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COMPARISON OF GEOSYNTHETIC MATERIALS AS SUBSTRATES ON COASTAL
STRUCTURES – GOLD COAST (AUSTRALIA) AND ARABIAN GULF
Bobbie Corbett1, L. Angus Jackson1, Timothy Evans1 and Simon Restall2
Coastal structures should be designed to minimize the risks to beach users and avoid negative impacts on the marine
environment. Past experience with sand-filled geotextile containers in the marine environment (i.e. submerged) shows
that they provide a permeable substrate that supports a diverse range of marine growth which differs from that found
on conventional “hard” structures. To quantify the potential benefits, comparative trials between different
geosynthetics at different depths have been undertaken in both in the hot high salinity waters of the Arabian Gulf
[UAE] and in the sub-tropical waters of the Pacific Ocean [Gold Coast, Australia]. Results indicated that high
strength non-woven type geosynthetics are most suitable for structures which are intended to provide ecological /
recreational benefits as they provide higher diversity and less hard growths which are not as user-friendly.
Keywords: geosynthetic, geotextile, substrate, ecology, marine growth, recreation, sand-filled geotextile containers
INTRODUCTION
Coastal structures should be designed to minimize the risks to beach users and avoid negative
impacts on the marine environment (Jackson 2010). Past experience with sand-filled geotextile
containers in the marine environment (i.e. fully submerged) shows that they provide a permeable three-
dimensional substrate that supports a diverse range of marine growth which differs from that found on
conventional “hard” structures. This was clearly observed during monitoring on the Narrowneck
artificial reef (Gold Coast, Australia) as well as a number of semi-submerged breakwater, reef and
groyne projects in the Arabian Gulf. This has lead to the recognition that coastal structures have the
potential to provide environmental benefits as well as associated recreational opportunities such as
snorkeling / diving. These objectives can be included in the engineering design process. There is a
wide range of high strength engineered geosynthetics available for use in coastal structures and this
paper investigates and compares the marine growth that is supported by various geosynthetics under a
range of conditions.
BACKGROUND
The first detailed monitoring and research into the marine habitat associated with SFG structures
was carried out at the Narrowneck reef on the Gold Coast, Australia (Jackson et al. 2004). This is a
Multi-Functional Artificial Reef (MFAR) designed to provide coastal protection and improved surfing.
After construction in 1999, monitoring showed that an extensive marine habitat had been unexpectedly
created and the reef also provided positive environmental benefits.
The thick needle-punched non-woven geosynthetics used in the construction of this reef provided
a porous surface for the embedding of sand and growth of a wide diversity of marine flora and fauna
[Jackson et al. 2004]. Observations by the Australian National Marine Science Centre indicate that “the
biological communities associated with Narrowneck Artificial Reef appear to enhance biodiversity and
productivity at a local scale and may also contribute to overall regional productivity.” (Edwards 2003).
It was also noted that the species communities are substantially different to other natural reefs in the
region with the presence of resident (benthic and demersal) fish and other species, such as juvenile
prawns, abalone, turtles, lobster that are not generally observed on nearby natural reefs that were
monitored to provide comparative data.
Figure 1. Turtles, kelp, long algae and scad at Narrowneck artificial reef
1 International Coastal Management, PO Box 7196, Gold Coast MC, Queensland, 9726, Australia
2 GSR International, Gold Coast, Queensland, 4216, Australia
INTERNATIONAL CONFERENCE ON COASTAL ENGINEERING
ICCE 2010 China
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TRIALS
A number of different geosynthetics were utilized on the Narrowneck reef project and it was clear
that there were differences in terms of the ecological development. Previous testing has been
undertaken comparing different geosynthetics, however their applicability to other locations and
conditions was unknown. As part of preliminary works on a number of projects, trials have been
undertaken comparing different geosynthetics in both the hot, high salinity waters of the Arabian Gulf
[UAE] and in the sub-tropical waters of the Pacific Ocean [Gold Coast, Australia]. Investigations have
also been structured to provide some insight into the potential impact of water depth and water flow on
growth.
Given the nature of the testing undertaken, general qualitative assessment of results is possible,
however a range of different factors were investigated (water depth, location, geosynthetic type) and
there were not sufficient samples for replication and as a result full statistical analysis of results is not
possible. The findings do, however, provide good indication of general results and behavior over a
relatively long time period.
