Lateral Embayments Research Articles

Numerical characterization of seiche waves energy potential in river bank lateral embayments

Lateral embayments or cavities built in the river banks are used to accomplish a two-fold mission: channel stabilization and enhancement of morphological diversity and habitat suitability. The presence of these lateral embayments in the shallow flow of rivers may trigger standing resonant waves in the cross-flow direction. These water surface oscillations, typical of enclosed water domains, are called seiches. Amplitude and frequency of seiches are jointly controlled by the geometry of the embayments and by the flow rate. The study of seiches from an energy harvesting perspective is herein considered. In this work, an in-house numerical simulation model is used to assess the seiche wave energy potential in a channel with multiple lateral cavities. The model, which is based on a high-order discretization of the shallow water equations, allows to reproduce the resonant phenomena using an Unsteady Reynolds Averaged Navier-Stokes approach. The numerical results indicate that there is a range of hydraulic conditions under which energy is concentrated in a dominant cross-flow resonant seiche wave. The optimal hydraulic conditions to operate a seiche wave energy recovery are shown and discussed. High flow rates alone do not control the energy recovery efficiency, actually, a tipping point in the embayment efficiency ratio is identified. The co-existence of multi-modal seiche waves challenges the dominance of the standing wave period of interest for energy harnessing.

Continuous and discontinuous gravity currents in open-channel embayments.

Embayments, as major storage zones in riverine environments, could be surrounded by gravity currents associated with industrial pollution, heavy impurities, and sewage. The accumulated contaminated matter in embayments significantly impairs aquatic habitats and influences the embayment performance. In this study, the three-dimensional vortical structures of continuous and discontinuous gravity currents in channels connected to an embayment are investigated by solving unsteady Reynolds-averaged Navier-Stokes with algebraic Reynolds stress model (ASM) which has been improved by the buoyancy effects. The lateral embayment with different configurations is simulated to investigate the exchange processes of the dense fluid between the lateral embayment and main stream. The accuracy and consistency of the developed model are checked using the experimental data of continuous and discontinuous gravity currents in the straight channel and mass exchange in the lateral embayment. The findings have revealed that the agreement between measured and simulated flow and concentration fields is reasonable. The current head propagation of the continuous gravity current inside the embayment is similar to the discontinuous gravity current head propagation. At early time, the continuous gravity current leads to rapid diffusion of the concentration in the embayment and main channel downstream. In the discontinuous gravity current with the embayment aspect ratio equal to 1.0, the dense fluid firstly fills the lateral embayment as the most volume of the dense fluid is trapped and then it flushes out into the main channel. The geometric aspect ratio of the lateral cavity slightly affects the exchange coefficient between the main channel and lateral cavity. The time-averaged exchange coefficient is 0.33 in the continuous gravity current.

On characteristic hydraulic times through hydrodynamic modelling: discussion and application in Patos Lagoon (RS)

Although the analyses of characteristic hydraulic times are important diagnostic tools for studying water mass exchanges and identifying areas prone to stagnation that are potentially subjected to eutrophication effects, their concepts and uses are often misinterpreted. This paper compares similarities and differences between widely used characteristic hydraulic times, CHT, known as Residence Time, Times of Water Renewal Rates and Water Age. A proper definition for each of these characteristic hydraulic times is stated to avoid the existing confusion with multiple concepts in the literature. Methodologies to compute these CHT through hydrodynamic modelling systems are presented and, in order to enhance understanding, applied to three idealized cases in steady flow channels: (1) Channel with uniform flow; (2) Channel with a lateral inflow; and (3) Channel with a lateral embayment. Finally, a practical example is discussed by applying the methodologies to the Patos Lagoon (RS). The results for the idealized channel cases are non-intuitive and this theoretical discussion clarifies the interpretation and uses of different timescales and outlines the Water Age as the more versatile and multifunctional timescale if compared to the others addressed here. The results for the Patos Lagoon exemplify the valuable information that CHT can offer for environmental management in natural water bodies.

Application of 3D-PTV to mass exchange in an open-channel flow past a lateral embayment

