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Franklin Local
Board
26 February
2019
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Sunkist
Bay Seawall - Remedial Work Options
File No.: CP2019/00859
Te take mō te
pūrongo
Purpose of the report
1. To
seek approval from the Franklin Local Board for the recommended replacement
solution for the eastern seawall at Sunkist Bay, Beachlands.
Whakarāpopototanga matua
Executive summary
2. In
the January 2018 storms, wave action caused erosion of part of the reserve
above the seawall at the eastern end of Sunkist Bay.
3. In
response to storm damage seawall, staff prioritised the seawall for renewal and
commenced investigation on how best to address the issue. Investigation was
funded from the January 2018 regional storm damage budget.
4. Staff
retained Tonkin and Taylor to assess the site and recommended best option for
minimising future erosion, protecting existing assets and maintaining safe use
of the beach for the community. Staff also requested that a grouted rock
seawall was considered as part of this assessment at the request of the
Franklin Local Board.
5. Tonkin
and Taylor report (attachment A) clearly recommends that the existing seawall
(a tipped rock seawall) is inadequate in terms of coastal protection and
outlines the effects of two replacement options.
6. This
report recommends that the existing tipped rock be replaced with a new rock
armour revetment seawall and seeks approval from the board to proceed with this
option as a renewal project.
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Ngā tūtohunga
Recommendation/s
That the Franklin Local Board:
a) note
the addition of Sunkist Bay tipped rock seawall renewal to the 2018/2019
Community Facilities renewal programme.
b) endorse
the remediation solution, which is to replace the existing tipped rock
seawall with an armoured rock revetment seawall.
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Horopaki
Context
7. Beachlands
is a fast-growing suburb of Auckland, and the beach at Sunkist Bay, including
the boat ramp, boat club, coastal walkway and park at the eastern end is well
used by locals and visitors to the area.
8. The
Franklin Local Board developed a Sunkist Bay Concept Plan (Attachment B) in
2017/2018 to inform future development of the reserve including renewals. Within
this concept plan, improvements to the seawall in terms of beach access and
visual amenity were highlighted, and retention of the carpark endorsed.
9. The
western end of Sunkist Bay is protected by a grouted rock
seawall constructed in 1997 by Manukau City Council along the toe of the cliff
that is the natural alignment of the coastline.
10. At
the eastern end of Sunkist Bay is reclaimed reserve and approximately 200 metres
of tipped rock was constructed by Manukau City Council in 1972 to protect the
reclamation. The reclaimed low-lying area has been developed to contain a
carpark, ramp and reserve area.
11. Sunkist
Bay is exposed to major storm-generated northerly and north easterly storm
surges, and wind generated waves that form in the Tamaki Strait. Waves that
reached higher than the seawall (wave overtopping) in the January 2018 storms
resulted in scour and erosion of the unprotected ground above the low crest of
the existing tipped rock structure. The storm damage triggered the need
for the coastal protection to be renewed.
12. Tonkin
and Taylor Limited were engaged to assess coastal erosion damage to the
structure and adjacent reserve, and to provide detailed design information to
effect replacement of the tipped rock protection. This investigation was
funded from the January 2018 Storm Damage budget.
13. The
investigation report also provides information on the relative benefits and
disadvantages of rock armour revetment, and at the request of the Franklin
Local Board (on behalf of the local community) also considers a grouted rock
seawall alternative.
Tātaritanga me ngā
tohutohu
Analysis and advice
14. Repair
to the existing tipped rock seawall is not a viable option due to the absence
of any geotextile and under layer material, the low crest and over-steepened
profile of the existing slope, and poor connectivity for access to the
foreshore via existing steps. The placement of additional rock material over
the existing tipped rock would not prevent migration of fines through the
existing rocks and ongoing settlement and undermining of the structure would
still occur, and it is recommended that the existing tipped rock be completely
rebuilt.
15. Two
replacement options have been considered for the renewal of the existing tipped
rock seawall that armours the coastal edge of the reclaimed reserve at the
eastern end of Sunkist Bay. Comparison of the rock armour revetment
option with the grouted rock seawall option is provided in the summary table
below.
