Download wave climate and coastal structures in the nile delta coast of egypt

Emirates Journal for Engineering Research, 18 (1), 43-57 (2013)
(Regular Paper)
WAVE CLIMATE AND COASTAL STRUCTURES IN THE NILE DELTA
COAST OF EGYPT
Moheb M. Iskander
Coastal Research Institute, National Water Research Center, Egypt, 15 El-Pharaana St., El-Shallalat, postcode 21514,
Alexandria, Phone +2 03 4844614, Fax +2 03 4844615, E-mail [email protected]
(Received August 2012 and Accepted February 2013)
‫ آﻢ ﻋﻠﻰ اﻟﺴﺎﺣﻞ اﻟﺠﻨﻮﺑﻲ اﻟﺸﺮﻗﻲ ﻟﻠﺒﺤﺮ اﻟﻤﺘﻮﺳﻂ وﺗﺘﻌﺮض ﻟﻠﻌﺪﻳﺪ ﻣﻦ اﻟﻤﺸﺎآﻞ‬240 ‫ﺗﻤﺘﺪ ﺷﻮاﻃﺊ دﻟﺘﺎ وادى اﻟﻨﻴﻞ‬
‫اﻟﻤﺮﺗﺒﻄﺔ ﺑﺎﻟﺰﻳﺎدة اﻟﺴﻜﺎﻧﻴﺔ وﺿﻌﻒ اﻟﺘﺨﻄﻴﻂ واﻻﺛﺎر اﻟﺠﺎﻧﺒﻴﺔ اﻟﻨﺎﺗﺠﺔ ﻋﻦ اﻧﺸﺎء اﻟﺴﺪ اﻟﻌﺎﻟﻲ ﺑﺎﻹﺿﺎﻓﺔ اﻟﻰ اﻟﺘﻐﻴﺮات‬
‫ وﻣﻦ اهﻢ‬.‫ وﺗﺼﻨﻒ دﻟﺘﺎ وادى اﻟﻨﻴﻞ ﻣﻦ اآﺜﺮ اﻟﻤﻨﺎﻃﻖ اﻟﻤﺼﺮﻳﺔ اﻟﻤﻌﺮﺿﺔ ﻟﻠﺨﻄﺮ ﺑﺘﺄﺛﻴﺮ اﻟﺘﻐﻴﺮات اﻟﻤﻨﺎﺧﻴﺔ‬.‫اﻟﻤﻨﺎﺧﻴﺔ‬
‫اﻟﺘﺤﺪﻳﺎت اﻟﺘﻰ ﺗﻮاﺟﻪ اﻟﺘﻨﻤﻴﺔ ﺑﺎﻟﻤﻨﻄﻘﺔ زﻳﺎدة ﻣﻨﺴﻮب ﺳﻄﺢ اﻟﺒﺤﺮ وﺗﻐﻠﻐﻞ اﻟﻤﻴﺎﻩ اﻟﻤﺎﻟﺤﺔ ﻓﻲ اﻟﻤﻨﺎﻃﻖ اﻟﺴﺎﺣﻠﻴﺔ‬
.‫ﺑﺎﻹﺿﺎﻓﺔ اﻟﻰ زﻳﺎدة ﻣﻌﺪل ﺣﺪوث اﻟﻨﻮات‬
‫واهﺘﻤﺖ هﺬﻩ اﻟﺪراﺳﺔ ﺑﺎﻟﻜﺸﻒ ﻋﻦ اﻟﺨﻮاص اﻟﻤﻮﺟﻴﺔ ﺑﺎﻟﻤﻨﻄﻘﺔ اﻟﺴﺎﺣﻠﻴﺔ اﻣﺎم دﻟﺘﺎ وادى اﻟﻨﻴﻞ وﻣﺪى ﺗﺄﺛﺮهﺎ ﺑﺎﻟﺘﻐﻴﺮات‬
‫اﻟﻤﻨﺎﺧﻴﺔ ﻋﻠﻰ اﻟﻤﺪى اﻟﻄﻮل وﻣﺪى ﺗﺄﺛﻴﺮ ذﻟﻚ ﻋﻠﻰ اﺗﺰان اﻟﻤﻨﺸﺂت اﻟﺴﺎﺣﻠﻴﺔ وﻗﺪ ﺗﻢ اﻻﺳﺘﻌﺎﻧﺔ ﺑﺒﻴﺎﻧﺎت اﻻﻣﻮاج اﻟﻤﻘﺎﺳﺔ‬
‫ ﻟﻠﺘﻌﺮف ﻋﻠﻰ ﺧﻮاص اﻻﻣﻮاج ﺑﺎﻟﻤﻨﻄﻘﺔ وﻣﻌﺪل اﻟﺘﻐﻴﺮ ﻓﻴﻬﺎ ﻋﻠﻰ اﻟﻤﺪى‬2010 ‫ اﻟﻰ‬1977 ‫اﻣﺎم اﻟﺪﻟﺘﺎ ﺧﻼل اﻟﻔﺘﺮة ﻣﻦ‬
‫ ﻟﺪراﺳﺔ ﺗﻮزﻳﻊ اﻻﻣﻮاج ﺑﺎﻟﻤﻨﻄﻘﺔ‬ImSedTran-2D ‫اﻟﻄﻮﻳﻞ آﻤﺎ ﺗﻢ اﻻﺳﺘﻌﺎﻧﺔ ﺑﺎﻟﻨﻤﻮذج اﻟﺮﻳﺎﺿﻲ ﺛﻨﺎﺋﻰ اﻻﺑﻌﺎد‬
‫وﺗﺤﺪﻳﺪ اﻟﻄﺎﻗﺔ اﻟﻤﻮﺟﻴﺔ اﻟﻤﺆﺛﺮة ﻋﻠﻰ اﻟﻤﻨﺸﺂت اﻟﺴﺎﺣﻠﻴﺔ وﻣﺪى ﺗﻐﻴﺮهﺎ ﻣﻊ اﻟﺰﻣﻦ وﻣﺪى ﺗﺄﺛﻴﺮ ذﻟﻚ ﻋﻠﻰ اﺗﺰان‬
.‫اﻟﻤﻨﺸﺂت اﻟﺴﺎﺣﻠﻴﺔ‬
The Nile Delta coast lies in the south eastern Mediterranean and extends about 240 km
alongshore. It suffers from many threats, due to population increase, uncontrolled development,
construction of Aswan High-Dam, and climate change. The Nile Delta is considered one of
Egypt’s most vulnerable areas to climate change. Sea level rise, salt-water intrusion and increase
in storm frequency and effect are considered the main challenges of climate change to any
development plans in the Nile Delta coastal zone.
