Figure 14. Conceptual sketch depicting the major differences of
hydrodynamic processes observed in tidal channels dissecting vegetated
(i.e., salt marshes, left columns) and unvegetated (i.e., mudflats,
right columns) intertidal plains. (a, b) Channel hydrodynamics during
below-bankfull water stages, with particular reference to early-flood
and late-ebb stages; (c, d) Channel hydrodynamics at the bankfull stage;
(e, f) Channel hydrodynamics during overbank stages.
6 Conclusions
This study contributes to the understanding of hydrodynamic flow
structures, and related morphodynamic evolution, in meandering channels
wandering through unvegetated tidal flats. Hydroacoustic measurements
were carried out, for several tidal cycles, at distinct locations along
a mudflat meander bend found within the macrotidal Yangkou tidal flat
(Jiangsu province, China).
The main conclusions of this research can be summarized as follows:
- Stage-velocity relationships in mudflat channels are different from
those observed in channels wandering through vegetated intertidal
plains (i.e., salt marsh and mangrove forests). Specifically, while in
the latter case both ebb and flood velocities tend to be higher for
above-bankfull water stages, in our study case we observed
significantly larger velocities when tidal flows remained confined
within the channel banks. This is likely because, in vegetated
intertidal plains, both frictionally-dominated flow propagation and
higher elevation of channel banks (relative to tidal excursions)
ensure flow confinement and high in-channel velocities even for
above-bankfull stages. In contrast, in unvegetated intertidal
mudflats, similar flow resistance within and outside channels and
lower elevation of channel banks produce widespread sheet flow at
above-bankfull stages and limit in-channel velocities due to reduced
flow confinement;
- Secondary currents appear to be mostly related to flood flows, and are
generally stronger during overbank stages. In some cases, however, the
orientation of secondary circulations is reversed compared to classic
flow models in meander bends. Poorly-developed secondary circulations
are observed at the bend apex. However, primary flow separation,
coupled with localized flow measurements that did not include the
entire channel cross-section, have likely limited our ability to
detect secondary circulation cells during our field measurements.
- Field data collectively suggest limited control of curvature-induced
helical flows on meander morphodynamics. This is most likely due to a
consistent phase lag between maxima of primary (i.e., streamwise) and
secondary (i.e., cross-sectional) velocities. Such a lag effectively
limits the landward (seaward) transfer of secondary flows during the
flood (ebb) phase, thus hampering the formation of coherent helical
flow structures along the entire meander bends. These findings support
the results of earlier studies that suggested that, in stark contrast
with both river and salt-marsh meandering channels, meander
morphodynamics in intertidal mudflats are poorly related to bankfull
hydrodynamics, in general, and curvature-induced helical flows in
particular.
- We suggest that other morphodynamic processes drive the evolution of
intertidal mudflat meander bends. Late-ebb tidal flows likely exert
strong control on meander morphodynamics due to sustained velocities
and pronounced seepage flows, which determine significant sediment
transport as well as both bank undercutting and collapses. These
effects are also possibly amplified by the absence of vegetation both
within and outside the channel, as well as by significant bioturbation
of the channel banks, which reduces bank resistance to erosion and
enhances seepage flow. In addition, storm waves and both episodic and
seasonal increases in discharges due to heavy rainfalls (e.g., related
to the monsoon season) and melting snows can compound the
morphological effects of late-ebb flows, producing abrupt morphologic
changes and pronounced channel migration.
Additional field and modeling efforts would be required to corroborate
the inferences presented in this study and to investigate how different
tidal ranges and channel-bank elevations (relative to characteristic
tidal oscillations) affect mudflat meander hydrodynamics and the related
morphodynamic evolution. Particularly, cross-sectional measurements of
tidal flow fields are needed to directly assess the scarce development
of curvature-induced helical flows, whereas repeated measurement of flow
fields during normal conditions and heavy rainfall events, coupled with
morphological monitoring of channel bank evolution, would help clarify
the relative importance of astronomic and meteorological forcings on the
morphodynamics of intertidal mudflat meanders.
Acknowledgments
This study was financially supported by the National Natural Science
Foundation of China (U2240220, 41625021), the Innovation Program of
Shanghai Municipal Education Commission (2019-01-07-00-05-E00027), the
China Scholarship Council (CSC) scholarships (202106190084) and Jiangsu
Special Program for Science and Technology Innovation (JSZRHYKJ2021006).
We thank Zhenqiao Liu, Wei Feng, Jianxiong Sun, and Dongyun Wei for
their help in field work. Special acknowledgments are given to Shibing
Zhu for his help in grain size analysis.
