74 THE IMPROVEMENT OF RIVERS. 



connected with the submerged mattress. Frequently a bank crib is put in also, con- 

 nected with the submerged cribs. 



The efficiency of this type of revetment when placed in caving bends is to a great 

 extent dependent upon the radius of the bends, the distance between them, and the 

 material of which the bank is composed. They are stated to be of little value in very 

 abrupt bends with sandy banks, and that where the banks are of clay and "buckshot," 

 the distance apart should not exceed 500 feet, and even then it may be necessary to 

 protect the intervals between them. 



Bank Heads. On the Mississippi and Missouri rivers there has been designed and 

 constructed in recent years, by Col. Amos Stickney, Corps of Engineers, U. S. A., a 

 form of revetment somewhat similar to the spur-revetment, and known as a "bank-head," 

 which consists of an isolated section of bank protection, of a size and distance apart 

 depending on the local conditions. It is placed on concave bends, and at the edge of 

 the bank. Its use is based on the theory * that by holding permanently certain points 

 of the bank at certain distances apart the force of the current will not be able to 

 seriously cut in between these protections, nor cut around them, owing to the short, 

 ness of the space in which its effects, such as scour, eddies, etc., have to work. On the 

 concave side of a sharp bend, for instance, if the points are too far apart, the velocity 

 of the current will sweep against the bank between, and cut it away until it can attack 

 and undermine the bank-head, but if the points are at the proper distance, the current 

 will work into the bank for a certain distance, and the erosion will then cease, as there 

 will not be room enough for further action. 



Experiments on the Missouri River showed that a considerable current of water 

 will pass around a fixed curve of 300 feet radius without causing violent eddies, and 

 that an angle of 30 to the current is approximately the one at which a soft bank can 

 approach a fixed point without much erosion. With these data as a basis the bank- 

 heads are built with a conical front, and a least radius of 300 feet. They are constructed 

 of ordinary materials, as brush, stone, etc., and provided with an ample protection 

 of riprap on the upper and lower sides. A large mass of riprap is also placed along the 

 river-face, and renewed as the water undermines it, and it has been found that this 

 action ceases after a certain time, as the stone having fallen over will gradually afford 

 a protection to the bank below the limit of scour. 



As far as they have been used these bank-heads have afforded very satisfactory 

 results. 



* Annual Report Chief of Engineers, U. S. A., 1897, p. 3537. 