The historical measured wave data from 1977 to 2010 are examined to investigate the effects of
climate change on wave climate in front of the Nile Delta coast. Also, the hydrodynamic
numerical model ImSedTran-2D has been used to describe changes in wave energy from place to
place and to check that existing coastal structures will remain effective.
Results show that there is an increasing trend in the mean significant wave height during the
period from 1985 to 2010 by a rate ranging from 2.6 to 2.9 cm/year. Increase in wave height
coincides with a decrease in wave period ranging from 0.01 to 0.26 sec./year. Wave energy in
front of the coastal structures within this area will increase by about 20% within high storms and
decrease by about 1 % within the normal conditions in the next 50 years. Nevertheless, most of
the Egyptian coastal structures are over designed and will not be affected by the increase in wave
energy due to the climate change.
Keywords: Climate change, Egyptian wave climate, numerical model, design wave, coastal
structures
1. INTRODUCTION
Climate change will take place over the next century
in spite of international efforts to reduce greenhouse
gas
emissions.
This
exacerbates
existing
environmental problems worldwide. As a result,
climate change research is changing from
understanding phenomena to impact assessment,
mitigation and adaptation strategies for the future
development of society. In general, the coastal zone
is particularly vulnerable, with expected impacts of
sea level rise, salt water intrusion and increasing
storm events in addition to existing problems such as
coastal erosion, subsidence, pollution, land use
pressures, and ecosystem deterioration.
Ocean waves and storm surges are considered among
the dynamic side issues of climate change. Long-term
changes of storm waves and surges are important for
coastal disaster prevention and reduction. Moreover,
stability of the coastal zone depends on wave
characteristics. Few researchers have conducted the
future wave climate projection by using wind-wave
models, in-situ measurements and satellite
data[1,2,3,4]. These studies have shown that the
averaged and extreme ocean wave climate changes
have not a general trend on both global and regional
43
Moheb M. Iskander
scales[5]. Large wave events are increasing at a
greater rate than mean wave heights[6].
The Mediterranean region is one of the sensitive
areas on Earth in the context of global climate
change, due to its position at the border of the
climatologically determined Hadley cell and the
consequent transition character between two very
different climate regimes in the North and the
South[7].
Mediterranean weather is highly seasonal in
nature and is strongly related to large-scale pressure
systems whose limits overstep the boundaries of the
Mediterranean area and extend towards the North
Atlantic, Eurasia and Africa[8]. The North Atlantic
Oscillation influences Mediterranean wave climate
with an instantaneous response[1]. In the
Mediterranean sea, the future significant wave height
(2071 to 2100) may be characterized by milder
marine storms and wave conditions all over the year
except in summer in some areas mean and extreme
significant wave height are higher[9]. These
influences may lead to great changes within coastal
regions, especially in heavily populated mega delta
regions such as the Nile Delta.
The Nile Delta coast of Egypt lies in the south
eastern Mediterranean, (Figure 1). It is classified as
one of the most vulnerable areas to climate change, as
well as the most important part of the country from a
socioeconomic viewpoint. While scarce research
work covered the wave climate changes adjacent to
North Africa and the Nile Delta coast. Abo Zed and
Gewilli[10] showed that there is a slight change (of
about 10 degrees) in the predominant wave direction
from 1985 to 2003 in front of the Rosetta coast, Nile
Delta, Egypt. There is a marked seasonality in the
wave climate, with higher waves in the winter and
lower waves in the non-winter period. The western
coast of Egypt has a more energetic wave climate
than the Nile Delta[11]. High-resolution wave
hindcasts were performed for the period 1958–2001
over the Eastern Mediterranean by using WAM
model within the HIPOCAS project.
