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Ōrākei
Local Board
19 September
2019
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Tāhuna
Tōrea Fish Dams protection project
File No.: CP2019/17495
Te take mō te
pūrongo
Purpose of the report
1. To
seek approval to cancel the Tāhuna Tōrea mangrove removal project
(Sharepoint ID 2308 in the Community Facilities 2018/2019 work programme), in light
of evidence that the activity together with increasing coastal inundation and
erosion along the western sandspit is likely to lead to damage of the fish
dams.
Whakarāpopototanga matua
Executive summary
2. The
Ōrākei Local Board approved the Tāhuna Tōrea mangrove
removal project as part of the 2015/2016 Local and Sports Parks Annual Work
Programme.
3. The
objective of the mangrove removal project is to preserve the Tāhuna
Tōrea Fish Dams at Tāhuna Tōrea Nature Reserve. Continued
mangrove colonisation at increasing densities within the embankments was viewed
as damaging to the integrity of the fish dams.
4. Resource
consent conditions were determined in consultation with Ngāi Tai ki
Tāmaki, and included the requirement for a test area (Stage 1) to be established
for partial mangrove removal to be undertaken with monitoring over 12 months.
As mangroves were removed, any effects on the fish dams, particularly on the
edges of the fish dam embankments, were assessed.
5. Monitoring
of the test area suggests that due to natural coastal erosion processes and
storm events, there is a risk to the fish dams of accelerated erosion and
damage should mangrove removal continue. As the fish dams are a significant
feature of the reserve, the local board agreed that mangrove removal was
detrimental to the integrity of the fish dams.
6. Staff
recommend that the mangrove removal at Tāhuna Tōrea be cancelled.
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Ngā tūtohunga
Recommendation/s
That the Ōrākei
Local Board:
a) approve
the cancellation of the Tāhuna Tōrea mangrove removal project
(Sharepoint ID 2308 in the Community Facilities 2018/2019 work programme),
noting that this activity and increased coastal inundaton and erosion along
the western sandspit is likely to lead to irreparable damage to the fish dams.
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Horopaki
Context
7. The
fish dams in the northern lagoon of Tāhuna Tōrea Nature Reserve are a
significant cultural heritage feature of the reserve, and are afforded
protection by the Auckland Unitary Plan. Over the years mangroves have
established and spread around the embankments of the dams and there was concern
that they were degrading the integrity of the dams by facilitating the
accumulation of sediment and inhibiting the growth of native reeds and rushes.
8. Ecologists
believed that removal of the mangroves would encourage the spread of rushes
across the exposed area of the fish dam and this would be a way of keeping the
integrity of the fish dams intact, and at the same time eliminate future
mangrove spread within the embankments.
9. The
project to remove mangroves from around the fish dams in the northern part of
Tāhuna Tōrea Nature Reserve was initiated in 2016 and the local board
allocated $65,000 from their LDI opex budget. An additional $62,000 of
LDI opex was approved in 2018/2019 with the primary objective of protecting the
Tāhuna Tōrea Fish Dams.
10. The
Council applied for resource consent to undertake mangrove management in three
stages and consent was granted in February 2017.
11. Conditions
on the consent were determined in consultation with Ngāi Tai ki
Tāmaki, and included the requirement of a test area (Stage 1) to be set up
where mangroves could be removed and the area monitored for 12 months. As
mangroves were removed, any effects on the fish dams, particularly on the edges
of the fish dam embankments, were assessed.
12. The local
board was then advised that the results of the monitoring suggested that due to
natural coastal erosion processes and storm events, there was a risk to the
fish dams that they would be more quickly eroded and damaged if further
mangroves were removed. As the fish dams are a significant feature of the
reserve, the local board agreed that mangrove removal was detrimental to the
integrity of the fish dams.
13. There has
been no further mangrove removal and on 20 June 2019 the local board resolved
to reallocate the remaining balance of $119,000 of LDI opex from the
Tāhuna Tōrea mangrove removal project to other projects.
Tātaritanga me ngā
tohutohu
Analysis and advice
14. The
purpose of the monitoring of Stage 1 was to measure and evaluate whether there
were any detrimental impacts on the fish dam caused by the removal and pruning
of the mangroves.
15. The
fish dam lagoon is subject to variable tidal inputs and has recently been
exposed to greater coastal process energies, particularly when high tides
coincide with storm events. The site is also subject to increasing variability
in sediment inputs, salinity, and wetting and drying periods. In the medium to
long term, further coastal erosion and projected sea level rise will increase
the frequency of the spit being breached and subsequent inundation of the site.
With exposure to higher coastal process energies, embankments are subject to
increased risk of erosion and or sediment accumulation infilling the area.
16. Mangrove
removal anticipated by the Stage 2 of the project would further increase the
erosion risk to the fish dam embankments, particularly where surface sediments
are exposed.
17. There is
no evidence to support the earlier hypothesis that light limitation due to
mangrove growth was the primary cause of rush decline over the site. In
addition, no expansion or decline of the existing footprint of rushes was
observed. There also appears to be no germination of new plants beyond the
existing rush cover along the pruned area of embankment 3 or elsewhere on site.
