Point-bar brink and channel thalweg trajectories depicting interaction between vertical and lateral shifts of microtidal channels in the Venice Lagoon (Italy)

Abstract

Tidal point bars are generally described as laterally accreting bodies, generated by lateral shift of meander bends, in which the point-bar brink (i.e. the break between bar top and bar slope) and the channel thalweg (i.e. the deepest part of the channel) shift horizontally toward the outer bank. The present study applies the concept of trajectory analysis at the point bar scale, focusing on the trajectories of point-bar brink and channel thalweg, in order to understand how vertical aggradation can interact with lateral migration to shape geometries of tidal point bars developed in a microtidal and highly aggrading salt marsh setting. We selected eight study-case meander bends, located in the Venice Lagoon and characterized by different pointbar morphologies, whose widths and depths range from 2 to 11 m and from 0.5 to 1.6 m, respectively. All the point bars were investigated through a high resolution facies-analysis carried out on closely-spaced sediment cores, collected along the bar axis. Location of bar brink and channel thalweg at different times defined specific trajectories, which were classified either as ascending or descending, and linear or non-linear. All brink trajectories are ascending, and show evidence of lateral shift of the bar brink under aggradational conditions of surrounding marshes. Development of non-linear brink trajectories is linked with changes in the ratio between vertical and lateral shift rates of the brink, which is in turn dictated by changes in local base level due to substrate compaction. Conversely, the thalweg trajectories can be either ascending or descending, reflecting an interaction between rates of lateral shift and aggradation/degradation of the channel floor. The brink and thalweg can either shift consistently (e.g., both trajectories are ascending) or incongruously (e.g., ascending brink vs. descending thalweg trajectory), reflecting different attitudes of the channel to maintain or increase its cross-sectional area.

Remotely-sensed planform morphologies reveal fluvial and tidal nature of meandering channels

“…We suggest that the observed diversity between TM and FM is largely due to differences in external forcings and intrinsic characteristics of the environments they wander through, as well as interactions thereof. In what follows, we argument this hypothesis based on review and www.nature.com/scientificreports www.nature.com/scientificreports/ physical interpretations of the ample literature on meandering channels, encompassing theoretical, numerical, and field studies, especially within the riverine framework [1][2][3][4][5][6][7][8][9][10][13][14][15][16][17][18][19][20][21][22] .…”

“…In tidal environments, most of the sedimentary products are related to tidal inundation and organic production 10,46 , and a more homogeneous soil texture is expected, owing to the continual sediment redistribution operated by tidal currents and to the quite homogeneous grainsize of the www.nature.com/scientificreports www.nature.com/scientificreports/ transported sediments 47 . In contrast, fluvial floodplains display a higher variability in terms of riparian flora and fauna [48][49][50] , pedogenic processes [51][52][53] , and both floodplain lithology and stratigraphy 5,19,54 . Moreover, self-formed lithological heterogeneities (i.e., generated by meander migration itself) of the plains hosting meandering channels are more pronounced in fluvial than in tidal landscapes, due to the greater capacity that meandering rivers have to autogenically modify their on floodplains (e.g., crevasse splays and oxbow lakes) 5,55,56 .…”

“…3, one can hardly disclose their fluvial or tidal nature. Thus far, only sedimentary facies of deposits associated with TM and FM have shown to be different, in spite of apparently similar large scale architecture of TM and FM point bars [16][17][18][19] . Differences in sedimentary facies are mainly due to the periodic flow reversal in TM, which produces distinctive sedimentary structures 16,17 .…”

An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon (Italy)

“…The terms point-bar brink and channel-thalweg trajectories were used in earlier articles to define the lateral and vertical shift of these two geomorphological elements along 2D vertical cross-sections (cf. Brivio et al, 2016;Finotello et al, 2018;Cosma et al, 2019). In the present study, this concept is developed into a three-dimensional perspective (Fig.…”

Active point bar development and river bank erosion in the incising channel of the lower Tisza river, Hungary

“…Likewise, point-bars have been the subject of various research (Dietrich and Smith, 1983;Hodskinson and Ferguson, 1998;Pyrce and Ashmore, 2005;Ikeda, 2013;Kasvi et al, 2013;Eke et al, 2014;Ghinassi et al, 2016;Wu et al, 2016;Cosma et al, 2019). The complex interplay of low-frequency high flows, and the quality and quantity of sediments, as well the geometry of rivers are significant in the deposition and erosion of a pointbar, just as in many other fluvial processes (Galia and Škarpich, 2013;Hagstrom et al, 2018;Moody, 2019).…”