THE SMALL TOWNof Kaikoura, New Zealand, has weathered 16 floods since 1923. With that track record, government leaders knew the town needed protection. But they also knew plunking down a slab of concrete in the middle of an area known for ecotourism could stir up a wave of complaints.
Located midway between Christchurch and Picton— on the rugged east coast of New Zealand's southern island—Kaikoura is a beautiful spot. But the town's tourist district sits directly in front of the Lyell Creek, which often receives runoff from the streams of Mt. Fyffe to the northwest. Several times, those waters have exceeded the height of the SH1 bridge that runs along the north end of the downtown area, flooding Kaikoura. In addition, water from the South Pacific Sea occasionally backs up into the creek. And when heavy rainfalls hit, flooding can happen immediately.
In late 1993, a storm wrought $12 million in property damage in just 12 hours, says Geoff Scholes, senior engineering officer at Environment Canterbury, which provides river engineering management services to Kaikoura. At the time, the only protection in place on the creek was an earth floodbank, which failed.
THE FLOODWALL'S PRIMARY FUNCTION WAS TO PROVIDE PROTECTION, BUT GIVEN ITS PROMINENT LOCATION, “IT HAD TO BE NICE TO LOOK AT.”
—JONATHON DOGGETT, DOWNER EDI WORKS PTY LTD, MELBOURNE, AUSTRALIA
Amid growing concern from property owners and insurers, Environment Canterbury reviewed the existing flood-management strategies and decided to erect a concrete floodwall.
“The wall had to be solid in order to retain the water when a flood occurred,” says David Elliott, senior structural engineer at Christchurch, New Zealand-based Connell Wagner, the structural consultant on the project.
The goal was to protect against floodwaters of up to 160 cubic meters per second—the maximum volume of water that could flow under the SH1 bridge, according to calculations by the project engineers.
Make It Pretty
Before construction could start, Mr. Scholes and others involved in the project had to gain support from Kaikoura's residents and business owners. Any flood-control structure would end up consuming part of the town's commercial area—and local business owners were understandably concerned. Moreover, the flood-wall was to be funded with public money. And of course, if a flood occurred, residents would depend on the wall for protection.
As a result, almost everyone in Kaikoura had suggestions for the project. “It was a high-profile job,” says Jonathon Doggett, west coast division manager at Melbourne, Australia-based Downer EDI Works pty Ltd., the primary contractor on the floodwall. “Everyone had strong opinions.”
So the team sat down and listened. One major point they picked up on: The floodwall's primary function was to provide protection, but given its prominent location, “it had to be nice to look at,” Mr. Doggett says.
Mr. Scholes and the project team took this to heart, adding a wavy top designed to evoke the waterways surrounding Kaikoura. “They were mindful of the aesthetic impact and came up with a design that had an artistic flair,” says Lynette Buurman, one of the owners of Dolphin Encounter, an ecotourism company based in Kaikoura. “Their willingness to listen to ideas and to consider the impact that the floodwall would have on the look and feel of the business center was the whole key to its success.”
The team also made it a point to show stakeholders the designs early on, Mr. Scholes says. When most businesspeople realized the wall would actually enhance the look of the town, they were more likely to support it. And once some of them voiced their support of the project, others came onboard, he adds.
Indeed, the businesspeople involved in the project used the construction of the floodwall as a catalyst to launch other projects to spruce up the area. Working with Environment Canterbury, they suggested to the Kaikoura District Council that the project incorporate an amphitheater and a new footbridge to the beach.
The council agreed and teamed up with Environment Canterbury. Although these projects had been under discussion, their schedules were accelerated to correlate with the construction of the floodwall, Ms. Buurman says.
WITH A
BUDGET OF
$500,000,
IT CERTAINLY
DOESN’T RANK
AS ANY
SOPHISTICATED
MEGA-
PROJECT.
Get to Work
With stakeholder support secured, construction could start, but timing would prove tricky. The risk of flooding was low in the summer, but that also just happened to be when most tourists hit town. To minimize the impact, work started in the spring.
First the team stabilized the bank on which the floodwall would be erected and lined the creek's banks with plants and retaining rocks to stop erosion. They also rerouted power lines and water-supply pipes that were located within the creek and impeding water flow.
To minimize both the cost and disruption to the community, the wall's concrete panels were precast at the team's own local yard, rather than on site. And although initial plans called for panels to be 8 meters long, the size was cut in half. Otherwise, they would have been too heavy to lift using the crane that fit in the narrow space between the river bank and the downtown area, Mr. Doggett explains. If the team had to use a larger crane, it would have to be located on the road in front of the businesses, and the panels actually lifted over the shops.
Once in place, each panel needed to match up with the ones on either side of it. However, the height of the wall gradually dropped from about 2.5 meters above ground at the upstream end to about 1 meter farther downstream. To achieve the protection and look the town needed, the panels were created in about a half-dozen different heights, Mr. Doggett says. While the overall effect is that of a single, continuous wall, each panel increases in height going up the river.
WHEN MOST BUSINESS-PEOPLE REALIZED THE WALL WOULD ACTUALLY ENHANCE THE LOOK OF THE TOWN, THEY WERE MORE LIKELY TO SUPPORT IT.
—GEOFF SCHOLES, ENVIRONMENT CANTERBURY, KAIKOURA, NEW ZEALAND
Although the new floodwall looks like a fairly simple design, it actually has to protect against several different flooding risks. If waters exceeded the height of the SH1 bridge, for example, the wall could end up keeping the water within the town. To avoid this, the design incorporates a “stop-log” section. Once water starts coming in over the bridge, the stop-logs can be removed, allowing the excess to flow out of the town and into the creek.
The floodwall also incorporates an adjustable weir located at a strengthened overflow point. In an overflow situation, floodwaters are moving so quickly that they could erode the wall foundations and cause the wall to fail. “It's like a waterfall cascading, and there's a lot of energy,” Mr. Scholes says. “The wall foundations are designed to withstand those forces in that strengthened section.” Because it would mean letting some water into the town, that step only would be taken if the integrity of the entire floodwall was undermined, he says.
During the process of erecting the wall, employees from Environment Canterbury were on call in case a flood was imminent and they would need to pile sandbags to protect the town.
Fortunately, that measure wasn't necessary.
The new wall, which runs for about 400 meters, was put in place in about six weeks. “The main challenge for us as the contractor was the fairly tight timeframe,” Mr. Doggett says.
As of September 2007, the new Lyell Creek floodwall hadn't yet been tested by rising waters, but it's already attracting attention. Earlier this year, the team earned an award for engineering excellence in projects that cost less than $1 million from Ingenium, a professional society for New Zealanders who work in support of the country's public assets.
The award credits the floodwall and its sub-projects as functional, cost-effective and enhancing the natural beauty of the town. “It was a very practical solution,” Mr. Doggett says. “It didn't win as a result of impressive design or technology.”
Indeed, with a budget of $500,000, it certainly doesn't rank as any sophisticated mega-project. But the project does show the power of the partnership forged among the people of Kaikoura, the public officials and the team charged with building the floodwall.
To highlight the community's ownership of the project, Kaikoura residents were asked to choose a phrase to be carved into the steps of the amphitheater. They agreed on a saying from the Runanga, part of the Maori or native New Zealand people: Ki Uta Ki Tai, which roughly translates to “the conservation and protection of the mountains to the sea.”
There is, after all, something to be said for letting nature take its course—once you've created a contingency plan. PM