The Island Bay foreshore: seawall, promenade, sand
dunes, park and marine drive
John
Robinson[1]
Introduction
Considerable changes are suggested for southern
Island Bay, affecting the beach, seawall, roads, footpaths and park. These deserve a full discussion of all the
facts.
Currently a seawall provides protection from
sand drift, shelter on the beach from northerlies and along the footpath from
southerlies. This has been a success
for 75 years until a section collapsed in 2013 due to inadequate maintenance as
reinforcing steel rods had rusted and not been replaced.
The wall is used to lean on, sit on, walk
on. The wide footpath is a popular
promenade and a site for festival stalls and a place to watch the blessing of
the boats. The road is part of the
coastal drive, used by tourists (including buses), locals and heavy trucks from
eastern suburbs to the Happy valley landfills.
The City Council has suggested that: “The walls collapse presents
a unique opportunity to consider
options on how the damage should be addressed.” [2] Two options are to destroy all those facilities, to reroute
traffic along suburban streets, and to block the park off from the beach by
artificial sand dunes. This would be
extremely disruptive and would threaten nearby residences with significant sand
drift. Many issues will be recognized
by locals who are familiar with this place.
These notes were prepared as a contribution to
preparation of a website and public presentation by the Island Bay Seawall Action Group,
formed after a June 2014 community meeting.
Basic information provided in September by Council (after many requests)
set the scene for a review of all aspects of the proposals. However the time line set by Council
demanded a change of strategy and these notes were not considered at the 1
October meeting. This is then an
individual contribution.
History
Since the environment was
modified by early settlement, we do not know what was ‘natural’. Before settlement there would have been
significant wetlands with low-lying swamp for some distance back from the beach. [3]
An early photograph of the Blue Platter Teahouse, built in 1906, shows
flat sand, with no dune system. [4]
Thus the construction of dunes will not restore
a natural system as suggested by the headings “Option D: Remove wall and restore coastal dunes” and “Option C3a
– Partially relocate wall and restore coastal dunes” [5] (here called option E). The only dune apparent (existing today as a
planted and protected area) was against a rock hill at the west end of the main
beach, which provided protection to the sand.
The rock base that protects those dunes is visible today.
Observations of the coast provide clear
information of the dynamic and vulnerable system. Planting of sand driven up against the wall at the eastern end of
Island Bay did not survive as the sand was driven away. Dunes at the eastern end of Princess Bay
have been susceptible to storms; although seemingly well established now they
had been washed away early this century before re-establishing over the past
decade. Sand evidently provides an
uncertain protective barrier.
The Esplanade, the road running along the
beach, is a section of the marine drive round the coast from Oriental
Bay to Happy Valley, constructed in the first third of the twentieth century,
thought to be a wonderful asset and one of the longest and most picturesque
marine drives possessed by any city in the world. [6] The coastal road at Island Bay is now a main
thoroughfare, used by locals, tourists and commercial traffic (including large
trucks travelling from the eastern suburbs to the landfills in Happy Valley).
In 1934 it was recognised that
Island Bay suffered severely from sand drifts in times of southerly storms and
a wall was constructed in 1936-37. [7] When repairs were carried out in 2002, the top of the wall was cleaned and
re-plastered to conceal the corroding steel rods. The erosion of the steel rods resulted in collapse of the wall in
a storm in 2013.
This is a listed heritage
seawall. “The seawall is an important heritage object
of aesthetic and social value” [8],
to lean or sit on when looking across the bay or eating a snack, even for
youngsters to run along. The footpath
is much used for promenading,
meeting by parked cars and youngsters cycling.
During the festival, stalls are along the wide footpath, and the path and wall provide a
favourite spot for watching the blessing of the boats. The wall provides wind protection to beach
users in a northerly and to users of the wide promenade in a southerly.
The solid base on which the wall sits has
provided stability over 76 years, and remains.
This may be compared with the failure of the
St Clair (Dunedin) esplanade in 2013 where sink-holes appeared in the pavement
when water came under the wall, which had not been adequately anchored to a basalt bedrock when constructed in
2004.
