WEST COAST RIVERS.

Patea Mail, 16 December 1880, Page 2

WEST COAST RIVERS.

A decision is now come to at Wanganui on the question of spending £30,000 or £-10,000 in improving that line river, by carrying training walls over the bar. The walls previously built are inside the bar, along’ the mud flats. A dredge is also about to operate on those flats, with the object of ploughing the surface, so that the outward current may carry the stuff away beyond the heads. Before the practicability of this plan is tested, a proposal was made last week to begin building walls to scour the bar ; one section of the Board contending that the bar is often worse than the worst part of the river, and therefore needs deepening first, while the other section insist it would be wasteful to spend a large sum in deepening the bar, which is already deeper than the mud flats inside, until the feasibility of dredging and scouring away those flats is clearly shown. The discussion was adjourned till Tuesday, and the Board decided on that day, by a narrow majority, to postpone for six months the question of extending training walls over the bar. The dredging Avill now proceed, and upon the success of this operation will depend the future policy of the Board. If the inner flats can be cleared away, the bar will be the next part to be deepened. If the flats cannot be permanently removed, why enter into the still more risky experiment of erecting training walls on the bar ?

Viewing the problem from a distance, with no churlish jealousy, we think the great breadth of the Wanganui river at its mouth is the real obstacle to its ready improvement. The principle of inducing a scour along the lines of two training walls is not readily applicable to a broad shallow stream. If the Wanganui river, at its mouth, could be divided into two channels, a strong scour within narrow lines might be induced along one outlet. The great breadth of the stream together with its ordinary sluggishness induces a slackness of the scour just where the river-water is backed up by the sea level; that is to say, just where the scour is most needed. This “ slowing ”of the outward scour causes the river-water to settle and deposit brought from the upper roaches. The general force of the outflow is not sufficient to carry this accumulating silt outward beyond the bar. If the wash of the sea outside the bar were less strong; if the Southern Ocean did not break on the coast in powerful rollers, the rivers flowing into that ocean would not have their silt thrown back in the form of a bar at each river mouth. The river problem on this Coast is a special one. Where the conditions arc favorable to the outflow being concentrated, the scour may r be so increased by narrowing the mouth as to overcome, on the average, the silting-up action of the sea outside. If the outflow of a broad-mouthed river cannot be confined within walls narrow enough to get the required increase of scour, the river-water must be slowed at some point near the mouth, and wherever the “ slowing” takes place, there, must the river’s downward silt bo deposited by natural settlement. These appear to bo the conditions of the navigation problem at Wanganui, and we cannot but watch its solution with keen and kindly interest.