9 8 THE IMPROVEMENT OF RIVERS. 



the highest banks do not exceed 40 feet, and that this height is only for a few hundred 

 feet at most. It is, then, practicable to make the few high levees of any dimensions 

 that may be necessary for safety with slopes of i to 10 if less will not do without 

 inordinate expense. No such proportions have ever been found necessary. About 

 i to 5 is the flattest slope that has ever 1>een used. It is not the great levees that break. 



"Even when ultimate grade is attained, say 3 feet above the 'potential high water' 

 of 1897, the average height of the levees will not exceed 18 feet, of which 3 feet will be 

 a margin against storms or accidental deficiencies, settling, etc., leaving a water-head 

 of 15 feet. Extreme high water will last only a few days; within a foot of extreme 

 height it may last three or four weeks. For such embankments, in ordinary soils, 

 slopes of i to 3, with banquettes, will be sufficient. ... As to cost, it may be said that 

 levees are the least expensive means of reclaiming overflowed lands that have ever been 

 proposed. . . . The idea that confining a river should cause it to deposit silt is so contrary 

 to reason that it is a wonder it ever obtained credence at all. Transporting power is 

 generally believed to be proportional to the square of the velocity. The confinement 

 of the stream unquestionably gives increased current. Undoubtedly, in retaining the 

 water within the channel we also retain the silt, but at the same time we retain the 

 vehicle by which to carry it, and give the vehicle greater capacity. 



"General Comstock's conclusion was that in the cases of the Po and the Rhine 

 the rise of bed, if any, was insignificant, and that in the case of the Mississippi there 

 is no evidence of any at all." 



Cost. A recent Government report shows that the United States and the States 

 of Arkansas, Missouri, Mississippi, and Louisiana have expended on the Mississippi 

 levees the sum of $39,600,000 during the past eighteen years. A levee engineer esti- 

 mates that their total cost has been over $50,000,000, of which the United States has 

 paid over $15,000,000. This is about $35,000 per mile. 



The report gives the following : 



Miles of levees in service 1,436.3 



Contents, cubic yards 150,595,000 



Required to complete the system, cubic yards 109,090,000 



Annual loss by crevasses, caving, and other causes 2} per cent. 



Foreign Levees. Mention has been made of the levees along the Po, the Theiss, 

 and the Loire, and owing to the extent and age of these works further information in 

 regard to them will be of interest. 



River Po. Usually the levees on the Po are located at a considerable distance 

 from the banks, but occasionally they come quite close, and at such places are protected 

 by revetments or fascines, an inner levee being frequently built behind them for fear 

 of accident. They extend up each side of each tributary to such distances as floods 

 are to be expected. In a word, the great submersible plain of the Po is guarded 

 by a network of embankments enveloping each affluent and laying out for each a minor 

 bed which must be followed by all the water falling in the valley. These levees are 



