Scarborough Reef Feasibility Final Report v1.2

Scarborough Beach Artificial Surfing
Reef Feasibility Study

Final Report
Final Report
September 2017
This report has been prepared under the DHI Business Management System
certified by Bureau Veritas to comply with ISO 9001 (Quality Management)
Approved by
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Approved by
Scarborough Beach Artificial Surfing
Reef Feasibility Study
Final Report
Click here to enter text.
Scarborough Beach (WannaSurf, 2015)
Project manager William Hibberd
Authors William Hibberd, Marcus Tree, Simon Mortensen
Quality supervisor Simon Brandi Mortensen
External Peer Review Ben Matson
Project number 43802163

Approval date 21/09/2017
Revision Final: 1.2
Classification Confidential
DHI Water & Environment Pty Ltd Level 2, 12 Short Street AU-4215 Southport Australia
Telephone: 61 7 5564 0916 Telefax: 61 7 5564 0946 dhi@dhigroup.com
CONTENTS
1 Executive Summary .................................................................................................... 4
2 Introduction ................................................................................................................. 7
2.1 Project Scope ................................................................................................................................. 7
3 Local Wave Climate Assessment ............................................................................... 8

3.1 Wave Climate Modelling ................................................................................................................ 8
3.1.1 Wave model input data ................................................................................................................ 12
3.2 Site Identification .......................................................................................................................... 12
4 Preliminary Artificial Reef Design ............................................................................ 14

4.1 Key Performance Criteria ............................................................................................................. 14
4.2 Reef Design Shape ...................................................................................................................... 15
4.3 Reef Location and Volume ........................................................................................................... 17
5 Artificial Surfing Reef Performance Assessment ................................................... 19

5.1 Surfing Condition Scenario Selection .......................................................................................... 19
5.2 Boussinesq Wave Modelling ........................................................................................................ 20
5.3 OPTISURF Surfing Amenity Modelling ........................................................................................ 22
5.4 Performance Assessment Results ............................................................................................... 23
5.4.1 Surfing Conditions at Scarborough Reef ..................................................................................... 23
5.5 Leeward Wave Reduction and Suitability for Learners ................................................................ 26
5.6 Swimming Safety ......................................................................................................................... 28
6 Preliminary Assessment of Constructability, Safety and Cost .............................. 30

6.1 Constructability............................................................................................................................. 30
6.1.1 Material ........................................................................................................................................ 30
6.1.2 Structural Stability ........................................................................................................................ 32
6.1.3 Construction Method .................................................................................................................... 32
6.2 Safety ........................................................................................................................................... 33
6.3 Cost .............................................................................................................................................. 34
7 Conclusions............................................................................................................... 38
8 Recommendations for Concept Design Phase ....................................................... 40
9 References ................................................................................................................. 42
10 Acknowledgement..................................................................................................... 42
FIGURES
Figure 2-1 Location of Scarborough Beach, wave rider buoys used for model validation, and complex
offshore bathymetric features ........................................................................................................ 7
Figure 3-1 Three-staged spectral wave modelling framework applied to generate a 1 year wave
climate for 2014 at Scarborough Beach ........................................................................................ 9
i
Figure 3-2 Flexible model mesh resolution throughout the local Scarborough Beach spectral wave
model. Model resolution ranged from approximately 700 m in deeper areas of low
bathymetric gradients to 20 m in shallower areas of high bathymetric gradients ........................ 10
Figure 3-3 Areas of 20 m model resolution were applied to resolve detailed shallow bathymetry
across the reefs and shoals between Garden Island and Rottnest Island (top image) and
at the study site at Scarborough Beach (bottom image) ............................................................. 11
Figure 3-4 Highly resolved detailed bathymetry at and offshore of Scarborough Beach ............................. 12
Figure 3-5 Offshore bathymetry at Scarborough Beach (left) and 2D area plot of average swell wave
height distribution for 2014 .......................................................................................................... 13
Figure 3-6 Wave rose plots showing annual Hs of the swell component extracted from Focal Zone 1
(Bottom plot) and Focal Zone 2 (middle plot) and Focal Zone 3 (top plot) .................................. 14
Figure 4-1 Crest level in relation to the historical measured water level recorded at Fremantle .................. 16
Figure 4-2 Scarborough Reef design ............................................................................................................ 16
Figure 4-3 Aerial view outlining the alignment of Scarborough Reef within Focal Zone 1 and the
proximity of the reef to Scarborough Amphitheatre ..................................................................... 18
Figure 5-1 BW model bathymetry with wave generation line ........................................................................ 21
Figure 5-2 2014 wave hindcast spectra from 20th of June 2014 at BW Model western boundary ............... 22
Figure 5-3 Instantaneous OPTISURF output showing rides over the ASR .................................................. 23
Figure 5-4 OPTISURF surfing track results for the Scarborough Reef left-hander ...................................... 24
Figure 5-5 OPTISURF surfing track results for the Scarborough Reef right-hander .................................... 24
Figure 5-6 Quantified wave height, wave count and ride length for surfing tracks derived from
OPTISURF for the left-hander and the right-hander .................................................................... 25
Figure 5-7 Quantified ride speed, wave count and ride length for surfing tracks derived from
OPTISURF for the left-hander and the right-hander .................................................................... 25
Figure 5-8 Instantaneous BW model output showing wave breaking on the ASR ....................................... 26
Figure 5-9 Differences in average significant wave height between the with the reef and without the
reef scenarios derived from the 30 minute wave condition. The black dashed line outlines
the reef extent .............................................................................................................................. 27
Figure 5-10 Averaged significant wave height with the reef derived from the 30 minute wave condition.
The black dashed line outlines the reef extent ............................................................................ 28
Figure 5-11 Differences in average wave driven current speed (m/s) between the with the reef and
without the reef scenarios derived from the 30 minute wave condition. The black dashed
line outlines the reef extent. The white dashed line presents the extent of the zone of
reduced swimming safety. The minimum distance to the zone of reduced swimming safety
from the 0m MSL contour is ~40 m ............................................................................................. 30
Figure 6-1 Construction method using BME applied for the construction of Parker Point Artificial Reef
(Western Australia) (Source: MScience (2012)) .......................................................................... 33
Figure 8-1 DHIs CFD model provides an exceptional accurate framework for safety assessment
detailed surf optimization of an artificial surfing reef (Mortensen 2009). ..................................... 40
Figure 9-1 Validation of the Scarborough local wave model against measured data from Rottnest
wave rider buoy for total wave component for the three (3) month validation period .................... 1
wave rider buoy for sea wave component for the three (3) month validation period ..................... 2
wave rider buoy for swell wave component for the three (3) month validation period .................. 2
wave rider buoy for total wave component for the full 2014 simulation year ................................. 3
Figure 9-5 Validation of the Scarborough local wave model against measured data from Cottesloe
wave rider buoy for total wave component for the three (3) month validation period .................... 3
wave rider buoy for sea wave component for the three (3) month validation period ..................... 4
wave rider buoy for swell wave component for the three (3) month validation period .................. 4
wave rider buoy for total wave component for the full 2014 simulation year ................................. 5
ii
TABLES
Table 3-1 Average significant wave height of the swell component at the three (3) focal zones ................ 13
Table 4-1 Reef design criteria ...................................................................................................................... 18
Table 5-1 Most common surf event parameters .......................................................................................... 20
Table 5-2 Selected representative surf event parameters ........................................................................... 20
Table 6-1 Preliminary cost estimate for Scarborough ASR ......................................................................... 35
APPENDICES
Appendix A: Spectral Wave Model Validation
Appendix B: OPTISURF Surfing Amenity Modelling
iii
1 Executive Summary
Scarborough is a popular metropolitan beach of Perth, Western Australia, located 14km
northwest of the city centre. Facing due west, Scarboroughs swell climate is heavily attenuated
by Rottnest Island and a series of offshore parallel limestone reefs, which absorb a significant
percentage of swell energy from the Indian Ocean and create a smaller swell climate compared
to exposed coasts such as Margaret River. Additionally, Scarborough and surrounding beaches
generally display relatively straight profiles which frequently result in close-out waves,
unsuitable for surfing.
The objective of the study was to investigate the feasibility of constructing an Artificial Surfing
Reef (ASR) at Scarborough Beach. The reef design presented in this report has been developed
by DHI experts based on a set of Key Performance Criteria defined by the client aimed at
providing a significant enhancement to the quality of local surfing amenity.
DHI were commissioned to investigate a top-tier design option involving an A-frame reef design
for this study. It is appreciated that the design can be scaled down in size and layout if
construction costs are later found to be prohibitive. One of the major potential cost reduction
options would be to reduce the A-Frame reef to a single wedge shaped structure, which would
reduce costs by almost 50%.
This feasibility study provides a clear and concise overview of proposed location, design, surf
performance, safety, construction method and cost of the proposed reef.
In this report the authors have done their outmost to address each item carefully and beyond
what would normally be expected for a feasibility design. Yet it is important to stress that, if the
project is found feasible to proceed to the next stage, a further detailed analysis will be required
of all aspects of detailed surfing optimization, structural stability, safety and environmental
impact assessment. Such further analysis is most often included as part of a Concept Design
study.
Three nearshore locations offering favourable natural wave conditions were identified, from
which the site closest to the Scarborough Amphitheatre was selected, due to its close proximity
to existing beach amenity. The surf reef design presented in this report has been optimised for
this location. Subject to minor adjustments it is expected to also be able to deliver a similar
increase in the surf quality at the two other locations.
A reef design was developed, that involved an A-Frame type surf break with a left-lander suited
for intermediate surfers and a right-hander aimed primarily at advanced surfers. Surf quality will
vary daily with natural variations in offshore wave conditions and tides. The reef is designed
specifically at producing a significant increase in the length of surf ride compared to current
conditions even in typical average quality wave conditions at Scarborough.
Numerical Boussinesq wave and OPTISURF simulations using one (1) representative natural
sea-state was carried out to document the surf reef performance. The study documented that
the reef was able to generate surf rides consistently longer than 40 m and with some rides in
excess of 120 m with pealing speeds suitable mostly for intermediate to advanced surfers. Local
wave focusing on the reef results in waves to be significantly larger compared to the open
beaches, especially at the initial take-off zone. Compared to existing conditions the reefs
capacity to generate significantly longer and noticeably larger surfable waves is expected to
significantly increase the quality of local surfing amenity to a level that is attractive to regional
surfing competitions. In this study the reef was only tested during average quality wave
conditions.
4
Executive Summary
During more infrequent episodes of large waves, the reef is expected to produce excellent
surfing conditions of very high quality but also with a potential increased risk of injury to the
surfer. Further testing and design optimizations in subsequent studies is strongly recommended
in order to minimize any unacceptable safety risks.
During times of very small waves the reef is expected to produce gentle surf conditions suitable
for beginners.
The modelling approach used in feasibility design provides a good insight into the natural
variation in surfing wave performance on the reef that can be expected during a typical surf
wave condition at Scarborough, but more wave events and detailed CFD modelling is
recommended for further optimization of performance and safety assessment.
During times of energetic wave activity the reef will provide a limited sheltered zone across the
shoreline to its lee, with noticeably smaller breaking waves compared to adjacent beaches. This
is expected to increase swimming safety substantially as it reduces the risk of injury from
plunging shore breaks. The reduced wave activity also means a reduction in rip-currents, which
can pull swimmers out to sea.
However at the same time the reef will generate strong return currents close to its edges, which
may pose a hazard to unseasoned swimmers but only if they get too close to the reef during
times of energetic wave activity. It is expected that swimming during such periods of energetic
wave activity would be uncommon.
If the reef construction is combined with proper marking of designated safe swimming areas
behind the reef, the overall level of swimming safety is expected to be increased in the lee of the
structure.
During periods of minor swell activity there will be negligible return currents. Within a few years
the reef is expected to provide excellent snorkelling and diving amenity.
As the reef is expected to generate a strong concentration of recreational water users, it is

