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velocities remained unchanged, caused a change of the storm surge heights 

 between -20 and +10 cm. The effect of sea level rise on storm surges is 

 thus very small, and the possible increase or decrease in the number of 

 severity of storms will be much more important for changes in storm surge 

 heights . 



Another feature which may be important is a change in the residual 

 transports, which will have impacts on the morphology and the marine 

 ecosystem, especially in the Wadden Sea. Model results show a change in 

 the residual transports when sea level rises by 1 meter (Fig. A. 12), but 

 results are still too inaccurate for quantitative use. 



Impact on Waves and Swell - A higher sea level and a greater water 

 depth caused less dissipation of waves and swell. Wave heights increased, 

 especially in shallow water. Many of the smaller waves, which break and 

 dissipate on shallow sandbanks in the present situation, will continue 

 unbroken in the case of increased water depth. For instance, calculations 

 of the wave height with the HISWA wave model near the Haringvliet sluices, 

 a location behind a shallow sandridge called the Hinderplaat, showed an 

 increase in wave height from 90 cm to 210 cm when the sea level rose 5 m 

 over an unchanged sea bed. However, if the sea bed were to rise as fast as 

 the sea level, the wave height would hardly change. In reality the outcome 

 will depend on the change of the morphology. 



Impact on the Morphology - The Netherlands coast can be roughly 

 divided into three different parts: the Delta area in the South with the 

 Eastern and Western Scheldt; the central coast between Hook of Holland and 

 Den Helder without important inlets; and the Wadden Sea area with the 

 inlets between the islands. The easiest part to study is the central 

 coast. 



The coastal defense of the central coast consists mainly of dunes. 

 The present situation is close to stable, but a sea level rise threatens to 

 destroy this delicate balance and will cause the coastline to retreat. The 

 present beach slope is about 1:60, so a rise of 1 meter may result in a 

 retreat of 60 m. A complicating factor is that the dunes are composed of 

 finer grain sand than that found on the beach and shore face. The expected 



