RIVER PORT DREDGING.

Manawatu Herald, Volume XXXIII, Issue 1004, 13 June 1911, Page 3

RIVER PORT DREDGING.

VALUE OF THE SAND PUMP. When he visited London, Mr P. J. Hennessy, Chairman of the Foxtou Harbour Board, was fortunate enough to secure a copy of Vernon-Harcourt’s work on rivers and canals, a treatise which is illuminative in its references to difficulties such as that which most often concerns our own port —the inconstant nature of a sand bar. This authority describes the operation and effect of the sandpump dredge in the Mersey, where certain conditions akin to those affecting the Manawatu River obtain. The Mersey, flowing through an extremely irregular estuary, emerges abruptly at New Brighton into Liverpool Bay, and flows through sandbanks and over a bar into the Irish Sea. The low-water channel gradually shoals through Liverpool Bay, till, in passing the crest of the bar, the depth has sometimes been reduced to nine feet below the lowest low water. The bar of the Mersey is about eleven miles beyond the actual mouth of the river at New Brighton, and prevents Liverpool being accessible at all states of the tide ; 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," says the writer, “the approach channels to Dunkirk and Calais, across the sandy foreshore in the open sea, have been greatly improved by dredging with sandpump dredges ; 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 in September, iSgo, 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 ton capacity in an hour, and dredging to a depth of 36 feet. These two dredgers succeeded in augmenting the depth over the crest of the bar, in the centre line of the navigable channel, from 12 feet to 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.” The dredging power was subsequently augmented, to cope with in-caving sand, and with the object of increasing the depth to 26 feet at lowest water and widening the channel to admit vessels of large draught at all states of the tide.” Suction dredging has been carried out at various Old World ports at costs ranging from 2'rd to 7*23d per cubic yard, including transportation of the material raised. Summing up the results of suction dredging operations, of which he has given examples, Vernon-Har-court remarks that they “ demonstrate the great improvements that can be effected by dredging in the navigable condition of tidal rivers, far exceeding the limits of depth obtainable by natural scour directed by training walls. . . . In seaports on tidal rivers, which form centres of trade, the increase in traffic which generally results from an increased depth in the river, and greater facility and safety of access, usually suffices to pay for the expenditure on the deepening and the costs of maintenance.”