Holloways Beach Groyne Preliminary Design, Costs & Benefit · 2019. 9. 20. · Holloways Beach Groyne Preliminary Design, Costs & Benefit i Executive Summary G:\Admin\B23732.g.mpb.HollowaysGroynes\R.B23732.001.02.HollowaysGroyneDesignBasis.docx

Holloways Beach Groyne Preliminary Design, Costs & Benefit

Reference: R.B23732.001.02.HollowaysGroyneDesignBasis Date: September 2019 Confidential

G:\Admin\B23732.g.mpb.HollowaysGroynes\R.B23732.001.02.HollowaysGroyneDesignBasis.docx

Document Control Sheet

BMT Eastern Australia Pty Ltd Level 8, 200 Creek Street Brisbane Qld 4000 Australia PO Box 203, Spring Hill 4004 Tel: +61 7 3831 6744 Fax: + 61 7 3832 3627 ABN 54 010 830 421 www.bmt.org

Document: R.B23732.001.02.HollowaysGroyneDesignBasis

Title: Holloways Beach Groyne Preliminary Design, Costs & Benefit

Project Manager: Matthew Barnes

Author: Matthew Barnes

Client: Cairns Regional Council

Client Contact: Iain Brown

Client Reference:

Synopsis: REVISION/CHECKING HISTORY

Revision Number Date Checked by Issued by

0 27th June 2019 MJA

MPB

1 24th July 2019

2 12th Sept 2019 DISTRIBUTION

Destination Revision

0 1 2 3 4 5 6 7 8 9 10

Cairns Regional Council BMT File BMT Library

PDF PDF PDF

PDF PDF PDF

PDF PDF PDF

Copyright and non-disclosure notice The contents and layout of this report are subject to copyright owned by BMT Eastern Australia Pty Ltd (BMT EA) save to the extent that copyright has been legally assigned by us to another party or is used by BMT EA under licence. To the extent that we own the copyright in this report, it may not be copied or used without our prior written agreement for any purpose other than the purpose indicated in this report.

The methodology (if any) contained in this report is provided to you in confidence and must not be disclosed or copied to third parties without the prior written agreement of BMT EA. Disclosure of that information may constitute an actionable breach of confidence or may otherwise prejudice our commercial interests. Any third party who obtains access to this report by any means will, in any event, be subject to the Third Party Disclaimer set out below.

Third Party Disclaimer Any disclosure of this report to a third party is subject to this disclaimer. The report was prepared by BMT EA at the instruction of, and for use by, our client named on this Document Control Sheet. It does not in any way constitute advice to any third party who is able to access it by any means. BMT EA excludes to the fullest extent lawfully permitted all liability whatsoever for any loss or damage howsoever arising from reliance on the contents of this report.

Commercial terms BMT requests the ability to discuss and negotiate in good faith the terms and conditions of the proposed terms of engagement, to facilitate successful project outcomes, to adequately protect both parties and to accord with normal contracting practice for engagements of this type.

http://www.bmt.org/

Holloways Beach Groyne Preliminary Design, Costs & Benefit i Executive Summary


Executive Summary

The benefits and costs of 20 unique strategies for shoreline management at Holloways Beach have been considered. Each strategy combines beach nourishment and groynes with the key variables tested including:

• Beach nourishment frequency;

• Groyne geometry (length and elevation);

• Groyne primary material (geofabric sand containers or rock);

• Capital and lifecycle costs;

• Alongshore benefit to the shoreline associated with avoided damages;

• Potential for undesirable erosion downdrift of the groyne; and

• Social and recreational values associated with the beach.

Based on the analysis and assumptions, the best performing strategy is two 30 m groynes constructed from rock to maximise the benefit of beach nourishment. The proposed locations for the groynes are:

• Oleander Street, representing an upgrade and replacement of the deteriorated geofabric sand container groyne constructed in 2002; and

• Pandanus Street (aligned with the beach access location), to provide benefit to the shoreline immediately to the south that is prone to beach lowering in front of the Hibiscus Lane rock revetment seawall.

The minimum crest elevation for the groynes is equivalent to the highest astronomical tide level. This is considered the preferred elevation in terms of providing alongshore benefit, avoiding damage during storms and minimising visual impact.

As part of the capital works, beach nourishment is required to ‘fill’ the groynes so that sand can ‘naturally’ bypass around the structures under the prevailing coastal processes. This is a desirable condition for Holloways Beach so that the potential for shoreline erosion downdrift of the groynes is minimised.

The metocean design basis for the proposed groynes has been established, combining:

• The existing knowledge of the local coastal processes;

• Numerical modelling of waves; and

• General design references, guidelines and standards.

The recommended rock armour for the groynes is larger than the rock used to construct the Hibiscus Lane seawall due to the increased exposure to waves. During storms with elevated water level and wave conditions, the proposed groynes will be directly exposed to wave action and overtopping. The larger rock is required to maintain stability and minimise the potential for displacement.

A preliminary geotechnical investigation infers the presence of ‘very soft to soft’ silty clay with limited bearing capacity at the seaward end of the proposed groynes. The site preparation works may require a ‘bridging layer’ to be constructed to improve the subgrade conditions. Prior to progressing to detailed design stage, further investigations are needed to define the extent and confirm the strength consistency of the silty clay material.

Holloways Beach Groyne Preliminary Design, Costs & Benefit ii Contents


Contents

Executive Summary i

1 Introduction 1

1.1 Background 1

2 Costs and Benefits 4

2.1 Introduction 4 2.2 Structure Assumptions 4

2.2.1 Groyne Dimensions 4 2.3 Cost & Benefit Assumptions 9

2.3.1 Key Assumptions 9 2.3.2 Assessment Scenarios 9

2.4 Assessment Results 13 2.4.1 Stage 1 Assessment: Groyne Dimensions & Material Type 13 2.4.2 Stage 2 Assessment: Groyne Field 16

3 Preliminary Design Considerations 19

3.1 General Design References & Guidelines 19 3.2 Basis of Design 19 3.3 Conceptual Cross Section 21

3.3.1 Crest Level Assessment 22 3.3.2 Preliminary Rock Sizing 23 3.3.3 Crest Width with Overtopping Assessment 24 3.3.4 Ease of Construction 24 3.3.5 Preliminary Design Criteria 24

3.4 Preliminary Geotechnical Investigation 25 3.5 Preliminary Design Drawings 26

4 References 27

Appendix A Coastal Processes A-1

Appendix B Survey (RPS 2019) B-1

Appendix C Geotechnical Investigation (Douglas Partners 2019) C-1

Appendix D Preliminary Design Drawings (GHD 2019) D-1

List of Figures

Figure 2-1 30 m groyne and alternative crest elevations 6

Holloways Beach Groyne Preliminary Design, Costs & Benefit iii Contents


Figure 2-2 40 m groyne and alternative crest elevations 7 Figure 2-3 50 m groyne and alternative crest elevations 8 Figure 2-4 Groyne Field Scenarios: Two Groynes (top) and Three Groynes (bottom) 17 Figure 3-1 Crest Elevation vs Water Level Design Condition (CIRIA; CETMEF; CUR,

2007) 22 Figure 3-2 Groyne Typical Cross Section 25 Figure A-1 Blended TC and non-TC tide plus surge extreme water levels for the present

climate at Cairns North (BMT & SEA 2019) A-1 Figure A-2 Ocean Swell with 9s Peak Period at Clifton Beach (BPA, 1984) A-3 Figure A-3 Holloways Beach Wave Parameter Timeseries: 11-year Hindcast A-4 Figure A-4 11-year Hindcast Wave Rose: Cairns Waverider Buoy Location (left) and

Holloways Beach (right) A-5 Figure A-5 Significant Wave Height and Wave Peak Period Scatter Plot A-6 Figure A-6 11-year Hindcast Extreme Value Analysis Significant Wave Height Return

Period Plots: All Data (right) and non-TC Data (left) A-7

List of Tables

Table 2-1 Estimate of Groyne Material Quantities 5 Table 2-2 Key Assumptions: Capital & Operational Costs 10 Table 2-3 Key Assumptions: Benefits 11 Table 2-4 Management Strategy Descriptions 12 Table 2-5 Stage 1 Rock Groyne Assessment Results 14 Table 2-6 Stage 1 Geofabric Sand Container Groyne Assessment Results 15 Table 2-7 Stage 2 Groyne Field Assessment Results 18 Table 3-1 Summary Basis of Design 20 Table 3-2 Preliminary Groyne Dimensions 25 Table A-1 Present climate water level statistics at Cairns North and Holloways Beach A-2 Table A-2 Tropical Cyclone Significant Wave Height Statistics at Holloways Beach (BMT

WBM, 2013) A-3 Table A-3 Top 10 Largest Hindcast Wave Heights at Holloways Beach A-6 Table A-4 Preliminary Design Wave Height and Period Statistics offshore from

Holloways Beach (bed elevation 5 m below AHD) A-7

Holloways Beach Groyne Preliminary Design, Costs & Benefit 1 Introduction


1 Introduction

This report considers groynes proposed for Holloways Beach and includes the following:

• Analysis of the costs and benefits of single and multiple groynes;

• The preliminary design basis for the proposed groynes;

• The findings of a preliminary geotechnical site investigation; and

• The preliminary design drawing set.

