermal energy in the field, its inefficient use in nature, and the uncertainty with which it and form can be related, thermodynamic equilibria concepts would appear, presently, to offer geomorphologists only very limited opportunities.
3 anastomosed river systems are described. Each reach consists of an interconnected network of low-slope, narrow and deep, straight to sinuous, stable channels that transport coarse sand and gravel. Channels are separated by levees and wetlands composed of silt/mud and vegetation. Gravel-bed braided channels occur upstream from each anastomosed system, joined by a transitional reach comprising stable, elongate, silt islands within braided channels. The 3 anastomosed reaches have formed upstream from elevating base levels caused by deposition of alluvial fans across trunk valleys. -from Authors
Although the Bélanger-Böss theorem of critical flow has been widely applied in open channel hydraulics, it was derived from the laws governing ideal frictionless flow. This study explores a more general expression of this theorem and examines its applicability to flow with friction and sediment transport. It demonstrates that the theorem can be more generally presented as the principle of minimum energy (PME), with maximum efficiency of energy use and minimum friction or minimum energy dissipation as its equivalents. Critical flow depth under frictionless conditions, the best hydraulic section where friction is introduced, and the most efficient alluvial channel geometry where both friction and sediment transport apply are all shown to be the products of PME. Because PME in liquids characterizes the stationary state of motion in solid materials, flow tends to rapidly expend excess energy when more than minimally demanded energy is available. This leads to the formation of relatively stable but dynamic energy-consuming meandering and braided channel planforms and explains the existence of various extremal hypotheses.
The paper outlines recent regime theory design equations for duned sand-bed canals and indicates limits of applicability. A simple problem is presented and compared with original Lacey equations. Reference to worked problems and extensions is given. The advantages of the method described include: (1) equations derived from data of self-adjusting channels of the type and size for which design is required; (2) provision of the three necessary and sufficient independent equations for finding equilibrium values of flow area, depth, and slope; (3) consolidation of the unmeasureables of the water-sediment complex and of the cohesive erodible-depositable sides into two overall bed and side factors that can be calculated from the running conditions of existing channels in equilibrium and used by engineers with field experience; and (4) simplicity.
So ubiquitous are curves in rivers and so common are smooth and regular meander forms that they have attracted the interest of investigators from many disciplines. Also, investigations of the physical characteristics of glaciers and oceans have led to the recognition that analogous forms occur in melt-water channels developed on glaciers and even in the currents of the Gulf Stream. The striking similarity in physical form of curves in these various settings is the result of certain geometric proportions apparently common to all, that is, a nearly constant ratio of radius of curvature to meander length and of radius of curvature to channel width (Leopold and Wolman, 1960, p. 774). This leads to visual similarity regardless of scale.
An analytical method has been developed for computing the self- adjusted geometry of sand-bed rivers based upon a sediment discharge formula, a flow resistance relationship, and the concept of minimum stream power. Regime rivers with low values of discharge Q and slope S are characterized by small width-depth ratios (less than 30) that are not sensitive to the change in slope or discharge. But for larger value of Q or S, or both, the results show that regime rivers may have large width-depth ratios sensitive to the changes in Q or S. As Q or S increases the width-depth ratio of a regime river may become so large that it indicates a braided channel pattern. -from ASCE Publications Abstracts
The theory of minimum energy and the minimum rate of energy dissipation have been applied to flow around bluff bodies, stability of falling bodies, statics and dynamics of gas bubbles, generation of ripples and dunes, and drag reduction by suspended load. These examples are used to illustrate how the minimization principles (variational principles) can be used to solve the problems of dissipative mechanical systems in static or dynamic equilibrium conditions. The minimization principle can be used independently or in tandem with the equations of motion to solve problems of a large degree of freedom. The equilibrium solution thus obtained has been shown to contain certain implicit information on the stability characteristics of the equilibrium state. For this reason, the method is not only a viable alternative to the vector mechanics method, but it also provides a relatively simple way of determining certain stability criteria.-from ASCE Publications Abstracts
A dynamical framework–that of regime theory–on which to base a study of the behavior of channels that have formed boundaries from their own transported material or material of like nature is given. The most important system of natural channels to which the theory can be applied is that of the rivers of alluvial plains. Rivers, in general, are not susceptible to quantitative analysis except for certain broad effects. The theory has grown from the unrivaled mass of data provided by the irrigation canal systems of northern India. These canals were built by British engineers in India at the end of the nineteeth century and were extended regularly until they formed the largest, composite body of canals in the world.
