Oosterschelde tidal flats
The shallow parts of the tidal flats in the Oosterschelde, which are important foraging areas for birds, are shrinking in area.
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Trends
In the period 1989-1994 the total area of tidal flats exposed at low water in the Oosterschelde decreased by 0.15%. This would seem to be a negligible decrease for the birds that forage on these flats, but in fact there have been big changes (see figure). In this period the area of the shallower parts (higher than 0.6 metres below the Amsterdam Ordnance Datum) has decreased, but there has been an increase in the deeper areas (lower than 0.6 metres below the Amsterdam Ordnance Datum). And for the birds that forage on the tidal flats it is the shallower areas that are important, as for most of the day they are exposed and are only incidentally submerged. The deeper parts are exposed for only a short time.
Causes
After the storm surge barrier was completed in 1986, the tidal range and current velocity in the Oosterschelde decreased. Before then there had been equilibrium between the sand transport from tidal channel to sand flat and vice versa. During flood tide, sand was transported to the flats; during ebb tide some of this sand was removed. Much of the removal occurred during the severe winter storms. The reduced current velocity in the channels has resulted in less sand being transported from these channels to flats in recent years. The transport from the flats to the channels continues largely unabated, however, and therefore the sand in the Delta is moving from the higher-lying areas to the lower-lying areas. This state of affairs is referred to as "sand starvation".
