Floods rising threat in Bay

Flood waters have surged through the Western Bay three times in the past five years. In the wake of the latest deluge, Julia Holmes takes a look at what is being done to keep the rising waters at bay and what we can do to protect our homes when the rains come.Floods rising threat in Bay%Weekend insight
In the event of heavy rainfall, Tauranga City Council wants our region to get a good soaking.
That might sound insensitive in light of last week's downpour that saw some homeowners frantically trying to rescue their valuables from rising floodwaters, but stormwater engineer John Palmer says it is the answer to Tauranga's wet weather woes.
To put it simply, we just don't have enough grassed areas to soak up all the rain.
The city's rapid development and infill housing means that there are fewer surfaces capable of absorbing water, like lawns, and more surfaces that the water runs straight off, such as large apartment buildings, driveways and paved yards.
"Historically, Tauranga relied on ground soakage, rather than being piped into the reticulated system. The stormwater system was only developed for streets," Mr Palmer says.
The council has been criticised about its drainage systems, with some residents claiming little has been done to alleviate the problem of flooding in the city. But Mr Palmer says the council is doing all that it can with the resources available. Work is prioritised and carried out as funding allows.
Council will next week consider stormwater works proposed for next year's annual plan, including Claremont Terrace, Balmoral Rd, Landscape Rd, The Mall, Karewa Parade, Pyes Pa Rd (near Aquinas College), Royal Palm Beach Estate and Courtney Rd.
For 15 to 20 years engineers have been collecting and mapping information about areas that have potential to flood. This information can be accessed at the council's customer services centre. This computer modelling tool is also used when considering building consents.
Flood-prone areas are identified by mapping both the contours of the land and information provided by people whose properties have flooded.
The problem is, says Paul Baunton, who manages the environmental engineering unit, not everyone reports flooding to the council because they fear it will affect the value of their property. "That makes it difficult to know accurately all the flood-prone land," he says.
"We provide people with advice on what they can and can't do. We manage that information and the risks it may raise, so the risk is not perpetuated," he adds.
Mr Palmer is keen to emphasise that the system shows "theoretical" flooding, should the stormwater system block or become overloaded. "It is a conservative, broad brush approach. The property's own documents will show if an event has been reported. We have to be quite careful. We don't want to cause unnecessary panic."
It is the older, established areas of Tauranga and Mount Maunganui that are most likely to flood, as stormwater systems in the newer areas, such as Bethlehem, Pyes Pa, Welcome Bay and Papamoa, have been designed from scratch to cope with water flows should the areas be built to capacity.
At Papamoa, because it is so flat, new houses must be built at least 500mm off the ground to avoid them getting swamped in a 50-year flood event. The sandy ground also helps as it soaks up the water better than in parts of Tauranga that are silt and clay-based.
Over the past 12 years, 40 retention ponds have been built at Welcome Bay, Bethlehem and Pyes Pa to hold excess stormwater. "When I started it was about getting in pipes as big as you could to get rid of the water as fast as you could. Now we try to contain it on site, not make it someone else's problem ... to hold it at a controlled rate and release it at a controlled rate," Mr Palmer says.
Sometimes people panic when they see them filling up, he adds, but that is what they are designed to do. And in many cases these waterways serve a dual purpose, with reserves and walkways built around them.
When new supermarkets and other large stores and carparks are built they are sometimes required to have underground soakage systems to stop rainwater running off and overloading drains.
Other initiatives include permeable paving, rain gardens - such as the one at Carlton Street reserve - and water tanks. Some properties in other parts of New Zealand are required to have water tanks that collect rainwater run-off from the roof, which is then recycled for toilets and laundry.
This is being considered for Papamoa East, says Mr Palmer, although he dreads to think what the public response would be should water tanks - and the accompanying cost - be made compulsory.
Personal responsibility is a hot topic for the stormwater team.
Most houses flooded last week when 142mm of rain fell in 28 hours, were hit because of "a lack of thought to on-site design", he says.
"You don't need a consent to build a driveway but you need to think of the consequences and make sure you're not providing a flow path into your building. It's about being aware.
"People say - why isn't the council doing something about the flooding? But their own actions, such as subdividing and creating extra storage space, can contribute to the problem."
However, he also recognises that current homeowners are not always at fault as it can be previous owners who designed these structures.
Omanu, Arataki, some parts of downtown Tauranga, Otumoetai, Pillans Point, Matua, Merivale and Gate Pa, particularly Courtney Rd, appear to be the areas worst hit last week.
Work is already underway at Courtney Rd but the new system, which will see a large concrete channel divert water to Waimapu estuary, will take four years to complete. Phone calls and letters to the council suggest about 20 houses in the Tauranga area were affected. This compares with eight homes in last year's April flood. "At Mount Maunganui a lot of properties' basements are below road level and the water gravitates towards them. If you are opposite a low point in the road, you are potentially at risk of inundation," says Mr Palmer.
The most flood-prone parts of the city are industrial sites, including Maleme St, Birch Ave, the Mount Maunganui industrial area and Merivale. "The land is not suitable for houses because it is below the minimum level required, so it has been taken over for industrial areas."
It is the third time in five years that the Western Bay has been hit by an unexpected deluge at this time of year, with floods also occurring in April 2000 and 2004.
"There seems to be a change in weather patterns. Very intense, localised events seem to happen at this time of year," Mr Palmer says.
And while we are fortunate that Tauranga does not have any main rivers running through the city and is therefore unlikely to ever experience flooding on a major scale, what we do experience may become more frequent.
"The question has to be asked whether changing weather patterns should be prepared for on a more regular basis. Should we reclassify a one-in-50-year to a one-in-20-year?" Mr Palmer says. According to NIWA scientist Charles Pearson, Pacific Oscillation, a weather pattern that brings dry weather for 30 years and then wet weather for the next 30, brought its rain cycle to the Bay in 2003.
"There will be larger floods each year in the Bay of Plenty ... and it is likely the floods will be higher and more frequent," he says.
Barry Low, manager for emergency management in Tauranga and the Western of Bay, says while we don't have any control over the elements, there are some measures that people can take to protect their homes and valuables - from keeping stormwater drains clear to storing valuable items at height.
"Check stormwater drains outside your house and keep them clean. It is seen as a council asset but it's your property that's going to be flooded. Take a bit of responsibility. You can't expect council contractors to keep them clean by the day," he says.
Those who know they are in flood-prone areas should keep about a dozen sandbags on hand to dam off basements and garages.
If your house does flood, turn off the electricity mains, he warns.
"If I'm a home owner it's my responsibility initially to do what I can. You've got to think about that. If this happens - what will I do?"