FLOODS IN NORTH

New Zealand Herald, Volume LXXII, Issue 22086, 16 April 1935, Page 12

FLOODS IN NORTH

ELIMINATING CAUSES VALUE OF AFFORESTATION COVERING BARREN SLOPES A policy of preserving existing hush and of re-afforesting the hills in parts of North Auckland as a means of preventing flooding has been brought into prominence by the frequent washouts during the past year. Actually, during the past nine months, six serious floods have occurred in that area alone, resulting in enormous loss to farmers, and in ■widespread damage to roads, railways and bridges. Four of these occurred in the winter, when heavy rainfall was more or less expected, but those in December and March came at a time when their damage was more heavily felt. At least six bridges have been carried away during the floods and many others have been damaged. The repair work and the cost of replacements is a constantly recurring expense and one that with tho further removal of hush is likely to increase. A scheme of tree planting in selected areas would probably pay for itself in the long run, while it- would provide considerable employment for a number of years. Similar work has been done in Europe for over a century and barren hillsides have in many places gvien rise to valuable forests that have repaid the cost of planting many times over. Great Run OH From Hills A number of reasons can be given for the prevalence of floods in North Auckland. In the first instance the largo block of high country stretching from Maungaturoto almost to Kaitaia acts as a great rain producer, condensing the warm westerlies, with consequent heavy precipitation. Much of the country is extremely steep, with quickly sloping valleys, composed for the most part of impervious clays with up to 100 per cent "run-off." Because of this almost the whole of any sudden rainfall, however heavy, is immediately discharged into the rivers. This is in marked contrast to the country in many other parts of the Dominion witli an equally heavy rainfall, but with soil that absorbs up to 75 per cent of the rainfall. The sudden change in» slope from the hills to the plains is also a factor in flooding, especially as many of the streams are abnormally sluggish through the presence of willows. In the lower country there is also the action of the high tides with strong winds, but actually most of the worst damage lias oecural in valleys beyond the reach of tidal waters. Visiting Experts' Advice The Government and the various drainage boards have already tackled the problems of opening up the streams in several areas, this undertaking giving work to a large number of men who would be otherwise unemployed. Care has to be taken not to remove the evil from one area to another by providing too rapid u flow to cause damage lower down in the drainage system. The slope of the hills is beyond tho control of man, but it is suggested that much can be done to correct the rapid run off from the clay slopes. The effect of forests on steep slopes is recognised in all the older established countries, and visiting experts have commented on the denuded hills in various parts of the Dominion. As recently as a few weeks ago Viscount Chaplin, who is agent to the Sutherland estates, expressed the opinion that the trees on the crests of the ridges in North Auckland should never have been removed and should certainly he replaced as soon as possible. Similar advice was given by members of the Empire forestry conference which visited New Zealand a few years ago. Arresting Soil Erosion "Twenty-five per cent of North Auckland should be under forest to conform with the established principles in similar countries," stated an expert yesterday. "Much of the North was certainly under kauri forest in the past, but this was cut out for timber and the remainder was burned, leaving a particularly impervious clay. Much of this is admirably suited to the growing of eucalypts, which have proved that they thrive where other trees would be too slow-growing. New Zealand is rapidly losing her hardwoods, the totara being the only one left in any quantity, and its replacement is an urgent matter." Tho effect of forest as a factor in arresting soil erosion and flooding is well known. The rain as it falls has to filter through the foliage of the trees and run down the trunks. When it reaches the soil it still has to soak down through the deep layer of humus, making its way gradually into the streams rising in the forest, with branches and rocks constantly impeding its progress. Thus heavy'rain, instead of pouring down hillsides'unchecked, carrying away valuable surface soil and swelling rivers, seeps gradually, to keep more or less constant level in the rivers.