This work presents the design and application of a Lagrangian measurement and analysis methodology, which is employed to study the flow and passive tracer exchange between a main channel and a lateral cavity in a laboratory experiment. For this purpose, a 3D-PTV system is implemented for which a static and dynamic experimental validation show that the use of a multiplane camera calibration technique and a recent trajectory linking strategy allow one to track the neutrally buoyant particles accurately in time. The resulting 3D particle trajectories are then used to quantify the 3D flow field and study the entrainment mechanisms between the main flow and the cavity using an Eulerian and a Lagrangian methodology, respectively. Analysis of the flow velocities at the geometrical interface between the main flow and the cavity indicates that the inflow is mainly concentrated at the downstream end of the cavity opening, closer to the bottom, while the particles tend to leave the cavity in the upstream part of the interface more uniform over the water depth. A vertical profile of the mass exchange coefficient is quantified based on the transverse velocity components at the interface, which confirms that the exchange intensity varies significantly with depth. More importantly, however, a novel Lagrangian trajectory classification strategy is proposed to study the transport of particles more in detail and overcome problems related to the time-dependent and 3D shape of the hydrodynamic boundary between the main channel and the cavity. Compared to the common Eulerian approach, this enables to refine the definition of mass exchange and exclude those particles that do not add to the net exchange. Subsequently a Lagrangian definition of the (depth-averaged) mass exchange coefficient is proposed, for which the current (preliminary) results indicate its potential to reliably quantify mass exchange.

The mandible of Kronosaurus queenslandicus Longman, 1924 (Pliosauridae, Brachaucheniinae), from the Lower Cretaceous of northwest Queensland, Australia

ABSTRACTThe complete mandible of the brachauchenine thalassophonean pliosaurid Kronosaurus queenslandicus Longman, 1924, is described for the first time from specimen KK F0630, discovered from the late Albian Allaru Mudstone, Rolling Downs Group, near Julia Creek, northwest Queensland. Previously undescribed anatomy results in several new features used to diagnose the taxon, including a mandibular symphysis exhibiting lateral embayments to accommodate overhanging premaxillary fangs and severe postsymphysis dentary constriction carrying embayments to accommodate large overhanging maxillary fangs. The presence of these embayments in specific areas on the lateral surface of the dentary and medial surface of the coronoid is a reflection of strongly developed anisodont dentition. The extension of a ventral dentary lamina posterior to a dorsal dentary lamina on the posterior margin of the dentary may represent a new character differentiating K. queenslandicus from K. boyacensis. Several differences are present between the mandible of KK F0630 and a previous composite reconstruction of the mandible of K. queenslandicus. The presence of six and a half pairs of functional alveoli within the mandibular symphysis in KK F0630 refutes prior research suggesting that K. queenslandicus bore three to four pairs of functional alveoli within the mandibular symphysis. A pathology exhibiting elongate grooves on the ventral surface of the right dentary is interpreted as a healed injury inflicted from the bite of a cretoxyrhinid lamniform shark. The discovery of KK F0630 further supports the notion that the late Albian Toolebuc Formation and Allaru Mudstone share similar fossil faunas.

Morphological resilience to flow fluctuations of fine sediment deposits in bank lateral cavities

Abstract Lateral cavities are built in the banks of rivers for several purposes: to create harbors, to capture sediment, to keep a central navigable channel (i.e., Casiers de Girardon in the Rhone river) or to promote the formation of aquatic habitats if a limited amount of sediment is captured, providing hydraulic and morphologic diversity (i.e., the case of Japanese Wandos). This work is focused on this latter purpose: promotion of hydraulic and morphologic diversity. In these scenarios, an increase in the flow discharge in the main channel may, however, re-mobilize the deposit of sediment inside these lateral embayments and cause a sudden increase of the sediment concentration and turbidity in the main channel. It is thus of interest to characterize the resistance and resilience of these sedimentary deposits when the main channel is subjected to high flow or flushing events. Laboratory tests were carried out for five different normalized geometries of the cavities installed in the banks of an open channel and for five hydrographs with different levels of unsteadiness. Water depth, sediment deposit mass, sediment concentration and area covered by the settled sediments were recorded throughout each experiment. Although sediment deposits established at equilibrium before the flushing events are different depending on the geometry of the cavities, generally, they are recovered after being flushed by the high flow events. It is shown that the resistance and resilience of the sediment deposits are strongly dependent on the flow field and the mass exchange between the main channel and the cavities. This mass exchange is governed by the geometry of the cavities and the magnitude of the hydrographs applied.

Influence of lateral embayments on suspended sediment transport under unsteady flow conditions

Local widening in a channelized river is a common practice in restoration projects. The lateral embayments built for this purpose in the river banks are partially filled up by fine sediments. This allows the formation of aquatic habitats with hydraulic and morphologic diversity. However, the design of these lateral cavities may be compromised by the fluctuations in the water discharge. To address this problem, systematic experimental investigations have been carried out with five different fluctuating hydrograph scenarios. Water depth, sediment concentration and area covered by the settled sediments are analysed in each experiment. The process of sedimentation in the lateral embayments proved to be, in general, resilient to flow fluctuations. However, there were several differences observed during the high flow phase fluctuation phase depending on the geometric configuration of the embayments and the applied hydrograph: (i) a higher peak in discharge means that more sediments are resuspended and a complete remobilization of the in-cavity sediment deposits is possible. (ii) Long and short cavities are more resilient to high flow events than cavities with an intermediate length.

