Multi-million dollar flood defences canvassed for climate change response
Flood walls, detention dams, a tidal gate and raising land were among options presented to the Nelson City Councillors to tackle flooding in the central city due to climate change. Engineers from the company Tonkin & Taylor told the council's infrastructure committee that about 3000 buildings were expected be at risk from freshwater flooding in the Maitai River flood plain by 2090 up from around 1800 today - in a 1 per cent AEP event (an event that has a one per cent chance of happening in any given year). Engineer Damian Velluppillai said the capital value of properties vulnerable to coastal inundation in the city was expected to grow from $2.2 billion to $3.1 billion, in a 1 per cent AEP event with 1m of sea level rise. He presented eight scenarios of combined measures that could be used to manage the risk. READ MORE: * Flood risk worse than council inundation maps suggest, lobbyists say * Council confirms new subcommittee members * Council urged to re-consider 'high risk' riverside development Six concepts were based on protection, including a possible 3.5m high riverside barrier at Nile St, stopbanks, widening the Maitai River channel upstream from about 15m to 30m, raising state highway 6, and raising ground levels within city streets. Detention dams at several upstream sites could store water beyond a 20 year event, and discharge water at a slower rate, Velluppillai said. A valley floor detention dam could do the work of the upstream dams combined, but also risked flooding the golf course near the Maitai Dam, and cutting off road access, he said. Feasibility of the sites still needed to be assessed, but any upper catchment dams could be about the size of the Hopua te Nihotetea detention dam in Whangarei, paid for through targeted rates. Velluppillai said 1m of sea level rise was forecast to extend the area at risk of saltwater inundation in central Nelson in a king tide, from around Bridge Street to Nile Street in the central city. A tidal gate at the Maitai River mouth could be lowered ahead of a storm surge, he said. There was no real silver bullet to adapting to the flood risk and a portfolio of responses was required, Velluppillai said. Measures to accommodate the risk in the medium term could include stormwater culvert flapgates, and raising part of the river bank and Collingwood St roadway. A managed retreat scenario highlighted people who could be moved first, as those potentially affected by a 1 per cent AEP storm surge event with 40cm of sea level rise. The 23 potential options came ahead of the release of flood maps for other catchments in the city, community consultation on climate change adaptation values and objectives, and changes to legislation that would affect adaptation. We need to understand where the communitys values are, Velluppillai said. But we also dont want to go to the community with a blank piece of paper. Guidance from the Ministry for the Environment outlined a spectrum of approaches should be considered in consultation with the community, rather than just protection or retreat, including flood warning systems, he said. A separate solution for freshwater flooding from the York catchment would be needed in most scenarios. Councillor Rohan O'Neill-Stevens questioned at what point council would be liable if any adopted defence measures failed. Tonkin & Taylor engineer John Rix said absolute protection was not possible. Council flood engineer Toby Kay said the risk of structures failing needed to be considered as part of any design process. Kay said most of the concepts presented were in the very high cost category, so over $10m. Councillor Kate Fulton was concerned there was no mention of Maitai restoration work done, and questioned how biodiversity efforts could be aligned with climate action. Councillor Rachel Sanson said she wanted to make sure council was taking a sufficiently long term view. The Ministry for the Environment, their advice is that we should be looking 2130 and beyond, and we shouldnt make any signifiant infrastructure investment into risk areas.