Tauranga Inner Harbour Sites
13 Motuhoa Island East
13.1 Setting
Site description
The coastal cliffs on the eastern side of Motuhoa Island trend north-east to south-west. The cliffs
rise up above the shore platform for approximately 6m to 8m at the north-eastern end to a
maximum of 18 m to 20 m towards the southern end. Slope angles typically stand between 60° to
90°, and there are several small areas where the cliffs overhang. At many places the cliffs are bare.
The shore platform at the base of the cliff is relatively high and extends nearly level out into
Tauranga Harbour. Small sandy beaches have formed at the toe of the cliff, and vegetation debris
has accumulated at the toe of the cliff along most of its length.
Geology
The Geological Map of the area indicates that the geology is dominated by early to middle
Pleistocene deposits, with the higher cliffs comprising deposits of the Pakaumanu Group and the
lower slopes and cliffs comprising Tauranga Group soils. These are described as:
•
Pakaumanu Group – Mostly welded undifferentiated Ignimbrite, with minor air fall
deposits.
•
Tauranga Group – Crystal and ash rich sediments, reworked from middle Quaternary
ignimbrites.
It is highly likely that the Tauranga group sediments will overly the Pakaumanu group at this
location.
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13.2 Geotechnical Issues
Failure method
It is difficult to see the model of failure at this location, as no detailed walkover could be
undertaken. Based on observations from afar, along with aerial photograph imagery and our
understanding of similar types of cliffs within the Inner Harbour, it is anticipated that slips will be
typically shallow or superficial. The slope failures appear too typified by a bare near-vertical
headscarpe, generally near the top of the slope. Slipped material collects at the base of the slope,
which ordinarily would provide support against on-going regression. However, this toe weighting
is removed by coastal processes to leave unsupported near vertical and vertical sea cliffs.
13.3 Coastal Processes
The Motuhoa Island site is protected from the prevailing winds but exposed to winds from the
periodic storms from the northeast round to the south. The mean fetch across the harbour to Mt
Maunganui is approximately 8 km. A fetch of this length given a 25 m/s wind storm (representative
of a 2% AEP event) would produce a theoretical significant wave height in the order of 1.4 m at high
tides. However, this site is fronted by tidal flats up to 1.6 km wide which will act to dissipate some
of the wave energy approaching this site. It is likely that tidal forces are also driving erosion with
the process of wetting and drying by tides particularly at higher spring cycles weathering the unconsolidated banks. Erosion rates along these cliffs are estimated to be up to 0.3 m/y, although as
banks tend to dropout in discrete events this erosion is likely to be episodic rather than at a
constant rate.
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Tauranga Inner Harbour Sites
MATUHOA ISLAND EAST
Ref
Option
Financial
Engineering
Environmental
Recreation
Health & safety and
Risk
Comment
1
Timber seawall
Not viable
Not appropriate
2
Rock revetment
Not viable
Not appropriate
3
Rock groyne
Not viable
Not appropriate
4
Submerged
breakwater
Not viable
Not appropriate
5
Mangrove
planting
Not viable
Not appropriate
6
Drainage
Not viable
Not appropriate
Note, very low financial benefit possible so no works can be economically justified.
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Opus International Consultants Ltd
Tauranga Inner Harbour Sites
14 Opunui Point
14.1 Setting
Site description
The coastal cliffs at Opunui Point trend east to west. The cliffs rise up above the shore platform for
approximately 8 m to 10 m and typically stand between 70° to 90°. At many places the cliffs are
bare. The shore platform at the base of the cliff is relatively high and extends nearly level out into
Tauranga Harbour. Small sandy beaches have formed at the toe of the cliff, and vegetation debris
has accumulated at the toe of the cliff along most of its length.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
14.2 Geotechnical Issues
Failure method
As per Motuhoa Island, determining the mode of failure is difficult due to a lack of access. Based
on observations from afar, along with aerial photograph imagery and our understanding of similar
types of cliffs within the Inner Harbour, it is anticipated that slips will be typically shallow or
superficial. The slope failures appear too typified by a bare near-vertical headscarpe, generally
near the top of the slope. Slipped material collects at the base of the slope, which ordinarily would
provide support against on-going regression. However, this toe weighting is removed by coastal
processes to leave unsupported near vertical and vertical sea cliffs.
14.3 Coastal Processes
The coastline of the Opunui Point site faces mainly into the southerly sector and is exposed to
prevailing winds from the south-west. The average fetch to the south-west is 3.5 km. A fetch of this
length given a 25 m/s wind storm (representative of a 2% AEP event) would produce a theoretical
significant wave height in the order of 0.9 m at high tides. Higher waves may come from the
southeast which has a longer fetch of approximately 6 km to Mt Maunganui, although winds rarely
generate from this direction.
A major tidal channel flows along the base of the cliffs at this site. A combination of wave energy
and tidal currents are likely sufficient to carry away cliff toe debris at this site. Tidal fluctuation
may also be adding to the erosion with the process of wetting and drying particularly at higher
spring cycles weathering the un-consolidated banks. Erosion rates along these cliffs are estimated
to be up to 0.4 m/y, although as banks tend to dropout in discrete events this erosion is likely to be
episodic rather than at a constant rate.
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Opus International Consultants Ltd
Tauranga Inner Harbour Sites
OPUNUI POINT
Ref
Option
Financial
Engineering
Environmental
Recreation
Health & safety and
Risk
Comment
1
Timber seawall
Not viable
Not appropriate
2
Rock revetment
Not viable
Not appropriate
3
Rock groyne
Not viable
Not appropriate
4
Submerged
breakwater
Not viable
Not appropriate
5
Mangrove
planting
Not viable
Not appropriate
6
Drainage
Not viable
Not appropriate
Note, very low financial benefit possible so no works can be economically justified.
