| Popis: |
The elevation and vegetation cover of salt marshes can contribute to wave attenuation in front of a levee, thus to coastal flood hazard reduction. The magnitude of such a contribution depends on the size, shape and cover of the marsh in relation to the normative hydrodynamic conditions. This function can change over time as the marsh grows or decays. Often, a high, mature marsh is appreciated because of the largest provision of flood hazard reduction compared to a lower and younger marsh. However, older marshes such as the Zuidgors along the Western Scheldt (Netherlands) might provide less ecological value (e.g., biodiversity) because they are lacking in dynamics and have a very homogeneous vegetation cover. For this reason, rejuvenation of older marshes is considered as a management option – but should not affect coastal safety unacceptably. Designs for marsh restoration or rejuvenation are often based on expert estimates. Because such an intervention has not often been carried out in similar environmental conditions, and because the development can be strongly determined by biophysical feedbacks, it is difficult to estimate in advance how the intervention will develop over time (decades). Especially quantitative substantiation of effects of different design variants on wave attenuation, siltation rate, plant and macrofauna establishment and possible use as a cattle grazing area is lacking.In order to provide such insight in advance, and thus be able to make a more effective design for the various functions together, a combination of a flood hazard assessment tool (Hydra-NL) and a numerical model that imitates the development of a salt marsh may be a solution. To this end, a combination of Delft3D-FM and a salt marsh development model (Brückner et al., 2020) was created, showing how vegetation colonizes the excavated area in different design variants: higher or lower elevation, more or less exposed to waves, and wider connection to the open waters of the Western Scheldt. In particular, the elevation and dimensions of the intervention strongly determine the expected bed level change, and thus further development. The excavation in its present form has no significant effect on flood safety. |