The model results show that the 50th percentile of
significant wave height has a decreasing trend in the
range of 0.2–2.2 mm/year all over the region. While
the 90th percentile shows a negative trend (less than 5
mm/year) in extreme wave regimes over the region,
with some slightly positive trends (0.5 mm/year) in
the Aegean Sea and along the coastline of Algeria,
Libya and Egypt[3].This study is a test to check the
effect of climate change on wave climate in front of
the Nile Delta coast and to identify the areas
vulnerable according to that. This target will be
achieved by examining historical wave data along the
Nile Delta coast to study changes in wave
characteristics with time. The hydrodynamic
numerical model ImSedTran-2D[13,14] will then be
used to describe changes in wave energy from place
to place. Existing protection works within these areas
44
will be monitored to identify the implications of this
altered wave climate.
2. NILE DELTA COAST
The coastal zones of Egypt extend for over 3,500 km
in length along the Mediterranean and Red Sea
coasts. The Mediterranean shoreline is the most
vulnerable to climate change due to its relative low
elevation especially within the Nile Delta coast[15].
The Nile Delta shoreline extends from Alexandria to
Port-Said with a total length of about 240 km and is
typically a smooth wide coast (Figure 1). It is the
most fertile land of the country accommodating
several millions of population with population
densities up to 1600 inhabitants per square kilometer.
It includes centres of significant economic
activity, hosts vital centres for summer tourism and
recreation areas as well as archaeological sites from
ancient civilization. It contributes 30–40% of
agricultural production and 60% of fish catch. Half of
Egypt’s industrial production comes from the
Delta[16].
The Nile Delta coast suffers from a high rate of
population growth, unplanned urbanization, land
subsidence, excessive erosion rates, sea level rise, salt
water intrusion, soil salinization, land use
interference, ecosystem pollution and degradation and
from a lack of appropriate institutional management
systems[17,18].
Egypt has been suffering from increased severity
and frequency of sand storms, dense haze and
flooding. These extreme events have had negative
socio-economic impacts on almost all sectors[19].
Statistical analysis of the tide measurements along the
Egyptian North coast during the period from 1943 to
2000 indicated that the rate of sea level rise ranges
between 1.6 mm and 5.3 mm included the land
subsidence (Table 1),[18,16]. It is estimated that
11.75% of the low land Delta regions will be affected
due to sea level rise by 2100. Increases in storm
events within the Nile Delta coastal zone will
complicate these problems and reduce the efficiency
of mitigation and adaptation measures
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
Wave Climate and Coastal Structures in the Nile Delta Coast of Egypt
Burullus
El Manzalla Outlet
Dam
ietta
Western Jetty
Bra
nch
Eastern Jetty
Dam
ietta
Bra
nch
Concrete Wall
Ras El Bar
Mediterranean Sea
Damietta
Ashdod
El Hamra
7
East Damietta
Ras El Bar Peninsula
New Damietta Harbour
Rosetta
Abu Quir Bay
Model part 2
Model part 3
Port Said
Abu Quir headland
ern
East
Detached Break Water
Jetty
Model part 1
Nile Delta
Wave gauges:
. W.
East B
West B. W.
2
W. P
rotect
ion
Wall
Navigation Channel
To Alexandria
0
To Rosetta
Maadia Outlet
10
20
30
S4DW CAS system OSPOS Wave Rider 40 km
3
Wall
ection
E. Prot
Gro
ins
n
Exten.sio
tty
of W Je
New
E. Jetty
e Wall
Concret
Groin
Borg El-Burullus
City
4
rbour
New Ha
Groin
s
1
Ma
adi
aV
illa
ge
Alexandria
El Dikheila harbor
6
Weste
rn Jett
y
5
Detached Break Water
Baltim Sea Resort
Rosetta Promontory
Burullus Lake Outlet and Burg El-Burullus
Village
Figure 1. Map of the Egyptian Mediterranean coast showing the Nile Delta coast, wave measurement stations,
selected areas for executing the numerical model and the coastal structures distribution along the area, modified from
Silem (2008)[12].
Table 1. Estimated average annual seal level rise (cm) relative to year 2000 sea level, after El Shinnawy et al., (2010).
City
Year
IPCC
Scenario
Alexandria
Burullus
Port Said
A1F1
2025
13.0
14.75
27.9
Coastal projects have been undertaken to protect
some parts of the Nile Delta coast through hard
structures as well as artificial nourishment that has
been applied at some sectors. For instance, the
Government of Egypt (GOE) spent about 170 million
US dollar to protect the three promontories of the
Nile Delta; Rosetta, Burullus and Damietta (Figure
1). It is unclear whether past approaches to managing
coastal zones, through the construction of hard
defences along the shoreline and through beach
nourishment, will remain cost effective in the face of
accelerated sea level rise and anticipated increases in
extreme weather events. In addition, increase in the
frequency and severity of storm surges will definitely
impact coastal structures[19].
3. METHODOLOGY
The directional wave climate obtained from a series
of field campaigns during the last decades will be
described and interpreted to find out the change in
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
2050
2075
34.0
37.5
68.8
55.0
60.3
109.6
2100
72.0
79.0
144.0
wave climate with time. ImSedTran-2D numerical
model is used to simulate the wave climate at the near
shore zone for two scenarios. The first is for present
conditions (existing sea level and wave climate). The
second assumes a sea level rise over 50 years in line
with IPCC and local estimates of sea level change
with the 50 years return period wave characteristics.
Also, stability of structures will be studied based on
the Hudson stability criteria[20].
