270 



slope of a beach consisting of this finer sand will be less than 1:60 and 

 consequently the retreat of the coastline may be as great as 80 to 150 m. 

 After such a retreat the dunes may not be strong enough to withstand a 

 design storm surge (design frequency 10'^) , resulting in the need to 

 strengthen parts of the dunes. A retreat of dunes and coastline would be 

 undesirable in many cases, due to the presence of harbours, other 

 constructions, and valuable dune areas, and so for large parts of the 

 Netherlands coast measures such as sand replenishment may be necessary. 

 For the coastal stretch protected by dikes (such as the Hondsbossche 

 Zeewering) strengthening is the only solution. 



The morphology of the Wadden Sea is very complex. Even the present 

 situation is difficult to model. Added to this uncertainty is the fact 

 that the Wadden Sea has still not attained equilibrium since the closure of 

 the Zuider Sea in 1932 (now IJssel Lake) . 



Presently there is a transport of sand from the North Sea into the 

 Wadden Sea of 30 x 10° m-^/year (standard deviation 15 x 10°) . If sea level 

 rises 1 meter in 100 years, an addition of 28 x 10" m^ sand per year is 

 needed for the sea bed to keep pace. So for the area at large, bottom rise 

 can probably keep up with this rate of sea level rise. So the intertidal 

 area will stay about the same size. This is very important for the ecology 

 of the area (see below) . 



It should be stressed once more that the morphology of the Wadden Sea 

 is very complex and that the sedimentation theory used here is very simple. 

 For example, of the origin of the present sedimentation (30 x 10° m-'/year) 

 only 25% can be traced to the coast of north Holland, the coasts of Texel 

 and Vlieland, and the adjacent North Sea, whereas the origin of the 

 remainder is more diffuse. Even at the present rate of sea level rise it 

 is not known in detail how sedimentation changes the morphology of the 

 Wadden Sea. 



In the Eastern and Western Scheldt of the Delta area, erosion will 

 probably occur and the intertidal area will probably decrease. But here, 

 too, it has to be stressed that the processes involved are very complex and 

 predictions difficult to make. 



