The Wainui-o-mata Waterworks
An engineering correspondent has favoured us with the following interesting notes on a recent private visit of inspection he made to the Wainui-o-mata waterworks:
First, as to why the dam broke. The water must have flowed over the by-wash first of all for some time before the breach was made in the dam itself. All the slope or “apron” of this is washed away. The theory is that at the bottom there is a rock, near which the concrete had not long been finished. This rock is at such an angle as might cause an eddy, and the supposition is that the eddy caused some of the stones to be washed out of the slope where the cement was not hard, and once started the whole apron would rapidly go like an arch without a keystone. From a careful examination of this part, I am inclined to think that that supposition is pretty near correct. I am told that the by-wash is about 83 feet wide, and the water would require to flow over it near 4 feet deep before it would be level with the top of the reservoir. This certainly is an exceedingly large provision for storm-water.
In reference to the breaking in the wall of the dam, it has been stated by Mr. Duncan Sinclair (who was inspector for the Corporation) that the water went over the top in waves and thus caused the breach. I have no hesitation in saying that this is utterly inconsistent with what presented itself to my own observations. I made a careful examination around, and there are none of those indications which must have appeared if such had been the case; and, furthermore, there were lying at and about the back of the wall loose boards in the same position in which they were before the rain, a thing which is utterly inconsistent with their having been exposed to a deluge of water sufficient to carry away a wall and cut a large channel.
What seems the probable thing is this: The place where the wall has been carried away is immediately in front of the spot where the contractor had cut a channel for his own use to carry off the water. The wall in this place was the last finished. The filling in with loose earth would not be so solid as the natural bank. Consequently, till the concrete had set and the earth consolidated, this part must be very much weaker than any other part of the dam, and as it stood at right angles to the force of the torrent I am not at all surprised that it went.
What made the matter worse was the fact that the trench had not been filled up by the contractor, and there was therefore a channel for the water to run in as soon as it got over, with a fall at the back of about twenty feet, and it is not at all surprising that a channel was soon cut through.
In reference to the cost, I may say that it is very much exaggerated; while of course engineers may spend a good deal of money, I shall be surprised if it costs as much as the smallest sum named. The concrete in the work I carefully examined, and it appears fairly good, with the exception of the coping in one place, which is utterly rotten.
Mr. Duncan Sinclair has expressed a decided opinion on the merits of the dam. Now those who know that gentleman will admit that, while he is no doubt capable of giving an opinion as to how many bullocks might draw a log out of the bush, his verdict on such a work as the dam is of little value, although I believe that from his presumed knowledge of the flow of the water, the Engineer lowered the top of the dam about a foot.
There has also been an opinion expressed by Councillor Petherick as to the race. Councillor Petherick holds very pronounced and perhaps valuable opinions on stray goats and horses, but I do not think that his bitterest enemy would accuse him of having sufficient training and experience in either earthwork or concrete to entitle his opinion on such points to any weight. Along the race there are one or two small places which might possibly slip, but each year lessens that probability, as in the case of the Rimutaka railway, and further, there is no other place where the race could be taken. Nor must it be supposed that because an accident has happened, due care has not been taken. The late Mr. Clark was originally consulted, and also Mr. Blackett at different times.
Tags: Newspaper Waterworks
