io THE IMPROVEMENT OF RIVERS. 



found, leads to most of the difficulties encountered in river navigation. Evidence of 

 it may be seen after every freshet in the deposits left in eddy water, or by allowing a 

 sample of it to settle for a short time. At the bottom of the vessel will be found a small 

 quantity of sediment which may be separated into two general classes, one of very fine 

 particles of mud and clay, and one of sand of varying grades of fineness. The materials 

 of the first class are easily kept in motion, being light, while those of the other grade 

 have a constant tendency toward the bottom, and will be precipitated in places where 

 the current velocity is reduced. Numerous experiments have been made from time 

 to time to determine the velocity at which the current begins or ceases to move various 

 materials, but there are several conditions entering into the problem which render the 

 data obtained of uncertain value. 



The experiments of Du Buat give the following results: 



Potter's clay o. 26 feet per second. 



Sand deposited by clay 0.54 " " 



Large angular sand 0.71 " " 



Gravel, size of peas 0.53 " " 



Gravel, size of beans 1.07 " " 



Round pebbles, as large as thumb 2.13 " " 



Angular flint stone of size of hen's egg 3.20 " " 



Formation. In order to grasp properly the influence of improvement works it is 

 necessary to understand the natural condition of rivers, and to bring about this under- 

 standing we will quote a very clear presentation of the subject by the Russian engineer 

 Janicki:* 



"Let us examine how the bends and the bars are formed. To better understand 

 the details of the process, let us suppose a plain, absolutely regular, and not horizontal, 

 but slightly inclined in a certain direction. Let us suppose, further, that in the direc- 

 tion of this inclination a canal is dug having a certain cross-section, a regular bottom, 

 a slope parallel to that of the ground, and regularly constructed side-slopes. Into this 

 canal we will admit a river at its maximum discharge. This river has a certain velocity 

 of current which undermines the banks and cuts out the bottom of the canal, and we 

 shall accordingly witness the following phenomena: the water begins by detaching 

 a few small particles from the bed and cutting away the two banks equally, but as in 

 nature no ground is absolutely homogeneous and of the same tenacity throughout, it 

 finally happens that at some point one bank yields sooner than the other. This first 

 undermining gives rise to a slip or mound which destroys the symmetry of the original 

 profile, and deflects the current toward the opposite shore. Soon this second shore, 

 in its turn, crumbles away just where the deflected current strikes it most powerfully. 

 This new mass of fallen earth cannot remain at the foot of the bank whence it came. 

 The current here being already increased, it is carried a little lower down stream and 



* Notes on the Navigability of Rivers. 1879. 