49
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Opus International Consultants Ltd
Tauranga Inner Harbour Sites
15 Opureora West
15.1 Setting
Site description
The coastal cliffs at Opureora West trend broadly north-west from the existing ferry terminal and
are west facing. The cliffs rise up above the shore platform for approximately 8m to 10m and
typically stand between 70° to 90°. At a few places the cliffs look to be bare. Small sandy beaches
look to have formed at the toe of the cliff, with vegetation debris accumulating at the toe, along
most of the length of the cliff.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
15.2 Geotechnical Issues
Failure method
As per Motuhoa Island, determining the mode of failure is difficult due to a lack of access. Based
on observations from afar, along with aerial photograph imagery and our understanding of similar
types of cliffs within the Inner Harbour, it is anticipated that slips will be typically shallow or
superficial. The slope failures appear to be typified by a bare near-vertical headscarpe, generally
near the top of the slope. Slipped material collects at the base of the slope, which ordinarily would
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Tauranga Inner Harbour Sites
provide support against on-going regression. However, this toe weighting is removed by coastal
processes to leave unsupported near vertical and vertical sea cliffs.
15.3 Coastal Processes
Opureora West is exposed to the west and is exposed to prevailing winds from this direction. The
average fetch to the south-west is 2.6km. A fetch of this length given a 25 m/s wind storm
(representative of a 2% AEP event) would produce a theoretical significant wave height in the order
of 0.70m at high tides. However this site is fronted by a sandy spit which would dissipate wave
energy as waves shoal over the shallow bathymetry.
Tidal fluctuation may also be adding to the erosion with the process of wetting and drying
particularly at higher spring cycles weathering the un-consolidated banks. Erosion rates along
these cliffs are estimated to be up to 0.2m/y, although as banks tend to dropout in discrete events
this erosion is likely to be episodic rather than at a constant rate.
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Tauranga Inner Harbour Sites
OPUREORA WEST
Ref
Option
Financial
Engineering
Environmental
Recreation
1
Timber seawall
Not viable
Not appropriate
2
Rock
revetment
Not viable
Not appropriate
3
Rock groyne
Not viable
Not appropriate
4
Submerged
breakwater
Potentially
viable
Only small structure
affordable, limiting
benefit
Fully visible at
low tide
5
Mangrove
planting
Viable
No issues. Would
reduce wave impact
and slow removal of
eroded material
6
Drainage
Potentially
viable
Borehole drains
horizontally to
reduce saturation.
May offer some
benefit locally within
upper sections of sea
cliff
Good ecological
habitat, fish
nursery, but
potential adverse
visual impact
No impact
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Potential
impacts on
boating and
connecting
ferry link
Poor, restricted
access and
potential
impact on
boating
No impact
Health & safety and
Risk
Comment
Watercraft grounding,
drowning risk
Not appropriate,
effectiveness, cost
and visual impact
Low, possible risk of
entrapment
Potentially most
appropriate
Some difficult access for
construction/maintenance
Potentially
appropriate
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
16 Opureora South
16.1 Setting
Site description
The coastal cliffs at Opureora South trend broadly west from the ferry terminal and are southern
facing. The cliffs rise up above the shore platform for approximately 8m to 10m and typically stand
at angles of between 70° and 90°. At a few locations it would appear as though there have been
slope failures within the cliffs. The shore platform at the base of the cliff appears to be relatively
high and extends nearly level out into Tauranga Harbour. Small sandy beaches have formed at the
toe of the cliff, and vegetation debris has accumulated at the toe of the cliff along most of its length.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
16.2 Geotechnical Issues
Failure method
Determining the mode of failure at this location is difficult due to a lack of access. Based on
observations from afar, along with aerial photograph imagery and our understanding of similar
types of cliffs within the Inner Harbour, it is anticipated that slips will be typically shallow or
superficial. The slope failures appear to be typified by a bare near-vertical headscarpe, generally
near the top of the slope. Slipped material collects at the base of the slope, which ordinarily would
provide support against on-going regression. However, this toe weighting is removed by coastal
processes to leave unsupported near vertical and vertical sea cliffs.
16.3 Coastal Processes
Opureora South is exposed to the south and is exposed to prevailing winds from this direction and
also the west. The average fetch to the south-west is 5.3km. A fetch of this length given a 25 m/s
wind storm (representative of a 2% AEP event) would produce a theoretical significant wave height
in the order of 0.60m at high tides. However this section of Opureora is also fronted by the same
sandy spit, which would dissipate wave energy as waves shoal over the shallow bathymetry.
Tidal fluctuation may also be adding to the erosion with the process of wetting and drying
particularly at higher spring cycles weathering the un-consolidated banks. Erosion rates along
these cliffs are estimated to be up to 0.10m/y, although as banks tend to dropout in discrete events
this erosion is likely to be episodic rather than at a constant rate.
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Tauranga Inner Harbour Sites
Opureora South
Option
Financial
Timber
seawall
Not viable
Not appropriate
Rock
revetment
Not viable
Rock groyne
Not viable
Potentially viable
once detailed site
inspection has been
completed
Not appropriate
Submerged
breakwater
Not viable
Not appropriate
Mangrove
planting
Potentially
viable
Drainage
Not viable
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Engineering
No issues. Would
reduce wave impact
and slow removal
of eroded material
Environmental
Good ecological
habitat, fish
nursery, but
potential adverse
visual impact
Recreation
Poor, restricted
access and
potential impact
on boating
Health & safety and Risk
Low, possible risk of
entrapment
Comment
Potentially most
appropriate
Not appropriate
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
17 Matua
17.1 Setting
Site Description
The shoreline at Matua trends roughly north-east to south-west and comprises coastal cliffs that
vary in height from about 12 m at the northern end, to a maximum of 24 m at the southern end.
Slope angles vary between approximately 50° and a maximum of 80° and are typically covered with
native trees and bush. At some locations these trees have been removed. At the base of the cliffs, a
shallow sandy beach extends out in to the Inner harbour, which at high tide is covered. A number
of coastal defence measures have been placed along the base of the cliffs at Matua, including rock
walls, gabion baskets and timber retaining walls. It should be noted though that many of these
erosion measures look as though they are not properly designed and appear to have been put
together by local residents.