1.1 Background

This work follows a detailed review of coastal processes and ongoing shoreline management undertaken at Holloways Beach (BMT 2018a) and a management options assessment report (BMT 2018b). The key findings of this previous work are summarised below.

Regional sediment transport patterns

• In geological timeframes, the Barron River has formed the most significant sediment source for the northern Barron River Delta beaches including Holloways Beach.

• The northern Barron River Delta beaches generally experience net longshore drift of sediments to the north. The net littoral drift system once extended north from the ‘old’ Barron River entrance at Ellie Point, however since the river entrance switched locations in 1939 it now extends northward from approximately Redden Creek.

• Since switching of the Barron River entrance in 1939, the delta continues to grow and a steady fluvial supply of sediment to Machans Beach (and subsequently Holloways Beach) is yet to be re-established.

• Southward littoral drift occurs along all the study region shorelines on occasions. South migrating sandbars on the ‘new’ Barron River delta indicate that a net southward drift of sediment occurs along part of Redden Island Beach.

• Cross shore transport of sediment occurs along all beaches. Onshore sediment transport causes the beaches to grow seaward and the smaller coastal creek mouths tend to become periodically infilled with sediment. Stormy conditions lead to erosion of beach sands that are transported in an offshore direction. During these conditions the beach elevation in front of hard structures tends to lower.

• Catchment flood conditions drive offshore sediment transport locally and the formation of entrance sandbars. These features then typically become reworked both onshore and northwards to form a shoreline bulge that progressively migrates northwards and diminishes in extent.

Long term trends

• The Barron River Delta continues to expand seaward and, together with the Redden Island southern shoreline, is experiencing progressive accretion.



• Machans Beach has experienced progressive erosion, prompting construction and recent refurbishment of a rock revetment seawall along the entire beach length.

• Periods of erosion and accretion have been recorded at Holloways Beach, with eroded conditions recorded in the 1950’s and 1990’s and accreted conditions recorded between the 1970’s to 1980’s. The shoreline position over the past 20 years or more has been relatively stable in comparison with past conditions on record.

• The zone of entrance instability at Barr Creek has reduced in width through time, with the entrance mouth now typically breaking out adjacent to the Machans Beach seawall. A ‘northern’ entrance channel that occurred in past times was associated with periodic sediment supply to southern Holloways Beach, and associated beach growth.

• Richters Creek entrance bars have generally grown over time.

Current approach to shoreline erosion management

• Holloways Beach has seen various shoreline erosion management measures since the 1960’s, including ad hoc seawalls to protect Hibiscus Lane properties, construction of a rock revetment of a more consistent design, installation of a geotextile sandbag groyne (no longer performing as intended) and multiple beach nourishment campaigns.

• The threat of erosion to Holloways Beach persists today with some areas displaying signs of long-term recession trends. Beach nourishment works have been undertaken periodically since 2004 following the upgrade and extension of the rock revetment along Hibiscus Lane. The sand is sourced from the mouth of Ritchers Creek and is delivered to the beach via a small cutter suction dredge and slurry pipeline.

• Nourishment sand is placed on the upper beach, gets reworked by the coastal processes and typically redistributed in both the cross shore and longshore directions. Considering the cross shore direction, upper beach profile data at Holloways Beach shows that the dynamically stable ‘natural’ slope is approximately 1V:10H. When nourishment sand is placed at a steeper slope the upper beach gradually adjusts to the natural slope which is in equilibrium with the prevailing coastal processes.

• Considering the longshore direction, nourishment material placed at the southern end of the beach will gradually provide benefit to the north as the sand is redistributed under the prevailing coastal processes.

• The volumetric change in sand across the upper beach can be inferred by comparing subsequent beach elevation surveys. Analysis of surveys between 2012 to 2017 suggests:

○ Approximately 20,000 m3/year of sand eroded from the upper beach between 2013 and 2014.

○ A positive change in beach volume between 2014 and 2015 following nourishment of the upper beach with 25,000 m3 of sand.

○ The 2014 nourishment campaign still had a positive benefit to the upper beach between 2015 and 2016.



○ Net erosion between 2016 and 2017 despite upper beach nourishment of 25,000 m3. The nourished profile was relatively steep in 2016 (to conceal the rock revetment at Hibiscus Lane) and had adjusted back to 1V:10H by 2017.

○ The total accretion volume based on estimates from the upper beach survey between 2012 and 2017 (62,097 m3) is similar to the total nourishment volume for this period (65,000 m3). This suggests that upper beach accretion is entirely due to nourishment and that little to no ‘natural’ accretion of the upper beach has occurred over the past 5 years.

○ During the same period, the net erosion volume is close to 82,000 m3, suggesting a net deficit of close to 20,000 m3 for the Holloways Beach upper profile since 2012.

Proposed approach to shoreline erosion management

The beach management plan proposed by BMT (2018b) included the addition of groynes to help maximise the benefit of future beach nourishment campaigns and maintain shoreline position at Holloways Beach. These proposed structures are considered further in this report.

Holloways Beach Groyne Preliminary Design, Costs & Benefit 4 Costs and Benefits


2 Costs and Benefits

2.1 Introduction

A simple cost benefit assessment of the groyne options at Holloways Beach has been undertaken. Shoreline erosion management actions lead to both capital and operating (or maintenance) costs. There are several different benefits which depend on beach condition in response to the management action. The key benefits considered in this assessment are:

• Avoided damages to land; and

• Recreational use of the beach and average value per visit.

Other potentially relevant costs or benefits not explicitly considered here include:

• Visual amenity, such as a view of the beach with or without the proposed groyne(s);

• Non-use benefits which refers to the value of the beach to residents even if they do not directly use it; and

• Discount rates to account for incurring costs in the present in return for benefits in the future.

Refinement or extension of this assessment could be undertaken as new information becomes available. For example, engagement activities associated with the ongoing Cairns CHAS project may produce outputs to inform several key assumptions regarding the non-tangible value of Holloways Beach and other beaches throughout the region.

Assessment has been undertaken in two stages:

(1) Testing the cost and benefits with respect to groyne length and primary material type (rock or Geofabric Sand Containers). This assessment is representative of a single groyne at Oleander St.

(2) Testing the cost and benefits with respect to multiple groynes. This assessment considers the groyne at Oleander St with an additional one or two groynes along the Hibiscus Lane shoreline.

2.2 Structure Assumptions

2.2.1 Groyne Dimensions The quantity of materials needed will vary for different groyne dimensions. The assessment considers three potential groyne lengths (30, 40 and 50 m measured perpendicular from the shoreline) and three elevations of the groyne head, corresponding to the following tidal datums:

• Highest Astronomical Tide (HAT), estimated to be 1.9 mAHD.

• Mean High Water Springs (MHWS), estimated to be 1.0 mAHD.

• Mean Sea Level (MSL), estimated to be 0.05 mAHD.

The cross-sectional area and footprint of the structure also depends on the crest width, batter slope and toe elevation (level founded). The following assumptions have been made:

• Crest width: 3.0 m.



• Batter slope: 1V:2H.

• Toe elevation (level founded): -1.64 (mAHD), equivalent to Lowest Astronomical Tide (LAT) level.

Conceptual illustrations of the groyne scenarios are provided in Figure 2-1, Figure 2-2 and Figure 2-3. Table 2-1 provides estimates of the quantity of rock (t) and 2.5 m3 Geofabric Sand Containers (GSCs) required for the groyne dimensions considered in the assessment. This information has been used to inform the capital cost estimate.

Table 2-1 Estimate of Groyne Material Quantities

Option Structure Volume (m3)

Rock Quantity (t) GSC Quantity (2.5m3 units)

50 m length, head at HAT 2105 3536 693

50 m length, head at MHWS 1728 2902 539

50 m length, head at MSL 1418 2381 404









Figure 2-1 30 m groyne and alternative crest elevations



2.3 Cost & Benefit Assumptions

2.3.1 Key Assumptions The data used to inform the assessment has come from various sources, including:

• Consultation with Council on the cost of dredging and beach nourishment at Holloways Beach and the cost to construct coastal protection structures at nearby locations.

• Consultation with local quarry owners on the cost of rock delivered to site.

• Consultation with Geofabrics Australia on the cost of GSCs delivered to site and labour requirements for filling and placement.

• A review of economic literature on the value of recreational beach use.

• Approximate land value based on valuation rates published by the Queensland Government.

The key assumptions are summarised in Table 2-2 and Table 2-3.

The consequences of reducing the groyne head elevation are as follows:

• Increased potential for damage, captured through the ‘operational costs’ assumption.

• Reduced updrift benefit of the structure, captured through the ‘updrift benefit scale factor’ assumption.

As indicated in Table 2-1, reducing the groyne head elevation also reduces the required material quantity and therefore capital cost. It is noted that the potential benefit associated with the decreased visual impact has not been considered but could be tested with key stakeholders and the outcomes used to refine this assessment.

2.3.2 Assessment Scenarios The Stage 1 assessments consider 18 unique management strategies, combining a single groyne option with beach nourishment. A ‘do nothing’ and ‘beach nourishment only’ management strategy is also considered. For the purpose of the assessment, the addition of a groyne is assumed to reduce the need for beach nourishment and double the time between nourishment campaigns. A summary of the management strategies is provided in Table 2-4, noting that each groyne strategy has been assessed using rock or GSCs as the primary material.

