Inundation levels and vegetation: keys to control peak flows in wetlands

Wetlands serve as coastal protection structures via hydrological and biogeochemical processes (Junk et al., 2013), preventing soil erosion (Barcelona et al., 2018) and promoting sedimentation and soil stabilization (Montakhab et al., 2012).  Wetlands contribute to mitigate the impacts of peak flows caused by pluvial or fluvial floods or storm surges. The increase in global warming will affect coastal areas with an increase in sea level and erosive processes (Reed et al., 2018), and an increase in the frequency of hydrometeorological phenomena such as coastal flooding and maritime storms (Hoggart et al., 2014). Inland wetlands are also to be increasingly affected by pluvial and fluvial floods (Kundzewicz and Pinskwar, 2020). It is then necessary to add knowledge on the impacts of both the wetland inundation level and the vegetation water resistance on hydrodynamics and sedimentary patterns in front of a peak flow to know the wetland benefits in front of flooding events. In this study, particle ladden floods were reproduced by flume experiments were a peak flow (of water height H) flowed into a wetland with a water height h (where H > h) populated with two natural species (Juncus maritimus and Arthrocnemum fruticosum). The peak flow was found to pass through different regimes with different sedimentation patterns: peak flow adjustment; peak flow; drag-dominated peak flow; ending to the gravity current regimes. During the peak flow regime, low-inundated wetlands induced higher sedimentation rates for the coarse sediment fraction than for the fine sediment fraction, while high-inundated wetlands resulted in similar settling rates for both sediment fractions, coarse and fine. Because the coarse portion has already settled, at greater distances sedimentation rates corresponded to the fine fraction and dropped monotonically along the flume.  It was also found that the presence of vegetation enhanced the sedimentation rates compared to bare soil conditions.

This finding demonstrates how crucial vegetation is to protect the bed and prevent bed erosion in coastal regions when facing peak flows and how higher inundation levels reduces the harmful effect of the front pass by enhancing the sediment deposition.

References

Barcelona, A., Serra, T., Colomer, J., 2018. Fragmented canopies control the regimes of gravity currents development. J. Geophys. Res-Oceans, 123, https://doi.org/10.1002/2017JC01314

Hoggart, S.P.G., Hanley, M.E., Parker, D.J., Simmonds, D.J., Bilton, D.T.,  Filipova-Marinova, M., Franklin, E.L., Kotsev, I., Penning-Rowsel, E.C., Rundle, S.D., Trifonova, E., Vergiev, S., White, A.C., Thompson, R.C., 2014. The consequences of doing nothing: The effects of seawater flooding on coastal zones. Coast. Eng. 87, 169–182. https://doi.org/10.1016/j.coastaleng.2013.12.001

Junk, W.J., An, S., Finlayson, C.M., Gopal, B., Kveˇt, J., Mitchell, S.A., Mitsch, W.J., Robarts, R.D., 2013. Current state of knowledge regarding the world’s wetlands and their future under global climate change: a synthesis. Aquat. Sci. 75, 151–167. https://doi.org/10.1007/s00027-012-0278-z.

Kundzewicz, Z.W., Pinskwar, I., 2022. Are Pluvial and Fluvial Floods on the Rise? Water 2022, 14, 2612. https://doi.org/10.3390/ w14172612

Montakhab, A., Yusuf, B., Ghazali, A. H., Mohamed, T. A., 2012. Flow and sediment transport in vegetated waterways: a review. Rev. Environ. Sci. Bio. 11(3), 275-287. https://doi.org/10.1007/s11157-012-9266-y