Three‐Dimensional Flow Structures and Morphodynamic Evolution of Microtidal Meandering Channels. Issue 7 (23rd July 2020)

• Record Type: Journal Article
• Authors: Finotello, Alvise; Ghinassi, Massimiliano; Carniello, Luca; Belluco, Enrica; Pivato, Mattia; Tommasini, Laura; D'Alpaos, Andrea
• Is Part Of: Water resources research. Volume 56:Issue 7(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-07-23
• Subjects: tidal meanders -- acoustic Doppler current profiler -- three‐dimensional flow -- velocity -- meander migration -- flow separation; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2020WR027822
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: The planform evolution of tidal meanders is driven by interactions between channel morphology and periodically reversing tidal flows, which feed back into the development of erosional and depositional patterns. However, the paucity of quantitative data has so far undermined detailed analyses about the geomorphic effects of tidal flows within tidal meanders. Here we aim to bond the morphodynamic evolution of tidal meanders with the structure of three‐dimensional flow that shapes them. By means of an acoustic Doppler current profiler, we have surveyed the flow fields over three different tidal meandering channels in the salt marshes at San Felice (Venice lagoon, Italy), each characterized by distinct planform morphology and evolutionary dynamics. Mutually evasive paths followed by the maximum ebb and flood streamwise velocities determine periodic changes in both the position and the orientation of curvature‐induced cross‐stream flows. These secondary flows can be locally disrupted by patches of submerged vegetation, especially in meanders of small size, with direct implications for the morphodynamic evolution of meander bends. The latter is further affected by flow separation in sharp bends. Flow separation effectively reduces channel width, enhancing bank erosion due to increasing flow velocities. Moreover, it creates low‐velocity zones of recirculating flows at both the inner and outer banks, thereby promoting the formation of point bars and concave‐bank benches, respectively. By relating three‐dimensional flow structure to patterns of channel change, our results provide a first step to unravel the relation between flows and forms within tidal meanders, whose planform characteristics may differ greatly from their fluvial counterparts. Key Points: Acoustic measurements of 3D flow structures in tidal meanders highlight pronounced asymmetries in the ebb and flood velocities Flow separation in sharp bends, controlled by the radius‐width ratio, can lead to unidirectional flow in some parts of the bar deposits Aquatic vegetation can influence the evolution of tidal meanders by disrupting and limiting the transfer of curvature‐induced helical flow