Formative conditions and sedimentary structures of sandy 3D antidunes: an application of the gravel step‐pool model to fine‐grained sand in an experimental flume

Antidunes and their sedimentary structures can be useful in reconstructing paleo‐hydraulic conditions, especially for large discharge events. However, three‐dimensional (3D) antidunes in sand‐sized sediments have not yet been studied extensively, as compared to either two‐dimensional (2D) antidunes or antidunes in gravel‐sized sediments. In this study, we estimated formative conditions of gravel step‐pool morphologies and applied them to the formation of 3D antidunes over a sand bed. Formative conditions are expressed in terms of a relationship between the water discharge per unit width and the bed slope. Flume experiments demonstrated that 3D mound‐like antidune configurations and their associated internal sedimentary structures could be preserved. Internal sedimentary structures were characterized by shallow lens‐like structures whose bases were erosional. Although gently‐dipping concave‐upward lamination was dominant, convex‐upward lamination was occasionally observed. The dimensions of lenticular lamina‐sets can be used to estimate antidune geometry. Thus if 3D antidunes can be interpreted in the stratigraphic record, it is possible to estimate the paleo‐hydraulic parameters such as water discharge and bed slope more precisely than previously. Copyright © 2010 John Wiley & Sons, Ltd.     

Morphological effects of local scouring in step–pool streams

Step–pool morphology characterizes many high‐gradient streams in a variety of natural settings, but formative processes and evolutionary dynamics are still poorly understood. In this paper, natural step–pool geometry is compared with steep alluvial channels where grade‐control structures such as check‐dams and bed sills make the stream profile resemble a natural stepped stream. Along these channels, local scouring due to falling jets forms plunge pools under each structure, analogous to natural steps determining the formation of pools. In order to test the hypothesis that natural pools are analogous to pools formed below grade‐control works with respect to their dimensions, shape and formative dynamics, 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern Italian Alps. Pools below grade‐control works featured a transitional zone between the scour hole and the downstream sloping bed, marked by a depositional berm. When geometric parameters such as maximum pool depth, length and step–berm distances are normalized to the jet virtual energy, no statistically significant differences were detected between natural and artificial systems. These results lend support to an upstream‐forced cascade model for step–pool formation, where the energy of falling jets controls the geometry of the pools, and is therefore regarded as the most important scaling‐independent variable. Copyright © 2005 John Wiley & Sons, Ltd.     

Numerical modelling of downstream migrating antidunes

A numerical model is presented that compute the geometrical dimensions and movement of downstream migrating antidunes. The model solves the Navier–Stokes equations together with the k‐epsilon turbulence model to find the water flow field over the bedforms. A two‐dimensional width‐averaged grid is used. The bed elevation changes are computed by solving the convection–diffusion equation for suspended sediments and bedload, together with the Engelund–Hansen sediment transport formula. The free surface is computed with an algorithm based on water continuity in the surface cells. Non‐orthogonal adaptive grids were used, moving vertically with the computed location of the bed and the free water surface. The numerical model was tested on data from a physical model study where regular downstream migrating antidunes had been observed. The numerical model started out with a flat bed and the trains of antidunes formed over time. Many of the physical processes observed in earlier studies were replicated by the numerical model. Four dune parameters were computed in the current tests: The antidune wavelength, height and celerity, together with the average water depth. The antidune wavelengths were best predicted with an accuracy of 3 to 8% compared with the measurements. The antidune heights were computed with a deviation of 11 to 25% compared with an empirical formula. The water depths over the antidunes were predicted with an accuracy of 3 to 9% related to the measured values. The average antidune celerity was the parameter with largest deviation: For the coarsest grid it was overpredicted with 37%. Copyright © 2017 John Wiley & Sons, Ltd.     

Bed-form-dependent pulsating flow in medano creek,southern colorado

Some sand-bed streams exhibit unsteady supercritical flow in the form of periodic bores that propagate downstream. The bores are formed by unstable antidunes, which store water while growing in amplitude and then release the stored water, when the antidunes wash out to form a plane bed. A cyclic process of antidune growth, oversteepening, antidune breaking, then bed-form washout produced bores on regular periodic intervals of roughly 15 s in Medano Creek, Great Sand Dunes National Monument, Colorado. This periodic bore-generating pulsating flow, is bed-form dependent, and it requires sufficiently high flow velocities and sediment transport rates to form unstable breaking antidunes. The Froude number maximum is 1.8, below that required for slug flow. Pulsating flow has been observed in flumes, arroyos, and rivers, and it may be more common than initially anticipated. Its high erosive capacity can influence both channel morphology and the character of associated sedimentary deposits.     

