Rainfall gauges indicate today’s deluge, which caused flooding from Khandallah to Kilbirnie, was a 1-in-30-year event.
The 28 April event was even more intense, with a 1-in-80-year likelihood, and was more narrowly focused on the southern and eastern suburbs.
Both short and sharp rainfall events created stormwater volumes well beyond the capacity of the city’s drainage network – much of which was built well over 100 years ago.
Wellington City Council Environment Committee Chairperson Iona Pannett says the Council is faced with a number of flooding ‘hot spots’ especially around low-lying areas of the city – such as the Basin Reserve area, Kilbirnie, the lower Aro Valley and parts of the CBD which were again hit hard today or two weeks ago.
“The Council is taking this issue seriously, flooding like this obviously causes distress and disruption to homeowners and businesses,” said Cr Pannett.
“Wellington Water is working with each affected party to find out the impact at their property and what short term protection measures can be taken. To help build a complete picture, we want to hear from everyone who has had their home affected by floodwater,” said Cr Pannett.
“We have been spending millions of ratepayers’ dollars in the past couple of decades to increase stormwater capacity in areas like Island Bay, Karori, Tawa and the Golden Mile.
“Now it’s clear we have more problem areas to deal with – so I look forward to the report being prepared by officers in response to my request on future solutions to these problems. It is likely that it may come with recommendations for a change in drainage spending so this is an issue that we can tackle as part of the 10-Year Plan deliberations.”
Cr Pannett says, contrary to many reports in the past couple of weeks, it appears that drainage network capacity – and not blocked drains and sumps – is the principal cause of the flooding. “We have been closely auditing the work by the contractors who are charged with keeping the drains clean.
“Generally we’re satisfied with their performance – and we know that, if anything, stormwater has been coming up out of the sumps rather than being blocked from going in. In many places the fact that manhole covers have been ‘popping’ is an indication that too much water is trying to go underground.”
That being said, some sumps today were blocked by impacts of the rainfall – autumn leaves and debris were washed into the drains by the heavy rain.
Cr Pannett says it is possible engineers may recommend some relatively inexpensive, but effective, flood mitigation measures such as paying for houses or buildings to be raised off the ground on higher piles, or for bunds to be built in some areas where practicable.
In the longer term, new pump stations or larger stormwater mains, particularly in the Newtown catchment – which runs from Melrose and Berhampore and through the Basin Reserve and under Kent Terrace to the harbour – may have to be considered and priced.
Cr Pannett says the City Council and Wellington Water are working on a city-wide plan to improve the overall stormwater network performance. “We have to take a careful and scientific approach - if we fix only part of a network without working to a plan, we risk simply transferring the problem, or making it worse, or wasting a lot of money.”
She says modelling of climate change and projected sea-level rises is playing an important part in the planning.
Wellington Mayor Celia Wade-Brown says the effective functioning of Wellington’s infrastructure is essential for the city’s health and economy. “We have budgeted more than $150 million in operational spending and over $56 million in capital spending over the next 10 years for stormwater assets alone,” she says.
“The combination of some older drains, more impermeable surfaces and increasing rainfall severity means that system modelling is more and more important. We need to spend the right amount in the right areas at the right time.
“For the longer term we must design our streets and sections to be more permeable and add more planting.”