strongly recommended that a life guard tower should be manned during daytime hours. An
electromagnetic shark deterrent system placed offshore the reef and a camera based real-time
forecast system of nearshore current should also be considered.
The reef should be constructed from rock using a barge-mounted excavator. Preliminary cost of
the structure is $16.9 million based on 2016 price estimates from local contractors, local weather
data and best practice estimates. Note that the price is largely proportional to reef volume and
an A-Frame reef requires almost twice the volume as a wedge shaped reef (individual left or
right-hander).
Further savings may be achievable if further studies identifies a substitute rock material to
granite. A potential option could be to further investigate the applicability of using limestone
which could provide a potential material cost saving of approximately 25%.
The overall cost of the reef is roughly linearly scalable with the volume of rock required as it
affects both the material price and the time required for placement. As a result the developed
reef design could be adjusted to fit a smaller budget simply by shortening the wings of the
structure. Such a modification would result in a shorter length of surf ride. Changes to the
shoaling platform or steepness of the structure is not recommended as it may compromise key
performance and safety aspects of the design. Based on budget availability further adjustment
of reef layout to fit a target budget may be recommended.
The artificial surfing reef is likely to result in a significant coastal response with a salient likely to
form in its lee and potential shoreline setback to the north. At its proposed location it is
considered likely that the resulting shoreline impact will be seen as beneficial by causing
accretion in front of existing beach infrastructure, while the setback will occur in an area with no
5
existing developments. However a further detailed study must be carried out in order to confirm
expected outcomes. If the coastal impact were found to be adverse, the reef location could be
located to one of the two other potential sites which would be deemed less sensitive to localized
shoreline changes.
Overall, it is considered that the artificial reef has the potential to provide a large increase in
surfing amenity for Scarborough Beach with a robust design framework for mitigating potential
adverse impact identified in subsequent stages of the project.
6
Introduction
2 Introduction
DHI were commissioned to undertake a feasibility assessment of an artificial surfing reef (ASR)
at Scarborough Beach in Perth, Western Australia (Figure 2-1).
Figure 2-1 Location of Scarborough Beach, wave rider buoys used for model validation, and complex
offshore bathymetric features
2.1 Project Scope

The aims of the study was to investigate the feasibility of constructing an ASR at Scarborough
beach. The assessment included;
An assessment of the best suitable location on Scarborough for hosting an

artificial surfing reef (feasibility screening study)
Preliminary design of the artificial surfing reef
Surfing performance assessment for one (1) typical surf wave event.
Preliminary assessment of constructability, safety and cost
7
3 Local Wave Climate Assessment
The local wave climate was assessed to provide a number of important inputs for the study;
To identify which nearshore areas along Scarborough Beach would be suitable

for an artificial surfing reef.
To quantify the distribution of nearshore wave conditions in order to provide input

into the preliminary reef design, such as orientation and crest level in accordance
to representative wave directions and heights.
To identify one (1) representative surfing condition for use in the surfing
performance assessment model scenarios.
3.1 Wave Climate Modelling

