Planning and design
Primary purpose of the Galgeplaat pilot was to investigate whether nourishment can compensate the erosion of a tidal flat. Furthermore, the pilot offers the opportunity to gain a better insight into the biological and morphological development of an intertidal flat and the relationship between biotic and abiotic parameters. This will help improving the design of new measures with respect to the mitigation of the ongoing erosion of tidal flats in estuaries like the Eastern Scheldt in the future.
Design conditions
At the planning and design phase, the challenge was to meet a number of conditions:
- The nourishment should preserve the ecological value of the tidal flat by compensating its erosion. By raising the flat, the emergence time is extended and birds have more time to forage.
- The nourishment should spread over the entire flat through natural sediment transport processes. Therefore the nourishment was planned at a location from where the prevailing waves and currents would be able to transport the sand.
- The area should be recolonised by benthic macrofauna after the nourishment has been put in place, e.g. by macrofauna larvae transported with the water and settling onto the nourishment. One of the key factors is the time needed for benthic recovery, which should be shorter than the lifetime of the nourishment (i.e. the time until the nourishment has been eroded away) (Ysebaert, 2009).
- The nourishment should have no significant negative effects on the commercial mussel beds in its vicinity. In order to prevent such effects during construction, conditions for the dredging operations were agreed upon with the mussel growers exploiting the nearby mussel beds:
- No increase in the turbidity of the water during the execution of the nourishment
- No uncontrolled discharge of water with sediment
- Only to be pumped during the tidal window of NAP – 0.60 m up to NAP + 0.40 m
Chosen design
One important argument in the design of the nourishment was the availability of sand from dredging activities in nearby navigation channels. Apart from this, the location for the nourishment on the Galgeplaat was chosen on the basis of the following criteria. It was:
- at a significant distance from the mussel beds;
- on the middle of the flat, thus preventing the sediment from being transported back into the channels too quickly;
- on a low-lying part of the flat, to generate a significant change in bed level;
- on the southern half of the flat, so that waves and currents from the dominant direction would transport the sediment onto the flat itself (i.e. in northern direction).