Poruma Sea Wall Feasibility Study - Torres Strait Regional Authority

Transcription

PORUMA SEA WALL FEASIBILITY STUDY
October 2011
Prepared for the Torres Strait Regional Authority by AECOM
AECOM
Poruma Sea Wall Feasibility Study
Prepared for
Torres Strait Regional Authority (John Rainbird)
Prepared by
AECOM Australia Pty Ltd
Level 8, 540 Wickham Street, PO Box 1307, Fortitude Valley QLD 4006, Australia
T +61 7 3553 2000 F +61 7 3553 2050 www.aecom.com
ABN 20 093 846 925
27 October 2011
60213761
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Revision 2 - 27 October 2011
AECOM
Quality Information
Document
Ref
60213761
Date
27 October 2011
Prepared by
Stuart Bettington
Reviewed by
Scott Snelling
Revision History
Revision
0
Revision
Date
Authorised
Details
Draft
1
16-May2011
11-Jul-2011
2
07-Nov-2011
Wave Return Walls Added
Final Issue
Name/Position
Signature
James Jentz
Associate Director
James Jentz
Associate Director
James Jentz
Associate Director
Signed Previously
Signed Previously
AECOM
Table of Contents
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
Introduction
Coastal Processes
2.1
Metocean
2.2
Sand Supply
2.3
Bathymetry
2.4
Sand Movements
2.5
Beach Rock
Identified Areas of Concern
3.1
Site Inspection
3.2
South western corner of island
3.3
Northern Foreshore East of Boat Ramp/Jetty
Seawall Design Inputs/Assumptions
4.1
Design Life
4.2
Climate Change
4.3
Design Event
4.4
Water Level
4.5
Water Depths
4.6
Wave Climate
4.7
Land Levels/Overtopping
4.8
Beach Levels
Seawall Design Solutions (SW corner)
5.1
Seawall Extent
5.1.1
Preferred option (310m)
5.1.2
Alternative short wall option (100m)
5.2
Conventional Rock Armour
5.3
Pattern Placed Concrete Seabee Units.
5.4
Sand Filled Geotextile Bags
5.5
Wave Deflection Wall
Beach Nourishment
6.1
Bypassing
6.2
Dredging
Opinion of Probable Construction Costs (OPCC)
7.1
Basis
7.2
Results
7.3
Explanation of Assumptions for Critical Items
7.3.1
Site Establishment
7.3.2
Rock Bedding and Armouring
7.3.3
Reinforced Concrete for Capping Layer
7.3.4
Seabee Units
7.3.5
Supply and Placement of Geotextile Sand Bags
7.3.6
Supply and Installation of Wave Deflector Walls
Summary and Recommendations
References
1
2
2
2
2
3
4
5
5
5
7
8
8
8
9
9
9
9
10
10
10
10
A
B
B
C
E
F
G
G
G
H
H
H
I
I
I
J
J
J
J
J
L
Appendix A
Conventional Rock Armour Sea Wall
A
Appendix B
Seabee Wall
B
Appendix C
Geotextile Bag Wall
C
Appendix D
Typical Section of the Wave Deflection Wall
D
AECOM
1.0
1
Introduction
Poruma (Coconut Island) is a low lying Coral Cay situated in the central Torres Strait. Ongoing erosion issues
have led the Torres Strait Regional Authority (TSRA) and the Torres Strait Island Regional Council (TSIRC) to
consider engineering solutions to protect critical community infrastructure. This report looks at the problems,
develops and assesses a number of solutions and provides cost estimates for these solutions.
Two areas of concern were raised during the site inspection. These were at the western end of island, near the
resort, and the area on the northern side of the island east of the jetty. The scope of this study is largely confined
to the western island erosion problem but some comment on the issues near the jetty has been included.
Reference should be made to Figure 1 for an aerial photograph of Poruma indicating the areas of concern.
Figure 1
Oblique aerial of Poruma indicating areas of concern April 2011
For the western part of the island detailed assessments, design and estimates for three seawall solutions have
been prepared. These include a conventional rock armour sea wall, a pattern placed concrete armour seawall
using Seabee units and a softer geotextile bag solution.
In addition rough estimates of volumes and costs for a maintenance dredge nourishment program have been
developed.
It should be noted that the seawall designs prepared are to stabilise the foreshore location and are not intended to
reduce the risk of the island to oceanic inundation.
AECOM
2.0
Coastal Processes
2.1
Metocean
2
There are two prevailing wind seasons in the Torres Strait. They are the dominant South Easterly winds that blow
for 9 months of the year from March to December, and the equally strong but less persistent North Westerly winds
that blow for 3 months from December to March. Associated with these winds are local seas and reef top
currents. The winds, seas and currents all contribute to the annual sand movements around the island.
Tides at the island are defined as semi diurnal, though they have a strong diurnal bias. The astronomical tidal
range is large at 4m, with a spring range of 2.4m. The tidal plans are:
Poruma Island Tides (Hydrographic Service - RAN)
Tidal Plane
Level (m above LAT)
HAT
4.4 (estimated)
MHWS
3.2
MHWN
2.2
MSL
2.0
MLWN
1.8
MLWS
0.8
LAT
0.0
Note: Datum levels in the Torres Strait are considered to be unreliable. There is some doubt over the exact levels
of tidal planes nominated in this table.
Significant tidal anomalies can occur in the Torres Strait. One cause of these are strong wind fields that force
water into the strait. Data from tide gauges in the area (ref Duce etal 2010) indicates an annual season variation
(on Booby Island) of approximately 0.5m is typical with the summer NW winds producing the higher water levels.
Another site on Goods Island indicated that sea level anomalies of up to 1.1m have been measured, though again
the large variations were in summer, while typical maximum monthly variations were less than 0.4m.
For seawall design these seasonal anomalies have not been incorporated into the assessment of water level.
Rather storm surge has been adopted for the design event. This decision was made based on the consideration
that measured water level anomalies relative to tidal forecasts are incorporated in the assessment of storm surge
and the inclusion of season fluctuations would be effectively doubling up on the impact of these anomalies.
2.2
Sand Supply
Sands on the island are carbonate based and are derived from coral and shells of the reef. Living coral reef
generates approximately 5kg of sand per square meter annually. This sand is washed across the reef platform
and supplies the islands beaches. This material is typically coarser than quartz based beach sand and results in
relatively steep beaches, as seen in Figures 4 and 6.
