90 THE IMPROVEMENT OF RIVERS. 



The prevention of a break from this cause consists in raising the grade of the 

 embankment by means of earth taken from the most convenient point and placed 

 upon the crown. Too often this point is the body of the levee or its banquette, because 

 everything else is under water at the time. Much of this raising of height is done with 

 teams and scrapers, but as the addition grows in height it also decreases in width, so 

 that it may be necessary to complete it with wheelbarrows. Another common method 

 of increasing the height is to set posts or drive stakes into the front edge of the crown 

 and against these set up planks. This structure is then backed up with earth. Proba- 

 bly this is the least expensive and quickest method of increasing levee height. Another 

 method largely resorted to is the placing of sacks filled with earth on the top of the 

 levee, sometimes in single tiers, sometimes piled to considerable height. They are 

 backed up with earth if considered necessary. 



(2) Leakage may arise from several causes, but one of the most common is the 

 crayfish. Burrowing animals of several kinds also work holes through or under a 

 levee, and once the water is started through these openings it rapidly cuts away the 

 embankment until it is checked. It is not probable, however, that all holes can be 

 ascribed to these causes, and part of them at least are chargeable to the decay of roots 

 in the ground. Whatever be their origin it is necessary to look after them promptly 

 when they begin to discharge muddy water, for then it is evident that erosion is taking 

 place within the body of the embankment, and that the opening is being enlarged. 



Even when levees are perfect in themselves there is often a continual transpiration 

 of water through the foundation soil during flood stages. This water, called "seep- 

 water," is not only injurious to the lands within the inclosed basin, but may also 

 become dangerous to the stability of the levee, should the base be somewhat narrow. 

 Hence the necessity for the banquette. It is not unusual to see the water break out 

 through the soil inside the levee and throw up considerable mounds of sand. These 

 eruptions are locally known as sand-boils, and as a general thing cease to throw out 

 anything but water after a short time. However, during a flood all these leaks, of 

 whatever nature or origin, may become quite dangerous, and then it is necessary to 

 arrest their flow. The method in most common use for this purpose, both in this 

 country and abroad, is to surround them with a small levee, the ends of which are joined 

 to the main embankment. This is called "hooping." This little bank is usually 

 built to a height about 2 feet below the river level, thus relieving the levee of a' portion 

 of the strain and reducing the erosive action within. 



In regard to "sand-boils," Mr. Starling says:* "Sand-boils are very common and 

 very alarming incidents of every high water, and have been especially prevalent since 

 the great accessions which have been made in recent years to the height of levees. At 

 their first occurrence a stream of water suddenly bursts through the ground, throwing 

 out volumes of sand of several cubic feet, or perhaps even of yards, which it distributes 

 around the circumference of the hole. Sometimes large numbers of these outbursts 



Levees of Mississippi, p. 13. 



