DEVELOPING “CUTS”

Star (Christchurch), Volume XLIV, Issue 683, 9 February 1933, Page 13

DEVELOPING “CUTS”

Work in Waimakariri Outlined. ENGINEER’S STATEMENT. Steady and balanced development of the system of “cuts” in the Waimakariri River up till the time when the water will be running entirely in the new channels is the aim of the Waimakarirr River Trust’s engineer (Mr H. W. Harris). Yesterday Mr Harris answered questions concerning recent criticism of the efficiency of the system of “cuts,” following a “ Star ” reporter’s inspection of the channel through which it is proposed that the water shall finally travel direct to the sea, through the sandspit at the mouth. Very little of the flood waters of February 1 found their way through the sandspit to the north of the deep line of 720 concrete blocks. Each of the blocks is ten and three-quarter tons in weight. -They have been sunk in the sand to form the south bank* of the river when the time comes when water will run entirely through the spit, in a direct line with the “cut” running from Stewart’s Gully. Channel at the Mouth. During the flood a channel ranging from four feet to twelve feet in width, and about two feet deep, was cut through the spit alongside the line of the blocks and about fifty yards south of the line through which it is anticipated the river will break when in high flood. In one or two high floods of the past, before the “cuts” were made, the river broke through at a point just north of the blocks; and a flood of about 100,000 cusecs, according to Mr Harris, is required to force the river through the lowest “cut.” Mr Harris agrees that very little water from the last flood went through the channel in the sandspit, and he states that any report that about twothirds of the volume of water went through there has been founded on a misapprehension. The recent flood was of about 40,000 cusecs, whereas it would require a flood of about 100,000 cusecs to gouge out a channel deep and wide enough not to to be covered in again by drifting sand. Any artificial opening up of the new channel would involve a tremendous amount of work, which the river could do itself when aided by a big flood. No Risk of Flooding. A flood of 100,000 cusecs, said Mr Harris, would not involve any serious risk of the flooding of neighbouring land. When the river was normal the

spillway of 1300 feet wide at Wright’s bank was above the level of the river, but, once the surface rose several feet, the upper “cut,” together with the height of the protective banks, could take 150,000 cusecs, whereas White’s bridge could not take 100,000 cusecs. The effect of such a volume of water going through would be to scour the “cut” deeper and make it available for the carrying of an evej larger volume. When there had been sufficient flooding to scour out the “cut” above the railway bridge to the same depth as the river bottom, the whole of the water, in both normal and flood times, would be taken hy the straightened stretch.

This result .-\yas not wanted, however, until the lower “cuts” had been developed sufficiently to take practically the whole of the flood water. If only the “cut” at Wright’s bank were fully developed, the shingle banked up above that would be taken through, but would come to rest again above Steward’s Gully, and the last condition would be worse than the first. Before the shingle began to move it was necessary to have a straight course so that the stones could be taken right out to the sea in one operation. There fore the time of the coming into operation of the whole system depended on the natural progress made with the formation of the channel through the sandspit.