At the top of the sea cliffs, is the region of Matua, a residential development on the western side of
Tauranga City. Privately owned gardens extend from the top of the cliffs to the residencies. Based
on the Tauranga City Council website, it wold appear as though all stormwater is piped away from
the area. The general topography of the area is trending downwards towards the coast line at this
location.
Geology
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Tauranga Inner Harbour Sites
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
Groundwater has not been observed issuing from within any of the sea cliffs at this location,
however, it is reasonably assumed that groundwater flows would be towards the sea cliffs along the
western side of Matua.
17.2 Geotechnical Issues
Failure method
Slope failures observed are typically shallow or surficial. The slope failures on the coastal cliffs are
typified by a bare near-vertical headscarpe, 2 – 3 m high, generally near the top of the slope.
The observations of slope stability at this site suggests that the steep slopes at this site are prone to
shallow slope failures, especially when they have had the vegetation coverage removed from them.
Coastal erosion of the shoreline has been restricted somewhat by the placement of the ad-hoc
retaining structures, but the effectiveness and longevity of these structures cannot be relied upon.
The removal of slipped material through coastal erosion appears to have slowed at this location.
Regression of slips appears to have slowed and this could be a result of development of pipe
stormwater drainage, along with the construction of the crude coastal defence system.
17.3 Coastal Processes
The Matua study is a generally sheltered site. It faces west to north-west, which although is exposed
to some of the most predominant winds it is sheltered by Te Puna in the west across a shallow
estuary. The average fetch to the north-west is approximately 5 km. A fetch of this length given a 25
m/s wind storm (representative of a 2% AEP event) would produce a theoretical significant wave
height in the order of 1 m at high tides. However this site is fronted by up to 400 m wide tidal flats
which would dissipate some of the wave energy.
A variety of ad-hoc seawalls have been installed along parts of this coastline and work to stabilise
the cliff toes. However, some are in a state of disrepair, and/or are low-lying and may be prone to
overtopping and scouring behind under the effects of future sea-level rise predictions. The heavy
vegetation cover of the banks in the aerial photographs made the exact rates of cliff erosion difficult
to assess. These were estimated to be up to 0.2 m/y, although as banks tend to dropout in discrete
events this erosion is likely to be episodic rather than at a constant rate.
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Tauranga Inner Harbour Sites
MATUA
57
Option
Financial
Engineering
Environmental
Recreation
Health & safety and Risk
Comment
Timber seawall
Viable
May be effective for
low-lying parts
Possibly
appropriate
Viable
Will provide possible
toe weight to higher
sea cliffs
Existing revetment
can be utilised
Potential to
limit beach
access
Potential loss of
access to beach
at low tide
Falling from height –
possibly require handrails
Rock revetment
Poor, linear
appearance, no
habitat
Possible adverse
impact, visually
Temporary
construction
access routes
issue
Slipping and falling in voids
- risk of injury
Appropriate
option
Rock groyne
Potentially
viable
Would extend into
deeper water
Submerged
breakwater
Viable
Visible at low
tide
Mangrove
planting
Viable
No issues. Little
impact on overall
erosion
No issues. Will
reduce but not
eliminate erosion at
toe of sea cliff
Drainage
Viable
Borehole drains
horizontally to
reduce saturation.
May offer some
benefit locally within
upper sections of sea
cliff
No impact
| August 2015
Not appropriate
Adverse visual
impact at this
popular location
Potential
impacts on
boating
Poor, restricted
access and
potential
impact on
boating
No impact
Watercraft grounding,
drowning risk
Not appropriate
Low, possible risk of
entrapment
Not appropriate
Some difficult access for
construction/maintenance
Potentially
appropriate
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
18 Rewarewa Place
18.1 Setting
Site description
The shoreline at Rewarewa Place trends roughly east to west and comprises coastal cliffs that vary
in height from about 17m at the eastern end, to a maximum of 22 m at the western end. Slope
angles vary between approximately 50° and a maximum of 80° and are typically covered with
native trees and bush. At some locations these trees have been removed and at a few places the
cliffs look to be bare, with slope failures evident. At the base of the cliffs, a shallow sandy beach
extends out in to the Inner harbour, which at high tide is covered. Locally slipped vegetation debris
has accumulated at the toe of the cliff.
At the top of the sea cliffs, is the region of Matua, a residential development on the western side of
Tauranga City. Privately owned gardens extend from the top of the cliffs to the residencies. Based
on the Tauranga City Council website, it wold appear as though all stormwater is piped away from
the area. The general topography of the area is trending downwards towards the coast line at this
location.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
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Tauranga Inner Harbour Sites
Groundwater
Groundwater has not been observed issuing from within any of the sea cliffs at this location,
however, it is reasonably assumed that groundwater flows would be towards the sea cliffs along the
southern side of Matua.
18.2 Geotechnical Issues
Failure method
The observations of slope stability at this site suggests that the steep slopes at this site are prone to
shallow slope failures, especially when they have had the vegetation coverage removed from them.
Coastal erosion of the shoreline has been restricted somewhat by the placement of the ad-hoc
retaining structures, but the effectiveness and longevity of these structures cannot be relied upon.
The removal of slipped material through coastal erosion appears to have slowed at this location.
Regression of slips appears to have slowed and this could be a result of development of pipe
stormwater drainage, along with the construction of the crude coastal defence system.
18.3 Coastal Processes
The Rewarewa Place study is a generally sheltered site, which faces the south across a shallow
estuary. The average fetch is approximately 0.80km. A fetch of this length given a 25 m/s wind
storm (representative of a 2% AEP event) would produce a theoretical significant wave height in
the order of 0.50m at high tides. However this site is fronted by significantly wide tidal flats which
would dissipate some of this wave energy.
