29

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certainty, what the ultimate net result would be. I should much fear, however, that the depths of the water .on the bar, although it is to be hoped that they might be frequently improved, would nevertheless bo subject to periods of shoaling of such frequency and duration as would approach very nearly to the existing state of things, and thus render it very questionable whether the real improvement of the entrance to the port would be commensurate with the outlay that would have to be incurred. Proposal for Permanent Works. Secondly. With regard to works of a character that would effect a permanent improvement in the entrance: I have given much consideration to this phase of the question; but Mr. Simpson's plan, completed in March of last year, valuable as 1 have already stated it to be, does not, alone, furnish all the data necessary for arriving at definite conclusions with respect to the exact lines which should be adopted for works of a permanent character, nor the kind of construction best adapted to meet the circumstances of the case; the means of framing a reliable estimate of cost are, consequently, not at present available. I should here remark that, although I made a general examination of the entrance when at Dunedin, in the early part of 1878, the questions then placed before me had reference to the Upper Harbour alone. 1 refer to this in explanation of the reason for my not having included in the memorandum left by me in the colony of information required, or noted in a separate document, the particulars necessary for framing a definite design, and furnishing an estimate of the cost thereof. Physical Features and Cause of Existing HJvils. The result of a careful study of this important branch of the question, by the light of the information at present available, has led me to the conclusion that the seas from the northward have had the effect of driving the sand, silt, &c, in suspension down into the bight on the southern shore between "Williams's Point and Harrington's Point, until stopped by the tidal currents passing into and out from the upper part of the estuary. Having travelled thus far they have come under the shelter of the southern shore, and therefore out of the influence of any such forces as can carry them back again, the result being the existence of the extensive " sand-flat " on the northern shore within the entrance, and, as I have already pointed out, the unsatisfactory condition of the channel just within the bar. As to the bar itself, its great length and the shallowness of the water upon it are both greatly aggravated by the presence of this sand-flat, seeing that the tidal streams are thereby so deflected as to cause them not only to turn at a right angle at about half a mile to the southward of Harrington's Point, but also —and this is by far the most serious cvil —to cause both the ingoing and outgoing tidal currents to flow in a direction which is practically parallel to that of the onshore breakers, and therefore to pass.(both inwards and outwards) along a course in which their power to combat the accumulations created by the waves is almost neutralized. Moreover, after passing Harrington's Point, and still more to the northward of Taiaroa Head, the scouring efF&ct of the currents is greatly reduced by reason of their diffusion over a considerable area. The depths in the channel between Harrington's Point and the sand-flat, where the width is about 1000 feet at low water, may be cited in proof of the sufficient power of the currents to create and maintain deep water if judiciously utilized. Remedial Measures Mecommended. Adopting this as the correct view of the origin of the evils at the harbour entrance' —and a lengthened and careful consideration of all the circumstances of the case has only served to confirm this reasoning —the key to the solution of the problem is not far to seek, and I feel assured that the only effectual and permanent method of treating the bar lies in the construction of two breakwater moles, so devised as to train and concentrate the tidal currents flowing into and ebbing out from the estuary, and thus to bring them to bear upon the bar in the manner and direction best suited to create and maintain a sufficient navigable depth for vessels of the largest class to enter and leave in all weathers. By reference to the accompanying drawings., and more especially to No. 2, it will be seen that I have laid down the lines which I should recommend for the two moles, supposing that the detailed investigation, to which I shall presently advert, serves to show that there are no local impediments in the way of rock or other material which cannot be removed at a reasonable cost, within a [depth of, sa} r, 26 feet at low water, which I regard as the minimum to be ultimately attained in the entrance, after making a sufficient allowance for " scend " or undulation, as well as for a depth under a vessel's keel to admit of proper steerage. The breakwater arm on the west side of the entrance would commence at the south-eastern point of the sandhills situated on the north shore of the estuary opposite to Harrington's Point, and run out straight in a North-north-east (magnetic) direction towards and over the site of the bar, and terminate in a depth of 28 feet at low water. The length from high to low water would be about 1,200 feet, and from low water to the outer extremity, or head, 5,000 feet, giving a total length of mole of 0,200 feet. The arm on the east side would commence at Taiaroa Head and run North by East (magnetic) for a length of 1,700 feet, terminating in a depth of about six fathoms at low water. According to the Admiralty chart, the tidal currents through the north channel within the bar run with a velocity of " from one to two knots " an hour at the point where the distance between the inner or south-western face of the bar and the coast line is greatest. Judging by the depths maintained in the north channel generally, it is only reasonable to assume that the velocity of the tidal currents running through the proposed entrance (i.e., on the line of the leading lights) will be from two to three knots. The currents would be deflected by the proposed works and directed continuously on one and the same line, and within a breadth confined to about 1,800 feet where it crosses the crest of the bar and 1,500 feet between the breakwater heads, where the strength of the currents would be brought to bear with the greatest effect in scouring the sea bed in the proposed entrance channel. In this way the two breakwater training moles shown on the drawings, and hereinbefore described, cannot fail to accomplish the object in view—namely, the establishment and maintenence of a depth in the entrance sufficient to admit of the largest vessels entering and leaving the port in all states of weather and tide.