2.3
Bathymetry
Poruma is a cigar shaped island that is approximately 2,000 m long on the East - West axis and 300m wide. It is
located on the NW corner of a coral reef platform that is 6,000m long in and 2,000m wide. The island location on
the reef platform reflects the prevailing SE wind conditions. To the North of the island there is only 100m from the
vegetation line to the edge of the reef. Reference should be made to Figure 2 for further details.
AECOM
Figure 2
3
Poruma Island and Coral Reef Platform (Note sand shoals over central reef)
The coral platform on the southern and eastern sides of the islands sits at approximately mean sea level (+2m
LAT); while the platform on the northern side of the islands is approximately 1 m lower near spring lower water. It
is understood that the reef platforms carries through under the island and this surface would provide a good
foundation for any works.
Drainage channels that funnel water off the reef during ebbing tides and from wave induced flux are present at a
few locations around the reef. One of these channels is to be found near the western end of the island and plays
an important role in the sediment budget of the island.
A drop off exists around the reef edge that slopes to deep water. Once sands passes over this edge it is lost to
the islands sand budget.
2.4
Sand Movements
Sand movements around the island have a seasonal bias. During the NW winds sand moves towards the east on
the north side of the island and the western end of the island develops a spit that pushes towards the south.
During the SE winds sand from the reef platform is forced towards the island and sand from the southern beaches
moves to the north and west, while the spit on the western extremity develops into a lobe of sand that pushes
north. Year to year the magnitude and extent of the sand movements varies and this leads to periods of build up
or erosion over the decades on various parts of the island.
Interference with the natural movements of the sand can upset the delicate balance of the sand budget in various
parts of the island. Examination of the beaches during the site visit (at the end of the NW wind season) and aerial
photography indicate that the construction of a channel and boat ramp, and the associated groynes and
breakwaters have lead to an interruption of the natural sand flow around the island. This development is
preventing the movement of sand eastward along the northern foreshore of the island. As seen in Figure 3 this
has resulted in a wide beach between the jetty and the western end of the island. To the east of the jetty and
ramp the beach is starved of sand.
The primary mechanism for sand loss from the reef is the channel to the west of the island. Sand on the
meandering western spit that drops into this channel is transported off the reef by strong ebb currents. It was
noted on site (ref Duce, etal 2010) that a shoal of sand stretches off in a NE direction from the western tip of the
reef and provides evidence of this process.
AECOM
Figure 3
2.5
4
Northern foreshore taken from western end of the island looking east towards Jetty April 2011(at approx. MSL = +2m LAT) –
Note this beach is wide when compared with other locations on the island.
Beach Rock
An important feature of the island is the extensive beach rock deposits. This material is formed when the buried
carbonate sands are exposed to alternating fresh and salt water in the tidal zone. This material is still forming
today as seen in the young (weak and pale) beach rock material found in some areas experiencing erosion. The
deposits of beach rock on the island can extend to elevations well above the present day tidal range, indicating
different sea levels in the past.
On Poruma the beach rock deposits offer a considerable degree of protection to the foreshore location through
the central parts of the island. That is to say despite the beach material eroding away the beach rock prevents
the shoreline from reseeding further, such as the northern foreshore area east of the jetty. This material is less
common and at lower levels over the western end of the island where ongoing erosion issues and large seasonal
fluctuations are causing concern.
Given the nature of the island areas lacking extensive beach rock are far more susceptible to significant erosion
events. It is likely that beach rock formations provide a useful insight into the past fluctuations in Island extent.
This is further supported by the extent of the southern foreshore dune that abruptly ends at the same location the
extensive beach rock formation ends. Based on this observation the western end of the Island, beyond Opeta
Street, could be considered vulnerable to significant coastal erosion.
It is understood that typically the beach rock is formed in thin layers (a few hundred mm) and of inconsistent
strength. The material cannot be relied on as structural base for brittle coastal works (e.g. Seabees) but strong
beach rock may be a suitable foundation for robust structures (e.g. rock or geotextile bags). In addition the beach
rock outcrops can offer solid tie in points for the end of the proposed seawall works.
AECOM
3.0
Identified Areas of Concern
3.1
Site Inspection
5
A site inspection was undertaken on Poruma on 5 April 2011 by Stuart Bettington from AECOM and Kevin Parnell
of JCU. During the site inspection two areas of concern were identified. The primary concern for the site
inspection was a 300m length of foreshore on the south western corner of the island where ongoing erosion
issues are threatening property. Local residents were also keen to discuss the problems impacting the northern
foreshore west of the dredge channel and jetty. The location and extent of these areas is indicated in Figure 1.
The site inspection was carried out at the end of summer in the transition from the NW winds to the SE winds. As
seen in Figure 1 the sand spit on the western end of the island is at its southern most extent. The tidal level in
this photo is low at approximately +1m LAT or 1m below MSL.
Other areas of concern discussed during the site visit have not been addressed in this report.
3.2
South western corner of island
The western end of the island has been subject to large fluctuations over the decades and historic erosion scarps
can be found that define past shore lines. In recent years erosion has removed areas of mature vegetation and
threatened near shore properties (the two bungalows of the resort are a particular concern).
During the site inspection the area west of the resort had undergone recent accretion up to level of approximately
Highest Astronomical Tide (HAT) or +4.4m LAT, and planting of this area by locals had been undertaken in the
hope of stabilising these gains. At the rear of this recently vegetated lobe is a small erosion scarp approximately
1m high that defines the recent coastal retreat. This erosion line is approximately 12 m from the resort bungalow
structures.
The southern foreshore of the western end of the island has an active erosion scarp with mature trees falling onto
the beach as seen in Figure 4. This erosion problem extended for approximately 250m to the start of the
extensive beach rock formation on the south coast. This can be identified in aerial photos and plans by a step in
the coast near the corner of Murray and Opeta Streets. Infrequent beach rock outcrops at the western end of the
island provide areas of less erosion and small cusped beaches have formed between them.
The erosion on the southern beaches is within 20m of the resort buildings and during the recent TC Yasi event
some stop gap measures were implemented buy the local council to protect the upper beach. A low seawall
constructed using small lightweight sand bags was constructed as shown in Figure 5.
