D. Mark Powell

Patterns and processes of sediment sorting in gravel-bed rivers

Sedimentological studies of coarse-grained alluvial rivers reveal patterns of bed material sorting at a variety of spatial scales ranging from downstream fining over the length of the long profile to the vertical segregation of a coarse surface layer at the scale of individual particles. This article reviews the mechanisms that sort bed material by size during sediment entrainment, transport and deposition and discusses some of the inter-relationships that exist between patterns and processes of sediment sorting at different spatial and temporal scales. At initiation of motion, sorting can arise from the preferential entrainment of the finer fractions from the heterogeneous bed sediments. Bedload grain-size distributions are modified during transport as different size fractions are routed along different transport pathways under the influence of nonuniform bed topography and associated flow patterns, and during deposition as the variable pocket geometry of the rough bed surface and turbulence intensity of the flow control the size of the particles that deposit. The review highlights the poor understanding of the many feedback linkages that exist between patterns and processes of sediment sorting at different scales and the need for a greater awareness of the spatial and temporal bounds of these linkages.

Flash-flood and bedload dynamics of desert gravel-bed streams

Comparatively little is known about the hydrology of desert flash-floods despite the extent of the worlds drylands. There is even less known about their sedimentary behaviour and particularly about the movement of coarse material as bedload. The results of an intense field monitoring programme carried out on an ephemeral gravel-bed stream in the northern Negev Desert are presented. In this semi-arid setting, flow duration analysis indicates that the channel is hydrologically active for 2% of the time, or about seven days per year, and that overbank flow can be expected for only 0·03% of the time—about three hours per year. Multipeaked flood hydrographs are the norm, reflecting many factors including the arrival of separate slugs of discharge from contributing subcatchments. The passage of the initial flood bore is surprisingly slow, but the rising limb of the flood hydrograph is rapid with a median time of rise of 10 minutes, in keeping with expected flash-flood behaviour. Bedload flux is high, averaging 2·67 kg s−1 m−1 during the period that the channel carries flow. This gives very high bedload sediment yield despite the infrequent and short duration of flood flows and matches the high yield of suspended sediment. The relationship between bedload flux and boundary shear stress is simple, in contrast with perennial gravel-bed streams, and the exponent of the log–log relationship is 1·52. Of great value is that the behaviour of the Nahal Eshtemoa corroborates a pattern established by the authors previously in a smaller tributary stream.

Flash floods in desert rivers: Studying the unexpected

Flash floods in desert ephemeral streams are unpredictable, infrequent, and shortlived. Although some observations of flood bores have been reported [e.g., Hassan, 1990], there is little quantitative data on the hydraulic and sediment transport dynamics of desert flash floods, despite the fact that desert streams pose many problems for river and reservoir management [Tolouie et al., 19937rsqb;. Furthermore, it has been suspected for some time that rates of sediment transfer might differ markedly in ephemeral and perennial rivers [e.g., Reid and Frostick, 1987; Schick et al., 1987]. Today, because of concern over prospective climate change, water resource issues are the subject of lively debate. Since current understanding and models of sediment dynamics derived from perennial streams may not be readily applicable to ephemeral flows, there is an urgent need for improved understanding of the dynamics of dryland rivers.

Streambed scour and fill in low-order dryland channels

This research has been funded by the Natural Environment Research Council (grant GR3/12754).

Dryland Rivers: Processes and Forms

Dryland alluvial rivers vary considerably in character. In terms of processes, high energy, sediment-laden flash floods in upland rivers contrast dramatically with the low sediment loads and languid flows of their lowland counterparts while from a form perspective, the unstable wide, shallow and sandy braid plains of piedmont rivers are quite different from the relatively stable, narrow, deep and muddy channels of anastomosing systems (Nanson et al. 2002). It is also apparent that few, if any, morphological features are unique to dryland rivers. The variety of dryland river forms and the absence of a set of defining dryland river characteristics makes it difficult to generalise about dryland rivers and raises questions about whether it is necessary (or even desirable) to consider dryland river systems separately from those in other climatic zones. Indeed, as noted in the introduction to this volume, the recent shift away from the study of morphogenesis within specific climatic regimes (e.g. Tricart and Cailleux 1972) towards the study of geomorphological processes per se (e.g. Bates et al. 2005) has largely undermined the distinctiveness of desert geomorphology. This is not to say rivers draining different climatic regions do not differ in aspects of their behaviour. They clearly do, as exemplified in several reviews of tropical (Gupta 1995), periglacial (McEwen and Matthews 1998) and dryland (Graf 1988; Knighton and Nanson 1997; Reid and Frostick 1997; Tooth 2000a) fluvial geomorphology. However, given the diversity of dryland river mor-

A New Conceptual Framework for Understanding and Predicting Erosion by Water from Hillslopes and Catchments

Problems have been widely encountered when trying to scale erosion models from the plot to hillslope or catchment scale. The application of a sediment-delivery ratio is frequently used to account for the overestimation of erosion at larger scales. However, this approach overlooks problems with the long-term definition of the sediment-delivery ratio and is generally incompatible with process-based modelling. Here we propose the use of a new framework based on the transport distance of individual particles. It is demonstrated that the particle-based approach predicts theoretically the relationship between catchment size and sediment yield as encapsulated within the sediment-delivery approach. Numerical approaches show that the model works well at the hillslope scale, and work is currently underway to validate the model at the catchment scale.

