Samenvatting

Introduction
In the harbor of Zierikzee there is a strong tidal influence. The difference between the water levels during high and low tide lies around the three meters. The harbors De Oude Haven and ‘T Sas fall dry during an extremely low tide. When these harbors fall dry, foundations will come in contact with oxygen, allowing them to rot. Furthermore, the sight of the harbors is not attractive during this situation. According to the Municipality of Schouwen-Duiveland the area around De Oude Haven and ‘T Sas are not sufficiently attractive. Therefor the municipality of Schouwen Duiveland has plans to develop the area around De Oude Haven and ‘T Sas in order to make it more attractive. The municipality of Schouwen Duiveland would like to realize a minimum water level in the harbor of Zierikzee by the use of a water retaining structure. This will prevent the foundations and the banks to be exposed during low water levels. This water retaining structure would be located under the bascule bridge. The retaining structure would “lock” the water in the harbors when the water on the Oosterschelde side of the structure has reached the desired maximum water level, likewise to the low tide. When the water level on the Oosterschelde side of the retaining structure lies within the allowed boundaries there will be a free connection to the Oosterschelde, the retaining structure is open. The minimal water level would make more activities in De Oude Haven and ’t Sas possible and make the area more attractive by decreasing the tidal differences, preventing exposure of the deteriorated quay walls and banks.

Research problem
Retaining structure and a reduction of the tidal influence influences the water in the harbors and this could have consequences for the water quality. This research is about analyzing whether water quality problems will arise when a minimal water level is implemented. Furthermore, solutions will be developed if the research points out that water quality problems will arise. In order to do this, the following main question will be answered in this report: “What are the consequences on the water quality when a minimal water level of +0,80 mNAP is implemented in the projected separated harbor of Zierikzee?”.

Method
By means of desk research, interviews and calculations the problems regarding the water quality were identified. Alternatives were developed, and the most suitable alternatives were chosen using a multi criteria analysis. The most suitable alternative was worked out into detail, calculations, drawings, implementation costs and an implementation strategy were developed.

Conclusion
The conclusion is split in two parts. Conclusions are made on the chemical water quality and the biological water quality.

Chemical water quality
By studying the water movement and refreshment on the water in the harbor a water quality problem has been identified when a minimal water level is implemented. In the Oude haven the velocity of the water is reduced and the refreshment time is increased. In other words, there is a chance on stratification in this area. An increase in chloride will decrease the oxygen concentration by reducing the saturation ability (Bayley, 1972) and by stratifying the water. The increase in N and P will increase algae growth. When the algae die, they will in turn decrease the oxygen concentration. In order to safeguard the water quality in the Oude haven a turbine is suggested to be installed to mix the water in the harbor. The water in the harbor is mixed by means of turbulent flow produced by the turbine. This movement prevents stratification of the water.

Biological water quality
In the present situation the harbors have an open connection with the Oosterschelde and therefore are subjected to the tide. Fish migration is possible through a leaking pipe in pumping station ‘t Sas. When the minimal water level will be created with the sluice doors the fish will not be able to migrate any more. This problem was seen as most relevant to study due to the fact that this negatively influences the water quality. The harbor is able to maintain the biological water quality considering the fish, by implementing a fish passage with a fish friendly pump. The fish will be able to migrate from the harbor to the polder and vice versa.