This structure has been vandalised (bags cut by kids) but despite this damage, and the small bags used the
seawall has withstood the conditions over the ensuing months. It is also apparent that despite the reflective
nature of seawall structures such as this sand has accreted at the base of the seawall.
Engineering solutions for this area have been assessed and costed as presented in later sections of this report.
AECOM
Figure 4
Erosion scarp on the North side of the western end of the island April 2011 (note undermined trees).
Figure 5
Temporary upper beach sandbag seawall near resort bungalows April 2011.
6
AECOM
3.3
7
Northern Foreshore East of Boat Ramp/Jetty
Although not part of the project scope an area of concern to the residents of Poruma is the northern foreshore
west of the jetty. Since the construction of the boat ramp and channel and associated groynes and breakwaters
the beach to the east has been starved of sediments during the summer NW winds. Over time this has resulted in
the loss of all but the upper beach along some 1000m of the foreshore. This section of the island has extensive
beach rock deposits that holds the coastline in the present location and offers a degree of protection to the land
behind the beach. Refer Figure 6 for details.
In response to the loss of beach and with concern for the road that backs the beach the locals have constructed a
makeshift sea wall along the upper beach. This structure consists of large coral boulders (taken from the channel
excavation), substantial pieces of concrete and smaller building materials (rubbish from demolitions). As a result
the structure is unsightly and when combined with the lack of sand makes the foreshore an unpleasant feature of
the island.
During the site inspection a number of ideas were suggested by the locals, including the construction of a more
formal (attractive) seawall, and the construction of a groyne field to trap sand. Given the extent of the area
considered, the extent of foreshore threatened will make construction of a more formal structure prohibitively
expensive. The construction of a groyne field would also be prohibitively expensive and would shift the erosion
problem to an area where the impact may be more severe (less substantial beach rock).
Figure 6
Foreshore east of jetty April 2011 with the tide approximately 0.5m below Mean Sea Level (+1.5m LAT). Note the significant
step in beach sand across the groyne.
Costed engineered solutions have not been assessed for this area as it is beyond the current scope. The
following recommendations are made from the limited knowledge of the design and use of the islands jetty/boat
ramp facility.
Clearly long term net littoral transport of sand on the northern side of Poruma is to the east, primarily driven by the
prevailing NW conditions that exist in the summer months. The construction of the boat ramp and groynes has
AECOM
8
adversely impact on the transport of sand along the northern foreshore of Poruma. To return the island to a more
natural balance this interruption of flow needs to be reduced. It is recommended that:
-
The eastern groyne should be removed. It appears that this structure has been built to prevent sand ingress
from the east; clearly it is not functioning as intended.
-
Consider the removal of the breakwater offshore from jetty. This structure generates a wave shadow in
which sand accumulates and acts as a partial groyne. If the removal of all or part of this structure can be
achieved without compromising wharf/boat ramp usability then it will improve sand mobility.
-
If the beach to the east does not recover sufficiently with the above measures then implement a program of
sand bypassing, taking sand from the lower beach on the western side of the boat ramp and depositing on
the upper beach approximately 50m east of the dredged channel. Any bypassing should only be done
during the NW seasonal conditions when sand transport is most active on the north side of the island. The
bypassing volumes would be modest and in line with natural transport mechanisms and would need to be
accessed on an ongoing basis. Bypassing can be achieved by either mechanical means (tip truck and
excavator) or with a slurry pump and buried delivery pipe.
The issue of the unattractive haphazard seawall needs to be addressed in conjunction with improving sand
transport on the island. It is also assumed that the funding will not stretch to the construction of a more formal
engineered seawall that is probably not required on this length of coast due to the substantial beach rock present.
It is recommended that:
-
Any large coral armour units won from the demolition of the groyne/breakwater plus any units already in use
in the seawall be used to construct a single layer seawall at a slope of 1:1.5 to bury the existing structure.
This work would cover a very limited length of seawall and should be used in areas of most concern.
-
In other areas it is recommended that sand be won from the western beach and used to bury the protection
works by creating a low dune. This dune would then need to be stabilised with vegetation.
4.0
Seawall Design Inputs/Assumptions
These assumptions have been used in the preparation of preliminary designs for the purposes of determining
costs. The sizing of armour and the height of the structure are impacted by the design inputs, with the design
particularly sensitive to water level.
It is recommended that these design assumptions be reviewed in more detail during the detailed design phase.
4.1
Design Life
A design life of 50 years has been adopted for the design of the seawall options, with a design end of life at 2070.
This is consistent with a normal commercial structure as set out AS 4997-2005 “Guidelines for the design of
maritime structures” and is equivalent to the design life that would be adopted for a residential structure in the
“Queensland Coastal Plan” (ref DERM 2011).
4.2
Climate Change
The primary area of concern relating to climate for the design of the proposed structure is sea level rise. For a
design horizon of 2070 the suggested sea level rise allowance in “Queensland Coastal Plan” is 0.5m. Note that if
a longer design life were being considered the nominated sea level by 2100 is 0.8m.
Other potential impacts relating increased storminess, reduction or increase in coral productions or changes to
seasonal patterns will have little to no impact on design.
It is noted that climate change impacts on low islands such as Poruma could be catastrophic. It is anticipated that
the coral platform on which the island is positioned will respond to changes in sea level by growing vertically in
line with the sea level changes. However, this will not provide a boost to the island level in the short term as the
gradual increase in island level as windblown sand is brought onto the island will take substantial time to be
realised.
AECOM
4.3
9
Design Event
As the function of the proposed structures offers a low degree of hazard to life or property a design event with a
0.5% AEP (Annual Exceedance Probability) or 200 year ARI (Average Recurrence Interval) has been adopted.
This is consistent with the defined design events in both AS 4997-2005 and the Queensland Coastal Plan.
4.4
Water Level
For most of Queensland storm surge associated with powerful weather systems such as cyclones combined with
tides define design water levels. The Torres Strait experience few cyclonic systems directly and less again that
are powerful enough to drive up significant surges. Recorded data show that significant water level variations can
occur due to remote systems and seasonal influences.
