us THE IMPROVEMENT <>/' RIVERS. 



been comparatively small, and it may be doubted whether the prolongation of the 

 jetties would make any material difference. In all doubtful cases it is probable that 

 the employment of a model similar to the one before referred to would orove of very 

 great value in indicating the proper solution of the problem. 



It must of course be borne in mind in planning the improvement of a river outlet 

 that designs which would be successful in one case might result in failure in another, 

 and that only a long and careful study of the conditions, supplemented by wide exjn - 

 rience, can indicate the best, methods to be adopted. Thus at the mouth of the Panuco 

 River, which enters the Gulf of Mexico at Tampico, two parallel jetties were completed 

 in 1892, about 1000 feet apart. The floods in this river are very variable, an interval 

 of three to five years sometimes passing between them, but their violence apparently 

 compensates for their rare occurrence. In 1893 a high rise came, and, confined between 

 the jetties, scoured out over a million cubic yards from the harbor, creating a deep 

 channel.* On the other hand, under different conditions, double jetties sometimes 

 have failed to produce any permanent impr, vement. 



Several examples are to be found, as on the Pacific coast, where single jetties have 

 resulted successfully, with, of course, a large saving in cost. Their theory has been espe- 

 cially developed in America by Professor L. M. Haupt, and described by him in various 

 papers.t Briefly stated, it is based on principles deduced from certain phenomena 

 of the flow of water in channels. Thus in a straight reach of river of alluvial bottom 

 the channel is usually shallow and often variable, while in a bend the same amount of 

 water directed along the curve of the bank will preserve a fixed channel of considerably 

 greater depth. According to the theory, therefore, a single jetty of curved trace, where 

 conditions are suitable for its application, and where it is properly located with regard 

 to littoral drift and other bar-building forces, should produce and maintain a channel 

 where one straight or two straight jetties would produce a shallower and more uncertain 

 one. The term "reaction jetty," or "reaction breakwater," has been applied to this 

 type of jetty, the plan of which, where fully developed, is a reversed or S curve, one part 

 being made concave to the channel at its outer end, the point of reversion being on or 

 near the bar, and the convex part lying inside the bar, and intended to cause a com- 

 pression or "head" there, and thus concentrate the effluent currents and prevent the 

 deposit of suspended matter. Where the jetty commences at a distance 1 from the shore 

 this feature is intended to arrest at the same time shoaling from littoral drift. The 

 jetty should as a rule be built, as other single jetties, on that side of the entrance from 

 which the sand-drift comes, just as a snow-fence is placed on that side of the road 

 whence the snow comes. 



The first known location where this type was tried under the conditions named 

 was at Aransas Pass, Texas, where the concave portion of the curve was constructed 

 in 1895, and was found to have produced, after four years, under conditions of a small 



* Transactions Am. Soc. C. E., vol. xlii.. p. 504. 



t Transactions Am. Soc. C. E., Journals of the Franklin Institute, Proceedings Am. Phil. Society, etc. 