The Stage 2 assessments tests the cost and benefits of management strategies that include multiple groynes. Each strategy assumes a ‘groyne field’ with a single groyne at Oleander St and one or two groynes along the Hibiscus Lane shoreline.



Table 2-2 Key Assumptions: Capital & Operational Costs

Key Assumptions Value Units

Beach Nourishment Capital Costs

Dredging and sand placement1 25 $/m3

Nourishment volume per campaign 20,000 m3

Nourishment planning & design costs per campaign 10,000 $

Rock Groyne Capital Costs

Design life 50 years

Rock delivered to site2 50 $/t

Rock sorting and placement 60 $/t

Rock groyne planning & design 100,000 $

Geofabric Groyne Capital Costs

Design Life 15 years

2.5 m3 Geofabric Sand Container standard3 152 $/unit

2.5 m3 Geofabric Sand Container vandal resistant3 285 $/unit

Geofabric Sand Container filling and placement onsite 400 $/unit

Geofabric Sand Container groyne planning and design 100,000 $

Operational Costs

Groyne maintenance, head at HAT 5% % of capital cost/p.a.

Groyne maintenance, head at MHWS 10% % of capital cost/p.a.

Groyne maintenance, head at MSL 15% % of capital cost/p.a. 1 estimate provided by Council, based on previous dredging and nourishment campaign 2 based on information provided by Boral Quarries 3 based on information provided by Geofabrics Australia



Table 2-3 Key Assumptions: Benefits

Key Assumptions Value Units

Recreational Use

Beach visits 10,000 p.a.

Average value per visit1 15 $

Avoided damages or loss

Stage 1 assessments: Beach compartment length (unprotected south of Oleander St) 350 m

Stage 2 assessment: Beach compartment length (Oleander St to southern extent of Hibiscus Lane seawall)

700 m

Land value2 600 $/m2

Updrift benefit of groyne (multiple of structure length) 3 m

Updrift benefit scale factor, head at HAT 1 -

Updrift benefit scale factor, head at MHWS 0.9 -

Updrift benefit scale factor, head at MSL 0.8 - 1 estimate based on Raybould et al. (2013) 2 estimate based on valuation rates published by Queensland Government



Table 2-4 Management Strategy Descriptions

Strategy ID Strategy Name Strategy Description

0 Do nothing No management activities

1 Beach nourishment only 20,000 m3 of beach nourishment occurs once every three years

2 50 m groyne, head at HAT 50 m groyne (rock or GSC), head at HAT, 20,000 m3 of beach nourishment occurs as needed

3 50 m groyne, head at MHWS 50 m groyne (rock or GSC), head at MHWS, 20,000 m3 of beach nourishment occurs as needed

4 50 m groyne, head at MSL 50 m groyne (rock or GSC), head at MSL, 20,000 m3 of beach nourishment occurs as needed









2.4 Assessment Results

2.4.1 Stage 1 Assessment: Groyne Dimensions & Material Type Assessment results for the management strategies are provided below. Table 2-5 presents the results for groynes constructed using rock and Table 2-6 presents the results for groynes constructed using GSCs. The ‘value of the management action’ is based on the difference between the ‘total life cycle benefits’ and ‘total life cycle costs’. The benefits of each management strategy are assessed against the ‘do nothing’ scenario. The ‘Benefit Cost Ratio’ indicates the value of the investment (a higher score represents better value for money).

The results indicate the following:

• The ‘do nothing’ strategy results in almost $29M in damages (costs) associated with the loss of beach and land to erosion.

• Compared to the ‘do nothing’ strategy, ‘beach nourishment only’ provides benefits to the community with a positive Benefit Cost Ratio. However, the residual damages are still relatively high compared to the options that combine beach nourishment with a groyne.

• In terms of materials, rock provides greater long term benefits over GSCs. This is due to the assumed design life of a GSC groyne being only 15-years meaning that the structure would be replaced three times over the 50-year total life cycle (i.e. incurring the capital costs three times).

• The best performing option is a 30 m groyne with head elevation at HAT.

• The difference in the benefits provided by the groynes dimensions considered is relatively minor and primarily influenced by the following assumptions:

○ Material quantities and associated capital costs; and

○ Life cycle costs and assumed damages associated with the groyne head elevation.



Table 2-5 Stage 1 Rock Groyne Assessment Results

Costs and Benefit Assumptions Do Nothing Beach Nourishment Only

Rock, 50m head at HAT

Rock, 50m head at MLWS

Rock, 50m head at MSL


Rock, 40m head at MLWS



Rock,30m head at MLWS


Groyne costs (design life 50 years)

Groyne length NA NA 50 50 50 40 40 40 30 30 30

Groyne function factor1 NA NA 1 0.9 0.8 1 0.9 0.8 1 0.9 0.8

Capital (delivery) NA NA $176,820 $145,110 $119,070 $141,456 $116,088 $95,256 $106,092 $87,066 $71,442

Capital (placement) NA NA $212,184 $174,132 $142,884 $169,747 $139,306 $114,307 $104,479 $104,479 $85,730

Planning & design NA NA $100,000 $100,000 $100,000 $100,000 $100,000 $100,000 $100,000 $100,000 $100,000

Life cycle costs (50 years)2 NA NA $972,510 $1,596,210 $1,964,655 $778,008 $1,276,968 $1,571,724 $526,428 $957,726 $1,178,793

Groyne total life cycle costs NA NA $1,461,514 $2,015,452 $2,326,609 $1,189,211 $1,632,362 $1,881,287 $836,999 $1,249,271 $1,435,965

Nourishment costs

Frequency (X yearly)3 NA 3 6 6 6 6 6 6 6 6 6

50-year costs NA $8,333,333 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667

Planning & design NA $166,667 $83,333 $83,333 $83,333 $83,333 $83,333 $83,333 $83,333 $83,333 $83,333

Nourishment total life cycle costs NA $8,500,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000

Management Benefits

Unmitigated costs, loss of beach4 $7,500,000 NA NA NA NA NA NA NA NA NA NA

Unmitigated costs, loss of land5 $21,420,000 NA NA NA NA NA NA NA NA NA NA

Residual damages with option6 $28,920,000 $18,000,000 $2,400,000 $2,580,000 $2,760,000 $2,760,000 $2,904,000 $3,048,000 $3,120,000 $3,228,000 $3,336,000

Total life cycle benefits NA $10,920,000 $26,520,000 $26,340,000 $26,160,000 $26,160,000 $26,016,000 $25,872,000 $25,800,000 $25,692,000 $25,584,000

Total value of management action $2,420,000 $20,808,486 $20,074,548 $19,583,391 $20,720,789 $20,133,638 $19,740,713 $20,713,001 $20,192,729 $19,898,035

Benefit Cost Ratio 1.28 4.64 4.20 3.98 4.81 4.42 4.22 5.07 4.67 4.50 1 used to scale the updrift benefit and estimate residual damages along unprotected shoreline between G1 and northern extent of the Hibiscus Lane rock revetment seawall 2 includes capital, planning & design and annual operational/maintenance cost assumptions (see Table 2-2) 3 general assumption that the groyne will reduce the need for beach nourishment campaigns; the actual need for beach nourishment would be determined by monitoring 4 loss of beach recreational and social use based on beach visits and average value per visit (see Table 2-3) 5 loss of land based on erosion prone area estimates (BMT 2019) and land value along unprotected beach compartment length (see Table 2-3); assumes entire length of unprotected shoreline 6 loss of land based on erosion prone area estimates (BMT 2019) and land value along unprotected beach compartment length (see Table 2-3) ; assumes reduced length of unprotected shoreline based on management action and groyne function factor



Table 2-6 Stage 1 Geofabric Sand Container Groyne Assessment Results

Costs and Benefit Assumptions Do Nothing Beach Nourishment Only

GSC, 50m head at HAT

GSC, 50m head at MLWS

GSC, 50m head at MSL


GSC, 40m head at MLWS



GSC,30m head at MLWS



Groyne length NA NA 50 50 50 40 40 40 30 30 30

Groyne function factor1 NA NA 1 0.9 0.8 1 0.9 0.8 1 0.9 0.8

Capital (delivery) NA NA $143,773 $115,178 $89,756 $115,083 $92,112 $71,858 $86,526 $69,198 $53,827

Capital (filling & placement) NA NA $277,200 $215,600 $162,000 $222,000 $172,400 $129,600 $166,800 $129,600 $97,200

Planning & design NA NA $300,000 $300,000 $300,000 $300,000 $300,000 $300,000 $300,000 $300,000 $300,000

Life cycle costs (50 years)2 NA NA $315,730 $496,167 $566,451 $252,812 $396,768 $453,281 $189,995 $298,197 $339,811

Groyne total life cycle costs NA NA $3,455,676 $3,756,483 $3,727,357 $2,966,318 $3,204,267 $3,182,462 $2,477,735 $2,656,650 $2,636,126

Nourishment costs

Frequency (X yearly)3 NA 3 6 6 6 6 6 6 6 6 6

50-year costs NA $8,333,333 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667 $4,166,667

Planning & design NA $166,667 $83,333 $83,333 $83,333 $83,333 $83,333 $83,333 $83,333 $83,333 $83,333

Nourishment total life cycle costs NA $8,500,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000 $4,250,000

