Control schemes re-emerge after more flooding

Press, 5 August 1983, Page 18

Control schemes re-emerge after more flooding

By

OLIVER RIDDELL

Recent devastating floods in Nelson and Marlborough have given a further impetus to the long-stand-ing campaign to reduce flooding in New Zealand by systematic flood control schemes.

A publication by the National Water and Soil Conservation Organisation sets out some of the work that has been done. It also provides graphic examples of what happens when flood control schemes are inadequate.

For example, flood control works on the Waimakariri were begun as early as 1859, but this did not prevent the flooded river rampaging through what are now Christchurch’s north-west suburbs in 1868.

Works continued with greater or lesser urgency for the best part of 100 years, with regular complaints that'not enough was being done, and major floods in 1926, 1950, and 1957 to reinforce the point. A new flood control programme for the Waimakariri was proposed in 1960 with three main aims:

• To cope with a 100-year flood discharge, estimated to be 4730 cubic metres per second.

• In the longer term, to deal with the rising shingle deposits on the river’s lower reaches. • To combat the erosion of flood control works.

Work began in 1963 on stopbank strengthening and new construction, new groynes, planting willows, bank protection, and building and replacing floodgates. The scheme was reviewed in 1976 and again in 1982. Work is expected to continue until 1988, but the major construction works are now essentially finished. Ironically, it now seems that the Waimakariri design flow of 4730 cumecs may represent a level of protection perhaps as high as a 200-year flood. This illustrates the difficulty, common throughout the country, of designing flood protection for an area in which flood records in most cases go back less than 50 years. The Waimakariri is fairly typical of what has happened everywhere. Not only has the work done lessened flooding risks in areas adjacent to the river, it has enabled large areas of former riverbed to be brought into farming production. . If North Canterbury has not experienced major river flooding for many years, other parts of the country have. In 1976. Wellington and the Hutt

Valley had what was described as a 1000-year flood. Miraculously, only one life was lost, but property damage came to ?34 million. The flatter Hutt Valley was found to be woefully ill-prepared to cope with such a flood. The suburbs of Pinehaven and Stokes Valley were declared disaster areas and more than a metre of water flowed through the Petone industrial area. Trees were uprooted, cars blocked drains, and culverts were quickly blocked with debris.

Two years later the Clutha River had what was called a 100-year flood. The river’s normal flow is 500 cumecs, but it reached 4500 cumecs — nine times its normal flow. There was severe damage to the towns of Kaitangata and Paretai, and to the river estuary region at Inchclutha.

The low-lying Hauraki Plains in the North Island are flood prone. There have been major catchment works installed since the catastrophic floods of 1910, but severe floods still occurred in 1936, 1954, and 1960.

There are now works to handle a 100-year flood. Even so, in 1981 these works were partly swept away, taking part of the village of Waikino with them.

In the deep south, the Mataura, Oreti, and Pomahaka Rivers have all been flood prone in recent years. The Pomahaka became so uncontrollable that the flourishing rural community of Kelso had to be abandoned to the river.

The main problem with the Southland rivers was found to be their restricted flood plain. Work to clear willows, gorse, and broom from the river bank has allowed the water to pass downstream without interruption and flooding. New Zealand is covered by a network of catchment authorities who have the front-line responsibility for flood control. Their decisions, however, have to be tempered by the availability of money from the Government and professional advice from the Ministry of Works. It is not easy for catchment boards to get the technical and financial backing they think they need. This is particularly the case on normally dry areas, such as Marlborough and Nelson.

When the countryside lies parched and brown for most of the time, it is hard to convince others that this tranquil landscape needs protection against raging floodwaters. People living in Marlborough and Nelson are the latest to be reminded that it does.