Comparison of hydraulic geometry between sand‐ and gravel‐bed rivers in relation to channel pattern discrimination

A comparison has been made between the hydraulic geometry of sand‐ and gravel‐bed rivers, based on data from alluvial rivers around the world. The results indicate a signi?cant difference in hydraulic geometry among sand‐ and gravel‐bed rivers with different channel patterns. On this basis, some diagrams for discrimination of meandering and braided channel patterns have been established. The relationships between channel width and water discharge, between channel depth and water discharge, between width–depth ratio and water discharge and between channel slope and water discharge can all be used for channel pattern discrimination. The relationship between channel width and channel depth can also be used for channel pattern discrimination. However, the accuracy of these relationships for channel pattern discrimination varies, and the depth–discharge relationship is a better discriminator of pattern type than the classic slope–discharge function. The cause for this difference has been explained qualitatively. To predict the development of channel patterns under different natural conditions, the pattern discriminator should be searched on the basis of independent or at least semi‐independent variables. The relationship between stream power and bed material grain size can be used to discriminate channel patterns, which shows a better result than the discriminator using the slope–discharge relationship. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal regime of a headwater stream within a clear‐cut,coastal British Columbia,Canada

This study examined the thermal regime of a headwater stream within a clear‐cut. The stream had a complex morphology dominated by step–pool features, many formed by sediment accumulation upstream of woody debris. Maximum daily temperatures increased up to 5 °C after logging, and were positively associated with maximum daily air temperature and negatively with discharge. Maximum daily temperatures generally increased with downstream distance through the cut block, but decreased with distance in two segments over distances of tens of metres, where the topography indicated relatively concentrated lateral inflow. Localized cool areas within a step–pool unit were associated with zones of concentrated upwelling. Bed temperatures tended to be higher and have greater ranges in areas of downwelling flow into the bed. Heat budget estimates were made using meteorological measurements over the water surface and a model of net radiation using canopy characteristics derived from fisheye photography. Heat exchange driven by hyporheic flow through the channel step was a cooling effect during daytime, with a magnitude up to approximately 25% that of net radiation during the period of maximum daytime warming. Heat budget calculations in these headwater streams are complicated by the heterogeneity of incident solar radiation and channel geometry, as well as uncertainty in estimating heat and water exchanges between the stream and the subsurface via hyporheic exchange and heat conduction. Copyright © 2005 John Wiley & Sons, Ltd.     

In-situ assessment of the spatial arrangement of step–pool units on eroded rills

Rills are generated on homogeneous hillslopes by the action of different discharges and evolve morphologically over short timescales due to a strong interaction between the flow and bed morphology. Such an interaction generates a reconfiguration of the bed geometry. Previous works suggest that bed geometry is often characterized by alternation between pools and flat reaches (steps). Each step–pool unit may contribute to hydraulic resistance and affects flow behaviour. The objectives of this work are (i) to assess different (innovative) techniques for the in-situ assessment of rill bed geometry, (ii) to use these techniques to assess the geometry of eroded rills in situ in order to determine the spatial arrangement in the bed macro-scale roughness and (iii) finally to analyse the role of slope and discharge as driving factors associated with the development of these macroforms. Roughly rectilinear, long rills were formed in the field as a result of combining different slope and discharges. Photogrammetry provided detailed digital elevation models (DEMs) before and after the experiments. The rills were morphologically characterized from the DEMs. In each rill, the presence of step–pools was identified from long profiles according mainly to morphological criteria published elsewhere, but with ad hoc critical threshold values more appropriate for small eroded channels. The minimum slope required for the development of step–pool units seems to be somewhere between 5 and 15%. Discharge seems to affect pool size or roughness amplitude. There does not seem to be a clear step–pool periodicity. However, external factors could have affected the normal growth and alternation of these structures. Identification of steps and pools from longitudinal elevation profiles can be objectively accomplished using a series of geometric rules originally proposed for rivers and large channels, and adapted to rills. © 2019 John Wiley & Sons, Ltd.     

