194 THE IMPROVEMENT OF RIVERS. 



action of winds and waves along the shores of the delta is favorable to the system, and 

 also any reduction in density of the sea-water, such as may be found in an inland sea. 



"(4) If the sea-bottom is flat; if a large proportion of the sediment is dense, so 

 that it is carried along the bed of the river or close to it ; if the outlet faces the prevalent 

 winds; and if no littoral current exists, it is possible that an improvement of the outlet 

 may not be practicable; and then recourse must be had to a side canal, starting off 

 from the river some distance up, and entering the sea beyond the influence of the allu- 

 vium of the river. 



" (5) The bars in front of the outlets of tideless rivers being formed by the deposit 

 from the river, vary in form according to the nature of the sediment brought down. 

 When the material is composed of particles of very variable density, it is gradually 

 sifted as the velocity of the current decreases, and gives a flat sea-slope to the bar. When, 

 on the contrary, most of the material is heavy, the bar has a flat river-slope, as in the 

 first case, formed by the gradual arrest of the sediment rolled along the bottom; but 

 as little of the material is carried beyond the crest of the bar the sea-slope is steep. 



" (6) The jetty system does not constitute a permanent improvement, for, sooner 

 or later, in proportion as the physical conditions are unfavorable or the reverse, a bar 

 is formed further out, and a prolongation of the jetties becomes necessary." 



The following views relating to jetty and harbor construction on the Gulf of Mex- 

 ico have been advanced, based on experience with the mouth of the Brazos River, 

 Texas:* 



"(i) With tidal harbors the slope of the surface in the jetty channel is inversely 

 as the length of the pass; and consequently, on the Gulf, where the tides are small 

 about one foot and the distance to deep water very great, the plan of improving 

 harbor entrances by confining tidal currents between jetties is a somewhat doubtful 

 experiment. 



" (2) Jetties, to produce a maximum result at a minimum cost, must be com- 

 pleted beyond the bar in a single season. The bar then acts as a submerged weir with 

 the strongest current on the outer crest, thus transporting all eroded material to a safe 

 distance from the entrance. 



" (3) Delays cause great increase in the cost of construction from damage to works 

 by storms, and the much greater distance the jetties have to be built seaward. 



" (4) The success of jetty improvements depends largely on the existence of 

 strong littoral currents in front of the harbor entrance; otherwise the advance of fore- 

 shore and bar would soon close any channel formed. 



" (5) In jetties at the mouths of rivers the strongest currents are at the outer end 

 of the channel, and in no case is it necessary to build the works to a greater depth 

 beyond the bar than that required in the channel. 



" (6) When scour takes place in the channel-bed its action is first noticeable at 

 the lower end of the section, and gradually works up stream. 



* Transactions Am. Soc. C. E., G. T. Wisncr; Pattern's Civil Engineering, p. 1312. 



