4 research outputs found
Sensitivity of interfacial hydraulics to the microtopographic roughness of water-lain gravels
Flow within the interfacial layer of gravel-bed rivers is poorly understood, but this zone is important because the hydraulics here transport sediment, generate flow structures and interact with benthic organisms. We hypothesized that different gravel-bed microtopographies generate measurable differences in hydraulic characteristics within the interfacial layer. This was tested using a high density of spatially and vertically distributed, velocity time series measured in the interfacial layers above three surfaces of contrasting microtopography. These surfaces had natural water-worked textures, captured in the field using a casting procedure. Analysis was repeated for three discharges, with Reynolds numbers between 165000 and 287000, to evaluate whether discharge affected the impact of microtopography on interfacial flows. Relative submergence varied over a small range (3.5 to 8.1) characteristic of upland gravel-bed rivers. Between-surface differences in the median and variance of several time-averaged and turbulent flow parameters were tested using non-parametric statistics. Across all discharges, microtopographic differences did not affect spatially averaged (median) values of streamwise velocity, but were associated with significant differences in its spatial variance, and did affect spatially averaged (median) turbulent kinetic energy. Sweep and ejection events dominated the interfacial region above all surfaces at all flows, but there was a microtopographic effect, with Q2 and Q4 events less dominant and structures less persistent above the surface with the widest relief distribution, especially at the highest Reynolds number flow. Results are broadly consistent with earlier work, although this analysis is unique because of the focus on interfacial hydraulics, spatially averaged 'patch scale' metrics and a statistical approach to data analysis. An important implication is that observable differences in microtopography do not necessarily produce differences in interfacial hydraulics. An important observation is that appropriate roughness parameterizations for gravel-bed rivers remain elusive, partly because the relative contributions to flow resistance of different aspects of bed microtopography are poorly constrained
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
Movements of a macroinvertebrate species across a gravel-bed substrate: effects of local hydraulics and micro-topography under increasing discharge
Flow refugia provide a mechanism that can explain the persistence of macroinvertebrate communities in flood-prone, gravel-bed rivers. The movement
behaviour of macroinvertebrates is a key element of the flow refugia hypothesis
but surprisingly little is known about it. In particular, little is known about how
local near-bed hydraulics and bed microtopography affect macroinvertebrate
movements. We used a novel casting technique to reproduce a natural gravel-bed
substrate in a large flume where we were able to observe the movement behaviour
of the cased caddisfly Potamophylax latipennis at different discharges. The
crawling paths and drift events of animals were analysed from video recordings
and used to classify sites on the substrate according to the type of insect
movement. We used Acoustic Doppler Velocimeter (ADV) measurements close
to the boundary to characterise the hydraulic conditions at different sites and a
detailed Digital Elevation Model (DEM) to characterise sites topographically.
Animals made shorter more disjointed crawling journeys as discharge increased,
although they tended to follow consistent paths across the substrate. As we
hypothesised, crawling behaviour was locally associated with low elevations, low
flow velocities and low turbulent kinetic energies, while sites that insects avoided
were characterised by higher elevations, velocities and turbulence. Discrimination
was greater at higher discharges. We suppose that these relations reflect the need
of animals to reduce the risk of entrainment and minimise energy expenditure by
avoiding areas of high fluid drag. As discharge increased there was a general
upward shift in the frequency distributions of local velocities and turbulent kinetic
energies. The animals responded to these shifts and it is clear that their different
activities were not limited to fixed ranges of velocity and turbulence. We assume
that the absolute hydraulic forces would become a limiting factor at some higher
discharge. At the discharges examined here, which are below those required to
generate framework particle entrainment, patterns of animal movement appear to
be associated with the animals’ experiences of relative velocities rather than
absolute hydraulic forces
Applications of close-range imagery in river research
This book offers a comprehensive overview of progress in the general area of fluvial remote sensing with a specific focus on its potential contribution to river management