ALBERT

Description: Pool-riffle sequences are geomorphological features of many streams, thought to contribute to the hydrodynamic variability necessary to support healthy habitat conditions. Due to this fact, the addition of artificial pools and riffles is a common alternative for restoration projects on channelized streams. In this paper, detailed three-dimensional (3-D) flow measurements conducted on a scale model of an existing pool-riffle design implemented as part of a restoration project is presented. The design incorporated the basic features of natural pool-riffle sequences but maintained the deepest part of the pool in the center of the cross section and away from the banks. Results showed that the 3-D flow patterns were qualitatively different for two discharge conditions tested. The lower discharge case was strongly affected by the topography, displaying a pattern consistent with a secondary flow generated by the curvature of the streamlines. The higher discharge case was less affected by the topography, presenting a secondary flow pattern similar to that observed over a flat bed and typically associated with turbulence anisotropy. Self-maintenance and flow variability were also investigated. Even though convergence of the values of bed shear stresses at pool and riffle sections with increasing discharge did take place, reversal conditions did not occur. The difference in flow structure with flow stage was also reflected in the spatial flow variability, the lower discharge displaying larger variability than the higher discharge. The higher discharge generated a level of variability comparable with the values obtained over a flat bed.

Description: The lake levels in Lake Michigan-Huron have recently fallen to near historical lows, as has the elevation difference between Lake Michigan-Huron compared to Lake Erie. This decline in lake levels has the potential to cause detrimental impacts on the lake ecosystems, together with social and economic impacts on communities in the entire Great Lakes region. Results from past work suggest that morphological changes in the St Clair River, which is the only natural outlet for Lake Michigan-Huron, could be an appreciable factor in the recent trends of lake level decline. A key research question is whether bed erosion within the river has caused an increase in water conveyance, therefore, contributed to the falling lake level. In this paper, a numerical modeling approach with field data is used to investigate the possibility of sediment movement in the St Clair River and assess the likelihood of morphological change under the current flow regime. A two-dimensional numerical model was used to study flow structure, bed shear stress, and sediment mobility/armoring over a range of flow discharges. Boundary conditions for the numerical model were provided by detailed field measurements that included high-resolution bathymetry and three-dimensional flow velocities. The results indicate that, without considering other effects, under the current range of flow conditions, the shear stresses produced by the river flow are too low to transport most of the coarse bed sediment within the reach and are too low to cause substantial bed erosion or bed scour. However, the detailed maps of the bed show mobile bedforms in the upper St Clair River that are indicative of sediment transport. Relatively high shear stresses near a constriction at the upstream end of the river and at channel bends could cause local scour and deposition. Ship-induced propeller wake erosion also is a likely cause of sediment movement in the entire reach. Other factors that may promote sediment movement, such as ice cover and dredging in the lower river, require further investigation. Copyright © 2012 John Wiley & Sons, Ltd.

Description: Past analytical studies of meander planform development have mostly focused on the complexity of the governing equations, i.e., hydrodynamics, and less so on the stream bank resistance to erosion, whose spatial heterogeneity is difficult to describe deterministically. This motivated the use of a Monte Carlo approach to examine the effects of floodplain soils and their distribution on planform development, with the goal of including bank erosion properties in the analysis. Simulated bank erosion rates are controlled by the resistance to hydraulic erosion of the bank soils using an excess shear stress approach. The spatial distribution of critical shear stress across the floodplain is delineated on a rectangular, equidistant grid with varying degrees of variability. The corresponding erodibility coefficient is computed using a field-derived empirical relation. For a randomly disturbed distribution, in which the mean resistance to erosion exponentially increases away from the valley centerline, two relevant parameters are identified: the standard deviation of the critical shear stress distribution, which controls skewness and variability of the channel centerline, and the cross-valley increase in soil resistance, which constrains lateral migration and also affects bend skewness. For a purely random distribution, migrated centerlines exhibit larger variability for increasing spatial scales of floodplain soil heterogeneity. For equal stochastic variability of the corresponding governing parameters, relating meander migration to hydraulic erosion of the bank soils produces more variability and shape complexity than the “classic” bank migration approach of Ikeda et al. (1981), which relates migration rate to excess velocity at the outer bank. Finally, the proposed stochastic approach provides a foundation for estimating a suitable spatial density of measurements to characterize the physical properties of floodplain soils and vegetation.