The local wave climate was generated using DHIs spectral wave model (MIKE21 SW) for the
full year of 2014. The year of 2014 was selected as it allowed DHI to use an existing offshore
hindcast dataset from its archives. The full year of historic wave conditions were considered well
suited for identifying typical variations in average surf conditions occurring at Scarborough.
The approach utilised a framework comprising of three separate models (as illustrated in Figure
3-1). Initially, DHIs Global Wave Model, with a one degree (~110 km) resolution was used to
resolve ocean-scale wave generation and growth processes and long period wave energy
components. Spectral wave energy transfer boundaries were extracted from the Global Wave
Model and used to force DHIs OzSea regional wave model that encompasses all Australian
waters. Ozsea further resolved wave generation and growth processes on a smaller scale
together with wave transformation across the south west Australian continental shelf. Spectral
wave energy transfer boundaries were extracted from the OzSea domain and applied to force
the fine-scale Scarborough Wave Model (Figure 3-2).
Local model resolution progressively increased from deeper to shallower depths. Figure 3-2
presents the distribution of model resolution bands ranging from 700 m to 20 m. Fine 60 m and
20 m resolution was applied to resolve the shallow reefs between Garden Island and Rottnest
Island (Figure 3-3). Resolving these features is important due to the wave climate prevailing
from the south west. Therefore, wave conditions along the Perth metropolitan coast will be
influenced by these shallow features. Sufficiently resolving the local bathymetry adjacent to
Scarborough Beach in high detail (Figure 3-4) was required to ensure accurate resolution of the
wave conditions locally within the study site.
To assess the performance of the Scarborough Wave Model, modelled wave data were
validated against measured data from the Rottnest and Cottesloe buoys, acquired from the
Department of Transport (WA), for a three month period; 1 March 2014 to 1 June 2014.
Modelled verses measured validation plots are presented in Appendix A.
8
Local Wave Climate Assessment
Figure 3-1 Three-staged spectral wave modelling framework applied to generate a 1 year wave climate
for 2014 at Scarborough Beach
9
Figure 3-2 Flexible model mesh resolution throughout the local Scarborough Beach spectral wave
model. Model resolution ranged from approximately 700 m in deeper areas of low
bathymetric gradients to 20 m in shallower areas of high bathymetric gradients
10
Figure 3-3 Areas of 20 m model resolution were applied to resolve detailed shallow bathymetry across
the reefs and shoals between Garden Island and Rottnest Island (top image) and at the
study site at Scarborough Beach (bottom image)
11
Figure 3-4 Highly resolved detailed bathymetry at and offshore of Scarborough Beach
3.1.1 Wave model input data

All three models were forced with 22.5 km resolution global winds sourced from the NCEP
Climate Forecast System Reanalysis (CSFR) database available from UCAR (2016). All three
domains used DHIs C-MAP bathymetry chart-based database. The regional and the local
model used the Department of Transport 5x5 m Lidar bathymetry survey data set in the Perth
metropolitan region (DoT, 2016).
3.2 Site Identification

Shallow features offshore of a beach (such as bomboras and reef pinnacles) can play a key role
in preconditioning incoming wave trains through wave shoaling and focusing. Such features can
concentrate wave energy inshore leading to increased wave heights at localised regions along
the beach. Locating the reef in a focal zone would make the most of the naturally amplified wave
energy, thus adopting a working with nature approach.
12
From Figure 3-5 three (3) key wave focusing zones were identified and can be observed in the
2D area plot of the averaged swell height derived from the 2014 wave climate. Wave heights are
approximately 10-20% larger at the focal zones compared to the adjacent beach.
Focal Zone 3 is located approximately 1.5 km north of the amphitheatre. Focal Zone 2 is
positioned more than 1 km north of the amphitheatre, is known as Thirds to local surfers. Focal
Zone 1 is located 200 m north of the amphitheatre. The average swell wave heights for each
focal zones are summarised in Table 3-1.
Table 3-1 Average significant wave height of the swell component at the three (3) focal zones
Focal Zone Average Hs of Swell [m]
Zone 1 1.15 m
Zone 2 1.2 m
Zone 3 1.16 m
Figure 3-5 Offshore bathymetry at Scarborough Beach (left) and 2D area plot of average swell wave
height distribution for 2014
13
Figure 3-6 Wave rose plots showing annual Hs of the swell component extracted from Focal Zone 1
(Bottom plot) and Focal Zone 2 (middle plot) and Focal Zone 3 (top plot)
4 Preliminary Artificial Reef Design
4.1 Key Performance Criteria

The following key performance criteria for the ASR were set by the client during this study to
guide the objectives of the feasibility study;
Should be an a-frame reef design to cater for intermediate (left hander) and
expert surfers (right hander)
A location in close proximity to Scarborough Amphitheatre is preferred
Structure must not be surface piercing at any time
Must not create no adverse coastal impact
14
Preliminary Artificial Reef Design
Create safe swimming / learner surfing conditions nearshore
A reef construction budget in excess of $10M is acceptable
A design schematic of the reef is presented in Figure 4-2 and key design parameters for the reef
are summarised in Table 4-1. The below sections discuss the components of the reef and
describe how the components produce enhanced surfing conditions.
DHI were commissioned to investigate a top-tier design option involving an A-frame reef design
for this study. It is appreciated that the design can be scaled down in size and layout if
construction costs are later found to be prohibitive. One of the major potential cost reduction
options would be to reduce the A-Frame reef to a single wedge shaped structure, which would
reduce costs by almost 50%.
4.2 Reef Design Shape

The reef is designed as an A-frame with a V-shaped crest, with each crest arm being 90 meters
long. The reef comprises of three key features;
A conical wave focusing toe
A 45 angled right-hand shoaling platform
A 40 angled left-hand shoaling platform
The focusing toe is designed to refract the incoming waves to the centre of the A-frame where
the wave will focus to a peak and subsequently break along both the left and right-hander
simultaneously. The focusing toe is an important aspect of the surf reef as it provides a
predictable take-off zone with consistently larger waves than in surrounding areas.
The focusing toe has a 1/12 bed slope designed to produce a plunging (barreling) wave of
medium intensity, suitable for intermediate to expert surfers.
The left-hand shoaling platform gradient tapers off from the focusing platform to a shallower
gradient of 1/20 at the end of the platform and has a peel angle of 45.
The left-hander is designed to provide waves suitable for intermediate level surfers. The
tapering of the gradient will result in the wave breaking intensity reducing with distance along the
reef platform.
The right-hand shoaling platform gradient progressively steepens from the focusing platform to a
gradient of 1/10 at the end of the platform and has a peel angle of 40. The right-hander is
designed to produce a faster breaking wave with a progressively increasing breaking intensity
(more hollow), suitable for advanced surfers.
From analysing the annual wave climate at the toe of the reef the wave direction was found to
be very close to due west (Figure 3-6). The reef was aligned to the mean swell direction (272)
derived over the annual wave climate.
From analysing the wave climate and considering the tidal range (Figure 4-1) the crest of the
reef is positioned at -1.3 mMSL to ensure wave breaking at the reef during typical surfing
conditions, while maintaining sufficient depth over the crest as a safety measure and to prevent
the reef from being exposed at lowest astronomical tide.
15
Figure 4-1 Crest level in relation to the historical measured water level recorded at Fremantle
Figure 4-2 Scarborough Reef design
16
Preliminary Artificial Reef Design
4.3 Reef Location and Volume

The location of the ASR was selected in order to maximize the effects of naturally generated
preconditioning of the incident wave train. For this purpose, two potential reef locations were
identified within the three separate wave focusing zones as identified from Figure 3-5.
Wave Focal Zone 1 was chosen over Focal Zone 2 as it had almost the same natural wave
focusing and was located in much closer proximity to the Scarborough Amphitheatre (Figure
4-3).
The offshore distance of the reef from the shoreline influences the relative size of the structure
and its level of interaction with nearshore coastal processes. If the ASR is placed very close to
shore, it will reduce the reefs capacity to transform the incoming waves before breakpoint, which
can create confused, uneven and dangerous (collapsing) wave breaking on the reef. In areas
with substantial near-shore sediment transport, the reef will also have to sustain a highly
dynamic coastal morphology.
Placing the ASR further offshore solves these issues but comes with a higher demand to reef
size and costs. A larger reef will allow for more influence over the train, causing more wave
refraction, wave shoaling and therefore wave amplification. This results in larger waves over the
structure compared to waves that would otherwise break on the natural beach.
Past multi-purpose surfing reef projects have demonstrated the requirement for sufficient size in
order to adequately influence the wave trains. Mortensen et al., (2015) includes a detailed
review of all multi-purpose surfing reefs constructed to date, with volumes ranging from 1475 m3
(Prattes Reef) to 65,000 m3 (Narrowneck), with Boscombe Reef at the second from largest;
13,000 m3.
In this feasibility study, it was chosen to place the ASR between the 9.5 m and 4.0 m depth
contour (MSL) so that the distance from the inshore tip of the reef to the 0 m (MSL) contour was
approximately 100 m. The resultant reef design has a volume 51,000 m3 and an area of
23,300 m2. The volume of Scarborough Reef is therefore within the upper range compared to
past projects.
The location of the ASR is an important consideration when estimating the level of shoreline
response to the adjacent coastline. In general the impact of the reef on the leeward beach
morphology will be reduced with increased distance offshore. However, the volume required to
build the reef increases with distance offshore that will consequently increasing construction
cost. In contrast, moving the ASR further inshore will save volume costs, but further increase
the shoreline impact.
A detailed shoreline assessment is strongly recommended for confirming the location and size
of the ASR developed in this feasibility study.
17
Figure 4-3 Aerial view outlining the alignment of Scarborough Reef within Focal Zone 1 and the
proximity of the reef to Scarborough Amphitheatre
An overview of reef design specifications are provided in Table 4-1.
Table 4-1 Reef design criteria
Design Specification Value
Position Focal Zone 1
Minimum distance from shoreline 100 m
Length (shore parallel) 220 m
Width 170 m
Inshore depth -4 m MSL
Offshore depth -9.5 m MSL
Crest level -1.3 m MSL
18
Artificial Surfing Reef Performance Assessment
Crest length 90 m
Toe angle 1/12
Volume 51,000 m3
Toe Area 23,300 m2
Peel Angle (right / left) 40 / 45
5 Artificial Surfing Reef Performance Assessment