Site specific data or estimates for design water levels during significant wave events are not available for Poruma,
and the scope of this study does not include allowance for a detailed Metocean assessment. Forecast design
water levels are provided in the publication “Storm Tide Threat in Queensland: History, Prediction and Relative
Risks” (ref Harper, 1998) for locations in Far North Queensland. In this document the forecast storm levels for
Cape Flattery and Lookout Point are HAT for a 100 year ARI event and 0.1m above HAT for a 500 year ARI
event. Thus adopted water level for a 200 year ARI event will be approximately HAT (4.4m LAT).
It is noted that this approach is based on spatial assessment of storm tide risks, considering storm frequency and
intensity plus near shore bathymetry. By comparison the “Queensland Coastal Plan” nominates a level of 2m
above HAT as at risk of coastal inundation. This approach is broad brush and conservative for the Torres Strait,
and as such has not been adopted for this study.
Over the reef water levels will be perched above surrounding sea levels due to wave setup. Conservatively an
allowance of 0.2m has been adopted for wave setup.
Applying wave setup (0.2m) with the design water level (4.4m LAT) and the adopted climate change sea level rise
(0.5m) yields a design water level at the island of 5.1m LAT.
To confirm the design estimate for sea level (above) has been compared with the forecast sea levels developed
by Bruce Harper for the TSRA. The estimates provided in the Bruce Harper assessment are focused on the
inundation of dwellings and are related to the minimum habitation levels. The design storm 200 year ARI surge
levels in 2050 at Poruma were estimated to be approximately 0.1m above the lowest habitation level. For Poruma
the lowest habitation level was nominated as 2.9m above MSL which is equivalent to 4.9m LAT. It follows that the
design storm surge level using these numbers is +5.0m LAT, and it confirms that the numbers provided above are
appropriate.
Although significant water level variations (over 1m) have been recorded in recent decades the probability that
these will occur in concurrence with a large astronomical tide and a significant wave event form the appropriate
side of the island is very unlikely. Thus the adoption of a water level only slightly above HAT is considered
reasonable for design.
4.5
Water Depths
For the purpose of design the reef top is considered to be flat at a level at or just below mean sea level. The
adopted reef level is 2.0m LAT. Subtracting the adopted reef level (2.0m LAT) from the design water level of 5.1m
LAT, a design event water depth of 3.1m is calculated.
4.6
Wave Climate
Again no site specific wave data is available. However, due to the extensive reef flats that surround the island
even moderate wave events will result in waves breaking on the reef edge and over the reef flat. That is to say
the waves reaching the shore are depth limited.
Studies of wave climates over flat reef tops revealed that the maximum sustainable wave height is 0.55 times the
depth of water. For a design water depth of 3.1m the largest design wave that can reach the island is 1.7m.
When this limiting wave height is considered the relevant design wave heights in the depth limited wave spectrum
are:
H 2% = 1.64m
(height exceeded by 2% largest waves)
AECOM
H 1/10 = 1.55m
(average height of largest 10% of waves)
H s = 1.35m
(significant wave height or average height of largest third of waves)
10
It is assumed that these waves will be short crested (choppy) with wave periods of less than 5 seconds.
Note that the wave height reaching the revetment defines the size of armour required for a stable revetment.
4.7
Land Levels/Overtopping
Land levels on the SW foreshore were taken from limited data sourced from the sewage scheme project being
undertaken by AECOM. The land levels along the length of the proposed seawall are relatively uniform at
approximately 4m AHD which equates to 6.0m LAT. This is approximately 900mm higher than the design water
level.
It is assumed that the seawalls will be founded on the solid reef platform at approximately 2.0m LAT (MSL). It is
possible that the rock or geotextile bag seawall solutions will be able to achieve savings by founding on higher
beach rock layers. The total sea wall height will be approximately 4.0m in the absence of shallow rock layers.
4.8
Beach Levels
It is anticipated that the seawall be a last line of defence and that under normal conditions hopefully the structure
will be buried in the beach. To construct the seawall beach material will need to excavate down to solid
foundations on either substantial beach rock or the reef platform (estimated to be 2.0m LAT under the beach).
Once the seawall is built the beach should be re-instated to its original level and profile.
It would be appropriate to undertake a dune stabilisation program after seawall construction over the crest of the
seawall.
5.0
Seawall Design Solutions (SW corner)
5.1
Seawall Extent
Three seawall options have been assessed, conceptually designed and costed. In all solutions it is assumed that
the seawall will be constructed on seaward face of existing or historic erosion scrap lines. For the areas west of
the resort this is the 1m high scarp approximately 12m in front of the bungalows, while for areas on the northern
foreshore this is the present day vegetation line. Two seawall extent options have been considered as shown in
Figure 7. These are a 310m long seawall and a lesser 100m long structure.
A 310m long seawall is the preferred solution, though it is likely that the costs may prevent this option from being
constructed. An alternative shorter option has been developed that only protects the western end of the area of
concern, basically protecting the corner of the island where the resort is located.
AECOM
Figure 7
Sewall extent options.
1
AECOM
5.1.1
Preferred option (310m)
The extent of seawall required to protect the eroding SW corner of the island is 310m. As discussed previously
this length of coast has little protection from beach rock and is experiencing ongoing erosion. With assets and
beach amenity at risk for entire length this is considered the preferred solution.
This solution would see the western end of the seawall finish just north of the resort bungalows (refer Figure 8)
where it will be protected by the extensive sand build up on the north western corner of the island. Additional
protection to this more exposed end can be achieved by turning the end of seawall to include a land tie back.
This tie back should be approximately 5m long, including seawall end details.
Note that for any bends, including the land tie back, a minimum radius for the crest of the bend shall be 5m.
The eastern end is positioned to finish in the lee of the extensive beach rock outcrops that occur in front of the
southern dune system (refer Figure 9).
Figure 8
Proposed western end of seawall – note historic erosion scarp (covered by vegetation).
A
AECOM
Figure 9
5.1.2
B
Eastern end of 310m seawall option
Alternative short wall option (100m)
Although protecting the full length of foreshore is desired the costs may be beyond the budget limits. A shorter
option has been considered that protects only the corner near the resort. This solution will not prevent erosion in
unprotected areas, but will act to reduce erosion rates in these areas as the beach reseeds. For this solution the
western end would have the same location (refer Figure 8), while the eastern end would finish behind a lesser
beach rock outcrop (refer Figure 9). Both ends of this structure would need to incorporate land tie backs.
