3

E.—7

This stream was described as more than 20 chains wide and 4 feet deep. If this were the case, it would prove that the river had blocked up the mouth of the Koau for a short time, and was discharging most of the water of the flood into the Matau, and through the breaches in its banks. This condition of the river still continues to a modified extent. In previous great floods the water swept over the point of land called the Ferry fiat, and flowed down the Koau branch. By the erection of the railway embankment, this waterway of 2700 feet was contracted to 676 feet, exclusive of the opening under the bridge. The effect of this contraction of the waterway should have been to raise the level of the river on the upper side of the embankment, which would cause an increased current through the bridge. The same effect would also take place through the flood openings, but aggravated by the smallness of the space compared with that of the bridge. We could not, however, get any direct evidence that the river was dammed up to any appreciable extent. A violent current through the bridge would strike against the mouth of the Matau and would wear away the small island in the channel and the banks of the island of Inch Clutha opposite. It does not appear likely, however, that this effect alone would cause the whole of the floodwater passing through the railway bridge to be forced down the Matau branch, which has a flat inclination and shallow crooked channel, in preference to the channel of the Koau, which has steeper inclination and a straight and deep channel. We would infer from this action of the water that some obstruction, like the bar already referred to, was preventing it from taking its natural course down the latter. This obstruction was observed by witnesses at the height of the flood, and the remains of the bar still partially obstruct the mouth of the Koau. It would almost be impossible to say positively what was the cause of this blocking up of the mouth of the Koau during the height .of the flood. Those witnesses who appear to have observed the river attentively at the time, assert that the great body of water flowing over the isthmus, which they said was more than 20 chains wide and 4 feet deep and running with a strong current, had the effect of checking the flow in that part of the Koau, between its mouth and the railway station. There can be little doubt that if the current in the Koau, between the railway bridge and the railway station, were checked at all during the height of the flood, a bar of silt and shingle would be formed across its mouth, which would have the effect of forcing the greater part of the flood down the channel of the Matau. Other witnesses attribute the blocking up of the mouth of the Koau to the effect of the railway embankment, which, by forcing more water down the river and sheltering the spit behind it from the course of the current, allowed the shingle and silt brought down by the flood to accumulate until it extended across the river. Taking the evidence of those persons who saw the flood in conjunction with the effects which occurred when at its height, we are of opinion that, so far as the damage to the island was increased by the unusual flooding of the Matau, this arose from the choking up of the mouth of the Koau ; that this might have, and probably had, several causes: —lst. A great body of the flood came over the isthmus or neck of land above the town. The river on the upper side of the isthmus is considerably higher than below it; consequently the water which passed over came down with a strong current and discharged directly against the flow of the Koau, thus checking its velocity, and allowed shingle and silt to be deposited in its mouth. 2nd. This action might possibly be assisted by the railway embankment, which, sheltering the sandspit behind it from the force of the current, would thus cause the extension of the spit by deposits swept through the bridge. 3rd and lastly. The breaches of the banks on the right and left side of the Matau, close to the head of the island, at once relieved the pressure of water tending to flow down the Koau, and permitted a still greater deposit of silt and shingle at its entrance. Of the causes above mentioned, it is very difficult to say which had the chief part in the damage to Inch Clutha. We are, however, clearly of opinion, that the overflow of the river at the isthmus or neck of land above the town, was not affected by the railway works, which are a mile below the isthmus. Taking into consideration the exceptional height of the flood of 1878, and that the town of Balclutha, the island of Inch Olutha, and adjacent districts, had not previously been entirely exempt from damage and inundation by lesser floods, it appears to us that we should not be justified in asserting that the railway works had been the chief cause of the damage sustained through the flood of 1878. That they assisted in aggravating the effects of the flood as experienced over the island is not impossible nor even improbable, but to what extent it is most difficult to say. 4th. The Public Works Department have undertaken some works above the town of Balclutha intended to prevent the encroachment of the river on its banks and the overflow across the township and peninsula generally, by which means the railway works will also be protected. As described by Mr. Blair, the scheme consists of: — Ist. The protection of the slopes of the'river by covering it with rocky material, and the deflection of the current by groins or spurs. 2nd. Carrying an embankment right round the river bank and the terrace on the peninsula, from the point where the river first overflows to a junction with the railway embankment near the foot of Glasgow-street. 3rd. Increasing the waterway of the railway flood openings on the ferry flat, and closing all the openings between Glasgow-street and the high ground south of the isthmus. By these means it is expected that the main portion of the township, as well as the railway, will be protected. Pending the result of this enquiry, the only portion of the work that has been put in hand is the protection of the river bank by rockwork. If durable stone of a large size had been procurable, we believe this of itself would be sufficient to withstand the encroachment of the river; but as such is not the case, the slope will probably require to be covered by more durable materials, or protected by groins that will deflect the current away from it. It must be borne in mind that if the isthmus is embanked, a very great body of water, which in high, floods found an outlet across this neck of land to the Koau branch, will be confined to the channel of the river, and the effects of the increased height and velocity of the flood, which will be occasioned by thus confining it, will require the other works to be carried out for the protection of the town and the head of the island below the bridge. Indeed, it would possibly have been better for the safety of the town and