Management Benefits

Unmitigated costs, loss of beach4 $7,500,000 NA NA NA NA NA NA NA NA NA NA

Unmitigated costs, loss of land5 $21,420,000 NA NA NA NA NA NA NA NA NA NA

Residual damages with option6 $28,920,000 $18,000,000 $2,400,000 $2,580,000 $2,760,000 $2,760,000 $2,904,000 $3,048,000 $3,120,000 $3,228,000 $3,336,000

Total life cycle benefits NA $10,920,000 $26,520,000 $26,340,000 $26,160,000 $26,160,000 $26,016,000 $25,872,000 $25,800,000 $25,692,000 $25,584,000

Value of management action $2,420,000 $18,814,324 $18,333,517 $18,182,643 $18,943,683 $18,561,733 $18,439,538 $19,072,265 $18,785,350 $18,697,874

Benefit Cost Ratio 1.28 3.44 3.29 3.28 3.63 3.49 3.48 3.83 3.72 3.72 1 used to scale the updrift benefit and estimate residual damages along unprotected shoreline between G1 and northern extent of the Hibiscus Lane rock revetment seawall 2 includes capital, planning & design and annual operational/maintenance cost assumptions (see Table 2-2); the actual need for maintenance would be determined by inspection 3 general assumption that the groyne will reduce the need for beach nourishment campaigns; the actual need for beach nourishment would be determined by 4 loss of beach recreational and social use based on beach visits and average value per visit (see Table 2-3) 5 loss of land based on erosion prone area estimates (BMT 2019) and land value along unprotected beach compartment length (see Table 2-3); assumes entire length of unprotected shoreline 6 loss of land based on erosion prone area estimates (BMT 2019) and land value along unprotected beach compartment length (see Table 2-3 ; assumes reduced length of unprotected shoreline based on management action and groyne function factor

Holloways Beach Groyne Preliminary Design, Costs & Benefit 16 Coastal Processes


2.4.2 Stage 2 Assessment: Groyne Field Assessment results for the groyne field management strategies are provided below. Following the outcomes of the Stage 1 assessment, 30 m groynes with head elevation at HAT and constructed from rock are considered. The scenarios are:

• 2 x 30 m groynes: One at Oleander St (G1) and one at Pandanus St (G2), see top panel of Figure 2-4.

• 3 x 30 m groynes: One at Oleander St (G1), one to the north of Pandanus St (G2) and one to the south of Pandanus St (G3), see bottom panel of Figure 2-4.

Table 2-7 presents the results which indicate the following:

• The ‘do nothing’ strategy results in over $50M in damages (costs) associated with the loss of beach and land to erosion. The increase in damages relative to the Stage 1 assessments is due to consideration of a longer length of shoreline.

• Based the assumptions that underpin the assessment, the two-groyne scenario is the best performing option. The additional capital costs associated with the third groyne does not provide proportionate benefits.



Figure 2-4 Groyne Field Scenarios: Two Groynes (top) and Three Groynes (bottom)



Table 2-7 Stage 2 Groyne Field Assessment Results

Costs and Benefit Assumptions

Do Nothing Beach Nourishment Only

Rock, 30m head at HAT x 2

Rock, 30m head at HAT x 3


Groyne length NA NA 30

Groyne function factor1 NA NA 1

Capital (delivery) NA NA $212,184 $318,276

Capital (filling & placement) NA NA $254,621 $381,931

Planning & design NA NA $200,000 $300,000

Life cycle costs (50 years)2 NA NA $3,501,036 $5,251,554

Groyne total life cycle costs NA NA $4,167,841 $6,251,761

Frequency (X yearly)3 NA 3 6 7

50-year costs NA $8,333,333 $4,166,667 $3,571,429

Planning & design NA $166,667 $83,333 $71,429 Nourishment total life cycle costs NA $8,500,000 $4,250,000 $3,642,857

Unmitigated costs, loss of beach4 $7,500,000 NA NA NA

Unmitigated costs, loss of land4 $42,840,000 NA NA NA

Residual damages with option6 $50,340,000 $39,420,000 $6,240,000 $5,160,000

Total life cycle benefits NA $10,920,000 $44,100,000 $45,180,000

Value of management action $2,420,000 $35,682,159 $35,285,382

Benefit Cost Ratio 1.28 5.24 4.57 1 used to scale the updrift benefit and estimate residual damages along shoreline between G1 and southern extent of the Hibiscus Lane rock revetment seawall 2 includes capital, planning & design and annual operational/maintenance cost assumptions (see Table 2-2) 3 general assumption that the groyne will reduce the need for beach nourishment campaigns; the actual need for beach nourishment would be determined by monitoring 4 loss of beach recreational and social use based on beach visits and average value per visit (see Table 2-3) 5 loss of land based on erosion prone area estimates (BMT 2019) and land value along unprotected beach compartment length (see Table 2-3); assumes entire length of unprotected shoreline 6 loss of land based on erosion prone area estimates (BMT 2019) and land value along unprotected beach compartment length (see Table 2-3 ; assumes reduced length of unprotected shoreline based on management action and groyne function factor



3 Preliminary Design Considerations

3.1 General Design References & Guidelines

The following general design and information references were applied in the preliminary designs:

General design references

• The Rock Manual (CIRIA; CETMEF; CUR, 2007).

• Coastal Engineering Manual. Engineer Manual EM-1110-2-1100. (U.S. Army Corps of Engineers (USACE), 2008).

• AS 4997:2005 – Guidelines for the design of maritime structures.

• AS 1170:2002 – Structural design actions.

Additional references

• BMT WBM (2013) Cairns Storm Tide Study Review.

• BMT (2019a) Cairns CHAS Phase 3: Storm Tide Hazard Refinement & Mapping.

• BMT (2019b) Cairns Region Erosion Prone Area Study: Stage 1 Hazard Assessment.

• MSQ (2019) Semidiurnal Tidal Planes.

3.2 Basis of Design

The basis of design information, considered generally applicable to any proposed coastal protection structure at Holloways Beach, is summarised in Table 3-1. Further detail regarding the adopted values and corresponding references are provided in the following sections and Appendix A. The result of a beach and nearshore survey and geotechnical investigation undertaken in July 2019 are also provided in Appendix B and Appendix C.



Table 3-1 Summary Basis of Design

Parameter Value Units/Values Reference

Structure Importance Category 1 (Low) Table 3.2 AS 1170.0:2002

Design life 50 Years Table 6.1 AS 4997-2005 - Normal Commercial Structure Design Event Annual Probability of Exceedance (Ultimate Limit State) Wind 1 in 100 Years Table 3.3 AS 1170.2:2002

Waves 1 in 200 Years Table 5.4 AS 4997-2005 – Function Category 1 Metocean Conditions

Water Levels

Tidal planes

HAT 1.86

mAHD Cairns Standard Port Semidiurnal Tidal Planes (MSQ 2019)

MHWS 0.98 MHWN 0.30

MSL 0.06 MLWN -0.18 MLWS -0.86

LAT -1.64

Storm Tide Levels

Event Tide + Surge (mAHD)

Wave Setup (m)

1 in 20 1.90 0.42 Storm tide levels – BMT WBM (2013) & BMT (2019b) Wave Setup - via Stockdon et al. (2006)1

1 in 50 1.98 0.51 1 in 100 2.04 0.51 1 in 200 2.15 0.59 1 in 500 2.52 0.59

Sea Level Rise By 2070 0.40 m BMT (2018)

Waves Offshore from Holloways Beach (~5 m depth)

Event Hs (m) Tp (s) 1 in 20 2.52 6

BMT WBM (2013) & additional modelling for this study

1 in 50 2.76 7 1 in 100 2.81 7 1 in 200 2.85 8 1 in 500 2.88 8

Groyne General Design Features

Minimum crest elevation 1.9 mAHD Minimum toe level (equivalent to LAT) -1.64 mAHD Structure head and batter slope 1V:2H Minimum rock density 2650 kg/m3 M50 armour mass 3.6 tonne Dn50 amour nominal diameter 1.1 m Minimum amour layer thickness 2 layers Minimum crest width 3Dn50 M50 Underlayer/Filter Rock (if used) 300 kg

1 1 in 10 beach slope assumed



3.3 Conceptual Cross Section

Coastal structures including seawalls and groynes, whether constructed from rock or geofabric containers, resist wave attack by the vertical mass of the armour units (individual rocks or sandbags) resisting the vertical uplift forces induced by the incoming wave. If an armour unit does not have enough mass, then it will be dislodged, and the structure will begin to fail. This is often a distressingly quick process as during storms waves attack the coastal structures every 8 to 10 seconds and can last for several hours or days. A common criterion is 5% damage which means when 5% of the rocks by number have been dislodged the structure is deemed to require maintenance. This does not necessarily mean the structure provides no further protection but, in the absence of maintenance, its performance is reduced.

The basic design for a groyne will include consideration of the following key elements:

• The crest level and width for the structure;

• The foundation level for the structure; and

• Armour rock sizing and any relevant under layers.

The crest level of the structure is traditionally designed to be above design storm water and wave runup level resulting in wave energy being absorbed by the sloping front face of the structure where gravity acts to keep the rock armour in place. However, this is not always practical and at Holloways Beach the foreshore levels and the desire to allow some sand to bypass the proposed groyne structures (to minimise shoreline erosion impacts downdrift of the structure), means that the some overtopping needs to be accounted for in the design.