It is shown that canals of the type considered provide formulas for the self-formation of width, depth, and slope. Rivers require two more formulas to give the general pattern of meandering. The developed laws fit into the framework of accepted hydraulics and generalize it. Although these laws apply to rivers, exactly as to canals, the disturbing influences in the former require judgment in the application of the formulas. Self-forming rivers cannot be described adequately in terms of the one flow formula of rigid-boundary hydraulics.
Richard Feynman's never previously published doctoral thesis formed the heart of much of his brilliant and profound work in theoretical physics. Entitled “The Principle of Least Action in Quantum Mechanics," its original motive was to quantize the classical action-at-a-distance electrodynamics. Because that theory adopted an overall space–time viewpoint, the classical Hamiltonian approach used in the conventional formulations of quantum theory could not be used, so Feynman turned to the Lagrangian function and the principle of least action as his points of departure. The result was the path integral approach, which satisfied — and transcended — its original motivation, and has enjoyed great success in renormalized quantum field theory, including the derivation of the ubiquitous Feynman diagrams for elementary particles. Path integrals have many other applications, including atomic, molecular, and nuclear scattering, statistical mechanics, quantum liquids and solids, Brownian motion, and noise theory. It also sheds new light on fundamental issues like the interpretation of quantum theory because of its new overall space–time viewpoint. The present volume includes Feynman's Princeton thesis, the related review article “Space–Time Approach to Non-Relativistic Quantum Mechanics” [Reviews of Modern Physics 20 (1948), 367–387], Paul Dirac's seminal paper “The Lagrangian in Quantum Mechanics'' [Physikalische Zeitschrift der Sowjetunion, Band 3, Heft 1 (1933)], and an introduction by Laurie M Brown. © 2005 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.
Variational formulations of the equations of motion applicable to irrotational, nonaccelerating, as well as gradually varied open channel flows are presented. It is shown that there exists a suitable functional, J, from which the equation of motion can be obtained as the condition of minimization with respect to the velocity profile. Minimization of J is shown to be equivalent to the minimization of the total rate of energy loss, E, providing that the discharge, Q, and the total head, H, at upstream and downstream ends of the river is known. Furthermore, if the speed of the movement of the river bed is small, then the total rate of energy loss is shown to be equal to the total stream power. Thus, under certain restrictive conditions the hydraulic problem reduces to the problem of minimizing total stream power. The minimization theory presented here is somewhat similar to the second writer's theory of minimum unit stream power.
