Tidal flushing of the Balgzand tidal flat ara, Dutch Wadden Sea

Summary

The Balgzand area is situated in the tidal basin drained by the inlet Marsdiep in the western Dutch Wadden Sea. According to ZIMMERMAN (1976), a representative flushing time scale of the Wadden Sea as a whole is about 14 tidal periods (about 7 days). However, for the waters surrounding the Balgzand tidal flat, being situated more closely to the North Sea, a flushing time of about 5 tidal periods should be expected. Now, in turn, the water mass that can be properly called “Balgzand water” (in accordance with its tidal mean position) is flushed with water from the surrounding deeper tidal channels. In order to interpret various (bio) chemical processes in the Balgzand area the flushing time scale of the renewal of Balgzand water with water from the surrounding channels should be calculated and, if possibble, the mechanism of water exchange should be revealed. An extensive survey of the area, in coorperation with the “Studiedienst Hoorn of Rijkswaterstaat”, was carried out in May 1977. (DE BOER, 1978; MANUELS, 1978).

As to the above-mentioned questions the survey revealed that:

• 1. The flushing of the Balgzand water mass can be calculated by means of reactive silicate as a tracer. Although this tracer is certainly not a conservative one (see VAN BENNEKOM, 1978) its distribution during the survey period was sufficiently stationary to calculate the flushing time scale from the silicate budget in a way analogously to that given by ZIMMERMAN (1974) for a comparable tidal flat area in the western Dutch Wadden Sea. Using figures kindly supplied by van Bennekom, the tidal mean silicate content of the Balgzand water mass amounts to 295 kmol. From measurements made in the various channels draining the Balgzand area, a tidal mean silicate loss of 95 kmol/ tidal period was estimated. From these figures a flushing time scale of about 3 tidal periods can be derived.

• 2. The flushing of tidal flat waters is strongly promoted by existence of tidal mean circulation as was also reported before (ZIMMERMAN; 1974, 1976). The tidal mean circulation in the Balgzand area is mainly characterized by a clockwise rotating tidal mean circulation, driving the water inwards from the Amsteldiep and outwards through the channels discharging in the western part of the Malzwing tidal channel (DE BOER, 1978).