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Prepared for the Torres Strait Regional Authority by AECOM

PORUMA SEA WALL FEASIBILITY STUDYOctober 2011

AECOM

Poruma Sea Wall Feasibility Study

J:\60213761\6. Draft Docs\6.1 Reports\2011 10.27-Poruma Sea Wall Feasbility Study.doc Revision 2 - 27 October 2011


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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© AECOM Australia Pty Ltd (AECOM). All rights reserved.

AECOM has prepared this document for the sole use of the Client and for a specific purpose, each as expressly stated in the document. No other party should rely on this document without the prior written consent of AECOM. AECOM undertakes no duty, nor accepts any responsibility, to any third party who may rely upon or use this document. This document has been prepared based on the Client’s description of its requirements and AECOM’s experience, having regard to assumptions that AECOM can reasonably be expected to make in accordance with sound professional principles. AECOM may also have relied upon information provided by the Client and other third parties to prepare this document, some of which may not have been verified. Subject to the above conditions, this document may be transmitted, reproduced or disseminated only in its entirety.

AECOM Poruma Sea Wall Feasibility Study


Quality Information Document Poruma Sea Wall Feasibility Study

Ref 60213761

Date 27 October 2011

Prepared by Stuart Bettington

Reviewed by Scott Snelling

Revision History

Revision Revision Date Details

Authorised

Name/Position Signature

0 16-May-2011

Draft James Jentz Associate Director

Signed Previously

1 11-Jul-2011 Final Issue James Jentz Associate Director

Signed Previously

2 07-Nov-2011 Wave Return Walls Added James Jentz Associate Director

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Table of Contents 1.0 Introduction 1 2.0 Coastal Processes 2

2.1 Metocean 2 2.2 Sand Supply 2 2.3 Bathymetry 2 2.4 Sand Movements 3 2.5 Beach Rock 4

3.0 Identified Areas of Concern 5 3.1 Site Inspection 5 3.2 South western corner of island 5 3.3 Northern Foreshore East of Boat Ramp/Jetty 7

4.0 Seawall Design Inputs/Assumptions 8 4.1 Design Life 8 4.2 Climate Change 8 4.3 Design Event 9 4.4 Water Level 9 4.5 Water Depths 9 4.6 Wave Climate 9 4.7 Land Levels/Overtopping 10 4.8 Beach Levels 10

5.0 Seawall Design Solutions (SW corner) 10 5.1 Seawall Extent 10

5.1.1 Preferred option (310m) A 5.1.2 Alternative short wall option (100m) B

5.2 Conventional Rock Armour B 5.3 Pattern Placed Concrete Seabee Units. C 5.4 Sand Filled Geotextile Bags E 5.5 Wave Deflection Wall F

6.0 Beach Nourishment G 6.1 Bypassing G 6.2 Dredging G

7.0 Opinion of Probable Construction Costs (OPCC) H 7.1 Basis H 7.2 Results H 7.3 Explanation of Assumptions for Critical Items I

7.3.1 Site Establishment I 7.3.2 Rock Bedding and Armouring I 7.3.3 Reinforced Concrete for Capping Layer J 7.3.4 Seabee Units J 7.3.5 Supply and Placement of Geotextile Sand Bags J 7.3.6 Supply and Installation of Wave Deflector Walls J

8.0 Summary and Recommendations J 9.0 References 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

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1.0 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.

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2.0 Coastal Processes

2.1 Metocean 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.

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Figure 2 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.

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Figure 3 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.

2.5 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.

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3.0 Identified Areas of Concern

3.1 Site Inspection 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.

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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.

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3.3 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

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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.

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4.3 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

4.6 Wave Climate

is calculated.

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

H

. When this limiting wave height is considered the relevant design wave heights in the depth limited wave spectrum are:

2% = 1.64m (height exceeded by 2% largest waves)

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H1/10

H

= 1.55m (average height of largest 10% of waves)

s

It is assumed that these waves will be short crested (choppy) with wave periods of less than 5 seconds.

= 1.35m (significant wave height or average height of largest third of waves)

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

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.

. This is approximately 900mm higher than the design water level.

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.

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Figure 7 Sewall extent options.

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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).

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Figure 9 Eastern end of 310m seawall option

5.1.2 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.

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Figure 10 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 seawall. Assuming that reasonable rock can be sourced the adopted rock armour density is 2.6t/m3

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

. This resulted in median primary armour of 300kg, resulting in a double layer armour thickness of approximately 1m.

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).

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Figure 11 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.