A rational method has been developed to predict the regime geometry of straight active gravel streams. The analytical model is based upon a resistance equation, a bed load equation, and the condition of minimum stream power for gravel streams in equilibrium. The analytical channel geometry so obtained is in general agreement with previously established relations and observations. The analytical channel width is proportional to the bank full discharge. Except for steep slopes, the width is essentially only a function of the discharge. The analytical depth increases with the discharge but decreases with the slope. On steep slopes, the width increases rapidly with the slope. This rapid increase in width accompanied by a decrease in depth indicates braiding tendency for steep gravel streams. As the bed load approaches zero at the lower boundary, this model for active streams reduces to the threshold theory. -from ASCE Publications Abstracts
The hypothesis of minimum stream power for stable alluvial channels has been used to derive a condition for alluvial channels in equilibrium. A method incorporating this condition with a flow-resistance formula and a sediment-discharge formula has been developed to compute the width, depth, and slope of stable alluvial channels for a given set of water and sediment discharges. Applying this method yields a design chart that provides the stable width and depth of alluvial canals with trapezoidal shape for a given set of water discharge, channel slope, sediment size, and side slope. Comparing data from some regime canals and small experimental canals has shown good agreement between the observed data and analytical predictions. -from ASCE Publications Abstracts
Extremal hypotheses proposed for river regime analysis are reviewed. These include Minimum Energy Dissipation Rate, Minimum Stream Power, and Maximum Sediment Transport. A lemma is proposed which proves the equivalence of variational formulations of related problems. Evidence of contradictions in the hypotheses is presented. The theory of minimum dissipation rate is examined and it is shown that it cannot be derived from the principles of mechanics. The physical basis of extremal hypotheses is analysed. A different analysis based on the transverse equilibrium theory is proposed. (from authors' abstract)
Yang's (1996) sediment transport theory based on unit stream power is one of the most accurate theories, but in his equations the use of product of slope and critical velocity instead of critical unit stream power is not suitable. Dimensionless critical unit stream power required at incipient motion can be derived from the principle of conservation of power as a function of dimensionless particle diameter and relative roughness. Based on a lot of data sets, this new criterion was developed. By use of this new criteria, Yang's (1973) sand transport formula and his 1984 gravel transport formula could be improved when sediment concentration is less than about 100 ppm by weight.
A general theory of minimum rate of energy dissipation for a class of open channel flows with or without the movement of sediment is proposed in this paper. This theory states that the rate of energy dissipation is a minimum under steady equilibrium or gradually varied flow conditions. The theory is derived from the Navier-Stoke's equations of motion for gradually varied open channel flow without sediment transport. It applies to turbulent and laminar flows as long as the inertia forces due to the time-averaged velocity distribution is small compared with the forces due to gravity and shear. The theory in different degrees of generality can be used to explain the fluvial processes from the movement of sediment to the change of velocity, slope, roughness, channel geometry, pattern, and profile of a river under an equilibrium condition or during the process of self-adjustment to reach an equilibrium condition. - from ASCE Publications Abstracts
The hydraulic geometry and the local features of straight, relatively wide self-formed channels composed of sandy beds are studied in laboratory flumes. Introduction of 1 loose and 1 rigid bank in experiments excludes the tendency toward meanders and provides good examples to test hydraulic regime relations. The transverse bed shapes in equilibrium have a universal profile regardless of initially-molded cross section, discharge of flow, and sand diameter. A theoretical analysis of secondary currents in straight channels provides reasonable interpretation of the local features of self-formed channels such as the longitudinal sand ridges, and this analysis suggests the insignificant effect of secondary currents on the size of self-formed channels. -from ASCE Publications Abstracts
This paper presents a critique of that magnitude-frequency concept which defines an 'effective', or 'dominant' event in terms of long-term sediment transport, and then associates this with channel morphology. It argues that the concept was associated with a research programme which emphasised engineering time scales and equilibrium forms, but which is now degenerating in geomorphology. The paper demonstrates that the concept has been protected by a series of ad hoc auxiliary statements and hypotheses, of which a critical one is that the effectiveness of an event is dependent on the initial morphological boundary conditions. This implies that the effectiveness of an event depends on morphology, as much as vice versa. A more appropriate geomorphological research programme is suggested, which emphasises the continuing and spatially-distributed feedback between form and process. This is applicable to a wide range of geomorphological problems, in both unconsolidated materials and hard rock, and is considered to lend itself to rigorous examination of observational evidence and critical, rational and scientific evaluation of alternative interpretations.