Figure 3-1 shows the main crest height versus water level scenarios which influence design considerations in wave overtopping cases. Comments on scenarios ranging from the bottom image to the top include:

• Non-overtopping structure – standard design case;

• Low-crested structure – often the worst design case; and

• Submerged structure – usually a lower standard of design required.

For the proposed groynes at Holloways Beach, where the crest level of the groynes is determined by the natural surface level of the foreshore, sand bypassing requirements and aesthetic values, the three scenarios will apply depending on the water level at the time of the storm wave action. For example, the non-overtopping case may apply when a storm occurs at low tide, the marginally overtopped case when the storm occurs at mid-tide and the submerged case will occur when the storm occurs at high tide with storm surge (such as during a tropical cyclone event).



Figure 3-1 Crest Elevation vs Water Level Design Condition (CIRIA; CETMEF; CUR, 2007)

The second consideration is the founding level of the structure which for sandy beaches is taken as the lowest eroded beach level. During storm events sand will typically be eroded from the beach by wave action to form offshore bars. As a minimum, the level of LAT can be used to estimate the eroded bed level.

The lowest possible beach level is important because design wave acting on the structure is a function of the depth of water (i.e. wave growth is limited by depth). The wave condition, together with the rock specific density, structure batter slope and foreshore slope are used to estimate the armour rock mass. Further detail in assessing these primary design criteria is given below.

3.3.1 Crest Level Assessment As discussed above, the proposed structures at Holloways Beach will have a crest level which, at various times, will fall within the three overtopped design categories shown in Figure 3-1 and will therefore need to be designed for the worst case. If this produces a structure size or cost which is unacceptable then a more moderate (less expensive) design could be adopted, recognising that more maintenance may be required during the working life of the structure.

At Holloways Beach, a non-overtopping structure would require a crest elevation at least 5 m AHD which is not practical due to the average level of the foreshore. A structure of this size would also greatly increase the volume of rock (and therefore cost) and negatively impact visual amenity, social and recreational values of the areas.



The analysis presented in Section 2 recommends a crest elevation at HAT which invokes the ‘overtopping structure’ design standards and the acceptance that the standard damage criteria (5% per design storm) may be exceeded in some years and hence maintenance costs elevated for that year.

3.3.2 Preliminary Rock Sizing The so-called Hudson formulae (1953, 1959) is given in Equation 1 and is a common method for armour rock sizing for non-overtopped, permeable structures:

𝑀50 = 𝜌𝑟𝑔𝐻𝑠

3

𝐾𝐷∆3𝑐𝑜𝑡𝛼

Equation 1

Where M50 is the median armour rock mass, ρr is the rock density, Hs is the significant wave height at the toe of the structure, Δ is the rock relative buoyant density, α is the slope of the structure and KD is the stability coefficient related to the wave condition (breaking or non-breaking) and rock type (rough angular or smooth).

The relevant parameters and initial rock sizing for proposed groynes at Holloways Beach are as follows:

• Hs at the toe of structure = 2.2 m (assuming a depth-limited condition).

• KD = 2 (recommended coefficient for breaking waves on angular rock).

• ρr = 2650 kg/m3 (lower limit for Cairns region metagreywacke, e.g. Patterson & Britton 2002).

• α = 1V:2H (head and batter slope).

• Δ = 1.59.

• M50 = 1.8 tonne.

Assuming the standard relationship between median rock mass and median nominal diameter (the equivalent cube size):

𝐷𝑛50 = (𝑀50

𝜌𝑟)

1/3

Equation 2

The armour rock nominal diameter is estimated to be 0.9 m for a non-overtopping, permeable structure at Holloways Beach.

For cases where the water level is at or above the crest of the structure, Kramer and Burcharth (2004)2 suggest the following ‘rule of thumb’ for estimating armour rock size in depth-limited wave conditions:

𝐷𝑛500.3ℎ

Equation 3

2 Cited in: CIRIA; CETMEF; CUR, (2007)



Where h is the water depth at the toe of the structure.

This gives a rock armour median nominal diameter of 1.11 m, assuming:

• h at the toe of structure = 3.73 m (1 in 100 ‘tide plus surge’ and scour to LAT).

Considering the relationship given by Equation 2, the corresponding armour rock mass for an overtopped structure is 3.6 tonne. As expected, the required rock size is larger for the low-crested scenario than the estimate obtained via the Hudson formula for the non-overtopping case.

3.3.3 Crest Width with Overtopping Assessment The standard design criteria for crest width (e.g. Van der Meer) is a minimum of 3Dn50 (i.e. three rocks wide) to allow for the interlocking of the armour. However, for overtopped structures a minimum width of six rocks is often used to allow the overtopping wave to break and be absorbed on the crest and not impact on the rear armour where gravity may promote rock dislodgement.

Structures with a width of 6Dn50 are not considered practical at Holloways Beach and therefore the initial design is for a standard 3Dn50 width using the nominal 3.6 tonne armour to achieve a ~3.3 m wide crest is proposed.

3.3.4 Ease of Construction Coastal structures are often layered, with armour rock and smaller sized underlying filter rock. For large structures this approach is usually cost effective, as the less expensive filter rock is used to fill the core of the structure. However, this construction approach usually requires the exclusion of tidal waters via bunding and is not always cost effective for low-crested structures where the underlayer volume is small relative to the armour layer.

In the case of low structures such as those proposed for Holloways Beach, it may be cost effective for the entire structure to comprise of armour rock. This simplifies construction by allowing truck back tipping to deliver rock to the end of the groyne without the need for different material types or bunding. The delivered armour rock is then sorted and individually placed by machine.

3.3.5 Preliminary Design Criteria Considering the above, the preliminary groyne dimensions are listed in Table 3-2. These are based on the local metocean conditions, longshore sediment transport processes, natural foreshore levels, rock density assumption and visual character. The typical cross section is shown in Figure 3-2. As previously noted, a non-overtopping structure is not considered viable at Holloways Beach and the proposed low-crested groynes may experience some damage during severe events, potentially incurring greater maintenance costs over the working life of the structure.



Table 3-2 Preliminary Groyne Dimensions

Groyne General Design Features

Groyne length* 30 m Minimum crest elevation 1.9 mAHD Minimum toe level (equivalent to LAT) -1.64 mAHD Structure head and batter slope 1V:2H Minimum rock density 2650 kg/m3 M50 armour mass 3.6 tonne Dn50 amour nominal diameter 1.1 m Minimum amour layer thickness 2 layers Minimum crest width 3Dn50 M50 Underlayer/Filter Rock (if used) 300 kg *measured seaward from the cadastral boundary; additional length may be required to tie-back to land

Figure 3-2 Groyne Typical Cross Section

3.4 Preliminary Geotechnical Investigation

Full details of a preliminary geotechnical investigation completed by Douglas Partners is provided in Appendix C. The key findings include:

• Relatively ‘poor’ ground conditions along the alignment of both proposed groynes.

• At the landward end of the proposed groynes (i.e. at the beach) granular sand was encountered to depths between 2.6-5.6 m, underlain by ‘soft to firm’ silty clay.

• At the seaward end of the proposed groynes ‘very soft to soft’ silty clays were inferred to extend up to 1.9 m below the seabed. Limited bearing capacity in this area of the site is expected.



• To improve the subgrade conditions, site preparation works may require a ~2.5 m ‘bridging layer’ to be constructed. This would involve large size rock boulders being pushed out onto the seabed and pressed individually into the seabed (using the bucket of a large excavator, or similar), with the excavator working out from the beach and progressively seaward. As the boulders are pushed into the seabed, additional boulders should be pushed on top and between the already placed material, with smaller sized rocks and cobbles used to fill the voids between larger boulders. Bridging works would continue until little or no movement is evident during track rolling using a large (at least 20 tonne) excavator.

The preliminary design drawings described below and provided in Appendix D assume a 2.5 m bridging layer along the entire length of the proposed structures. Prior to progressing to the detailed design stage, further investigations are needed to define the extent and confirm the strength consistency of the silty clay material, bridging layer quantities and preferred construction methodology.

3.5 Preliminary Design Drawings

A preliminary design drawing set prepared by GHD is provided Appendix D, showing:

• Locality;

• General arrangement;

• Groyne detail plans;

• Groyne longitudinal sections; and

• Groyne typical cross section.

As discussed above, further geotechnical site investigations are needed to allow specification of the bridging layer and finalise the design.



4 References

BMT WBM (2013). Cairns Storm Tide Study Review. Report prepared for Cairns Regional Council. R.B19102.001.02.

BMT (2018a). Holloways Beach Coastal Processes Final Report, prepared for Cairns Regional Council.

BMT (2018b). Holloways Beach Options Assessment Final Report, prepared for Cairns Regional Council.

BMT (2019a). Cairns Region Erosion Prone Area Study: Stage 1 Hazard Assessment, prepared for Cairns Regional Council.

BMT (2019b). Cairns CHAS Phase 3: Storm Tide Hazard Refinement & Mapping, prepared for Cairns Regional Council.

CIRIA, CUR, CETMEF (2007). The Rock Manual, the use of rock in hydraulic engineering (2nd edition). C683, CIRIA, London, 1267pp.

Maritime Safety Queensland (2019). Tidal Planes. http://www.msq.qld.gov.au/Tides/Tidal-planes

Stockdon, H.F., Holman, R.A., Howd, P.A. and Sallenger Jr., A.H. (2006). Empirical parameterization of setup, swash, and runup. Coastal Engineering, 53, pp. 573-588.