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Effects
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Armoured
Rock Revetment
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Grouted
Rock Seawall
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Beach
character and visual
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Similar
to existing rock character, with improved visual appearance as engineered
armour rock revetment is constructed from interlocking rocks at a uniform
slope.
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Change
in visual character at eastern end of beach, but similar to existing western
seawall. Likely to require hand rail/balustrade due to height above beach
which may impede views from the reserve.
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Beach
levels
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The
slope, permeability and surface roughness of stacked interlocking rock
dissipates wave energy and reduces wave reflection. Scouring of beach
(loss of sand) is less likely.
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Solid
seawalls have higher incidence of wave energy reflection. Grouted rock
seawall would create more turbulence, particularly from wave reflection in
the eastern corner Scouring of beach (loss of sand) likely.
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Beach
access (from road to sand)
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Similar
for either option – steps can be incorporated into both types of
structure
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Beach
access (alongshore)
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Similar
footprint to existing structure, no extension over foreshore and no change in
beach width.
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Potential
increase in beach width if alignment of new seawall is pulled landward and
rock removed from foreshore.
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Effects
on beach sand (and founding depth)
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More
dissipative structures have less wave reflection, which means less potential
for beach lowering along toe of structure. Shallower founding possible.
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Deeper
founding is required to account for wave reflection and potential scour
effects that can lower beach sand levels along the toe of the structure.
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Planning
implications
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Repair
of existing lawful reclamation either a Permitted Activity i.e. can proceed
without a resource consenting process or Restricted Discretionary activity.
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New
hard protection structure discretionary activity. New Resource consent
required, delaying remediation and adding cost to project delivery.
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Adaptive
management potential
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Easier
to reuse materials for future variations of protection.
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Expensive
to adapt for future sea level rise.
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Wave
overtopping
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More
dissipative structure due to slope, permeability and surface roughness so
more wave energy is absorbed and there is less wave overtopping.
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Less
dissipative structure due to solid face and generally steeper slope that
reflects more wave energy and has generally higher wave overtopping rates.
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Design
life
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Similar,
50 years
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Maintenance
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Similar
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Indicative
Cost (Physical Works Only)
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$3,000
per lin. m
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$8,000 per lin. m
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16. Replacement
with a rock armour revetment option is recommended to remediate the storm
damage and scour above the existing tipped rock. The proposed rock armour
revetment option will;
i. dissipate
more wave energy, be less reflective and have lower wave overtopping rates than
the alternative grouted rock wall option. Minimising the risk of beach/sand
loss.
ii. be
more readily adapted for future sea level rise.
iii. enable
improved public access with steps through the rock armour revetment to provide
safe connection to the foreshore from the reserve.
iv. accommodate
improved public access and amenity landward of the revetment crest should the
local board chose to invest in this enhancement in the future.
v. be able
to progress as repair to the existing lawful reclamation meaning that
remediation can commence with minimal delay and risk to further erosion and
public safety is minimised.
Ngā whakaaweawe me
ngā tirohanga a te rōpū Kaunihera
Council group impacts and views
17. This
recommendation has been informed with input from the following council
departments: Engineering Technical Services; Community Services; Community
Facilities, and Regulatory Services.
Ngā whakaaweawe
ā-rohe me ngā tirohanga a te poari ā-rohe
Local impacts and local board views
18. The
remediation of the tipped rock seawall at Sunkist Bay is of high interest to
the local community. Community feedback on the Sunkist Bay concept plan
(Attachment B) highlighted the importance of safe public access to the
foreshore and along the reserve, and a community desire to enhance connectivity
with beach area. The visual character of Sunkist Bay is also recognised
as an important aspect of the amenity of the area.
19. The
strength of community feeling is further demonstrated through a petition
received by Council on 12 December 2018 (Resolution number FR/2017/213) that
requested funding allocated to be “firstly invested in the building of
a permanent seawall along the eroding boulder edge foreshore in Sunkist
Bay.” (Refer to Attachment C)
20. The
Tonkin and Taylor options report was workshopped with the Franklin Local Board
on 4 September 2018. Some members of the board reiterated the community
interest in an alternative grouted seawall option. The board also expressed
interest in opportunities to enhance recreational and visual amenity values as
part of the project.