Habitat modeling to assess fish shelter design under hydropeaking conditions

Two-dimensional (2-D) numerical simulation and habitat suitability curves for brown trout in the Lima River in northern Portugal were used to predict the velocity suitability index (VSI) for various fish shelter configurations. The modeling results were plotted against fish frequentation rates obtained from previous research for juvenile brown trout captured in the Tannenbach River on the Swiss Plateau, and tested in an artificial flume equipped with a lateral embayment. New shelter configurations were analyzed numerically to assess habitat availability for juvenile and adult Lima River trout. Good agreement was achieved between the experimental results and the hydrodynamic simulation results, highlighting similarities between the preferences of brown trout from different regions. The obstacle shape and its placement at the shelter entrance greatly influence the habitat availability. The results of this study include an easy-to-use method to assist water managers in selecting appropriate fish shelter conf...

Flow structure and mean residence times of lateral cavities in open channel flows: influence of bed roughness and shape

Natural lateral cavities in open channels are important because their lower water velocities promote water quality and provide refugia for organisms. Little is known about the influence of natural cavity shapes and roughness on flow structure and exchange dynamics. We investigated the effects of cavity shape (semi-circular, backward conic, and forward conic) and bed roughness on the flow structure and mean residence time (MRT) of a lateral cavity in a flume. All cavity shapes have a flow field dominated by a one-gyre recirculation pattern, contrasting results of rectangular cavities at similar Reynolds number and aspect ratios. Transverse velocity energy spectra indicate the flow is dominated by large-scale quasi-2D coherent structures. Fundamental frequencies of mixing layer vortex shedding are fastest for forward conic cavities and slowest for backward conic cavities. Fluid enters cavities at shallower mixing layer depths, and fluid exits cavities at deeper mixing layer depths. MRTs are smaller for hydraulically smooth cases and forward conic cavities due to higher recirculation velocities. MRTs are larger for rough bed cases and backward conic cavities. Rough flow cases have a strong correlation to a predictive MRT relationship derived by Jackson et al. (Water Resour Res: 10.1002/wrcr.20272 , 2013) (R 2 = 0.77); however, this predictive model does not work well for smooth cavities. Two MRT relationships were derived for smooth lateral cavities and both have strong power-law correlations to normalized MRT. Understanding cavity shape and bed roughness effects will provide a guideline for designing lateral embayments in stream restoration projects.

Attractiveness of a lateral shelter in a channel as a refuge for juvenile brown trout during hydropeaking

Peak power production in hydroelectric storage power plants results in frequent and intense flow variations in the rivers downstream of the plants. Fish populations can be negatively impacted when subjected to these so-called hydropeaking phenomena. In researching mitigation solutions, shelters in the riverbanks of channelized rivers have been identified as a means of protecting fish from excessive flow velocities. These shelters were studied systematically using juvenile brown trout (Salmo trutta fario) in an experimental configuration in which a straight channel was equipped with a lateral embayment. The purpose of the experiments was to generate hydrodynamic hydropeaking conditions in the channel that are undesirable for juvenile trout, thereby causing them to enter the shelter. The flow velocity distribution in the intersection plane between the main channel and the lateral shelter was found to be a significant parameter for attracting fish to the shelter. The utilization rate of trout in the shelter was used as a performance indicator. Using a basic rectangular shelter configuration without forced water exchange between the shelter and the channel, the utilization rate was only 35 %. This rate was more than doubled by introducing a deviation groyne to force water exchange between the channel and the shelter. The position and orientation angle of this groyne were systematically varied to maximize the utilization rate. Maximum utilization rates approaching 90 % were obtained for an optimum configuration in which an island-type groyne was placed in the shelter. The results of the systematic channel tests showed the potential of the shelter to attract fish. Such a shelter could be used in channelized rivers both for morphological revitalization and to improve fish habitats. As a next step in this research, prototype shelters will be built on a natural river and monitored for 2–3 years under a hydropeaking flow regime.

Distribution of Kepone in the James River Estuary

An analysis is presented defining the distribution of an organic chemical in the water and bed of an estuarine system. The present structure of the model is a tidally averaged two‐dimensional (longitudinal‐vertical) main channel interacting with lateral embayments. The water column, segmented into two layers to incorporate the typical estuarine circulation, interacts with a bed which is also segmented into two layers, an active transport layer and a deeper stationary bed. Adsorption‐desorption kinetics are included which, in the case of Kepone distribution of the James River, are assumed to be at equilibrium. Both steady‐state as well as time‐variable conditions are analyzed to calibrate the model to reproduce presently observed concentrations of Kepone. A procedure of analysis to address the problem of organic chemicals in estuaries has been structured. The procedure involves a series of analytic and computational steps, relating to the fluid transport, the solids distribution, and the concentrations of ...