Channel nourishments to feed the eroding flats in the Eastern Scheldt

Abstract

The tidal channels in the Eastern Scheldt basin, are out of equilibrium due to the reduced tidal prism as a consequence of the construction of the Storm Surge Barrier in 1986. An estimated 500 million cubic meters of sediment is required in order to reduce the cross-sectional areas of the tidal channels such that the system reaches a new equilibrium. This phenomenon is referred to as sediment starvation. The sediment starvation has been causing severe erosion of the intertidal areas which are the only available sources of sediment to feed the tidal channels because the storm surge barrier practically blocks sediment import into the estuary. The intertidal areas form the habitats for the benthic commu- nity, foraging ground for wader birds, and rest area for aquatic animals. In addition to their ecological value, the intertidal areas are valuable wave dampers and therefore important for flood protection of the hinterland. Their erosion thus harms the ecosystems as well as the flood safety of the hinterland.
Mitigation measures have been carried out in the form of directly nourishing the intertidal areas (Roggen- plaat, Galgenplaat, and Oesterdam). These nourishments have had detrimental effects on the ecology that remained for a period of 5 years after the implementation. Nourishing the channels, hence reduc- ing the cross-sectional area of the channels, can feed the intertidal areas gradually, thus preserving or increasing the ecological value of the intertidal area. Nourishing the channels can succeed according to the theory on channel-shoal interaction and the experience from the Western Scheldt; however, this has not been studied yet for the Eastern Scheldt.
In this study, the effects of a nourishment in a tidal channel have been evaluated for different nourishing methods, volumes and locations in the Eastern Scheldt Estuary. The objective of this research is to answer the following research question: Is a tidal channel nourishment in the Eastern Scheldt a feasible way of supplying the channel’s surrounding intertidal areas?
To answer this question, we applied a 2DH numerical model (ScalOost) that runs in the Delft 3D soft- ware. For this research, the forcing consists out of tidal elevations as well as a wind climate. For reasons of simplicity and to limit larger computational time, wind waves are excluded from the model’s forcing. The numerical model is capable of simulating the hydrodynamic effects and the morphody- namic evolution of a tidal channel nourishment. For four channels, the effect of nourishing on the velocity magnitude is studied for different ways of nourishing (elevating or narrowing the channel). The model results show that the considered nourishments cause a local increase of the velocity magnitude and an additional flow on the surrounding intertidal area during flood. According to the analyses of the computed hydrodynamics, the Krabbenkreek and the Brabantsche Vaarwater channels show the most potential considering the velocity magnitude increases and velocity direction changes. There- fore, these two cases were analysed separately in form of two case studies with an in-depth hydro- and morphodynamic analysis for various nourishment designs.
For the Krabbenkreek, a nourishment of 2 million cubic meters increases the maximal flow velocities in the order of 0.15 m/s, such that the critical velocity for sand transport (0.45 m/s) was exceeded over a larger part of the channel; to approximately 750 meters further landwards. The period in which the critical velocity is exceeded, increased by 15 to 60 minutes per tidal cycle. The results of the morphodynamic simulations indicate that 2.5% of the initial nourishment erodes over the first year, of which 80% settles above the MLW-line. Bearing in mind the model’s limitations, it is concluded that a tidal channel nourishment in the Krabbenkreek feeds the intertidal area at a slow pace such that the ecology is not adversely affected.
The Brabantsche Vaarwater and its two main bends were used to study the effect of secondary flow on sediment transport and eventually on the behavior of a nourishment. Historic data, as well as theoretical analysis, indicate that in both bends, the centrifugal effect is dominant over the Coriolis effect for gen- erating secondary flow. Model results confirmed this observation, yet the dominance of the centrifugal effect is larger in the second bend. As outer bends tend to erode and inner bends to accrete, the outer bends were nourished (750 & 920 ∗ 103 cubic meters) in order to use the secondary flow to transport sediment towards the inner bend and eventually onto the intertidal area. The simulation results show that for both bends, 3% of the initial nourishment erodes after the first year of which 80% accreted on both the inner and outer bend. The morphodynamic simulation results do not confirm the dominance of the centrifugal forces on sediment transport, as larger accretion rates than those simulated on the inner bends were expected. Although the results were not as expected, the nourishments did increase the velocities which increased the suspended sediment concentration in the channel and as simulation results show, sedimentation in sheltered areas.
A tidal channel nourishment in the Eastern Scheldt has been proven to be a potentially successful way of indirectly nourishing the channel’s surrounding intertidal areas. However, the accretion rates were predicted in the order of 2% per year, whereas this would be 100% if directly nourished. Furthermore, the impact of a tidal channel nourishment on the sediment starvation in the whole basin is small con- sidering the proposed volumes in this research only represent 0.4% of the actual sediment demand. Nourishing a tidal channel should be considered in view of maintaining ecological values.