A variety of ad-hoc seawalls have been installed along parts of this coastline and work to stabilise
the cliff toes. However, some are in a state of disrepair, and/or are low-lying and may be prone to
overtopping and scouring behind under the effects of future sea-level rise predictions. The heavy
vegetation cover of the banks in the aerial photographs made the exact rates of cliff erosion difficult
to assess. These were estimated to be up to 0.06m/y, although as banks tend to dropout in discrete
events this erosion is likely to be episodic rather than at a constant rate.
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Tauranga Inner Harbour Sites
REWAREWA PLACE
Option
Financial
Timber
seawall
Not viable
Rock
revetment
Potentially
viable
Rock groyne
Not viable
Submerged
breakwater
Viable
Mangrove
planting
Viable
Drainage
Viable
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Engineering
Environmental
Recreation
Health & safety and Risk
Comment
Not appropriate
Will provide
possible toe weight
to higher sea cliffs
Existing revetment
can be utilised
Possible adverse
impact, visually
Temporary
construction access
routes issue
Potential loss of
access to beach
at low tide
Slipping and falling in voids risk of injury
Appropriate option
Not appropriate
No issues. Little
impact on overall
erosion
No issues. Will
reduce but not
eliminate erosion at
toe of sea cliff
Borehole drains
horizontally to
reduce saturation.
May offer some
benefit locally
within upper
sections of sea cliff
Visible at low tide
Adverse visual
impact at this
popular location
No impact
Potential
impacts on
boating
Poor, restricted
access and
potential impact
on boating
No impact
Watercraft grounding,
drowning risk
Potentially
appropriate
Low, possible risk of
entrapment
Potentially
appropriate
Some difficult access for
construction/maintenance
Appropriate option
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
19 Hairini West
19.1 Setting
Site Description
The shoreline at Hairini West trends roughly north-east to south-west and faces north-west. The
shoreline at this location comprises two distinct slope types. At the lower level, ground levels
extend gently from the shoreline at slope angle in the order of 5° for a distance of about 25 – 30 m,
from where, slope angles steepen to approximately 40° to 45° to a higher plateau, some 30 - 35 m
above the shoreline. The slopes at this location have a cover of native trees and bush.
At the base of the cliffs, a shallow sandy beach extends out in to the Inner harbour, which at high
tide is covered. A number of coastal defence measures have been placed along the shoreline at this
location, including rock walls, gabion baskets and timber retaining walls. It should be noted
though that many of these erosion measures look as though they are not properly designed and
appear to have been placed by local residents.
At the top of the plateau, is a large open reserve, beyond which are a number of sub-divisions. It
would appear as though stormwater drainage is piped away from this plateau based on the
Tauranga City Council website.
Geology
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Tauranga Inner Harbour Sites
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
19.2 Geotechnical Issues
Failure method
It would appear that the steeper slopes set back from the shoreline represent historic slip scarp
locations. The slipped material at the base of the slip zones have come to rest forming the gently
inclined slopes which extend down to the shoreline. Coastal erosion of these gently inclined
shoreline slopes has been restricted by the placement of the ad-hoc retaining structures, but the
effectiveness and longevity of these structures cannot be guaranteed.
19.3 Coastal Processes
The Hairini site is situated within a low energy environment within Waimapu Estuary. It faces
north-west with an average fetch across the estuary of 1.2 km. A fetch of this length given a 25 m/s
wind storm (representative of a 2% AEP event) would produce a theoretical significant wave height
in the order of 0.2 m at high tide. Although the tidal flats of the estuary would dissipate much of
the wave energy. It is sheltered in the northern end by a road bridge which has effectively split the
estuary.
This site is more likely to be affected by the influence of the tide, and ebb tide currents may cause
localised turbulence and scouring along the riprap along this road. A variety of low ad-hoc seawalls
have been installed along parts of this coastline. However, some are in a state of disrepair, and/or
are low-lying and therefore may be prone to overtopping and scouring behind under the effects of
higher water levels. The process of wetting and drying of unprotected banks by tides particularly at
higher spring cycles will also weather the un-consolidated banks. Cliff erosion rates along this site
are estimated to be up to 0.1 m/y, although as banks tend to dropout in discrete events this erosion
is likely to be episodic rather than at a constant rate
There are also some low-lying areas along this site which will become susceptible to inundation
under future sea-level rise predictions.
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Tauranga Inner Harbour Sites
HAIRINI WEST
63
Option
Financial
Engineering
Environmental
Recreation
Health & safety and
Risk
Comment
Timber seawall
Only viable
at small
scale
Effective for low
sections
Poor, linear
appearance, no
habitat
Potential to
limit beach
access
Falling from height –
possibly require handrails
Possibly
appropriate
Rock revetment
Potentially
viable
Low energy site,
smaller scale
appropriate
Possible adverse
impact, visually
Potential loss of
access to beach
at low tide
Slipping and falling in voids
- risk of injury
Potentially
appropriate
Rock groyne
Not viable
Submerged
breakwater
Viable
No issues. Will
reduce but not
eliminate erosion
Mangrove
planting
Viable
No issues. Will
reduce but not
eliminate erosion
Drainage
Viable
Borehole drains
horizontally to
reduce saturation.
May offer some
benefit at upper
sections of sea cliff
| August 2015
Not appropriate
Adverse visual
impact at this
highly visible
location
Adverse visual
impact at this
highly visible
location but
ecological value
No impact
Potential
impact on
boating
Low, possible risk of
entrapment
Small rock
revetment is better
option
Potentially
restricted
access and
potential
impact on
boating
No impact
Low, possible risk of
entrapment
Potentially
appropriate
Some difficult access for
construction/maintenance
Potentially
appropriate
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
20 Maungatapu West
20.1 Setting
Site Description
The shoreline at Maungatapu West trends roughly north-east to south-west and faces north-west.
