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Kashyap S.,University of Ottawa | Constantinescu G.,University of Iowa | Rennie C.D.,University of Ottawa | Post G.,Northwest Hydraulic Consultants Ltd | Townsend R.,University of Ottawa
Journal of Hydraulic Engineering | Year: 2012

Flow within an alluvial channel bend is significantly affected by channel geometry, including curvature ratio (bend radius/channel width, R/B) and aspect ratio (channel width/flow depth, B=H). High curvature bends (R/B ≤ 3) can experience substantially more erosion than milder curvature bends. This study employs a three-dimensional Reynolds-Averaged Navier-Stokes (RANS) model to investigate the effects of curvature ratio and aspect ratio on bend flow with respect to a high curvature (R/B =1.5) base case in a 135° bend. Experimental data are used to validate the RANS model predictions for the high curvature base case with a flat bed (FB) and an equilibrium deformed bed (DB). Five curvature ratios (1.5, 3, 5, 8, and 10) and four aspect ratios (5.00, 6.67, 9.09, and 12.50) are investigated. Results show that a decrease in R/B or B=H for the FB cases results in a strong increase in total circulation of the regions associated with the primary cell of cross-stream circulation (G {Thorn}), an increase in maximum bed shear stress, and an increase in the contribution of the cross-stream component to the total magnitude of bed shear stress. The values of R/B and B=H also affect the structure of the cross-stream flow. The primary cell of cross-stream circulation splits into two clockwise-rotating cells at low R/B values and the cell situated closer to the inner wall induces strong ejections of vorticity. At high R/B values, a secondary counter-clockwise rotating cell forms at the outer bank. At lower B=H values, the primary cell splits into two clockwise-rotating cells. This study shows that the position and size of regions of high bed shear stress, and thus the capacity of the flow to entrain sediment, depend strongly on bend curvature. © 2012 American Society of Civil Engineers. Source


Jamieson E.C.,University of Ottawa | Post G.,University of Ottawa | Post G.,Northwest Hydraulic Consultants Ltd | Rennie C.D.,University of Ottawa
Earth Surface Processes and Landforms | Year: 2010

Experimental results of the mean flow field and turbulence characteristics for flow in a model channel bend with a mobile sand bed are presented. Acoustic Doppler velocimeters (ADVs) were used to measure the three components of instantaneous velocities at multiple cross sections in a 135° channel bend for two separate experiments at different stages of clear water scour conditions. With measurements at multiple cross sections through the bend it was possible to map the changes in both the spatial distribution of the mean velocity field and the three Reynolds shear stresses. Turbulent stresses are known to contribute to sediment transport and the three-dimensionality inherent to flow in open channel bends presents a useful case for determining specific relations between three-dimensional turbulence and sediment entrainment and transport. These measurements will also provide the necessary data for validating numerical simulations of turbulent flow and sediment transport. The results show that the magnitude and distribution of three-dimensional Reynolds stresses increase through the bend, with streamwise-cross stream and cross stream-vertical components exceeding the maximum principal Reynolds stress through the bend. The most intriguing observation is that near-bed maximum positive streamwise-cross stream Reynolds stress coincides with the leading edge of the outer bank scour hole (or thalweg), while maximum cross stream-vertical Reynolds stress (in combination with high negative streamwise-cross stream Reynolds stress near the bend apex) coincides with the leading edge of the inner bank bar. Maximum Reynolds stress and average turbulent kinetic energy appear to be greater and more localized over the scour hole before final equilibrium scour is reached. This suggests that the turbulent energy in the flow is higher while the channel bed is developing, and both lower turbulent energy and a broader distribution of turbulent stresses near the bed are required for cessation of particle mobilization and transport. © 2010 John Wiley & Sons, Ltd. Source


Kura P.K.,University of British Columbia | Kura P.K.,Northwest Hydraulic Consultants Ltd | Alila Y.,University of British Columbia | Weiler M.,Albert Ludwigs University of Freiburg
Water Resources Research | Year: 2012

