Flood letter:
WRSAG were not consulted on this matter. We were told in 2023 that flood modelling had been done but were not aware of when the modelling was done, the purpose of the modelling, or the model parameters. HAL stated up to 400 homes could get flooded under the scenario used. The information was withheld by council claiming it was too contentious for ratepayers to see. We are now aware councillors had this information as early as 2018.
WRSAG was subsequently advised in 2024 new flood modelling was completed now showing up to 1900 properties could get flooded. We were not advised if this meant floor levels were flooded or just the land surrounding homes. We understand this is the modelling now being presented.
Letters have been sent to 3500 property owners. Council stated the purpose of the letters is for response and preparedness to flooding. Council has yet to explain why 3500 owners got letters if only 1900 got flooded, or to what extent.
During the drop-in sessions one owner was provided a report by HAL we suspect is the report withheld from WRSAG and ratepayers as being too contentious. This report references about another dozen reports also withheld (see LGOIMA request Flood Letters documents).
Council claims it followed Tauranga City Councils processes when it issued the 3500 letters. WRSAG has obtained Tauranga City Councils conference paper to the 2015 Asia Pacific Stormwater Conference explaining how they released their flood modelling. TCC state this was done on a ‘no secrets policy’ and was ONLY ISSUED TO THOSE HOUSES THAT DID FLOOD after TCC had completed modelling including planned future improvements so owners would see council was working to mitigate flooding. TCC scrubbed out flooding of 100mm depth. WRSAG claim TCDC did not follow TCC process.
Download all these here along with further LGOIMA responses. Flood Letters documents
WRSAG has researched other councils – one example is QLDC. They issued letters in 2022 https://www.qldc.govt.nz/2022/october/22-10-07-new-stormwater-flood-hazard-maps-released/
Along with the flood model report they released the DHL review https://www.qldc.govt.nz/media/o4fewuog/north-wanaka-model-update-and-flood-extent-mapping-nov2021.pdf
DHL state they have followed the flood model best practice guide produced by NIWA for Ministry for the Environment. https://environment.govt.nz/assets/Publications/Files/preparing-for-future-flooding.pdf
The TCDC HAL report does not state they have followed this guide or that they have validated the model by on site surveying during actual flood conditions of lesser AEP rainfall events working up to the 2%AEP and 1%AEP.
WRC states that 1%AEP modelling is not a requirement under the Whangamata CSDC 105667 but if it were, WRC should be provided information in yearly reports (reference LGOIMA in flood letters documents).
Auckland City Council released this Model Review USER GUIDE https://www.epa.govt.nz/assets/Uploads/Documents/Fast-track-consenting/The-Hill-Ellerslie/application/Attach-H22-Model-Build-Appendix-L-Auckland-Council-Model-Review.pdf
Auckland Councils review shows good use of flood model mapping. It is an application guide. The guide states that the modelling begins with a 50%AEP (peak rainfall intensity over the last 2 years) with site survey to validate the model in selected areas representative of Auckland. They followed with modelling for 20%AEP, 10%AEP, 5%AEP and then the 2%AEP. This provides validation of the model variables to better represent possible flooding. Infrastructure deficiencies become known and future planning has a base to work from.
What we understand of the 1%AEP is to take into account future variables including global warming (2.1degC by 2090), sea level rise, higher intensity rainfall (>20% than last 100-year event), future development of land and options for infrastructure to manage the future by 2090.
The flood model at 1%AEP DOES NOT IN ANY WAY REPRESENT FLOODING AS AT TODAYS WEATHER OR ENVIRONMENT SETTINGS.
The model is to provide a tool to assess on best case probability the projected change in our environment, coupled with likely future development, and mitigated by future infrastructure improvements. If we model this way we have planned outcomes for the future.
WRSAG does not accept the letters, the flood model maps being provided to owners, or the explanations purporting to flooding comply with the NIWA/MfE best practice guide.
TCDC has not surveyed affected properties to validate the model and grade against Councils flood complaint form adverse risk policy. The following image is a copy of Whangamata Ward Flood Complaint form. This is TCDC 2020 adverse effects policy. It is noted the adverse effects are categorised into 6 levels of adverse effect. Level 1 is the lowest with Nil Effects expressed as No effects at all. Level 6 is Unacceptable Adverse Effects expressed as Extensive adverse effects that cannot be avoided, remedied or mitigated. It stands to reason that possible minor flooding on a property by the year 2090 would never create a reason to prevent development or the use of that property to an extent a warning is required on the LIM to protect future owners. The flood documents include a valuers opinion on this.
