Search

The widespread distribution of tidal creeks and channels that undertake meandering behaviour in modern coasts contrasts with their limited documentation in the fossil record, where point-bar elements arising from the interaction between a mix of both fluvial and tidal currents are mainly documented. The sedimentary products of tidal-channel bend evolution are relatively poorly known, and few studies have focused previously on specific facies models for tidal point bars present in modern settings. The present study improves understanding of tidal-channel meander bends through a multidisciplinary approach that combines analyses of historical aerial photographs, measurements of in-channel flow velocity, high-resolution facies analyses of sedimentary cores and three-dimensional architectural modelling. The studied channel bend (12 to 15 m wide and 2 to 3 m deep) drains a salt-marsh area located in the north-eastern sector of the microtidal Venice Lagoon, Italy. Historical photographs show that, during the past 77 years, the bend has translated seaward ca 15 m. Results show that the channel bend formed on a non-vegetated mud flat that was progressively colonized by vegetation. Seaward translation occurred under aggradational conditions, with an overall migration rate of 0.2 to 0.3 m/year, and was promoted by the occurrence of cohesive, poorly erodible outer bank deposits. Ebb currents are dominant, and translation of the channel bend promotes erosion and deposition along the landward and seaward side of the bar, respectively. Tidal currents show a clear asymmetry in terms of velocity distribution, and their offset pattern provides a peculiar grain-size distribution within the bar. During the flood stage, sand sedimentation occurs in the upper part of the bar, where the maximum flow velocity occurs. During the ebb stage, the bar experiences the secondary helical flow that accumulates sand at the toe of the bar. Lateral stacking of flood and ebb deposits has caused the formation of localized coarsening-upward and fining-upward sedimentary packages, respectively. This article is protected by copyright. All rights reserved.

This paper reports on a laboratory study that aims to reproduce a tidal channel network, in order to enhance the understanding of the morphodynamic evolution of the channel characteristics as the network expands and finally reaches equilibrium. A high-resolution laser system scanned the bed topography at different time steps creating multiple digital elevation models of the channel network. Two hundred and seventy individual channel segments are analyzed and cross-correlated in terms of their width, depth and length. The laboratory results show positive linear correlations between depth and width as well as between length and width of channel segments of the network configuration at final equilibrium. In a downstream direction, channels appear to widen more than they deepen, indirectly a sign that discharge has a stronger control on channel width than on depth. In contrast to fluvial drainage networks that commonly display fractal and scale-invariant behavior, the geometric properties of the experimental tidal creek network shows scale dependence. Channel attributes exhibit consistent patterns of exponentially decreasing abundance, with increasing creek length, depth and width. The nature of the observed exponential distributions within creek attributes (width, depth, length) allows for statistical predictability of relative creek attribute dimensions downstream and through time. Copyright © 2010 John Wiley & Sons, Ltd.

The interaction of flow with an erodible bed in alluvial rivers and deep-sea channels gives rise to a wide range of self-formed morphologies, including channels, ripples, dunes, antidunes, alternate bars, multiple-row bars, meandering and braiding. As the flow is invariably turbulent in field manifestations of these morphologies, there has been a tendency to assume that turbulence is necessary for them to form. While turbulence undoubtedly has an important influence when it is present, it is not necessary for any of these features. Indeed, all of these features can be formed by the morphodynamic interaction of purely laminar or nearly laminar flow with an erodible bed. This paper provides a survey and synthesis of a wide range of laminar or near-laminar flow analogues of morphologies observed in the field. Laminar-flow analogues of turbulent-flow morphologies cannot and should not be expected to satisfy dynamic similarity in terms of all relevant dimensionless parameters. What is of more significance is the convergence of the underlying physics. It is illustrated in this paper that many existing theoretical frameworks for the explanation of turbulent-flow morphodynamics require only relatively minor modification in order to adapt them to laminar flows.