26 International Conference for Seaports & Maritime Transport

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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
SHORELINE RESPONSE FOR LONG, WIDE, AND DEEP
SUBMERGED BREAKWATER OF ALEXANDRIA CITY, EGYPT
Bahaa El-sharnouby (1), Akram Soliman (2)
(1)
Professor of Ports and Marine Structures,
Dean Port Training Institute - Associate Professor of Ports and Coastal Engineering
(1)
Faculty of Engineering, Alexandria University, Alexandria, Egypt
(2)
College of Engineering and Technology, Arab Academy for Science and Technology and
Maritime Transport, Alexandria, Egypt
E - mail: (1) [email protected] - (2) [email protected]
(2)
Abstract
In order to stabilize the eroded beach, along the eastern part of Alexandria shoreline,
a submerged breakwater was constructed. The breakwater system consists of one main parallel
part and two overlapping parts approximately 150 to 300 meters offshore. Total length of the
breakwaters is about 3000 meters while depth ranges from 2.5 to 8.5 meter. Width of the
breakwater crest ranges from 36 to 46 meters with crest level 0.5 meter less than the low sea
water level. The construction of the breakwater has been completed in July 2007. The response
of that system is unusually amazing. The beach width varied from 50 to 150 meter compared
to 0.0 to 20 meter before installation with stable sediment leeward of the breakwater under
storm conditions for nearly two years. The breakwater system showed tremendous efficiency
concerning wave transmission and shore protection at storm times.
This paper documents the rubble mound submerged breakwater effects with relation to its
nature and configuration. Details about the design procedures is presented in attached with a
monitoring program which has been applied for the last two years to measure the performance
of the submerged breakwater. The paper also included the research work which is under
progress now to study using of the same techniques to solve erosion problems in other areas of
Alexandria coastline. Also the research work will be extent to study using of other userfriendly alternatives such us the submerged artificial reefs. Finally, a conclusion of the paper
will be summarized and presented.
1. Introduction
Alexandria coastline suffers from many erosion problems along its coastline
which result from natural and human activities in the coastal zone.
Significant erosion occurs along most of Alexandria’s beaches as a result of
sediment starvation, coastal processes and sea level rise. The beach width
gradually decreased due to the action of the waves and currents. With the
time, some beaches have disappeared totally and the wave action has
attacked the toe of seawalls (Soliman & Reeve, 2007). Flooding problems
appeared also in these eroded areas affecting the traffic flow. In December
2003, the shoreline of Mandara beach (Alexandria city, Egypt) had suffered
serious erosion. The sand beach was vanished with wave scouring
underneath the road seawall. Waves and sea sand jumped over the broken
seawall to the Cornish road (Figure 1).
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
Figure 1: Mandara beach at 2003 storm
There are many factors contributed to that severe coastal erosion, including:
 Incident waves, storm events, and the phenomena of Sea Level Rise (SLR).
 Elimination of sources of organic sediments as a result of water pollution.
 Instability of northern delta cost due to the lack of Nile river sedimentation.
 Loss of a considerable part of the sand beach due to the enlargement of the
Cornish road toward the sea.
During the last years, many alternative solutions for shore protection were
adopted for Alexandria city, Egypt. Figure (2) presents one of these solutions
which have been constructed at Loran beach. As can see from Figure 2, the
revetment protected the shoreline against further erosion but the beach profile
was completely reinforced.
Figure 2: Reinforcement of beach profile by concrete revetment
Groins can be another solution to protect Alexandria coastline. Groins can
minimize instability but added no good to the already eroded beaches. This can
be shown clearly on Geleem area at Figure (3).
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
Figure 3: A satellite image for groins for Geleem and San-Stefano Areas,
(Google Earth, 2009)
Beach nourishment protected, temporally, against flood and showed quick
results in Sheraton area but lasted for short time and needed to be done
periodically. Other coastal structures have been also used at Alexandria
coastline. For example, Seawalls were effective at reducing erosion landword
of structure in Chatpi area, but caused erosion in front of the structure.