COMMENT:
Councils adverse effects policy does not distinguish between property damage, land damage or physical harm to people or animals. It could be concluded that level 4 has more than minor adverse effects could include damage to dwellings. This risk is currently insurable so can be remedied.
Tauranga City Council scrubbed from the modelling flood depths to 100mm. There are a few reasons for this.
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LiDAR in 2013 was claimed to be sensitive to + or – 50mm.
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Compliant floor levels must be at least 100mm above paved areas and up to 225mm above soil.
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Compliant floor levels must be 150mm above the crown of the road or 225mm above natural ground with a 1:20 unimpeded slope to a natural waterway.
This means TCC were considering the limitations as to accuracy of LiDAR and until surveyed floor levels would be higher than flood water. The process of scrubbing out is to ignore temporary flooding of ground up to floor levels in a 1%AEP event because no damage will be done to walls or floors. The survey should include freeboard of S traps as water entering waste water systems could overwhelm pump capacity and cause up-flows in waste systems to lower lying homes.
The flood letters fail to explain the deficiencies in our existing stormwater network, what is required, and when council intends making improvements.
The flood letters fail to consider future development of private land and the adverse effect of extra impervious run-off that may occur.
The flood letters fail to mention or include any future stormwater infrastructure to mitigate flooding to the year 2090 to allow for that development
The flood model maps being provided to owners in the drop-in sessions includes this disclaimer. Thames Coromandel District Council give no warranty in relation to the data (including accuracy, reliability, completeness or suitability) and accepts no liability (including, without limitation, liability in negligence) for any loss, damage or costs relation to any use of the data. WRSAG claims councils’ admission that modelling is not accurate or reliable means the modelling is NOT in accordance with best practice guide for modelling, does not meet the required reasonableness or accuracy test within s44A, so cannot satisfy the 'good faith' provisions in the new s44A LGOIMA 2025.
WRSAG claims issuing 3500 flood letters on the same basis means it is a ‘blanket decision’ so unlawful. Decisions must be specific, and letters must relate to that property.
The NIWA and MfE best practice guide for modelling does not state property files should be tagged as the response to flood modelling. The response to flood modelling is for plans to mitigate future flooding in larger storm events allowing for expected future development and environment changes.
Council made a clear statement to WRSAG that none of the communications with owners challenging the flood model letters would be placed on property files. Last week council changed its mind and stated in writing it was going to add any correspondence to the property file. This pollutes our LIM’s with false information.
WRSAG supports flood modelling providing it encompasses all the provisions in the best practice guide including:
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Model must include all flood risks including Wentworth and Otahu rivers,
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Is validated to at least the 50%AEP and 10%AEP,
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Includes lists of current deficiencies in existing stormwater system
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Options on how to correct infrastructure deficiencies (improvements) for existing development
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Including projected sea level rise (and inundation of),
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Running the 1%AEP rainfall intensity to model various improvement options for each of the existing catchment areas within Whangamata to demonstrate future development opportunities are possible
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AND that this is all made available to the public following TCC ‘no secrets policy’.
WRSAG claims council already has flood warnings on about 400 properties as one of the outcomes of the Opus 2003 public flood survey. Council needs to explain why it still allowed these properties to be developed and why the recommendations in the Opus report were not followed.
Owners wanting to develop their properties now have to work out how to deal with additional ‘red tape’ where the modelling shows only ‘minor adverse flooding’.
The flood letters is misdirected as owners cannot fix up councils’ poor infrastructure. This is understood to be frustration. Legally now notices are on property files owners cannot develop properties under s31 of RMA solely because council has failed to provide stormwater solutions recommended by Opus in 2003 and the numerous reports since.
WRSAG accepts some properties are likely to be adversely affected by potential flooding in both the 2%AEP and 1%AEP to the extent the properties cannot be developed unless either the land is raised, or council install infrastructure. Council must front up and advise these owners and explain what infrastructure is required and/or provide the infrastructure. This is especially true for new developments like the Camp Site.
The surface flooding in the main is a man-made problem because development proceeded without mitigating the adverse effect. This is not a natural hazard as defined in RMA. It is man made. What is truly disappointing is council has collected substantial contributions for subdivisions and development but has not used that money for the purpose it was collected.
Council must withdraw all letters and notices placed on property files. If council want to issue letters and place notices they must do so in accordance with accepted guidelines and on a factual basis.