A suggestion of closing a
stretch of the Esplanade and introducing a dune system along the beach was
considered in 1995 and again in 2004-5. [9] The suggestion of road closures was turned
down as the community wanted to preserve the sea wall. [10] Discussion in 2005 then concerned two
stages: 1, preserve the existing wall and roads and 2, move the Esplanade to
the north, cutting into Shorland Park and Reef Street Reserve, a with dune area
along the expanded beach edge. [11] The first stage was favoured.
The failure of a section of the
wall in 2013 and concerns with sea-level rise have reawakened interest and a
number of options have been proposed.
Protection of the area near the beach may be provided by either a
seawall (existing or in a new position) or by a dune system, together with a
new road system.
There may be uncertainty due to
the impact of sea level rise and the movement of sand in an active marine
environment. A possible future beach
profile is based on the assumption that
as sea level is raised, the equilibrium profile is moved upward and
landward. This assumption may, or may
not, hold. The beach profile has
remained stable since the construction of the seawall. “The bay is relatively stable in its
present form with no long-term erosion or accretion trends discernable,
although is likely subject to limited storm erosion processes.” [12]
Sand doesn’t sit still. Not on the land, not in the sea.
There is uncertainty concerning the impact of
introduced sand, and Tonkins and Taylor asked “how does a sparse
modern sand cover behave on an exposed and indented coast; how does it change
through time, and what are the processes causing spatial and temporal
variability?” In the sea along this
active coast sand is frequently mobilised by the tides and southerly swell, and
Island Bay has the modern sand cover restricted to the bay head. The semipermanence of the sand cover
supports the role of stable currents such as the tides in forming and maintaining
the scours. [13]
The vigorous natural forces of wind and water
may then re-establish the previous system, and remove introduced, artificial
dunes and beach. Both may well fail.
Options
The following options have been proposed. [14] They are illustrated in figures in the
reports, which provide further information.
Option A: Retain wall in present alignment
The seawall would be maintained in its present alignment with the
damaged section repaired and maintained as required in the future.
Cost estimate $550,000 - $1,150,000.
Pro:
Preserves the listed heritage
seawall and footpath.
Preserves the beachside section
of the marine drive.
The seawall provides protection
against winds and sand drift.
Con:
Along a 200 m section, the wall is likely to require more frequent
maintenance and repair in the future. This may include raising of wall crest
levels, potentially by 0.5 m before 2065 and by 1 m before 2115 and/or works to
improve the walls foundations.[15]
Points of information:
Option A is the lowest cost but
ongoing repair and upgrade of the wall will likely be required due to higher
sea levels and beach recession due to climate change. [16]
Option B: Beach replenishment to protect wall
The seawall would be maintained
in its present alignment with the damaged section repaired. Additional sand
would be added to the beach to build a 5 m wide, RL 2.5m high berm along a 300
m beach length between the stormwater outfalls to the south and headland to the
east. This would require the addition of some 12,500 m3 of sand.
Cost estimate $1,700,000 - $2,900,000.
Pro:
Preserves the listed heritage
seawall and footpath.
Preserves the beachside section
of the marine drive.
The seawall provides protection
against winds and sand drift.
Points of information:
A significant volume of imported
sand is required for Option B which may provide amenity benefits due to the
wider beach but may also have adverse ecological impacts. The option is high
cost due to the volume of sand required and may require ongoing maintenance
and/or future upgrade of the seawall. [17]
The coast is a
dynamic system and the beach may return to its previous form, with the loss of
any added sand.
Option C: Relocate wall to stable beach planform
The straight seawall between the
outfalls and surf club would be realigned to the natural beach planform and the road and pavement realigned fully or
partially behind the wall.
Cost estimate $1,900,000 - $2,200,000.
Pro:
Although following a different curve, there will be a seawall, footpath
and coast road along the beach.
