IMPROVEMENT OF RIVER OUTLETS. 125 



"(7) In fresh-water streams flowing directly into the Gulf salt-water currents up 

 stream often exist, where the surface current is flowing seaward; and consequently 

 surface velocities are no sure measure of the discharge of scouring action. 



" (8) When the East Jetty of the Brazos River was completed 2000 feet in advance 

 of the West Jetty, the main current at ebb-tide flowed past the end of the unfinished 

 jetty at nearly right angles to the jetty channel. The shoaling which then took place 

 on the bar, and the subsequent deepening when the jetty was extended, very plainly 

 indicate that one jetty would not be very effective for channel-making at ports of 

 this class." 



One of the most scientific attempts to study the effects of jetties or training-walls 

 was made by Professor Vernon-Harcourt in 1886, when a relief model, about nine feet 

 long, of the estuary of the Seine was constructed on a scale of i to 40,000 horizontal 

 and i to 400 vertical.* The bottom of the river was represented by a very fine sand 

 containing a small admixture of peat. The model was tilted back and forth to repre- 

 sent the ebb and flow of the tides, about 25 seconds being the proportionate time for 

 each period. Training-walls made of tin were then placed in accordance with differ- 

 ent plans of improvement, and their effects on the movement of the bottom could be 

 clearly traced for the different positions, and the results of theories analyzed accord- 

 ingly. 



It is understood that the German Government has since made use of similar models 

 in the study of the improvement of rivers. 



Methods of Improvement. There are two general methods in use for bettering 

 navigable conditions at the mouths of rivers: (i) by jetties; (2) by canals connect- 

 ing the deep water of the river with that of the sea. In addition to the results from 

 natural forces channels have frequently to be maintained by dredging. Theoreti- 

 cally, the bar across the mouth may be removed by increasing either the current 

 velocity or. the volume of the water. The latter might be accomplished by closing some 

 of the several passes, but this may not be advisable, as all the material brought down 

 would be then deposited at the extremity of the navigable channel. It may be neces- 

 sary, therefore, to increase the velocity over the bar, and jetties are employed for this 

 purpose. They are located so as to contract the natural section and give direction to 

 the current, but as they do not lessen the quantity of material brought down it is 

 evident that a new bar will be formed farther out unless there are cross-currents to 

 remove it or a great depth of water in which it can be deposited. This will necessi- 

 tate an extension of the jetties. At the mouth of the Mississippi the crest of the bar 

 was more than two miles at South Pass and five miles at South-west Pass beyond the 

 outlets, and it was necessary in the improvement of the former to carry the jetties out 

 from the shore to remove it. 



To quote some examples bearing on the foregoing statements, in the improve- 



* Improvement of the Maritime Portion of Rivers, L. F. Vemon-Harcourt, 1892, and Report on Engineering, 

 Paris Exposition, 1889, Wm. Watson, p. 653. 



