Page:Transactions NZ Institute Volume 5.djvu/136

112 great a cost as to exclude the probability of their being carried out either by the residents on the flooded area or the Government; while there is one which has been mooted upon several occasions, but dismissed almost as soon as mentioned, which appears to me worthy of more consideration. Having had for some time the feeling, scarcely raised to the position of an opinion, that the proposal to form a store reservoir at the Taieri Lake had not received sufficient attention, I took the opportunity, while in that neighbourhood last December, of spending some hours in making a careful examination of the physical features at the outfall, and I now propose to investigate how far the damming back of the waters of that part would tend towards the prevention of those disastrous floods to which the Taieri plain has been subjected in late years. Unfortunately, I have not at command sufficient information either as to the rainfall or the configuration of the ground, to give exact quantities in dealing with the whole of this subject; but for purposes of preliminary enquiry we may find enough either from direct observation or from general laws which may bear upon the subject. In the following calculations I have been much indebted to the elaborate survey executed in connection with Mr. J. T. Thomson's report upon the subject in 1870, which has been kindly placed at my disposal. Other parts are filled in from the general map of the Province, and by personal observation.

Before considering the case I shall glance briefly at the nature and proportions of the evil, for without a knowledge of these we cannot judge of the feasibility of any proposed remedy.

Referring then to the map of Otago, we find that the lower Taieri plain lies at the mouths of the Taieri and Waipori rivers and the Silverstream, all discharging large quantities of water during floods, especially the former river, which has an outpour per minute through the gorge at Outram, even at its lowest, nearly equal to that of the Clyde in Scotland between Glasgow and Port Glasgow; but having a fall very much greater, the cross section is correspondingly less. By computing the drainage areas of these local rivers, we find that into the basin occupied by the Waipori and Waihola lakes, and the Taieri plain, there is discharged that portion of the rainfall over 2065 square miles of country, which has escaped evaporation or absorption by plants or porous strata, the relative areas being—

The Silverstream being comparatively small, and the waters of the Waipori being discharged into the lake of that name, with an effect upon the floods