Chute channel dynamics in large,sand‐bed meandering rivers

Meander bends of many large, sand‐bed meandering rivers are partitioned by chute channels that convey permanent flow, and co‐exist with the mainstem for decades. As a first step toward understanding the dynamics and morphodynamic implications of these ‘bifurcate meander bends’, this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics. Regression models developed for the Strickland River, Papua New Guinea, the lower Paraguay River, Paraguay/Argentina, and the Beni River, Bolivia, revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate (the rate at which a bend elongates in a direction perpendicular to the valley axis trend). Image analyses of all rivers and field observations from the Strickland suggest that the majority of chute channels form during scroll–slough development. Rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank, resulting in widely‐spaced scrolls with intervening sloughs that are positively aligned with primary over‐bar flow. The rivers plot in order of increasing chute activity on an empirical meandering‐braided pattern continuum defined by potential specific stream power (ω_pv) and bedload calibre (D₅₀). Increasing stream power is considered to result in higher bend extension rates, with implications for chute initiation. In addition, chute stability is shown to depend on river sediment load relative to flow discharge (Qs/Q), such that while the Beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power, the formation of stable chute channels is suppressed by the high sediment load. This tendency is consistent with previous experimental studies, and results in a planform that is transitional between single‐thread meandering and braided. Copyright © 2011 John Wiley & Sons, Ltd.     

Particle path length distributions in meandering gravel‐bed streams: results from physical models

In gravel‐bed rivers with well‐de?ned pool–bar morphology, the path length of transported bed particles must be, at least during ‘channel‐forming’ ?ows, equal to the length scale of the morphology. This is the basis for some methods for estimating bed material transport rates. However, previous data, especially from ?eld tests, are often strongly positively skewed with mean much shorter than the pool–bar spacing. One possible explanation is that positively skewed distributions occur only in channels lacking distinct pool–bar topography or only at lower discharges in pool–bar channels. A series of ?ume experiments using ?uorescent tracers was used to measure path length distributions in low‐sinuosity meandering channels to assess the relation with channel morphology and ?ow conditions. At channel‐forming ?ows, 55 to 75 per cent of the tracer grains were deposited on the ?rst point bar downstream of the point of tracer input, with 15 per cent passing beyond the ?rst bar. Path length distributions are symmetrical with mean equal to the pool–bar spacing and can be described with a Cauchy distribution. In some cases there was a secondary mode close to the point of tracer introduction; this bimodal distribution ?ts a combined gamma–Cauchy distribution. Only when discharge was reduced below the channel‐forming ?ow were frequency distributions unimodal and positively skewed with no relation to the pool–bar spacing. Thus, path length distributions become more symmetrical, and mean path length increases to coincide with pool–bar spacing, as ?ow approaches channel‐forming conditions. This is a substantial modi?cation of existing models of particle transfer in gravel‐bed rivers. Copyright © 2003 John Wiley & Sons, Ltd.     

Alluvial architecture in headwater streams with special emphasis on step–pool topography

Alluvial mountain streams exhibit a range of channel forms: pool–riffle, plane bed, step–pool and cascades. Previous work suggested that these forms exist within discrete, and progressively steeper slope classes. Measurements conducted at over 100 sites in west‐central and central Idaho confirm that slope steepens progressively as one moves from pool–riffle, to plane bed, to step–pool, and finally to cascades. Median slope for pool–riffle topography is 0·0060, for plane beds 0·013, for step–pools 0·044, and for cascades 0·068. There is substantial overlap in the slopes associated with these channel forms. Pool–riffle topography was found at slopes between 0·0010 and 0·015, plane beds between 0·0010 and 0·035, step–pools between 0·015 and 0·134, and cascades between 0·050 and 0·12. Step–pools are particularly striking features in headwater streams. They are characterized by alternating steep and gentle channel segments. The steep segments (step risers) are transverse accumulations of boulder and cobbles, while the gentle segments (pools) contain finer material. Step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers. This result differs from past studies that have reported channel slope to be the dominant control on step wavelength. The presumed geometry and Froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step–pools are formed by antidunes. Copyright © 2000 John Wiley & Sons, Ltd.     

Minimum froude number and the equilibrium of alluvial sand rivers

Rivers adjust towards an equilibrium condition, the stability of which depends upon a set of controlling factors expressed by the Froude number. As alluvial river channels approach stable conditions, the Froude number of the channel flow will tend to attain a minimum value which reflects minimum bed material motion and maximum channel stability, under the constraints imposed by water discharge, sediment load, and particle size. Computer simulations for sand bed rivers show that the Froude number of the flow tends to a minimum value when the equilibrium river tends to a certain hydraulic geometry. Evidence from 57 alluvial sand material rivers and stable canals shows that this simulated hydraulic geometry with minimum Froude number corresponds to the natural equilibrium state.     