Tauākī whakaaweawe
Māori
Māori impact statement
21. The
coastal environment of Sunkist Bay is of interest to a number of mana whenua
groups. The appropriate mana whenua groups will be consulted in relation
to the proposed remediation option should the board approve the addition of the
project to the Franklin Local Board work programme.
Ngā ritenga
ā-pūtea
Financial implications
22. The
indicative cost estimate for the rock armour revetment is $970,000, which
includes professional services, physical works for coastal erosion
protection, internal charges and contingency.
23. The
cost estimate includes consenting; however does not include any unforeseen
consent conditions that may have to be met before being granted. Should
this risk materialise, a funding variation will need to be sought from within
the renewal programme budget.
24. The
estimate does not include additional features to enhance public access along
the reserve, landscape design features or delineation of parking and vehicle
access on the adjacent reserve. Should the board wish to broaden the
scope of the project to engage specialist landscape designers to provide
options for enhanced public access and visual amenity in Sunkist Bay reserve,
additional funding would be required.
25. The
allocation for this project from within renewals programme is as below:
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2018/2019
– Coastal Renewals
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$100,000
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2019/2020
– Coastal Renewals
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$870,000
(provision will be allocated to the board in the 2019/2020 Annual Plan to
fund the balance).
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26. Additional
Discretionary Works – Walkway improvements and/or landscape design
features is out of scope for this project. These may be funded using LDI Capex
or alternative sources of funding.
Ngā raru tūpono me
ngā whakamaurutanga
Risks and mitigations
27. Temporary
safety fencing has been erected above the scoured reserve edge to mitigate
potential risks to pedestrians from existing damage and to avoid vehicles
parking close to the top of the bank and worsening damage from existing
erosion; however this temporary solution does not eliminate risk.
28. If
no works are undertaken there is a risk that, in the event of any future storm
events, there may be additional wave overtopping induced scour of reserve and
damage to the carpark area at Sunkist Bay.
29. If
the Franklin Local Board does not approve the recommended option and instructs
staff to proceed with a grouted seawall option the following risks have been
identified:
i. Increased
wave overtopping rates experienced for the grouted rock seawall because it is
more reflective and less dissipative than rock armour revetment.
ii. Increased
time and cost associated with consent application for grouted rock seawall, and
risk that consent application may be declined.
iii. Unforeseen
consent conditions could impact on scope and budget, and may trigger future
budget variation.
iv. The rock
armour revetment structure is more adaptable to future management options to
address wave overtopping and sea level rise than a grouted rock seawall that
would be more expensive
to adapt in the future.
Ngā koringa ā-muri
Next steps
29. Following a
decision by the Franklin Local Board on the staff recommendation to replace the
tipped rock seawall with a armoured rock revetment seawall, the next steps will
be:
1. Tonkin + Taylor
Ltd to commence stakeholder consultation, and proceed with detailed design and
tender documents.
2. Tender release on
a competitive basis
3. Tender negotiation
and award
4. Physical works
5. Practical
completion and handover.
30. The Franklin
Local Board may wish to extend the project to include development of additional
landscaping features above the rock armour revetment to enhance public access
along the reserve, any parks amenity structures or delineation of parking and
vehicle access on the adjacent reserve. In this instance, the board would
need to request a report seeking additional funding to engage specialist
landscape designers to provide options for enhanced public access and visual
amenity in Sunkist Bay reserve.
Ngā tāpirihanga
Attachments
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No.
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Title
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Page
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a⇩ 
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Remedial Works, Sunkist Bay - Tonkin + Taylor Ltd
Report dated August 2018
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11
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b⇩
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Sunkist Bay Concept Plan December 2017
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65
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c⇩ 
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Sunkist Bay Petition
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69
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Ngā kaihaina
Signatories
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Author
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Rodrigo Pizarro - Senior Property Manager
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Authorisers
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Rod Sheridan - General Manager Community Facilities
Nina Siers - Relationship Manager
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