There are outstanding issues surrounding tidal influx and elevation of the
remaining embankments relative to the open mudflat that are likely impacting
the distribution of the rushes along the remnants of the embankments in
addition to competitive exclusion impacts from the mangroves.
18. Monitoring
on a quarterly basis has identified that the on-going impacts of coastal
inundation and storm surge at the site are likely to present a greater risk to
the survival of the fish dam structures.
19. The
project’s main objective was to protect the Tāhuna Tōrea Fish
Dams and initial analysis concluded that mangroves are the primary and sole
driver degrading the fish dams.
20. However,
observation of coastal processes during the monitoring period has made it clear
that tidal inundation and encumbent changes in silt and salinity levels,
combined with insufficient erosion protection of the western sandspit will play
a significant role in changing the integrity of the fish dams and the
surrounding environment.
21. Further,
experts concluded during the monitoring of Stage 1 that the additional factor
of mangrove removal will contribute to exacerbating these effects by enabling
increased exposure to coastal processes.
22. No
recommendations regarding reinforcement of the western sandspit have been made
to mitigate the increasing breaching and erosion that is occurring as hard
engineering solutions are not considered to be appropriate for this site.
23. At
a workshop on 7 February 2019 the preliminary findings of the draft monitoring
report were discussed with further information required in relation to the coastal
processes and iwi consultation. On review in April 2019 options to consider
planting or hard options to protect the western sandspit from increased
breaching and erosion were not recommended by Coastal and Geotechnical Services
staff.
24. Ngāi
Tai ki Tāmaki have been informed of the recommendations, and Community
Facilities staff await their response as to whether they support the
recommendations of the monitoring report highlighting the predominant risk to
the fish dams of increased tidal inundation and erosion, and whether they wish
to arrange a hui to further explore options to protect the fish dams.
25. Findings of the monitoring report and coastal assessment have been discussed
with the Tāhuna Tōrea Rangers on 14 May 2019. At
its 15 May committee meeting, theTāhuna Tōrea
Rangers pass that “The preservation of the existing Maori fish
dams is to be given the highest priority and the Orakei Local Board is to be
asked to pursue the removal of mangroves and to take other necessary measures
to ensure this preservation”. The Rangers assert that effects are
limited to only “inland mangroves located where stiller water
sometimes laps”.
26. The
reallocation of the 2018/2019 LDI opex budget of $62,000 for the Tāhuna
Tōrea mangrove removal project and the reallocation of the LDI opex carry
forward budget of $57,000 for the Tāhuna Tōrea mangrove removal
project was approved on 20 June 2019.
27. On the
basis of the findings from the monitoring period and current coastal processes,
it is recommended that no further mangrove removal is undertaken and that the
project be removed from the local board’s work programme.
Ngā whakaaweawe me
ngā tirohanga a te rōpū Kaunihera
Council group impacts and views
28. This
report has been prepared as a collaborative effort by staff from Community
Facilities, Engineering and Technical Services and consultants from Morphum
Environmental and Kokea.
Ngā whakaaweawe
ā-rohe me ngā tirohanga a te poari ā-rohe
Local impacts and local board views
29. The
proposal has been considered by the local board in a series of workshops from 5
October 2017 to 7 February 2019.
30. Staff
have explained the findings of the Stage 1 monitoring to the reserve’s
local care group, the Tāhuna Torea Rangers, but they do not support the
direction recommended on ceasing mangrove removal from around the fish dams.
Tauākī whakaaweawe
Māori
Māori impact statement
31. Ngāi
Tai ki Tāmaki were consulted from the inception of the project in 2015 and
provided direct input into the preservation of the taonga, the Tāhuna
Tōrea dish dams. This has been through providing direct input into
the resource consent conditions and subsequent approval of the mangrove removal
management plan.
32. Ngāi
Tai ki Tāmaki also provided cultural monitoring of the physical works, and
are currently reviewing the draft Tāhuna Tōrea Stage 1 Monitoring
Report recommendations.
Ngā ritenga
ā-pūtea
Financial implications
33. The
budget for the mangrove removal project has been reallocated, but further
funding could be allocated by the local board in discussions for the 2020/2021
work programme to ensure the fish dams are protected as much as possible.
Ngā raru tūpono me
ngā whakamaurutanga
Risks and mitigations
34. The
resource consent is active for up to 10 years on the condition that the first
stage monitoring has been completed. It does not negate consideration for
future strategies to protect the Tāhuna Tōrea Fish Dams, however the
resource consent does not consider the coastal processes that have been
identified as the key risk to the fish dam environment.
Ngā koringa ā-muri
Next steps
35. Complete
resource consent conditions which include the mangrove removal Stage 1
monitoring report and Ngāi Tai ki Tāmaki hui minutes and direction.
Ngā tāpirihanga
Attachments
There are no attachments for
this report.
Ngā kaihaina
Signatories
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Author
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Johan Ferreira, Manager Project Delivery
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Authorisers
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Rod Sheridan - General Manager Community Facilities
Adam Milina - Relationship Manager - Albert-Eden &
Ōrākei Local Boards
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