As at Hairini West, the shoreline at this location comprises two distinct slope types. At the lower
level, ground levels extend gently from the shoreline at slope angle in the order of 5° for a distance
of about 20 m to 35 m, from where, slope angles steepen to approximately 40° to 45° to a higher
plateau, some 30 m to 35 m above the shoreline. The lower slopes at this location are either
developed with residential housing, or have a cover of native trees and bush. According to the TCC
website, stormwater drainage appears to be piped however this is not conclusive. The steeper slope
sections are covered in native bush and trees. At the top of this steeper slope are more residential
properties.
At the base of the cliffs, a shallow sandy beach extends out in to the Inner harbour, which at high
tide is covered. A number of coastal defence measures have been placed along the shoreline at this
location, particularly rock and concrete walls, although it looks as though these erosion measures
are not properly designed again.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
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20.2 Geotechnical Issues
Failure method
It would appear that the steeper slopes set back from the shoreline represent historic slip scarp
locations. The slipped material at the base of the slip zones have come to rest forming the gently
inclined slopes which extend down to the shoreline. Coastal erosion of these gently inclined
shoreline slopes has been restricted by the placement of the ad-hock retaining solutions, but the
effectiveness and longevity of these structures cannot be guaranteed.
20.3 Coastal Processes
The Maungatapu site is on the other side of the road bridge to Hairini along the Maungatapu
Peninsula. This is also within a relatively low energy estuarine environment. This shoreline faces
north-west across the estuary which has an average fetch of 2.4 km. A fetch of this length given a
25 m/s wind storm (representative of a 2% AEP event) would produce a theoretical significant
wave height in the order of 0.8 m at high tide. Although the tidal flats of the estuary would
dissipate much of the wave energy. Waves from the north may also affect the site, although it is
unclear how the bridges to the north will affect their passage, and these are likely to refract and
diffract over the shallow tidal flats and Motuopuhi Island in front of this site.
Localised turbulence and scour may occur at the southern end of this site with flood tidal currents
and/or waves interacting with the rock rip-rap associated with the road bridge.
A variety of low ad-hoc seawalls have been installed along parts of this coastline. However, some
are in a state of disrepair, and/ or are low-lying and may be prone to overtopping and scouring
behind under the effects higher water levels. The process of wetting and drying of unprotected
banks by tides particularly at higher spring cycles will also weather the un-consolidated banks. Cliff
erosion rates along this site are estimated to be up to 0.1 m/y, although as banks tend to dropout in
discrete events this erosion is likely to be episodic rather than at a constant rate. There are also
some low-lying areas along this site which will become susceptible to inundation under future sealevel rise predictions.
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Tauranga Inner Harbour Sites
MAUNGATAPU WEST
66
Option
Financial
Engineering
Environmental
Recreation
Health & safety and Risk
Comment
Timber seawall
Potentially
viable
Potential to
limit beach
access
Potential loss of
access to beach
at low tide
Not appropriate
Viable
Poor, linear
appearance, no
habitat
Possible adverse
impact, visually
Falling from height –
possibly require handrails
Rock revetment
Would need to be a
significant height at
some locations
Relatively sheltered
site, 2m height by
4m base
Slipping and falling in voids
- risk of injury
Potentially
appropriate
Rock groyne
Not viable
Submerged
breakwater
Viable
No issues. Will
reduce but not
eliminate erosion
Mangrove
planting
Viable
No issues. Will not
have enough effect
Drainage
viable
Borehole drains
horizontally to
reduce saturation.
May offer some
benefit at upper
sections of sea cliff
| August 2015
Not appropriate
Adverse visual
impact at this
highly visible
location
Adverse visual
impact at this
highly visible
location but
ecological value
Potential
impact on
boating
Low, possible risk of
entrapment
Rock revetment
is better option
Potentially
restricted
access and
potential
impact on
boating
Low, possible risk of
entrapment
Not appropriate
No impact
No impact
Some difficult access for
construction/maintenance
Potentially
appropriate
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
21 Maungatapu Central
21.1 Setting
Site description
The shoreline at Maungatapu Central trends roughly north-east to south-west and faces northwest. The slope up from the shoreline stands at about 50° to 60° from the horizontal and range in
height from 16m to a maximum of 20m. The slopes are generally vegetated with shrubs and native
grasses, whilst also containing a number of large mature native trees along both the crest and base
of the slope. At the base of the cliffs, a shallow sandy beach extends out in to the Inner harbour,
which at high tide is covered. This section of coastline has been heavily protected by rock walls,
gabion baskets and timber retaining walls. It should be noted though that many of these protection
measures look as though they are not properly designed and appear to have been put together by
local residents.
At the top of the sea cliffs, is the region of Maungatapu, a residential development. Privately owned
gardens extend from the top of the cliffs to the residencies. Based on the Tauranga City Council
website, it wold appear as though all stormwater is piped away from the area. The general
topography of the area is trending downwards towards the coast line at this location.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
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21.2 Geotechnical Issues
Failure method
The observations of slope stability at this site suggests that the steep slopes would have been prone
to shallow slope failures in the past. However, the extensive residential development along the
crest of this cliff line, combined with the re-engineering of the slope profile that appears to have
occurred, disguises the number of slip failures and true rate of erosion at this location.
In addition, coastal erosion of the shoreline has been restricted somewhat by the placement of the
ad-hoc retaining structures, although the effectiveness and longevity of these structures cannot be
relied upon. Never-the-less, regression of slips and rates of coastal erosion at this location have
been difficult to accurately predict due to the amount of engineering that has occurred at this
location.
21.3 Coastal Processes
This site is located centrally along the north-western side of the Maungatapu Peninsula and is
affected by winds form the northerly sector. The average fetch to the north-west is 1.6km. A fetch
of this length given a 25 m/s wind storm (representative of a 2% AEP event) would produce a
theoretical significant wave height in the order of 0.65m at high tides. Waves from the north may
also affect the site, although it is unclear how the bridges to the north will affect their passage.
