CHAPTER IV. 



v 



PROTECTION OF BANKS. 



Objects. The purpose of the protection of banks is the prevention of erosion, and 

 may include one or more of the following objects: 



1. The reduction of the quantity of traveling sediment and consequent lessening 

 of deposit in the river-bed. 



2. The maintenance of a permanent minor bed of normal depth and width in the 

 bends. i -"H - - - - *-*/ 



3. The protection of property, wharves, landings, etc. 



4. The protection of levees built along the banks. 



5. The prevention of cut-offs which may affect a river's regime and also leave 

 important commercial centers without means of transportation. 



Rivers as a rule have not a sufficient capacity of bed to carry off their waters at 

 flood times, and this leads to a constant effort at widening by cutting away the banks, 

 particularly on the concave sides at bends. The material thus eroded is moved along 

 in the channel until it finds lodgment, when it obstructs low-water navigation in the 

 form of a bar or shoal. As the bank is torn away the river shifts its position toward 

 that side, and year by year its low-water channel, as well as its bank line, is moving. 

 The floods undoubtedly scour out the bed at the same time they cause the banks to cave, 

 but by reason of this continual shifting and alteration of channel this scouring does very 

 little good. That done one year may be obliterated the next, and a new chute cut out 

 nearer the bank. The result is that the great amount of work done by the river itself 

 toward providing a good channel is ineffective, and it is necessary, in order to take 

 advantage of this work and to reduce the quantity of material to be transported, to 

 permit no encroachment upon the banks nor movement of the channel. In other 

 words, the shores must be held to a fixed line, and thus give a permanent, if restricted, 

 passage for the water, and one which must not be allowed to silt up with material cut 

 from the banks. 



Character and Kinds. It is important in laying out protecting works that they 

 should be planned so that they will not modify too greatly the regime of the river and 

 thus bring currents upon points heretofore uninjured. They should be strictly defen- 

 sive, because, while riparian owners are compelled to endure losses from floods, they 

 have good grounds for complaint when the construction of works brings serious injury 



upon them. In fact the Government is often held responsible in the public mind for 



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