Tidal channel meandering and marsh erosion

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Erosion of sheltered marshes

Salt marshes located within estuaries are generally sheltered from strong wave action. Marsh vegetation in sheltered estuarine zones tends to colonize mudflats adjacent to the main tidal channel system. As the marsh cliff progresses towards the main tidal channels, it becomes increasingly vulnerable to the scouring effect of tidal currents at the base of the marsh cliff. In this situation, advance and retreat of the marsh cliff are regulated by the dynamics of channel meandering[1][2].

Dynamics of channel meandering

Fig. 1. Schematic representation of tidal flow and sediment transport in a channel bend. a: flood flow path; b: ebb flow path; c: horizontal pattern of residual fluid flow (blue) and net sediment transport (yellow); d: vertical cross-channel pattern of residual flow (blue) and net sediment transport (yellow). Red = erosion; Green=accretion.

Channel meandering results from inertia, the natural propensity of the flow to follow with delay the curvature of a bending channel. The flow is then concentrated at the outer channel bend just downstream of the location of maximum channel curvature. At this location the channel wall is subject to strong shear stress. If the channel wall consists of soft sediment it will erode and so the channel meander will enlarge, see Fig. 1a and 1b. When the outer channel bend is formed by a marsh cliff, shear-induced erosion will primarily attack the cliff base; subsequent cliff collapse by sliding or toppling is often preceded by tension cracks propagating down from the cliff top when the critical tensile strength of the bank soil is exceeded. Several other geotechnical processes contribute to bank instability, notably tidal water level variations and delayed soil pore pressures compared to the external water level. This results in excess pore water pressure on the bank slope, outward seepage and development of tension cracks during water level drawdown (ebb) and bank soil saturation during rising water, which causes loss of matric suction and decreased soil shear strength [3][4].

Figure 1c shows a schematic picture of the residual horizontal flow circulation at a channel bend. Superimposed on the tidal flow, the residual flow circulation produces a strong net sediment transport that contributes to removing eroded sediment from the outer channel bend. The residual sediment transport converges at the inner channel bend where a point bar develops. Tidal flow at a channel bend also produces a residual flow circulation in the vertical cross-channel plane, which provides an additional contribution to sediment transfer from the outer to the inner channel bend (Fig. 1d). The channel bend configuration thus produces a feedback to fluid flow and sediment transport that enlarges the channel meander. When a meander becomes too large, eventually a chute channel (shortcut channel through the point bar) will develop that cuts off the meander[5].

A numerical model simulation of the development of tidal channel meanders is shown in the article Estuarine morphological modelling. This simulation illustrates the inherent instability of straight non-curving channels; meandering can be triggered by infinitesimal small irregularities in the channel alignment. For a mathematical treatment of this phenomenon the reader is referred to the literature, e.g., [6][7]. The inherent instability of meandering channels can produce shifts in the channel pattern with a high degree of randomness, as illustrated in Fig. 2.


Fig. 2. Dynamic meander pattern of tidal channels in the eastern part of the bay of Mont-Saint-Michel. Tidal channel are dark blue. Sentinel satellite images.


Illustration of marsh erosion by channel meandering

Meandering of channels that are deeply scoured in well-consolidated sediment is a very slow process. Lateral channel shifts in one year are typically less than about 10% of the channel width[8]. Observed rates of marsh retreat are of the same order of magnitude; reported values are in the range 0.1-3 m/yr [9].

Channels in tidal basins filled with fine non-cohesive sediment display a highly dynamic meander pattern. An example is the megatidal bay of Mont-Saint-Michel (France). A large tidal flat system at the bay head contains a high proportion (about 50%) of very fine carbonate sand (0.1-0.15 mm) and carbonate silt (so-called 'tangue', 0.03-0.09 mm) which is low-cohesive and highly mobile[10]. The tidal channels therefore remain shallow and can easily shift over great distances in response to small perturbations. The highly dynamic nature of the channel pattern is illustrated in Fig. 2, showing channel shifts of more than 100 m in a single month.

Marsh vegetation in the bay of Mont-Saint-Michel tends to progress from the upper tidal flats to the interior of the bay. The bay area occupied by salt marshes has strongly increased over the past century. Their extent is limited by the meandering channel system. Fig. 3 illustrates marsh retreat in the southern part of the bay caused by meandering channels that undermine and eventually erode the marsh cliffs. See also French case studies: Upper tidal flat evolution in the bay of Mont-Saint-Michel (NW France).


Fig. 3. Marsh retreat in the southern part of the bay of Mont-Saint-Michel caused by an adjacent meandering tidal channel in the period 2019-2022, and marsh accretion in the north. The left triangle indicates the Mont-Saint-Michel and the right triangle the Pointe du Grouin; the dotted yellow line is drawn for reference. Tide channels are dark blue. Marsh vegetation is green. Sentinel satellite images.


Related articles

Dynamics, threats and management of salt marshes
Salt marshes


References

  1. ↑ Gabet, E. J. 1998. Lateral migration and bank erosion in a saltmarsh tidal channel in San Francisco Bay, California. Estuaries 21: 745–753
  2. ↑ Ladd, C.J.T., Duggan-Edwards, M.F., Pagès, J.F. and Skov, M.W. 2021. Saltmarsh Resilience to Periodic Shifts in Tidal Channels. Front. Mar. Sci. 8:757715
  3. ↑ Johansson, J. 2014. Impact of Water-Level Variations on Slope Stability. Licentiate Thesis, Lulea University of Technology
  4. ↑ Zhao, K., Coco, G., Gong, Z., Darby, S. E., Lanzoni, S., Xu, F., Zhang, K. and Townend, I. 2022. A review on bank retreat: Mechanisms, observations, and modeling. Reviews of Geophysics 60, e2021RG000761
  5. ↑ Kleinhans, M.G. and van den Berg, J.H. 2011. River channel and bar patterns explained and predicted by an empirical and a physics-based method. Earth Surface Processes and Landforms 36: 721-738
  6. ↑ Ikeda, S., Parker, G. and Sawai, K. 1981. Bend theory of river meanders. Part 1. Linear development. J. Fluid Mech. 112: 363-377
  7. ↑ Dronkers, J. 2017. Dynamics of Coastal Systems. Advanced Series on Ocean Engineering 41. World Scientific
  8. ↑ Finotello, A., Lanzoni, S., Ghinassi, M., Marani, M., Rinaldo, A, and D'Alpaos, A. 2017. Field migration rates of tidal meanders recapitulate fluvial morphodynamics. PNAS 115: 1463-1468
  9. ↑ Fagherazzi, S., FitzGerald, D.M., Fulweiler, R.W., Hughes, Z., Wiberg, P.L., McGlathery, K.J., Morris, J.T., Tolhurst, T.J., Deegan, L.A., Johnson, D.S., Lesser, J.S. and Nelson, J.A. 2022. Ecogeomorphology of Salt Marshes. Ch. 8.14 of Treatise on Geomorphology (ed. Shroder, J.F.), Elsevier
  10. ↑ Larsonneur C. 1989. La Baie du Mont Saint-Michel. Bull. Inst. Geol. Bassin Aquit. 46: 1-75



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