RIVER AND BAR DREDGING.

Manawatu Herald, Volume XXXIII, Issue 1006, 17 June 1911, Page 4

RIVER AND BAR DREDGING.

Following are further excerpts 1 from Vernon-Harcoutt’s articles on '< dredging in the river Mersey and 1 the action of the sand - pump dredge, a synopsis of which appeared in Thursday’s issue : The river Mersey, draining an area, in conjunction with the Weaver, of 1,722 square miles, flows through an extremely irregular estuary into the sea. The river, which is narrow and winding near Warrington, gradually expands below, but is contracted at the rocky gap of Runcorn, below which the estuary again widens out, increasing rapidly in width below Hale Head, opposite which the Weaver flows in ; and 1 attaining a maximum width of about three miles in front of Ellesmere Pott, it contracts again, and flows in a comparatively narrow channel, with a minimum width of about three-quarters ot a mile, between Liverpool and Birkenhead. The river emerges abruptly at New Brighton into Liverpool Bay, and flows through sandbanks and over a bar into the Irish Sea. The depth of the lowwater channel exhibits similar variations, for it is shallow and variable down to Garston, below which it deepens to a minimum depth of about 50 feet in the narrow channel past Liverpool, and gradually shoals through Liverpool Bay, till in passing the crest of the bar, the depth has sometimes been reduced to about nine feet below the lowest low water. The flood tide enters the estuary across the bar, and also to a smaller extent from the west along the narrow Rock channel, which is prevented from becoming deeper by a ledge of rock across its entrance to the main channel. Above the narrow neck, the flood tide pursuing a straight course, maintains a deep blind channel in the direction of Eastham along the Cheshire shore. The ebb channel, on the contrary, whilst constantly shifting its course in the upper part of the inner estuary, always emerges near Garston on the Lancashire shore, and following along that shore, only joins the flood-tide channel between Dingle Point and Pluckington Bank. Influence of Inner Estuary on Mersey Outlet Channel. The wide inner estuary, together with the neck, has an area of 22,500 acres, of which 17,300 acres become bare at low-water spring tides ; and it forms a vast natural sluicing basin which, when filled 1 during an equinoctial spring tide rising 29 feet at the bar, has a tidal volume ot 710 million cubic yards flowing out past New Brighton on the ebb, and 281 million cubic yards at a low neap tide. This large volume of tidal water, being directed and concentrated by the narrow neck in flowing into Liverpool Bay, maintains the outlet channel through the sandbanks, which gradually shoals as the outflowing current spreads out with a reduction in velocity ; and the sands are only prevented from forming a continuous strand across the bay by the tidal ebb and flow, for the fresh-water discharge of the river only amounts on the average 10 about 2,500,000 cubic yards per tide, which could only maintain a comparatively insignificant channel. The tidal capacity of the inner estuary is to some extent preserved by the slight preponderance the fresh - water discharge gives to the ebb, but mainly by the changes in the channel, which, by fretting the sandbanks periodically, prevent their consolidation and growth, and by stirring up the sand facilitate its removal by the current. The inexhaustible supply of sand in the bay, and the prevalence of north-westerly winds blowing into the estuary, together with the difference in the velocity of the current in the neck and above, render it probable that most of the sand which encumbers the inner estuary has come from outside; and as in this instance the maintenance of the outlet channel depends on the maintenance ot the tidal capacity of the inner estuary, the possibility of a continuance of the accumulation of sand inside is not devoid of interest. If the process is still in operation, It is a slow one, for the decennial surveys of the estuary, though exhibiting fluctuations, do not indicate any marked change; but they are not exact enough, nor have they been carried on long enough, to decide the question. The depth over the bar has remained fairly uniform for several years, exhibiting an average of about ten feet below low water of the lowest tides, with slight variations which may be accounted for by changes in the position and in the cross-section of the channel; and the area of the cross-section of the channel, rather than the maximum depth, would be the true measure of the scouring capacity of the ebb current. The bar appears to have gradually shifted seawards between 1840 and 1880, indicating a strengthening of the ebb current, which must have been due to the regulation of the narrow channel by the formation of quays on each side, and also to the deterioration of the Rock channel, thereby checking the diversion of a porlion of the ebb in that direction. Any increase in the tidal capacity of the inner estuary would have had a similar effect ; but beyond the temporary influence of a large fret, further accretion is more probable than erosion. Since 1880, however, a retrograde movement of the bar seems to have commenced, for the bar in 1890 was more than half a mile inside of its position in 1880, and approaching the place which it occupied in 1870. Dredging the Mersey Bar.—The bar of the Mersey differs from the

bars which existed iu front of the Tyne and the Tees, in being much further out from the coast, being about eleven miles beyond the actual mouth of the river at New Brighton, owing partly to the large mass of sand encumbering Liverpool Bay, and also to the powerful ebbing current from the inner estuary. This distance, and the situation ot the bar in relation to the coast-line preclude the possibility of resorting to converging breakwaters tor lowering the bar. A training wall across the sandbanks, in continuation of the left bank of the river beyond the Rock lighthouse, would undoubtedly concentrate the flood and ebb tides in the main channel by closing the Rock channel and other subsidiary outlets, and consequently lower the bar ; but such a work would be costly on shifting sandbanks and iu an exposed situation. The bar, however, prevents Liverpool being accessible at all states of the tide ; and Atlantic liners, which have crossed the ocean at a high speed, are liable to be delayed outside the bar if arriving near low water.

Since 1876,the approach channels to Dunkirk and Calais, across the sandy foreshore in the open, sea, have been greatly improved by dredging with sand-pump dredgers; and more recently a similar system has been adopted with success at Ostend. The results achieved in dredging sand in the sea at these places and elsewhere, suggested the expediency of giving the system a trial on the Mersey bar ; and iu September, 1890, a sand-pump dredger was set to work on the bar. Two sand pumps were at first fitted up on two steam hopper barges, capable of filling the hoppers of 500 tons capacity in an hour, and dredging to a depth of 36 feet. These two dredgers succeeded iu augmenting the depth over the crest of the bar, in the centre line of the navigable channel, from 12 feet to 18*4 feet below low water of the lowest tides, by the beginning of 1892, by the removal of 657,000 tons of sand, equivalent to about 438,000 cubic yards. During the next year and a-half, however, though the dredgers removed nearly three times this quantity, the depth remained stationary, partly owing, doubtless, to sands from the sides being drawn down the steepened slopes, and partly to deposit in the deepened channel, which will always require periodical removal. Accordingly, a sand-pump hopper dredger was specially built for the work, capable of conveying 3000 tons ot sand in its eight hoppers, which can be filled in three-quar-ters of an hour. The vessel can convey its load to the depositing