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. 
DESIGN. 
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. 