The performance of the artificial reef and the impact of the reef on the surfing conditions in the
lee of the reef were assessed by simulating a representative wave event expected to produce
favourable surfing conditions at Scarborough Beach. The wave event was simulated in DHIs
non-linear 2D Boussinesq Wave Model MIKE21 BW (BW) without the reef and with the reef in
place. The resultant BW model wave field outputs were used as inputs into DHIs OPTISURF
model for the quantification of surfing wave quality index parameters required for the
performance assessment.
5.1 Surfing Condition Scenario Selection

An analysis of surf event recurrence in 2014 was undertaken to determine the wave condition
for the BW model. In order to provide a realistic quantification of the frequency and duration of
the surfable events lower bounds were established where surfable waves were expected to be
present at Scarborough. In addition it was set as a requirement that surfing events could only
occur during daylight hours and during episodes of offshore wind or light onshore wind. A
summary of the surf criteria requirements are given below.
Minimum Surfable Criteria

Minimum Significant Wave Height (Hs) of 0.5 m
Minimum Spectral Peak Period (T p) of 8 seconds
The wind direction must originate from the East (90) or wind speed must be less than
6 m/s
The event must last for at least 2 hours
Occur during average daylight hours between 6am and 6pm
Surfing at Scarborough is expected to be possible outside of these conditions, but is not

expected to represent favourable surfing conditions.
Wave parameters were extracted from 2014 wave hindcast at the approximate location of the
reef toe in the Scarborough Beach spectral wave model and winds from Swanbourne were
provided by the Bureau of Meteorology.
The three (3) most common surf event parameters are described in Table 5-1. A representative
surf event with parameters within the most populated category (category 1) was selected for the
ASR performance assessment. Parameters for the representative event that occurred on the
20th of June 2014 are presented in Table 5-2.
19
Table 5-1 Most common surf event parameters
Binned Event Rank No. Hs Swell [m] Tp Swell [s] DirP Swell []
1 1.0 1.5 14 15 265 - 270
2 1.0 1.5 14 15 270 - 275
3 0.5 1.0 13 14 260 265
Table 5-2 Selected representative surf event parameters
Hs [m] Tp [s] DirP []
Swell 1.28 14.1 270
Wind Waves 0.59 6.8 270
Total 1.39 14.1 270
5.2 Boussinesq Wave Modelling

MIKE21 BW is one of the most advanced wave models currently available in the industry and is
ideally suited to the calculation of wave transformations and breaking in nearshore coastal
areas.
The BW model was used to provide a detailed assessment of wave breaking on the artificial
surfing reef taking into account the influence of the regional bathymetry and a realistic natural
irregular sea state.
The BW model computational domain was discretised by a Cartesian grid aligned with true north
with an extent of 2600 m by 4000 m and a horizontal grid spacing of 1.5 m. Coordinate system
was MGA-50 with the vertical datum referenced to mean sea level.
Model bathymetry was generated from the same set of Lidar bathymetry used for the spectral
wave models; Department of Transport 5x5m Lidar bathymetry survey (DoT, 2016). BW model
bathymetry with superimposed wave generation line is shown in Figure 5-1.
20
Figure 5-1 BW model bathymetry with wave generation line
The simulation period for each model run was 40 minutes in order to allow for sufficient model
warm up (approximately 10 minutes) and an adequate time window for OPTISURF and spectral
analysis.
The wave boundary condition was applied as an internal wave generation line close to the
western boundary represented by a fully directional and irregular sea state. This was derived
from the full spectral output of the 2014 wave hindcast. The boundary condition spectra for the
20th of June 2014 is shown in Figure 5-2. A second order correction scheme was applied to the
incident wave train to prevent the release of spurious harmonics in the weakly non-linear wave
conditions at the boundary.
21
Figure 5-2 2014 wave hindcast spectra from 20th of June 2014 at BW Model western boundary
Maximum water depth was set to 16.5 m to ensure a constant water depth along the wave
generation line and limit the potential for deep water instabilities. If necessary an area within
75 m of the wave generation line was interpolated to provide a smooth transition to the
surrounding bathymetry. Sponge layers of the wave length were applied at the western
boundary and 1/8 of the wave length at the northern and southern boundaries to prevent non-
physical reflections of energy back into the domain.
A moving shoreline was applied in this BW model to resolve wave run up. With a moving
shoreline no minimum water depth is required and model accuracy near the shoreline is
improved.
5.3 OPTISURF Surfing Amenity Modelling

BW model wave field outputs were used as inputs into DHIs OPTISURF model for the
quantification of surfing wave quality index parameters. Thickness of whitewater (broken
waves, or rollers) is output from the BW model and analysed in OPTISURF. OPTISURF tracks
the propagation of the rollers to determine where the pocket or optimal surf position is for each
step in time. This provides a series of rides each with corresponding speeds, distances and
wave heights. A snapshot of OPTISURF results are shown in Figure 5-3. Further detail is
discussed in Appendix B.
22
Figure 5-3 Instantaneous OPTISURF output showing rides over the ASR
5.4 Performance Assessment Results
5.4.1 Surfing Conditions at Scarborough Reef

For the purpose of comparison the OPTISURF surfing track results for the right-hander and left-
hander have been presented in separate figures despite being generated from the same model
simulation. The spatial area surfing track results of the left-hander and right-hander are
presented in Figure 5-4 and Figure 5-5, respectively. The results show the ridable waves start to
break at the focusing toe and peel along the contours of each shoaling platform. Simultaneous
wave breaking on the left-hander and right-hander can be observed in Figure 5-8. As
anticipated, larger waves were found to break in deeper water, further offshore on the focusing
toe. Conversely, smaller waves were found to break in shallower depths closer to the crest.
A quantitative summary of wave height, wave count and ride length for ridable waves tracked at
the left-hander and the right-hander is presented in Figure 5-6. A quantitative summary of ride
speed and ride length is summarised in Figure 5-7.
The right-hander and the left-hander generates rides up to 120 m and 100 m in length,
respectively, with a majority of wave face heights between 1.8 m to 2.6 m. The majority of the
waves > 60 m in length return speeds between 25 and 35 km/h. The surfing criteria returned for
the majority of the rides would provide a substantial level of surf amenity suitable for
intermediate to advance surfers at Scarborough Beach.
The mean ride speeds produced by the right-hander was overall higher, which is a result of the
smaller peel angle.
23
Figure 5-4 OPTISURF surfing track results for the Scarborough Reef left-hander
Figure 5-5 OPTISURF surfing track results for the Scarborough Reef right-hander
24
Figure 5-6 Quantified wave height, wave count and ride length for surfing tracks derived from
OPTISURF for the left-hander and the right-hander
Figure 5-7 Quantified ride speed, wave count and ride length for surfing tracks derived from OPTISURF
for the left-hander and the right-hander
25
Figure 5-8 Instantaneous BW model output showing wave breaking on the ASR
5.5 Leeward Wave Reduction and Suitability for Learners

