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Modelling sea level rise impact on salt marsh/mangrove-mudflat
morphodynamics
Ü.S.N Best1, M. van der Wegen 12 , J Dijkstra 2, B.W. Borsje 23, Dano Roelvink124 & P.W.J.M. Willemsen 23
1
UNESCO-IHE Institute for Water Education, best2@unesco-ihe.org, mvw@unesco-ihe.org,
2 Deltares, Jasper.Dijkstra@deltares.nl
3University of Twente, b.w.borsje@utwente.nl, p.willemsen@utwente.nl 4
TU Delft University of Technology, dro@unesco-ihe.org
Introduction
Over the past decade, strategies such as the Building with Nature have made coastal managers aware that only the combination of hard and soft techniques can ensure a sustainable and stable coastline. In many tropical and subtropical regions whose coast comprise of highly dynamic mudflats, the inclusion of vegetated foreshores within coastal protection strategies has been critical towards stabilizing the coastline. However in many of these countries, an effective understanding of the interaction between the vegetation and morphodynamics and the processes which result in the long term cyclic erosion-sedimentation patterns is limited by resources. Therefore the use of a schematized model, which couples the vegetation growth model with the morphodynamic modelling of Delft3D provides the framework for universal applicability and will enhance the knowledge on the triggers for the cyclic processes and the resilience of these coastlines to sea level rise.
Methodology
The vegetation growth model was developed using Matlab, which was then coupled with a 2DH depth averaged Delft3D model including wave action and tides. For the salt marsh species, Spartina and Salicornia, the growth model was based on that of Temmerman, et al. (2007) whereas the mangrove growth model is based on approach of van
Maanen, et al. (2015). For both models the initial establishment is randomized over the grid cells, followed by the growth, diffusion and decay of the vegetation in areas of high stresses. The Spartina and Salicornia were coupled with the morphodynamics every three months and based on simplified conditions similar to the Western Scheldt whereas the mangroves were coupled every year with similar forcings to that of tropical countries whose coasts receive sediments from the Amazon Basin. The trachytope application of Delft3D Flow
represented the change in bed roughness and the effect on flow well for the Spartina-Salicornia
models. Figure 1: From the top, Bed Level without vegetationi,
with vegetationii, Sea level rise (SLR) with vegetationiii
Results, Discussion and Conclusions
The marsh-mudflat system develops towards equilibrium within 100 years. Model results even show a characteristic cliff at the mudflat-marsh interface. Imposing a gradual rise in sea level incises and widens the channels and eventually drowns the system. The cliff continues to shift landward as seen by the continuous deepening in front of the marsh. Without vegetation, the formation of levees is quite noticeable. Model results provide new insights into possible impacts of sea level rise essential to address vulnerability of mangrove and marsh coasts to sea level rise.
References
Temmerman S, Bouma T, Van de Koppel J, Van der Wal D, De Vries M, Herman P (2007) Vegetation causes channel erosion in a tidal landscape. Geology 35: 631-634
van Maanen B, Coco G, Bryan KR (2015) On the ecogeomorphological feedbacks that control tidal channel network evolution in a sandy mangrove setting. Proc. R.
