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NUMERICAL MODEL AS A TOOL TO INVESTIGATE COASTAL
PROBLEMS IN EGYPT
Ahmed Sayed Mohamed Ahmed
Dr. Eng., Researcher, Hydraulics Research Institute,
National Water Research Center, Delta Barrage 13621, Egypt
Tel: +202-2188268 , Fax: +202-2189539
E-mail: [email protected]
ABSTRACT
The results of recent numerical applications for the Delft3D model were accumulated.
The numerical applications modeled problematic areas along the Egyptian shoreline.
Among these problems are the occurrences of rip currents, erosion or accretion in spite
of the presence of countermeasures at these locations.
Investigating the results proved the inadequacy of such measures at the problematic
sites. It was thus recommended to model the problematic areas numerically before
implementing any countermeasure that might produce undesired impacts on the
shoreline. It was further recommended to simulate numerically the Damietta
promontory, where sustainable projects are liable to be encompassed. This was done to
predict shoreline changes in order to apply a countermeasure that will maintain the
shoreline and cease the sedimentation of the access channel of the Damietta port.
Keywords: Nile Delta, coastline erosion and sedimentation, Rosetta Promontory,
Damietta Promontory, rip currents, coastal processes and morphology, numerical
simulations, Delft3D.
INTRODUCTION
The total length of the Egyptian Mediterranean coastline is about 995 km. The Nile
Delta shoreline, which forms 1/3 of the total length, faces serious impacts due to the
cutting off of the Nile sediment by the commission of Aswan High Dam. On the other
hand, the part from Alexandria to El-Sallum, rip current problems started to arise due
to the substantial development of coastal regions represented by many tourist villages
and resorts during the last two decades. The purpose of such development was to
provide suitable swimming conditions. However, due to natural complex coastal
processes and irregular bathymetries, some villages suffer from irrelevant and
dangerous conditions. Such undesired swimming conditions were created by rip
currents.
Researchers focused on the Nile Delta only and quantified the erosion and accretion
rates. Also, coastal structures were proposed based on limited field measurements.
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The objective of this paper is to discuss the results of the numerical model (Delft3D)
that simulated eroding or accreting problematic areas or areas suffering from rip
currents.
This paper thus displays the following:
• An overview of the coastal problems along the Mediterranean Coast
• The field measurements and numerical model construction
• The results of numerical simulations for the rip current problem and erosion and
sedimentation of Rosetta
COASTAL PROBLEMS RESEARCH OVERVIEW
This section reviews the researches that were executed to study problematic areas
suffering from erosion, accretion and rip currents.
Erosion and Accretion Problems
Toma and Salama [1] conducted a bathymetric survey ten years after the completion of
AHD that included Abu Quir Bay, Rosetta mouth and Burullus Outlet. They compared
it to the 1992 Admiralty chart. They found that Abu Quir bay has undergone erosion
and the highest values were in the middle of the Bay, assuming that some sorts of
eddies play an active role in sediment transportation from the Bay. They concluded
that erosion was not directly related to damming of the Nile. Extensive erosion of the
Rosetta promontory was observed due to damming the Nile River. Toma and Salama
also concluded that the Rosetta canyon off Rosetta mouth plays a significant role as a
sink for the eroded sediments transported offshore from Abu Quir Bay and Rosetta
fan.
Frihy [2] used image analysis technique and analyzed two aerial photographs taken
before and after the construction of AHD in 1955 and 1983, respectively. The analysis
aimed at quantifying the erosion and accretion rates along the Egyptian Mediterranean
Coast including Rosetta and Damietta promontories and Burullus-Baltim.
Frihy [2] stated that he found the maximum erosion rate at Rosetta promontory was
114 m/yr while it was 31 m/yr at Damietta promontory. This was based on the fact that
Rosetta and Damietta mouths receive insignificant amounts of water and sediments
only twice a year during periods of irrigation. The erosion rate at Burullus-Baltim was
9m/yr. The eroded sediments have been deposited at the eastward of the promontories
by the longshore currents. Frihy also observed the development of the Damietta spit
eastward of the Damietta promontory, and concluded that the eroded sediment from
the promontory is the main source for the spit formation.