Open Research
The data sets generated and/or analyzed during the current study are
freely available athttps://doi.org/10.6084/m9.figshare.20161733.v2
References
Azpiroz-Zabala, M., Cartigny, M. J. B., Sumner, E. J., Clare, M. A.,
Talling, P. J., Parsons, D. R., & Cooper, C. (2017). A General Model
for the Helical Structure of Geophysical Flows in Channel Bends.Geophysical Research Letters, 44 (23), 11,932-911,941.https://doi.org/10.1002/2017GL075721
Bathurst, J. C., Thorne, C. R., & Hey, R. D. (1977). Direct
measurements of secondary currents in river bends. Nature,
269 (5628), 504-506.https://doi.org/10.1038/269504a0
Bayliss-Smith, T. P., Healey, R., Lailey, R., Spencer, T., & Stoddart,
D. R. (1979). Tidal flows in salt marsh creeks. Estuarine and
Coastal Marine Science, 9 (3), 235-255.https://doi.org/10.1016/0302-3524(79)90038-0
Bever, A. J., & MacWilliams, M. L. (2016). Factors influencing the
calculation of periodic secondary circulation in a tidal river:
numerical modelling of the lower Sacramento River, USA.Hydrological processes, 30 (7), 995-1016.https://doi.org/10.1002/hyp.10690
Blanckaert, K. (2009). Saturation of curvature-induced secondary flow,
energy losses, and turbulence in sharp open-channel bends: Laboratory
experiments, analysis, and modeling. Journal of Geophysical
Research: Earth Surface, 114 (F3).https://doi.org/10.1029/2008JF001137
Blanckaert, K. (2011). Hydrodynamic processes in sharp meander bends and
their morphological implications. Journal of Geophysical Research:
Earth Surface, 116 (F1).https://doi.org/10.1029/2010JF001806
Blanckaert, K., & de Vriend, H. J. (2003). Nonlinear modeling of mean
flow redistribution in curved open channels. Water Resources
Research, 39 (12).https://doi.org/10.1029/2003WR002068
Blanckaert, K., Kleinhans, M. G., McLelland, S. J., Uijttewaal, W. S.
J., Murphy, B. J., van de Kruijs, A., et al. (2013). Flow separation at
the inner (convex) and outer (concave) banks of constant-width and
widening open-channel bends. Earth Surface Processes and
Landforms, 38 (7), 696-716.https://doi.org/10.1002/esp.3324
Boon, J. D. (1975). Tidal discharge asymmetry in a salt marsh drainage
system1,2. Limnology and Oceanography, 20 (1), 71-80.https://doi.org/10.4319/lo.1975.20.1.0071
Bouma, T. J., Vries, M. B. D., Low, E., Kusters, L., Herman, P. M. J.,
Tánczos, I. C., et al. (2005). Flow hydrodynamics on a mudflat and in
salt marsh vegetation: identifying general relationships for habitat
characterisations. Hydrobiologia, 540 (1-3), 259-274.https://doi.org/10.1007/s10750-004-7149-0
Bridge, J. S., & Jarvis, J. (1982). The dynamics of a river bend: a
study in flow and sedimentary processes. Sedimentology, 29 (4),
499-541.https://doi.org/10.1111/j.1365-3091.1982.tb01732.x
Brivio, L., Ghinassi, M., D’Alpaos, A., Finotello, A., Fontana, A.,
Roner, M., & Howes, N. (2016). Aggradation and lateral migration
shaping geometry of a tidal point bar: An example from salt marshes of
the Northern Venice Lagoon (Italy). Sedimentary Geology, 343 ,
141-155.https://doi.org/10.1016/j.sedgeo.2016.08.005
Brooks, H., Möller, I., Carr, S., Chirol, C., Christie, E., Evans, B.,
et al. (2021). Resistance of salt marsh substrates to near-instantaneous
hydrodynamic forcing. Earth Surface Processes and Landforms,
46 (1), 67-88.https://doi.org/10.1002/esp.4912
Chant, R. J. (2002). Secondary circulation in a region of flow
curvature: Relationship with tidal forcing and river discharge.Journal of Geophysical Research, 107 (C9).https://doi.org/10.1029/2001JC001082
Chen, J. D. (2016). Sediment dynamic process on tidal flat under
windy conditions. (Master), Nanjing University, Nanjing.
