Page:Transactions NZ Institute Volume 5.djvu/142

118 could be kept back for even twelve hours, their most destructive effects would be moderated. It would also be necessary to have reservoirs upon the Sutton, Deep, and Lee streams, perhaps more so than in the Taieri, in proportion to their areas, as the features of their catchment basins are such as to show many indications of rapid flood-producing streams.

The mode of flood prevention I have examined in this paper is one which has been much adopted upon the continent of Europe, and notably upon the river Loire, which I have already referred to as standing remarkably high as a flood-producer. Above the particular part where the discharge I have referred to was gauged, we have seen that it ranks nearly three times as intense as the Taieri; yet to moderate these waters a weir sixty-five feet high was erected in 1711, which did immense service in the floods of 1846. They topped it, however, by a height of about five feet, but were still sufficiently restrained to lessen considerably the damage which otherwise would have been sustained.

The advantages which the prevention of the flow of the waters upon the lower plain possesses over any scheme of embankment, either along the present channel or any new one, are so evident as scarcely to require remark. Besides being much cheaper, it possesses an advantage in this, that even if carried out to a partial extent it produces general benefit to all the land which has hitherto been liable to inundation; but by the method of embankment upon the plain, intended to shut the water off particular parts, these portions are protected only by aggravating the evil upon other spots, both by the increased depth of the water and the heightened current.

One objection to this method has been so often urged that, paradoxical though it may appear, I believe that had it been founded upon facts, they would, ere this, have been recognized as an argument for its immediate adoption. I refer to the belief that, supposing such a work were erected, the lake would quickly be silted up by tailings derived from the diggings, so that the bottom being raised the weir would speedily become useless. Now the area proposed to be occupied by the reservoir is presently about as much exposed to those deposits as it would be then, and though some parts are so acted upon to a considerable extent, yet had the evil been of such proportions as to be practically felt, a necessity would have existed ere this for the immediate erection of a weir at the outlet, to counteract the shoaling process, and thus prevent a more rapid discharge of the water than would be consistent with its natural condition. An examination of the locality, however, would convince anyone that there is but little to fear from this evil assuming dangerous proportions; for, taking the Naseby diggings alone, it will be seen that even after about nine years of extensive sluicing operations, during which the heaviest flood on record has been experienced, the greatest distance to