Frihy et al. [3] also noted that the promontories erosion begun with the construction of
Aswan Low Dam in 1902, but the development of Aswan High Dam effectively
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eliminated the flow of river water into the Mediterranean and cut off all delivery of
sediment to the coast. Frihy et al. [3] also studied the pattern of sediment transport
along the Nile Delta using field data. Their analysis concluded that the constructed
jetties at the inlets of Idku and Burullus lakes considerably interrupted the littoral drift
and resulted in shoreline erosion at the downdrift of the jetties. Furthermore, they
observed sand deposition at the entrance of the lakes leading to a major hazard for
navigation process. Frihy et al. [3] also mentioned the shoreline erosion at Ras El Bar
and the reason for building three groins and sand nourishment in 1970 in order to
impede the shoreline erosion. Frihy et al. [3], Frihy et al. [4], Fanos et al. [5], Fanos
[6], El-Raey et al. [7], Stanley [8], Stanley and Warne [9] and Ahmed [10] identified
that the coastal zones along the Nile Delta Coast that undergo potential shoreline
erosion are the Rosetta and Damietta promontories.
Fanos et al. [5] showed that erosion was not the only trouble affecting the Nile Delta.
High rates of easterly longshore transport which cause shoaling of the Nile mouths and
the outlets of the northern lagoons (lakes) are also series problems as they directly
affect coastal navigation and eco-system of the lakes and consequently fish production.
Fanos et al. [5] reviewed the coastal problems along the Nile Delta from Alexandria to
the east at Rafah.
All the above researchers used limited field measurements and based on that they
proposed a countermeasure and nobody performed physical or mathematical model
scenarios to solve the coastal problems.
On the other hand, Ahmed et al. [11] and Ahmed [12] discussed the erosion problem
of Rosetta promontory while the sedimentation problem was discussed by Ahmed et
al. [13] and Ahmed [14].
Rip Currents Problem
The coastal stretches from Alexandria to the west at El-Sallum have different
problems. Those coastal stretches have been in a dynamic equilibrium until their
coastal areas were developed thus emphasizing the necessity for safe swimming water
demand. The investors of the Egyptian coastal resorts which received many complains
concerning the rip currents that endanger the swimmers at the resorts.
The rip currents are relatively shore-normal. The generation of the rip currents is due
to the irregular bathymetry, wave conditions and beach properties.
De Vroeg et al. [15] suggested submerged structures as a measure to mitigate the rip
currents problem and to produce reasonable swimming conditions. Their studies
utilized both physical and mathematical models. However, the submerged structures
lead to negative impacts on the nearshore regions.
Although De Vroeg et al. [15] studied the rip current using the hybrid approach which
comprised both physical and mathematical modeling, the rip current was relatively
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Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt
mitigated but the proposed countermeasure produced negative impact on the adjacent
nearshore zone.
Ahmed [16] discussed rip currents problems and some alternatives to improve the
swimming conditions.
SIMULATING PROBLEMATIC AREAS NUMERICALLY
The establishment of numerical models requires field data. The following section
explain field measurements that have been performed to simulate the erosion and
accretion problem at Rosetta promontory and rip current problem at 84.5km west of
Alexandria. The field data have been collected from Hydraulics Research Institute
(HRI), Coastal Research Institute (CoRI) and Egyptian Authority for Shore Protection
(SPA) to three problem areas, Figure 1.
Alexandria
Rip current
location
Rosetta
Figure 1: Location of the problematic areas
As for the rip current problem, about 4.5 km alongshore and 2.0 km offshore was
surveyed. The current velocity and sand properties were measured at three stations.
The wave data for the rip current problem were obtained from De Vroeg [17]. As for
the sedimentation and erosion problems at Rosetta promontory, field survey has been
performed by SPA and CoRI, Egypt. The bathymetric survey utilized for the study was
that conducted in September 2001 and October 2002. The wave data were the
averaged wave climate of six years between 1985 and 1990. The sediment grain sizes
at the nearshore zone were between 0.16 mm and 0.24 mm. The field data were
analyzed for the model establishment. The study areas of both problems were
simulated using Delft3D model.
