GAUGING FLOOD LEVELS

Press, Volume XCI, Issue 27632, 13 April 1955, Page 12

GAUGING FLOOD LEVELS

Canterbury Research On Rivers USE OF AUTOMATIC EQUIPMENT So that, over a long period, more information can be accumulated about the hydraulics of Canterbury rivers, existing automatic gauges for recording the height of the water which have fallen into disrepair are being reconditioned, and the installation of a further series of gauges—about 10 — is planned. The automatic gauges cost about £2OOO each. The circular concrete towers are 30 to 50 feet high and more, according to the flood flow of a particular river. Inside each tower is a float which indicates the height of the river on an automatic clockwork recorder. This gives a continuous record of the level of the water. The installation of an automatic gauge is not, however, the final answer to hydraulic questions. Before the gauge can be of real value, physical gaugings of a river have to be taken with current meters at various levels so that the ‘'stage discharge curve” can be plotted. When this has been done, it is possible to calculate how much water a river is discharging at a particular level, but the determination of the discharge curve may be a long-term project. Long-term Observations Mr R. R. Dawber, Christchurch soil conservation and rivers control engineer to the Ministry of Works, yesterday said that, though observations had been started in the Karamea river in 1935, no stage discharge curve had yet been worked out. “These instruments become really useful after 50 years, and much more useful after 100 years,” he added. In the Nile river, a continuous record of flood levels had been kept for 2000 years, apart from a period of 400 years in the Middle Ages, said Mr Dawber. These observations had revealed a regular flood cycle of about 77 years. The unsatisfactory operation of gauges in Canterbury in the past is believed to have resulted largely from Back of experience among those who erected them. To overcome this problem, the Ministry of Works in Canterbury now has its own hydraulic construction party—the first in New Zealand—to repair and build gauges. A further problem has been to obtain the necessary specialists to man the unit, which has to work in isolated localities; and its strength has. varied between one and four. Much of the work now being undertaken was authorised as long ago as 1947, but has been delayed by the labour shortage. So far, the work has been confined mainly to reconditioning existing gauges. An automatic gauge in the Ashley river, which had become silted up at the bottom, has lately been restored to working order. A gauge in the Hurunui, which was damaged by vandals a few years ago, has also been reinstated. New Installations A new gauge is being constructed on the Conway. A gauge has also recently been built on Waimate creek in South Canterbury, to facilitate the study of the effects of soil conservation measures in the Waimate catchment. Further gauges are planned for the Hurunui, Hope, Waiau, Buller rivers, possibly the Clarence river, and on the Rakaia, where an existing instrument has proved unsatisfactory. The new gauges take from four to six months to build. There are gauges at many other points. The North Canterbury Catchment Board has an automatic gauge on the Waimakariri at Sheffield; there are three on Lake Ellesmere; one on Lake Heron; one on the Rangitata; one on the upper Opihi; and one on/the Opuha, a tributary of the Opihi. At all the hydro-electric stations there are gauges. In addition, there are a number of staff gauges (sticks) in use. The long-term results of these hydraulic observations, in relation to rainfall and other factors like the nature of the country, will be invaluable in assessing the storage possibilities of rivers for hydro-electric schemas; the type of flood protection needed in the light of the size and frequency of floods; and the suitability of rivers for irrigation purposes, on which low flow has an important bearing. Some of the first gauges installed in Canterbury were for observations—mainly of low flow—for irrigation.