Design and Construction Methodoloy of a Novel Multi- FunctIonal Artificial Reef for Dubai

Abstract

Numerical and physical modeling of waves, currents, sediment transport and morphological response were performed for a large range of artificial reef designs. The effectiveness of each design was evaluated in terms of both coastal protection and surfing amenity. The design of the reef shape progressed from a generic delta-shaped reef through a series of lens-shaped configurations and eventually led to a unique composite shelf-lens form. The new design has a number of significant advantages over earlier designs: Most notably, the shelf serves to reduce wave refraction until the waves impinge on the lens, resulting in larger peel angles along the breaker line and therefore a more surfable wave. Additionally, the new shape assists in focusing waves onto the lens shape closer inshore, resulting in an extended length of surfable wave as well as wave shadows on both sides of the reef that effectively increases the length of coastline protection. The beneficial influence of the sheltering effect of the reef is demonstrated by morphological predictions using state of the art numerical and physical modeling methods, which also demonstrated the sensitivity of reef performance as a function of its relative position offshore. Finally, the identification of a location for the proposed reef, the design of the reef and its construction methodology, as well as the environmental response to the proposed use of sand-filled geotextile containers for constructing the reef lens shape are discussed.

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DESIGN AND CONSTRUCTION METHODOLOGY OF A
NOVEL MULTI-FUNCTION ARTIFICIAL REEF FOR DUBAI
Francois Smit
1
, Gary P. Mocke
1
and Angus L. Jackson
2
Numerical and physical modeling of waves, currents, sediment transport an d mor pho logi ca l
respon se were perfor med for a large rang e of artificial re ef d e si gn s. T h e e ff ec t iv en e ss o f e ac h d es i gn
was evaluated in terms of both coastal protection and surfing amenity. The design of the reef shape
p ro gr es se d f r om a g e ne ri c d el ta -shaped reef through a series of lens-shaped configurations and
eventually led to a unique composite shelf-lens form. The new design has a number of significant
a dv an t ag es o ve r e a rl ie r d es i gn s: M os t n o ta bl y, t he s he lf s er ve s to r ed u ce wa ve r ef ra c ti on u nt i l t h e
waves impinge on the lens, resulting in larger peel angles along the breaker line and therefore a more
surfable wave. Additionally, the new shape assists in focusing waves onto the lens shape closer
inshore, resulting in an extended length of surfable wave as well as wave shadows on both sides of
t he re ef t ha t e f fe ct iv e ly i nc r ea se s t h e l en g th of c oa st li n e p ro t ec ti o n. Th e b e ne fi c ia l i nf l ue nc e o f t h e
sheltering effect of the reef is demonstrated by morphological predictions using state of the art
numerical and physical modeling methods, which also demonstrated the sensitivity of reef
performance a s a function of its relat ive posi tion offshore. Finally, the identification of a location for
the proposed reef, the design of the reef and its construction methodology, as well as the
environmental response to the proposed use of sand-filled geotextile containers for constructing the
re ef le ns s h ap e a re d i sc us s ed .
INTRODUCTION
Background
Submerged “reef” type breakwaters have been the subject o f extensive
investigation (Kawasaki and Iwata, 1998, Saito et al., 2002) as a possible
alternative to conventional emerged breakwater structures for coastal protection.
In addition to o ffe rin g potential cost savings by allo wing for a reduction in
structure volumes and bre akwater element sizes/weights, submerged structures
can beneficially reduce wave induced currents and gradients in sediment
transport by allowing limited wave energy transmission over the structures.
Besides helping to avo id excessive sand trapping behind such structures, this
effect may also translate into a reduced danger to swimmers and improved water
quality. There is also the aesthetic advantage of a submerged structure not
obstructing desirable coastal views as well as potential environmental impacts in
providing habitats for marine life.