3.1 Wave Database
Actual measurements have been made along the
Egyptian Mediterranean coast at nine stations by
Coastal Research Institute (CoRI); El Hamra, El
Dikheila harbor, Abu Quir Headland, Abu Quir bay
(two locations), Burullus, Damietta harbor, and east
& west of Ras El Bar, (Figure 1). Also, Israel’s
measured wave data at Ashdod , at a depth of 20 m, is
used to fill the gap in the seventies. Table 2
summarizes these wave monitoring campaigns.
45
Moheb M. Iskander
Measurements were made using an Offshore
Suspended Pressure System (OSPOS), a Wave Rider
buoy, a Cassette Acquisition System (CAS) and an
S4DW Current and Wave Meter. Frihy[21] concluded
that there is a strong correlation (a coefficient of 0.82)
between significant wave heights for wave records
measured at Abu Quir bay and Damietta harbor.
According to that wave data of each station was
analyzed separately to get the change of wave climate
within this station with time. These results are used to
represent the changes in wave climate in the Nile
Delta.
OSPOS wave data were collected at Abu Quir
Headland, Burullus and east of Ras El Bar during the
period from 1971 to 1983. OSPOS is essentially
pressure meters which measure variations in pressure
caused by the waves. The wave direction is assumed
to be the same as that of the wind. CoRI experienced
many problems analyzing these records manually by
using the zero up-crossing method[22,23]. The
available OSPOS wave data only describe some
storms, and are insufficient to enable any trend
analysis. So, the Wave Rider data at Ashdod, which
were installed by Israel Ports Authority during the
period from 1977 to 1980, were used to represent the
wave climate within the seventies. The Ashdod waves
measured were analyzed by computer digitizing two
pen-and-ink records per day, applying standard
spectral analysis techniques and computing
significant wave heights and periods[24]. Digitizing
accuracy was determined to be ± 0.5 sec. and ± 5 cm
for wave periods and heights, respectively[25].
Table 2. Wave data sources along the southeastern Mediterranean coast.
No.
Location
1
El Hamra
El Dikheila
Harbor
Abu Quir
Headland
Abu Quir
Bay
Abu Quir
Bay
2
3
4
5
Position
Lat. 30o 55.9' N;
Long. 28o 50.1' E
Lat. 31o 08.326’ N;
Long. 29o 48.826’ E
--------o
Lat. 31 22.3273' N;
Long. 30o 13.503' E
Lat 31o 23.5667’ N;
long 30o 15.5167’ E
Instrument
Depth
Period
Duration
(month)
Meas.
Interval
S4DW
8.0
July 1998 to May 1999
11
20 minutes 2.0hr.
S4DW
17.0
40
20 minutes 4.0hr.
OSPOS
6-8
----
20 minutes 6.0hr.
CAS
18.5
Sep 1985 to Dec, 1990
64
S4DW
14.1
Dec. 2003 to Nov. 2005
24
March 1992 to March
1995
Some records during the
period from 1971 to 1977
Some records during the
period from 1972 to 1983
Sep. 1997 to June 1999
June 2001 to March 2004
Nov. 2009 to Sep. 2010
6
Burullus
----------
OSPOS
6-8
7
Damietta
Harbor
Lat. 31o 30.4316' N;
Long. 31o 45.5994' E
S4DW
12.0
West Ras ElBar
East of Ras
El-Bar
Ashdod,
Israel
Lat. 31o 31.4201' N;
Long. 31o 49.1847' E
CAS
7.0
May 1985 to Dec. 1990
68
-------------
OSPOS
6-8
Some records during the
period from 1972 to 1977
----
--------------
Wave rider
20
Jan. 1977 to Des. 1980
30
8
9
10
The S4DW data of El Dikheila and El Hamra
harbors were collected in the lee of a headland or
breakwater, which means that these gauges represent
local conditions, and did not give an unfiltered view
of the wave climate within these periods. Also west
of Ras El Bar wave data were collected in relatively
shallow water which suffers from refraction,
diffraction, shoaling and breaking and is not suitable
to be used in this study. In Abu Quir bay, the CAS
system data, which was moored at 18.5 m depth
during the period from 1985 to 1990, is used to
represent the wave climate within the eighties. CAS
system consists of three pressure sensors which are
fixed under the water level on the legs of a Gas
Platform in Abu Quir bay about 18 km from the
46
34 minutes each
6.0hr.
20 minutes 4.0
hr.
-----
20 minutes 6.0hr.
22
33
10
20 minutes 4.0
hr.
34 minutes each
6.0hr.
20 minutes 6.0
hr.
3.0 hr.
shoreline. They are spaced 7 m apart in a triangular
array and send their data to an encoder. A special
program is used to analyze the row data to get wave
characteristics
by
the
use
of
Fourier
transformation[26].
The wave climate within the last two decades is
described by the S4DW wave data, which were
collected in front of Damietta harbor at 12.0 m depth.
The S4DW consists of an electro-magnetic currentmeter with a pressure sensor. These are converted to
wave measurements using a sampling frequency of 2
samples/sec. Wave direction is derived from the two
components of the wave orbital currents. A wave
program utilizes the measured pressure information at
a point to compute the statistics of sea surface
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
Wave Climate and Coastal Structures in the Nile Delta Coast of Egypt
elevation by the use of Fourier transformation.
Information about the directional properties of the
wave field is obtained from the phase differences
observed between the surface elevation and the two
components of velocity[27].