Differences in the average significant wave height with and without the reef is presented in
Figure 5-9. The degree of wave amplification over the reef (positive values exceeding 0.4 m)
and subsequent wave shadowing (negative values lower than 0.4 m) in the lee of the structure
are both clearly depicted. As a result, smaller waves will be present behind the reef. The
reduction of wave height in the lee of the structure is presented in the plot of the averaged
significant wave height derived over the 30 minute wave condition (Figure 5-10). These smaller
waves will be more approachable for beginner surfers as opposed to the larger waves
experienced without the reef and also along the adjacent beaches with the reef in place.
Therefore, the reef looks to provide a sheltered zone of reduced wave heights more suitable for
beginners.
26
Figure 5-9 Differences in average significant wave height between the with the reef and without the
reef scenarios derived from the 30 minute wave condition. The black dashed line outlines
the reef extent
27
Figure 5-10 Averaged significant wave height with the reef derived from the 30 minute wave condition.
The black dashed line outlines the reef extent
5.6 Swimming Safety

Introducing fixed structures that alter the wave field within the nearshore will modify wave driven
currents leeward of the structure. Differences in net current speed within the nearshore zone
with and without the reef are presented in Figure 5-11. The net current is not to be confused with
a constant current but is caused by large periodic water movements over the reef caused by
bores formed by breaking waves.
The highest increase in current speed (>0.9 m/s) is located along the crests of the reef structure
as a result of the wave breaking at the reef. Regions of increased longshore current speed in the
order of 0.2-0.7 m/s prevail from the end of each crest. In the present model the reef structure is
currently represented as impermeable. Introducing the porosity of the rock armouring layers in
subsequent studies will induce a substantial amount of energy dissipation, which is expected to
reduce current magnitudes.
The longshore currents are a residual effect of the intense wave breaking driven currents over
the reef. The longshore currents dissipate with increased distance from the end of the reef, in a
28
northerly and southerly direction, dissipating over a distance of approximately 350 m in each
direction. The residual currents remain ~40 m from the shoreline. Landward of the 40 m exists a
margin of currents that remain relatively unchanged in velocity.
Within the lee of the reef a shadow zone of reduced current speeds (by <0.1m/s) exists. This
reduction in current speed is attributed to the reduction in wave heights and resultant wave
shadow leeward of the reef, indicted in Figure 5-9 and Figure 5-10.
In regard to swimming safety, the reef has introduced higher current speeds that persist beyond
~40 m offshore and ~350 m north and south of the reef. These currents are in the order of
magnitude that will influence swimming conditions. It should be noted that the return currents
are almost parallel to the beach, which does impose the save level of risk as opposed to an out-
going rip current.
structure during normal wave conditions.

Effective management of beach uses will need to be actioned by the lifeguards. In order to
reduce the impact of the reef on swimmer safety no swim zones a set distance north and south
of the reef could be implemented.
The level of assessment of the wave driven current magnitudes against existing conditions is
limited to the single wave event assessed in this study. Undertaking further assessment of
current magnitudes caused by the reef against a wide range of typical conditions are important
to enable further assessment of the level of impact of the reef on swimming safety in
comparison to existing conditions.
29
Figure 5-11 Differences in average wave driven current speed (m/s) between the with the reef and
without the reef scenarios derived from the 30 minute wave condition. The black dashed
line outlines the reef extent. The white dashed line presents the extent of the zone of
reduced swimming safety. The minimum distance to the zone of reduced swimming safety
from the 0m MSL contour is ~40 m
6 Preliminary Assessment of Constructability, Safety and Cost
6.1 Constructability
This section outlines the justification of the proposed material selection, construction method
and sequence of the ASR at Scarborough.
6.1.1 Material
To date the construction and longevity of functional submerged control structures (or reefs, for
coastal protection and/or improved surfing amenity) that meet and maintain the intended key
30
Preliminary Assessment of Constructability, Safety and Cost
design objectives has proven challenging. An extensive review was undertaken in Mortensen et
al., (2015) that addressed seven existing submerged control structures around the world.
Of the seven assessed structures, Cables Reef in Western Australia was the only reef
constructed using rock (granite), while the remaining reefs were constructed using geotextile
containers; Boscombe (UK), Prattes (USA), Opunake (New Zealand), Mount Maunganui (New
Zealand), Kovalum (India) and Narrowneck (Australia).
Cables Reef was considered successful in producing a significant increase in surfable waves on
an annual basis in the short (Bancroft, 1999) and in the long-term (Pattiaratchi, 2007). The use
of granite stone to modify an existing limestone platform at Cables has been proven to create a
stable artificial surfing reef that has continued to function according to the design specification.
There has been no report of maintenance required since construction. There were issues
regarding the accuracy of initial placement with some rock rolling off the existing limestone
platform. Nonetheless, such difficulties should not be experienced if deployed onto a shallow
gradient sand bottom bed such at Scarborough.
For each geotextile application, negative reports outlining either one or more of the following
were discussed;
placement inaccuracies
weight induced container settlement
settlement due to storm bar movement
tearing causing damage beyond repair resulting in the loss of sand
container movement
fill valve failure resulting in the loss of sand
material degradation and fragmentation
anthropogenic induced damage (for example by anchors, propellers and spear
guns)
For all geotextile designs, the containers were reported to be prone to moving, settlement and
failing structurally. The resulting impacts were in some instances due to the complexity of the
initial reef design being too difficult to build or, if built to specification, the shape was not
maintained over time. If a reef alters from the specific design then the reef will no longer perform
to the design specification and will therefore pose a risk of not continuing to meet the intended
project objectives. When such failures have occurred at existing geotextile multi-purpose reefs,
costly on-going maintenance work has been required, for example: Narrowneck (2001 - 2006),
Boscombe (2010 - 2012) and Mount Maunganui (2007 ongoing).
Several stability issues arose in a number of previous projects due to the reef being positioned
too shallow and placed in the active, highly mobile profile causing the containers to shift, sink
and alter from the design shape, thus reducing functionality. All geotextile container designs
have experienced significant problems with breakage, burial and shifting of containers
commencing from the time of construction. For Narrowneck, Maunganui and Boscombe, the
surfing quality at all three reefs has diminished over time due to structural changes in the initial
design shape. At the time of writing none of these reefs have worked for years.
Based on the extensively documented failures of geotextile containers, it is highly recommended

geotextile containers are not considered for the construction of an ASR at Scarborough. In
addition to the confirmed challenges with obtaining the required design shape, it is considered
alarming that all constructions reviewed to date have experienced significant structural failures
of its geotextile containers, in most cases destroyed any produced surfing amenity after a few
years.
Based on the findings of the comprehensive review outlined in Mortensen et al., (2015) it is
recommended that an ASR at Scarborough is to be constructed from rock. Based on local
availability and the proven success of durability and material integrity from Cables Reef, the
31
preliminary recommended rock type is granite. Further investigations of using cheaper
alternative rock sources such as limestone is recommended but beyond the scope of the current
project.
6.1.2 Structural Stability

The reef design should comprise of a core layer, a filter layer and an amour layer. Rock sizing
depends on the nearshore wave heights during an extreme storm event. Most typically, a 100
year return period event will be chosen for such a structure as an ASR. The depth limiting wave
height at the toe of the reef is 7.6 m, but the actual maximum wave height is likely to be
significantly less due to sheltering from offshore reef formations.
Palm Beach Shoreline Project Concept Design (DHI, 2014) previously estimated that a rock
armouring layer of 1.1 m D50 was required based on an extreme significant wave height of 6.5
m (Hmax ~9 m). The concept design estimate was based on design formula originally developed
for steeper sloped breakwaters, as opposed to the more gentle slopes used for the artificial
reefs.
In support of the above claim, recent physical model tests for the Palm Beach projects adopting
a 100 yr ARI design significant wave height of 6.2 meters (Hmax ~9 m) found that armouring
rock sizes could be limited to a diameter (D50) of 0.9 m and a core layer of ~0.5 m.
Further studies are required to confirm a detailed rock sizing for the Scarborough ASR, but it is
considered likely that rock armour sizes can be reduced to less than 0.9 m D50 due to a milder
wave climate and heavier rock density compared to the Palm Beach SCS.
6.1.3 Construction Method

