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. This occurs for flood and ebb flow alike, see Fig. 1a and 1b. If the outer channel bend is formed by a marsh cliff, shear-induced erosion will primarily erode 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[3]. Several other geotechnical processes contribute to bank instability, notably tidal water level variations and delayed soil pore pressures relative to the external water level. During ebb (water level drawdown), the bank slope is subject to excess porewater pressure, resulting in outward seepage, suction stress in the unsaturated zone and development of tension cracks. During rising water, the bank soil becomes saturated and the porewater pressure increases, resulting in loss of matric suction and decreased soil shear strength[4][5]. Due to these mechanisms, the outer channel bend will erode, and the channel meander will thus become larger. Marsh vegetation stabilizes channel bend cliffs in several ways; it can retard the erosion process, but generally will not prevent it[5].

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 (collapsed bank soil) 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 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[6].

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., [7][8]. Explicit modeling of the various geotechnical processes contributing to channel meandering is quite complicated and hampered by knowledge gaps[9]; mainstream numerical morphodynamic models use crude parameterizations that require field data for calibration. 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[10]. Observed rates of marsh retreat are of the same order of magnitude; reported values are in the range 0.1-3 m/yr [11].

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[12]. 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
Morphology of estuaries


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. ↑ Langendoen, E.J. and Simon, A. 2008. Modeling the evolution of incised streams. II: streambank erosion. J. Hydraul. Eng. ASCE 134: 905–915
  4. ↑ Johansson, J. 2014. Impact of Water-Level Variations on Slope Stability. Licentiate Thesis, Lulea University of Technology
  5. ↑ 5.0 5.1 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
  6. ↑ 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
  7. ↑ Ikeda, S., Parker, G. and Sawai, K. 1981. Bend theory of river meanders. Part 1. Linear development. J. Fluid Mech. 112: 363-377
  8. ↑ Dronkers, J. 2017. Dynamics of Coastal Systems. Advanced Series on Ocean Engineering 41. World Scientific
  9. ↑ Lai, Y.G., Thomas, R.E., Ozeren, Y., Simon, A., Greimann, B.P. and Wu, K. 2015. Modeling of multilayer cohesive bank erosion with a coupled bank stability and mobile-bed model. Geomorphology 243: 116–129
  10. ↑ 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
  11. ↑ 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
  12. ↑ Larsonneur C. 1989. La Baie du Mont Saint-Michel. Bull. Inst. Geol. Bassin Aquit. 46: 1-75



The main author of this article is Job Dronkers
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