to THE IMPROVEMENT OF RIVERS. 



i, and the land slope 4 to i to within 5 feet of the crown; thence to the crown it is 

 aj to i. 



" For levees from 15 to 20 feet in height the crown is 8 feet, the river slope is 3 to i, 

 the first 8 feet of the land slope from the ground is 6 to i, the next 6 feet 4 to i, and 

 thence to the crown 2} to i. 



" In the upper districts 10 per cent of the height, both in wheelbarrow and team 

 work, is required for shrinkage. 



"These standard sections are expected to withstand the water to within 3 feet of 

 the crown of the levee, without excessive saturation or change of form, and to give 

 unqualified protection under all normal conditions of foundation and materials of 

 construction. 



"When subjected to water above the 3-foot line, though they are intended to 

 remain intact, they cannot be considered, either theoretically or practically, standards 

 of excellence. 



"Without taking into account the effect of waves on exposed levees, which neces- 

 sitates recourse to special slopes and methods of protection, planking, revetments, 

 etc., the whole question of standard section depends on the permeability of the embank- 

 ment and foundation, that is, the extent of seepage, or percolation, and the best form 

 and method for overcoming it in different materials. 



"In 'buckshot' or clay, which is practically impermeable, the section might be 

 given a strictly theoretical form, dependent alone on the height of the water and the 

 weight of the buckshot; allowing some crown merely for increasing the height in time 

 of excessive flood, the slopes being plane surfaces with an inclination sufficient to 

 insure the required weight to counteract the hydrostatic pressure and the angle of 

 repose of the material. 



" In cases of permeable materials, light clays, sand, and loam, the levee becomes 

 partly saturated when subjected to high water, the line of demarcation between satu- 

 rated and dry soil descending in a hydraulic gradient varying in inclination with the 

 soil of which the levee is composed, and being probably very irregular in trace because 

 of the lack of homogeneity of the material in the body of the levee. 



"In surface soils, subject to direct rainfall or percolation, from adjacent watered 

 areas, the ground-water stands at a level dependent on the composition of the soil, 

 both physical and chemical, the natural and artificial voids, and the hydrostatic pressure. 

 In nearly all soils, remote from intersecting fissures, wells, or streams, the line or plane 

 of saturation is parallel with the surface of the ground, following the inclination of 

 hill and valley. Where wells, fissures, or river-beds occur in the surface soil, the line 

 of moist material, or plane of upper surface of saturation, is inclined towards 

 the fissure, well, or river, the degree of inclination depending on the consistency 

 of the soil. 



"The power of soils to resist the pressure of water is due to their specific gravity, 

 fineness of comminution, cohesiveness, and the irregularity of individual particles. 



