D.—4

1889. NEW ZEALAND.

SURVEYS OF NEW PLYMOUTH HARBOUR (REPORTS OF), BY THE CHIEF SURVEYOR, NEW PLYMOUTH.

Presented to both Houses of the General Assembly by Command of His Excellency.

Memorandum from the Chief Surveyor, New Plymouth, to the Surveyor-General, Wellington. New Zealand Survey Department, Ist April, 1889. I have the honour to forward the enclosed report on the soundings taken in the vicinity of the breakwater. The tracing is sent by separate parcel. Thomas Humphries, The Surveyor-General, Wellington. . Chief Surveyor.

Report on Soundings taken at New Plymouth Breakwater with a View of ascertaining the Extent of Sand-accumulation in the Vicinity. The original intention was to survey the sandspit at the entrance of the harbour, and determine its limits in such a manner that its future progress, both as regards direction and mass, might from time to time be ascertained. I very soon found that the spit was the visible result of a more extensive trouble, and that any survey limited to the spit would be of little service or value if the evil of which it was but the outcome was not examined. The survey has therefore been much more exhaustive than at first intended. It will be necessary to make a few explanatory remarks on the map which accompanies this report. First, all depths of water shown on it are in feet, and are the results of soundings taken by myself on the 17th, 20th, and 28th instant, and are consequently on the top of any sand-accumula-tion. Second, the spit and vicinity are shown in contour-lines, each representing a certain depth of water. This depicts the shape of the spit more clearly than a mass of soundings, and is far more comprehensive and less confusing. Third, the area over which sand has deposited since Mr. Jones's survey in 1877 is shown, and thickness is shown in red figures— i.e., so much less water at those places than in 1877, before the breakwater had commenced. It will be noticed that the bank of sand inside of the breakwater, known as the spit, extends 800 ft. due south from the end, and at 250 ft. out is 300 ft. in width between the 6ft. low-water lines, and 200 ft. at the outer end. On the inner side—that is, against the wharf—the bank is very steep, shoaling from 12ft. to 6ft. in a boat's length. It is also abrupt against the channel, but shelves away very slowly on the outside out to the 3 and 4 fathom lines, I,oooft. from the inner edge. In other words, the base of the spit is about 800 ft. in length and a little over I,oooft. at its widest part. There is now a clear channel of 450 ft. in width between its end and the "Hawea" wreck, with a depth across of from 12ft. to 10ft. at low water, spring-tides. Along the centre of this channel (now used by the steamers) right up to the wharf there is nothing less than 12ft. On the inside of the breakwater, and close to it, there is a deposit 15ft. near the outer end, decreasing to Bft. opposite the wharf. This is of no great width—some 30ft. or 40ft.—and has probably been washed over from the outside in heavy weather. Around the wharf and inshore it has shoaled about 2ft.; but this undoubtedly took place some years ago. It appears to be chiefly a mud deposit ■ — debris from the railway and stockyard embankments, and spoil-heaps, which were injudiciously tipped below high-water mark inside the harbour. The amount of sand and mud deposited at the spit and inside the breakwater is calculated at 260,000 cubic yards. The soundings outside have disclosed the forming of another spit, or deposit of sand, running directly out to sea due north from the end of the breakwater for 1,400 ft. At I,oooft. out the deposit is 4ft. 6in. thick, but, being in 5- and 6-fathom water, will not be disturbed again. Out from the end of the breakwater, and in the direction of the proposed ultimate extension, the sanddeposit is thin, and runs out at 900 ft. On the outer or seaward side of the breakwater the sand is banked up against it—at the outer end as much as 15ft. above the base, and rarely in any case less than 10ft. throughout the whole length, leaving no more than 2ft. 6in. of water a few feet from it for the whole of the outer 1,200 ft. Nearer in it is, of course, less. This bank slopes seaward, running out about 500 ft.