Coarse sediment transport in mountain streams in Colorado and Wyoming,USA

Since the early 1990s, US Forest Service researchers have made thousands of bedload measurements in steep, coarse‐grained channels in Colorado and Wyoming, USA. In this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes, including step–pool, plane‐bed, pool–riffle, and near‐braided channels. This effort builds upon previous work where we applied a piecewise regression model to (1) relate flow to rates of bedload transport and (2) define phases of transport in coarse‐grained channels. Earlier, the model was tested using bedload data from eight sites on the Fraser Experimental Forest near Fraser, Colorado. The analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied. The earlier results were, however, derived from data collected at sites that, for the most part, have quite similar geology and runoff regimes. In this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions. The results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs. The transition from primarily low rates of sand transport (phase I) to higher rates of sand and coarse gravel transport (phase II) occurs, on average, at about 80 per cent of the bankfull (1·5‐year return interval) discharge. A comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase I transport. Moreover, the movement and capture of the D₁₆ to D₂₅ grain size of the bed surface seems to correspond with the onset of phase II transport, particularly in systems with largely static channel surfaces. However, while there were many similarities in observed patterns of bedload transport at the 19 studied sites, each had its own ‘bedload signal’ in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system. Published in 2005 by John Wiley & Sons, Ltd.     

Effects of sediment pulses on bed relief in bar‐pool channels

To further develop prediction of the range of morphological adjustments associated with sediment pulses in bar‐pool channels, we analyze channel bed topographic data collected prior to and following the removal of two dams in Oregon: Marmot Dam on the Sandy River and Brownsville Dam on the Calapooia River. We hypothesize that, in gravel‐bed, bar‐pool channels, the response of bed relief to sand and gravel sediment pulses is a function of initial relief and pulse magnitude. Modest increases in sediment supply to initially low‐relief, sediment‐poor cross‐sections will increase bed relief and variance of bed relief via bar deposition. Modest increases in sediment supply to initially high‐relief cross‐sections, characteristic of alternate bar morphology, will result in decreased bed relief and variance of relief via deposition in bar‐adjacent pools. These hypothesized adjustments are measured in terms of bed relief, which we define as the difference in elevation between the pool‐bottom and bar‐top. We evaluate how relief varies with sediment thickness, where both relief and mean sediment thickness at a cross‐section are normalized by the 90th percentile of observed relief values within a reach prior to a sediment pulse. Field measurements generally supported the stated hypotheses, demonstrating how introduction of a sediment pulse to low‐relief reaches can increase mean and variance of relief, while introduction to high‐relief reaches can decrease the mean and variance of bed relief, at least temporarily. In general, at both sites, the degree of impact increased with the thickness of sediment delivered to the cross‐section. Results thus suggest that the analysis is a useful step for understanding the morphological effects of sediment pulses introduced to gravel‐bed, bar‐pool channels. Copyright © 2015 John Wiley & Sons, Ltd.     

Quantifying the Morphological Print of Bedload Transport

Bedload and river morphology interact in a strong feedback manner. Bedload conditions the development of river morphology along different space and time scales; however, by concentrating the flow in preferential paths, a given morphology controls bedload for a given discharge. As bedload is a non‐linear response of shear stress, local morphology is likely to have a strong impact on bedload prediction when the shear stress is averaged over the section, as is usually done. This was investigated by comparing bedload measured in different bed morphologies (step‐pool, plane bed, riffle‐pool, braiding, and sand beds), with bedload measured in narrow flumes in the absence of any bed form, used here as a reference. The initial methodology consisted of fitting a bedload equation to the flume data. Secondly, the morphological signature of each river was studied as the distance to this referent equation. It was concluded that each morphology affects bedload in a different way. For a given average grain shear stress, the larger the river, the larger the deviation from the flume transport. Narrow streams are those morphologies that behave more like flumes; this is particularly true with flat beds, whereas results deviate from flumes to a greater extent in step‐pools. The riffle‐pool's morphology impacts bedload at different levels depending on the degree of bar development, considered here through the ratio D₈₄/D₅₀ which is used as a proxy for the local bed patchiness and morphology. In braiding rivers morphological effects are important but difficult to assess because width is dependent on transport rate. Bed morphology was found to have negligible effects in sand bed rivers where the Shields stress is usually sufficiently high to minimize the non‐linearity effects when hydraulics is averaged over the section. Copyright © 2015 John Wiley & Sons, Ltd.     