Data from each station is analyzed separately and
used to get the change in wave characteristic with
time for its specific duration. According to frihy et
al., 2008 these results can be used to represent the
change in wave characteristics along the Nile Delta.
3.2 ImSedTran-2D Model
In the current study, the ImSedTran-2D model[13,14] is
applied to determine wave distribution along the
study area under the effect of changes in bed
morphology and wave characteristics for the next 50
years, which corresponds to the coastal structures’
lifetime. Model input consists of wave characteristic,
bed morphology, coastal structures and bed sediment
characteristics. Expected changes in wave
characteristics have been obtained related to the
available data within the study area. Future
morphology of the study area has been predicted
according to the effect of sea level rise only using the
A1F1 scenario as shown in table 1.
The governing equations used to determine wave
direction and wave height distribution for refraction
calculations are summarized as follows:
K cos θ
∂
∂x
(
∂θ
∂x
ρgH
8
+ sin θ
∂K
∂x
2
C G Sinθ ) +
= − K sin θ
∂
∂y
(
ρgH
8
∂θ
∂y
+ cos θ
∂K
∂y
→1
2
C G Cosθ ) = 0 → 2
Whereas x and y axes are the alongshore and
offshore directions, respectively, K is the wave
number, θ is the angle between wave crest and the
bed contour, ρ is water mass density, g is the gravity
acceleration, H is the wave height, and
CG is the
group velocity.
The wave diffraction calculation based on the
Kraus[28] solution is used to simulate the wave
condition in the vicinity of the coastal structures. The
wave height at the location in question is simply the
product of the specified partially refracted incident
wave height and diffraction coefficient. The angle of
the wave crest is computed assuming a circular wave
front along any radial; this angle is then refracted
using Snell’s law. Throughout the refraction and
diffraction schemes, the local wave heights were
limited by the value 0.78 of water depth. Calculations
of the wave distributions are based on shoaling
processing, refraction, diffraction, and depth limited
breaking. Model is designed to take the actual wave
data measurements at any point offshore of the
breaking point.
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
4. RESULTS
4.1 Nile Delta Wave Climate
Wave action along the coast is seasonal in nature,
with storm waves (winter) starting from mid October
to March; the summer (swell) season covers the
months from June to mid October. The spring season
covers the months April and May. In general, there
are sixteen storms per year, of which seven are fairly
strong, with high winds and heavy rain[23,29].
Statistical analysis of the waves recorded in Abu
Quir bay, at 18.5 m depth between 1985 and 1990,
shows that waves had significant wave height of 1.91
m, average wave height of 1.12 m, and average peak
wave period of 6.0 sec. originated from the NW
(Table 3). Figure 2 shows the monthly wave
characteristics in Abu Quir bay. It is clear that in
more than 50% of the year, the wave heights range
between 0.5 and 1.5 m and the wave periods range
between 5 and 7.0 sec. In summer, about 50% of the
waves come from the NW direction while in winter
50% of the waves oscillate between NNW and
WNW. In spring and early summer, the predominant
wave direction may be changed to NE direction.
In front of Damietta harbor at 12.0 m depth
between 1997 and 2010, waves had significant wave
height of 1.02 m, average wave height of 0.56 m,
average peak wave period of 6.3 sec. originated from
the NW (Table 3).
Figure 3 shows the monthly wave characteristics
in front of Damietta harbor. It is clear that in more
than 75% of the year, the wave heights are less than
1.0 m. More than 50% of the year, the wave periods
range between 5.5 and 6.5 sec. and it rarely (less than
1% of the year) increases than 9.0 sec. In summer,
about 50% of the waves oscillate between NNW and
WNW directions, while in winter 50% of the waves
oscillate between N and NW directions.
4.2 Change in Wave Climate
The spatial distribution between wave characteristics
and time is used to identify the changes in wave
climate, (Figures 4&5&6). Results show that there is
a general increasing trend in the mean significant
wave height during the period from 1985 to 2010 by
using all Hs value ranging from 2.6 to 2.9 cm/year,
(Figure 4).
This trend follows a very small decreasing trend of
the mean significant wave height by 0.29 cm/year
during the seventies, (Figure 5-A)
47
Moheb M. Iskander
Sum.
Win.
Total period
Table 3. Wave characteristics along the Nile Delta coast.
Location
Duration
Measured Depth
Hs
Hav
Tp av.
Direction
Maximum
condition
Abu Quir Bay
1985 – 1990
18.5 m
1.91 m
1.12 m
6.0 sec.
NW
Damietta Harbor
1997 - 2010
12.0 m
1.02 m
0.56 m
6.3 sec
NW
Hs= 5.44, Tpc= 12.8 sec., WNW
Hs= 4.47, Tsc= 5.6 sec., NW
Monthly Hs
1.24 – 3.18 m
0.5 – 2.16 m
Monthly Tp Av.
4.5 – 7.8 sec.
4.4 – 8.3 sec.
Monthly Hs
1.15 – 2.12 m
0.43 – 1.12 m
Monthly Tp Av.
4.9 – 6.8 sec.
4.5 – 7.3 sec.
wave
6
1%
Significant wave height (m).
5
m e a n-S t. Dv.
25%
4
M e dium
Mean
3
75%
2
m e a n+S t. Dv.
99%
1
M a x.
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Mon the s
14
Peak wave period (sec.)
12
1%
m e a n-S t. Dv.
