Review of existing multi-functional artificial reefs

Abstract

Costs, benefits and performance of existing surfing and multi-functional artificial reefs have varied considerably. This paper compares details of the 6 reefs completed or under construction to determine the total costs and performance as well as construction issues. The investigation was undertaken as part of the design and construct contract for a multi-functional artificial reef project.

Full text

AUSTRALASIAN CONFERENCE ON COASTS AND PORTS 2007
1
Review of Existing Multi-Functional Artificial Reefs
Leslie (Angus) Jackson1 and Bobbie B Corbett1
1International Coastal Management, Queensland
Abstract
Costs, benefits and performance of existing surfing and multi-functional artificial reefs have varied considerably.
This paper compares details of the 6 reefs completed or under construction to determine the total costs and
performance as well as construction issues. The investigation was undertaken as part of the design and construct
contract for a multi-functional artificial reef project.
1 Introduction
Coastal engineers have been aware of the need to
include surfing into design of coastal structures and
there are numerous examples in Australia, and
worldwide, where coastal protection works have
improved surf amenity. Most artificial “surf” reefs to
date have been constructed in Australia. However, the
design of such structures is difficult as there is a lot of
conflicting information on the performance of the
existing reef projects.
Despite considerable interest in multi-functional
artificial reefs (MFAR), only 4 have been totally
completed to date, and a further 2 commenced:
Completed as at 1-3-07
• Bargara, Queensland, Australia 1997
• Cables, Western Australia, Australia 1998-99
• Narrowneck, Queensland, Australia 1999-00
• Pratte's Reef, El Segundo, USA 2000-01
Near Completed as at 1-3-07
• Mount Maunganui, NZ 2005 - ??
Construction commenced as at 1-3-07
• Opunake, NZ 2006 -???
Detailed technical monitoring reports have been
published on all of the completed reefs, except
Bargara. This published technical data, with
additional research and observations, has been used in
this review of the following reef characteristics:
• Location and site conditions (waves and tides)
• Design - size and shape
• Construction materials and methods
• Costs [total & $/m3]
• Performance
• Coastal protection / salient
• Safety
• Amenity created e.g. surfing, diving and
fishing
• Comments and lessons learned
2 Bargara Reef
2.1 Location and site conditions
Bargara is located at the northern end of Hervey Bay
in Queensland. Waves are generally < 1 m. The most
common occurring swell is 0.2 – 0.4m with periods of
6 – 9 sec [BPA, 1986]. Tides are semi diurnal with a
spring tidal range of about 2.5m
2.2 Design
The site is on the north side of a headland and is
effectively a ½ V. The objective was to smooth the
existing bathymetry to give a break that was rideable
without abrupt interruptions [Pitt, 2005].
No modeling was carried out. Local knowledge was
used to relocate / break boulders that were observed to
be adversely impacting on the break. These boulders
were also used to fill holes to further improve the wave
quality for surfing. The initial works were monitored
on the “full scale model” and additional works have
been “designed” [Figure 1]. [Redgard, 2006]
Figure 1 Bargara Reef Phase 3 [Source: Greg Redgard]
2.3 Construction materials and methods
The rocky headland was groomed at low tide using a
large excavator to move the existing basalt boulders
(Figure 2). No additional materials were required to be
imported. The rock volume moved is very difficult to
estimate, but is approximately 300m3.
Figure 2 Excavator moving boulders at low tide
(Source: Greg Redgard)