5.2
Conventional Rock Armour
Normally a conventional double layer rock armour sea wall is the most economical and robust solution and thus is
usually preferred for coastal protection where suitable sized rock is available. For Poruma no substantial rock is
available on the island so the rock will need to be imported. For a relatively small project such as this rock would
need to be sourced from an established large rock quarry, preferably with experience supplying armour rock
grade material. A quarry exists on Horn Island and rock armour can be produced on demand. An economically
attractive alternative to sourcing rock off the island is the mining of the reef material such as the armour used
around the boat ramp and jetty. This option has been discarded for this study on the grounds of environmental
and community concern, however, it is a commonly used armour material on tropical islands in developing
countries.
Rock armour is durable and massive seawalls built of large rock are robust, which means that even if conditions
exceed design the seawall though damaged will remain functional in some form. This is an important feature in a
location with little metocean data and considering possible impacts of climate change into the more distant future.
AECOM
Figure 10
C
Rock armour seawall partially buried at rear of beach - Moreton Bay
Rock armour units were sized using Hudson Formula and checked using the Van der Meers Formulae and a
revetment slope of 1:1.5 (ref. CIRIA 2007). In the design allowance has been made for minor damage to the
3
seawall. Assuming that reasonable rock can be sourced the adopted rock armour density is 2.6t/m . This
resulted in median primary armour of 300kg, resulting in a double layer armour thickness of approximately 1m.
A geotextile filter layer shall be laid under the rock to prevent movement of sand through the seawall armour. This
geotextile material shall be a needle punched non-woven fabric Class E (per NSW RTA Guideline). A suitable
material that meets specifications is Elcomax 1200R with a drop height of up to 1m for the armour. To ensure
complete coverage a minimum overlap of 0.5m at sheet edges is required. After the revetment is constructed the
beach is reinstated over the structure, as per Figure 10.
The design cross-section and suggested armour grading are presented in Appendix A. The ends of seawall can
be finished abruptly with no special details other than land lie backs were required.
5.3
Pattern Placed Concrete Seabee Units.
Pattern placed armour behaves as a mattress, with the units held in place by the units surrounding them. This
allows significant savings in volume/mass of armour required over rock to achieve an equivalent level of
protection. Concrete armour units such as Seabees also offer a neat visually pleasing solution. Because of the
relatively light units involved and reduced volumes of material required seawalls of this type are an attractive
option in remote locations such as the Torres Strait Islands. Figure 11 presents a view of Seabee seawall
constructed on Boigu Island. These were installed on the northern foreshore in the late 1990s and have
successfully withstood climatic conditions since. One of their main reasons why Seabee walls were adopted at
Boigu was the opportunity for local labour to assist in the construction process, hence reducing the cost of the
project. The Seabee units were cast on site and Boigu locals were employed by the Principal Contractor to pour
and place them (under supervision).
AECOM
Figure 11
D
Seabee Seawall on Boigu Island.
Seabees armour units rely on interlocking to hold them in place, and a single unit offers little resistance to wave
attack. If a seawall of this type is damaged the seawall can quickly unravel, resulting in a catastrophic failure of
the structure. Because of this failure mechanism seawalls constructed of relatively light interlocking units such as
Seabees are said to have a brittle failure mechanism. During construction care is required to ensure that this
interlocking between units is achieved.
To help ensure that the seawall remains intact strong edges are required. The toe of the seawall needs to be well
founded, which on Poruma means it will need to be cut into the reef platform. The crest and ends require suitable
fixing with a concrete beam.
Other issues with this type of sea wall include high wave run-up and high reflectivity. This heightened wave action
on the face discourages sand build up on the seawall.
Using standard tables from the University of NSW Seabees Design Manual (ref. UNSW 1997) for a seawall at a
slope of 1:1.5 with an armour porosity of 35% the nominated design is a single layer of 0.3m high by 0.3m wide
units weighing 27kg each. This armour is laid over 0.3m thick layer of 60 to 160mm secondary rock armour.
Beneath the secondary a light geotextile layer is required to ensure that the sub soil profile remains stable. For
this a suitable material would be Elcomax 360R.
The design cross-section for a Seabee seawall is presented in Appendix B.
AECOM
5.4
E
Sand Filled Geotextile Bags
A semi permanent coastal protection unit is the sand filled geotextile bag. Unlike the sand bags used to protect
the resort area from the impact of TC Yasi these units are relatively large and the constructed of long lasting
vandal resistant geotextile. The behaviour of these units is similar to rock armour in that the mass of the
individual elements is the primary source of strength for the seawall and as such the structure is considered
robust.
The big advantage this type of unit offers is that large quantities of heavy material are not required to be imported,
bags can be brought in on pallets and the fill is taken from the local beach. It should be noted that because the
bags are buried back in the beach there is not net loss of volume by using in-situ sand. A bag being filled on the
beach is presented in Figure 12.
Figure 12
Filling of 0.75m3 geotextile bags during construction
Another advantage of this solution is that the seawall can be removed a later date by tearing the bags open and
emptying contents before disposing of the bags. Rock or Seabee seawalls require considerable effort to
dismantle.
A big disadvantage of this type of unit is the limited life of the geotextile bag. The material slowly breaks down
when exposed to UV radiation (sun shine). At this time manufacturers have had bags in exposed beach locations
for a couple of decades and they now say that the bags will last for at least 25 years fully exposed, though
anticipate longer life as the existing installations are holding together well. It is anticipated that the seawall will be
buried for much of the design life. However, if exposed to the sun to ensure that a structure of this nature lasts for
the nominated design life of 50 years a double layer solution has been developed plus heavy duty vandal resistant
bags are specified in exposed locations. Ultimately, however, the bags would degenerate and at that future time
options for a new structure would need to be assessed. An example of longer term placement with a similar
cross-section is presented in Figure 13.
AECOM
Figure 13
F
Stockton Beach Surf Club Seawall - During construction (l) and 13 years later (r).
A possible solution to the limited life of the bag is to mix cement into the sand during the filling phase. Once the
bags are wet the cement sets leaving a solid element that will persist long after the geotextile of the bags has
degraded. This solution has been discarded as it will add considerably to cost and if a concrete unit revetment is
desired then the Seabee seawall is a representative solution.