Chen, X., Zhang, C., H. Townend, I., Paterson, D. M., Gong, Z., Jiang,
Q., et al. (2021). Biological Cohesion as the Architect of Bed Movement
Under Wave Action. Geophysical Research Letters, 48 (5),
e2020GL092137.https://doi.org/10.1029/2020GL092137
Choi, K. (2010). Rhythmic Climbing-Ripple Cross-Lamination in Inclined
Heterolithic Stratification (IHS) of a Macrotidal Estuarine Channel,
Gomso Bay, West Coast of Korea. Journal of Sedimentary Research,
80 (6), 550-561.https://doi.org/10.2110/jsr.2010.054
Choi, K. (2011). External controls on the architecture of inclined
heterolithic stratification (IHS) of macrotidal Sukmo Channel: Wave
versus rainfall. Marine Geology, 285 (1-4), 17-28.https://doi.org/10.1016/j.margeo.2011.05.002
Choi, K. (2014). Morphology, sedimentology and stratigraphy of Korean
tidal flats – Implications for future coastal managements. Ocean
& Coastal Management, 102 , 437-448.https://doi.org/10.1016/j.ocecoaman.2014.07.009
Choi, K., Hong, C. M., Kim, M. H., Oh, C. R., & Jung, J. H. (2013).
Morphologic evolution of macrotidal estuarine channels in Gomso Bay,
west coast of Korea: Implications for the architectural development of
inclined heterolithic stratification. Marine Geology, 346 ,
343-354.https://doi.org/10.1016/j.margeo.2013.10.005
Choi, K., & Jo, J. (2015). Morphodynamics of tidal channels In the Open
Coast Macrotidal Flat, Southern Ganghwa Island In Gyeonggi Bay, West
Coast of Korea. Journal of Sedimentary Research, 85 (6), 582-595.https://doi.org/10.2110/jsr.2015.44
Christiansen, T., Wiberg, P. L., & Milligan, T. G. (2000). Flow and
Sediment Transport on a Tidal Salt Marsh Surface. Estuarine,
Coastal and Shelf Science, 50 (3), 315-331.https://doi.org/10.1006/ecss.2000.0548
Coco, G., Zhou, Z., van Maanen, B., Olabarrieta, M., Tinoco, R., &
Townend, I. (2013). Morphodynamics of tidal networks: Advances and
challenges. Marine Geology, 346 , 1-16.https://doi.org/10.1016/j.margeo.2013.08.005
Cosma, M., Finotello, A., Ielpi, A., Ventra, D., Oms, O., D’Alpaos, A.,
& Ghinassi, M. (2020). Piracy-controlled geometry of tide-dominated
point bars: Combined evidence from ancient sedimentary successions and
modern channel networks. Geomorphology, 370 .https://doi.org/10.1016/j.geomorph.2020.107402
Cosma, M., Lague, D., D’Alpaos, A., Leroux, J., Feldmann, B., &
Ghinassi, M. (2022). Sedimentology of a hypertidal point bar
(Mont-Saint-Michel Bay, north-western France) revealed by combining
lidar time-series and sedimentary core data. Sedimentology,
69 (3), 1179-1208.https://doi.org/10.1111/sed.12942
D’Alpaos, A., Lanzoni, S., Marani, M., Bonometto, A., Cecconi, G., &
Rinaldo, A. (2007). Spontaneous tidal network formation within a
constructed salt marsh: Observations and morphodynamic modelling.Geomorphology, 91 (3-4), 186-197.https://doi.org/10.1016/j.geomorph.2007.04.013
D’Alpaos, A., Lanzoni, S., Marani, M., Fagherazzi, S., & Rinaldo, A.
(2005). Tidal network ontogeny: Channel initiation and early
development. Journal of Geophysical Research-Earth Surface,
110 (F2).https://doi.org/10.1029/2004JF000182
D’Alpaos, A., Finotello, A., Goodwin, G. C. H., & Mudd, S. M. (2021).
Salt Marsh Hydrodynamics. In Salt Marshes (pp. 53-81).https://doi.org/10.1017/9781316888933.005
Dietrich, W. E. (1987). Mechanics of flow and sediment transport in
river bends. In K. S. Richards (Ed.), River Channels: Environment
and Process (Vol. 18, pp. 179-227). Institute of British Geographers,
Special Publication: Blackwell Oxford.
