Scour Features at Wood Bundles

Structures like blunt-nosed chevrons, log deflectors and double-winged log frames help in modifying the flow regime in the channel by concentrating the flow and increasing navigability. Moreover, they create scour pools in the downstream stilling basin, which can be used either as fish refuge or as an in-stream storage site for previously dredged material. In this respect, the use of wood debris in the channel in the form of wood bundles has gained attention for the ability of these structures to integrate into the surrounding fluvial habitat and to divert the flow partially towards the central part of the channel when placed in curves. Considering the absence of studies dealing with wood bundles as a restoration structure, the aim of this paper is to analyse the scour mechanism and equilibrium scour morphology of wood bundles in straight and curved channels. In doing so, a wide range of hydraulic conditions, structure positions and configurations were tested. Thereafter, dimensional analysis was carried out to derive useful empirical relationships to predict the maximum scour depth and length as well as the maximum dune height based on a novel, equivalent Froude number, which accounts for the effects of channel curvature and structure position. Moreover, the various resulting scour morphology types were classified, and conditions of their existence were determined depending on the abovementioned Froude number and other key hydraulic parameters.

Use of Multidimensional Models to Investigate Boundary Shear Stress through Meandering River Channels

Three-dimensional hydraulics were simulated through a wide range of synthetically generated meandering river channels to determine how channel curvature and width would correlate with the maximum boundary shear stress. Multidimensional models were applied, similar to a computational flume to simulate a wide range of 72 meandering channels, developed from sine-generated curves. Cannel sinuosity ranged from 1.1 to 3.0 and included five consecutive meander bends. Longitudinal slopes of the various channels spanned four orders of magnitude, while bankfull discharges spanned three orders of magnitude. Using results from one-half of the simulation sets, an empirical correlation was found to predict the maximum boundary shear stress as a function of dimensionless ratios of channel curvature and width. The remaining simulation sets were used for verification. Multidimensional models were used to simulate channel hydraulics to efficiently investigate a wide range of channel sinuosity, width/depth ratios, bankfull discharges, and valley slopes. When simulating such a wide range of channel conditions, multidimensional models offer a more efficiency method of generating consistent datasets than either field studies or physical modeling. This paper demonstrates how multidimensional models can be used to identify important hydraulic relationships that are otherwise difficult to determine.

Flow in groovy curved channels with thin gap

A pressure-driven viscous flow through groovy curved channels of small width compared to the groove wavelength is studied. The Reynolds number is assumed to be very small, such that the flow is dominated by the viscous and the pressure-gradient forces. The effects of the channel geometry on the inertial free flow are analyzed. Two distinct flow directions are considered: (i) flow transverse to the grooves and (ii) flow longitudinal to the grooves. The velocities for both flow directions are obtained, and their distributions are found to be significantly affected by the grooves and channel curvature. The axial pressure gradient for the transverse flow is examined as a function of the amplitude and the phase difference. The results further indicate that the flow rate can be increased by the grooves for longitudinal flow, irrespective of the phase difference, unlike transverse flow This is because the latter is more affected by grooves for the same radius of curvature and phase difference.

The Straightening of a River Meander Leads to Extensive Losses in Flow Complexity and Ecosystem Services

To assist river restoration efforts we need to slow the rate of river degradation. This study provides a detailed explanation of the hydraulic complexity loss when a meandering river is straightened in order to motivate the protection of river channel curvature. We used computational fluid dynamics (CFD) modeling to document the difference in flow dynamics in nine simulations with channel curvature (C) degrading from a well-established tight meander bend (C = 0.77) to a straight channel without curvature (C = 0). To control for covariates and slow the rate of loss to hydraulic complexity, each of the nine-channel realizations had equivalent bedform topography. The analyzed hydraulic variables included the flow surface elevation, streamwise and transverse unit discharge, flow velocity at streamwise, transverse, and vertical directions, bed shear stress, stream function, and the vertical hyporheic flux rates at the channel bed. The loss of hydraulic complexity occurred gradually when initially straightening the channel from C = 0.77 to C = 0.33 (i.e., the radius of the channel is three-times the channel width), and additional straightening incurred rapid losses to hydraulic complexity. Other studies have shown hydraulic complexity provides important riverine habitat and is positively correlated with biodiversity. This study demonstrates how hydraulic complexity can be gradually and then rapidly lost when unwinding a river, and hopefully will serve as a cautionary tale.