AUSTRALASIAN CONFERENCE ON COASTS AND PORTS 2007
2.4 Cost
Costs incurred to date have been ~A$10,000, including
approvals, but not including considerable time
contributed by the community (Redgard, 2006). This
is equivalent to ~A$30/m3.
2.5 Performance
No scientific monitoring has been done. It is obvious,
however, that the work is producing an improved and
safer, longer point break [Figure 3] near high tide with
swells of over about 1m and light and / or offshore
winds. The number of surfable days is relatively low,
but expectations of the surf quality do not appear
unrealistic and it is seen as successful in improving the
surf quality and increasing the number of surfable
days. (Redgard, 2006)
There have been no reports found of any serious
injuries. The reef works would have had no significant
affect on coastal processes on the rocky headland.
Figure 3 Surf at Bargara (Source: Greg Redgard)
2.6 Comments
The following comments and conclusions can be
made:
• The emphasis has been to “improve” local
conditions, not create a new surfing location. With
realistic expectations, the project is generally seen
as successful at a local level.
• The reef only works at high tides.
• The avoidance of expensive modeling and use of
community involvement, simple construction
methodology and local equipment has resulted in
a low total and unit cost.
• The site is rough boulders, but there have been no
reports of injuries.
• Monitoring would be very beneficial.
3 Cables Reef
3.1 Location and site conditions
Cables Reef is located at Perth, Western Australia.
Tide is diurnal with a spring tidal range of 0.4m.
Mean wave conditions are characterized by a
significant wave height (Hs) of 2.0m and a spectral
mean wave period (Tm) of 8.8s although there is
considerable seasonal variability (Lemme et al, 1999).
3.2 Design
Engineering for the final design and construction of
the project was coordinated by the WA Department of
Marine and Harbours. The Centre for Water Research
at the University of Western Australia assisted the
Department in the design aspects. A large number of
comprehensive studies were undertaken and the
outcomes have been published (Pattiaratchi, 1997).
The final reef shape (Figure 4) chosen was a
“Boomerang shaped” reef with a nose ½ angle of
approx. 45deg. Crest height was set at -1m LAT for
safety. To minimize volume, the reef was on a natural
nearshore rocky reef.
Figure 4 Cables Reef - initial shape and extension
(Source: tender documents, 1999)
3.3 Construction materials and methods
Detailed construction design was undertaken by the
WA Dept of Transport with project management by
Egis consulting. The reef was constructed with
5,500m3 of 1.5t and 3.0t stone. The contract was
awarded to WA Limestone with construction being
undertaken using a barge transporting the granite stone
material from Fremantle Harbour (Figure 5).
Figure 5 Excavator placing rock at Cables (Source:
Pattiaratchi, 2003)
3.4 Cost
The total cost was $1.8M (DPI, 1999). This is
equivalent to ~A$327/m3.
3.5 Performance
Monitoring of the reef has been carried out by
Bancroft [1999] and Pattiaratchi [2003]. They
concluded that the reef was performing according to
design, with swell as low as 0.5m breaking on the reef
in low tides. In 1999, the reef was considered to be
“surfable” 142 days of the 178 days it was breaking.
(Figure 6)

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Figure 6 Surf at Cables
3.6 Comments
The following comments and conclusions can be
made:
• The location has abundant swells and a low tide
variation that are suitable for a surfing reef.
• The project appears to have “improved” surfing
conditions when swell, tide and wind conditions
are suitable.
• The reef does provide a quality surf wave at times,
but it has not gained a reputation as a great surf
spot. Part of the reason for this appears to be that:
• There are a number of good surf breaks in the
area
• When conditions favour the reef, a number of
other local breaks work as well
• The reef takeoff area is about 300m offshore
[other natural breaks are closer to shore]
• No reports of injuries have been noted.
4 Narrowneck Reef
4.1 Location and site conditions
Narrowneck Reef is located on the Gold Coast,
Queensland. It is part of the Northern Gold Coast
Beach Protection Strategy (Gold Coast City Council).
The site experiences high wave energy and a nett
northerly sand transport rate of ~500,000m3 pa.
Average Hs is about 1.0m but Hm has exceeded 12m
since construction. Tide is semi-diurnal with a spring
tidal range of 1.3m
4.2 Design
The primary function of the reef was coastal
protection, with “improved surfing” as a secondary
objective. The final design (Figure 7) was determined
by ICM based on the recommendations of the Uni of
Waikato , additional numerical and physical modeling
by WRL [Uni of NSW] and Griffith Uni and extensive
monitoring.
Figure 7 Narrowneck design; black = original
footprint, colours = design contours after monitoring
The reef is a flared V-shape with a nose ½ angle of
~13deg separated by a bridged central paddle channel.
Design crest height was reduced to -1m LAT [from
0.0m LAT recommended by Uni of Waikato] due to
concerns regarding safety – surfers and rips. The reef
volume is very large [approx. 70,000m3].
4.3 Construction materials and methodology
ICM developed the construction methodology and the
contract was awarded to local firm McQuade Marine.
The reef was constructed using over 400 mega sand-
filled geotextile containers supplied by ELCO
Solutions [then SoilFilters Australia], filled, and
dropped into place using a hopper dredge, Faucon
[Figure 8].
Figure 8 Placing containers from a split hull hopper
dredge at Narrowneck
The seabed at the inner section of the reef can vary by
up to 2m due to the migration of the storm bar. A very
large storm bar had formed over the back half of the
reef prior to construction and a sequenced construction
was undertaken with top-up after migration of the bar
shoreward [and resulting “settlement” of the reef].
4.4 Cost
The total cost of the reef to date, including design
studies, top-up and replacement of damaged containers
is A$2.8M (US$2.1M). This equates to a unit rate of
~A$40/m3.
4.5 Performance
Considerable monitoring of the reef has been carried
out and a number of monitoring reports have been
published (GCCM, 2004). Despite a number of storm
wave events, the reef has proven been effective in
stabilizing the beach and a salient is generally present
(Turner, 2006).
Wave breaking occurs ~50% of the time – generally
for waves >1m at MLWS and >1.8m at MHWS. Good
surf [Figure 9] is experienced regularly on the reef
when wind, waves and tides are suitable. It is noted
that the surf very rarely looks like either the numerical
or physical models as there is often multi-swell / wave
conditions and wind factors. When there is a clean
swell without wind, the modeling is replicated in the
real world.

