SALT MARSH DRAINAGE 



145 



Diking is necessary, as a rule, only when the fall of the tide is 

 not sufficient to draw all the water out of the ditches, when the 

 drainage ditches pass through material of such a nature that it is 

 impracticable or unduly expensive to keep them open or when the 

 area in question is too low to be drained. 



Diking implies the construction and installation of tide-gales, 

 since it is necessary that the streams and ditches carrying water 

 be given an opportunity to discharge. The main purpose of the 

 diking is to keep out all but the very highest tides, so that low 

 areas will not constantly be covered with water and so that the 



Dike 



Fig. 102. — Method for control of abandoned coastal rice-field. 



water-table in the ground will remain at a sufficient depth. 

 Otherwise, a comparatively slight rain-fall or a high tide would 

 result in pools that would remain for days. 



The dike should, as a rule, be located from 25 to 50 feet from 

 the edge of the water, so as to leave a narrow strip of marsh to 

 protect it from wave action. It should be sufficiently high to 

 keep out all but the very highest of high tides, and proportion- 

 ately wide. In constructing the dike, due allowance must be 

 made for shrinkage. 



CONSTRUCTING THE DIKE 



In constructing the average dike, mud and sods are the chief 

 materials. A core ditch, about 1 foot wide and 1 foot or more 

 deep, should first be excavated along the center line of the pro- 



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