Assessing the effect of vegetation‐related bank strength on channel morphology and stability in gravel‐bed streams using numerical models

Bank strength due to vegetation dominates the geometry of small stream channels, but has virtually no effect on the geometry of larger ones. The dependence of bank strength on channel scale affects the form of downstream hydraulic geometry relations and the meandering‐braiding threshold. It is also associated with a lateral migration threshold discharge, below which channels do not migrate appreciably across their floodplains. A rational regime model is used to explore these scale effects: it parameterizes vegetation‐related bank strength using a dimensionless effective cohesion, C_r*. The scale effects are explored primarily using an alluvial state space defined by the dimensionless formative discharge, Q*, and channel slope, S, which is analogous to the Q–S diagrams originally used to explore meandering‐braiding thresholds. The analyses show that the effect of vegetation on both downstream hydraulic geometry and the meandering‐braiding threshold is strongest for the smallest streams in a watershed, but that the effect disappears for Q* > 10⁶. The analysis of the migration threshold suggests that the critical discharge ranges from about 5 m³/s to 50 m³/s, depending on the characteristic rooting depth for the vegetation. The analysis also suggests that, where fires frequently affect riparian forests, channels may alternate between laterally stable gravel plane‐bed channels and laterally active riffle‐pool channels. These channels likely do not exhibit the classic dynamic equilibrium associated with alluvial streams, but instead exhibit a cyclical morphologic evolution, oscillating between laterally stable and laterally unstable end‐members with a frequency determined by the forest fire recurrence interval. Copyright © 2008 John Wiley & Sons, Ltd.     

Flow reversal over a natural pool–riffle sequence: a computational study

A computational study is presented on the hydraulics of a natural pool–rif?e sequence composed of mixed cobbles, pebbles and sand in the River Lune, northern England. A depth‐averaged two‐dimensional numerical model is employed, calibrated with observed data at the ?eld site. From the computational outputs, the occurrence of longitudinally double peak zones of bed shear stress and velocity is found. In particular, at low discharge there exists a primary peak zone of bed shear stress and velocity at the rif?e tail in line with the local maximum energy slope, in addition to a secondary peak at the pool head. As discharge increases, the primary peak at the rif?e tail at low ?ow moves toward the upstream side of the rif?e along with the maximum energy slope, showing progressive equalization to the surrounding hydraulic pro?les. Concurrently, the secondary peak, due to channel constriction, appears to stand at the pool head, with its value increasing with discharge and approaching or exceeding the primary peak over the rif?e. The existence of ?ow reversal is demonstrated for this speci?c case, which is attributable to channel constriction at the pool head. A dynamic equilibrium model is presented to reconstruct the pool–rif?e morphology. A series of numerical modelling exercises demonstrates that the pool–rif?e morphology is more likely produced by shallow ?ows concentrated with coarse sediments than deep ?ows laden with low concentrations of ?ne sediments. It is concluded that channel constriction can, but may not necessarily, lead to competence reversal, depending on channel geometry, ?ow discharge and sediment properties. Copyright © 2003 John Wiley & Sons, Ltd.     

Velocity characteristics along a small step–pool channel

This paper summarizes measurements of velocity along three reaches of a small mountain channel with step–pool bedforms. A one‐dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph. Measurement cross‐sections were located upstream from a bed‐step, at the step lip, downstream from the step, and in a uniform‐gradient run. Data analyses focused on characteristics of velocity profiles, and on correlations between velocity characteristics and the potential control variables bedform type, reach gradient and flow depth. To test the hypothesis that velocity characteristics are related to channel bedform types, ANOVA and ANCOVA tests were performed for the average velocity and coefficient of variation of point velocity data. Results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge. Velocity became more variable as stage increased, particularly at low‐gradient reaches with less variable bed roughness. Velocity profiles suggest that locations immediately downstream from bed‐steps are dominated by wake turbulence from mid‐profile shear layers. Locations immediately upstream from steps, at step lips, and in runs are dominated by bed‐generated turbulence. Adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation, whereas favourable pressure gradients at steps are suppressing turbulence. The bed‐generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake‐generated turbulence and form drag of step–pool bedforms. Copyright © 2000 John Wiley & Sons, Ltd.     

