10 BULLETIN 304_, U. S. DEPARTMENT OF AGRICULTURE. 



wave action from the main river channel. Many levee failures have 

 occurred because of poor material, unstable foundation, or erosion. 

 It is usually poor economy and often disastrous to place the levee too 

 near the river channel in order to reclaim a few more acres of land. 

 The ground is quite often lower nearer the river channel, and besides 

 the danger of failure due to erosion, the maintenance expense on levees 

 exposed to wave and current action is always heavy. Government 

 engineers in charge of the Mississippi River work now require that 

 the borrow pits made in levee construction be at least 100 feet from 

 the bank of the river, thus making the minimum distance from the 

 levee to the bank not less than about 150 feet. Consideration must, 

 be given to the question of leaving outside the levee a sufficiently 

 wide flood channel. The conclusion reached in any particular case 

 will depend upon the conditions existing upon the opposite banks of 

 the stream, and no definite rules can be stated. 



The height of the levees on districts along the Illinois and Missis- 

 sippi Rivers varies all the way from zero at their ends against the 

 high ground to 20 feet in places, but they are in general between 

 8 and 12 feet high. These heights have been fixed with reference to 

 the height of the flood stages in the rivers above the land upon which 

 the levee is located. The top of the levee should be at least 3 feet 

 above highest expected stage. 



The dimensions which a levee must have in order that it may be 

 durable and sufficiently strong depend upon the kind of material used, 

 the method of construction, and the nature of the exposure of the 

 levee to the action of waves and currents. A tenacious, clayey soil 

 or gumbo will in general make the best levee. Experience has shown 

 that the least dimensions which a finished and compacted levee should 

 have are a width on top of 6 feet and a slope on each side of 3 hori- 

 zontal to 1 vertical. Where the location is such that the water 

 remains against the outer side of the levee for only short periods of 

 time, and where the material is exceptionally good, a total slope of 

 5 to 1 might be divided equally between the two sides. If the mate- 

 rial used is very sandy, or if the bank is to be exposed to wave action 

 or strong current, flatter slopes must be used. In this connection it 

 has been the experience on many districts that while coarse, pure 

 sand will erode very readily it will not give trouble from settlement 

 or sliding on the land side during high water. On the other hand, a 

 mixture of fine sand and clay often will become saturated and semi- 

 liquid during prolonged high water and disastrous slides may occur 

 very suddenly. By making the slopes of the levees not steeper than 

 3 to 1 the all-important maintenance of the levees is much facilitated. 