Assessment of meander hydro-morphodynamics using modelling approaches in an amazonian river

<p>The Madre de Dios river basin belongs to the Amazon river basin, until the study area has an approximate area of ​​47070 km<sup>2</sup>. In recent years serious problems of lateral undermining are occurring on the right bank of the river, bringing with it problems in nearby towns and the layout of the interoceanic highway that connects to the country of Brazil. The case study is the Meander “La Pastora”, whose right margin is constituted by a compact clay material, which was affected by local undermining phenomena, also the approach of thalweg and sedimentation in the left margin was caused by causes of deforestation of the basin and increased solid contribution. Since 2015, measures have been built that have the function of mitigating erosion on the right bank and recovering the affected area. Using two-dimensional numerical modelling, ADCP/multibeam bathymetric surveys and limnimetric records, the hydrodynamic conditions and sediment transport will be evaluated by the hand of a results from physical modelling and inclusion of structural measures, estimating erosion and sedimentation areas that may have been in the meander. BASEMENT and IRIC NAYS2DH-FASTMECH models were used which simulated the flow conditions in different minimum and maximum hydrological scenarios compared with physical modelling results and field data, considering sediment flux corrections in curved channels with significant secondary flow motions and lateral erosion to precisely capture the complex flow field induced by channel curvature and riverbank gravitational effects.</p>

Intratidal and Subtidal Circulation in a Tropical Estuary during Wet Season: The Maroni, French Guiana

Observations of water level, current velocity, river discharge, wind and salinity were collected in the Maroni estuary, on the border of French Guiana and Suriname during the wet season of 2018 to explore subtidal circulation patterns. Measurements are complimented by the application of analytical models with an aim to diagnose forcing mechanisms responsible for producing subtidal flows during the day of data collection and to extrapolate these findings to other time periods with variable wind and river forcing. Subtidal along-channel flows were found to be dominated by river discharge, with seaward directed velocities found throughout the channel section reaching 40 cm s − 1 . This pattern was altered with strong southwesterly winds, which produced and inverse gravitational circulation pattern despite the elevated river discharge. Secondary, or cross-channel flows, displayed a three-layer vertical structure in the main channel due to a combination of channel curvature and tidal asymmetry in the lateral baroclinic pressure gradient. The pressure gradient was produced by a salinity intrusion front that only manifested in the channel during flood tide. This is the first comprehensive study of tidal and subtidal flow dynamics in the Maroni estuary.

Interaction between curvature-driven width oscillations and channel curvature in evolving meander bends

We study the morphodynamics of channel width oscillations associated with the planform development of river meander bends. With this aim we develop a novel planform evolution model, based on the framework of the classical bend theory of river meanders by Ikeda et al. (J. Fluid Mech., vol. 112, 1981), that accounts for local width changes over space and time, tied to the local hydro-morphodynamics through a two-way feedback process. We focus our attention on ‘autogenic’ width variations, which are forced by flow nonlinearities driven by channel curvature dynamics. Under the assumption of regular, sinusoidal width and curvature oscillations, we obtain a set of ordinary differential equations, formally identical to those presented by Seminara et al. (J. Fluid Mech., vol. 438, 2001, pp. 213–230), with an additional equation for the longitudinal oscillation of the channel width. The proposed approach gives insight into the interaction between autogenic width variations and curvature in meander development and between forcing and damping effects in the formation of width variations. Model outcomes suggest that autogenic width oscillations mainly determine wider-at-inflection meandering river patterns, and affect their planform development particularly at super-resonant aspect ratios, where the width oscillation reaches its maximum and reduces meander sinuosity and lateral floodplain size. The coevolution of autogenic width oscillation and curvature occurs through temporal hysteresis cycles, whereby the peak in channel curvature lags behind that of width oscillation. Width oscillation amplitudes predicted by the model are consistent with those extracted from remotely sensed data.