7 8 THE IMPROVEMENT OF RIVERS. 



along the rivers, some of those in the most exposed positions having slopes of 

 10 to i. 



In regard to section, Hewson in his work on levees before referred to, has the 

 following: "The presentation of equal strength at all parts of the levee does not require 

 a greater width even under the most unfavorable circumstances than (whatever may 

 be the proper width of crown) side slopes from each side of crown at a rate of i foot 

 horizontal to i foot vertical. Such a section may be said to be, in general, the section 

 of uniform strength. The strength of anything being the strength of its weakest 

 part, an excess of strength at any one part is, it is almost needless to observe, a waste 

 of material in leveeing, and consequently a waste of money. 



" In practice, however, it is impossible to conform to the section of uniform strength 

 in levees, seeing that the controlling consideration rests in the standing angle of the 

 material. The standing angle of clay has been set down at 8 inches base to i foot in 

 height ; and, therefore, may be held to conform closely to the section of perfect economy 

 of material the section of uniform strength. Twenty-one inches of base for every 

 12 inches of height being the standing slope of sand, that material is seen in the excess 

 of its natural section over the section of equality of strength to involve in leveeing 

 a very large waste of material, and, therefore, of money. In a levee having a 3-foot 

 crown, a 2i-foot base, and a height of 5.2 feet, the area of cross-section is 62.4 square 

 feet. This levee, it must be recollected, is one of equal strength; and, therefore, 

 measuring its effective strength by its weakest part its 3 -foot crown we find the 

 limit of its actual resistance to be, when made of sand, as 3 feet X 95 pounds, or 285. 

 A clay levee of 2.11 feet crown, sloped down at the standing angle of clay to a base of 

 9 feet for 5.2 feet in height, contains within it the slope of uniform strength, and con- 

 sequently its crown being its weakest part, the limit of its effective resistance is as 

 2.11X135, or 284.9. This clay levee of 2-feet crown and 9-feet base presents, then, 

 precisely the same resistance to water-pressure as does the sand levee of the same 

 height, having a crown of 3 feet and a base of 21 feet. The cross-section of the clay 

 bank in this case is 29 square feet; while, as has been said above, that of the sand is 

 62 square feet. But practice goes still further in increasing this disproportion between 

 the different quantities necessary in levees of sand and in corresponding levees of clay. 

 The standing angle, as presented in theory, must be deviated from in both sand and 

 clay in order to meet, in practice, the contingencies of floods and rains. Lighter, looser, 

 and less adhesive than clay, the flattening of slopes in sand below that of the 

 angle or slope of repose must be much more considerable in practice than that in 

 the heavy concreted and adhesive bank of clay, in order to resist without endangering 

 the effective strength or stability of the bank the active washes of rains and waves. 

 . . . Confining the equation of the materials to the simple fact of the difference between 

 their strict standing angles, 26 yards of clay are seen to be equal, in a levee of 5 feet 

 height, to 58 yards of sand, in accomplishing the object of all levees, namely, effective 

 resistance to floods." 