Detached amerced breakwater with narrow crest was a complete failure in
Celopatra area with adverse results. Sub-aerial parallel rubble mound
breakwater (4.0 meter above water level) was effective at controlling erosion in
Mandara area (from Miamy beach to Montaza palace). However, it had a quite
severe adverse impact on beach amenity and aesthetics .The installation of the
emerged breakwater in 2005 led to a rapid deterioration in water quality as can
seen from Figure (4).
Figure 4: Contamination of water at the leeside of emerged breakwater,
Miamy area
The shoreline near the end of the breakwater suffered from severe wave attack
and the breakwater was repeatedly extended to protect against wave scouring
(Figure 5). After the construction of about 60% of the breakwater, Alexandria
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
government inquired a complete re-evaluation of the project which was
suspended.
Figure 5: Broken seawall due to wave attack
2. Submerged breakwater for shore protection
The studies which had been done upon the request of Alexandria government
showed that the emergent breakwater should be removed and replaced by a
submerged one. Elsharnouby et al. (2006) suggested a submerged breakwater
system constructed with natural stones and armored with concrete blocks
(Figure 6).
Figure 6: Submerged breakwater system for Miamy-Montaza area
The sub-aerial breakwater was remolded and stored in site in order to allow for
construction of the submerged breakwater as a replacement of the sub-aerial
one. Figure 7 presents the preliminary breakwater cross section and modified
cross section (submerged breakwater) using the same material.
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
Figure 7: Rubble moud submerged breakwater
2.1. Design of the submerged breakwater
Design characteristics of submerged breakwater (Elsharnouby et al, 2007)
shows at Figure 8 include:
 Length of the breakwater (Lb) and offshore distance (x)
 Crest width (B) and incident wave length (Lw)
 Water depth (d) and structure height (h).
 Incident wave height (H) and freeboard (F).
Figure 8: Submerged breakwater characteristics
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
The findings of this previous study (Elsharnouby et al, 2007) can be
summarized as follows:
1- Flushing time of water mass in the breakwater lee ranges from 4 to 6
days in calm weather conditions.
2- The current velocities behind the submerged breakwater are about 5
times of those in case of emergent breakwater and from 50% to 100%
of currents in open sea.
3- The submerged breakwater provides transmission coefficient ranging
from 0.28 to 0.36 in storm conditions using Friebl and Harris
formulation (Fribel et al, 2006).
4- The shore line is well protected from wave attack providing a width of
beach sand not less than 30 meter.
5- Wave height is about 0.50 meter or less in the leeside of the submerged
breakwater for at least 90% of the year days.
6- Continuous submerged breakwater provides better shore line stability
with a 60% decrease of the total eroded volume.
7- Accretion will take place within 12 months after installation.
2.2. Breakwater configuration
For this particular location (3000 meter length from Miamy to Montaza), the
breakwater consists of three segments with two overlaps as shown before at
Figure 6. No direct openings are allowed to prevent unequal shore line
changes. The breakwater depth varies from 2.5 to 8.5 meter. Five cross
sections at different locations are considered according to the depth and wave
height. Outer slope is 1:2 and inner slope varies from 1:3 to 1:5 in order to
alleviate turbulence in the lee side. For depths 3 and 5 meter, the breakwater is
armoured with two layers of 3 ton tetrapods blocks at the sea side while one
layer of 5 ton concrete cubes is adequate as armour layer in the lee side (Figure
7). For depth 8.5 meter, 5 ton tetrapods blocks and two layers of stone concrete
cubes are used.
Filters of greater stone size are used at both sides to protect the breakwater
body made from fine natural stones (10-300 Kg).No horizontal filter is placed
between armour layer and the core since no significant vertical forces are there
(Zaki, 2007)
The width of the breakwater crest is 36 meter at depth 3 to 5 meter and 46
meter at depth 8.5 meter. The width at bed level is 66 and 92 meters. The free
board is kept constant for all cross sections with a 0.5 meter value. The 0.5
meter value is the minimum distance which insures both best efficiency and
circulation requirements in the lee side.
