The Wanganui River.

Manawatu Herald, 8 March 1894, Page 3

The Wanganui River.

Mr Leslie Reynolds in his report on the Wanganui rivor thus significantly refers to the past labours of the Harbour Board, and it is worth bearing in mind when " improvement" are considered to the Manawafcu river : — The approximate discharge of the Wanganui River during normal conditions as given by Mr Lloyd Hassel, 0.E., in his report, of May, 1885. amounts to from 30'J,000 cubic feet to 400,000 cubic feet per minute ; whereas during springs the discharge from tidal water in the estuary below the bridge probably amounts to about 750,000 cubic feet per minute. From these figures ib will be seen what irreparable injury to the entrance would arise from the silting of the estuary, and yet it is only by contracting the channel and encouraging the flats to silt up that a navigable ohannel can be maintained from the Heads to the town. From an engineering point of view, there can be no question, I think, that the site for harbour construction should be at Castlecliff, and that the work of improvement in the eatuary should never have been undertaken: ; and it appears to me that before further development of the channel is entered upon, the Board ihould seriously consider and determine definitely what the ultimate possibilities of the harbour are, before irrevocably sacrificing them to a light draft channel up to the town only at .great cost. A navigable channel through the ostuary undoubtedly points to a confined track, extending over the entire distance from the bar to the town, and enormous loss of scouring volume therefrom. What the possibilities of this estuary are as contained in works at the entrance, lam not in possession of sufficient data to definitely calculate — but of this I am certain that a very much deeper and better harbour can be maintained at Castlecliff than can ever be looked for further up the estuary, and that when the river channel has been deepened to its practical limits of improvement, probably eight to nine feet below l.w.s. tides, the loss of depth due to decreased tidal compartment will prevent vessels drawing the minimum draft of the channel inside the estuary to enter over the bar. There are several instances of estuary harbours having been completely ruined by the contraction of inside channels. There *te sufficient evidence to show that the channel seaward of the training walls is shoaling, and this is the natural result of a confined channel opening out to practically unlimited breadth. The scouring depth of a chanriej in in proportion to its width, • so that the heavier particles of silt . or shingte borne along the channel between the walls drop out and deposit where the currents are allowed to fan out. It is only a matter of a few years before the training walls will require to be extended right down- to the entrance of the estuary, if a ohannel from 7ft to Bft at l.w. is to be maintained up to the present wharf. So far, I do not consider much contraction of the tidal compartment has resulted from the existing training walls, as the sectional area of the channel between them is not nearly sufficient to carry the bulk of the water passing into and out from the estuary, which consequently finds ingress and egrets overthe flats ; but further development of the ohannel will only be precnred, as before stated, at an •normoui sacrifice of the tidal compartment.