Compromise vital for river control

MOVEMENT: This picture shows indentations left by a mechanical ripping tool during river work downstream of Gladstone Bridge six months earlier. Vegetation in the channel is causing gravel to catch and build up. A shows ripping when the beach is exposed loosens gravel allowing it to gradually move downstream preventing erosion of the right-hand bank. B shows backwater habitat for wading birds and fish, undisturbed by GWRC river work. C shows pool areas, undisturbed by GWRC river work.

WAIRARAPA, Land of Glistening Waters, just wouldn't be Wairarapa without our many rivers, streams and lakes. Our abundant freshwater brings benefits and risks to the individual landowners and communities that live next to our waterways. Managing Wairarapa's rivers often means finding a balance between cultural, recreational, ecological and economic concerns, with a high standard of community flood protection.

Settlers to the region established early townships on the valley floor beside the Ruamahanga River and its tributaries. Town forefathers made flood protection decisions in a very local way, reacting to individual floods, not able to look too far into the past or future. They found solutions for homes, businesses, infrastructure and farmland with the knowledge, skills and resources of their era.

Today, river management in New Zealand takes into account a wider range of information: the natural process and character of rivers and land, historic human modifications and region-wide goals for flood protection, ecological restoration, land and river use. River engineers make decisions with a very different set of tools.

Rainfall and river flow data, more knowledge about the geology of our region, a better understanding of how ground and river water behaves, as well as research into climate change, help river managers keep water where people want it: in a "design corridor" - an agreed pathway through town and country.

Strengthening all the river edges and constantly constraining the water into a largely man-made channel would be an expensive approach. Leaving the river to return to its natural course is not acceptable to our communities.

Using the design corridor, an area we are prepared to let our rivers move within, is an effective compromise.

Wairarapa has a fine example of a New Zealand braided river system. We think it's beautiful and maintaining its natural character, with riffles and backwaters for fish to breed, is important to us.

Work to keep water away from homes and property, as well as protect vital infrastructure such as bridges and urban water supply pipelines, goes on year round. GWRC Flood Protection Department solutions consider a range of community concerns, from ecological to economic.

River depth and flow, personnel safety and cost efficiency are always factors in the timing of our work programmes. But we do consider many other aspects, such as fish breeding cycles or bird nesting seasons.

In Wairarapa, a team of four engineers and seven river works staff manage about 250km of waterways.

Right now, you might see our staff clearing crack willow debris blockages in the eastern tributaries of the Ruamahanga or undertaking routine vegetation control work.

During the winter, our focus is on assessing the condition of the Flood Protection assets in the Wairarapa, which are valued at more than $150 million.

Winter often sees us undertaking repair work. In spring 2015, we will be planting thousands of trees in the river corridors across Wairarapa to support river edge erosion control.

Our major maintenance time is in the summer months when river levels are low. We can be seen aligning the river channels, removing problem vegetation, or managing various gravel extraction operations.

The Flood Protection Department uses a range of methods to keep water within the river design channels.

The choice of river management technique needs careful consideration.

Clearing debris from the more narrow, upper parts of rivers, or weed and silt from the man-made drainage systems of the lower valley, takes place in the drier months of the year. Clear channels help prevent water washing away productive farm land and moving this material downstream, where it might not be wanted.

The river channel can change as part of a natural process of course. There are kilometres of riverbank, naturally, yet slowly, eroding. This material will inevitably be deposited lower down in the valley as the river meanders and loses energy.

High river flows will move gravel from north to south along the valley. Wairarapa hasn't had a flood large enough to bring much gravel into the river system from the headwaters in the past 15 years. Gravel movement is a long-term process.

-Part Two of this feature will be published in tomorrow"s edition