10
25%
M e dium
8
Mean
75%
6
m e a n+S t. Dv.
99%
4
M a x.
2
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Mon th e s
100
Wave direction from North.
80
60
1%
40
m e a n-S t. Dv.
20
25%
M e dium
0
Mean
-20
75%
-40
m e a n+S t. Dv.
-60
99%
-80
-100
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Monthe s
Figure 2. Wave characteristics in Abu Quir bay illustrating all the available observations during the period from
1985 to 1990 at 18.5 m depth
.
48
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
Wave Climate and Coastal Structures in the Nile Delta Coast of Egypt
5
Segnificant wave height (m).
4.5
1%
4
m e a n-S t. Dv.
3.5
25%
3
M e dium
2.5
Mean
2
75%
1.5
m e a n+S t. Dv.
1
99%
0.5
M a x.
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Monthe s
11
10
Significant wave period (sec.)
1%
9
m e a n-S t. Dv.
25%
8
M e dium
7
Mean
75%
6
m e a n+S t. Dv.
5
99%
M a x.
4
3
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Monthe s
200
Wave direction from North
150
1%
100
m e a n-S t. Dv.
50
25%
M e dium
0
Mean
-50
75%
-100
m e a n+S t. Dv.
99%
-150
-200
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Monthes
Figure 3. Wave characteristics in front of Damietta harbor illustrating all the available observations during the
period from 1997 to 2010 at 12.5 m depth.
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
49
Moheb M. Iskander
A Student t test has been used to assess the
statistical significance of wave height trend[30]. The
null hypothesis of zero-slope is rejected for the above
cases at a 1%- level of significance. The monthly
significant wave height (one monthly value) increases
with a rate of 3.65 cm/year while the monthly
maximum wave height increases within a wider range
of 7.3 to 11 cm/year during the period from 1985 to
2010 (Figure 5-B&C).
Increase in wave height during the period from
1985 to 2010 coincides with a decrease in wave
period ranging from 0.01 to 0.26 sec./year. The null
hypothesis of zero-slope is rejected for the above
cases at a 1%- level of significance. The decrease in
wave height during the period from 1977 to 1980
coincides with an increase in wave period of 0.029
sec./year (Figure 6).
Check of seasonal effects shows that seasonal
wave climate has insignificant effect on the general
increase trend but affects only the increased value. In
winter, mean significant wave height during the
period from 1985 to 2010 increases by a rate ranging
from 1.8 to 3.3 cm/year, (Figure 4), while the wave
period decreases by a rate ranging from 0.01 to 0.37
sec./year (Figure 6-B&C). During summer, the
increase rate of wave height ranged from 1.1 to 2.6
cm/year and the decrease rate of wave period ranged
from 0.037 to 0.22 sec./year (Figures 4&6).
Significant wave heights of 2.0 m were chosen to
characterize storm and non storm waves. A similar
wave height limit was used by Frihy[11]and Carmel[31]
to characterize winter and non-winter wave climates
on the Mediterranean coast of Egypt and Israel.
Figure 7 illustrates the distribution of annual storm
percentage of occurrence with time. The results show
a normal distribution of the storms with time but the
available data are not enough to identify any trends in
storm frequency.
4.3 Design Wave Conditions
The long-term statistics of these measurements and
the probability analyses provide useful information
regarding design wave conditions for coastal
engineers. The approach used here to calculate
extreme wave height and associated return periods
follows the methods discussed in Issacson and
Mackenzie[32].
Significant Wave height (m).
6
Linear (all data)
Abu Quir Bay wave Gauge-18.0 m depth
5
Linear (Winter)
Linear (summer)
4
3
2
1
0
Jul-85
Feb-86 Aug-86 Mar-87 Sep-87 Apr-88 Nov-88 May-89 Dec-89 Jun-90
Jan-91
Significant wave height (m).
Date & Time (day)
5
Damietta wave gauge-12
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Mar-97
Nov-98
Jun-00
Feb-02
Linear (All data)
Linear (Winter)
Linear (Summer)
Oct-03
May-05
Jan-07
Sep-08
May-10
Date & Time (day)
Figure 4. Change in wave height with time at Abu Quir and Damietta stations during the period
from 1985 to 2010.
50
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
Wave Climate and Coastal Structures in the Nile Delta Coast of Egypt
Monthly Wave height (m).
1.2
1
0.8
0.6
0.4
0.2
Ashdod w ave gauge- 20.0 m depth.
0
Aug-76 Mar-77 Oct-77 Apr-78 Nov-78 May-79 Dec-79
Date & Tim e (day)
Jun-80
Jan-81
Aug-81
6
Monthly Wave Height (m)
Hmean (m)
5
Hs (m)
Hmax (m)
4
3
2
1
Abu Quir Bay wave Gauge-18.0 m depth
0
Nov-84
Sep-85
Jul-86
May-87
Feb-88
Dec-88
Date & Tim e (day)
Oct-89
Aug-90
Jun-91
Monthly Wave Height (m).
5
Hmean (m)
Hs (m)
4
Hmax (m)
3
2
1
0
Dec-96
Damietta w ave gauge-12
Jul-98
Mar-00
Nov-01
Jun-03
Feb-05
Oct-06
Jun-08
Jan-10
Sep-11
Date & Tim e (day)
Figure 5. Change in monthly average, significant and maximum wave height with time at the three measured stations
during the period from 1977 to 2010.
