PROTECTION OF BANKS. 73 



vals. One of these is placed close to the outer edge of mat, the others at 30, 37, 38, 

 and 42 feet respectively. 



Ballasting can begin after 600 feet have been finished. If the mattresses are not to 

 be more than a thousand feet in length no ballasting need be done until it has all been 

 completed. The stone is wheeled from barges and placed along the transverse poles, 

 loading the entire floating mat until only the poles are above water. By then bring- 

 ing the stone barges immediately over the mattress and unloading them the structure 

 is sunk to the river-bed. The shore mattress may be ballasted at leisure from barges 

 or otherwise. 



Owing to scour along the outer edge many of the mattresses of this type were 

 damaged, and other defects also developed. In order to overcome these difficulties 

 a more flexible and durable form of mattress has been used for the outer edge, com- 

 posed of fascines or bundles of brush i foot in diameter and in lengths of 50 to 100 feet, 

 tightly pressed and bound together at 3-foot intervals. 



The regular fascine mattress may be constructed in two different ways, one with 

 the fascines normal to the bank and the other with them parallel to it. The latter is 

 generally considered to be the more flexible type. The fascines are made by putting 

 the brush in two layers with the butts in opposite directions, always breaking joints. 

 These layers are drawn together by chains at 8-foot intervals, and then bound by wire 

 into a bundle about 12 inches in diameter. These are then woven into mattresses and 

 sunk in a manner not unlike that described for the ordinary woven type. The slopes 

 above the mattresses are usually paved with stone. 



Surveys of revetted reaches seem to prove that where the bank is protected, no 

 matter how strong the revetment, the channel is deepened just outside the subaqueous 

 work, and under its outside edge, causing it to take a steep grade. If the mattress is 

 built with sufficient flexibility, strength, and compactness, its edge will slowly settle, 

 and the ultimate result will be the steepening of the subaqueous slopes, without destroy- 

 ing the efficiency of the work. 



Spur Revetments. In 1884 a continuous revetment in New Orleans harbor was 

 broken up by the river after sinking, and this led to the introduction of submerged 

 spurs normal to the bank and placed at intervals of from 500 to 1600 feet, usually at 

 salient points. These structures consist of a woven mattress foundation of the width 

 deemed advisable, and extending out into the stream usually beyond the deepest water 

 and protected on the edges with a narrow cribwork of willow poles filled with rock. In 

 the case mentioned additional cribs were sunk one on top of another, at a distance of 

 about 70 feet from its up-stream edge, and affording a base of about 60 feet and a top 

 width of about 22 feet. This cribwork was about 300 feet long. Similar work was put 

 in at Memphis later on and at other places. 



After sinking the mattress and cribs the portion of the bank opposite to and above 

 it should be graded to a flat slope and covered with a revetment of willow and stone, 