UAE TRIAL
Test setup
The UAE trial (Jackson et al. 2005) utilized the following commercially available geosynthetics:
• A polyester staple fibre needle punched non woven
• A composite dual layer mixed denier needle punched non woven
• A split film high strength polypropylene woven
Figure 2. Location of deployment of UAE samples
Samples were deployed in Dubai (United Arab Emirates) on a navigation beacon within the Gulf
at two different depths. Samples were placed in March 2005 and retrieved in July (after 4 months) and
November (after 8 months).
Results
The results showed that there was considerable difference between the growth on the woven and
non-woven samples. The non-woven samples tended to promote soft growths, with an initial
dominance of macro-algae and later development of sponges and ascidians (sea squirts). The woven
sample was dominated by bivalve / mollusk growth. This type of “hard” growth was similar to
observations of growth on rock and other conventional structures in the vicinity of the trial and the pile
to which the samples were attached.
The non-wovens showed a much higher biodiversity and productivity than the woven sample,
particularly the composite non-woven, which supported crabs, annelids, polychaetes and shrimp after
only 4 months.
The full results are presented by in a separate paper by Jackson et al (2005).
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Figure 3. Non-woven composite sample after 4 months
Discussion
The trial confirmed that, even in the hot, high salinity conditions of the Gulf, geosynthetics can
still provide an effective substrate for marine growth and that the type of geosynthetics used
determines the type of growth. Similar to other trials, the woven geotextiles supported “harder”
growths which were considered to be unsuitable for coastal structures where recreational benefits
might be encouraged. While both non-woven geosynthetics provided improved habitat suitable for
recreational usage, the composite layer geosynthetic provided the most diverse habitat.
AUSTRALIA TRIAL
Test setup
For the Gold Coast (Australia) trial, four different geosynthetic materials were chosen (Figure 4).
• A polyester staple fibre needle punched non woven (ELCOMAX® 1200R)
• A composite dual layer mixed denier needle punched non woven (ELCOMAX® 1209RP)
• A staple fibre woven geosynthetic (Grassroots)
• A composite of heavy-duty polypropylene fibres with a thin backing layer of polyester /
polypropylene fibre geotextile (Drainmat)
The ELCOMAX® 1200R and 1209RP are standard products that have been successfully utilized
in coastal structures (including the Narrowneck artificial reef). The Grassroots and Drainmat products
had not been previously trialed and are designed primarily for turf reinforcement and drainage
respectively, but both geosynthetics have an open structure that was expected to provide a suitable
substrate. A woven type geosynthetic was not included due to its poor performance in other trials.
The samples were compared to the concrete marina piles to which the samples were attached. The
samples were secured around the pile using cable ties with the more complex or “open” side away
from the pile.
The trial location was within the calm waters of the Gold Coast Broadwater. Samples were placed
at two locations within the Southport Yacht Club marina (close to the channel and close to the shore).
The difference between the two sites was the exposure to waves and currents (and associated water
quality). Samples were deployed at two depths, -2.5m LAT (~1m above the seabed) and -1m LAT
(just below the pontoon).
The samples were placed in April 2006 and retrieved in February 2009 after an almost 3 year
deployment. It is noted that retrieval occurred during the summer period.
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Figure 4. Geosynthetic 300mm square samples (ELCOMAX 1200R, ELCOMAX 1209RP, Grassroots, Drainmat)
Figure 5. Location of deployment of Australian samples
Top of Pile - Sample 1
Bottom of Pile - Sample 1
Top of Pile - Sample 2
Bottom of Pile - Sample 2
LAT
Seabed
Pontoon Pontoon
Pile
Figure 6. Configuration of sample deployment
ELCOMAX® 1200R ELCOMAX® 1209RP
GRASSROOTS DRAINMAT
channel
Broad
water
Pacific
Ocean
Southport
Yacht
Club
-1.0m LAT
-2.5m LAT
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Results
After 3 years deployment, there was substantial growth on all the samples. While the piles were
dominated by oysters or barnacles, there was a predominance of a range of “soft” growths on the
geosynthetics, including sponges, algaes, ascidians (sea squirts) and anthozoa. Bivalves and oysters
were also present on some samples, but these were predominantly on the edges or back of the sample
(adjacent to the pile) and would not be expected to be present on a structure.