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
51
Moheb M. Iskander
Segnificant Wave period (sec.)
8
7.5
7
6.5
6
5.5
Ashdod w ave gauge- 20.0 m depth.
5
Aug-76
Mar-77
Oct-77
Apr-78
Nov-78
May-79
Dec-79
Jun-80
Jan-81
Aug-81
Date & Time (Day)
14
Peak Wave period (sec).
Linear (All data)
12
Linear (Winter)
10
Linear (Summer)
8
6
4
2
Abu Quir Bay w ave Gauge-18.0 m depth
0
Jul-85
Feb-86 Aug-86 Mar-87 Sep-87
Apr-88
Nov-88
May-89
Dec-89
Jun-90
Jan-91
Date & Time (Day)
12
Significant Wave period (sec.)
Damietta w ave gauge-12 m
Linear (All data)
10
Linear (winter)
Linear (Summer)
8
6
4
2
0
Mar-97
Jul-98
Dec-99
Apr-01
Sep-02
Jan-04
May-05
Oct-06
Feb-08
Jul-09
Nov-10
Date & Time (Day)
Figure 6. Change in wave period with time at the three measured stations during the period from 1977 to 2010.
52
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
Wave Climate and Coastal Structures in the Nile Delta Coast of Egypt
Storm percentage of occurrence/year
20
16
12
8
4
0
1984
1985
1986
1987
1988
1989
1990
1991
Strom percentage of occurrence/year
Date (year)
7
6
5
4
3
2
1
0
1996
1998
2000
2002
2004
2006
2008
2010
2012
Date (year)
Figure 7: Yearly percentage of occurrence of storms getting from wave data collected at two stations during the
period from 1985 to 2010.
In Abu Quir bay, at 18.5 m depth, the results showed
that a wave height of 7.60 m can be expected to occur
once in fifty years with a maximum height of 8.10 m
occurring at least once every 100 years. Since there
was no single wave period associated with the
extreme wave height, periods of the largest waves
recorded are associated with the estimated extreme
wave heights (wave period 10.7 to 12.8 sec.). The
predominant storm wave direction varies between
NW and WNW. Estimates of extreme wave heights
are summarized in table 4
Table 4. Estimated extreme wave heights related to the wave measurements at 18.5 m depth in Abu Quir bay during the period
from 1985 to 1990.
Return periods (year)
1
2
5
10
20
30
40
50
100
Wave Height (m)
4.70
5.20
5.90
6.40
6.90
7.20
7.40
7.60
8.10
4.4 Wave and Coastal Structures
The ImSedTran-2D model is used to determine wave
distribution in the nearshore zone and in front of the
coastal structures along the study area. The Nile Delta
bed morphology is taken from the bathymetric map of
May 2010 which covered the area from the backshore
zone to (-6.0 m depth) below the mean water level
surveyed by Coastal Research Institute. The
remaining contours to (-18.0) m depth are from the
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
most recently available full Delta survey, of May
1986, surveyed by Misr Offshore Services and
Surveys. The characteristic wave conditions were
obtained from the wave measurements in front of
Nile Delta coast, (tables 3&4). The future
morphology of the study area has been predicted
according to the effect of sea level rise only for A1F1
scenario (Table 1).
To apply the model, the Nile Delta is divided into
three parts; the western part Rosetta promontory, the
middle part Burullus and the eastern part Damietta
53
Moheb M. Iskander
• The sea level rise has a reasonable effect for the
case of maximum wave condition. The wave energy
in front of the coastal structures within this area will
increase by about 20% after 50 years.
The stability checks for the Egyptian coastal
structures are got from applying Hudson criteria and
reviewing the physical model reports of the coastal
projects. It can be concluded that most of the
Egyptian coastal structures constructed between 3.0
to 4.0 m depth will be affected by 3.0 to 3.5 m wave
height maximum. Obviously, these structures
designed to face 5.0 to 6.0 m wave height and remain
stable if water depth in front of the structure reaches
7.0 to 8.0 m. It means that most of the Egyptian
coastal structures are over designed and will not be
affected by the increase in wave energy due to
climate change. However, the change in wave
characteristics may affect the sediment transport,
shoreline orientation and wave overtopping which
will affect the efficiency of the coastal structures.
(Figure 1). For each part, the following modules are
built:
1. Recent morphology with the average wave
condition (Hav= 1.12, T= 6.0 sec., NW direction,
Depth= 18.0 m) (Table 3).
2. Recent morphology with the maximum wave
condition in 50 years (Hmax= 7.60, T= 10.7 sec.,
NW direction, Depth= 18.0 m) (Table 4).
3. Future morphology (recent bathymetry + the sea
level rise in 2050 (Table 1)) with the average
significant wave condition.
4. Future morphology with the maximum wave
condition in 50 years.
Figure 8 is an example of the model results
which deduced that:
• The effect of sea level rise for the case of average
wave condition is negligible except for the
shoreward movement of the breaking point. The
wave energy in front of the coastal structures within
this area has a slight decrease with time, will reach
1% after 50 years.
