68 THE IMPROVEMENT OF RIVERS. 



damages resulting entirely from causes over which it has no control, if such injuries 

 have occurred in proximity to public works. It is thus of first importance to have a 

 thorough knowledge of the reach to be protected, as time and money both may be 

 wasted through a lack of acquaintance with the currents, etc. 



The revetments constituting the protecting works of river banks comprise two 

 distinct forms. In the one the bank is covered completely for the entire length of the 

 space to be protected; in the other, the space is divided into sections by separate 

 revetment-dikes placed at considerable distances apart, the bank between being left 

 uncovered. The revetments themselves comprise two distinct parts, one below and 

 the other above low water; the first is inaccessible and invisible, and must support the 

 second, which is exposed to all the changes of weather and water, and is open to inspec- 

 tion during the dry season. As the portion under water must act as a foundation for 

 that higher up and cannot be readily inspected and repaired, it should be built in a 

 substantial manner. A stable foundation is a requisite for good bank protection, since 

 if it is composed of soft or perishable material the subsidence of the upper part will 

 assuredly result. It may be built of stones thrown into the water and allowed to take 

 a natural slope. In many cases the first stones placed will settle into the river-bed, 

 but this settlement will eventually cease as additional material is put in. To avoid 

 this subsidence it is a common practice to dredge a trench into which the stones are 

 dropped, forming a toe. This, if carried to a sufficient depth, insures at once the sta- 

 bility of the revetment, and usually requires less material for its accomplishment. It 

 is needless to remark that large stones are better than small ones for this purpose, 

 particularly along the water face. 



The use of round and sheet piles in connection with broken stone is quite common 

 also in these foundations, and their employment usually lessens the quantity of stone 

 required, and permits the work above water to proceed without danger of future settle- 

 ment, if they are driven well below the limit of erosion. A base is obtained in this way 

 which is capable of resisting pressure from both sides, and of assisting in distributing 

 the loads over soft soils. Whatever woodwork is used in the foundation should be 

 constantly submerged, in order to avoid the alternations of wet and dry; if timber 

 must be used in exposed positions it should be only in those where it can be seen and 

 repaired during each low-water season without disturbing the foundations. 



A revetment of dry masonry, or of masonry laid in mortar, is sometimes used as 

 a foundation, and as a protection above low water as well, but it is more expensive 

 than loose stone and timber, and in the majority of cases more difficult to build and 

 to maintain. The difference in cost between dry masonry and masonry laid in mortar 

 is not great. Brick and concrete are also employed for the purpose of bank protec- 

 tion. In all these forms it is necessary to prepare a foundation-apron of broken stone 

 or gravel which will prevent erosion. 



Masonry protection works, whether of stone, concrete, or brick, are not generally 

 employed on account of their cost, except at cities or great industrial works. They 