A further significant advantage of submerged structures is the potential amenity
value of a reef designed to enhance surfing conditions. Fo llowing on from the
pioneer ing work of Walker (1 974) into the fundamental physical parameters
constituting a quality surfing wave or break, numerous recent studies have
1
MaSTconsult, P.O.Box 64135, francois@mastconsult.com, Dubai, United Arab Emirates (formerly with
Coastal Management Section, Dubai Municipality)
2
International Coastal Management, Gold Coast, Australia
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attempted to elucidate the science of relevant physical processes (see Scarfe et
al., 2003 for a review). These have included efforts directed at classifying reef
morphological forms as a function of wave response (Mead and Black, 2001,
Scarfe et al., 2003).
Of the three surf reefs constructed to date only the Gold Coast Artificial Reef at
Narrowneck in Australia appears to have incorporated an optimized design
process considering “surfability” parameters (Black and Mead, 2001, Turner at
al., 2001). This has resulted in a lens shape reef configuration that differs from
the delta shape forms implemented at Cable Station in Western Australia
(Pattiaratchi, 2000) and Pratte’s Surfing Reef at Dockweiler Beach in California
(Borrero, 2003). Only limited information exists (Ronasinghe et al., 2001;
Borrero, 2003) on the performance of these reefs, with indications being that
neither coastal protection nor surfing functionality has been conclusively
demonstrated.
Although some level of physical and numerical modeling was carried out for the
above referenced reef projects, there is no published record of the design
evolution. Mocke et al. (2003) demonstrated how mathematical and physical
modeling of wave and current processes associated with a comprehensive range
of reef configurations can be used to arrive at an original composite design that
fully satisfies surfability requirements. This investigation demonstrated that all
of the reef shapes adopted to date have a number of inherent deficiencies that
result in poor surfing conditions, along with negligible or inefficient coastal
protection performance.
Dubai Multi-Function Artificial Reef
The new Multifunction Artificial Reef (MFAR) shape proposed by the authors
has further been refined using state of the art numerical and physical
morphological modeling methods, confirming its suitability for coastal
protection purposes. The reef has also been comprehensively evaluated for
locations along the coast of Dubai, highlighting the impor tanc e of ensuring that
a submerged structure is optimally sited.
REEF DESIGN EVOLUTION
Surfability Evaluation
In order to perform evaluations of reef designs based on numerical or physical
model test results, several parameters need to be extracted and evaluated,
namely: H
b
, wave breaking height along the breaker line; α, peel angle along the
breaker line; V
s
, surfer velocity along the breaker line; L
s
, length of ride; ξ
b
,
Iribarren number along breaker line that provides an indication of the type of
breaking; L
prot
, length of coast protected by the reef (defined as the distance
immediately behind the reef where wave height was reduced to less than the
wave height on the open coast adjacent to the reef)
Most of these parameters are inter-dependant, with the most important indicator
for surfability being the peel angle. As a wave propagates towards shore, the so-
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called peel point marks the lateral movement of breaking along the wave crest.
As schematized in Figur e 1 the peel angle α is measured between a line joining
two peel points (in time or space) and the unbroken wave crest orientation. Peel
angles vary between 0° and 90°, with a zero peel angle corresponding to what is
referred to as a “close out” where the wave breaks simultaneously along the
entire crest. Such a wave is completely impossible to negotiate by a surfer, and
being a common feature of most beaches around the world, reflects the shortage
of quality surf locations internationally. As peel angles increase the speed of
breaking along the crest, which approximates the surfer velocity V
s
, decreases to
a speed that can be negotiated by experienced surfers. This occurs ar ound
α = 30° to 45°, with the optimal peel angle for most recreational surfers
considered to be in the range 45°-65°.
Figure 1. Peel angle definition
Surfability Shape Evolution
Delta Reef - At the outset of the design a basic delta-shaped reef was defined
that could be modified through the change of various parameters, such as nose
half-angles, side slopes, length of sides, height, reef orientation etc. A total of 36
different delta-shaped reefs were evaluated in this manner using various
components of the MIKE21 numerical modeling suite as well as the 2D
Boussinesq wave model FUNWAVE (Kirby et al, 1998). Analysis of model
results indicated that it is impossible to achieve a peel angle in the surfable range
of 40º to 60º without red ucing the nose angles to 30º or less. This is due to
amount of refraction of the waves that occur before breaking, resulting in a wave
front that is too closely aligned to the bathymetric contours. This agrees with the
findings of Black and Mead (2 001) during design studie s for the Narrowneck
Artificial Surfing Reef. Small nose angles result in a narrow elongated reef that
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must be longer and located in deeper water than wider reefs to provide both
effective coastal protection and surfing functionality. In order to protect a longer
stretch of coast, such a narrow reef could be duplicated, i.e. a similar second reef
installed a distance alongshore immediately next to it. Such an option is however
likely to be uneconomical due to the large fill volumes required.