3495000
3495000
1.2
3490000
1.1
3490000
1
0.9
3485000
3485000
0.8
0.7
3480000
3480000
0.6
0.5
3475000
3475000
240000
245000
250000
255000
260000
265000
240000
245000
250000
255000
260000
265000
3495000
3495000
9
8
7
3490000
3490000
6
5
3485000
3485000
4
3
2
3480000
3480000
1
0
3475000
3475000
240000
245000
250000
255000
260000
265000
240000
245000
250000
255000
260000
265000
Figure 8: Wave distribution across Rosetta promontory with: (A) the recent morphology for average wave
conditions, (B) the 50 years sea level rise for average wave conditions, (C) the recent morphology due to a
maximum 50-year wave condition, and (D) the 50 year sea level rise due to maximum wave condition in 50
years. Wave height is denoted by the color scale in meters and wave direction by arrows.
54
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
Wave Climate and Coastal Structures in the Nile Delta Coast of Egypt
5. DISCUSSION
The available data show an increasing trend in the
mean significant, monthly significant and monthly
maximum wave height during the period from 1985
to 2010. The trend calculated from all the available
data seems more reasonable than that from the
monthly values for this case. The monthly significant
and the monthly maximum wave height are calculated
if more than 50% of the measurements are available,
which affected the results and may be did not give
fully confident results. The storm trends are not clear
because the southeast Mediterranean is particularly
susceptible to changes in the frequency of storms due
to annual differences in time of the seasonal
displacement of the subtropical zone, and variations
in number of winter cyclones[33].
The above result is different from Musić and
Nicković[3], who show that the median significant
wave height has a decreasing trend in the range of
0.2–2.2 mm/year all over the east Mediterranean
region. It may be due to the lack in simulation of the
high storms and swell within the WAM model. In
addition, his paper concludes that the model shows
better performance for the buoy in the open sea
compared to the coastal buoys. A possible
underestimate of the wind speed components due to a
lack of resolving small, sharp scale features in the
wind pattern, may affect the WAM-computed values
of wave height[34]. However, there are also problems
with the data used in this study as these data are
neither continuous nor representative of whole years.
So, it is very important to compare the result of
WAM model in HIPOCAS project with the actual
measurements in front of the Egyptian coast to
identify the reason of inconsistent results.
The increase in wave height during the period
from 1985 to 2010 coincides with a decrease in wave
period ranging from 0.01 to 0.26 sec./year. It means
that there is a movement towards more local sea
waves than the swell waves. The statistical
significance of these negative trends has been shown
at the 1%-level.
S4DW wave gauge takes the current direction
near the bed as wave direction, while the CAS system
calculates the wave direction from the phase
difference between the signals of two sensors. It
means that the S4DW wave gauge does not give the
exact wave direction. So the changes in wave
direction trend cannot give the real situation for this
case.
6. CONCLUSIONS AND
RECOMMENDATIONS
This work is a step to study the effect of climate
changes on wave climate within the Nile Delta coast.
All the available data in Abu Quir bay, Damietta and
Ashdod have been analyzed separately to understand
Emirates Journal for Engineering Research, Vol. 18, No.1, 2013
the change in wave climate with time. More
investigations are required to assess the actual change
rate or direction. Also, the ImSedTran-2D numerical
model is used to check the effect of change in wave
climate in the nearshore zone and in front of the
coastal structures. The main results show that:
• There is an increasing trend in the mean
significant wave height during the period from
1985 to 2010 by a rate ranging from 2.6 to 2.9
cm/year. The statistical significance of these
positive trends has been proved at the 1%-level.
This trend followed a very small decreasing trend
of the mean significant wave height by 0.29
cm/year during the seventies.
• The increase in wave height during the period
from 1985 to 2010 coincides with a decrease in
wave period ranging from 0.01 to 0.26 sec./year.
It may be due to increase in sea wave and
decrease in the swell waves. The statistical
significance of these negative trends has been
proved at the 1%-level. The decrease in wave
height during the period from 1977 to 1980
coincided with an increase in wave period of
0.029 sec./year.
• The predominant wave direction oscillated
between NNW and WNW directions during the
period from 1985 to 2010. The only exception is
within the spring season and some times the early
summer season. Within this period, the
predominant wave direction may be changed to
NE direction.
• Wave distribution obtained from ImSedTran-2D
model after 50 years shows that the wave energy
in front of the coastal structures within this area
increases by about 20% within high storms. While
it decreases by about 1% for the normal wave
condition.
• Stability check within the coastal structures
lifetime shows that nearly most of the Egyptian
coastal structures are over designed and will not
be affected by the increase in wave energy due to
the climate change.
• Wave data should be collected in a consistent,
operational manner, over tens of years with
complete annual coverage to get the actual
changes of wave characteristics with time but
until that, the worst scenario should be taken into
consideration in designing coastal structures.
• It is recommended to execute a comparative study
between the result of this project and the result of
HIPOCAS project[3] in front of the Nile Delta
coast to unify the results.
• It is very important to study the combined effect
of increase in sea level rise and wave energy on
sediment transport and the resulting effect on
coastal structures.
55
Moheb M. Iskander
ACKNOWLEDGEMENTS
This research is supported by Coastal Research
Institute (CoRI), Swedish International Development
Cooperation Agency (SIDA) and Swedish
Metrological and Hydrological Institute (SMHI). I am
deeply grateful to Dr. Philip Axe from SMHI for
supervising, and revising this work. Also, thanks to
Prof. Dr. Ibrahim El Shinnawy, Alfy Fanos and Abu
Bakr Abu Zed from CoRI for providing me with
wave observations in Egypt.
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