232 



The National Geographic Magazine 



Pile Dikes for Protection against Marine Erosion 



is afforded by allowing the swollen river 

 to discharge a large part of its burden 

 into the adjacent countr}'. This is ef- 

 fected by providing at suitable points 

 low dikes over which the water will run 

 on reaching the danger line. These low 

 dikes or weirs, known in Dutch as 

 ' ' overlaten, ' ' might well be compared to 

 safety-valves. Their location is chosen 

 in such a manner that the water dis- 

 charged into the open country will do 

 comparativel}^ little harm, and, being 

 confined by dikes especially designed 

 for that purpose, is made to find its way 

 to some low point farther down the 

 river or near the latter' s mouth. 



Overlaten existed many years ago on 

 all the large rivers, and although their 

 use has saved the country much damage 

 and expense, it has proved a serious evil 

 in another way. It is obvious that 

 whenever a river at the flood stage is 



deprived at some point along its course 

 of a large share of its burden the natural 

 consequence is a lessening of the cur- 

 rent below such a point and the deposit- 

 ing of a vast amount of sediment. Not 

 only is this deposition of sediment at a 

 time when the river transports a max- 

 imum amount a very serious evil, but 

 the slackening of the current also offers 

 most favorable conditions for the for- 

 mation of ice jams. During the past 

 years all the overlaten have been abol- 

 ished with the exception of one on the 

 Meuse, known as the Beerse Overlaat, 

 which exceeds two miles in length and 

 has been known to discharge with a . 

 head of three feet during severe floods, 

 the river at such times being relieved of 

 more than a third of its total flow. The 

 policy at present is to increase the ca- 

 pacit}" of the river channels by deepen- 

 ing and widening the mouths,, and in- 



