EMBANKMENTS. 221 



will flow with the greater force. The latter consideration is sometimes 

 of importance in muddy rivers, as the slower the water flows, the more 

 quickly does mud accumulate in the bed of the river; and they thus get 

 choked up in places, causing the water to overflow. But if such rivers 

 are confined by embankments, a greater force is given to the water ; and 

 the mud is either thrown up on the spaces between the bed of the river 

 and the banks, or is carried away. 



Care should be taken to avoid forming any sharp points in the banks, 

 as these cause a resistance to the water, which either works into the 

 banks at that point, or the water is thrown to the opposite side with great 

 force, causing it to undermine there. 



Great difficulty is often felt in draining lands adjacent to rivers which 

 overflow their usual course, especially in procuring an outfall for the 

 main drains without admitting the water from the river to the fields 

 adjoining. In the first place, the drain should not be cut straight 

 across the bank towards the water, as is very frequently done, where 

 the water from the drain flows against the water in the river, and is 

 thus prevented getting freely away ; and besides, in such a position the 

 mouth of the drain is very apt to be choked up with foreign matters. 

 The drains should be cut along the bank to a certain extent, and run 

 into the river with the flow of the current, and not against it, as then 

 the flowing current will tend to draw the water away from the drain, and 

 keep any mud, branches, &c., from lodging there. 



In order to prevent water flowing into the drains from the river, 

 a system of flood-gates is generally adopted. These are made on the 

 principle of a valve. A wall of masonry is built round the mouth of 

 the drain, leaving an opening like a doorway in the centre of any size 

 thought necessary; in this opening is firmly fixed a frame of wood- work, 

 on which a door is hung from the top,, but is not fixed in any way at 

 either the sides or the bottom, so that when water flows from the 

 mouth of the drain, the door or valve is forced open and the water 

 escapes; and again, when the water rises in the river, it flows against the 

 valve and keeps it shut while it is up, thus preventing the water from 

 entering the drains. These doors and frames should be made of strong 

 well-seasoned wood, and firmly fixed in their place. They should be 

 fixed so that the water from without will not be able to get underneath 

 them, and so lift them up. It is always the most secure to have two 

 such doors at the mouth of each drain one upon the outside and at the 

 mouth of the drain, and the other a few feet within the drain, and both 

 hung in the same way. Where this is done, if at any time the outer door 

 should be lifted up by "the waves, the water will be stopped by the inner 

 door. Care, on the other hand, should be 'taken to observe that the 