A numerical model is described which simulates bank erosion in a
straight channel composed of noncohesive sediment. The model predicts
the distribution of boundary shear stress, cross-channel sediment
transport rates and the evolution of the bed topography. When erosion
produces a bank slope which exceeds the angle of repose, widening occurs
by a planar bank failure. Equilibrium channels produced by the model
have flat beds and curved bank regions which are similar to the
classical cosine stable bank profile. Equilibrium values of
dimensionless depth are inversely proportional to the slope, as
suggested by previous studies. The model also reproduces the exponential
cross sections created during laboratory experiments. However, as the
computations proceed, the exponential profiles slowly develop a flat bed
and a curved bank region, suggesting that past flume experiments of
channel widening may have frequently been terminated before a stable
equilibrium form had evolved.
A review of the important stream morphologic theories and hypotheses published before 1971 indicates that none provides a satisfactory explanation to the formation of meandering streams. The law of least time rate of energy expenditure introduced by Yang1) is used to explain the formation of meandering streams. This law can explain why stable unbraided channels always follow a smooth sinuous course. It also explains how the variation in water discharge, sediment concentration, channel geometry, channel slope, valley slope, geological constraints, etc., should change the characteristics of meandering channels. The arguments made in this paper are strongly supported by laboratory and field observations.
Phillips, J.D., 1992. The end of equilibrium? In: J.D. Phillips and W.H. Renwick (Editors), Geomorphic Systems. Geo-morphology, 5: 195-201. An overview of the proceedings of the 1992 Geomorphology symposium on Geomorphic Systems reveals several trends. First, there is a pervasive concern with the presence and nature of geomorphic equilibria. However, the focus is not on detection of a single, stable equilibrium condition for geomorphic systems, but on the behavior of systems away from equilibrium, the presence and prevalence of dis-and nonequilibrium forms, and the presence of multiple equilibria for a given system. A second major theme is the nonlinear behavior ofgeomorphic systems. This nonlinearity provides a bridge between existing threshold concepts in the discipline and the emerging study of earth surface phenomena as complex nonlinear dynamical systems. In connection with this, the critical role of feedback mechanisms in geomorphic systems emerges as a third general theme. Finally, a number of papers implicitly or explicitly raise the possibility of universality -i.e., that there are universal properties of geomorphic system behavior independent of location-and time-specific controls. Taken as a whole, the symposium papers are interpreted as signalling the possible end of classic equilibrium studies in the sense of searching for a single or typical equilibrium response to a given set of processes or controls. Conceptual frame-works which emphasize, or at least include, nonequilibrium responses and multiple equilibria are on the ascension. His-torical perspectives suggest that these new developments are not necessarily inconsistent with prevailing and traditional geomorphic thought.
The hypotheses of minimum energy dissipation, minimum stream power, and
minimum unit stream power are summarized and compared. Their derivation
from analogies with laminar flow and linear thermodynamics is criticized
on the grounds that these situations differ fundamentally from river
flows, which are usually highly turbulent and strongly nonlinear. The
authors' empirical hypothesis of maximum friction factor seems
preferable to the minimization hypotheses because it is compatible with
the known behavior of turbulent flows and nonlinear processes, it is
applicable with a wider range of independent variables, and it is more
in keeping with trends shown by experimental data under all constraints.
The minimization hypotheses seem likely to give incorrect predictions
when flow rate and depth are independent variables. The empirical
success of the minimization hypotheses is confined to situations in
which they predict similar behavior to the maximum friction factor
hypothesis; they may be considered as special cases of this more general
hypothesis.
Various aspects of Newton's second law of motion were discussed. Newton's second law of motion, F = ma, is the soul of classical mechanics. The left-hand side of the equation has no independent meaning. Yet, the law is full of meaning, by the highest standard. It proves itself useful in demanding situations. The paradox deepens when the force is considered from the perspective of modern physics. Astute observers have commented to eliminate force even before the emergence of relativity and quantum mechanics.