One option of construction has been considered in this feasibility study and involves a barge
mounted excavator (BME), using a shallow draft flat barge with a long reach crawler excavator
on the deck (as per the example in Figure 6-1). This option was the recommended option, due
to price, outlined in the Palm Beach Shoreline Project Concept Design (DHI, 2014). The
alternative option for Palm Beach was construction via a side stone dumping vessel (SSDV) with
a deck mounted crane (or excavator) to construct the crest.
Barge Mounted Excavator (BME)

The BME option would apply a similar construction method as used for Cables Reef but with a
larger long reach excavator and a larger barge. Two 2000t capacity barges 55-60m in length
and 15-20m in width would be appropriate for construction. Barges of similar size have
approximate drafts of 0.5m (unloaded), 2.8m (fully loaded) and less than 1.5m for 1000t loads.
The two barges would be equipped with flood lights and would operate 24 hours a day. Each
barge would require a tug for towing and repositioning purposes at all times the barge is in
operation. A primary barge would remain at the site for the duration of construction positioned by
a temporary pre-installed four point mooring system for stability and rock placement accuracy. A
secondary supply barge would transport rock to the primary barge.
Rock could be sources from a local quarry such as WA Limestones Byford quarry (WA) and
transported by road to a suitable marine loading facility with sufficient stockpiling space such as
the Australian Marine Complex (Henderson, WA). Material can be loaded onto the supply barge
via articulated dump trucks using a temporary ramp and transported to the site. Assuming a
sailing speed of 4 knots and a sailing distance of 35km the journey to the site would take
approximately 5 hours. Using an excavator on the supply barge the material would be
transferred onto the primary barge. Applying this method would prevent the primary barge from
disconnecting from the mooring system, increasing efficiency of the rock placement.
32
The core would be comprised of rocks with a preliminary estimated D50 of ~0.5 m and would be
constructed first followed by the armour layer. The scour and core material could be deployed
with a bucket or deployed from the side of the barge. The armour units would be deployed
individually by the long reach excavator fitted with a grab and positioned with the aid of GPS
instrumentation coupled with an onboard computer model of the reef. This would allow for
accurate rock placement tolerances in good weather conditions. The primary barge would
operate fully laden (1940t) for the majority of the core layers and half laden (1000t) for the
armour layer. The shallower draft of the barge at half laden would enable close positioning of the
barge for the construction of the armour layer.
A support vessel would remain with the primary barge to aid in staff shift rotation and undertake
bathymetry surveys for construction monitoring and design fulfilment. Subject to weather this
work strategy should produce an average production rate of 1940t of core material per 15-20
hours and 1000t of armour material per 15-20 hours.
Figure 6-1 Construction method using BME applied for the construction of Parker Point Artificial Reef
(Western Australia) (Source: MScience (2012))
Standby Duration
The BME rock transfer and placement will be limited by environmental conditions. The maximum
significant wave height and wind speed the barge would operate in is approximately 1.5 m and
25 knots, respectively. Analysis of measured wind recorded at Swanbourne and modelled
significant wave height over the period of 2014, wave heights and wind speeds were confirmed
to be consistency lower in summer than in winter (January to March and December). The
number of standby days were estimated based on the days during which the wind or wave
conditions exceed 80% (as a conservative operational threshold) of the wind or wave threshold
values within the four month summer period. Of the 121 days assessed, 17% were found to
exceed the conservative operational threshold. A conservative rounding was applied and 20% of
the construction day count was assigned as standby. This approach resulted in an estimated 14
day standby duration.
6.2 Safety
Safety has been one of the key considerations in the design of the reef. A number of key design
safety measures are outlined below;
33
To reduce the risk of a surfer being trapped between the reef and the breaking
waves, 5 years of historical tide data measured at Fremantle was analysed to
assure a sufficient water level over the reef crest at all times. A crest level of -
1.3m MSL was selected to ensure at least ~0.5m of water remain over the reef
crest even during infrequently experienced low water levels.
A relatively mild bed slope was chosen for the focusing toe to increase the
horizontal distance from the point of the wave crest overturning and the crest of
the structure. Hence, in the case a surfer falls off upon take off and is projected
(pitched) forward the surfer will land in deeper water compared to a case in
which the waves were to break on a steeper toe gradient. Additionally, the mild
bed slope means incoming swells have a longer distance to adjust to the change
in water depth, which reduces the risk of water draining across the structure,
lowering the water level and potentially exposing the reef.
Rock is the recommended material for the construction of Scarborough surfing
reef. Rock was used for Cables Reef (Western Australia). To DHIs understanding
no surf accidents involving hitting the bottom has been reported at Cables Reef,
while Boscombe remains closed due to safety hazards caused by shifting
geotextile containers forming cavities large enough to trap an adult human being.
6.3 Cost
An indicative estimate of the cost for constructing Scarborough reef inclusive of a 20%
contingency allowance is $16.9M. A breakdown of the costing is provided in Table 6-1. The
presented costs are preliminary and are sensitive to the assumptions applied and are subject to
further refinement that would take place during a concept design phase. The cost of construction
per m3 of volume is $332/m3. It should be noticed that the price is largely proportional to reef
volume and an A-Frame reef requires almost twice the volume as a wedge shaped reef (left or
right-hander).
The historical unit costs for comparable previous reefs projects are Boscombe ($390 {2009}),
Cables ($302 {1999}) and Kovalam ($370 {2010}) (Mortensen et al., 2015). It should be noted
that adjusting the historical Cables reef costs to present day values provides a unit cost of
approximately $600/m 3 for the Cables reef (FV(3%/1,17,$1,800,000,1).
A significant portion of the cost is assigned to the material alone. Granite has been selected as a
result of the successful application for Cables Reef. A potential significant cost reduction would
be to use limestone. Limestone is 25% cheaper than granite ($40 per tonne, ex. quarry) and is
used extensively on Perths metropolitan coastline for breakwater and groyne construction.
However, the application of limestone for an ASR would be subject to a review of its lasting
functionality and durability in the marine environment and structural integrity tests of the quarry
specific source material.
The rate of rock deployment largely impacts the construction duration and therefore also
influences construction costs. A potential for cost saving could involve assessing the feasibility
of operating two long reach excavators on the primary barge. This could consequently reduce
excavator manoeuvrability during placement. However, reducing the amount of rock stockpiled
on the barge could mitigate this restriction. A reduction of stock piling on the barge would require
more trips undertaken by the supply vessel. Increased supply trips would require a barge
loading plant be located closer to Scarborough Beach. A potential option would be Hillarys Boat
Harbour, pending the feasibility of the construction of a temporary loading facility suitable for the
operation coupled with material stockpiling capacity. The rock would be sourced from a quarry
closer to Hillarys Harbour, such as Italia Stone Groups Northern Perth quarry (availability of
rock type, rock cost and transport costs have not been investigated).
Another major contributing lump sum cost is the mobilisation/demobilisation fee of the barges
from Dampier due to the shortage of barges that meet the required specifications currently
34
available in the Perth metropolitan region. In the case barges can be sourced locally the
mobilisation/demobilisation fee will be reduced.
Table 6-1 Preliminary cost estimate for Scarborough ASR
No. Item Unit Rate ($) Qty Cost ($) Allowed Cost/Source
1. Rock material
1. a Armour Rock supply (d50 2t) t 55 114,240 6,283,200 WA Limestone (email)
1.b Volume of rock converted using SSD m3 2.8 51,000 WA Limestone (email)
density of 2.8t/m3
1.c Packing Void (20%) % 0.80 WA Limestone (email)
2. Transport of rock material (quarry to wharf)
2.a Transport of rock: Byford Quarry to t 14 114,240 1,645,056 WA Limestone (email)

Australian Marine Complex (Fremantle
Harbour). 40km distance. Inclusive of
return journeys.
2.b Rate of $18/50km/Tonne. Cost for km 0.36 40 Australian Marine