Linking Short- and Long-Term Soil—Erosion Modelling

Soil erosion by overland flow is a significant process over large areas of the Earth. It leads to specific forms of landform development over both short and long time scales. In some cases, the landscape can be dramatically modified in a matter of hours, as a result of an extreme storm event. Understanding soil erosion is therefore fundamental in being able to explain the geomorphology of these areas. The soil is also a fundamental resource for human food supplies, and its loss means direct and indirect impacts on sustainability. Off-site effects of erosion can be significant both for pollution, particularly when chemical fertilisers and pesticides have been used, and for siltation of reservoirs and other structures. In extreme cases, persistent erosion can lead to a total loss of productivity, leading to desertification. The understanding of soil erosion therefore also has important practical implications.

Structural properties of mobile armors formed at different flow strengths in gravel-bed rivers

Differences in the structure of mobile armors formed at three different flow strengths have been investigated in a laboratory flume. The temporal evolution of the bed surfaces and the properties of the final beds were compared using metrics of surface grain size, microtopography, and bed organization at both grain and mesoscales. Measurements of the bed condition were obtained on nine occasions during each experiment to describe the temporal evolution of the beds. Structured mobile armors formed quickly in each experiment. At the grain scale (1–45 mm; 9 ≤ Ds50 ≤ 17 mm where Ds50 is the median surface particle size), surface complexity decreased and bed roughness increased in response to surface coarsening and the development of the mobile armor. Particles comprising the armor also became flow aligned and developed imbrication. At a larger scale (100–200 mm), the surface developed a mesoscale topography through the development of bed patches with lower and higher elevations. Metrics of mobile armor structure showed remarkable consistency over prolonged periods of near-constant transport, demonstrating for the first time that actively transporting surfaces maintain an equilibrium bed structure. Bed structuring was least developed in the experiments conducted at the lowest flow strength. However, little difference was observed in the structural metrics of the mobile armors generated at higher flows. Although the range of transport rates studied was limited, the results suggest that the structure of mobile armors is insensitive to the formative transport rate except when rates are low (τ* ≈ 0.03 where τ* is the dimensionless shear stress).

Differentiated suspended sediment transport in headwater basins of the Besor catchment, northern Negev

ABSTRACT Alexandrov, Y., Balaban, N., Bergman, N., Chocron, M., Laronne, J.B., Powell, D.M., Reid, I., Tagger, S., and Wener-Frank, I. 2008. Differentiated suspended sediment transport in headwater basins of the Besor catchment, northern Negev. Isr. J. Earth Sci. 57: 177–188. An extensive water and sediment monitoring network has been established during the past two decades in the Nahal Besor catchment of the northern Negev. Its primary purpose is to measure water and sediment fluxes at different hydrological scales and thus assist in understanding the complexity of sedimentary dynamics when these are assessed at the outlet of the trunk stream. Water and suspended sediment monitoring systems have been developed to provide discrete and continuous records, from which material fluxes have been calculated. The network involves 3 upland catchments—Nahal Eshtemoa, Nahal Sekher, and a sub-catchment of Nahal Bikhra—differing in size and/or rainfall regime. Suspended sediment rating curves for the small (0.66 km

Does the permeability of gravel river beds affect near-bed hydrodynamics?

The permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near-bed flow dynamics, and thus on fluid advection at the sediment-water interface. This study provides the first quantification of this effect for water-worked gravel-beds. Laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography. The turbulence over permeable beds is less intense, more organised and more efficient at momentum transfer because eddies are more coherent. Furthermore, turbulent kinetic energy is lower, meaning that less energy is extracted from the mean flow by this turbulence. Consequently, the double-averaged velocity is higher and the bulk flow resistance is lower over permeable beds, and there is a difference in how momentum is conveyed from the overlying flow to the bed surface. The main implications of these results are three-fold. First, local pressure gradients, and therefore rates of material transport, across the sediment-water interface are likely to differ between impermeable and permeable beds. Second, near-bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary. Third, more sophisticated flow resistance models are required for coarse-grained rivers that consider not only the bed surface but also the underlying permeable structure. Overall, our results suggest that the effects of bed permeability have critical implications for hyporheic exchange, fluvial sediment dynamics and benthic habitat availability.