Con:
Loss of part of Shorland Park and and Reef Street Reserve (used
as a dog run).
Points of information:
Option C is technically feasible
although also high cost due to the costs of removing and rebuilding a long
section of seawall. The option is sustainable in the long-term as the
replacement wall can be built to withstand wave forces and higher sea levels if
recession back to the wall ever occurs (potentially after 2065). [18]
The coast is a
dynamic system and sand the beach may return to its previous form, with the
loss of any added sand.
Option D: Remove wall and restore coastal dunes
As noted above, this heading is not accurate as there is no indication
that this area was a dune system such as that proposed.
The straight seawall between the outfalls and surf club and the backing
road and footpath would be removed. The
footpath and part of the Esplanade would be removed, with coastal traffic
rerouted along Brighton Street and Trent
Street, the Parade and Derwent Street. Once the seawall, backing road
and park are removed and cleared, sand would be imported and shaped into a dune
system, stabilised using wind-fencing and planted. Where the existing seawalls
are truncated, the wall ends should be turned inland to ensure they are not
outflanked.
Cost estimate of $850,000 - $1,500,000 does not include the cost of
rediversion of roadway and utilities inland.
The Tonkins and Taylor comment that “Option
D is lower cost” is inaccurate. With these costs included D and E will
probably be the more expensive options
Pro:
The new planted dune system may provide
protection to rising sea levels.
Con:
The new planted dune system may not provide
protection to rising sea levels and may be eroded or swept away.
Loss of the listed heritage seawall and footpath, and the
beachside section of the marine drive moving heavy traffic through
intersection.
Loss
of easy parking by the beach
Loss of access from the
park. Significant areas of Council
Reserve including Shorland Park would need to be returned to natural dune system
for this option, although it is noted that southern parts of Shorland Park are
already dune (currently protected by the seawall. [19] To be successful the sand dune system would
have to be continuous and fenced off, like the dunes at the west end of the
beach, and not available for recreation.
While it has been suggested that dunes will connect Shorland Park to the
sea, this is the very opposite; the
illustrative figure shows no path through the extended dune area. Were such a path introduced, it would
provide a channel for sand drift.
The effectiveness of any gap for allowing sand flow was evident when a
southerly moved sand through the current gap in the seawall in a storm.
Sand inundation may become
problematic as the dune system attempts to ‘roll back’ over backing land. [20]
There is currently no suggestion
of how traffic flow would be managed.
Traffic is rerouted along Trent Street, which is narrow for large
vehicles when cars are parked on both sides.
Some figures show trees along an avenue, which would narrow the road
further and remove parking spaces.
There would need to be traffic control at the intersection of Trent
Street and The Parade. The added
traffic, including significant numbers of heavy vehicles would create
difficulties in the narrow section of Derwent Street.
Points of information:
Option D is … technically
feasible, although space is limited between the future beach alignment and
private property at the end of Reef St. Wind-blown sand and dune roll-over with
future beach recession may be problematic and require wind fencing and/or
maintenance and issues associated with traffic re-routing would require
detailed assessment. [21]
Option
E: Partially relocate wall and restore coastal dunes
As with option D, this heading is not accurate as there is no
indication that this area was a dune system such as that proposed.
The straight seawall between the
outfalls and intersection of the main road and Reef street would be
removed. The footpath and part of the Esplanade would be removed, with coastal
traffic rerouted along Reef Street, the Parade and Derwent Street. Once the seawall,
backing road and park are removed and cleared, sand would be imported and
shaped into a dune system, stabilised using wind-fencing and planted. Where the
existing
seawalls are truncated, the wall ends should be turned inland to ensure they
are not outflanked.
Cost estimate of $1,000,000 - $1,800,000 does not include the cost of
rediversion of roadway and utilities inland.
Pro:
The new planted dune system may provide protection
to rising sea levels.
Con:
Loss of the listed heritage seawall and footpath, and the
beachside section of the marine drive.
