326 happ. STREAM-CHANNEL CONTROL [Ch. 18 



of major floods eastward to the Gulf through Lake Pontchartrain in 

 order to give added protection to the city of New Orleans; the river 

 has been shortened about 170 miles by cutoffs and opening chutes; 

 several reservoirs have been constructed for alleviation of floods in 

 parts of the Alluvial Valley affected by the tributary St. Francis and 

 Yazoo rivers. An additional west-side auxiliary floodway was planned 

 between the Arkansas and Red rivers, but there was considerable op- 

 position to this feature and it was later eliminated from the plan. 



Most of the 30,000 square miles of alluvial lands are now protected 

 by about 2,679 miles of levees which reach heights of 30 feet in places 

 and have a volume aggregating about 1,238 million cubic yards. The 

 levees confine flood waters and hence increase stage heights ; according 

 to a tabulation given by Matthes (1948) the increase in stage heights 

 was 6 to 15 feet at various points of measurement, before channel 

 shortening by cutoffs. 



Permeable pile dikes,* of heavier construction than those which 

 proved inadequate during earlier years, are the principal means of 

 training and contracting the channel, restricting the low-water channel 

 to about its normal average width and thus eliminating local sections 

 of excessive width where shoaling on the crossing bars was most 

 troublesome. The pile dikes are set considerably below the top of the 

 river banks, but sedimentation around them, followed by growth of 

 vegetation on the sedimentary deposits, is usually expected to build up 

 the diked area to full bank height. Sand-fill dams and dikes, built by 

 hydraulic dredging, are used in many places to direct the current or 

 close off secondary channels; these sand dams are not expected to be 

 permanent but serve a temporary purpose (Ferguson, 1940, p. 9) . The 

 concave banks on the outside of bends are revetted to prevent excessive 

 caving, chiefly by concrete mattresses of various types sunk below 

 low water from barges, and by concrete or asphalt paving above low 

 water. Underwater paving with a sand-asphalt mixture has been tried 

 experimentally (Senour, 1948). In 1948 it was reported that 163 miles 

 of operative bank revetments were in place. 



Early channel cutoffs for navigation improvement were not success- 

 ful (Elliott, 1932, p. 280; Ferguson, 1940, p. 15), but a natural cutoff 

 in 1929 and 15 artificial cutoffs between 1932 and 1943, together with 

 dredging and enlargement of chutes, shortened the low-water channel 

 about 170 miles, or about 25 percent, between Memphis and Baton 

 Rouge, with very satisfactory results. The channel shortening is 



* In American engineering practice, the term "dike" is usually applied to rows 

 of wooden piles fastened together by timbers or cables. 



