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Secretarial notes: a
comprehensive update on the new flood model, including its calibration,
differences from the previous model, and the integration of new LIDAR data
was provided.
o
Explained that new LIDAR data was captured in July, followed by
QA and integration into the model in late July, with adjustments made in
August to correct riverbed fitting issues. This improved the accuracy of the
terrain representation in the model.
o
Bertrand described calibrating the model against three
historical flood events (August 2014, January 2011, March 2007), ensuring the
model's river level predictions were within 200 millimetres of observed
peaks, and confirming the model passed QA checks.
o
The new model uses a 2D rain-on-grid approach with the latest
rainfall dataset (Herds v4), covering the entire catchment, while the 2019
model used a 1D/2D approach with an older rainfall dataset and limited
catchment coverage. The new model also incorporates council stormwater assets
and rural culverts.
o
Following Cyclone Gabrielle, the team adopted a new standard to
model extreme events, running scenarios with double the 100-year plus climate
change flow, aligning with national recommendations for assessing flood risk
under future climate conditions.
·
Flood Mapping Results and Ground Truthing: Discussed the
flood extent and depth maps, highlighted differences between the 2019 and
2025 models, and emphasized the need for ground truthing to validate model
outputs.
o
Compared flood extents and depths for key areas (Waipapa,
KiwiFresh, Stone Store) between the 2019 and 2025 models, noting increased
flooding in some areas due to improved grid-based modelling and LIDAR
coverage.
o
NRC stressed that ground truthing is essential to confirm model
accuracy, as local observations sometimes differ from model predictions, such
as the sports field's flood status. Acknowledged that only two weeks had been
available for review, and ground truthing would be a next step.
o
Presented depth maps in 0.1 or 1 metre increments and discussed
the availability of velocity maps, which help identify areas of high flow and
inform infrastructure planning.
o
Assumptions regarding dam water levels and the timing of
rainfall events were discussed as model limitations, with Bertrand noting
that initial dam levels are estimated and can affect attenuation during flood
events.
·
Sensitivity Analysis and Tidal Influence: Presented
sensitivity analyses on the influence of tidal levels at the Stone Store,
explaining how tidal conditions interact with river flooding and identifying
areas most affected by tidal surges.
o
The team ran model scenarios with both static and oscillating
tides, including storm surge conditions, to assess the impact on flood
extents, particularly in the lower catchment near the Stone Store and Kemp
House.
o
Results showed that tidal influence is significant in red and
orange zones near the river mouth, while areas further upstream are
predominantly affected by rainfall-driven flooding. The difference between
static and oscillating tide scenarios was found to be minimal for most areas.
o
The analysis highlighted the need for targeted flood protection
in tidal-influenced zones and informed the group about the importance of
considering both riverine and tidal flooding in future mitigation planning.
·
Next Steps and Business Case Development: NRC outlined
the next steps, including drafting a Council paper, seeking funding, engaging
consultants for a business case, and conducting cost-benefit analyses for
flood mitigation options.
o
The Rivers team will draft a paper for the December Council
meeting to seek funding for engaging Toby Kay to develop a business case for
flood risk reduction options, aiming to inform the Long Term Plan by August
2026.
o
Riley has been instructed to update cost estimates for the K3A
dam, and a Quantity Surveyor will be engaged to refine total cost estimates,
considering past discrepancies between initial and final project costs.
o
A gap analysis will be conducted with Toby and the team to
identify missing hydrology or geotechnical information needed for the
business case, with financial modelling to be performed in-house.
o
Discussions included the need for a cost-benefit analysis to
determine the value of flood mitigation for beneficiaries, consideration of
targeted rates, and the impact of current Council policies requiring
developers to fund infrastructure for new developments.
·
Policy, Funding, and Stakeholder Considerations: Discussed
policy implications, funding mechanisms, and stakeholder responsibilities for
flood mitigation, including the role of targeted rates, developer
contributions, and the need for inter-council coordination.
o
Discussed past and current policies on targeted rates for flood
infrastructure, noting that previous rates were community-specific and that
any new rate would require consultation and potentially be structured to
reflect differential benefits.
o
FNDC Council resolutions require that comprehensive flood
mitigation for new developments be designed and funded by developers, with no
cost to Council, raising questions about how to allocate costs and benefits
between existing and future landowners.
o
NRC CE emphasized the need for coordination between Regional
and District Councils, especially regarding development levies and the
distribution of benefits from flood protection works.
o
The discussion highlighted ongoing issues with local drainage
and stormwater systems, particularly in the Waipapa industrial area, and the
need for significant upgrades to address both current and future flood risks.
·
Peer Review and Publication of Model Results: Confirmed
that the flood model has undergone peer review, with further review by Toby
Kay planned, and discussed the timeline and prerequisites for publishing
model results.
o
The model was peer reviewed by SK Engineering, with Toby Kay
engaged for an additional review to ensure confidence in the model outputs
before public release.
o
Louisa explained that the model results are not yet published
due to the need for ground truthing and QA/QC, and an estimated publication
date will be provided once these steps are complete.
·
Wastewater Pump Station Flood Risk: Fred Terry raised
the issue of the wastewater pump station in the lower catchment being located
in a flood zone, highlighting the need for Council action to protect this
critical infrastructure.
The pump
station at the lower section of Amokura Drive, which serves the Kerikeri Road
area, is at risk of flooding, and past incidents of overflow during heavy
rainfall were noted, indicating the need for protection works.
Moved (Carr / Working
Group)
To accept and support the
next steps and to add a 9th step “To model effect of bund to
protect Waipapa Industrial Estate”. The motion was accepted by all of
the working group members with no objections.
Agreed action points:
·
NRC Secretariat to circulate the presentation slides with
the meeting minutes
·
NRC Rivers team to conduct ground truthing of the flood
model results by gathering feedback from people on the ground to confirm and
improve confidence in the model outputs
·
NRC Rivers Team to determine and communicate an estimated
timeline for publishing the flood maps on the NRC website after ground
truthing and QA/QC are completed
·
NRC Rivers Manager to draft a paper for the December Council
meeting seeking funding to engage Toby Kay for development of a business case
on flood risk reduction options for Kerikeri, including cost estimates and
timelines.
·
NRC Rivers Manager to update the cost estimates for the K3A
dam and obtain a total cost estimate, including engaging a Quantity Surveyor
Engineering Group for detailed numbers (Riley).
·
NRC Rivers Manager to perform a gap analysis to identify
missing information, including additional hydrology or geotechnical data
needed to inform business case options development for K3A and spillway.
·
Run financial modelling scenarios in-house to determine the
impact of flood mitigation options on rates, considering government grants
and funding sources.
·
Include a cost-benefit analysis in the business case to
assess what land would be enabled for development and the associated benefits
for landowners and funding mechanisms.
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Kerikeri-Waipapa River Working Group