The hydraulics of hydraulic geometry has been sadly neglected by most geomorphologists. The links are explored first for a station variation and then for downstream hydraulic geometry. There are comments on maximum efficiency channels and bank stability in mobile bed channels. Downstream hydrology is empirically more regular than at a point variation, though harder to explain rationally. - K.Clayton
Mathematical model for defining stable width and depth of straight gravel rivers is presented, in which it is revealed that the major agency for maintaining stable channel is lateral diffusion of longitudinal fluid momentum due to turbulence. The primary quantities which determine the stable channel geometry are found to be flow discharge, longitudinal free surface slope, the median size of bed material, and gradation of the bed material. Increasing gradation is found to increase the depth and decrease the width. Introduction of one loose and the other rigid bank in laboratory experiments provides good examples to test the model, in which four kinds of sand were used to test the gradation effect. These laboratory data support the mathematical model presented herein. The theory also performs well when it is applied to natural gravel rivers.
The extremal hypotheses of minimum stream power, minimum unit stream
power, minimum energy dissipation rate, and maximum sediment transport
rate, when combined with conventional sediment transport and flow
resistance equations, lead to conclusions incompatible with
observations. For wide, straight, unconstrained alluvial reaches in
equilibrium, these conclusions include that the Einstein sediment
discharge and Shields entrainment function are nearly constant, the
magnitude of the particular constants depending only on the hypothesis
and equations used, whereas data from flumes and natural rivers show
that both expressions are highly variable in stable channels. Constancy
of the Einstein and Shields expressions provides, in fact, a sufficient
but unnecessary condition for channel stability. In the maximum friction
factor hypothesis there is no maximum for friction factor when channel
width, depth, and slope are dependent variables. Variational principles
may one day supply a solution to the problem of alluvial channel
stability, but current formulations of the mentioned hypotheses require
redefinition.
The theory of minimum rate of energy dissipation states that a system is in an equilibrium condition when its rate of energy dissipation is at its minimum value. This minimum value depends on the constraints applied to the system. When a system is not at equilibrium, it will adjust in such a manner that the rate of energy dissipation can be reduced until it reaches the minimum and regains equilibrium. A river system constantly adjusts itself in response to varying constraints in such a manner that the rate of energy dissipation approaches a minimum value and thus moves toward an equilibrium. It is shown that the values of the exponents of the hydraulic geometry relationships proposed by Leopold and Maddock for rivers can be obtained from the application of the theory of minimum rate of energy dissipation in conjunction with the Manning-Strickler equation and the dimensionless unit stream power equation for sediment transport proposed by Yang. Theoretical analysis is limited to channels which are approximately rectangular in shape. The width and depth exponents thus derived agree very well with those measured in laboratory experiments by Barr et al. Although the theoretical width and depth exponents are within the range of variations of measured data from river gaging stations, the at-station width adjustment of natural rivers may also depend on constraints other than water discharge and sediment load.
Sand-dominated ridge-form anabranching rivers on the Kimberley Plateau in northwestern Australia are a new type of alluvial channel system not previously described or explained in detail. This paper examines the morphology, sedimentology and stratigraphy of this type of river, and proposes mechanisms for their formation and maintenance. Alternating with relatively narrow, bedrock reaches of valley are wider, depositional, alluvial reaches that support roughly parallel, very elongated, steep-sided and treed sandy ridges of approximately floodplain height. These ridges subdivide the total flow system into remarkably straight, canal-like, anabranching channels. In this seasonally arid environment there is an abundance of riparian vegetation, in places growing chaotically over wider sections of stream bed. It is proposed here that the formation of ridges results in a reduction of flow resistance with an increase in depth. The ridges concentrate the flow and compensate for the less efficient flow conditions associated with these well-vegetated alluvial reaches; trees generally do not survive on the bed of the higher-energy channels. Enhanced velocities and increased bed shear act to maintain or increase water and sediment flux in these alluvial reaches. The formation of the channels and intervening ridges may be aided by the development of double flow helices in each anabranch, similar to that described for much smaller ridge features observed in laboratory flumes. Anabranches are also observed having formed by channel avulsion scouring new channels into adjacent floodplains, particularly in widened sections of valley near tributary junctions.