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5.4 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

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.

geotextile bags during construction

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.

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Figure 13 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.

Utilising design information developed for geofabrics (ref Hornsey, etal, 2011) a design utilising 0.75m3

Appendix C

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 .

5.5 Wave Deflection Wall 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.

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6.0 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 past experience would indicate volumes in the order of 10,000m3

6.2 Dredging

/annum, moved during the summer months, would be required to make this solution viable.

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 around the island today an initial program would need to supply approximately 40,000m3 (assumes 20m3/m for 2000m of foreshore) with sand delivered all around the island. Ongoing requirements would need to be assessed at that time, however a program of delivering say 20,000m3

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

sand once every 5 years for example would provide sufficient frequency to minimise risk of property loss through beach erosion.

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 volume of 3000m3

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.

and would take only a couple of days to complete the dredge works assuming that the delivery pipes establishment do not hold it up.

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.

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Figure 14 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.

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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 $1100/m3

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:

for 20mm gravel screenings was tendered by the successful contractor in the Masig Island sewerage scheme.

- Materials supply costs (delivered to the barge on Horn Island):

- Gravel Screenings (20mm) - $82/m3

- Rock Armour (greater than 500mm diameter) - $78/m

; 3

- Filter Rock (nominally less than 300mm diameter) - $63/m

; and 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 as $130,000 or $200/m3

Including an allowance of 20% for contractor overheads and the approximate nature of the quotations, the unit rate for supply and transport of rock to site is approximately $330/m

. The barge is based in Cairns and hire costs would be inclusive of sailing time to Horn Island return.

3

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 rate is $730/m3

As a conservative measure, a rate of $800/m

. 3 has been adopted for budgeting purposes.

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7.3.3 Reinforced Concrete for Capping Layer

In the recently constructed Mabuiag Island sewerage scheme a concrete driveway was constructed for the access to the sewerage treatment plant. The rate tendered by the successful contractor for this work was $1,500/m3

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 project was tendered. As a consequence, a rate of $2,500/m

and this included supply and transportation of materials, excavation, placing of forms, fixing of reinforcement and pouring of concrete.

3

7.3.4 Seabee Units

has been adopted for the OPCC.

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:

- Supply, transportation and installation of unreinforced concrete into moulds: $2,000/m3

- Installation of Seabee blocks: 25% of the cost to make the Seabee unit;

;

- Approximately 67 Seabee blocks can be formed from 1m3

- Approximately 17 units/m

of concrete; and

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.

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Table 2 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.

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9.0 References CIRA, CUR and CETMEF, 2007. “The Rock Manual. The use of Rock in hydraulic engineering (2nd

Department of Environment and Resource Management, 2011. “Queensland Coastal Plan”. Queensland Government, Brisbane.

edition)”. Report C683, CIRIA, London.

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.

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Appendix A

Conventional Rock Armour Sea Wall

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Appendix A Conventional Rock Armour Sea Wall For a revetment slope of 1:1.5 and armour density of 2.6t/m3

M

the design median armour is:

50

Dn

= 300kg

50

Double Layer Thickness = 1.0m

= 0.49m

In-situ armour mass = 1.42t/m

Suggested grade for rock armour (ref CIRIA 2007):

3

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.

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Appendix B

Seabee Wall

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Appendix B Seabee Wall For a revetment slope of 1:1.5 and armour density of 2.3t/m3

D = 300mm

and armour porosity of 35% the design Seabee armour is:

R = 300mm

d = 160mm

M = 26kg

No. = 17units/m2

Secondary armour or bedding layer shall be 300mm thick and comprise rock:

D50

In the range D

= 100mm

min = 65mm to Dmax

Bedding layer thickness = 300mm

= 165mm

In-situ armour mass = 1.5t/m

To ensure foundations do not move a geotextile

3

The structure shall be constructed on a geotextile cloth underlay with a minimum overlap between sheets of 0.5m. Recommended suitable geotextile is light material such as Elcomax 360R

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Appendix C

Geotextile Bag Wall

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Appendix C Geotextile Bag Wall For a revetment slope of 1:1.5 the design revetment based on the published data for the Elcorock product shall comprise 0.75m3

Bags shall be filled using in-situ sand.

bags filled to manufactures specifications.

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.

The structure shall be constructed on a geotextile cloth underlay with a minimum overlap between sheets of 0.5m. Recommended suitable geotextile is a medium grade material such as Elcomax 600R.

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Appendix D

Typical Section of the Wave Deflection Wall

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