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Were the recent North Island floods naturally occurring events?

From New Zealand Tree grower November 2015

The recent North Island floods seem to be treated as if they were naturally occurring events. Concentrated heavy rain resulted in farm damage and flooding of low lying areas, such as Whanganui city, with silt laden water. What is of concern to conservation minded foresters is the absence of any media discussion that human activity may have been a significant contributing factor.

In earlier times the clearance of indigenous forest in the upper catchment regions was recognised as a major contributory cause of lowland flooding. In 1938 there was a series of major North Island floods.Two floods appear to have been particularly important.There was the flash flood on the 18 February at Kopuawhara, north of Wairoa, in which 21 Ministry of Works employees were drowned. Only two months later, there was a major flood in the plains of the Esk River, north of Napier. These and other floods resulted in the 1941 Soil Conservation and Rivers Control Act. As well as building stop banks on the flood plains there were tree plantings, mostly of poplar and willows, in the erosion prone catchment regions. Esk and later Mangatu Forests were established to reduce flooding and erosion in the Esk andWaipaoa rivers.

The government cannot claim it has not been warned of flooding and erosion problems that will result from indigenous forest clearance of steep high country. Over the last 150 years there have been numerous warnings from concerned forester environmentalists. The first serious warning was published in the 1877 report of the New Zealand visit from1875 to 1877 of the professional forester, Captain Inches Campbell Walker. On page 91 of Michael Roche’s History of New Zealand Forestry there is the following quote from Campbell Walker’s report −

I should view with very greatest anxiety any clearing of the hills which form the dividing range of the Island and am convinced that it would be followed sooner or later, by the most disastrous results, both in the shape of deterioration of the climate, dangerous floods and drying up of springs and sources of rivers.

Forest clearance of steeper backcountry and subsequent conversion to farmland is a major contributor to lowland flooding with silt laden water. There should be greater awareness that and lowland flooding has been made worse by human actions.

In large part because of the political power of landowners, solutions may be difficult to implement on a significant scale.The erosion prone land could be retired, in other words, such land to be abandoned and left to revert to an indigenous vegetative cover. Farm profitability may be actually increased because erosion land may not be very productive. The erosion prone land could be planted with a production tree species. However, although there may be a short term gain, older stands could be unstable because of the underlying soil instability.Tree harvesting may present erosion problems and may be costly. Another option is for erosion land to be planted with manuka, especially bred to maximise the production of nectar for manuka honey.

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Disclaimer: Personal views expressed in this blog are those of the writers and do not necessarily represent those of the NZ Farm Forestry Association.

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