A tidal channel run close to this site and it appears that the wave and tidal energy is sufficient to
remove debris from the toe of the cliffs over time. The process of wetting and drying of banks by
tides particularly at higher spring cycles will also weather the un-consolidated banks. Cliff erosion
rates along this site are estimated to be only 0.04m/y, although the slowness of this rate is likely to
be due to the amount of slope modification and engineering that has occurred during development
of the residential area. It has also made defining the rate of erosion somewhat problematical at this
location. It is highly likely that if this development had not occurred then rates of erosion at this
location would be significantly greater.
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Tauranga Inner Harbour Sites
MAUNGATAPU CENTRAL
Option
Financial
Timber
seawall
Not Viable
Not appropriate
Rock
revetment
Not Viable
Not appropriate
Rock groyne
Not Viable
Not appropriate
Submerged
breakwater
Viable
Mangrove
planting
Viable
Drainage
Viable
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Engineering
Environmental
Recreation
Health & safety and
Risk
Comment
Not likely to have
enough effect. Site
at low water line,
but risk of loss
with channel
migration. Will
slow erosion, not
stop
No issues. Will
not have enough
effect
Visual impact at
low tide, may form
habitat for
kaimoana
fish/shellfish
Obstruct
boating
Watercraft grounding,
drowning risk
Not appropriate
Adverse visual
impact at this
highly visible
location but
ecological value
Low, possible risk of
entrapment
Not appropriate
Borehole drains
horizontally to
reduce saturation.
May offer some
benefit at upper
sections of sea cliff
No impact
Potentially
restricted
access and
potential
impact on
boating
No impact
Some difficult access for
construction/maintenance
Potentially appropriate
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
22 Maungatapu Marae
22.1 Setting
Site Description
The shoreline at the location of Maungatapu Marae is a short section that points north in to
Rangataua Bay. The slope up from the shoreline stands at about 50° to 60° from the horizontal
and range in height from 16m to a maximum of 20 m. At the top of the slopes are numerous sports
fields and open spaces. It is not known what drainage may exist, if any at this location. The slope
itself is generally vegetated with shrubs and native grasses. In addition there are a number of large
mature native trees along both the crest and base of the slope.
A number of slips are evident at various locations along this section of shoreline, with the failed soil
mass including trees and bush extending out across the beach below.
At high tide, the water level covers the sandy beach and active removal of slip debris is occurring.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
22.2 Geotechnical Issues
Failure method
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Tauranga Inner Harbour Sites
Slope failures observed are typically shallow or superficial. The slope failures on the coastal cliffs
are typified by a bare near-vertical headscarpe, generally near the top of the slope.
The observations of slope stability at this site suggests that the steep slopes at this site are prone to
shallow slope failures, which are exacerbated by the undercutting of the slopes by the wave action
once failure has occurred, coastal erosion removes the slipped soil mass and the process of
undercutting continues.
22.3 Coastal Processes
This site is at the tip of Maungatapu Peninsula and is affected by winds form the northerly sector.
The average fetch to the north-west is 2.1 km. A fetch of this length given a 25 m/s wind storm
(representative of a 2% AEP event) would produce a theoretical significant wave height in the order
of 0.7 m at high tides. Waves from the north may also affect the site, although it is unclear how the
bridges to the north will affect their passage. The State Highway 2 road bridge to the north-east is
likely to disrupt the passage of waves from the north-east. This road bridge has also worked as a
groyne blocking the longshore drift of sediment into the estuary. The low tide beach where the
bridge abuts onto the shore has accreted up to approximately 100 m since its construction.
Tidal channels run close to this site on both sides of the point, although tidal flats of up to 65 m
protrudes from the tip. It appears that the wave and tidal energy is sufficient to remove debris from
the toe of the cliffs .over time. The process of wetting and drying of banks by tides particularly at
higher spring cycles will also weather the un-consolidated banks. Cliff erosion rates along this site
are estimated to be up to 0.2 m/y, although as banks tend to dropout in discrete events this erosion
is likely to be episodic rather than at a constant rate.
The area in front and adjacent to the Marae is low-lying and will become susceptible to inundation
under future sea-level rise predictions.
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Tauranga Inner Harbour Sites
MAUNGATAPU MARAE
Option
Financial
Engineering
Timber
seawall
Not viable
Rock
revetment
Viable
High cliffs would
require large
structure
Will provide
possible toe weight
to higher sea cliffs
Rock
groyne
Not viable
Submerged Viable
breakwater
Mangrove
planting
Viable
Drainage
Viable
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Environmental
Recreation
Health & safety and
Risk
Comment
Not appropriate
Possible adverse
impact, visually
Temporary
access routes will
need
consideration
Potential loss
of access to
beach at low
tide
Slipping and falling in voids
- risk of injury
Appropriate option, possibly in
conjunction with drainage
Not appropriate
Not likely to have
enough effect. Site
at low water line,
but risk of loss
with channel
migration. Will
slow erosion, not
stop
Inadequate effect
Visual impact at
low tide, may
form habitat for
kaimoana
fish/shellfish
Borehole drains
horizontally to
reduce saturation
No impact
Obstruct
boating
Watercraft grounding,
drowning risk
Not appropriate
Not appropriate
No impact
Some difficult access for
construction/maintenance
Appropriate option
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
23 Te Hono Street North
23.1 Setting
Site description
The shoreline at Te Hono Street North trends roughly north-west to south-east, before turning
north to south at the location of Rotary Park.
The slope up from the shoreline stands at about 50° to 60° from the horizontal and range in height
from 7m to a maximum of approximately 24m. The slopes are generally vegetated with shrubs and
native grasses, whilst also containing a number of large mature native trees along both the crest
and base of the slope. At the base of the cliffs, a shallow sandy beach extends out in to the Inner
harbour, which at high tide is covered. This section of coastline has been heavily protected by rock
walls, gabion baskets and timber retaining walls. It should be noted though that many of these
protection measures look as though they are not properly designed and appear to have been put
together by local residents.