Paired watershed studies have limited researchers wishing to disentangle road and harvesting effects on peak flows or to study management schemes other than the existing scenario. The outcomes of many paired watershed studies examining peak flows have also recently been challenged since only an approach that pairs peak flows by frequency can adequately evaluate the effects of harvesting on peak flows. This study takes advantage of a model that has been developed and extensively tested at a site containing a rich set of internal catchment process observations to examine the isolated and combined effects of roads and harvesting on the peak flow regime of a snow-dominated catchment for return periods of up to 100 years. Contrary to the prevailing perception in forest hydrology, the effects of harvesting are found to increase with return period, which is attributable to the uniqueness of peak flow runoff generation processes in snow-dominated catchments. Planned harvesting (50% harvest area) is found to have a significant effect (9%-25% over control) on peak flows with recurrence intervals ranging 10-100 years. Peak flow frequency increases after harvesting increase with return period, with the largest events (100 year) becoming 5-6.7 times more frequent, and medium-sized events (10 year) becoming 1.7-2 times more frequent. Such changes may have substantial ecological, hydrological, and geomorphological consequences within the watershed and farther downstream. Study findings suggest that peak flow regimes are fairly tolerant to the current level of harvesting in this particular watershed but that further harvesting may affect this element significantly. Copyright 2012 by the American Geophysical Union. Source


Van Meerveld H.J.,VU University Amsterdam | Baird E.J.,Simon Fraser University | Baird E.J.,Northwest Hydraulic Consultants Ltd | Floyd W.C.,British Columbia Ministry of forests | Floyd W.C.,University of British Columbia
Water Resources Research | Year: 2014

Unpaved resource roads have the potential to produce large amounts of sediment and can negatively impact water quality and aquatic ecology. In order to better understand the dominant controls on sediment generation from unpaved resource roads, we did 23 large-scale rainfall simulation experiments on a road section in the Honna Watershed, Haida Gwaii, British Columbia, Canada. The experiments were performed with different rainfall intensities (4-52 mm/h), with and without traffic. Precipitation intensity was the dominant control on the amount of sediment generated from the road surface; the total mass of sediment increased linearly with precipitation intensity. The number of passages of loaded logging trucks during an experiment was the second most dominant control on the total amount of sediment generated from the road surface. Elevated sediment concentrations in road surface runoff persisted for 30 min following the passage of loaded logging trucks during low intensity (<8 mm/h) rainfall events and for much shorter periods at higher rainfall intensities. The mass of sediment generated by the passage of a loaded truck increased with precipitation intensity. Passages of empty logging truck did not result in sediment pulses, except during very high rainfall intensities. Seven small-scale rainfall simulation experiments on other parts of the road, however, highlight the large spatial variability in sediment production from the road surface, suggesting additional experiments are required to better describe and predict sediment production from different road sections. © 2014. American Geophysical Union. All Rights Reserved. Source


Kuras P.K.,University of British Columbia | Kuras P.K.,Northwest Hydraulic Consultants Ltd | Alila Y.,University of British Columbia | Weiler M.,Albert Ludwigs University of Freiburg | And 2 more authors.
Hydrological Processes | Year: 2011

Hydrologic models have increasingly been used in forest hydrology to overcome the limitations of paired watershed experiments, where vegetative recovery and natural variability obscure the inferences and conclusions that can be drawn from such studies. Models are also plagued by uncertainty, however, and parameter equifinality is a common concern. Physically-based, spatially-distributed hydrologic models must therefore be tested with high-quality experimental data describing a multitude of concurrent internal catchment processes under a range of hydrologic regimes. This study takes a novel approach by not only examining the ability of a pre-calibrated model to realistically simulate watershed outlet flows over a four year period, but a multitude of spatially-extensive, internal catchment process observations not previously evaluated, including: continuous groundwater dynamics, instantaneous stream and road network flows, and accumulation and melt period spatial snow distributions. Many hydrologic model evaluations are only on the comparison of predicted and observed discharge at a catchment outlet and remain in the 'infant stage' in terms of model testing. This study, on the other hand, tests the internal spatial predictions of a distributed model with a range of field observations over a wide range of hydroclimatic conditions. Nash-Sutcliffe model efficiency was improved over prior evaluations due to continuing efforts in improving the quality of meteorological data collection. Road and stream network flows were generally well simulated for a range of hydrologic conditions, and snowpack spatial distributions were well simulated for one of two years examined. The spatial variability of groundwater dynamics was effectively simulated, except at locations where strong stream-groundwater interactions exist. Model simulations overall were quite successful in realistically simulating the spatiotemporal variability of internal catchment processes in the watershed, but the premature onset of simulated snowmelt for one of the simulation years has prompted further work in model development. © 2011 John Wiley & Sons, Ltd. Source

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