USACE (2008). Coastal Engineering Manual. Engineer Manual EM-1110-2-1100. U.S. Army Corps of Engineers.

http://www.msq.qld.gov.au/Tides/Tidal-planes

Holloways Beach Groyne Preliminary Design, Costs & Benefit A-1 Coastal Processes


Appendix A Coastal Processes

A.1 Metocean

Numerical and statistical modelling and existing information from previous studies have been used to define the metocean conditions offshore from Holloways Beach. Key outcomes of the metocean assessments are provided below.

A.1.1 Extreme Water Level Statistics The combined extreme water level hazard due to each of the independently derived TC and non-TC events has been recently estimated for the Cairns region as part of the ongoing CHAS project (BMT & SEA 2019).

The resulting combined ‘tide plus surge’ return period curve for Cairns North Beach (yellow) is shown in Figure A-1, together with the non-TC (blue) and TC (red) components. This illustrates that, due to the significant difference in slopes, the effect of blending is simply to provide a smoothed transition between the two independent probabilities of exceedance near the 0.7% AEP intersection point. This slightly increases the 1% AEP ‘tide plus surge’ water level in comparison to the comparison to the TC only water level statistics previous reported by BMT WBM (2013). Above and below the transition region (approximately between 3% and 0.6% AEP) the blended water level statistics are dominated by the TC and non-TC populations respectively.

As the non-TC tide plus surge levels are only available for the single long-term tide gauge site in Cairns Harbour adjustment to other sites of interest can be achieved by applying the estimated HAT ratios, which have been derived from published tidal plane data (MSQ 2019) and interpolation between sites as required. The appropriate adjustment for Holloways Beach is estimated to be 0.95. The resulting adjusted ‘tide plus surge’ water level is provided in Table A-1.

Figure A-1 Blended TC and non-TC tide plus surge extreme water levels for the present climate at Cairns North (BMT & SEA 2019)



Table A-1 Present climate water level statistics at Cairns North and Holloways Beach

ARI (years) Cairns North (mAHD) Holloways Beach* (mAHD)

20 2.00 1.90

50 2.08 1.98

100 2.15 2.04

200 2.26 2.15

500 2.65 2.52

*estimated by applying an adjustment ratio of 0.95

A.1.2 Wave Statistics On a regional scale, the Great Barrier Reef (GBR) partially shelters the North Queensland coastline from the deep ocean waves propagating westward from the Coral Sea. Trinity Opening is a natural channel to the north-east of the study area which allows some swell to penetrate the GBR lagoon from the Coral Sea, albeit with significantly attenuated energy.

On a more local scale, Cape Grafton shelters the Cairns northern beaches from the south-easterly waves generated within the GBR lagoon. Fetches within the GBR lagoon are generally limited to 30-50 km by the large mid shelf reef complexes. Non-cyclonic winds rarely exceed 25 knots and locally generated sea wave heights are typically less than 1.4 m. East-south-easterly sea waves within the 3-5 second period band are the most prevalent wave energy component measured at the Cairns Waverider buoy (BPA, 1984). Waves approaching the study area from the east-southeast are refracted as they propagate into Trinity Bay.

Due to the complex arrangement of reef passes, fetch lengths and local topography and bathymetry, the wave climate in the study area can at times be multi-modal, meaning that it is made up of multiple component wave trains with distinct wave periods and directions. Figure A-2 is a photograph showing an instance of ocean swell with 9 second peak period propagating into Clifton Beach from the Trinity Opening to the north-east. The proposed Holloways Beach is located approximately 10 km southeast of Clifton Beach and similarly exposed to occasional north-easterly wave conditions.

Wave statistics for Holloways Beach are provided in the following sections. The TC associated wave statistics have been taken from an existing study that involved Monte Carlo simulation of 50,000 years of TC activity across the Cairns region (BMT WBM 2013). The non-extreme wave statistics are based on new analysis and modelling.



Figure A-2 Ocean Swell with 9s Peak Period at Clifton Beach (BPA, 1984)

Tropical Cyclone Waves

Significant wave heights associated with TC activity have been previously assessed and reported by BMT WBM (2013). The wave heights from the output location at Holloways Beach are summarised in Table A-2.

Table A-2 Tropical Cyclone Significant Wave Height Statistics at Holloways Beach (BMT WBM, 2013)

ARI (years) Significant Wave Height (m)

20 2.52

50 2.76

100 2.81

200 2.85

500 2.88

Wave Hindcast

A wave hindcast was performed for an 11-year period from 2007 to mid-2018. This period includes several notable wave events and includes 5 of the top 10 peak wave conditions recorded by the Cairns Waverider buoy that was installed in 1975.

Hourly timeseries of wave parameters (significant wave height, wave peak period and wave peak direction) were derived from the hindcast at a location in 5 m water depth (below AHD) and approximately 1 km offshore from Holloways Beach. Timeseries plots of the wave parameters for the hindcast period are shown in Figure A-3. This data is the basis for establishing wave statistics and extreme value analysis presented in below.



Figure A-3 Holloways Beach Wave Parameter Timeseries: 11-year Hindcast



Distribution of Wave Height and Direction

The distribution of significant wave height and direction from the hindcast simulation at the Cairns Waverider Buoy and offshore from Holloways Beach are shown in Figure A-4. Significant wave heights greater than 1.4 m are rare and occur from the northeast to southeast directions at the Cairns Waverider buoy location. Due to the orientation of the bathymetric contours at Barron River delta, prevailing waves from the southeast refract towards the easterly sector as they propagate to Holloways Beach. The most frequent direction for extreme waves at both locations is the northeast, with occasional north-northwestly waves extreme waves at Holloways Beach.

Figure A-4 11-year Hindcast Wave Rose: Cairns Waverider Buoy Location (left) and Holloways Beach (right)

Wave Period

A scatter plot of wave period versus significant wave height at Holloways Beach is shown in Figure A-5. This indicates the longer period (>8 seconds) ocean swell waves tend to be associated with significant wave heights less than 0.5 m. The locally generated sea waves are typically less than 1.0 m in height with a wave period within the 2 to 6 second band.

For waves generated by local winds, it is suggested that a wave steepness of around 1 in 50 would be a reasonable (upper bound) estimate of peak wave period for design conditions greater than or equal to the 1-yr ARI. Following this approach, estimates for design wave height and period pairs are provided below in Table A-4.



Figure A-5 Significant Wave Height and Wave Peak Period Scatter Plot

Hindcast Extreme Value Analysis

A Peak Over Threshold (POT) approach was used to select independent peak wave height conditions from the continuous hindcast dataset. Peak significant wave heights greater than a threshold (in the range 1.0–1.3 m) were selected for extreme value analysis (EVA). A period of 5 days between peaks was adopted to ensure independence of events. The top 10 largest wave heights from the 11 year hindcast are summarised in Table A-3. Four of the top 10 peak wave heights can be attributed to TC events, and the largest hindcast wave height was due to TC Ita in April 2014.

Table A-3 Top 10 Largest Hindcast Wave Heights at Holloways Beach

Rank Date Event Name Peak Hs (m)

1 11/04/2014 TC Ita 1.30 2 2/02/2011 TC Yasi 1.28 3 13/03/2015 TC Nathan 1.20 4 2/03/2008 1.14 5 19/03/2015 1.13 6 20/02/2007 1.10 7 12/04/2012 1.09 8 5/02/2007 1.08 9 9/04/2007 1.07

10 23/01/2013 TC Oswald 1.07

To derive significant wave heights corresponding to the design recurrence intervals relevant to the project, EVA was performed on the POT sample of independent peak wave heights (Coles, 2001). Generalised Pareto distributions were fitted to the POT exceedances using a Maximum Likelihood estimator fitting technique. Distributions were fitted to population samples representing all hindcast wave height peaks (‘all data’) and only non-TC event wave height peaks (‘non-TC data’). The EVA return level plots showing the distributions is shown in Figure A-6.



Figure A-6 11-year Hindcast Extreme Value Analysis Significant Wave Height Return Period Plots: All Data (right) and non-TC Data (left)

Design Wave Parameters

The best-estimate significant wave heights and peak wave period for the 1, 20, 50, 100, 200 and 500-yr ARIs offshore from Holloways Beach is presented in Table A-4. This table includes wave statistics based on non-TC, combined and TC-only populations. Table A-4 may over estimate wave heights at the head of a new groyne structure if the wave growth is depth-limited. This will be investigated further using up-to-date survey and geotechnical data during the design process. The critical wave direction is from the north to north-easterly sector.

Table A-4 Preliminary Design Wave Height and Period Statistics offshore from Holloways Beach (bed elevation 5 m below AHD)

ARI (years)

Hsig (m) hindcast fitted distribution (non-TC data)

Hsig (m) hindcast fitted distribution (all data)

Hsig (m) Tropical Cyclone, BMT WBM (2013)

Tp (s), estimated from Figure 3-10

1 1.01 1.00 - 5 to 6

20 1.18 1.35 2.52 5 to 6

50 1.25 1.43 2.76 6 to 7

100 1.29 1.5 2.81 6 to 7

200 - - 2.85 7 to 8

500 - - 2.88 7 to 8

A.2 Coastal Sediment Transport

Natural longshore sediment transport at Holloways Beach occurs in a net northerly direction at an estimated rate between 5,000 to 10,000 m3/year (BPA 1984; EPA 2002), assuming sediment supply into the beach compartment from the south. It is noted that these are potential rates based on the wave climate and assume sand is ‘available’. BMT (2018) assessed upper beach monitoring data between 2012 and 2018 which showed a deficit of close to 20,000 m3 during that period, implying that the actual longshore transport rate may have been significantly lower due limited sand supply. The width of the potentially ‘active zone’ is relatively narrow and estimated to be up to 30 m during prevailing south-easterly conditions (BPA 1984).