Significant areas of Council
Reserve including Shorland Park would need to be returned to natural dune
system for this option, although it is noted that southern parts of Shorland
Park are already dune. [22]
There is currently no suggestion
of how traffic flow would be managed.
Some figures show trees, an avenue along Reef Street, which would remove
parking spaces. There would need to be
traffic control at the intersection of Reef Street and The Parade, which is
currently a bus turnaround. The added
traffic, including significant numbers of heavy vehicles would create
difficulties in the narrow section of Derwent Street.
Points of information:
This option reduces the risk of future erosion
impacting assets and infrastructure by only partial removal of the seawall
compared to the full removal option. [23]
Immediate modification to the stormwater
outfalls are not envisioned and have not been costed into the present project
but modifications may be required during future upgrades to the stormwater
system. [24]
Wastewater (sewage) pipes from Houghton system run up Reef Street, and
those from Owhiro Bay turn north at the south-west corner of Shorland
Park. These can be left as they are.
References
Carter
L and Lewis K 1995.
Variability of the modern sand cover on a tide and storm driven inner
shelf, south Wellington, New Zealand.
New Zealand Journal of Geology and Geophysics, 38:4, 451-470.
Evening Post
1887 to 1939. Articles and letters
relating to the Island Bay esplanade and sea wall.
Findlay, M
2013. Rampart on the sea: the Island
Bay sea wall. South Coast Historical
Society, magazine number 9 (check).
Wellington
City Council 1995. Preliminary review
of beach management options for Island Bay.
Wellington
City Council 2005a. Island Bay
foreshore: archaeological issues.
Wellington
City Council 2005b. Minutes of Strategy
and Policy Committee, 9 November.
Wellington
City Council 2005c. Island Bay five
year restoration plan.
Wellington
City Council 2005d. Set of
illustrations.
Wellington City Council 2014a. Information is
on line at http://wellington.govt.nz/your-council/projects/island-bay-seawall-replacement/
and http://wellington.govt.nz/your-council/projects/island-bay-seawall-replacement/reports-and-concept-designs,
including two reports at Island
Bay Seawall Alternatives Analysis - Coastal Processes Assessment (2.5MB
PDF) and Island
Bay Smart Project - Seawall Alternatives Analysis - Additional Option (1.2MB
PDF).
Wellington City Council 2014b. Island
Bay Seawall: DRAFT Project Plan; A
project to develop options to address the damage to the Island Bay Seawall
caused by the June 2013 Storm.
Tonkin and
Taylor May 2014a. Island Bay seawall
alternatives analysis, coastal processes assessment. For Wellington City Council.
Tonkin and Taylor June 2014b. Island Bay smart project - seawall
alternatives analysis -
additional option. For
Wellington City Council.
[1] Email johnrob@paradise.net.nz
[2] Wellington City Council 2014b
[3] Tonkin and
Tayor May 2014, terrain
model figure 7
[4] Tonkin and
Tayor May 2014, photograph
figure 4
[5] Tonkin and
Tayor May 2014 and June 2014
[6] Dominion
Post 1905 to 1936, various letters and articles.
[7] Findlay 2013
[8] Wellington
City Council 2005b and 2005c
[9] Wellington
City Council 1995
[10] Celia
Wade-Brown, private communication
[11] Wellington
City Council 2005d
[12] Tonkin and
Tayor May 2014
[14] WCC website: http://wellington.govt.nz/your-council/projects/island-bay-seawall-replacement/. Options A to D are from Tonkin
and Taylor May 2014. Option E is the
additional option of Tonkin and Taylor June 2014.
[15] Tonkin and
Tayor May 2014
[16] Tonkin and
Tayor May 2014
[17] Tonkin and
Tayor May 2014
[18] Tonkin and
Tayor May 2014
[19] Tonkin and
Tayor May 2014
[20] Tonkin and
Tayor May 2014
[21] Tonkin and
Tayor May 2014
[22] Tonkin and
Tayor June 2014
[23] Tonkin and
Tayor June 2014
[24] Tonkin and
Tayor June 2014