There were a range of species present, including:
• Worms
o Tubeworm
o Roundworm
o Fan worm
o Flat worm
o Polychaete worms
• Nudibranch
• Crabs (Mud crabs and Portunid crabs)
o Megalopa (final larval stage)
o Juvenile crabs
o Adult crabs
• Prawns
o Juvenile prawns
o Adult prawns
• Shrimp
• Brittlestars
• Seaspider
• Fish (gobi)
Impact of Geosynthetic type
All geosynthetic samples developed “soft” growth suitable for contact during recreational usage.
In general, the composite 1209RP and Drainmat samples exhibited the greatest coverage of growth.
They also displayed higher numbers and diversity of other species in comparison to the Grassroots and
1200R. This was anticipated as the open structure of the geosynthetic outer layer has greater overall
surface area and the voids provide greater habitat complexity and protection during the initial growth
stages.
It was also noted that the Grassroots sample deteriorated substantially during the long trial period
and its suitability in long-term coastal applications appears to be limited.
Figure 7. Deteriorated Grassroots sample
Impact of Water Depth
The shallower samples would likely experience greater exposure to light being closer to the
surface (despite shadowing from the pontoon) and lower turbidity being further from the seabed. As
could be expected, top samples for both locations exhibited a greater coverage of algaes (for outer pile,
four times that of the bottom samples). The bottom samples experienced a greater ingress of fine
material from the seabed.
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Figure 8. Samples from (a) top of outer pile and (b) bottom of outer pile after removal of major growth
Impact of Water Flow
The outer pile experienced higher currents (and possibly correspondingly improved water quality).
For the concrete piles, the higher flow allows a predominance of barnacle growth (filter feeders)
compared to the inshore pile which is dominated by oysters (which are capable of feeding in lower
flow conditions). The samples on the outer pile experienced much higher growth (25 – 100%
coverage) than the samples in calmer water (10 – 35% coverage). In general, species diversity on the
outer pile was approximately double that of the inshore pile.
COMPARISON
Unlike the UAE samples (which were retrieved after a much shorter period), the Australian
samples were left in over a longer timeframe and were retrieved in summer after almost three years
(Figure 9). Conditions between the two sites were also very different. It is clear that geosynthetics can
support significant growth and habitat in a wide range of the conditions. Regardless of location, non-
woven geosynthetics appear to support a dominance of “soft” type growths which are considered most
suitable for structures where recreational aspects of the structure are desirable. While the overall
performance of the geosynthetics differs depending on conditions, the geosynthetics with a more
“open” structure consistently provide greater productivity and diversity of growth and resident species.
Figure 9. ELCOMAX 1009RP after (a) 3 years, Gold Coast and (b) 4 months, UAE
CONCLUSION
The comparison of trials at both sites and observations of growth on actual sand-filled geotextile
structures were generally consistent. For design, selection of the most suitable geosynthetic is
essential. This will depend on a number of factors, including the importance of environmental
benefits, local conditions and anticipated usage. Where ecological performance is an important aspect,
deployment of samples in the specific location will provide a site-specific indication of performance of
various geotextiles. It is generally considered that high strength non-woven type geosynthetics are
most suitable for structures which are intended to provide ecological / recreational benefits as they
provide higher diversity and less hard growths which are not as user-friendly. To further increase
diversity, consideration should be given to selection of a geosynthetic with a more open structure [e.g.
composite type geotextiles that have a “hairy” outer layer with significant voids].
ACKNOWLEDGMENTS
Geotextile samples tested as part of the Australian trial were provided by Geofabrics.
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REFERENCES
Edwards, R. 2003. An investigation into the biodiversity of a geotextile artificial reef, Narrowneck,
Gold Coast, Qld. University of New England Honours Thesis
Jackson, L.A. 2010. Design and Construction of low crested reef breakwaters using sand-filled
geotextile containers. Geosynthetics and Modern Materials in Coastal Protection and Related
Applications. IAHR
Jackson, L.A., Restall, S., Corbett, B.B. and Reichelt, R.E. 2005. Monitoring of Geosynthetics in
Coastal Structures, Proceedings of 1st International Conference in Coastal Zone Management and
Engineering in Middle East
Jackson, L.A., Reichelt, R.E., Restall, S., Corbett, B., Tomlinson, R. and McGrath, J. 2004. Marine
Ecosystem Enhancement on a Geotextile Coastal Protection Reef – Narrowneck Reef Case Study.
Proceedings of the 29th International Conference on Coastal Engineering. ASCE
