THE FLOODS IN THE CLUTHA RIVER. (REPORT OF THE COMMISSION ON)

Appendix to the Journals of the House of Representatives, 1880 Session I, E-07

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THE FLOODS IN THE CLUTHA RIVER. (REPORT OF THE COMMISSION ON)

Presented to both Houses of the Ge?ieral Assembly by Command of His Excellency.

The Commissioners to the Hon. the Minister for Public Works. Sir,— Dunedin, May, 1880. Acting under your letter of instructions dated the 29th January, 1880, we proceeded to the Clutha District on. the 12th April and examined the town of Balclutha and the island of Inch Clutha, with the object of ascertaining the nature and extent of the damage caused by the flood of September, 1878; and in this we were assisted by the plans and reports on the same subject, prepared by Mr. H. P. Higginson and the Public Works Department shortly after the occurrence of the flood. We also took evidence from the settlers as to the extent of the damage suffered by them, and their opinion on the causes of it ; we examined the railway bridge and embankments, the breaches made in the banks of the island and in the isthmus above the town, and the present condition of the river, due attention being directed to the nature of the damage sustained by the island and the town ; we also examined the works, in course of construction by the Government, intended to protect the town, the railway station, and railway embankments. After duly considering the evidence given us by the settlers, and founding our judgment on it and our own observations, we have the honor to submit the following report arranged in the order in which the instructions from you were issued to us, which were as follows : — Ist. As to the causes and extent of the floods in the lower Clutha valley, and the nature and extent of damage therefrom, more particularly with reference to the floods of September, 1878. 2nd. As to whether the flood of 1878 was higher than any previously recorded, and if so, to what extent. 3rd. As to whether the damage to the township of Balclutha or the districts adjoining the island of Inch Clutha was aggravated by the existence of the Railway works in the vieinit}'. 4th. As to the character of the various works now in progress, their probable usefulness and effect, and the various means for permanently protecting the township of Balclutha, the railway, and Inch Clutha, and adjoining districts from similar damage in future, together with an estimate of the cost of the necessary works. Ist. The causes of the flood of September, 1878, were the melting of the winter snow on the western mountains, accompanied by a warm rain from the westward. The weather at the time was fine at Balclutha, but intelligence of the rising flood was sent from stations higher up the river by telegraph. The flood reached its greatest height on Monday, the 30th September, but on the previous Sunday the river had already overflowed its banks above Balclutha, covering the peninsula between the town and the hills with a stream represented as over 20 chains wide, 4 feet deep. The water flowed across the town and down the streets, scouring out large holes in them. The flow was checked by the railway embankments for a short time, the water accumulating slightly from this cause ; but upon the railway embankments giving way, the water subsided about 4 inches, but gradually rose again until the culmination of the flood on the next day. During the continuance of the flood a great deal of the land was washed away from the banks of the river at the bend above the town. It was soon noticed that an unusual volume of the flood was flowing down the Matau Branch, and this appears to have caused most of the damage to the island and the land on both banks of the Matau. From levels taken by Mr. H. P. Higginson, it appears that the banks of the Matau are higher than those of the Koau. The former branch is exceedingly crooked, its course being about six miles longer than the Koau, measuring from the railway bridge to the sea. The banks of both branches are much higher than the centre of the island, and the land has an inclination generally from the Matau towards the Koau, and also from its upper end at Balclutha towards the sea. On the left bank of the Matau the country has a general fall towards Stirling and Kaitangata. The consequence of these features of the island was, that when the flood rose to the height of the banks ml the Matau branch, the water rushed over all the low places, making breaches in the soft earth of which they are composed, extending to the low ground immediately behind, thus opening channels for large streams of water to flow in upon the island. In places the banks were cut away by the strong current until lower ground was exposed, when the same result followed. Vast quantities of water then flowed over the island, taking a course towards the Koau branch, and also towards the sea. Quantities of I—F. 7.

