Tidal flat nourishment – Galgeplaat

Lessons Learned

Several lessons-learned have been obtained on the field of morphology, ecology dynamics and (predictive) modelling 

For morphology

The nourishment has a relatively long-lasting effect on the height and emergence time at the nourished area itself. On the other hand, the nourishment only has a limited effect on the surrounding area on the flat. Several factors can explain this limited redistribution:

  • The nourishment is situated at a location with relatively calm hydrodynamic conditions. The edges of the intertidal flats received the brunt of wave activity. The wave energy is dissipated in shallower waters towards the middle of the flats where the nourishment is located.
  • There were no severe storms during the observation period. Eight storms (wind force 8 Beaufort or higher) occurred after the nourishment was in place in 2008-2009 and three storms in 2010, but no storm with wind force 10 Beaufort or more.
  • The size of the nourishment is relatively small compared to the entire flat and therefore has had no large impact so far on the surrounding flat.
  • The circular shape of the nourishment with its bounding ‘wall’ does not allow sediment to be transported easily to the surrounding flat. The slightly larger grain size may also increase stability of the nourishment. A more open, less compacted and less flat nourishment could increase the sediment transport from the nourished area.
  • The bed level at which the nourishment is placed also influences the sediment spreading. Model results show that a nourishment erodes faster when placed at a lower part of the shoal.

For ecology

The nourishment buried and killed all benthic macrofauna. The recolonisation of the nourished area started directly after the nourishment was finished. After 2-3 years, the recolonisation appears to be most successful on the lower parts of the nourishment. The recolonisation seems to be driven by:

  • the water content of the sediment during low tide;
  • the slope, enlarging or reducing drainage of bed.

Although the emergence time was negatively correlated with benthic recolonisation on the Galgeplaat nourishment, this factor seems less relevant and correlated with the water content (dry vs wet areas) of the sediment. Shaping the nourishment with bulldozers is not recommendable, as this might additionally compact the sediment.

For modelling

  • A 2D (depth-averaged) hydraulic model is a useful tool for decisions on the location and shape of the nourishment and to assess the important driving processes for sediment distribution around the nourishment.
  • The simulations showed that locally generated waves play an important role in the transport of sediments around and on the flats and nourishment. This transport should be investigated further by looking at the resuspension processes involved due to both currents, waves and wave-current interaction.
  • There are many biogeomorphological processes influencing sediment transport and morphology on the Galgeplaat and the absence of these processes in the model will influence transport rates and thus the simulated morphological evolution of the flats.