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level rise will cause seepage to start. See Fig. A. 15 where the present 

 seepage rate is given together with the rate when sea level would be 5 m 

 higher. Simple calculations show an Increase of the total seepage of 200 

 to 300% when sea level rises 5 m. The amount of fresh water needed for 

 flushing of the polders will then increase by about 50%. In dry stammers 

 this extra amount is not available . 



Impact on Water Management - A main problem for water management has 

 been discussed above, namely the saltwater intrusion. Another important 

 item is the drainage of the surplus of water. This is especially the case 

 during winter when precipitation is large and evaporation is small. Where 

 we are dealing with areas well below sea level there is only one solution 

 for the drainage, namely pumping. In case of a higher sea level all 

 pvimping stations have to be rebuilt because the surplus water of polders 

 has to be pumped upwards over a greater height and stronger pumps will be 

 needed. There are also areas that drain naturally by gravity. The 

 solution here seems to be building of new pumping stations when sea level 

 rises. However, in the Netherlands a very extensive area is still drained 

 by gravity, with a very large discharge, where this solution might not be 

 the best one. I am talking of the IJssel Lake and the area (15,000 km^) 

 that in turn drains into the IJssel Lake. Moreover, the river IJssel comes 

 into the lake and brings about 10% of the discharge of the river Rhine to 

 the lake. The IJssel Lake with an area of 1200 km^ is drained naturally 

 into the Wadden Sea by two sluices in the enclosing dam. The water level 

 of the lake is kept 50 cm below mean sea level during winter and 20 cm 

 below mean sea level during summer. This makes natural drainage possible 

 during low tide. In a recently performed study it has been found that up 

 to a sea level rise of about 50 cm it is possible to keep on draining the 

 IJssel Lake naturally without raising the water level of the lake. A 

 substantial further increase of the sea level would, for example, make it 

 necessary to build an enormous pumping station with a capacity of 300 x 10 

 m-^/day. This great capacity is needed to keep the water level of the 

 IJssel Lake below 1 . 30 m (above mean sea level) during extreme discharges 

 of the river Rhine (the design discharge of the river Rhine is 16,500 



