6t THE IMPROVEMENT OF RIVERS. 



for navigation, in diverting the principal current from the heads of the spurs, and 

 pushing it forward into the open bed to the line of the greatest depth. Barges and 

 rafts floating with the current, and fleets of boats in tow have ceased to be carried 

 against the heads of these spurs, and are now kept by the natural forces in the middle 

 of the channel, or at least at a sufficient distance from the banks. The submerged 

 sills have thus caused the disappearance of one of the inconveniences which could be 

 urged against the system of contraction by spurs that of forming obstructions dan- 

 gerous to navigation. The improvement was so marked that sharp bends, whose 

 rectification was formerly demanded, are now free from danger, and they form part 

 of the plans adopted for the improvement of the river. 



The sill which we have described is usually somewhat short, but in places where 

 considerable scour has been produced, or is to be feared, they no longer have as their 

 object the protection of the spur-dikes, but are constructed with a view to the regulari- 

 zation of the bottom, and consequently of the slope. Hence the works on the Elbe 

 have in a great measure made the depths uniform. Formerly this river, as in most 

 streams, had a series of pools, more or less deep, separated by rapids. To-day the 

 bottom has a nearly uniform depth. It is true that the velocity of the current has been 

 increased in the parts corresponding to these pools, but the advantage of a regular 

 depth of water over the whole route, and the other advantages which result from it, 

 are such that even this inconvenience is not of great consequence. 



Submerged sills, which may render such considerable service as permanent works, 

 are not less useful as a preparation for other works, and for their economical execution. 

 The work of regularization by spur-dikes is not carried out immediately, as is the case 

 when longitudinal dikes are built. Their construction in Germany is accompanied 

 by methods and rules which permit a considerable liberty of action on the part of the 

 engineer. Hence, when an improvement is liable to lead to a great displacement of 

 the bed, especially in the concave parts of a stream, the dikes are commenced on a 

 short length of river, and stopped at a provisional curve. Time is then taken to 

 watch the effect produced. If the action of the current attacks the bottom at the 

 outer ends of the spurs, they are prolonged by sills which form a foundation for the 

 succeeding part of the dike, and which in the meantime immediately stop the scour 

 and cause deposits. This gradual construction, or experimenting with the dikes, is 

 done not only as regards their width, but also as regards their height, and when a 

 certain depth has been obtained, submerged works are commenced, which at a later 

 period, are built higher in proportion as the expected effect is produced. Remarkable 

 effects of deposits are thus obtained, and dikes which could only have been constructed 

 at great cost in deep water, are built gradually, and are finished on a bottom that has 

 been raised without difficulty and at a small cost. 



On convex curves the results are very rapid, but on concave curves the system 

 of spur-dikes gives less satisfactory results. Scour at the heads of the dikes always 

 takes place, and it cannot be clurkrd except by the precaution.? and the gradual 



