Conservation Comment: Ecological principles of river behaviour must be observed

Gordon Park, pictured from No 3 Line on July 7, 2006 - flood running through the Whanganui floodplain forest. PHOTO/JIM CAMPBELL

Before people arrived in what is now Whanganui, there was a shifting mosaic of land and water. The Whanganui River and its tributaries flooded periodically, changing their courses, sometimes flowing against the base of St John's Hill and Aramoho, other times towards "Wanganui East". Silt was deposited, either to become vegetated or remobilised in later floods. There is an obvious past course of the Whanganui, roughly from where SH4 meets Kaimatira Rd and then to No 3 Line and down what is now Matarawa Stream to Kowhai Park.

Seemingly flat, the floodplain would have had many minor ridges and terraces that the slowly rising land lifted above all but the largest floods. The terraces offered safe building sites in the 19th and early 20th centuries, ie above Anzac Parade.

Occasionally flooded land would have supported swamps of harakeke (flax), toetoe, sedges and shrubs. Swamp forest established eventually where destructive floods were rare. Almost the only visible record of the vegetation in those pre-human times is the 8ha of swamp forest that is now Gordon Park Scenic Reserve.

Floods on a wide, vegetated floodplain have lower water levels and slower flows than would otherwise occur.

Building stopbanks narrows the floodplain, resulting in higher water levels and faster flows. Moreover, the natural vegetation on the floodplain was adapted to survive floods, much as bamboo groves in Kowhai Park do today. It trapped silt and absorbed some of the flood's kinetic energy. The impacts of the Whanganui's floods since European settlement have been because the ecological principles of river behaviour are not understood - or ignored.

The lower Whanganui's course has been confined by roads, walls, wharves - and stopbanks. Land clearance in its catchment delivers water faster than in pre-human times, along with higher silt loads. Roads and slips add more.

The floodplain, mostly lacking wetland vegetation, cannot absorb either the silt or the water's energy.

So what should be done to mitigate flood impacts in the Whanganui? Some far-sighted planning is presented by Environment Canterbury (ECan). It says: "Historically Canterbury, New Zealand and many other places around the world have relied on structural protection works, such as stopbanks, to 'protect' floodplain communities ... This historic reliance on structural protection works has created a false sense of security and encouraged a much higher level of development on floodplains than otherwise would have occurred had the structural protection not been put in place ... Inevitably, structural protection measures (like stopbanks) are overwhelmed ... Over the last 20 years there has been an increased realisation worldwide that full reliance on structural measures, such as stopbanks, to prevent flood damages is unrealistic and not cost effective. The emphasis now is to find a balance between measures that keep floodwaters away from people and those that keep people away from floodwaters." (http://ecan.govt.nz/advice/emergencies-and-hazard/flooding/). ECan's strategy goes on to list measures that modify flooding, including vegetating the river berms and stopbanks to reduce erosion, and advocating for controls on new developments in areas with a high risk of flooding.

ECan's strategy doesn't cover existing buildings on floodplains, but for the Waikanae River, the Greater Wellington Regional Council offers: "House raising: this applies to existing properties on flood-prone land. It will be funded by the WRC ... [and] Building Relocation". How does Horizons' strategy stack up?

Before committing ourselves, we need to compare the costs of raising stopbanks, which will inevitably fail, with costs of raising and moving flood-prone houses.

-Colin Ogle is a retired ecologist