AUSTRALASIAN CONFERENCE ON COASTS AND PORTS 2007
Figure 9 Surf at Narrowneck (Source ICM)
Prior to initial settlement of the reef [with a crest
height of -0.5m LAT], the reef formed a very hollow
but hazardous wave that often sucked dry at the break
point. Prior to top-ups, when crest heights have been
lower than -1.5m LAT in locations, waves tend to be
more spilling.
It has also been observed that the reef interacts with
the adjacent bar formations, creating more favourable
natural conditions. With the normal sand bars, the
waves tend to break on the reef and then link into the
shorebreak, significantly extending ride length [rides
over 200m have been recorded].
The reef has provided a suitable substrate for
development of a diverse ecosystem and has become a
popular location for fishing and diving. As a result it
has been designated as a no anchoring zone. The type
of geotextile used promotes soft growths [such as
algaes] that do not present a safety hazard to surfers.
4.6 Comments
The following comments and conclusions can be
made:
• The project appears to have achieved the objective
of improving surfing conditions when swell, tide
and wind conditions are suitable.
• The reef does provide a quality surf wave at times,
but it has not gained a reputation as a great surf
spot. Part of the reason for this appears to be that:
• Very high expectations and initial media
‘hype’ followed by negative media.
• There are a number of world-class surf breaks
in the area
• When conditions favour the reef, a number of
other local breaks work as well
• The reef takeoff area is about 250m offshore
[other natural breaks are closer to shore]
• Selection of geotextile type influences the type of
marine flora and fauna.
• Suitable construction methodology using efficient
gear and experienced operators resulted in a very
low unit cost.
• If constructed on a sandy seabed, fluctuations in
the seabed can have significant impacts on reef
“settlement” and performance
• Crest levels are important for safety
considerations, although no reports of injuries
have been noted.
• Reef generally improves surf quality on adjacent
bars. It is possible for the reef break to link with
the bar break and extend the ride length.
• Vessels can damage sand-filled geotextile
containers.
• The surface of the reef is now rough. Wave
quality is not affected by hollows it is affected by
isolated high spots.
• Despite a number of storm wave events (Hm up to
12m) coastal protection has been effective.
5 Pratte's Reef
5.1 Location and site conditions
Pratte’s reef is located at El Segundo, California. The
wave climate is generally <1m. Tides are semi-
diurnal with a tidal range of approx 1.6m.
5.2 Design
The surfing reef was designed by Skelly Engineering.
The shape [Figure 10] is a delta type with a nose ½
angle of 45deg and crest height of -1.8m MLLW [later
raised to -0.9m MLLW].
Figure 10 Pratte’s design
5.3 Construction materials and methodology
Two types of woven geotextiles were used – polyester
and polypropylene. For safety and budget constraints,
the reef was designed and constructed of ~14t
sandbags that could fit snugly into the back of a
standard tip truck tray. Once filled, they were loaded
onto the barge and towed to site to be placed by a
barge-mounted crane [Figure 11]. Final reef volume
was only ~1,350m3.
Figure 11 Placement of sandbags at Pratte’s

