The accuracy of a river bed moulding/casting system and the effectiveness of a low-cost digital camera for recording river bed fabric

Digital photogrammetry has been used to develop and test an artificial river bed moulding and casting system, which allows the pebbles within a coarse grain river bed to be recreated for hydraulic research in a laboratory flow channel or flume. Imagery of both the original streambed and the cast facsimile was acquired using a non-metric Kodak DCS460 digital camera and digital elevation models and orthophotographs were derived and compared to assess the accuracy of the moulding and casting system. These comparative tests proved to be critical in modifying and developing the system. Additional imagery was obtained in the field using a non-metric Olympus C3030 “compact” digital camera to assess whether far cheaper camera technology could deliver data appropriate for such comparative examinations. Internal calibration parameter sets and data that were generated were compared with data obtained by the non-metric Kodak DCS460. These tests demonstrate that digital sensors built around highquality 35 mm professional camera bodies and lenses are required for comparative examinations and for similar system development

Comment : photographic techniques for characterizing streambed particle sizes

Comment : photographic techniques for characterizing streambed particle size

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. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Diel patterns of suspended sediment flux and the zoogeomorphic agency of invasive crayfish

The role of biotic forcing in fluvial geomorphology is understudied. This paper investigates the suggestion that the activities of signal crayfish (Pacifastacus leniusculus) can increase suspended sediment fluxes in rivers. Previous field work, supported by mesocosm experiments, suggests that crayfish nocturnalism can cause night time increases in turbidity, but field data are limited to a 16-hour period from a single site where suspended sediment time-series are not available. Here, field data collected over a 28-day period on the Brampton Branch of the River Nene, UK, are used to quantify the impact of diel fluctuations in suspended sediment concentration on sediment fluxes. Field observations and laboratory experiments are used to evaluate the likelihood that crayfish, which are abundant in this river, are responsible for the diel patterns. Turbidity and water stage were measured at 2-minute intervals at a single site. Water was sampled for suspended sediment concentration on a diel cycle and during storm events. A relation between suspended sediment concentration and turbidity, along with a local discharge time-series, permitted calculation of sediment flux and sediment loads. Aquarium experiments with one or two crayfish were used to directly observe the relative impacts of crayfish activity and conspecific interactions on sediment suspension. Over the 28-day period, turbidity and suspended sediment exhibited a strong diel pattern, characterised by night-time increases in the frequency and magnitude of spikes in the turbidity data and by increases in ambient suspended sediment concentrations. Small diel fluctuations in stage were also measured, but the rises in stage were out of phase with turbidity and there was no correlation between stage and turbidity at any frequency. In the absence of a credible hydraulic explanation for the increases in night-time suspended sediment concentration, several lines of evidence, including results from the aquarium experiments, strongly suggest that crayfish are the most likely cause. We estimate that crayfish activity contributed at least 20% of the suspended sediment load over the 28-day period (which included two moderate floods) and this proportion was 47% during the days when baseflow conditions prevailed. This work extends and strengthens the argument that crayfish are important zoogeomorphic agents with potential implications for managing fine sediment pressures. It also highlights the need to better understand the link between crayfish activity, sediment suspension and downstream dispersal, particularly the catchment-scale distribution and seasonality of such impacts

Structural properties of mobile armours 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)