Page:Transactions NZ Institute Volume 5.djvu/140

116 For this purpose it will be necessary, in the absence of particular data, to assume that the Waipori river and Silverstream delivered their waters at the same rate as the Taieri, in proportion to area; and for the general purpose of this investigation that will be enough. This would show a total outpour from these sources of 5,532,900 cubic feet per minute; and deducting from that amount 1,186,900 cubic feet, which could flow towards the sea, the amount of water which would be dammed back would be 4,548,000 cubic feet per minute, representing the rate of rise of the flood. But it has been ascertained that on this occasion 4,585,996,800 cubic feet were the total flood waters, so that dividing the one quantity by the other we would have a period of flood equal to seventeen and a half hours. Of course, this is to be regarded as simply a hypothetical statement which will represent only the average rise and length of time resulting from that, for it is very likely that the rise would be at this rate only for a few hours, which would consequently necessitate a more lengthened period to produce the same total accumulation from a smaller rate of increase.

We have now to consider the amount of water which it would be desirable to prevent flowing upon the plain by the Taieri river, in order that the floods may not attain to such an extent as to overflow the banks. It has been found that the damage caused about the West Taieri has been the consequence of the small section of the river further down not being sufficient to carry off the whole waters as they arrived; and it has been shown by Mr. Thomson that the smallest section has been able to pass down 1,173,744 cubic feet per minute; but it might not be safe to charge it with even this quantity, so we will leave a considerable margin by taking it only to the extent of 900,000 cubic feet, looking to store the remainder in the Upper Taieri lake, or upon some of the tributaries of the river, such as the Deep, the Sutton, and the Lee streams. It will be observed that this makes no provision for the waters of the Silverstream, or the Waipori river. The first of these could probably be stored in the lagoon near its junction with the main river, and which is evidently the natural flood moderator of that stream, or in some reservoir higher up; while the large flow from the Waipori would be allowed to spread over that and the Waihola lakes. The general result, then, may be arrived at thus:—