River Control – A Major Problem
New Zealand is favoured with a generous and well distributed rainfall exceeding 45 in. over about 70 per cent of the country and with only a very small area with less than 25 in. In some districts, such as the west coast of the South Island and the western central plateau of the North Island, annual rainfall exceeds 100 in. with extremes over 200 in. on the West Coast and Fiordland.
The topography and soils over most of the country generally favour a high run-off both in total flow and in peak flood discharges. There are, however, in the North Island some areas where flood peaks are greatly reduced by porous pumice soils, while on some river systems the natural lake storage in headwaters helps to control flooding. The general condition of the country has had an appreciable effect on the magnitude and incidence of flooding, due in some areas to such factors as the clearing of steep hill country, excessive burning, overgrazing by domestic animals, and grazing and browsing by animal pests.
Severe storms are likely to occur at any season of the year, arid extreme falls of 12 in. to 20 in. in 24 hours and up to 40 in. over four to five days have been experienced in certain areas. It is therefore not surprising that river control is a major problem in New Zealand, with total run-off and peak flood discharges being among the highest in the world.