Waipu, Paparoa flood maps released

10 May 2019, 10:58 AM

Landowners in Waipu and Paparoa are the latest to be issued with new Northland Regional Council maps showing the potential effect floods – including large ‘one in a century’ type events – could have on them.

The council has released flood maps for 26 catchments over the past few years to raise awareness of where flood hazard areas are and help inform public and district council decision-making.

The Waipu properties involved in the latest flood mapping sit within the regional council’s Coastal South constituency (represented by Councillor Rick Stolwerk) and the Paparoa properties within the Kaipara constituency (represented by Cr Penny Smart).

Councillor Stolwerk says the council has been methodically working its way through the region, initially focussing on reducing flood risks in more than two dozen catchments deemed to be facing the highest flood threats. (District councils have also produced some stormwater flood maps for specific settlements.)

Councillor Stolwerk says the Waipu catchment, which covers a roughly 220 square kilometre area – and the smaller Paparoa catchment, which covers just 41.8 square kilometres – had now also been modelled.

The models forecast the predicted impacts of a 10-year, 50-year and 100-year floods and take into account the latest Central Government climate change projections on storm rainfall intensity and sea level rise.

“Technically, there’s a 10 percent chance of 10-year flood happening in any year, a two percent chance of a 50-year flood and a one percent chance of a hundred-year flood over 12 months.”

However, Cr Stolwerk says across New Zealand, most authorities only have useable, scientifically valid data stretching back 30 to 50 years which can affect the accuracy of predicting the probability of extreme events.

“The new Waipu and Paparoa maps are based on detailed survey data from the relevant river and floodplain areas, hydrology assessments of rainfall profiles, tidal conditions and catchment conditions and computer modelling of floods to determine the likely extent of flooding.”

He says as with other parts of Northland, residents in both catchments are already familiar with heavy rainfall events and how these affect properties in their respective areas.

“The risk from floods relates mainly to the depth and speed of floodwater, although other issues like debris can increase the potential risk. Additionally, factors such as localised high intensity rainfall, overland flows and extremely high tide levels can also elevate the degree of risk.”

Over the next few days, the regional council will be writing to the roughly 600 landowners within the flood mapped areas (500 of them from Waipu and 100 from Paparoa) outlining the latest flood risk to their properties and directing them to the new maps.

“We’ve had a number of actual floods (including in June 1997, March 2007 and January 2011) which have been very useful from a computer modelling point of view and we’ve been able to use these real-life experiences to validate some of our flood models.”

Councillor Stolwerk says identifying areas at risk of flooding helps local authorities and landowners manage flood risks.

“Councils are required to help manage risks for land that’s susceptible to flooding, via regional policy, district plans, and building and resource consents.”

The Regional Policy Statement for Northland (2016) – a regional council document – contains policies for managing land in flood hazard areas. Likewise, district councils need to consider flood hazard information when processing resource consents and building consents.

“They need to ensure that decisions on what’s built, and where, is based on the best information available.”

The new flood maps can be viewed in GIS via the regional council’s website: www.nrc.govt.nz/naturalhazardsmaps

Meanwhile, general information about flood mapping is available at: www.nrc.govt.nz/floodmaps

Alternatively, landowners with questions about the maps – or who want to provide the council with any additional information they have regarding flooding at their properties – can contact the council’s Rivers Team on freephone (0800) 002 004.