1880. NEW ZEALAND.

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fine silt and sand were carried by this water and spread over the upper half of the island, in some places to a depth of many feet; most of the fences were swept away ; ditches used for draining the farms were filled up, and roads destroyed ; houses were inundated to the height of several feet, and some destroyed. After the flood had commenced to subside, the water flowed off the island into the Koau branch, causing other breaches to be cut where it fell over the left bank into that branch at a lower level, continuing to do so till such time as the Inch Clutha Road and River Board were able to construct stopbanks to arrest it. To a less extent the same thing happened on the left banks of the Matau, the floods inundating the land as far as Stirling, flowing back into the Matau, near Stirling bridge, and over the Kaitangata swamps ; but the mainland does not appear to have suffered damage to the same extent as the island. The railway sustained damage to a considerable extent'—part of the flood openings, some culverts and cattle stops, and the embankment beyond the railway station for a length of 15 chains, being washed away. A part of the high embankment between the bridge and the flood opening was also damaged, and narrowly escaped being destroyed. The suspension road bridge at the town had the centre piers scoured out, the wire ropes being cut in order to clear the river of the wreck. All road bridges, from Balclutha to the lakes, were destroyed with the exception of the Victoria bridge and the Kawarau, and the Cromwell and Bendigo bridges over the Clutha. Different opinions were elicited from witnesses as to the nature of the damage to the land from the silt deposited by the floods. In some places there was more sand mixed with the silt than in others ; and the greater the quantity of sand the more permanent would be the injury to the land from this deposit; but it was acknowledged by several farmers that, after an interval of two or more years, the deposit of silt would become fertile soil and yield good crops and grass. Where that is the case there will be no permanent damage done to the land from this cause. Previous floods had inundated the land in a similar manner, and deposited silt on it; the whole of this land, which is exceedingly fertile, no doubt owing its existence to the deposits of a succession of floods. 2nd. All the witnesses were of opinion that the flood of 1878 was the highest which had occurred since the country was settled. The extent to which this flood rose above those of former years was variously estimated at from 20 inches to 2 feet in and about the town. One witness thought the flood of 1866 was higher in the lower part of the Matau, which he attributed to the bursting of the banks during the flood of 1878 by which the water was relieved and subsided. We think that all the evidence went to show that the flood of 1878 was considerably higher than any previously observed. At Clyde it rose 17£ feet above the highest known flood mark, and the fact that nearly all the bridges were destroyed seems sufficient evidence to prove that the flood of 1878 rose above any previously known flood mark. With regard to the Clutha island, there was evidence given that previous floods had covered the land to nearly the same extent, but that, as the water on these occasions had risen over the island from the lower branch of the Koau, the same amount of damage had not been caused. 3rd. It is our opinion that the existence of the railway works had no appreciable influence in aggravating the damage to the township of Balclutha. This damage was caused by the overflow of the river at the bend above the town ; and it does not seem possible, considering the inclination and velocity of the flood, that the railway embankments should influence the surface of the water nearly a mile above them. Every previous great flood had inundated the site of the town and the isthmus between it and the hills; but in former times the ground had been protected by flax, scrub, and tussock, and on this occasion the scouring out of holes had originated where the ground was stripped of its covering of vegetation in streets and side ditches. Any damage occasioned by submergence was due to the greater height of this flood. In considering the question whether the railway works had any influence in aggravating the damage to the Clutha Island and the land on the left bank of the Matau, it is necessary to have a correct idea of the condition of the two channels of the Matau and Koau before the occurrence of the flood. The Matau branch is exceedingly crooked compared to the Koau, and is nearly 6 miles longer, the fall to the sea being the same in both. As a consequence of its greater length, the slope of the surface of the water in the Matau must be flatter than it is in the Koau. This would cause the current to be stronger in the latter, which would have the effect of deepening and widening the channel. This was actually the case, and the Koau, with a straighter and shorter course, used to carry the largest streams in times of heavy flood. The water, flowing over the low point of land called the Ferry, flat and over the neck of land between the town and the hills, would augment still more the volume of the Koau branch. It is shown, by levels taken on any line at right angles to the fall of the country, that-the surface of the Matau is higher than that of the Koau, and this is proved conclusively from the circumstance that, with the exception of four, all the breaches in the banks of the Matau were caused by water flowing on to the island, while all the breaches in the banks.of the Koau were caused by the same water flowing off the island into the river. This difference was probably less before the Matau had received its present great increase of water, but evidence was given that floods in the Koau river had the effect of sweeping across the island and breaking the banks, as occurred on this occasion from the flood in the Matau. The entrance of the Matau was formerly narrow, and there was a small island in the middle of it. During the flood of 1866, this island was partially washed out and the mouth of the Matau widened. The narrow entrance to the Matau, and the wide and deep entrance to the Koau had, previously to 1878, prevented the floods from forcing their way into the Matau. After the flood of 1878 had subsided, it was found that the little island had disappeared, and had given place to a wide and deep channel, while the entrance to the Koau, on the contrary, had become narrow and shallowed. It was given in evidence that, during the height of the flood, a bar of shingle and silt was formed across the mouth of the Koau, with only a shallow depth of water on it, which afterwards was partially removed by the current. Several witnesses distinctly asserted that the part of the Koau immediately south of the town, from its mouth to the railway station, was stagnant for three days during the height of the flood, and they attributed this state of things to the great flow of water coming over the isthmus from the river above the town.