In order to solve the peel angle and coastal protection conundrum a number of
modifications were evaluated, consisting of dual-sloped delta and lens shapes.
Results indicated that whilst the peel angle could be improved slightly it was not
sufficient to provide acceptable surf breaks whilst providing a reasonable length
of coastal protection.
BH Reef - All reefs tested up to this point had failed the design criteria because
of either the amount of refraction under gone prior to breaking (small peel
angles) or the limited coastal protection offered. In their bathymetric
classification of quality surfing breaks Mead and Black (2001) discuss the
function of a platform intermediate to a deeper water area and the shallow
breaking zone. A platform has the desired minimal effect on the wave
orthogo nals, but the relatively shallow bathymetry has the disadvantage of
allowing “close out” breaking of large waves, making it difficult to design an
appropriate platform level for the full range of possible wave and water level
combinations. This is, however, less of a disadvantage in the fetch limited wave
regime of Dubai, where the depth over the shelf can be judiciously determined
to not allow breaking under the full range of wave conditions. Mocke et al
(2003) presented an evaluation of a so-called “piriform” platform-lens design.
Improvements to this design has since been made by extending the lens section
shoreward and reducing the shelf width slightly. This resulted in an increase in
the ride length as well as an increase in the amount of wave energy dissipated
over the reef. Figure 2 depicts the evolution of the reef from the initial delta
shape to the present design dubbed the “bishop’s hat” or BH reef.
Figure 3 depicts typical wave height results as well as extracted peel angles,
surfer velocities, Iribarren numbers at breaking and wave heights at breaking for
the BH reef. Results indicate that the significant coastal protection is provided
over approximately 250 m of coastline.
Examining the predicted distribution of surfability parameters shown in Figure 3
a number of notable features are evident. With an incident wave height of 1.4m
it can be seen that wave heights are amplified by more than 20% at the likely
“take off” point A. Magnitudes remain above the incident wave height over the
remainder of the surf zone, with notable peaks at Points B and D. With a
relatively large peel angle of 60º point B would be an excellent area for “ cu t
back” type manoeuvres or for less experienced surfers to catch the wave. There
then follows an area of smaller peel angles around point C before a “bowl” wave
around point D where incident wave amplification exceeds 35%. Thereafter
follows a section of around 50m where a large peel angle remains relatively
constant but wave heights gradually diminish. This would be an appropriate area
for novice surfers as it is also close to shore.
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The sensitivity of the BH reef response to a number of key parameters (wave
direction, water level and ramp level) was tested and results confirmed that the
reef would provide shore protection and surfable waves for most of the expected
combinations of parameters. The other key sensitivity analysis relates to the
position of the reef relative to the shore, which influences the morphological
response.
Morphological Response
Numerical Modeling - The MIKE21 CAMS model was used to evaluate the
morphological response of the beach to the reef. As the simulation of
morphological response is a time-consuming modelling effort, a condensed (30
d a y ) wave climate was compiled from thirteen years of hindcast waves to
represent a “morphological year” (providing the same morphological response
that would occur if a typical year of waves were to be applied). For simulations
made on an open beach section this reduced wave climate produced a net
longshore sediment transport equivalent to known rates. Modelling results
indicated the formation of a salient behind the reef with an increase in bed
elevations behind the reef observed for a distance of up to 50 m from the
original shoreline.
Physical Modeling - A combination of 14 model wave and water level
conditions were determined for physical model testing. The first series of tests
used an offshore wave direction of 310ºN, which is approximately 15º off the
shore perpendicular direction. The second series of tests used an offshore shore
perpendicular direction of 295ºN. For each direction, a long duration, random
200 250 300 350 400 450 500 550
300
350
400
450
500
550
600
650
700
delta
shelf
platfor
m-lens
pirifor
m-lens
BH Reef
dual-
slope
lens
Figure 2. Evolution of reef configuration .
