Working Together to Reduce Flooding
Winter 2006
Hutt City is a great place to live. But history shows us that flooding is a constant danger to our city and our people we live on the most densely populated floodplain in the country. Hutt City Council, in partnership with Greater Wellington Regional Council, has the responsibility to reduce the risk of flooding.
The flash floods that caused widespread damage in our city in February and August 2004, and earlier last year, were alarming, damaging and dramatic evidence of the forces of nature. It reminds us all that we need to be constantly aware of the high risk of flooding in our city.
I take the threat seriously and am committed to doing what we can for the safety of our residents. The Council has a number of projects to protect our city from flooding, and has significantly increased its capital spending on stormwater defences. While we have made great progress, there is still much to do.
I want to take this opportunity to share with you a picture of our work, completed or in progress. I trust you will find it interesting and informative, and gain a greater understanding of the issues of flood risk management. With your awareness of the issues, we can do so much more to make our city an even better place to work, live and play.
David Ogden
Mayor - On Behalf of Hutt City Council
Hutt River Overflows
The Hutt River is the soul of our city. But even before the first European settlement in 1839, people have known about the rivers propensity to overflow. In just 50 years from 1840, 20 major floods were recorded in our city. Even now, a heavy downpour of rain can cause our river to overflow in minutes.
Eastbourne Upgraded
Thanks to improvements to drainage and a new pumping station, the flood risk to Eastbourne is now once in 50 years.
Rainwater doesn't have to go far from Eastbourne's bushclad hills to the sea, but until recently, the suburb often flooded due to massive amounts of rainwater overcoming the drainage system.
To reduce the flood risk more drains have been installed, or made bigger and better. Other flow paths now have to be available in case drains become overloaded. A pumping station was also installed at Pukatea Street to help the stormwater on its way.
Did you know?
Global warming might double the risk of heavy rainfalls within 25 years.
Stopbanks & Floodwalls in Lower Hutt
The lowest reaches of the Hutt River have historically been vulnerable to floods. The first stages of a major project to protect this area are now reaching completion.
The old stopbank beside Alicetown was removed and replaced with a more watertight stopbank which is also higher and wider. Larger stormwater pipes run underneath. Reinstatement of footpaths, fences and native planting will be completed by September 2006.
Concrete floodwalls have been installed on the stopbanks by the Ewen Bridge and at Ava Park. Construction of the Opahu Stream Pump Station and floodgate is now well advanced. Over the next few months, rock lining of the eastern stopbank next to Ava Rail Bridge will begin.
When the final work is completed, in 2009, flood protection in this area will increase from once in 100 years to once in 440 years.
Battling Pondweed at Waiwhetu
Cape Pondweed is choking Waiwhetu Stream. And not only is it unsightly, it slows the flow of water in a flood.
Spraying or excavating Cape Pondweed (Aponogenton distachys) manually or mechanically, causes some damage to the stream environment, and the weed grows back. Growing trees to shadow the stream would reduce weed growth longer term. But then the trees themselves become a potential debris and so make the risk of flood greater.
Complex Issues for Waiwhetu
After the devastating flooding of the Waiwhetu and Awamutu Streams in 2004, public consultation will decided on the most effective and least disruptive way to lessen the flood risk.
Housing and business developments over the past 80 years has made flood prevention difficult. There is limited scope to expand the Waiwhetu Stream channel, and decades of heavy metals and pesticides being discharged into the stream has created 29,000 cubic metres of highly contaminated riverbed. Also the many low bridges over Waiwhetu Stream could be submerged and turn into dams in a major flood.
High tides and southerly winds at the wrong time back up floodwaters, so flood gates and a pumping station may be needed to prevent backflow into Awamutu Stream. In extreme floods Hutt Park could be used as a ponding area, and houses in some streets could get individual protection rather than the high cost measures of safeguarding a whole district.
The Waiwhetu Stream Working Group has already organised thousands of native trees to be planted on the stream banks by community and schools to help absorb extra water. The Waiwhetu options are for different extents of stream-bed excavation. The deeper and longer the excavation, then the lower the containing stopbanks and walls can afford to be. The options for the Awamutu centre on the tolerable frequency and depth of flood ponding in Hutt Park, against the capacity to pump that water over into the Waiwhetu.
Excavating Gravel from Hutt River
Stopbanks are one way to help limit flooding. Another way is to extract gravel deposited by the river. In the higher reaches of the Hutt River, the speed of water collects gravel from the riverbed. Further down, by about Belmont, the river slows and drops the gravel it is carrying onto the riverbed.
Since the January 2005 flood, about 90,000 cubic metres of gravel has been extracted from dry beaches along the Hutt River. Greater Wellington Regional Council has now applied for a resource consent to excavate gravel from below the water level, thus lowering the actual riverbed and further reducing flood risk.
Stopbank at Hathaway Avenue
Hathaway Ave, about one kilometre north of Hutt Hospital, is vulnerable to Hutt River flooding.
Three options of where to place stopbanks are now being assessed for feasibility. A public meeting will be held in October to present the findings. By the end of 2006 a protection option will be chosen.
Work at Black Creek, Wainuiomata
Black Creek has overflowed three times in recent years. Flood protection measures began in 2005. The water channel has been widened by up to four metres and work is soon to begin on creating a wetland site beside the creek.
Work will begin in the Spring on the next stage. This is to lay larger drains and widen channels. At the moment, in heavy rain, the water runs off buildings and roads faster than the drainage system can cope with. When the work is complete, it's estimated that Black Creek will overflow only once in a hundred years.
Largest Floods in Valley history
In the past 150 years many large floods have swept through the Hutt Valley:
1855 - River rose 600mm, higher than had ever been recorded before. Destroyed third Hutt Bridge. Killed 300 sheep and destroyed large areas of crop.
1858 - Most disastrous flood experienced by European settlers. Nine lives lost, new Hutt Bridge damaged. Heavy livestock and crop losses.
1878 - Two large floods. Few details recorded but flood led to first meeting of Hutt River Board in March 1879.
1893 - Two large floods. One inundated Petone 1.2m deep. Floods led to Petone stopbank being built in 1894.
1898 - Two large floods again, including the largest ever recorded - the Hutt River flood flow was 2000 cubic metres per second, and rose 900mm in 30 minutes.
1931 - Manor Park and Haywards bridges swept away. Taita Gorge road washed out. Vast amounts of gravel deposited.
1939 - Worst flood of 20th century. Swept away unprotected areas north of Belmont and in Mangaroa Valley. Stock drowned.
1976 - Widespread surface flooding throughout Hutt Valley in December rains.
2004 - Two large floods. At Waiwhetu flooding levels were unprecedented.