Description: Wind stress over the ocean depends on the sea surface roughness which is determined by the sea state. On one hand, underdeveloped wind seas, rougher than their fully developed counterpart, increase the drag. On the other hand, the presence of swell can modify wind stress by modifying the wind sea roughness. This latter mechanism is believed to have a great impact at high winds whenever underdeveloped local waves coexist with swell. Detailed measurements of wind stress and wavefield in fetch-limited growth conditions were made in an area subjected to strong and persistent winds. Through the analysis of wavefield observations, it is found that the presence of swell dampens the short wind waves. The observed attenuation is greater for younger wind seas and decreases as the wind waves become older. Results obtained from modeling the interaction of wind waves and the air flow above point out that the attenuation of short wind waves causes a reduction of the wave-supported stress, which in turn decreases the total wind stress.

Description: Meander migration and planform evolution depend on the resistance to erosion of the floodplain materials. To date, research to quantify meandering river adjustment has largely focused on resistance to erosion properties that vary horizontally. This paper evaluates the combined effect of horizontal and vertical floodplain material heterogeneity on meander migration by simulating fluvial erosion and cantilever and planar bank mass failure processes responsible for bank retreat. The impact of streambank failures on meander migration is conceptualized in our RVR Meander model through a bank armoring factor associated with the dynamics of slump blocks produced by cantilever and planar failures. Simulation periods smaller than the time to cutoff areconsidered, such that all planform complexity is caused by bank erosion processes and floodplain heterogeneity and not by cutoff dynamics. Cantilever failure continuously affects meander migration, because it is primarily controlled by the fluvial erosion at the bank toe. Hence, it impacts migration rates and meander shapes through the horizontal and vertical distribution of erodibility of floodplain materials. Planar failures are more episodic. However, in floodplain areas characterized by less cohesive materials, they can affect meander evolution in a sustained way and produce preferential migration patterns. Model results show that besides the hydrodynamics, bed morphology and horizontal floodplain heterogeneity, floodplain stratigraphy can significantly affect meander evolution, both in terms of migration rates and planform shapes. Specifically, downstream meander migration can either increase or decrease with respect to the case of a homogeneous floodplain; lateral migration generally decreases as result of bank protection due to slump blocks; and the effect on bend skewness depends on the location and volumes of failed bank material caused by cantilever and planar failures along the bends, with possible achievement of downstream bend skewness under certain conditions. Therefore, floodplain stratigraphy must be accounted for when estimating meander migration within floodplains. Results also suggest that the generation of slump blocks due to mass failure mechanisms and their effect on rates of bank erosion must be included explicitly in models of meander migration: tweaking the bank erodibility parameters for fluvial erosion to indirectly account for those processes cannot sufficiently reproduce their effects on meander migration patterns, because slump block generation depends on river planform configuration, direction of migration, and location of the river bends with respect to the floodplain material patches.

Description: New experimental data on bedform initiation under unidirectional, oscillatory, and combined flows are presented to gain quantitative insight into bedform genesis from artificially-generated defects on a flat sediment-laden bed. Planform changes revealed in time-lapse photography, allowed the study of the evolution of the downstream and upstream edges of defects from their initial geometric center. Based on this temporal data set and flow velocity profiles, it was observed that combined flow bedforms share the same bedform initiation processes as unidirectional and oscillatory flows, which are reflected directly in the generation of similar geometric patterns regardless of the hydrodynamic conditions. The development of these bed defects is strongly coupled to the direction and magnitude of the shear stress applied to the bed throughout the wave cycle. Under current-dominated combined flow conditions, no defect propagation occurred in the upstream direction, despite the presence of flow reversal. In addition, spectral analysis of the evolution of the downstream and upstream edges of the defects demonstrated that: (i) periodicity in the defect growth pattern scales with the wave period for pure oscillatory flows, (ii) wave-dominated combined flows possess a period-driven peak with respect to defect growth in experiments with relatively short wave periods, but this peak is absent in experiments possessing relatively long wave periods (T 〉15 s), and (iii) current-dominated combined flows and pure unidirectional flows do not display a period-driven peak in defect growth. These results suggest that the occurrence of long-period combined flows bedforms may be under-represented in the sedimentary record.