3
Utilising design information developed for geofabrics (ref Hornsey, etal, 2011) a design utilising 0.75m geotextile
bags at a slope of 1:1.5 was developed. The design cross-section for a sand filled geotextile bag seawall is
presented in Appendix C.
5.5
Further to Section 4.7, a wave deflection wall constructed at the crest of the seawall can provide additional
immunity against overtopping and thus provides a more robust sea wall solution.
It should be noted that the crest levels of the proposed seawall options compare favourable against the design
water level and therefore, a wave deflector wall is not a mandatory engineering requirement for stabilisation of the
foreshore. However, a wave deflection wall would reduce the risk of overtopping failure and associated erosion
immediately behind the seawall. Because the waves will run over unpaved areas behind the seawall, the wave
deflector acts to reduce this erosion.
A ‘T’ shaped concrete wave deflection wall 0.7m high has been considered as a possible addition to the seawall
design. Based on the design conditions described previously it is expected that wave bores up to 0.35m high
could breach the seawall during a design event, and a structure 0.7m high is considered sufficient to handle this
wave bore. A typical section is presented in Appendix D.
The wall height of 0.7m is also considered a convenient height for people to sit on the wall safely and does to
greatly impede access or amenity of the water front.
Another optional feature worth considering is the inclusion of a footpath. The width of the base of the T can be
expanded to allow easy access for people along the side of the wall and to help manage erosion around the base
of the wave wall. The footpath can be located on either side of the wall, but will improve the wave deflection
characteristics if it is located on the seaward side allowing the seawall to be further back from the crest of the
structure.
The wave deflection wall is presented on the crest of the three alternate seawall designs (see Appendix A to
Appendix C). The proximity of the wave wall to the crest and the nature of the overtopping wave both impact on
the immunity offered by the wave deflection wall. The close proximity and already high run-up levels of the
Seabee solution will mean that the wave deflection wall in this design will offer slightly less immunity than for the
other two designs.
Note that an isolated section of wave deflection wall will not prevent inundation from still water levels above land
level.
AECOM
6.0
G
Beach Nourishment
Beach nourishment has been suggested as a possible solution to foreshore erosion issues on Poruma. It could
be used to bolster the sand reserves on threatened foreshores, though there are significant issues to consider.
Two possible sources of nourishment are to take sand from a beach experiencing heavy build up sand and deliver
it to a location that is starved of sand (bypassing), or to dredge sand from offshore deposits and deliver via
pumping to areas in need of sand.
6.1
Bypassing
Taking of sand from one beach on Poruma to supply another beach would generally be discouraged, and is not
recommended as a solution to problems on the SW corner of the island. This option may be appropriate when a
structure interrupts the natural movement of sand and moving the sand is simply bypassing this interruption. As
discussed previously this is the case at the dredge channel and jetty, where the interruption of sand transport has
left the beach on the NW corner of the island very wide at the expense of the northern foreshore east of the jetty.
If this solution were adopted then the volume moved would need to be assessed on an ongoing basis, though
3
past experience would indicate volumes in the order of 10,000m /annum, moved during the summer months,
would be required to make this solution viable.
6.2
Dredging
An option that has been discussed is the dredging of sand from shoals that exist off the NW corner of the reef
platform. The shoal is supplied by sand washed from the reef platform at the western end of the island and as
such should have similar properties to the material found on the beaches today.
The nourishment program would need to incorporate an initial delivery to re-instate beaches to a “healthy” profile
and then an ongoing program to replace material lost from the system. To rectify problem areas as they exist
3
3
around the island today an initial program would need to supply approximately 40,000m (assumes 20m /m for
2000m of foreshore) with sand delivered all around the island. Ongoing requirements would need to be assessed
3
at that time, however a program of delivering say 20,000m sand once every 5 years for example would provide
sufficient frequency to minimise risk of property loss through beach erosion.
To deliver sand in this situation a Trailer Suction Hopper Dredge (TSHD) with pump out facilities would be
required. There are a number of commercial operators that have such equipment, though the scale of the
dredges and the delivery rates would be high relative to the scale of the project being considered (refer Figure
14). The Port of Brisbane dredge periodically undertakes maintenance dredging operations in North Queensland
for authorities such as Ports North and it is an obvious contender for the project. This dredge has a hopper
3
volume of 3000m and would take only a couple of days to complete the dredge works assuming that the delivery
pipes establishment do not hold it up.
In addition mobilisation costs to the Torres Strait for what is small project would be prohibitive. For this solution to
be viable a dredge would need to be in the vicinity and able/willing to undertake the project. The most obvious
solution is to liaise with authorities undertaking Dredging in North Queensland and come to a shared cost
arrangement. Relevant authorities include DERM who maintain access to boat harbours, and Ports North who
are responsible for a number of small ports in the area.
Issues relating to the small scale of the project with multiple delivery points, versus the remote location and
exposed dredging area all combine to make this solution technically difficult utilising the existing commercial
dredging fleet. It is likely that the only way dredging at Poruma on a regular basis can be achieved is if a small
TSHD with pump out facilities were available to work specifically on small dredge projects in the region.
AECOM
Figure 14
H
Medium sized TSHD delivering sand via floating pipes (l) and discharges of slurry from pipe (r).
Another major constraint to a dredge operation is the approvals process. For capital dredging beyond any defined
port limits the approvals could be difficult to obtain and the process both expensive and drawn out.
At this stage, given the constraints outlined above dredging is not seen as practical long term solution to the
erosions issues on the SW corner of the island, and cost estimates have not been prepared. This option may be
attractive if a suitable dredge were available to the regional authorities on a regular basis.
7.0
Opinion of Probable Construction Costs (OPCC)
7.1
Basis
An opinion of probable construction costs (OPCC) was prepared for the proposed works to determine the financial
implications of the proposed options. The tender schedules used as part of the original Boigu Island Seabee wall
construction works was used as a basis for the cost estimates and were modified to suit all three seawall options.
This approach provides some surety that major project elements have not been disregarded. A considerable
amount of investigation went into the preparation of the original schedule and accordingly it documents in some
detail the actual steps the construction contractor will need to take in order to complete the works.
The OPCC includes the following items:
-
Establishment and dis-establishment;
-
Setting out the works;
-
Clearing and grubbing;
-
Earthworks including excavation and filling;
-
Supply and installation of geotextile, rock layers, geotextile sand bags or Seabee wall units (depending on
the option);
-
Supply and installation of a wave return wall and associated footpath (measured as a separate item);
-
Compliance assessment testing; and
-
As constructed records.