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wave test was first run to develop the beach to an equilibrium state. A number
of short monochromatic tests were then run to investigate the surfing conditions.
-150 -100 -50 0 50 100 150
distance along breakerline (m)
0
1
2
3
4
5
Iribarren No
-150 -100 -50 0 50 100 150
distance along breakerline (m)
0
20
40
60
80
peel angle (deg)
-150 -100 -50 0 50 100 150
distance along breakerline (m)
0
4
8
12
16
20
V
s
-150 -100 -50 0 50 100 150
distance along breakerline (m)
1
1.2
1.4
1.6
1.8
2
H
b
(m)
Figure 3. Predicted surfability parameters along the breaker line (H= 1.4 m, T=6 s).
Physcial Model Morphological Response - Test 1 was a random wave test
intended to develop beach plan shape. The test was run for a total of 8 hours.
Figure 4 shows a photo of the beach shape at the end of test 1. The plan-shape
was measured at increasing intervals over the duration of the test. The
asymmetric salient that developed in the lee of the reef is clearly visible. Once
the beach had reached equilibrium state (after approximately five hours of
testing), wave conditions and wave-induced currents were measured.
Physical Model Surfability - The remaining tests in each series used
monochromatic wave c onditio ns to investigate the surfing conditions on the reef.
Figure 5 shows images extracted from the video footage that were used to
determine break rate and peel angle. Examining images taken at two known
A
B
B
C
C
D
D
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times, t = 1 and t = 2, the velocity of the breaking point was used to define the
breaking rate, V
b
. The angle between the breaking rate velocity vector and the
incident wave crests was used to define the peel angle, α.
Figure 4. Beach shape at end of Test 1.
DUBAI REEF
Environmental Conditions
Situated on the south-eastern end of the Arabian Gulf, the coastal regime of
Dubai is characterized by weak tidal currents, a tidal range of approximately 1.0
m - 1.75 m and the occurrence of north-westerly (shamal) winds that blow along
the length of the gulf. These shamals are the principal wave generating force and
mainly occur during the winter months. During shamal conditions wave periods
are typically in the range of 7 s – 8 s, with significant wave heights in the range
1 m to 2 m. These events typically last for 1 to 3 days. Sediment transport is
predominantly wave-driven and generally directed from southwest to northeast.
Potential transport rates vary between 20,000 m
3
/yr in the northeast to
40,000 m
3
/yr in the southwest, depending on the orientation of the coastline.
Reef Environmental Aspects
The intention fro m the outset of the reef design was to construct the lens from
sand-filled geotextile containers, primarily because of surfer safety concerns
should rock have been used. A second possible benefit of the use of geotextiles
was the potential increase in marine biodiversity observed such as was observed
and monitored at the Narrowneck reef on the Gold Coast, Australia. (Jackson et
al. 2004 and Edwards, 2003).
Reef
Salient
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To determine if the results at Narrowneck are indicative of similar benefits that
could be achieved in the warmer Arabian Gulf conditions, studies involving
different geosynthetics were carried out in the Arabian Gulf. Data w a s also
obtained from rock, concrete and steel structures near to the test site to
determine the marine habitat that has been created by these structures and
materials. Three different commercially available geosynthetics were chosen: A
non woven; a composite dual layer non wove n; a split film high strength woven.
The samples were deployed at various depths near the initially proposed MFA R
site off Umm Suqeim 2, Dubai. The depths were selected to represent critical
depths on the proposed MFAR.
Despite initial deployment in early summer, the most stressful season, rapid
marine growth was observed. Preliminary sampling was undertaken after 4
months. The samples were retrieved by divers, divided in half and one half
returned to allow for further development. Further visual inspections were
undertaken at 7 and 8 months. As expected, there was considerable difference
between the growth on woven and non-woven samples. Predominate species
identified after 4 months were:
• Woven: Barnacles, about 90% coverage; Red algae, about 5% coverage
• Single layer non-woven: Red Algae, >90% coverage; Ascidians
(seaquirts), about 10% coverage; Crabs (2 species)
Figure 5. Extracted images from oblique (top) and overhead (left) video showing wave
transformation over the reef. By tracking the breaking point of individual waves at
time t=1 (top, left) and t=2 (bottom, left) an estimation of the peel angle can be made.
