204 report — 1877. 



fixed on to the tops of the hickory pegs ; and thus sufficient solidity is obtained to 

 permit of the mattresses being handled. 



These mattresses, 75 feet long and 40 feet wide, are sunk in horizontal layers, with 

 one edge resting against the piles on the side towards the channel. To sink them 

 they are loaded with just sufficient stone to carry them down. 



It is stated that the sediment deposited by the water would of itself suffice to 

 sink them after its exposure in the water for a week. On these wider mattresses 

 narrower ones are then placed, with one edge close against the piles, and so on in 

 succession ; they thus form a series of steps, till the surface of the water is reached, 

 when a pile is driven close against the outer edge of the top mattress through the 

 under layers to hold them all in place. The top mattress is then covered with stone 

 to about two feet above the surface ; and when the top sinks down, as it does 

 gradually and unevenly, it is again raised by stone or by mattresses and stone. 



It is intended eventually to cover the whole with stone ; the sloping sides will be 

 covered with large rough stones thrown in, and the top surface with stones laid 

 with more regidarity. 



This is the method of constructing the more permanent jetties ; where more 

 temporary effects are desired to be obtained, as for the purpose of directing the flow 

 of the current during the construction of the permanent work, the mattresses are 

 applied sometimes vertically, sometimes at a steep incline. 



The peculiarity of this mode of construction in the Mississippi is, that at first the 

 jetty is very pervious to water ; consequently a portion of the water which comes 

 down the Pass flows through the interstices of the wickerwork in its progress 

 through the jetties, and thus the scouring effect of the water is gradually lost. 



But the water is so full of silt that by degrees the interstices become filled up, 

 and the jetty wall is thus gradually made impervious to water : in proportion as the 

 work is thus strengthened, the scouring power of the water on the channel will 

 increase. 



The operation of the current in the channel is therefore necessarily gradual, and 

 its effect increases with the consolidation of the banks. 



At the mouth of the Pass, the east jetty is 12,100 feet in length ; and the west 

 jetty, in consequence of the greater prolongation of the west bank of the Pass, is 

 about 7,660 feet in length. 



At the head of the Passes there is an island about mid-channel ; this has been 

 connected with the east side of the channel, so as to confine the water in one channel ; 

 and from this island, and from a point on the western shore, jetties have been 

 carried up the stream across the shoal at the head of the Pass for a length of about 

 2,500 feet. 



At the Grand Bayon the area of a section of the channel, at high water, contained 

 about 24,000 square feet, whereas at the southern extremity of the Pass a corre- 

 sponding section had an area of section of about 14,000, the diminished section 

 being due to the diminished volume of water consequent upon a portion being ab- 

 stracted by the Grand Bayon. 



A dam has been erected across the Grand Bayon, as well as one across a small 

 bayon lower down the Pass, so that the whole of the water which enters the 

 head of the Pass now flows through the jetties at the mouth into the Gulf of 

 Mexico. 



Since the closing of the Grand Bayon, the advance in width and depth of the 

 channel below that point has been steady and regular ; and the increased volume of 

 water issuing from the mouth of the Pass is gradually producing its effect, by 

 tending to make the section of the lower part of the Pass equivalent to the normal 

 section above. 



The third object to be attained by the works, viz. to control the flow of water 

 through the South-west Pass and the Pass a Loutre, is in progress, but not yet 

 complete. A sill has been placed across the head of the South-west Pass to prevent 

 scour. It consists of an apron of mattresses. It runs from a point on the west 

 shore of _ the South-west Pass to a point near the head of the western jetty. A 

 line of piles were driven and mattresses were sunk against the upperside. These 

 mattresses are 70 feet wide, and are sunk side by side so as to make the apron 70 feet 