40km Complex (email)
3. Stockpiling of rock material at AMC
3. a Australian Marine Complex - day 972 68 66,096 Australian Marine

stockpiling material Complex (email)
3.b Based on cap of 25% material m2 0.12 8100 Australian Marine

stockpiled to a height of 1.6 m Complex (email)
($0.12/m2/day)
4. Loading wharf hire at AMC
4.a Australian Marine Complex - Loading day 2,860 68 194,480 Australian Marine
Wharf dry hire fee Complex (email)
5. Plant cost for loading rocks from stockpile onto barges
5.a 2 x Excavator (assuming 8 hr days). day 5,200 68 353,600 Rawlinson Construction

Inc. Labour. Handbook (2016)
5.b 2 x Dump truck (assuming 8 hr days). day 6,560 68 446,080 Rawlinson Construction
Inc. Labour. Handbook (2016)
6. Barge hire
6.a Barge mobilisation/demobilisation (from item 85,000 2 170,000 AIS Brokers (email)
Onslow)
6.b 2 x Barge hire. excl. crew. (24hr days) day 5,000 68 340,000 AIS Brokers (email)
6.c 2 x Excavator for barge. Incl. labour. day 14,880 68 1,011,840 Rawlinson Construction
(24hr days) Handbook (2016)
7. Tug hire
35
7.a 2 x Tug hire (includes crew and vessel day 26,000 68 1,768,000 Indian Ocean Shipping
mileage (24 hr) Agencies (email)
7.b 2 x Fuel for tug (assuming 2500 litre / day 6,500 68 442,000 Indian Ocean Shipping
days) fuel @ $1.30 Agencies (email)
8. Support vessel hire and survey equipment
8.a Support/survey vessel hire and crew day 10,800 68 734,400 SMEC 2014 (Palm
(24hr) Beach Shoreline
Protection Project -
Concept Design Phase)
8.b Survey equipment (24hr) day 320 68 21,760 SMEC 2014 (Palm
Beach Shoreline
Protection Project -
Concept Design Phase)
9. Standby
9.a Wharf hire, plant costs, barges with day 46,592 14 633,651 AIS Brokers (email)
excavators, support vessel and survey
equipment
9.b Barge mooring day 800 14 10,880 AIS Brokers (email)
10. Contingency
10.a Contingency (20% of fee) item 2,824,209 1 2,824,209 AIS Brokers (email)
Total $16,945,252
Table 6-1 is subject to the following assumptions;
Operation will be 24 hours / day (loading plant staff excepted)

Daily fuel consumption by tug estimate at 2500 litres per day
Diesel cost of $1.30 per litre
During standby conditions tugs can be off-hired if a berth is provided for the
barges
50% of the barge surface area is available for loading material
Material can be stacked on the barge to a height of 1.6m
A conservative void ratio of 20% was assumed for the material cost calculation
A conservative void ratio of 40% was assumed for the transportation and
construction stage calculations
1 minute placement cycles per armour layer rock unit
1 minute placement cycles per core layer units
Barges will be loaded with 1960 t of material for scour and core construction and
1000 t for amour construction
Material sourced from WA Limestones Byford Quarry
The material will be loaded at the Australian Maritime Complex (Henderson, WA)
Barge sailing speed of 4 knots
Sailing distance from Australian Maritime Complex to Scarborough Beach of
35km
Mobilisation/demobilisation costs incur a cost of $170,000 per barge for barges
supplied form Dampier
36
Table 6-1 is subject to the following exclusions;
Barge crew
Deck fit out and four point mooring spread
Construction of loading ramp for dump truck loading onto barge
Towing plans
Procurement and contract management costs
Cost to repair damaged stockpile area
37
7 Conclusions
The objective of the study was to investigate the feasibility of constructing an Artificial Surfing
Reef (ASR) at Scarborough Beach. The reef design presented in this report has been developed
by DHI experts based on a set of Key Performance Criteria defined by the client aimed at
providing a significant enhancement to local surfing amenity.
This feasibility study provides a clear and concise overview of proposed location, design,
performance, construction and cost of a surfing reef. It does not include an in-depth analysis of
all aspects of detailed surfing optimization, marine structural design or environmental impact
assessment that will be required if the project is to proceed to the next stage.
DHI has utilized experiences made from similar projects supported from highly sophisticated
numerical models to support their recommendations, but more site specific in-depth technical
work will be required to support further engineering works.
In its current layout both the left and right-hander surfing waves provide a significant increase in
the level of surfing performance compared to the adjacent beaches during a typical average
quality Scarborough surf event, but further detailed studies will be required to assure optimal
performance, especially for the right-hander, where margins between fast barrelling waves and
close-outs become very fine. Other detailed reef features such as down the line back-door
barrel sections are also subject to further detailed assessment.
surfing amenity for Scarborough Beach.
During more infrequent episodes of large waves, the reef is expected to produce excellent
surfing conditions of a very high quality but also with a potential increased risk of injury to the
surfer. Further testing and design optimizations in subsequent studies is strongly recommended
in order to minimize any unacceptable safety risks.
During times of very small waves the reef is expected to produce gentle surf conditions suitable
for beginners.
The modelling approach used in feasibility design provides a good insight into the natural
variation in surfing wave performance on the reef that can be expected during a typical surf
wave condition at Scarborough, but more wave events and detailed CFD modelling is
recommended for further optimization of performance and safety assessment.
However at the same time the reef will generate strong return currents close to its edges, which
may pose a hazard to unseasoned swimmers but only if they get too close to the reef during
times of energetic wave activity. It is expected that swimming during such periods of energetic
wave activity would be uncommon.
structure.
38
Conclusions

The reef should be constructed from rock using a barge-mounted excavator. Preliminary cost of
the structure is $16.9 million based on 2016 price estimates from local contractors, local weather
data and best practice estimates. Note that the price is largely proportional to reef volume and
an A-Frame reef requires almost twice the volume as a wedge shaped reef (left or right-hander).
Further savings may be achievable if further studies identifies a substitute rock material to
granite. A potential option could be to further investigate the applicability of using limestone
which could provide a potential material cost saving of approximately 25%
The overall cost of the reef is roughly linearly scalable with the volume of rock required as it
affects both the material price and the time required for placement. As a result the developed
reef design could be adjusted to fit a smaller budget simply by shortening the wings of the
structure, which would affectively result in shorter surf rides. Changes to the shoaling platform or
steepness of the structure is not recommended as it may compromise key performance and
safety aspects of the design. Based on budget availability further adjustment of reef layout to fit
a target budget may be recommended.
The artificial surfing reef is likely to result in a significant coastal response with a salient likely to
form in its lee and potential shoreline setback to the north. At its proposed location it is
considered likely that the resulting shoreline impact will be seen as beneficial by causing
accretion in front of existing beach infrastructure, while the setback will occur in an area with no
existing developments. However further detailed studies must be carried out in order to confirm
expected outcomes. If the coastal impact were found to be adverse, the reef location could be
located to one of the two other potential sites which would be deemed less sensitive to localized
shoreline changes.
surfing amenity for Scarborough Beach with a robust design framework for mitigating potential
adverse impact identified in subsequent stages of the project.
39
8 Recommendations for Concept Design Phase
The feasibility study provides a clear overview of the overall design aspects, construction
methodology and costs of the artificial surfing reef for Scarborough Beach. If the project is to
proceed to Concept Design stage, the following engineering studies will be highly
recommended.
Comprehensive Stakeholder consultations to confirm KPCs
Detailed coastal impact assessment study clarifying impact of surfing reef on

nearby beach and optimize potential for enhanced coastal protection of
vulnerable areas. Comprehensive assessment of safe swimming conditions.
Detailed bathymetry survey at reef location including core sampling to confirm

bed geotechnical properties, benthic coverage and sand layer thickness.
Minimum 30-year wave hindcast study to confirm structural design wave

conditions.
Physical modelling to confirm rock size requirements.
Further reef optimisation using Boussinesq Wave and OPTISURF on a wider

spread of representational surfing conditions to assure maximum performance.
Detailed reef optimisation using Computational Fluid Dynamic (CFD) to fine-tune

sensitive high-performance criteria such as surfer touch-bottom risks, optimize
barrel sections and investigate the incorporation of detailed features such as
down the line back door sections. Figure 8-1 illustrates the level of detailed output
produced by DHIs CFD model.
Review of alternative rock materials applicability for specific application including

