IMPROVEMENT OF RIVER OUTLETS. 127 



of successful works of this kind the Seine, Maas, Clyde, Tyne, Tees, and the Fen 

 Rivers. 



On some rivers the training-walls are placed parallel to each other, while on others 

 they gradually converge. Their direction necessarily depends to some extent upon 

 the natural line of the river and the situation of the towns along its banks. The 

 authority just quoted states that the advantage of the system of converging over the 

 system of parallel jetties is, that additional tidal capacity is provided close to the 

 entrance, which promotes the scour through it, and also that deposit tends to take place 

 within the harbor, where it is easily removed, instead of at the actual mouth. The 

 increase in the width of channel between the Seine walls averages 5 feet in 1000 feet; 

 in the Clyde and Tyne 10 feet in 1000 feet; and in the Scheldt 20 feet in 1000 feet. 



Materials. Jetties have been built of masonry, of timber cribs filled with stone, 

 of brush or log mattresses loaded with stone, of piles driven into the river-bed, and 

 connected together by timbers, of piles and stone combined, of fascines secured by 

 piles and weighted with stone, of furnace slag or cinders, of iron cylinders spaced some 

 distance apart, reinforced at their bases by dikes of riprap rising slightly above the 

 water, and of earth protected by stone. The strength, stability, and durability of 

 brush-and-stone and pile jetties in exposed situations is not always sufficient, and more 

 durable materials are required. The log mattresses are rafts composed of trunks about 

 ten inches in diameter, held together by tie-poles about four inches in diameter spiked 

 or bolted to the main timbers. On the top of this structure brush and sawmill slabs 

 are placed in layers to a thickness of twelve to twenty-four inches. When ready, the 

 mats are towed into position and sunk by piling on stone. Sometimes several layers 

 of mattresses are sunk one upon another. Many jetties are built of rubble thrown in 

 loose, while others are made of masonry on a foundation prepared of mattress-work. 



Results. The results realized from jetties have been to a considerable extent 

 disappointing, although in a number of instances the benefits derived from them have 

 been very great. In many cases their failure has been due to a lack of proper location 

 and design, but in the majority of instances the works have been considered experi- 

 mental from the start, and were built with the hope, not the assurance, that they would 

 prove satisfactory. The problems have frequently been taken up and studied, with 

 the experience gained elsewhere as a guide, and yet the final result has been to disprove 

 many of the theories advanced, and in some cases a final abandonment of the works 

 has taken place and a new start has been made, based on new ideas. There is no doubt 

 of the great value of tidal scour when judiciously directed, but it is a difficult matter 

 to determine in advance the effect any structure placed in a stream will have upon its 

 current and bed. The improvement effected at the mouth of the river Liffey (Dublin 

 harbor) shows how converging piers, arranged so as to receive a large quantity of tidal 

 water within the area they inclose, and discharging it at low water through a con- 

 tracted opening, can increase the depth at the entrance; but in the Wear, Yare, and Adour. 

 where the works simply guide the outlets into deep water, the increase in depth has 