In the design process, special care has been shown the following factors
affecting the performance of a submerged breakwater, the freeboard (F)
,relative crest width (B/Lw) , and offshore distance X. It was found that crest
width 36 meter and 46 meter are fairly good for a relative width (B/Lw) close to
unity of maximum efficiency.
No physical model has been done for the submerged breakwater. One
completer part of the breakwater in the deepest location (8.0 meter) was
installed in 2006, one year before the installation of whole breakwater system
.The breakwater of 220 meter long, 8 meter depth and 46 meter crest width was
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
hit by the highest wave recorded at Alexandria coastline in the last 100 years
(7.0 meter wave height in deep water) in December 27th 2006. Figure 9 shows
the amazing efficiency of the breakwater in storm-condition. The shore line
and sea-wall remained safe along the 220 meter breakwater with noticeable
good quality of water in the leeside in summer.
Figure 9: Efficiency of the submerged breakwater at depth 8.0 meter in
storm time
3. Monitoring program
The Miamy-Montaza submerged breakwater was installed in 2007-2008 to
control the beach and shore line problems. The performance of the submerged
breakwater system after its complete construction has been measured through
24 months of monitoring for the shoreline, sand beach, sea wall stability, cross
shore, and wave transmission.
The following has been observed:
 Pre-breakwater installation, the beach width was the greatest at 20 meter.
The shoreline continued to retreat until it reached the sea wall in several
locations with zero beach width.
 Few months after installation, the shoreline accreted an average of 100
meter (Figures 10, 11 and 12). No erosion happened and no beach
nourishment needed at any point along the 3000 meter length of the
shoreline protected by the submerged breakwater.
 Wave height all round the year never reached 0.5 meter in the leeside as
theoretically predicted. The submerged breakwater system showed much
more efficiency during storm times than was anticipated.
 Sea walls remained untouched and waves never came close to the shoreline.
 So far, Shoreline after accretion remained unchanged.
 Water quality has been improved and signs of contamination were
disappeared.
 Swimmers were absolutely safe and no drowning cases were claimed for
first time for so many years.
 The beach is good for swimming, fishing, and relaxation for almost all year
around (Figure 13).
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
Figure 10: Comparison between the beach width after and before the
construction of the submerged breakwater at Mandra beach, Alexandria,
Egypt
Figure 11: Comparison between the beach width after and before the
construction of the submerged breakwater at Miamy beach, Alexandria,
Egypt
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
Figure 12: Comparison between the beach width after and before the
construction of the submerged breakwater at Montaza beach, Alexandria,
Egypt
Figure 13: Comparison between the water quality after and before the
construction of the submerged breakwater, Alexandria, Egypt
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
The excellent performance of the submerged breakwater can be due to the
following:
1- Method of construction (land end method).
2- Effect of shoaling (wave non-linearity).
3- Strong breakwater foot.
4- Minimum value of freeboard (F) and maximum value of breakwater crest
(B=Lw) regardless of construction height or incident wave height.
5- Continuity of breakwater and site layout.
4. Future work
The 3000 meter rubble mound submerged breakwater at Alexandria is
considered one of the longest submerged breakwaters all over the word. This
breakwater solves the problems of beach erosion and storm attacks at this area.
There are still many other areas of Alexandria coastline which still suffer from
the same problems. A research project is under progress now to study using of
the same techniques to solve erosion problems in other areas of Alexandria
coastline. Other alternative which can be used is the submerged artificial reefs.
More recently, submerged artificial reefs applications have varied widely,
including: aquaculture production; coastal protection (Seaman and Jensen,
2000); and habitat protection (Baine, 2001). Artificial reef materials must last a
minimum of 30 years to provide ecological services economically, and also be
non-toxic to the marine environment (Grove et al., 1991).
5. Conclusion
During the last few years, Alexandria’s coastline faces many flooding
problems. In winter of 2003 to 2006, many surge storms struck the
Alexandrian coastline, water and sand overtopped the seawall and destroyed
many parts of it. In the coming years, this kind of storms are expected to
increase due to sea level rise. The option of raising the level of the defenses is
not feasible, both from the reduction in amenity and also the costs, as existing
infrastructure would have to be sacrificed to accommodate larger defenses.