longevity, durability and structural integrity testing
Figure 8-1 DHIs CFD model provides an exceptional accurate framework for safety assessment
detailed surf optimization of an artificial surfing reef (Mortensen 2009).
40
Recommendations for Concept Design Phase
41
9 References
Bancroft, S. (1999) Performance monitoring of the cable station artificial surfing reef, BEng
(Hons) thesis, Department of Environmental Engineering. The University of Western Australia.
DHI (2014) Palm Beach Shoreline Project Concept Design Final Report Prepared for City of
Gold Coast. March 2014.
DOT (2016) DOT Hydrographic/Bathymetric Surveys [available online at]

http://catalogue.beta.data.wa.gov.au/dataset/composite-surfaces-index/resource/2bf241af-d5ae-
3c78-951d-0050cb29a8ea [last access 21/09/2016]
Mortensen, S. B., Hibberd, W. J., Kaergaard, K., Kristensen, S. E., Deigaard, R. and Hunt, S.
(2015) Concept Design of a Multipurpose Submerged Control Structure for Palm Beach, Gold
Coast Australia, Proc. Australasian Coasts And Ports Conference 2015, Auckland, New
Zealand.
Mortensen, S. B, and Henriquez, M. (2009) Advanced Numerical Modeling of Artificial Surfing

Reefs, South Africa, Proc. International Multi-Purpose Reef Conference 2009, Jefferys Bay,
South Africa.
MScience (2012) A practical guide to the construction and management of artificial reefs in
northwestern Australia [available online at] www.mscience.net.au/wp-
content/uploads/2015/06/AR-Guide-Current_online.pdf [Last accessed 23/06/2016]
Pattiaratchi, C. (2007) The Cables Artificial Surfing Reef, Western Australia, Shore and Beach,
75. (4) 80-92.
Rendle,E., and Davidson,M., (2012) An evaluation of the Physical Impacts and Structural
Integrity of a Geotextile Surf Reef Coastal engineering 2012
UCAR (2016) NCEP Climate Forecast System Reanalysis (CFSR) Selected Hourly Time Series
Products [available online at] http://rda.ucar.edu/datasets/ds093.1/#!description [last access
14/01/2016]
10 Acknowledgement
An acknowledgement of our gratitude is to be extended to the Department of Transport
(Western Australia) for granting the use of the bathymetric survey data and the measured wave
data acquired from the Rottnest Island and Cottesloe stations.
42
APPENDI CE S
APPENDI X A Spect ral W ave Mode l Vali dat io n
Spectral Wave Model Validation Plots
A Spectral Wave Model Validation Plots

The regional and local models were validated against the Rottnest wave buoy (located in 48m
MSL water depth) and the Cottesloe wave buoy (located in 17m MSL water depth). Validation
plots from of the local wave model are presented below.
Figure 10-1 Validation of the Scarborough local wave model against measured data from Rottnest wave
rider buoy for total wave component for the three (3) month validation period
A-1
rider buoy for sea wave component for the three (3) month validation period
rider buoy for swell wave component for the three (3) month validation period
A-2
rider buoy for total wave component for the full 2014 simulation year
Figure 10-5 Validation of the Scarborough local wave model against measured data from Cottesloe wave
rider buoy for total wave component for the three (3) month validation period
A-3
rider buoy for sea wave component for the three (3) month validation period
rider buoy for swell wave component for the three (3) month validation period
A-4
rider buoy for total wave component for the full 2014 simulation year
A-5
A-6
APPENDI X B O PT I SURF Surf ing Amenit y
Mode lli ng
OPTISURF Surfing Amenity Modelling
B OPTISURF Surfing Amenity Modelling

DHIs surfing analysis program OPTISURF was used to calculate key surfing amenity
parameters based on the outputs from the Boussinesq wave model. OPTISURF utilises the
instantaneous wave field output to track the moving steep transition zone between unbroken
and broken waves, which most often marks the optimum position (surf term: the pocket) for a
surfer during a wave ride.
For each time step the program keeps track of all active surf rides occurring in the domain. Note
that some waves offer the possibility for multiple rides to be executed. This can include a
simultaneous left and right hand ride created by a breaking wave peak. Other waves can break
over multiple sections before reaching the shoreline.
For each active surf ride, the program keeps a track of the minimum speed the surfer will have
to maintain in order to keep ahead of the breaking wave front. The program also logs time series
of the wave face height and the wave steepness at the position of the surfer and the surfers
ground speed. The maximum ground speed achievable depends on both the detailed wave
breaking characteristic and the skill level of the surfer. Maximum surfer speeds of more than 10
m/s are uncommon. Yet some fast wave sections are still passable if the surfer can predict
them in advance and move faster during prior wave sections.
If the maximum surf speed exceeds a predefined maximum, the particular section of the wave is
considered to be closing out or breaking too fast to be surf able and the surf ride is terminated.
Consequently, if possible, a new ride is initialised for subsequent wave sections. Due to the site
specific nature of maximum surf speeds, it is often recommended to compare threshold values
to site specific measurements and observations.
B-1
OPTISURF Surfing Amenity Modelling
C-1

Scarborough Reef Feasibility Final Report v1.2

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Scarborough Reef Feasibility Final Report v1.2

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Scarborough Beach Artificial Surfing

Reef Feasibility Study

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Scarborough Beach (WannaSurf, 2015)

Project manager William Hibberd

Authors William Hibberd, Marcus Tree, Simon Mortensen

Quality supervisor Simon Brandi Mortensen

External Peer Review Ben Matson

Project number 43802163

3 Local Wave Climate Assessment ............................................................................... 8

4 Preliminary Artificial Reef Design ............................................................................ 14

5 Artificial Surfing Reef Performance Assessment ................................................... 19

6 Preliminary Assessment of Constructability, Safety and Cost .............................. 30

8 Recommendations for Concept Design Phase ....................................................... 40

Appendix B: OPTISURF Surfing Amenity Modelling

As the reef is expected to generate a strong concentration of recreational water users, it is

2.1 Project Scope

An assessment of the best suitable location on Scarborough for hosting an

Preliminary design of the artificial surfing reef

Preliminary assessment of constructability, safety and cost

To identify which nearshore areas along Scarborough Beach would be suitable

To quantify the distribution of nearshore wave conditions in order to provide input

3.1 Wave Climate Modelling

3.1.1 Wave model input data

3.2 Site Identification

Focal Zone Average Hs of Swell [m]

4 Preliminary Artificial Reef Design

4.1 Key Performance Criteria

A location in close proximity to Scarborough Amphitheatre is preferred

Structure must not be surface piercing at any time

Must not create no adverse coastal impact

Create safe swimming / learner surfing conditions nearshore

A reef construction budget in excess of $10M is acceptable

4.2 Reef Design Shape

A conical wave focusing toe

A 45 angled right-hand shoaling platform

A 40 angled left-hand shoaling platform

Figure 4-2 Scarborough Reef design

4.3 Reef Location and Volume

An overview of reef design specifications are provided in Table 4-1.

Table 4-1 Reef design criteria

Design Specification Value

Position Focal Zone 1

Minimum distance from shoreline 100 m

Length (shore parallel) 220 m

Inshore depth -4 m MSL

Offshore depth -9.5 m MSL

Crest level -1.3 m MSL

Toe angle 1/12

Toe Area 23,300 m2

Peel Angle (right / left) 40 / 45

5 Artificial Surfing Reef Performance Assessment

5.1 Surfing Condition Scenario Selection

Minimum Surfable Criteria

Minimum Spectral Peak Period (T p) of 8 seconds

The event must last for at least 2 hours

Occur during average daylight hours between 6am and 6pm

Surfing at Scarborough is expected to be possible outside of these conditions, but is not

1 1.0 1.5 14 15 265 - 270

2 1.0 1.5 14 15 270 - 275

3 0.5 1.0 13 14 260 265

Table 5-2 Selected representative surf event parameters

Hs [m] Tp [s] DirP []

Swell 1.28 14.1 270