The proposed groynes at Holloways Beach are designed to incept the longshore sediment transport and hold the updrift the shoreline in a more seaward position. The duration to ‘fill’ the new structures is related to the natural longshore sediment transport rate, noting that following construction of a new groyne beach nourishment is often used to accelerate the filling process. When groynes reach a ‘full’ condition, sand can bypass the structure to the downdrift shoreline. This is a desirable condition for Holloways Beach so that the potential for downdrift shoreline erosion is minimised.

The volume of sand required to fill a 30 m long groyne at Holloways Beach is estimated at approximately 4,000 m3. Assuming a natural supply of sand is available, the previous longshore sediment transport estimates suggest a single 30 m groyne would fill within one year. During this period the shoreline downdrift of the structure would be starved of sand.

Holloways Beach Groyne Preliminary Design, Costs & Benefit B-1 Survey (RPS 2019)


Appendix B Survey (RPS 2019)

O

l

d

S

a

n

d

B

a

g

G

r

o

y

n

e

TBM Nail in Conc4.14

1.07

0.98

0.82

0.97

3.30

3.24

1.03

0.93

3.40

3.45

0.78

0.92

3.61

3.79

0.63

-0.97

-0.96

-0.99

-0.98

-0.94

-0.97

-0.90

-0.94

-0.80

-0.88

-1.53

-1.56

-1.51

-1.52

-1.60

-1.61

3.03

3.04

2.86

3.29

3.26

3.37

2.92

3.09

3.47

3.18

2.79

2.63

2.63

2.64

2.16

1.63

1.441.45

0.56 0.99

1.67

1.61

1.86

1.34

1.35

-0.51-0.19

-1.28

-0.95

1.92

-1.51

-1.44

-1.60

-1.67

-1.67

-1.70

-1.61

-1.71

-1.73

-1.77

-1.69

-1.76

-1.80

-1.70

-1.62

-1.71

-1.72

-1.70-1.59

-1.82-1.83

-1.84

-1.83

-1.94

-1.99

-1.85

-1.91

-1.93

-1.69

-1.67

-1.67

-1.72

-1.73

-1.72

-1.68

-1.72

-1.69

-1.59

-1.73

-1.68-1.96

-1.71-1.66

-1.63

-1.85

-1.91

-1.94-1.95-1.93

-1.85

-1.90

-1.82

-1.73

-1.81

-1.96

-1.93

-1.96

-1.89

-1.92

-1.81

-1.90

-1.55

-1.72 -1.73

-1.73-1.68

-1.80 -1.74

-1.74

-1.80

-1.73 -1.87

-1.91

-1.91

-1.68

-1.68

-1.75

-1.82

-1.75

-1.75-1.73

-1.77

-1.78

-1.80

-1.88

-1.71

-1.70

-1.67

-1.65

-1.66

-1.64

-1.71

-1.74

-1.71

-1.76

-1.67

RP73503914

-1.00

-1.00

-1.00

0.00

0.00

0.00

1.00

1.00

2.00

2.00

3.00

3 .00

-1.64

-1.64

-1.64

-1.64

6

0.06

0.06

0.06

86

1.86

1.8

C

a

s

u

a

r

i

n

a

S

t

r

e

e

t

LegendNatural Surface

Hydrographic

Top of Bank

Change of Grade

Fence

Contour - Major

Contour - Minor

Contour - H.A.T (1.857m)

Contour - M.S.L (0.057m)

Contour - L.A.T (-1.643m)

-1.78

-1.78

TreeT 0.4mS 6mH 9m

PROJECT MANAGER

COMPILED

SURVEYED

CAD REF

OF

SHEETS

SHEET SHEET SIZE

AMENDMENTS

A. Solomon

NB 1/7/2019

RMS

PR144129-1.DWG

PR144129 Detail.mjo

1

2 A3

SCALE IS APPLICABLE ONLYTO THE ORIGINAL SHEET SIZE.

metres

0 5 10 15 20 25

1:500(A3)

DRAWING NO.SCALE ISSUEDATE

© COPYRIGHT PROTECTS THIS PLANUnauthorised reproduction or amendment not permitted. Please contact the author.

RPS Australia East Pty LtdACN 140 292 762135 Abbott StPO Box 1949CAIRNS QLD 4870T +61 7 4031 1336F +61 7 4031 2942W rpsgroup.com

GHDDetail Survey

Proposed GroynesHolloways Beach

Sheet 1 of 21:500 5/7/2019 PR144129-1

IMPORTANT NOTE1. This plan was prepared for the sole purposes of the client for the specific

purpose of producing a detail plan. This plan is strictly limited to the Purpose and does not apply directly or indirectly and will not be used for any other application, purpose, use or matter. The plan is presented without the assumption of a duty of care to any other person (other than theClient) ("Third Party") and may not be relied on by Third Party.

2. RPS Australia East Pty Ltd will not be liable (in negligence or otherwise) forany direct or indirect loss, damage, liability or claim arising out of or incidental to:A. Third Party publishing, using or relying on the plan;B. RPS Australia East Pty Ltd relying on information provided to it by the

Client or a Third Party where the information is incorrect, incomplete,

inaccurate, out-of-date or unreasonable;C. any inaccuracies or other faults with information or data sourced from a

Third Party;D. RPS Australia East Pty Ltd relying on surface indicators that are

incorrect or inaccurate;E. the Client or any Third Party not verifying information in this plan where

recommended by RPS Australia East Pty Ltd;F. lodgement of this plan with any local authority against the

recommendation of RPS Australia East Pty Ltd;G. the accuracy, reliability, suitability or completeness of any

approximations or estimates made or referred to by RPS Australia EastPty Ltd in this plan.

3. Without limiting paragraph 1 or 2 above, this plan may not be copied,distributed, or reproduced by any process unless this note is clearly

displayed on the plan.4. Scale shown is correct for the original plan and any copies of this plan

should be verified by checking against the bar scale.

3. The title boundaries as shown hereon were not marked at the time of survey and have been determined by plan dimensions only and not byfield survey. If not able to be so located, services have been plotted fromthe records of relevant authorities where available and have been notedaccordingly on this plan. Where such records either do not exist or areinadequate a notation has been made hereon.

4. Prior to any demolition, excavation or construction on the site, the relevant authority should be contacted for possible location of further underground services and detailed locations of all services.

NOTESLevel Datum:Origin of Levels:

Meridian:

Origin of Coordinates:

Contour Interval:Index:

AHDPSM 38435RL 3.318

PSM 38435E 365648.699N 8137008.954

0.2m1.0m

R

o

c

k

W

a

l

l

R

o

c

k

W

a

l

l

3.36

1.27

3.19

3.32

1.36

1.15

0.95

0.85

0.82

0.70

-0.93

-0.89

-0.89

-0.86

-0.85

-0.85

-0.84

-0.91

-0.95

-1.03

-1.53

-1.60

-1.68

-1.67

-1.61

-1.65

3.07

1.35

3.08

2.99

3.75

2.77

3.21

3.66

3.95

4.09

4.07

3.25

2.58

2.45

2.38

2.10

2.093.24

3.16

2.04

1.71

1.503.00

1.362.76

2.003.05

2.92

2.872.57

1.482.64

1.17

2.16

2.64

2.64

3.02

3.48

3.48

3.64

3.86

3.61

3.65

-1.70

-1.76

-1.72

-1.73

-1.68

-1.69

-1.76

-1.72

-1.59-1.78

-1.68

-1.75

-1.80

-1.75

-1.77

-1.72

-1.69

-1.65 -1.81

-1.78

-1.91

-1.64

-1.72

-1.70

-1.78

-1.66

-1.76

-1.65

-1.63

-1.64

-1.56

-1.98

-1.90

-1.60

-1.61

-1.69

-1.72

-1.71

-1.69

-1.67

-1.72-1.87

-1.88

-1.73

-1.75

-1.73 -1.73

-1.83

-1.82-1.89

-1.84

-1.81

-1.70

-1.71

-1.69

-1.70

-1.68

-1.71

-1.62

-1.60

-1.63

-1.60

-1.64

-1.72

-1.69

-1.67

-1.89

-1.79

-1.86-1.73

-1.74

-1.78

-1.71

-1.65

RP709285

RP709285

RP709285

RP742713

11

10

48

47

-1.00

-1.00

-1.00

0.00

0.00

0.00

1.00

1.00

1.00

2.00

2.00

2.00

3.00

3.00

3.00

3.00

4.00

4.00

-1.64

-1.64

-1.64

-1.64

-1.64

-1.64

-1.64

0.06

0.06

0.06

1.86

1.86

1.86

P

a

n

d

a

n

u

s

S

t

A

c

c

e

s

s

H

i

b

i

s

c

u

s

S

t

LegendNatural Surface

Hydrographic

Top of Bank

Change of Grade

Fence

Contour - Major

Contour - Minor

Contour - H.A.T (1.857m)

Contour - M.S.L (0.057m)

Contour - L.A.T (-1.643m)

-1.78

-1.78

SCALE IS APPLICABLE ONLYTO THE ORIGINAL SHEET SIZE.

metres

0 5 10 15 20 25

1:500(A3)

DRAWING NO.SCALE ISSUEDATE

© COPYRIGHT PROTECTS THIS PLANUnauthorised reproduction or amendment not permitted. Please contact the author.

