3

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works, they do not appear to have taken it into their consideration, there being, consequently, a considerable amount of injustice suffered by the people on the left bank. In order to keep flood water off the land, it must be confined between embankments erected upon both sides of the river, and thus confine a large body of water which previously spread itself across the country. The space allowed between these banks and their height must therefore be so proportioned as to contain it; but at the same time, should both sides of the river be embanked in the manner hitherto adopted, such space would be too confined, and would result in the flood level being raised many feet, to the great risk of destruction to the bridges spanning the river, the railway works, as well as the increased risk of damage resulting from breaches in the embankment, owing to the elevation of the flood water above the plain. Consequently, in order to economize land in the width between the embankments, also to reduce the cost and risk in constructing them so high, it would be advisable, if possible, to arrest a portion of the high floods in the upper parts of the Taieri, thus giving time for the escape of water from the lower areas. With this view, the sectional area of the river and the height of embankments shown upon the accompanying plans have been calculated to convey a flood equal to that of 1868, as gauged by Mr Thomson, less the amount withheld in storage reservoirs, as will be hereafter explained. With regard to the flood discharge of the Taieri River, Mr Thomson, in his report, states that he gauged it at Outram, after the flood of 1868, and gives the amount as 6,700,000,000 cubic feet per twenty-four hours, which is equivalent to a depth of 166 inches, running off the drainage area of 1,730 square miles in the same time: this is equal to 4,653,000 cubic feet per minute, or 2,690 cubic feet per minute per square mile. This discharge appears to be an exceedingly high one from so large a drainage area, spread over a period of twenty-four hours. Messrs. Reid and JJuncans gauged the surface velocity of the river above Outram Railway Bridge, where the flood waters were confined between two embankments, while the flood of 1879 was at its maximum, and from which they have calculated the discharge to be 3,750,000 cubic feet per minute. From the same surface velocity and cross section supplied to us by them we make the discharge slightly less, owing no doubt to our having allowed a less mean velocity; such discharge is equivalent to a depth of 134 inches running off. After careful consideration we have deemed it advisable to adopt Mr Thomson's measurement as the maximum flood discharge to be provided for at the moment of highest flood where proposing works upon the Plains required to withstand an extreme case. The maximum flood discharge per twenty-four hours we have taken at 4,020,000,000 cubic feet, or 1,613 cubic feet per minute per square mile; or a depth of X inch of rain running off. The flood discharge of the lower Taieri channel through the gorge we have endeavoured to arrive at as accurately as possible, though we can at best only form a rough approximation from observations, cross sections, and levels, taken in so short a time as was at our disposal. The flood discharge at high water we make 1,837,739 ; and at low water, 2,540,056 ; or an average of, say, 2,188,717 cubic feet per minute, which is equivalent to 3,152,000,000 per twenty-four hours. In the following table we have shown the different areas, with the estimated discharges per twentyfour hours, and equivalent depths of rainfall running off, the quantities being in millions of cubic feet:—

Area Square Depth Run off Millions Cubic Feet Miles. in Inches. per 24 Hours. Upper Taieri, above lower dam ... 800 TOO 1,860 L'iwer Taieri, below lower dam ... 930 TOO 2,160 Waipori ... ... ... ... 290 125 842 Silverstream ... ... ... 50 T5O 173 Total flood-water discharged on plain... ... .., ... 5,035 Average discharge of Lower Taieri channel ... ... ... 3,152 Daily accumulation on the plain ... ... ... ... 1,883

The total maximum amount of flood water lying on the lower end of the plain, as recorded by Mr. Thomson, being 4,580,000,000 cubic feet, divided by the daily accumulation, shows that the water should occupy nearly three days in rising to its greatest height. This result would, however, be somewhat modified by the state of the tide and direction of the wind at the mouth of the river. The method of constructing storage basins in the upper reaches of rivers has been practised in certain parts of Europe, notably on the affluents of the River Loire, where in 1711, at the dyke of Pinay, seven miles above Roanne, and at the Chateau de la Roche, works of this description were erected. The dam of Pinay was referred to by the late Emperor Napoleon in a letter to M. Rouher, then Minister of Public Works, after his visit to the inundated districts in the Valley of the Loire in 1856, wherein he cited it as an example; it having been the means of saving Roanne both in 1846 and 1856 from complete disaster, having kept back the flood water at the dam to a height of 70 feet above the ordinary river level. Many other examples could bo named, the most prominent of which is, perhaps, the dam of Fahrens, across the Valley d'Enfer, above the Town of St. Etienne, which was constructed of rubble masonry 170 feet in height, at a cost of nearly £200,000. As, however, this work serves the double purpose of protecting the town from inundation, as well as a water supply for manufacturing and domestic purposes, it is remunerative, paying, some years since, 2\ per centum on the outlay. It is not generally advisable to combine these two duties, their requirements being in direct opposition. For protective purposes the reservoir is only required to hold a quantity equivalent to a maximum flood, when in order to render it again serviceable for the same purpose the stored water must be discharged rapidly, while for useful purposes a certain quantity must be always maintained, thereby reducing its storage powers necessary to intercept a flood. By constructing a work very much larger than is required for protective purposes, it could be made to fulfil both duties, but at a heavy cost, only warranted under exceptional circumstances.