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5.4 Costs
The total budget for this project was only US$300,000
(~A$285/m3).
5.5 Performance
Detailed monitoring was undertaken [Borrero and
Nelsen, 2003]. While they noted that Pratte’s has
generally not performed to expectations, there are
some good photos of wave breaking at the reef [Figure
12], and public comments suggest that it is “epic” in
the right conditions [1-5 times per year].
Figure 12 Surf at Pratte’s
Surveys and dive inspections show that seabed
variations and damage to the bags were likely
responsible for significant lowering of the reef,
affecting the wave breaking.
5.6 Comments
The following comments and conclusions can be
made:
• The project is generally not seen as successful
• There are a number of good surf breaks in the area
• Insufficient budget to make the reef of adequate
size.
• Seabed fluctations and damaged containers
lowered the reef - major cause of the reduced
effectiveness for surfing
6 Mount Maunganui
Mt Maunganui is located on the west coast of New
Zealand’s north island. Waves are generally <1m high
and tidal range is >2.5m. The reef was designed by
ASR using [small scale] physical and numerical
modeling. It is a basic V shape with a nose ½ angle of
~40deg and a crest height of -0.4m LAT. (see design at
www.asrltd.co.nz)
The reef is being constructed using sand-filled
geotextile containers strapped to a webbing grid in two
halves prior to deployment on the seabed by divers and
filling by a pump powered by a barge-mounted
excavator. As at March 2007, construction has been
very slow due to site conditions and costs [NZ$1.6M]
had been considerably over budget [NZ$0.8M]. The
reef is presently partially covered with a storm bar and
construction is only 80% completed. (see construction
newsletters at www.asrltd.co.nz)
7 Opunake
Opunake is located on the west coast of New
Zealand’s north island. Two reefs are proposed and
construction of the first reef started in March 2006.
The reef is being constructed of large sand filled
geotextile containers filled using a pump and hopper
located on the headland. Construction is presently
awaiting suitable conditions for the construction
methodology. (see design at www.asrltd.co.nz)
8 Conclusions
It is possible to combine coastal protection and
“improved” surf conditions at a reasonable cost.
However, in most cases, public expectations have not
been fulfilled.
In the design, the following issues are important:
• Crest height is important for surf, safety and in
determining coastal protection.
• The size and location of the reef is important.
• Public expectations need to be realistic.
• Construction methods need to practical - A very
smooth surface is not necessary, but isolated high
spots should be avoided.
• Seabed changes need to be considered.
• Numerical modelling tends to overstate the
performance of surf reefs - Wind, wave and tide
range and seabed levels may limit surfability to
certain conditions.
9 References
Bancroft S. 1999. Performance Monitoring of Cable
Station Artificial Surfing Reef. University of WA
undergraduate thesis
Beach Protection Authority. 1986. Hervey Bay
Beaches
Borrero and Nelsen. 2003. Results of a Comprehensive
Monitoring Program at Pratte’s Reef. 3rd International
Surfing Reef Symposium.
DPI. 1999. Report on Major Achievements
GCCM. 2004. Summary of Narrowneck Monitoring to
June 2004. prepared for Gold Coast City Council
Lemme AJ. Hegge BJ. Masselink G. 1999. Offshore
Wave Climate, Perth (Western Australia) 1994 - 96.
CSIRO Marine and Freshwater Research Vol. 50, 2.
Pattiaratchi, C. 1997. Design Studies for an Artificial
Surfing Reef at Cable Station, Western Australia. 1st
International Surfing Reef Symposium
Pattiaratchi, C. 2003. Performance of an Artificial
Surfing Reef: Cables Station, Western Australia.
COPEDEC
Pitt A. 2005. Redgard the Reef Renovator. Artificial
Surf Reef Conference.
Redgard, G. 2006. pers comm.
Turner IL. 2006. Analysis of Shoreline Variability,
Seaasonality and Erosion/Accretion Trends: Feb 06-
July 06. prepared for Gold Coast City Council

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Table 1: Reef comparison
SURF "REEF"
PROJECTS
Date
constructed
COUNTRY VOL
[m3]
approx
TYPE total
A$
$/m3
A$
Construction method Tide
Range
[approx]
Average
Wave
climate
Hs
Completed Projects
Bargara 1997 Australia
300 Rock $10,000
$33 Reprofiling existing rocks on
headland with excavator at low
tide
3.7m <1m
Cables 1998- 99 Australia
5,000 Rock $1,400,000
$280 Rock placed with excavator
from barge
0.8m Summer
1-2m
winter
1.5 -
2.5m
Narrowneck 1999-2000 Australia
70,000 SFGC
non-
woven
$2,800,000 $40 150 - 450t mega sand filled
containers filled in hopper
dredge and dropped.
2m 1m
Prattes 1999-01 USA
1,350 SFGC
woven
$385,000 $285 14t sand filled containers
filled on shore, loaded on barge
and placed by crane from barge
1.6m <1m
Partially
Constructed
est
Mount Maunganui 2005 -?? NZ 6,000 SFGC
non-
woven
$1,454,545 $242 mega sand filled containers
attached to web, anchored and
filled in situ
[20% construction outstanding]
>2.5m <1m
Opunake 2006 -?? NZ
? SFGC
non-
woven
$760,000 ?? mega sand filled containers
attached to web, anchored and
filled in situ
[construction stalled?]
>3m