Page:Transactions NZ Institute Volume 5.djvu/138

114 particularly the conditions of the Taieri drainage ground, with a view to ascertain, if possible, in what respects it facilitates the rapid discharge of the rain or melting snow, and it will then be seen that the chief features likely to promote this are the generally mountainous character of the catchment area, with the exception of the Upper Taieri plain, the steep inclination of the ridges, the nature of the rocks, and the general steep declivity of the bed between the upper and lower plains. On the other hand the upper of these plains forms a natural basin about 280 square miles in extent, but of this only about one and a half square miles are an open lake, the remainder being deposits of shingle of various depths lying upon impervious clay, and capable of being a store reservoir only to the extent of the interstices between the stones. That portion, in fact, resembles a huge sponge, acting with the open lake in retaining the water, and preventing to a certain extent its sudden rush down the channel towards the lower parts. The lake lies at the flank of the Lammerlaw or Rock and Pillar range; and finds along with the Kyeburn an outlet through a narrow gorge, at one place not wider than 110 feet, but unfortunately its low level limits the capacity of the whole reservoir as a flood moderator. The point now is, to consider the practicability of raising that outlet to such an extent as to store the greatest flood waters that are likely to occur, and release them only at such a rate as shall not be prejudicial to the low grounds lying below Outram.

The flood of 4th February, 1868, being the greatest on record, I shall take as the standard one, seeing that it is necessary in any remedial works that may be proposed to provide against the occurrence of an evil at least equal to that already experienced.

The circumstances of rainfall attending the floods of January and February of that year, at least so far as we can judge the Taieri basin by observations taken at Dunedin, were almost such as to lead us to expect that no such floods are likely to occur for very many years. Upon examination of the meteorological tables prepared by Dr. Burns and the Meteorological Department, it will be found that the unprecedented nature of the January and February floods was more owing to the extreme degree of saturation in which the ground must have been by months of previous rain, than to even the heavy rains of any one particular day. Doubtless these were very heavy, but not so much so as several days both before and since, which did not produce the same rise in the water. For the months of October, November, December, January and February, the rainfall ranged from 5.0 to 8.078 in., thus showing a continuance of wet weather unequalled, I believe, in the history of the settlement. In the case of the flood of January, 1870, which did not rise so high as the one we are specially considering, the rainfall recorded in one day exceeded that shown upon 4th February, 1868, but that