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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

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island of Inch Clutha if the river had been diverted through the isthmus before the bridge was erected, but, as it would have involved interference with so many conflicting interests, it was impracticable. The objects for which any new works would be required are to reduce the undue quantity of water in the Matau branch and prevent the possibility of the flood again forcing its way in that direction, to prevent the scouring of the banks at the head of the island, and by diverting the main body of the river into the Koau to restore it to its original depth and width. The whole of the peninsula on which the town is built, together with the isthmus up to the foot of the hills, was formerly an outlet for the waters of a heavy flood. As the works proposed for the protection of the isthmus and the town, if successful, will have the effect of closing this outlet, it will be perceived that the next high flood will present increased difficulties in confining it within safe bounds, and under these circumstances any works which are undertaken must be of a very substantial character. Mr. H. P. Higginson, in his report to the Chairman of the Board of Conservators of the Clutha river, dated the 12th March, 1879, has indicated the method which, we think, would be the most feasible for the protection of the Matau branch from the effects of floods breaking into it, and to exclude much of the stream which at present flows down it, so as to restore the Koau to its original depth and width. This work should be extended far enough to protect the head of the island from being overtopped and breached by the floods. Mr. Higginson describes this work as being the construction of a training bank composed of rocky material placed in a certain position at the bifurcation of the two branches of the river. Where constructed along the river bank it would take the form of a rock-faced protection wall, backed up behind with an earthern bank. As this work reaches the main stream, it curves into and up the same until the desired effect is produced. Such portion would be a simple embankment of rocky material faced with the largest of the stone allowed to settle into the bed of the river, the nose or extreme point being well protected with harder rock or concrete blocks as a defence against the scour of the river when in flood. This rock-faced bank should be extended as far as necessary to protect the head of the island, or to the end of Section 10 Block XIII. Anything like a complete protection for the township of Balclutha, Inch Clutha, and the railway must embrace all the works proposed by the Public Works Department, as well as those proposed by Mr. Higginson. Furthermore, unless the whole is carried out, it will be inadvisable to do certain portions, for safety to one place may only be secured by endangering another. The protection of the slopes above Balclutha will not harm any other place, neither will the works at the head of the Matau branch proposed by Mr. Higginson ; but the construction of the embankment round Balclutha will certainly necessitate the extension of the railway flood openings, and possibly the construction of Matau protective works. On the other hand, if the bank is not made at the isthmus, it would be advisable to make flood openings in the railway embankment south of the station, where the breach was made by the last flood. The following is an approximate estimate of the cost of the works above described : — Completing protection of slopes ... ... ... ... ... £7000 Embankment round Balclutha ... ... ... ... ... 500 Extending Railway Mood openings ... ... ... ... 3500 Total for works proposed by Public Works Department ... 11,000 Matau Protective Works ... ... ... ... ... 20,000 Total for complete protection ... ... ... ... £31,000 The cost of making the necessary flood openings south of the station, should the other alternative works not be carried out, will be about £3,500. We also think, that under any circumstances, it would be advisable to extend the present flood openings on to the ferry flat by say eight chains, which will cost about £2,500. We have, &c, C. Napier Bell, M. Inst. C.E. H. P. Higginson, M. Inst. C.E. W. N. Blair, M. Inst. C.E. The Hon. the Minister for Public Works, Wellington.