Floating Dams.

Clutha Leader, Volume II, Issue 90, 30 March 1876, Page 6

Floating Dams.

As the protection of the banks ofthe Clutha River and the turning of the current in certain places, mnst shortlyj engage attention in this quarter, thd the following* from the ' Marlborough Express' regarding Mr Douslin's floating dam scheme will be read with interest : — , At the meeting of the Rivers Board on Wednesday evening, Mr Douslin exhibited a very elaborate plan,, illustrating the working* of his scheme for affecting the course of rivers. lie accompanied it with an explanatory statement upon the construction of a floating dam, of which the following is an abstract. We may premise that the dam is a series of logs connected together so as to form a long line, and this is anchored in. the stream at a certain angle, thereby diverting the water, and rising with it. Its action is however, explained below. Mr Douslin considers that it would be a certain and cheap mode of dealing with the Opawa Breach. He objected at first to the publication of the plan, which is the result of several years deliberation, and therefore original, as he purposed, in case the Board declined to try the experiment, trying- to get some other body in another part, of the Colony to do so. Ultimately he changed his. mind, and determined to allow a description to be published, so that it might be fully considered before the Board met again. Mr Douslin proposes to construct his dam with white pine logs or trees' from 40 to 50 feet long, and from two to three feet in diameter. These logs would be conpled together with ringbolts, with one end overlapping the next log. The ring in the bolt head is intended to secure the rope or chain. The logs are to float about one-third out of the water, and be fixed at an oblique , angle. The dam thus constructed must next be moored to black birch logs 12 feet long and 18 inches diameter, sunk 'in the ground. '""' A trench four feet deep should be dug in the shingle into which the log is put, after, securing it by putting the chain round the middle, and the trench filled in ag-ain. In case of any difficulty from water rising* in the trench he proposes other methods of securing the log, into which it is not necessary for us to enter. He cousiders such a dam could be constructed at an outside price often shillings per lineal foot, and .portions only built at a time, as circumstances required, or means were forthcoming, as it could be lengthened at any time as thought necessary. What Mr Douslin claims for his scheme is that the floating dam would curb the current; not simply the water coming-in contnct with the dam, but the full depth of the river, causing the shingle brought down by the stream to deposit on the lower side of the dam, and ultimately forming a massive shingle-bar, at the same time scooping out directly under the clam. In many cases where works have been erected of a permanent nature, that is to say works that have been built on the shingle-bed, as those in Canterbury and the Hutt, and notably at the old Wairau Breach, these have all failed because the water scooped out the shingle under the foundations ; it is therefore clear that similar results will follow with the floating clam, that is to say, the scooping of the shingle beneath it, and the curbing of the water -after striking ag'ainst it. It follows as a certain consequence that a large deposit of shingle will take place. Although Mr Douslin cousiders it impossible to be otherwise, he says suppose that the clam does not cause a bar of shingle to form behind it, yet it will divert the current, and cause a large body of water to run in a parrallel line, with it, which may be run off in a branch or river. It. njust also be taken into account that there will be a cross current, the current running parallel with the stream, which also assists in curbing the current in the vicinity of the dam, which causes an eddv. In another placq in this issue we have copied an article from tlie Builder, of' October 16th, 1875, giving an account of the terminus of the Eastern Bengal railway by a flood, which sufficiently proves that' any fixed dam, or undertaking of any kind built on a shingle bed with the view of suddenly stopping a current, must end in a failure. The spur or dam built on the Ganges was constructed with large blocks of cement concrete, the out-side faced, with masoniy, and' connected with the main .land witk briolc ballast, ancf solid clay hoarting. Mr Douslin refers ' to~tne"se works for the sake of drawing a comparison between them and his scheme. fje maintains thatthe floating dam will gradually work out a channel, and from a shingle bar at the back/thereby grad-' uaily curbing the current! Such would not,, however be the result of a dam erected on the single bed, but in that J case if], brought to a certain 1 point and j there stopped with tlie whole force, of the. water directed against it, it must , eitner undermine the structure, pr carry j it'a\ya'y ill together.. Much! willdepend*! - upon ; the force ; of the current^ .'and how ;: long the flood keeps up, as to determining the length of time for turning the river, effectually, or such portion as may be thought necessary. In order to regulate the proportion- of water to be directed down such river beds, the Wairau

arid the/Opawa'j.ft! ipduld^phly )ie nec§s-j sary to increase or Uimimsh the length, of the i dam. 1 ., r ,; -, „ r „-r ._,,. ... „,,.. j It is to be hoped that ' the Rivers^ Board will \yelljan4 duly consider, this scheme before their next meeting. ; We do not pretend, to have any special knowledge of the subject ourselves, but the earnestness land 'zeal of our fellow- ' townsman, who has been at considerable trouble iri the- matter, as well as its simplicity aud cheapness, deserves that it should be fairly and thoroughly investigated. - . .