Costs calculated from the OPCC are summarised in the section below.
7.2
Results
Reference should be made to the OPCCs attached to this report. Separate schedules have been created for
each alternative to enable simple comparison. Both seawall lengths have also been considered and separate
schedules have been prepared for each. A summary of the OPCC appears in the following table.
AECOM
I
Table 1 - OPCC Summary
Length
Rock Seawall
Seabee Seawall
Sand Bag Seawall
100m
$1,403,195
$1,530,974
$1,110,106
310m
$4,276,079
$4,606,607
$3,347,378
From the OPCC, it is clear the vast majority of the costs are contained in only a few items. Given the associated
quantities have not been calculated with a great degree of accuracy and are large, small inaccuracies in
construction rates can have a significant bearing on the total cost. Quantities are based on typical sections for
each seawall option.
Further information surrounding the basis of the rates is discussed below. This information was gathered to
mitigate the risk of inaccuracies in calculated construction costs, and to ensure selected rates were site specific.
7.3
Explanation of Assumptions for Critical Items
7.3.1
Site Establishment
Based on experience with previous construction works conducted in the Torres Strait the cost for site
establishment can be generally estimated to be 5% of the project costs, not including project management.
7.3.2
Rock Bedding and Armouring
Rock in the Torres Strait represents a significant construction cost specifically because it cannot be won locally in
the outer islands and transport costs to import acceptable materials to site are expensive. For example a rate of
3
$1100/m for 20mm gravel screenings was tendered by the successful contractor in the Masig Island sewerage
scheme.
Previous seawall works in the Torres Strait confirmed supply, transportation and installation rates. This was done
through discussions with the quarry at Horn Island for material supply costs, Seaswift for transportation costs and
a local civil contractor experienced in remote and Torres Strait construction works. Results are summarised
below:
-
Materials supply costs (delivered to the barge on Horn Island):
-
Gravel Screenings (20mm) - $82/m ;
-
Rock Armour (greater than 500mm diameter) - $78/m ; and
-
Filter Rock (nominally less than 300mm diameter) - $63/m .
3
3
3
This indicates the supply costs for rock armour are approximately comparable to the rate for gravel screenings,
however the rate for filter rock is approximately 25% cheaper
-
Barging costs: Seaswift suggested they have a barge capable of transporting approximately 1100t payload.
Depending on tidal levels, this barge may also be capable of carrying up to 1500t. Barge costs were quoted
3
as $130,000 or $200/m . The barge is based in Cairns and hire costs would be inclusive of sailing time to
Horn Island return.
Including an allowance of 20% for contractor overheads and the approximate nature of the quotations, the unit
3
rate for supply and transport of rock to site is approximately $330/m .
A loader, an excavator and a supervisor will be required to unload the barge and place rocks on the seawall.
Weekly rates are presented below:
-
Loader: $150/hr
-
Excavator: $150/hr ; and
-
Supervisory Staff and Overheads (20%):
Allowing a ten week construction schedule and 20% profit margin, the total supply, transportation and installation
3
rate is $730/m .
3
As a conservative measure, a rate of $800/m has been adopted for budgeting purposes.
AECOM
7.3.3
J
Reinforced Concrete for Capping Layer
In the recently constructed Mabuiag Island sewerage scheme a concrete driveway was constructed for the access
3
to the sewerage treatment plant. The rate tendered by the successful contractor for this work was $1,500/m and
this included supply and transportation of materials, excavation, placing of forms, fixing of reinforcement and
pouring of concrete.
These work items are considered similar to what will be undertaken as part of construction of the reinforced
concrete capping. This rate has been factored to account for any price increases since the Mabuiag Island
3
project was tendered. As a consequence, a rate of $2,500/m has been adopted for the OPCC.
7.3.4
Seabee Units
The Seabee units are constructed of unreinforced concrete and can be poured on site using prefabricated
moulds. The unit rate for each block was calculated from first principles assuming the following:
3
-
Supply, transportation and installation of unreinforced concrete into moulds: $2,000/m ;
-
Installation of Seabee blocks: 25% of the cost to make the Seabee unit;
-
Approximately 67 Seabee blocks can be formed from 1m of concrete; and
-
Approximately 17 units/m .
3
2
It is envisaged that pouring the concrete for the Seabee blocks would be slightly more labour intensive than the
concrete capping layer, however it would not require additional forming or fixing of reinforcing. Consequently the
unit rate would be slightly cheaper. The adopted unit rate for concrete and the cost for construction yielded an
assumed cost of $38 per Seabee unit.
7.3.5
Supply and Placement of Geotextile Sand Bags
Geofabrics Australia was contacted to provide start up and supply rates for geotextile sand bags as well as
geotextile underlay. The rates provided include the cost of the material shipped to the island as well as the
necessary equipment to fill and close the sand bag units. The cost has been factored to represent the
contractor’s rate which includes filling and closing the sand bags, placing, overhead and profit. It is noted that the
Geofabrics will include assistance with equipment hire and staff training.
Each of the three seawall options requires the placement of a geotextile underlay. The rate provided by
Geofabrics Australia was again factored to represent the contractor’s rate which includes placement, overhead
and profit.
7.3.6
Supply and Installation of Wave Deflector Walls
In the recently awarded construction contract for the TSIRC Regional Asset Sustainability Project, a wave
deflector wall was included in the scheme at Iama. The work elements included in this item are comparable to
what is required at Poruma. As a result, the rate tendered by the successful contractor has been used to
calculate the total cost.
Quantities have been estimated based on the typical wave deflector wall section.
8.0
Summary and Recommendations
Three seawall designs applied to two possible extents have been considered. As listed in the summary table the
three design options each have advantages and disadvantages that can be considered independently to assess
the best design solution for Poruma.
AECOM
Table 2
K
Summary Table
Options
Rock Seawall
(0.5m armour)
Seabee Seawall
Geotextile Bag Seawall
Advantages
Very robust, aesthetic
appeal, permanent, and
vandal proof.
Offers a solution that ties in
easily with paving works, well
suited to a wave deflector if
extra protection is required,
and requires significant
amounts of manual labour
(local employment).
