DIKES AND THEIR EFFECTS. 61 



river-bed, or else with a view to raising it at points where too great depths exist. They 

 are built in nearly the same way as the spur-dikes which rise above the water-surface, 

 and of the same kind of material, and usually start from low-water mark, inclining in 

 direction up stream at an angle varying between 60 and 80. The top slope as it goes 

 out into the river is about i in 10 to 15 for a short distance and then decreases consider- 

 ably until it connects with the opposite shore. 



The earliest example of submerged spurs is found on the Ruhr, a little river which 

 falls into the Rhine at Ruhrort, and to which the coal mines of Westphalia gave an 

 unusual importance in the system of transportation routes prior to the construction 

 of railroads. The Ruhr was made navigable for a length of 46^ miles by a canalisation 

 comprising eleven locks and dams. It happened that one of these pools was scoured 

 out, and the slope in low water disappeared, so that the miter-sill of the lock at the 

 head of the pool was uncovered, and navigation was stopped. The engineers restored 

 the slope by constructing throughout the pool a series of submerged transverse dikes, 

 which, even before the filling up of the bottom, divided the total fall so as to restore 

 the depths necessary for navigation during low water. 



Great use was made of submerged spurs in the improvement of the Elbe, and it 

 may be said that the German engineers have been naturally led to them by the system 

 of works which they have adopted. They have aimed at improving the channel by 

 contractions, by creating in the natural bed a minor bed whose width, after taking 

 into account the degree of resistance of the bottom, is regulated in accordance with 

 the conditions of slope and of discharge. Instead, however, of controlling this minor 

 bed by longitudinal dikes, they have created it by building spurs which extend into 

 the stream from each bank with a slight up-stream inclination, and terminate on the 

 line adopted for the desired bank of the minor bed. As might have been expected, 

 the heads of the spurs were usually attacked by the current, and scour produced, 

 threatening the existence of the spurs, and destroying the regularity of the channel. 

 The engineers were thus led to prolong their spurs under water, advancing into the bed 

 of the river in order to protect them and limit the effect of the scour. 



The works of regulation on the Elbe are as follows: 



First: next to the bank is the spur-dike intended to contract the natural bed. 

 Usually this dike at its root on the bank is at a height of 8 feet above low water, and 

 at its outer end at a height of 6{ feet above low water. 



Second, in prolongation of this is the submerged spur or sill which limits the scour 

 caused by the dike proper and protects its head. The starting-point of the sill is about 

 5 feet below low water, and the top has a slope which may be from i foot in 25 to i 

 foot in 12. 



The works thus placed have completely answered the expectation of the engineers. 

 The alluvial deposit has filled up the places that had been scoured out, and has 

 permitted a reclamation of the spaces between the spurs. The works have thus been 

 secured definitely, and other results have been produced which are still more important 