Step–pool evolution in the Rio Cordon,northeastern Italy

The principle that formative events, punctuated by periods of evolution, recovery or temporary periods of steady‐state conditions, control the development of the step–pool morphology, has been applied to the evolution of the Rio Cordon stream bed. The Rio Cordon is a small catchment (5 km²) within the Dolomites wherein hydraulic parameters of floods and the coarse bedload are recorded. Detailed field surveys of the step–pool structures carried out before and after the September 1994 and October 1998 floods have served to illustrate the control on step–pool changes by these floods. Floods were grouped into two categories. The first includes ‘ordinary’ events which are characterized by peak discharges with a return time of one to five years (1·8–5·15 m³ s^?1) and by an hourly bedload rate not exceeding 20 m³ h^?1. The second refers to ‘exceptional’ events with a return time of 30–50 years. A flood of this latter type occurred on 14 September 1994, with a peak discharge of 10·4 m³ s^?1 and average hourly bedload rate of 324 m³ h^?1. Step–pool features were characterized primarily by a steepness parameter c = (H/L_s)/S. The evolution of the steepness parameter was measured in the field from 1992 to 1998. The results indicate that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events. During this period, bed armouring dominate the sediment transport response. However, following an extraordinary flood and unlimited sediment supply conditions, the steepness factor can suddenly decrease as a result of sediment trapped in the pools and a lengthening of step spacing. The analogy of step spacing with antidune wavelength and the main destruction and transformation mechanism of the steps are also discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

Hydraulic geometry and maximum flow efficiency as products of the principle of least action

Basic flow relationships have previously been seen to be insufficient to explain the self‐adjusting mechanism of alluvial channels and as a consequence extremal hypotheses have been incorporated into the analyses. In contrast, this study finds that by introducing a channel form factor (width/depth ratio), the self‐adjusting mechanism of alluvial channels can be illustrated directly with the basic flow relations of continuity, resistance and sediment transport. Natural channel flow is able to reach an optimum state (Maximum Flow Efficiency (MFE), defined as the maximum sediment transporting capacity per unit available stream power) with regard to the adjustment of channel form such that rivers exhibit regular hydraulic geometry relations at dominant or bankfull stage. Within the context of MFE, this study offers support for the use of the concepts of maximum sediment transporting capacity (MSTC) and minimum stream power (MSP). Furthermore, this study indicates that the principle of least action is able to provide a physical explanation for the existence of MFE, MSTC and MSP. Potential energy is minimized and consequently sediment transport is maximized in alluvial channels. Copyright © 2000 John Wiley & Sons, Ltd.     

Riffle‐pool morphometry and stage‐dependant morphodynamics of a large floodplain river (Vereinigte Mulde,Sachsen‐Anhalt,Germany)

Riffle‐pool sequences are a common feature of gravel‐bed rivers. However, mechanisms of their generation and maintenance are still not fully understood. In this study a monitoring approach is employed that focuses on analysing cross‐sectional and longitudinal channel geometry of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany) with a high temporal and spatial resolution, in order to conclude from stage‐dependant morphometric changes to riffle and pool maintaining processes. In accordance with previous authors, pool cross‐sections of the Mulde River are narrow and riffle cross‐sections are wide suggesting that they should rather be addressed as two general types of channel cross‐sections than solely as bedforms. At high flows, riffles and pools in the study reaches changed in length and height but not in position. Pools were scoured and riffles aggraded, a development which was reversed during receding flows below the threshold of 0·4Q_bf (40% bankfull discharge). An index for the longitudinal amplitude of riffle‐pool sequences, the bed undulation intensity or bedform amplitude, is introduced and proved to be highly significant as a form parameter, its first derivative as a process parameter. The process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation. It is indicated by increasing longitudinal bed amplitudes. According to the observed dynamics of bed amplitudes, maintenance of riffle‐pool sequences lags behind discharge peaks. Maximum bed amplitudes may be reached with a delay of several days after peak discharges. Increasing bed undulation intensity is interpreted to indicate bed mobility. Post‐flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools. This type of transport behaviour is referred to as disconnected mobility. The comparison of two river reaches, one with undisturbed sediment supply, the other with sediment deficit, suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions. It is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload. Copyright © 2011 John Wiley & Sons, Ltd.