Proceedings of the International Conference Coastal Structures 2007, Venice, Italy.

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• Composite non-woven: Red Algae, about 30% coverage; Ascidians
(seaquirts); Crabs; Barnacles (juvenile); Annelids; Polychaetes;
Sponges; Shr imp
A further custom sample was deployed during the 7 month observations. After 8
months the samples appeared similar although the non-woven, composite layer
sample at -3.5m water depth clearly showed a trend towards an increase in
sponge type / sea squirt type growth and a decrease in macro-algae coverage.
Inspections showed that rock and concrete structures were not providing any
significant habitat improvement. This may be due to a predominance of sea
urchins on these structures that limit the establishment of marine flora. The
results confirmed that geosynthetics can be used as an effective substrata to
increase biomass, particularly of important species such as prawn lavae. The
predominant “soft” growth on both of the non-woven geosynthetics indicated
that either would be suitable for the shallow sections of the proposed surf reef.
Figure 6 depicts photo graphs of the marine gr owth on the non-woven samples as
well as the sea urchin predominance on rock in the vicinity. Additionally, the
marine biologists considered that the crab population evident in these samples
was likely to be responsible for controlling the incidence of urchins. While both
non-woven geosynthetics provided habitat for good species numbers and
diversity, the composite layer geosynthetic provided the most diverse habitat. As
such, it would provide the best substrate for ecological enhancement and a dive
reef. The “har d” growth and potential for urchins on the woven sample indicates
that it is not suitable for the shallow sections of a surf reef and would provide
the least diversity for a dive reef.
Figure 6. Sea urchins on rock (left), marine growth on composite non-woven after
four (middle) and eight (right) months.
Reef Location
A number of possible locations requiring shore protection along Dubai’s rapidly
evolving coastline were evaluated for suitability for constructing an artificial
reef. Eventually a location near the southern entrance to Dubai Marina was
identified as the most suitable. This location has experienced exacerbated
erosion since the reclamation of Palm Jumeirah and construction of the Dubai
Marina breakwaters. The suggested location of the reef and a plot of historical
surveyed shorelines are depicted in Figure 7.
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Figure 7. Proposed Dubai Multi-Function Artificial Reef location.
Reef Design
Following a hydrographic survey at the site, the design of the ramp, geotextile
containers and construction methodology commenced. A rock platform for the
ramp was proposed, as this would be easiest to create accurately to the required
level. On top of this custom-made geotextile containers were designed to be able
to reproduce the theoretical lens shape as accurately as possible. The containers
were designed so as to ensure stability (dry weight > 10-15 t); ensure
smoothness through joints along contours; avoid corners and gaps in breaking
regions through utilizing customized V-shape reinforced nose joints; avoid sand
leakage through overlapping of units b y at least 1 m. The vertical design
tolerance was specified as 0.2 – 0.6 m, depending on the position of each
container, with the tolerance increasing with increasing depth. The reef layout
and two cross-sections are depicted in Figure 8.
The construction methodo logy developed involved: Constructing a causeway
and sheetpile along the back (landward end) of the platform; placing rock for the
back of the platform; constructing a temporary construction platform using sand
from nearby construction sites; constructing the rock platform; construct the first
level of containers for the lens from the construction platform and a barge;
construct levels 2 and 3 of the stepped lens; removing the causeway and
spreading excess sand behind the reef.
Construction costs for the reef were estimated at $2.69 million at the specified
site. The estimated volume of the reef is 41,000 m
3
translating into a cost of
$65/m
3
. Typical costs in the UAE for groyne construction (full core and rock
Proceedings of the International Conference Coastal Structures 2007, Venice, Italy.

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Figure 8. Reef geotextile container layout and reef cross-sections.
Proceedings of the International Conference Coastal Structures 2007, Venice, Italy.

