Page:Transactions NZ Institute Volume 5.djvu/141

Rh So that the flood, under these conditions, would accumulate at the rate of 443,000 cubic feet per minute, which would spread over the areas of these two lakes, and raise their surface about three feet nine inches in a twenty-four hours flood. Even though there were such a rise, no serious damage would result to the adjoining lands; but in this calculation there are two elements, which are taken at extreme figures, viz., the length of time, and the rate of discharge by the Waipori, which is much in excess of that estimated by Mr. Thomson, so that we may reasonably presume that even under such circumstances as those of the 1868 floods, the rise would be very much less than that I have stated.

The quantity to be stored above Outram would evidently be the difference between the largest flood delivery there in the 1868 floods, and the quantity which I have already named as likely to get past the least capable section of the river, between there and the East Taieri bridge, amounting to 3,200,000 cubic feet per minute. Now this would be derived from different districts, the comparative areas of which are—

But the configuration of the country to the east of the river is unfavourable for storing a large quantity of water upon any of the tributary gullies, and the basin of the Taieri lake is more favourable, so that we must calculate upon having none upon the eastern portion, but store a correspondingly larger amount upon the Taieri lake. Taking therefore three-fourths of the total for that part, and the remaining fourth to be stored in small reservoirs upon the Sutton, Deep, and Lee streams, the Taieri lake would require to have its outlet so raised as to enable it to store as much as 3,002,400,000 cubic feet in eighteen hours. Now the area of that lake and part of the Kyeburn valley which would be affected so far up as the present ford on the Dunedin road is about 91,846,260 square feet, so that the increase of depth would amount to thirty-four feet. This additional depth would be required over the whole area named; but owing to the fall of the valleys, this could not be got on an average without raising the dam to a height above the bank at the bridge, and thus requiring a considerable extension in length. Probably a more economical method might be by a smaller dam at that part, and other two at the outlets from the lake proper, by which means suflicient storage might possibly be obtained for the waters of floods less than that of February, 1868, and also sufficient to reduce a similar one to safe limits, for if they