This leads to the proposed deployment of submerged offshore rubble mound
breakwater and/or submerged offshore artificial reefs, to induce wave breaking
and energy dissipation, to limit the wave heights reaching the beach. (Soliman
and Reeve, 2009). A submerged breakwater system was installed to protect the
sea –shore of Miamy –Almontaza area in Alexandria, Egypt in 2007-2008. The
3000 meter long breakwater body is made from fine stones and armoured with
tetrapods blocks. The crest width equals nearly a wave length and the crest
level is 0.5 meter below water level. The estimated shore line changes based on
theoretical analysis are far from reality. Natural sedimentation formed a beach
with a 100 meter average width in the leeside of the breakwater. No erosion
occurred for two years at any point and sea – wall remained untouched by
waves in storm conditions.
The water mass between the submerged breakwater and the beach circulates
well with very gentle and light waves even in storm times. The breakwater in
either shallow (3 meter) or deep (8 meter) locations has consistent stability.
The accumulated beach has shown a stable line for the last two years.
6. References
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26th International Conference for Seaports & Maritime Transport
" Integration For a Better Future "
1- Baine, M. (2001) Artificial reefs: a review of their design, application,
management and performance. Ocean and Coastal Management, 44:
pp.241-259.
2- Deamrich, Mai, and Nina “ Wave transmition at Rubble mound structure”
first German –Chinesse joint symposium on coastal and ocean
Engineering-German, 2002.
3- Elsharnouby, B., Zaky. M. and Abdelfattah, S. “Experimental and
theoretical Analysis of impervious submerged breakwater” second
conference on civil Engineering, Military technical college ,Cairo, Egypt,
November 1994.
4- Elsharnouby, B. and Abdrabbo, F. ” Re-evaluation of Miamy-Montaza
Breakwater project ” Technical report , Faculty of Engineering,
Alexandria, Egypt ,2006 .
5- Elsharnouby, B., Nagy, H. and Abdrabbo, F. ” Stability of shore line for
Miami –Montaza area “ Technical note, Faculty of Engineering,
Alexandria, Egypt, 2007.
6- Friebl, Harris, and Masce “Re-evaluation of wave transmission coefficient
formulae from submerged breakwater physical models “ Institute of
technology ,Hoboken ,New Jersey ,U.S.A, 2006
7- Grove, R.S., Sonu, C.J. and Nakamura, M. (1991).Design and Engineering
of Manufactured Habitats for Fisheries Enhancement. In: W.J. Seaman and
L.M.E. Sprague (Editors), Artificial Habitats for Marine and Freshwater
Fisheries. Academic Press, Inc., San Diego, California. pp. 109 – 149.
8- Seaman, W.J. and Jensen, A.C. (2000), Purposes and Practices of Artificial
Reef Evaluation. In: W.J.E. Seaman (Editor), Artificial reef evaluation:
with application to natural marine habitats. CRC Marine Science Series,
CRC Press LLC, Boca Raton, Florida, USA. pp. 1 – 20.
9- Soliman, A., and Reeve, D. (September, 2007) “Artificial Submerged
Reefs: A solution for Erosion Problems along Alexandria Coastline,
Egypt?” Proceeding of the IMA 2nd Conference on Flood Risk
Assessment, Plymouth, UK, pp: 41-50.
10- Soliman, A., and Reeve, D. (September, 2009) “Applying the Artificial
Submerged Reefs techniques to reduce the Flooding Problems along the
Alexandria Coastline” Proceeding of the Institution of Civil Engineers
Conference 2009 "Coasts, Marine Structures and Breakwaters",
Edinburgh, Scotland, UK, pp: 1-12.
11- Zaki, A. “stability of armour layer blocks of submerged breakwater
“Master thesis , Faculty of Engineering ,Alexandria ,Egypt ,2007 .
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