12
armour) are about $55/m
3
. The per volume cost for the larger (71,000 m
3
)
Narrowneck reef was about 30/m
3
.
CONCLUSIONS
This paper has attempted to illustrate that the design of a multifunction artificial
reef that provides both coastal protection and surfing amenity is not a trivial
exercise. This is primarily because the narrower form reef designs that suit
surfability requirements would provide only limited wave sheltering for the
adjacent coast. Through a systematic evaluation of generic reef forms it is found
that neither delta shape or lens forms are suitable designs, despite their adoption
in the only actual reef constructions to date. A composite reef configuration
(dubbed the BH reef) consisting of a lens superimposed on a platform is found
to provide optimal benefit, as demonstrated through an exhaustive modeling
investigation that includes morphodynamic predictions. Although the BH reef
design is considered to have wide application, a number of site specific factors
require consideration, including environmental aspects and incident wave and
tidal conditions. This was illustrated through presenting a design fo r a
recommended Dubai location. Any reef should ideally be sited to take advantage
of any natural wave focusing and would potentially require modifications to
accommodate a preferred range of wave conditions.
REFERENCES
Black, K. and Mead, S., 2001. Design of the Gold Coast Reef for Surfing, Public Amenity and
Coastal protection: Surfing Aspects, Journal of Coastal Research Special Issue No 29.
Borrero, J.C. and Nelsen, C., 2003. Re sul ts of a c omp rehe nsi ve mon ito rin g p rog ram at Pr att e’s
R ee f. . Proc. of the 3rd International Surfing Reef Symposium, Raglan, New Zealand,
Kawasaki, K and Iwata, K., 1998. Numerical analysis of wave breaking due to a submerged
breakwater in three dimensional wave field. Proceedings of the 26
th
International Conference of
Coastal Engineering, Copenhagen Denmark.
Kirby, J.T., Wei, G., Chen, Q., Kennedy, A.B., Dalrymple, R.A., 1998. FUNWAVE 1.0 Fully
Nonlinear Boussinesq Wave Model Documentation and User’s Manual, Research Report CACR-
98-06, Univ of Delaware.
Mead, S.T. and Black, K.P. , 2001. Field studies leading to the bathymetric classification of world –
class surfing breaks. Journal of Coastal Research, Special Issue No. 29, pp. 5-20.
Mocke, G.P., Smit, F., Fernando, S. and Al Zahed, K.M. , 2003. Coastal protection and amenity
value of an arti ficia l surf r eef for D ubai . Proc. of the 3rd International Surfing Reef Symposium,
Raglan, New Zealand
Ranasinghe, R., Hacking, N., Evans, P., 2001. Multi-functiona l artifici al su rf break s – a r ev ie w.
Report for Center for Natural Resources, NSW Dept of Land and Water Conservation. ISBN 0
7347 5192 3
Saito, M., Watanuki, A., Nishigori, W., Hanzawa, M., 2002. Stability of new type submerged
breakwater and its effect on ecosystem. Abstract of 28
th
International Conference on Coastal
Engineering, Cardiff, Wales, UK, poster paper no.33.
Scarfe, B. E., Elwany, M. H. S., Mead, S. T., and Black, K. P., 2003. The Science of Surfing Wave s
and Surfing Breaks - A Review. March 7, 2003. Scripps Institution of Oceanography Technical
R ep or t. http://repositories.cdlib.org/sio/techreport/17
Walker, JR. (1974). Recreational surfing parameters. LOOK Laborat ory TR-30, University of
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Proceedings of the International Conference Coastal Structures 2007, Venice, Italy.

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KEYWORDS – CSt07
Abstract acceptance number
DESIGN AND CONSTRUCTION METHODOLOGY OF A NOVEL MULTI-
FUNCTION ARTIFICIAL REEF FOR DUBAI
1
st
Author: Smit, Francois
2
nd
Author: Mocke, Gary, P.
3
rd
Author: Jackson, Angus, L.
Geotextiles
Morphodynamic Response
Multifunctional Artificial Reef
Numerical Modelling
Physical Modelling
Surfing
Wave transformation
Proceedings of the International Conference Coastal Structures 2007, Venice, Italy.