RPS Australia East Pty LtdACN 140 292 762135 Abbott StPO Box 1949CAIRNS QLD 4870T +61 7 4031 1336F +61 7 4031 2942W rpsgroup.com

GHDDetail Survey

Proposed GroynesHolloways Beach

Sheet 2 of 21:500 5/7/2019 PR144129-2

IMPORTANT NOTE1. This plan was prepared for the sole purposes of the client for the specific

purpose of producing a detail plan. This plan is strictly limited to the Purpose and does not apply directly or indirectly and will not be used for any other application, purpose, use or matter. The plan is presented without the assumption of a duty of care to any other person (other than theClient) ("Third Party") and may not be relied on by Third Party.

2. RPS Australia East Pty Ltd will not be liable (in negligence or otherwise) forany direct or indirect loss, damage, liability or claim arising out of or incidental to:A. Third Party publishing, using or relying on the plan;B. RPS Australia East Pty Ltd relying on information provided to it by the

Client or a Third Party where the information is incorrect, incomplete,

inaccurate, out-of-date or unreasonable;C. any inaccuracies or other faults with information or data sourced from a

Third Party;D. RPS Australia East Pty Ltd relying on surface indicators that are

incorrect or inaccurate;E. the Client or any Third Party not verifying information in this plan where

recommended by RPS Australia East Pty Ltd;F. lodgement of this plan with any local authority against the

recommendation of RPS Australia East Pty Ltd;G. the accuracy, reliability, suitability or completeness of any

approximations or estimates made or referred to by RPS Australia EastPty Ltd in this plan.

3. Without limiting paragraph 1 or 2 above, this plan may not be copied,distributed, or reproduced by any process unless this note is clearly

displayed on the plan.4. Scale shown is correct for the original plan and any copies of this plan

should be verified by checking against the bar scale.

3. The title boundaries as shown hereon were not marked at the time of survey and have been determined by plan dimensions only and not byfield survey. If not able to be so located, services have been plotted fromthe records of relevant authorities where available and have been notedaccordingly on this plan. Where such records either do not exist or areinadequate a notation has been made hereon.

4. Prior to any demolition, excavation or construction on the site, the relevant authority should be contacted for possible location of further underground services and detailed locations of all services.

PROJECT MANAGER

COMPILED

SURVEYED

CAD REF

OF

SHEETS

SHEET SHEET SIZE

AMENDMENTS

A. Solomon

NB 1/7/2019

RMS

PR144129-1.DWG

PR144129 Detail.mjo

2

2 A3

Holloways Beach Groyne Preliminary Design, Costs & Benefit C-1 Geotechnical Investigation (Douglas Partners 2019)


Appendix C Geotechnical Investigation (Douglas Partners 2019)

Holloways Beach Groyne Preliminary Design, Costs & Benefit D-1 Preliminary Design Drawings (GHD 2019)


Appendix D Preliminary Design Drawings (GHD 2019)

LOCALITY PLANNOT TO SCALE

N

HOLLOWAYS BEACH

CASUARINA STREET

HIBISCUS LANE

WIS

TARI

A ST

REET

CASSIA STREET

PANDAN

US LAN

E

GROYNE LOCATION 01

GROYNE LOCATION 02

BARRO

N RIVE

R

BARR

ON R

IVER

This Drawing must not beused for Construction unlesssigned as Approved

Date

CheckDrafting

DateDrawnRevisionNoA1

Rev:Drawing No:

Original Size

Title

Project

Client

Check

DesignerDrawn

Scale

DesignConditions of Use.This document may only be used byGHD's client (and any other person whoGHD has agreed can use this document)for the purpose for which it was preparedand must not be used by any otherperson or for any other purpose.

DO NOT SCALE

Note: * indicates signatures on original issue of drawing or last revision of drawing

Level 8, 15 Lake Street, Cairns QLD 4870 AustraliaPO Box 819, Cairns QLD 4870T 61 7 4044 2222 F 61 7 4044 2288E [email protected] W www.ghd.com.au

Plot Date: Cad File No:10 September 2019 - 9:07 AM G:\6000-6999\6400-6499\6440 Holloways Beach Groynes\DESIGN\CAD\DWG\6440-C001.dwgPlotted by: Matthew Tickner

(Project Director)Approved

JobManager

ProjectDirector

PRELIMINARY

CAIRNS REGIONAL COUNCIL

HOLLOWAYS BEACH GROYNES

COVER, LOCALITY & DRAWING SCHEDULE

6440-C001

A

M. TICKNER M. TICKNER

G. APPLIN G. APPLIN

AS SHOWNA PRELIMINARY MJT GA GA



6440

DRAWING LISTDRG No. DRAWING TITLE

6440-C001 COVER, LOCALITY & DRAWING SCHEDULE

6440-C002 NOTES

6440-C003 GENERAL ARRANGEMENT

6440-C004 GROYNE DETAIL PLANS

6440-C005 GROYNE LONGITUDINAL SECTIONS

6440-C006 TYPICAL GROYNE SECTIONS AND DETAILSHOLLOWAYS

BEACH

This Drawing must not beused for Construction unlesssigned as Approved

Date

CheckDrafting

DateDrawnRevisionNoA1

Rev:Drawing No:

Original Size

Title

Project

Client

Check

DesignerDrawn

Scale

DesignConditions of Use.This document may only be used byGHD's client (and any other person whoGHD has agreed can use this document)for the purpose for which it was preparedand must not be used by any otherperson or for any other purpose.

DO NOT SCALE

Note: * indicates signatures on original issue of drawing or last revision of drawing

Level 8, 15 Lake Street, Cairns QLD 4870 AustraliaPO Box 819, Cairns QLD 4870T 61 7 4044 2222 F 61 7 4044 2288E [email protected] W www.ghd.com.au

Plot Date: Cad File No:10 September 2019 - 9:12 AM G:\6000-6999\6400-6499\6440 Holloways Beach Groynes\DESIGN\CAD\DWG\6440-C002.dwgPlotted by: Matthew Tickner

(Project Director)Approved

JobManager

ProjectDirector

PRELIMINARY



NOTES

6440-C002

A

M. TICKNER M. TICKNER

G. APPLIN G. APPLIN

AS SHOWNA PRELIMINARY MJT GA GA

SURVEY CONTROL NOTES

1. SURVEY ORIGIN - LEVEL DATUM : AHD(D)ORIGIN OF LEVELS : PSM 38435

RL 3.318MERIDIAN : MGA ZONE 55ORIGIN OF COORDINATES : PSM 384235

E: 365648.699N: 8137008.954

GENERAL NOTES

1. ALL WORKS MUST BE CARRIED OUT IN ACCORDANCE WITH THE RELEVANTLOCAL AUTHORITY SPECIFICATIONS UNLESS NOTED OTHERWISE.

2. THE CONTRACTOR IS TO ENSURE A COPY OF THE OPERATIONAL WORKSAPPROVAL IS AVAILABLE ON SITE. THE SITE FOREMAN IS TO ENSURE ALLWORKS ARE UNDERTAKEN IN ACCORDANCE WITH THE APPROVAL.

3. THE LOCATIONS OF SERVICES HAVE BEEN APPROXIMATED FROM THEKNOWN POSITIONS. PRIOR TO ANY DEMOLITION, EXCAVATION ORCONSTRUCTION ON SITE, THE CONTRACTOR MUST CONTACT RELEVANTAUTHORITIES FOR POSSIBLE LOCATION OF FURTHER SERVICES ANDDETAILED LOCATIONS OF ALL SERVICES.

4. EXISTING SERVICES ARE TO BE PROTECTED FROM DAMAGE DURINGCONSTRUCTION. WHERE NECESSARY THE CONTRACTOR SHALL CONFIRMTHE DEPTH/HEIGHT TO EXISTING SERVICES BEFORE COMMENCING WORKS.THE SUPERINTENDENT SHALL BE CONSULTED WHERE THE CONTRACTORCONSIDERS SPECIFIC PROTECTION WORKS NECESSARY TO PROTECT THESERVICE.

5. ALL DIMENSIONS AND RADII ARE EXPRESSED IN METRES (UNO).

6. EXISTING CONTOURS, LEVELS AND FEATURES SHOWN ON THE DRAWINGSARE INDICATIVE ONLY AND ARE BASED ON SURVEY DRAWINGS AND DATAPROVIDED BY BRAZIER MOTTI SURVEYORS.

EROSION AND SEDIMENT CONTROL NOTES

1. NO EARTHWORKS SHALL COMMENCE ON ANY

Documents

Holloways Beach Groyne Preliminary Design, Costs & Benefit · 2019. 9. 20. · Holloways Beach Groyne Preliminary Design, Costs & Benefit i Executive Summary G:\Admin\B23732.g.mpb.HollowaysGroynes\R.B23732.001.02.HollowaysGroyneDesignBasis.docx