Robust, low visual impact,
easy to traverse and
comfortable to sit on, can be
dismantled and removed
more easily if desired, and
low volumes of imported
material
Disadvantages
Large volumes imported
material, rubbish build up,
vermin habitat, and difficult
to traverse.
Requires high quality
construction, significant
volumes of imported material,
brittle design, very high wave
run-up and reflection, and not
consistent with natural
appearance of foreshore.
It can be vandalised, will
eventually degrade, and
higher wave run-up than rock
armour.
Cost for 100m
(resort only)
$1.4 M
$1.5 M
$1.1 M
Cost for 310m
$4.3 M
$4.6 M
$3.3 M
Based on the above summary table above and in consideration of the expectations of the residents of Poruma we
recommend a seawall constructed from geotextile bags be adopted as the preferred solution. This
solution has the lowest construction costs and offers a solution that will have a minimal impact on beach access
and amenity.
The costs associated with construction of the wave deflector wall have been broken into the following items. Note
the additional reinforced concrete footpath is not mandatory however may increase amenity to the local
community:
-
wave deflector wall unit:
-
additional reinforced concrete for footpath:
Addition of the wave return wall increases the costs of the preferred solution by approximately $150,000 for the
100m wall option and $430,000 for the 310m wall option. From Section 5.5 the wave return wall is not mandatory
and represents a major investment for a minor increase in protection. Based on a seawall crest level of RL4.0m
AHD, there is approximately 900mm freeboard between the top of the wall and the design water level.
Given the dynamic nature of the coastline on Poruma and natural feel of the island the use of Seabees is not
recommended. These lightweight units will require a higher degree of supervision and possible maintenance that
will not be readily supplied on the island.
The main area of concern is that the geotextile bag structure presents is that the option is not a “permanent”
solution, with the bags degrading slowly over time due to UV radiation and being vulnerable to physical damage.
Not with standing that the structure is expected to survive beyond the design service life of 50 years, if a more
permanent solution is desired then a rock seawall is recommended as the alternative solution.
The alternate lengths of seawall have been prepared based on the likely reality of budget constraints. We
recommend that the 310m long seawall solution be adopted. This will offer protection to the full extent of
coast currently at risk from erosion. The shorter option will protect the tourist bungalows and offer a degree of
protection to the area to the west by providing a man made headland to stabilise the foreshore position.
The use of ongoing regular beach nourishment from offshore deposits is not seen as a viable option to foreshore
stability at this time.
It is recommended that consideration be given to removing some elements of the marine facility (e.g. eastern
groyne) to reduce the impedance to sand movement around the island.
AECOM
9.0
L
References
nd
CIRA, CUR and CETMEF, 2007. “The Rock Manual. The use of Rock in hydraulic engineering (2
Report C683, CIRIA, London.
edition)”.
Department of Environment and Resource Management, 2011. “Queensland Coastal Plan”. Queensland
Government, Brisbane.
Duce, S.J., Parnell, K.E., Smithers S.G., and McNamara K.E., 2010. “A Synthesis of Climate Change and Coastal
Science to Support Adaptation in the Communities of the Torres Strait – Synthesis Report” prepared for the
Marine and Tropical Science Research Facility. Reef and Rainforest Research Centre, Cairns, Australia.
Harper B., 1998. “Storm tide threat in Queensland: History, prediction and relative risk”. Queensland Government
Department of Environment and Heritage, Conservation technical report no. 10, Brisbane.
Hornsey W.P., Carley J.T., Coglan I.R. and Cox R.J., 2011. “Geotextile sand containers shoreline protection
systems: Design and application”. Geotextiles and Geomembranes, Elsevier, Netherlands.
University of New South Wales, 1997. “Seabees Design Manual V2.2”. Water Research Laboratory, Australia.
AECOM
Appendix A
Conventional Rock
Armour Sea Wall
AECOM
Appendix A
a-1
Conventional Rock Armour Sea Wall
3
For a revetment slope of 1:1.5 and armour density of 2.6t/m the design median armour is:
M 50 = 300kg
Dn 50 = 0.49m
Double Layer Thickness = 1.0m
3
In-situ armour mass = 1.42t/m
Suggested grade for rock armour (ref CIRIA 2007):
Extreme Lower Limit ELL (M<5%)
125 kg
Nominal Lower Limit NLL (M<10%)
190 kg
Mean mass Mem
285 kg
Nominal Upper Limit NUL (M>70%)
380 kg
Extreme Upper Limit EUL (M>97%)
570 kg
The structure shall be constructed on a geotextile cloth underlay with a minimum overlap between sheets of 0.5m.
Recommended suitable geotextile is a heavy grade material such as Elcomax 1200R.
AECOM
a-2
AECOM
Appendix B
Seabee Wall
AECOM
Appendix B
b-1
Seabee Wall
3
For a revetment slope of 1:1.5 and armour density of 2.3t/m and armour porosity of 35% the design Seabee
armour is:
D = 300mm
R = 300mm
d = 160mm
M = 26kg
2
No. = 17units/m
Secondary armour or bedding layer shall be 300mm thick and comprise rock:
D 50 = 100mm
In the range D min = 65mm to D max = 165mm
Bedding layer thickness = 300mm
3
In-situ armour mass = 1.5t/m
To ensure foundations do not move a geotextile
Recommended suitable geotextile is light material such as Elcomax 360R
AECOM
b-2
AECOM
Appendix C
Geotextile Bag Wall
AECOM
Appendix C
c-1
Geotextile Bag Wall
For a revetment slope of 1:1.5 the design revetment based on the published data for the Elcorock product shall
3
comprise 0.75m bags filled to manufactures specifications.
Bags shall be filled using in-situ sand.
Bags laid in a double layer. Based on manufactures (Geofabrics) recommendations:
-
Exposed bags (top layer) shall be of vandal resistant 809RP geotextile.
-
Covered bags (lower layer) shall be of standard 900R geotextile
Expected bag placement density for a 4m high seawall is 12.2 bags per meter run.
Recommended suitable geotextile is a medium grade material such as Elcomax 600R.
AECOM
c-2
AECOM
Appendix D
Typical Section of the
AECOM
d-1

Poruma Sea Wall Feasibility Study - Torres Strait Regional Authority

Transcription

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