At the top of the sea cliffs, are the residential developments along Te Hono Street. Privately owned
gardens extend from the top of the cliffs to the residencies. Based on the Tauranga City Council
website, it would appear as though all stormwater is piped away from the area. The general
topography of the area is trending downwards towards the coast line at this location.
Geology
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The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
23.2 Geotechnical Issues
Failure method
The observations of slope stability at this site suggests that the steep slopes would have been prone
to shallow slope failures in the past. However, the extensive residential development along the
crest of this cliff line, combined with the re-engineering of the slope profile that appears to have
occurred, disguises the number of slip failures and true rate of erosion at this location.
In addition, coastal erosion of the shoreline has been restricted somewhat by the placement of the
ad-hoc retaining structures, although the effectiveness and longevity of these structures cannot be
relied upon. Never-the-less, regression of slips and rates of coastal erosion at this location have
been difficult to accurately predict due to the amount of engineering that has occurred at this
location.
23.3 Coastal Processes
This site is located along the north-eastern side of the Maungatapu Peninsula and is affected by
winds from the northerly sector. The average fetch to the north-east is 2.4km. A fetch of this length
given a 25 m/s wind storm (representative of a 2% AEP event) would produce a theoretical
significant wave height in the order of 0.75m at high tides. Waves from the north may also affect
the site, although it is unclear how the State Highway bridge to the north will affect their passage.
Localised turbulence and scour may occur at the northern end of this site with flood tidal currents
and/or waves interacting with the rock rip-rap associated with the road bridge.
A tidal channel runs close to this site and it appears that the wave and tidal energy is sufficient to
remove debris from the toe of the cliffs over time. The process of wetting and drying of banks by
tides particularly at higher spring cycles will also weather the un-consolidated banks. Cliff erosion
rates along this site are estimated to be only 0.11m/y, although the slowness of this rate is likely to
be due to the amount of slope modification and engineering that has occurred during development
of the residential area. It has also made defining the rate of erosion somewhat problematical at this
location. It is highly likely that if this development had not occurred then rates of erosion at this
location would be significantly greater.
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Tauranga Inner Harbour Sites
TE HONO STREET NORTH
Option
Financial
Timber
seawall
Not viable
Rock
revetment
Viable
Rock groyne
Not viable
Submerged
breakwater
Viable
Mangrove
planting
Viable
Drainage
Viable
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Engineering
Environmental
Recreation
Health & safety and Risk
Comment
Not appropriate
Will provide
possible toe weight
to higher sea cliffs
Possible adverse
impact, visually
Temporary access
routes will need
consideration
Potential loss of
access to beach
at low tide
Slipping and falling in voids risk of injury
Appropriate option,
possibly in
conjunction with
drainage
Not appropriate
Not likely to have
enough effect. Site
at low water line,
but risk of loss with
channel migration.
Will slow erosion,
not stop
No issues. Will not
have enough effect
Borehole drains
horizontally to
reduce saturation
Visual impact at
low tide, may form
habitat for
kaimoana
fish/shellfish
Obstruct boating
Watercraft grounding,
drowning risk
Not appropriate
Adverse visual
impact at this
highly visible
location but
ecological value
No impact
Potentially
restricted access
and potential
impact on
boating
No impact
Low, possible risk of
entrapment
Not appropriate
Some difficult access for
construction/maintenance
Appropriate option
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
24 Te Hono Street South
24.1 Setting
Site description
The shoreline at Te Hono Street South trends roughly west to east, with slopes up from the
shoreline standing at about 50° to 60° from the horizontal. Slopes are typically 25m in height at
this location. The slopes are generally vegetated with shrubs and native grasses, whilst also
containing a number of large mature native trees along both the crest and base of the slope. At the
base of the cliffs, a shallow sandy beach extends out in to the Inner harbour, which at high tide is
covered.
At the top of the sea cliffs, are the residential developments along Te Hono Street. Privately owned
gardens extend from the top of the cliffs to the residencies. Based on the Tauranga City Council
website, it would appear as though all stormwater is piped away from the area. The general
topography of the area is trending downwards towards the coast line at this location.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
Based on our observations these Quaternary Deposits would appear to be underlain by welded and
non-welded Ignimbrites.
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24.2 Geotechnical Issues
Failure method
In areas where the welded Ignimbrite is exposed, the slope failures on the coastal cliffs are typified
by a bare near-vertical cliffs, with undercutting occurring at the toe. Some support is provided to
the cliff face by the significant tree root system from above. The slipped material collects at the
base of the slope, which ordinarily would provide support against on-going regression. However,
this toe weighting is removed by coastal processes to leave unsupported near vertical and vertical
sea cliffs.
In other areas where the welded Ignimbrite is not evident, then the dominant slope failure
observed is typically shallow or superficial. These slope failures are typified by a bare near-vertical
headscarpe, generally near the top of the slope.
The observations of slope stability suggests that regardless of the mode of failure at this location,
failures are exacerbated by the undercutting of the slopes and removal of slipped material by the
wave action. Removal of the slipped soil mass causes the process of undercutting and
destabilisation to continue.
24.3 Coastal Processes
Te Hono Street South is exposed to the south and is exposed to prevailing winds from this
direction. The average fetch to the south-west is 0.73km. A fetch of this length given a 25 m/s wind
storm (representative of a 2% AEP event) would produce a theoretical significant wave height in
the order of 0.45m at high tides.
Tidal fluctuation may also be adding to the erosion with the process of wetting and drying
particularly at higher spring cycles weathering the un-consolidated banks. Erosion rates along
these cliffs are estimated to be up to 0.02m/y, although as banks tend to dropout in discrete events
this erosion is likely to be episodic rather than at a constant rate.
