A flood-prone land
Settlers in New Zealand commonly chose to live next to rivers and lakes, as these were a source of fresh water, and the adjacent plains usually had fertile soil. Consequently, about two-thirds of New Zealanders now live in areas that are naturally prone to flooding. Nearly 70% of towns and cities with populations of over 20,000 have river flood problems.
Human activity has also increased the likelihood of flooding. Large areas of native forest were cleared by both Māori and European settlers, leading to a more rapid run-off of rain into stream networks, and to erosion that raised the levels of river beds. In urban areas, buildings, footpaths and roads have replaced ground that would normally soak up falling rain. This leads to surface flooding during heavy rain, and increases the run-off into storm drains, causing higher water levels in local streams.
At Avonhead lived one Mister Bray,
Who every morning used to say
‘I should not be much surprised today
If Christchurch city were swept away
By the rushing, crushing, flushing, gushing Waimakariri River.’
This 1860 poem by Canterbury politician and poet Crosbie Ward refers to the engineer William Bray, who predicted that the Waimakariri River would break its banks and run through Christchurch. His prediction came true in February 1868, when the river overflowed into the Avon River and other old stream channels, flooding parts of the city.
Legislating flood control
Early attempts at controlling rivers were piecemeal – to protect a homestead, individuals would build stopbanks that sometimes deflected river flow onto neighbouring properties. The River Boards Act was passed in 1884, setting up a national network of river management. But a single river was often controlled by several different boards.
In April 1938, a flood in Hawke’s Bay left the lower Esk valley buried under metres of silt. The disaster spurred an effort to control floods, resulting in the Soil Conservation and Rivers Control Act 1941. This pioneering legislation, as well as the later Water and Soil Conservation Act of 1967, authorised catchment boards to deal with complete river systems – both carrying out river control works and controlling erosion in their catchments.
In 1991, the Resource Management Act turned over flood control to regional and territorial authorities as part of their obligation to avoid or mitigate natural hazards.
Controlling the rivers
A number of techniques are used in New Zealand to control rivers. Thousands of kilometres of stopbanks have been built to keep high river levels within the channels; this has allowed dense settlement of areas such as Christchurch, the Hutt Valley and the Heretaunga Plains. Christchurch, for example, is situated mainly on a flood plain. It occupies part of the huge alluvial fan of the Waimakariri River, which has frequently changed course, sometimes shifting as far south as Lake Ellesmere. Stopbanks now keep the Waimakariri in one place along the northern edge of the fan.
Stopbanks, however, may also cause problems. Near Franz Josef, stopbanks built to protect the township narrowed the bed of the Waiho River. The river bed built up so much that in 2011 the SH6 bridge had to be raised to stay clear of the river. In 2016, the river breached the stopbank, flooding two hotels and a campground.
Solution – or problem?
In 1877, Hawke’s Bay County began to plant willows along river margins to help prevent bank erosion. Less than 20 years later, in 1894, the county set up a sub-committee to deal with ‘the willow nuisance’. The trees, which impeded the flow of rivers during floods, continued to be a worry for the next half-century.
One method of preventing rivers overflowing their stopbanks is to lower the river bed by removing gravel. Speeding the passage of flood waters through an area can involve straightening river channels and removing obstructions such as vegetation. Another method of flood control is damming or diverting flood water until river levels drop. To protect communities downstream, some river stopbanks have floodgates that can be opened to deliberately spill flood water onto less densely settled farmland. The Moutoa Sluice Gates, for example, divert excess water from the Manawatu River into a floodway that rejoins the river 10 kilometres downstream.
New Zealand’s large dams were built for power generation, irrigation and water supply, but they have at times buffered major floods.