15.4.3: E.4.3- Impact to the (semi-) closed basins
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For the effect within the closed basin only the most important environmental problems are discussed. The problems appear to be mainly dependent on the type of the closures.
The Haringvliet estuary is closed by sluices which only allow fresh river water flowing out and no flow from the sea to the former estuary can occur through the sluices. The basin thus became a fresh water reservoir. As the cross-sectional area of the former estuary is too large for only discharging the river water, accumulation of fluvial sediment occurs in the basin after the closure. In the first years after the closure the quality of the fluvial sediment was poor. Therefore polluted sediments are now present in the bottom of the basin, which forms an environmental problem.
In case of Grevelingen the former estuary was fully closed by a dam, and became a salt water lake. The only exchange of water between the lake and the sea takes place via a siphon structure. Due to the lack of sufficient refreshment of the water in the basin water quality problem starts to occur in the basin in the recent years.
The Eastern Scheldt has kept its estuarine characteristics because it is semi-closed by a storm surge barrier. Under normal conditions the barrier is open allowing tidal flow through it. Only during severe storms the barrier is closed protecting the area behind it from the high sea. However, the construction of the barrier did reduce the cross-sectional area. In order to limit the decrease of the tidal range in the estuary the basin area has been reduced by additional engineering works. The end result is that only limited change of the tidal range occurs after the closure. However, the tidal prism and the strength of the tidal flow have significantly decreased. As a consequence the inter- tidal flats in the estuary suffer from serious erosion (Fig. E.9). This is an environmental problem as the inter-tidal flats are used by birds for finding food and resting when they are dry.