Dietrich, W. E., & Smith, J. D. (1983). Influence of the point bar on
flow through curved channels. Water Resources Research, 19 (5),
1173-1192.https://doi.org/10.1029/WR019i005p01173
Dinehart, R. L., & Burau, J. R. (2005). Averaged indicators of
secondary flow in repeated acoustic Doppler current profiler crossings
of bends. Water Resources Research, 41 (9).https://doi.org/10.1029/2005WR004050
Engelund, F. (1974). Flow and Bed Topography in Channel Bends.Journal of the Hydraulics Division, 100 (11), 1631-1648.https://doi.org/10.1061/JYCEAJ.0004109
Fagherazzi, S., Hannion, M., & D’Odorico, P. (2008). Geomorphic
structure of tidal hydrodynamics in salt marsh creeks. Water
Resources Research, 44 (2).https://doi.org/10.1029/2007WR006289
Ferguson, R. I., Parsons, D. R., Lane, S. N., & Hardy, R. J. (2003).
Flow in meander bends with recirculation at the inner bank. Water
Resources Research, 39 (11).https://doi.org/10.1029/2003WR001965
Finotello, A., Canestrelli, A., Carniello, L., Ghinassi, M., &
D’Alpaos, A. (2019). Tidal flow asymmetry and discharge of lateral
tributaries drive the evolution of a microtidal meander in the Venice
Lagoon (Italy). Journal of Geophysical Research: Earth Surface,
124 (12), 3043-3066.https://doi.org/10.1029/2019JF005193
Finotello, A., Capperucci, R. M., Bartholomä, A., D’Alpaos, A., &
Ghinassi, M. (2022). Morpho-sedimentary evolution of a microtidal
meandering channel driven by 130-years of natural and anthropogenic
modifications of the Venice Lagoon (Italy). Earth Surface
Processes and Landforms, n/a (n/a), 1-17.https://doi.org/10.1002/esp.5396
Finotello, A., D’Alpaos, A., Bogoni, M., Ghinassi, M., & Lanzoni, S.
(2020). Remotely-sensed planform morphologies reveal fluvial and tidal
nature of meandering channels. Scientific Reports, 10 (1), 54.https://doi.org/10.1038/s41598-019-56992-w
Finotello, A., D’Alpaos, A., Lazarus, E. D., & Lanzoni, S. (2019). High
curvatures drive river meandering: COMMENT. Geology, 47 (10),
e485.https://doi.org/10.1130/G46761C.1
Finotello, A., Ghinassi, M., Carniello, L., Belluco, E., Pivato, M.,
Tommasini, L., & D’Alpaos, A. (2020). Three‐Dimensional flow structures
and morphodynamic evolution of microtidal meandering channels.Water Resources Research, 56 (7).https://doi.org/10.1029/2020WR027822
Flemming, H.-C., & Wuertz, S. (2019). Bacteria and archaea on Earth and
their abundance in biofilms. Nature Reviews Microbiology, 17 (4),
247-260.https://doi.org/10.1038/s41579-019-0158-9
Folkard, A. M. (2005). Hydrodynamics of model Posidonia oceanica patches
in shallow water. Limnology and Oceanography, 50 (5), 1592-1600.https://doi.org/10.4319/lo.2005.50.5.1592
Friedman, G. M. (1962). Comparison of moment measures for sieving and
thin-section data in sedimentary petrological studies. Journal of
Sedimentary Research, 32 (1), 15-25.https://doi.org/10.1306/74D70C36-2B21-11D7-8648000102C1865D
Friedrichs, C. T. (2011). Tidal Flat Morphodynamics: A Synthesis. In J.
D. Hansom & B. W. Fleming (Eds.), Treatise on Estuarine and
Coastal Science (Vol. 3, pp. 137-170). Amsterdam, Netherlands:
Elsevier. https://doi.org/10.1016/B978-0-12-374711-2.00307-7
Friedrichs, C. T., & Aubrey, D. G. (1988). Non-linear tidal distortion
in shallow well-mixed estuaries: a synthesis. Estuarine, Coastal
and Shelf Science, 27 (5), 521-545.https://doi.org/10.1016/0272-7714(88)90082-0
Friedrichs, C. T., & Perry, J. E. (2001). Tidal salt marsh
morphodynamics: a synthesis. Journal of Coastal Research , 7-37.https://www.jstor.org/stable/25736162
Frothingham, K. M., & Rhoads, B. L. (2003). Three-dimensional flow
structure and channel change in an asymmetrical compound meander loop,
Embarras River, Illinois. Earth Surface Processes and Landforms,
28 (6), 625-644.https://doi.org/10.1002/esp.471
Gabet, E. J. (1998). Lateral Migration and Bank Erosion in a Saltmarsh
Tidal Channel in San Francisco Bay, California. Estuaries, 21 (4).https://doi.org/10.2307/1353278
Gao, S. (2019). Geomorphology and Sedimentology of Tidal Flats. InCoastal Wetlands (pp. 359-381): Elsevier.