Flood risk and development on the Takaka floodplain


Takaka has not experienced an extreme flooding event since 1983 and changes to the urban area and the river environment, as well as anticipated exacerbation effects caused by climate change (estimates are that the 200yr ARI would become the 100yr ARI by 2090) mean that the hazard risk situation was not well understood.

TDC needed to better understand the hazard risk situation and hold discussions with affected parties and the community to provide input into (i) land use decisions regarding current occupation and future growth (ii) emergency management planning (iii) Council asset and infrastructure management (e.g., wastewater treatment plant); to respond to the hazard risk.

Partners and key personnel

  • TDC: staff (science, policy and engineering), Ward Councillors and Golden Bay Community Board
  • Several specific landowners, (one with an informal stop bank which affected the flood hazard and others potentially affected by response options)
  • Aurecon (modelling work consultants)
  • Local Takaka residents and business owners and the wider Golden Bay community
Overall approach
  • Hydraulic modelling of the flood hazard. This was iterative as initial modelling raised key questions that where addressed by additional work. A 200yr ARI was used as a proxy for climate change impacts affecting Takaka over the 100 year planning timeframe
  • Preparation of discussion document including maps describing the modelling results and a simple evaluation of possible response options to the hazard risk information
  • An open day presentation of (i) hazard information from the model results and (ii) a range of possible response options (e.g., engineered flood protection, land use and building controls, closure of residential zones, maintaining status quo)

The open day was advertised using radio, local new paper, councillor and community board networks, and letters to directly affected parties. The day was split in two, (i) a morning session for landowners affected by potential responses to the hazard risk (ii) open to anyone. A PowerPoint presentation was done several times over the day and a static display of information was available. Expert staff attended all day to answer questions and a smaller static display was left at the council offices following the open day. Staff took note of questions and concerns raised.

  • A public feedback period (approx. 10 weeks) invited responses to general questions about hazard knowledge in the area; acceptability of risk; and appetite for the various response options. Feedback was low so further direction was sought from the Golden Bay Community Board
  • Information from the process was used to enhance hazard prediction data, inform asset management, and feed into LIMS and building consents. Council’s current decision regarding land use controls is to rely on existing zoning and development rules for extreme events (100 – 200ARI) and to investigate potential for engineering responses to smaller events (less than 20yr ARI). This may be revisited.
Key points about approach

Modelling was useful in several ways:

  • Outputs, including maps and animations, were very useful in communicating the hazard risk to attending residents
  • The modelling approach also allowed for easy assessment of the issues and effects of possible response options
  • There was general public acceptance that the model adequately reflected the flood hazard, and matched anecdotal evidence from landowners. There were requests to extend the modelling work and include outputs in planning maps.

Public engagement – some benefits and some lessons

  • The open day format allowed attendees to absorb information and gain answers to questions at their own pace. It avoided over dominance by those more vocal.
  • There was some positive feedback which showed appreciation for a greater understanding of the flood risk so that good decisions could be made.
  • Feedback was surprisingly low and most concentrated on the response options.
  • Notification by a mass mail out may have been more effective at gaining attention
  • A two ‘half-day’ format for the open day would have provided more flexibility.
  • Providing take-home pamphlets summarising information would have been useful

The importance of understanding community and history

  • Subsequent assessment of the local community suggests apathy about the hazard, an independent 'we’ll cope”'attitude in people, and expectation that existing engineering works were already dealing with the issue may have affected interest
  • Misinformation was spread within the community regarding Council’s hazard response intentions with a persistent perception Council had already decided to construct a stop bank. This was likely bedded in prior negative experiences some residents had with TDC and preceding authorities
  • A better understanding of the community make-up and targeted discussions with key personalities may have helped address this issue
Things to consider
  • Acquisition of LIDAR contour data has made modelling and development of map and animation resources much more affordable
  • There are good benefits to be had in utilising model outputs and analysis software (e.g. WaterRide) to create maps and animations to assist in communicating the hazard
  • Be aware of and plan engagement around community existing perceptions, attitudes and history with the local authority
  • Separate the presentation of hazard and risk information with the discussion of response options. This avoids people giving feedback based on their reaction to the cost and impact of response treatments rather than their view of the hazard and the risk
  • Local communication networks can be useful in disseminating information but they can also spread misinformation quickly
  • It is important, (although challenging) to communicate the limitations and accuracy within models (they are only an estimates of the real situation), particularly where these ‘best estimates’ are being used to make decisions
  • It is difficult to adequately include a flexible allowance for climate change projections in long planning time-frames, as this is an inherently variable aspect
When to use this approach? Projects involving hazard identification and communication/discussion with the community.
More information Environment and Planning department – Tasman District Council [link]