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Tauranga Inner Harbour Sites
TE HONO STREET SOUTH
Option
Financial
Timber
seawall
Not viable
Not appropriate
Rock
revetment
Not viable
Not appropriate
Rock groyne
Not viable
Not appropriate
Submerged
breakwater
Viable
No issues. Will
reduce but not
eliminate erosion
Mangrove
planting
Viable
No issues. Will not
have enough effect
Drainage
Viable
Borehole drains
horizontally to
reduce saturation
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Engineering
Environmental
Adverse visual
impact at this
highly visible
location
Adverse visual
impact at this
highly visible
location but
ecological value
No impact
Recreation
Health & safety and Risk
Comment
Potential impact
on boating
Low, possible risk of
entrapment
Small rock revetment
is better option
Potentially
restricted access
and potential
impact on
boating
No impact
Low, possible risk of
entrapment
Not appropriate
Some difficult access for
construction/maintenance
Appropriate option
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
25 Matapihi East
25.1 Setting
Site Description
The shoreline at the location of Matapihi East extends along the north-western side of Rangataua
Bay. The slope up from the shoreline stands at about 50° to 60° from the horizontal and are
generally vegetated with shrubs and native grasses. In addition there are a number of large mature
native trees along both the crest and base of the slope. However, at some locations trees along the
crest of the slope had been removed. A number of slips are evident at various locations along the
shoreline, with the failed soil extending out across the beach below.
At the top of the slopes is a reserve, beyond which is SH2. At its closest, SH2 is about 10 - 15 m
from the crest of the slopes. A number of surface tension cracks are visible within 1 m or 2 m from
the crest of the slope.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
25.2 Geotechnical Issues
Failure method
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Slope failures observed are typically shallow or superficial. The slope failures on the coastal cliffs
are typified by a bare near-vertical headscarpe, generally near the top of the slope.
The observations of slope stability at this site suggests that the steep slopes at this site are prone to
shallow slope failures, which are exacerbated by the undercutting of the slopes by the wave action
once failure has occurred, coastal erosion removes the slipped soil mass and the process of
undercutting continues.
25.3 Coastal Processes
This site is a straight coastline which fronts the road reserve of the SH 2 as it passes the estuary
known as Te Tahuna O Rangataua. This coastline is one of the most sheltered of the study sites. It
faces south-east, from which very few winds originate, and has a small fetch of 2 km over a shallow
estuary. A fetch of this length given a 25 m/s wind storm (representative of a 2% AEP event) would
produce a theoretical significant wave height in the order of 0.65 m at high tides. Although the tidal
flats of the estuary would dissipate much of the wave energy. This site is more likely to be affected
by tides and tidal currents. A large channel runs very close to the banks along the length of this site,
which may be acting to scour the toe of the banks.
The process of wetting and drying of unprotected banks by tides particularly at higher spring cycles
will also weather the un-consolidated banks. Cliff erosion rates along this site are estimated to be
up to 0.2 m/y, although as banks tend to dropout in discrete events this erosion is likely to be
episodic rather than at a constant rate.
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Tauranga Inner Harbour Sites
MATAPIHI EAST
81
Option
Financial
Timber seawall
Not viable
Not appropriate
Rock revetment
Not viable
Not appropriate
Rock groyne
Not viable
Not appropriate
Submerged
breakwater
Not viable
Not appropriate
Mangrove
planting
Potentially
viable
Drainage
Not viable
| August 2015
Engineering
No issues, but little
impact on overall
erosion
Environmental
Good ecological
habitat, fish
nursery, but
potential adverse
visual impact
Recreation
Little impact at
this location
Health & safety and Risk
Low, possible risk of
entrapment
Comment
Potentially
appropriate
Not appropriate
Opus International Consultants Ltd
Tauranga Inner Harbour Sites
26 Whareoa Marae
26.1 Setting
Site Description
This section of the coast line trends north-west to south-east, with the shoreline having a southwest aspect. This section of shoreline is flat and low-lying, with a maximum height of only 2 m
above the shoreline. At this location there is a small sandy beach, which at high tide is only about
2 m wide. Above the shoreline is a grassed area on which the Marae is located. Localised
undercutting of this grassed area is occurring, presumably at high tide.
Geology
The Geological Map of the area indicates that the geology is dominated by Quaternary fluvial
deposits of the Tauranga Group. This comprises alluvium pumice, crystal and ash rich sediments
re-worked from middle quaternary ignimbrites.
26.2 Geotechnical Issues
There are no geotechnical issues/coastal cliffs associated with this site.
26.3 Coastal Processes
Whareroa Marae a beach site situated near Tauranga Airport. Although this site faces into the
predominant south-west wind it is sheltered by the rock rip-rap revetment associated with the Te
Awanui Road bridge. Therefore waves affecting this site propagate from the south to south- east,
the average fetch across the estuary from this direction is approximately 2 km. A fetch of this
length given a 25 m/s wind storm (representative of a 2% AEP event) would produce a theoretical
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Tauranga Inner Harbour Sites
significant wave height in the order of 0.65 m at high tides. Although the tidal flats of the estuary
would dissipate much of the wave energy. Wave action has shown to cut into the low grassy
shoreline at high tides. Small groynes have been constructed along the eastern section of the beach
which has experienced historical erosion of approximately 0.1 m/y.
A small tidal channel runs across the front of the beach which may work to scour sand material
from the low tide beach. The dominant longshore sediment transport direction appears to be
associated with the outgoing ebbtide as evidenced by the sediment mounding up on the eastern
side of these small groynes.
This site is very low-lying and will become a susceptible to inundation under future sea-level rise
predictions.
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Tauranga Inner Harbour Sites
WHAREROA MARAE
84
Option
Financial
Timber seawall
Not viable
Not appropriate
Rock revetment
Not viable
Not appropriate
Rock groyne
Not viable
Not appropriate
Submerged
breakwater
Not viable
Not appropriate
Mangrove
planting
Potentially
viable
Drainage
Not viable
| August 2015
Engineering
No issues. Very little
impact on overall
erosion
Environmental
Good ecological
habitat, fish
nursery, but
potential adverse
visual impact
Recreation
Restricted
access due to
plant growth
Health & safety and Risk
Possible risk of entrapment
Comment
Potentially
appropriate
Not appropriate
Opus International Consultants Ltd