E.—9.

16

Source of Supply and Travel of Shingle. Mr. Weber states that the bar consists of river gravel intermixed with limestone boulders, and he correctly adds that the gravel derives its origin from the Tula Tuki, the mouth of which river is situated about six miles south-east of Napier Bluff. Immense quantities of shingle exist over a consider' able portion of the district in which the Tuki Tuki takes its rise, and through which it flows; the supply of the shingle drift which incumbers the entrance at Napier may therefore be considered as practically inexhaustible When inspecting the locality I carefully investigated the conditions which govern the travel of the shingle, and concur in the views expressed by Mr. Weber on this head in his memorandum of 20th March of last year, and his report of 25th March. The preponderating direction and force of the waves impinging on the shore being from south to north, a northerly travel of the shingle from the Tuki Tuki is thereby generated. Following the sinuosities of the coast, the shingle passes across the entrance to the harbour, the deep indentation of the coast north-eastward of Napier Bluff being favourable to the more sluggish travel and deposition of shingle at the head of the bight, and in the vicinity of the harbour entrance. As an instance of the rapidity with which the shingle travels along the shore, I may refer to two facts given by Mr. Weber, namely, that the construction in 1875 of an experimental groin on the eastern shore, 200 feet in length, occupied six weeks, and that within a few days after its completion the shingle had made up to its end. Again, between the commencement of the pier works in July, 1876, and the date of his memorandum of 20th March, 1879, Mr. Weber estimates that 240,000 cubic yards of shingle had been trapped between the eastern pier and the Bluff", notwithstanding that the mouth of the Tuki Tuki, the source of the shingle supply, was closed for nearly eighteen months during that period, the beach from Tuki Tuki to Napier Bluff having been greatly denuded of shingle in the interval. Standing out prominently as one result of past experience, it may be stated that when the travel of the shingle was temporarily suspended by the experimental groin, the bar remained good, and the same effect was produced when the construction of the eastern pier for a time outstripped the growth of the shingle. Easterly winds bank up the bar. Fortunately, however, the prevailing winds are from north-west and south-west, or practically off-shore, at this port, and hence the intervals of obstructive wave-action are of less frequent occurrence than the periods of fine weather, although, as I have before explained, the northward travel of the shingle, which owes its initiation to wave-action impinging obliquely on the beach, proceeds with but little interruption. Bacleivater. The only agency which can in this case be relied upon for keeping open a navigable channel through the bar and shingle drift, is that of the currents produced by the alternate filling and emptying of the immense tidal reservoir of Ahuriri Lake. The area of this backwater is 9,300 acres, and the capacity of its tidal compartment during mean tides is fully 000,000,000 cubic feet; this large area being filled and discharged twice in every twenty-four hours, or thereabouts. There is also the fresh water from the Esk and the Tutaekuri, having an aggregage watershed of 406 square miles, which discharge into Ahuriri Lake; but the useful scouring effect to be looked for from this source is infinitesimal when compared with the daily discharge from the tidal compartment. Moreover, the fresh-water contribution is subject to fluctuations, and liable to periods of great drought on one hand and excessive rainfall on the other. Although some temporary beneficial effect on the entrance would probably be produced by the out-run of a heavy freshet, nevertheless, if a sufficient navigable depth in the entrance is to be permanently maintained so that the trade of the port can be carried on without interruption, this can only be secured by framing works upon the basis that the channel shall be kept clear by tidal water, independently of the fresh. This is quite practicable, seeing that the cui'rents between the piers, due to the filling and emptying of the tidal reservoir, run at fully six knots an hour during portions of the flood and ebb, as has been shown by observations made for the purposes of this report; and this result is borne out by calculation here, the rivers at the time the observations were made having been exceptionally low. It is satisfactory to be able to state that an examination of the documentary records of different dates, so far as they are comparable, bear out the remarks in Mr. Weber's report with reference to the absence of any material changes in the bed of the tidal basin during the last twenty-eight years. This augurs well for the permanence of the scouring power. Works already executed. Eeverting to the condition of the port when the Napier Harbour Act came into operation in 1876, I may observe that the works executed since that time consist of two parallel training piers at the entrance, placed 400 feet apart, and running in a northerly (magnetic) direction, the head of the east pier extending 870 feet further seaward than that of the west. The minimum depth in the fairway in March of last year (the date of the survey) was 7 feet 6 inches at low water, or 13 feet at high water of spring tides. At that time, in consequence of the accretion of shingle eastward of the east pier, the low-water line of the shore had advanced to within 110 feet of the end of that work, the shingle stretching to the westward across the entrance and passing the head of the west pier during easterly winds. It was a wise provision in executing these piers to provide an overlap of the west pier by the east to the extent just described, as this enables most of the shingle to continue its normal travel along the coast across the mouth of the harbour, without being trapped by the western work and deposited in the channel, as would have been the case had the terminations of the two piers been more nearly abreast of each other. The works throughout consist of timber piling and framing, filled in with rubble stone, the outermost 400 feet on the east side, and 200 feet on the west side, being formed of greater width than the inner portions. The contract for the execution of these piers was signed in July, 1876, and they were completed at a cost of about £60,000 in April, 1878. Mr. Carruthers, the late Colonial Engineer, was the designer and Engineer-in-chief of the works, and Mr. Weber the Resident Engineer. Mr. Carruthers's design was based on sound principles—namely, the fixing of the channel, and concentration and training of the efflux and reflux currents along a properly proportioned and definite