Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/602

Rh 578 RIVER ENGINEERING estuary. The scouring current predominates at one time [ in one part and at another time elsewhere, so that slack : water is never permanent in any part of the estuary, and accretion cannot progress for a long period without dis- turbance. When, however, a channel is permanently fixed by training walls, the condition of the estuary is com- pletely transformed. The flood tide, indeed, conies in with its burden of silt as before, rising sooner up the improved channel, and therefore dispersing with a somewhat gentler flow over the rest of the estuary. The ebb tide, however, is mainly concentrated along the trained channel, especi- ally when it attains its maximum scouring efficiency towards low water. Accordingly, whilst the flow in the trained channel is increased, stagnation occurs more or less over the rest of the estuary, and silting-up inevitably occurs, resulting eventually in a large reduction of tidal capacity. The accretion, moreover, is not confined to the portion of the estuary behind the training walls, but gradually creeps down, on each side of the estuary, for a considerable distance beyond the ends of the walls. Low training walls hardly rising above the adjacent sandbanks have been tried with the object of preventing this accre- tion, but the improved flow in the trained channel and the reduced velocity elsewhere still promote accretion behind the low walls, and the deposit, rising first along the shores of the estuary, gradually attains high-water level over a great portion of the area at the back of the training walls, and from thence slopes down to the top of the walls on each side of the channel. Though the pro- cess of accretion is less rapid with low training walls, the ultimate result is only delayed and not prevented, as clearly manifested by observations on the Seine and other rivers, so that the view formerly entertained by some engineers, that if training walls were kept down to the level of the existing sandbanks no accretion would take place, has been proved to be erroneous by the results of experience ; and it may be accepted as a general law that training walls, whether high or low, inevitably lead to accretion if the flood tide is charged with silt. Remarks The most careful consideration should be given to all retrain- ^ ne conditions of an estuary before training works are commenced, for when once begun they must be eventually carried out to deep water ; and, if ports exist along the shores of the estuary, they are liable to be injured by the accretion resulting from the works unless the trained channel can be led close along them. The training works of the Seine estuary, though very advantageous to the inland port of Rouen, are compromising the approach channels to Honfleur and Havre, and have silted up the port of Harfleur, so that the extension of the training walls to Honfleur has been urged in the interests of that port, whilst large works have been executed to preserve its entrance, and a new direct channel into the sea is being proposed for Havre. Though the training works on the Dee have not been carried out hitherto in a judicious direction, having been formed mainly with a view to land reclamation, it would be advantageous for the ports of Chester and Connah's Quay to extend these works towards the sea, as there are no ports below the present limits of the training walls to be injured by the effects of their pro- longation, and the navigable channel would be much improved, provided the works were carried out to deep water. The training walls in the Ribble estuary must eventually be extended, even beyond the limits at present authorized, if a good navigable channel is to be secured to Preston, but these works will produce an entire trans- formation in the estuary, of which large portions have been already reclaimed as a consequence of the works already accomplished. The Mersey estuary would need a very comprehensive scheme for its improvement, owing to falls. the very defective natural condition of the estuary, and the situation of the ports along its banks. The mere training of the channel in the upper estuary, for the bene- fit of the up-river ports, would result in the reclamation of the wide estuary between Runcorn and Liverpool, and thus deprive the channels between Liverpool and the sea of their natural scouring reservoir of tidal water ; whilst the training of a channel below Liverpool out to the bar would necessitate very extensive works in deep water and in an exposed situation. Dredging. The improvements effected within recent Dredgh years in the ordinary dredging machinery, and the intro- duction of the sand-pump dredger, have facilitated and cheapened dredging operations to such an extent that some of the most remarkable river improvements have been effected by dredging. The great increase in depth realized on the Tyne and the Clyde has been effected by means of steam bucket-dredgers aided by hopper barges, whilst the maintenance of the entrance channel to St Nazaire on the Loire, and the deepening of the approaches to Dunkirk and Calais, have been accomplished by sand pumps, which have the advantage of being able to work when exposed to moderate waves. Dredging merely con- sists in removing material from the river bed and thus enlarging and deepening it ; and the extent to which this method of improvement may be carried simply depends upon the economical consideration as to how far the improvement of the traffic on the river by an increase of depth will afford an adequate return for the outlay. Dredging, however, furnishes a cheap method of excava- tion owing to the small cost of carriage by water. Dredg- ing, being a purely artificial means of improvement, generally necessitates regular maintenance ; whereas the improvement from scour effected by jetties and training walls is permanent, being realized by natural means. Frequently, however, training walls and jetties are supple- mented by dredging, for the walls and jetties render the deepening by dredging easier and more permanent; whilst, on the other hand, dredging enables a greater depth to be attained, and even maintained, than could have been effected by scour alone. The improvements on the Tyne and on the Clyde have mainly resulted from very extensive dredging operations, but they have been aided by training walls on the Clyde, and by the Tynemouth piers on the Tyne, which protect the entrance channel from drift and the dredgers from waves, and concentrate the scour over the bar. The three methods of improvement described above have been re- sorted to on the Tees : for training walls have been formed through the wide estuary below Middlesborough for fixing the channel ; converging jetties are being constructed for sheltering the channel from wave-borne sand, and for directing the scour over the bar ; and dredging is being employed for deepening the trained and sheltered channel. On the Maas also, and at Charleston, dredging is being used for attaining a depth for navigation which the jetties alone were unable to produce. River Tyiie Improvement Works. The Tyne has a drainage area of 1053 square miles ; its tidal Plat< flow extends 18 miles from its mouth, and the range of spring figs. tides at its outlet is 14f fret. Being by nature a small winding irregular river, with little tidal capacity and no estuary, its depth was small a7id variable, and a bar existed at its mouth, which opens directly on to the sea-coast. The first improvement works, com- menced in 1843, consisted in training the river by cross jetties, subsequently connected by low training walls, so as to regulate the width and consequently the depth of the river. As, however, the volume of water in the river was small, the scour was not adequate to eflect a great improvement in the depth ; the bed of the river between Newcastle and the sea, in 1860, was in many places above low-water level ; and the depth on the bar at low tide was only 6 feet (fig. 9). The piers at the mouth were commenced in 1856,