Description: The development of bedforms under unidirectional, oscillatory and combined-flows results from temporal changes in sediment transport, flow and morphological response. In such flows, the bedform characteristics (for example, height, wavelength and shape) change over time, from their initiation to equilibrium with the imposed conditions, even if the flow conditions remain unchanged. These variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record. Hence, understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record. This article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional, purely oscillatory and combined-flow conditions, which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples. The present results yield a unified model for bedform development and equilibrium under unidirectional, oscillatory and combined-flows. The experimental results show that the processes of bedform genesis and growth are common to all types of flows, and can be characterized into four stages: (i) incipient bedforms; (ii) growing bedforms; (iii) stabilizing bedforms; and (iv) fully-developed bedforms. Furthermore, the development-path of bedform growth exhibits the same general trend for different flow types (for example, unidirectional, oscillatory and combined-flows), bedform size (for example, small versus large ripples), bedform shape (for example, symmetrical or rounded), bedform planform geometry (for example, two-dimensional versus three-dimensional), flow velocities and sediment grain sizes. The equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition. This article is protected by copyright. All rights reserved.

Description: The two-photon absorption (TPA) coefficient β and the nonlinear index of refraction n 2 for bulk cuprous oxide (Cu 2 O) direct gap semiconductor single crystal have been measured by using a balance-detection Z-scan single beam technique, with an excellent signal to noise ratio. Both coefficients were measured at 790 nm using a 65 fs laser pulse at a repetition rate of 90.9 MHz, generated by a Ti:Sapphire laser oscillator. The experimental values for β were explained by using a model that includes allowed-allowed, forbidden-allowed, and forbidden-forbidden transitions. It was found that the forbidden-forbidden transition is the dominant mechanism, which is consistent with the band structure of Cu 2 O. The low value for β found in bulk, as compared with respect to thin film, is explained in terms of the structural change in thin films that result in opposite parities of the conduction and valence band. The n 2 is also theoretically calculated by using the TPA dispersion curve and the Kramers-Kronig relations for nonlinear optics.

Description: [1]  Past analytical studies of meander planform development have mostly focused on the complexity of the governing equations, i.e., hydrodynamics, and less so on the stream bank resistance to erosion, whose spatial heterogeneity is difficult to describe deterministically. This motivated the use of a Monte Carlo approach to examine the effects of floodplain soils and their distribution on planform development, with the goal of including bank erosion properties in the analysis. Simulated bank erosion rates are controlled by the resistance to hydraulic erosion of the bank soils using an excess shear stress approach. The spatial distribution of critical shear stress across the floodplain is delineated on a rectangular, equidistant grid with varying degrees of variability. The corresponding erodibility coefficient is computed using a field-derived empirical relation. For a randomly disturbed distribution, in which the mean resistance to erosion exponentially increases away from the valley centerline, two relevant parameters are identified: the standard deviation of the critical shear stress distribution, which controls skewness and variability of the channel centerline, and the cross-valley increase in soil resistance, which constrains lateral migration and also affects bend skewness. For a purely random distribution, migrated centerlines exhibit larger variability for increasing spatial scales of floodplain soil heterogeneity. For equal stochastic variability of the corresponding governing parameters, relating meander migration to hydraulic erosion of the bank soils produces more variability and shape complexity than the “classic” bank migration approach of Ikeda et al. (1981), which relates migration rate to excess velocity at the outer bank. Finally, the proposed stochastic approach provides a foundation for estimating a suitable spatial density of measurements to characterize the physical properties of floodplain soils and vegetation.

Description: ABSTRACT An in-house fully three-dimensional general-purpose finite element model is applied to solve the hydrodynamic structure in a periodic Kinoshita-generated meandering channel. The numerical model solves the incompressible Reynolds-Averaged Navier-Stokes (RANS) equations for mass and momentum, while solving the k-ε equations for turbulence. The free surface is described by the rigid-lid approximation (using measured water surface data) for flat (smooth-bed) and self-formed (rough-bed) conditions. The model results are compared against experimental measurements in the “Kinoshita channel” where three-dimensional (3D) flow velocities and turbulence parameters were measured. This validation was carried out for the upstream-valley meander bend orientation under smooth (flat bed) condition. After validation, several simulations were carried out to predict the hydrodynamics in conditions where either it was not possible to perform measurements (e.g. applicability of the laboratory acoustic instruments) and to extrapolate the model to other planform configurations. For the flat smooth-bed case, asymmetric (no skewness) planform configuration was modeled and compared to the upstream-skewed case. For the self-formed rough-bed case, prediction of the hydrodynamics during the progression of bedforms was performed. It appears that the presence of bedforms on a bend induces: [1] the natural secondary flow of the bend is disrupted by the presence of the bedforms, thus depending on the location of the dune, secondary flows might differ completely from the traditional orientation, [2] an increment on both the bed and bank shear stresses, having as much as 50 % more fluvial erosion, and thus, a potential increment on the migration rate of the bend. Implications on sediment transport and bend morphodynamics are also discussed along the paper. This article is protected by copyright. All rights reserved.