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Delft3D model which, is developed by Delft Hydraulics, is properly calibrated for the
Egyptian coastal problems (De Vroeg et al. [11] and Ahmed [18]). Delft3D is a
process-based model that includes wave, current, transport and bottom modules. More
details concerning the theoretical background and input parameters can be found in the
user manual, Delft Hydraulics [19].
The model was operated to simulate the two case studies in order to model various
alternatives to mitigate the pre-mentioned problems. Conventional countermeasures
were thus proposed and investigated.
This section introduces the results obtained from different researches based on
numerical modeling as follows:
Rip Current Problem (84.5 km west of Alexandria)
A reach located 84.5km west of Alexandria was investigated and simulated
numerically. The reach stretches 4.5 km along the coast. Figure 2 shows the computed
flow field at the area of interest. The circle indicates the zone where rip current occurs.
The wave condition, which was prescribed in the model, was 2.30m high, with 8.0s
wave period. The direction was relatively normal to the shoreline.
3 4 1 7 0 0 0 .0
North coordinate (m)
3 4 1 6 9 0 0 .0
3 4 1 6 8 0 0 .0
3 4 1 6 7 0 0 .0
3 4 1 6 6 0 0 .0
Rip current zone
3 4 1 6 5 0 0 .0
132000
132100
132200
132300
132400
132500
1.0 m/s
132600
132700
132800
East coordinate (m)
Figure 2: Flow velocity field for the present situation
In this study, alternatives were proposed to improve the swimming conditions; they
were similar to that recommended by De Vroeg et al. [11]. Other alternatives were
further given in Ahmed [12] and Ahmed et al. [20]. The proposed alternatives were to
produce minor impact on the shore. The following alternatives were investigated:
• Segmented submerged breakwaters, with crest level 3.0m below the MSL
• Segmented submerged breakwaters with crest level 1.0 m below the MSL
• Segmented emerged detached breakwaters
The results are presented in this paper as follows:
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• The flow velocity field for the submerged breakwaters with crest level 3.0m
below MSL are shown in the left panel of Figure 3, the offshore currents are
worsen while the submerged breakwaters with 1.0m crest level below the MSL
showed reasonable results, as demonstrated in Figure 3 right panel, but still the
rip current exist.
• The flow velocity field for the emerged breakwaters. The rip current problem is
properly vanished as the emerged detached breakwaters are employed as shown
in Figure 4, left panel. The emerged detached breakwaters were simulated to
predict the impact on the shoreline.
• The morphological evolution caused by the detached breakwater construction,
Figure 4, right panel.
Rip current
Figure 3: Flow velocity field overlaid with the nearshore bathymetry for
the submerged breakwaters 3.0m (left) and 1.0m (right) below MSL
Figure 4: Flow velocity field for emerged breakwaters (left) and
morphological evolution after one yare (right)
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From the study, it was found that
• Severe erosion is observed at the adjacent nearshore zone causing negative
impacts on the shore.
• Segmented emerged breakwaters are not feasible to mitigate the rip current
problem. This conclusion is controversy to the recommended alternative by De
Vroeg et al [11].
This leads to a conclusion that the conventional measures are not always proper to
mitigate a problem such as the rip current problem.
Erosion Problem at Rosetta Promontory
The shoreline erosion problem is located at the southwest of Rosetta promontory.
Photos 1 and 2 reveal the severe erosion of the shoreline that took place in December
2002.
Photo 1 (left): shoreline erosion just southwestward of Rosetta dike.
Photo 2 (right): the shoreline erosion at 750 m southwestward of Rosetta dike.
The proposed countermeasure for the shoreline erosion was to produce a stable
condition to the existing dike located west of Rosetta promontory. Several alternatives
were explained by Ahmed [14]. In this paper, the results of two alternatives are
presented. The two proposed measures were as follows:
• Detached breakwaters constructed at water depth of 4.0 m
• Series of groins with 150m long and 400/600 m spacing
The results presented in this paper are as follows:
• The morphological evolution for the groins and detached breakwaters is shown
on Figure 5. It can be observed that the detached breakwaters result in severe
local scour in front of the existing dike, while the groins produce sedimentation
that expected to stabilize the existing dike.
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Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt
Although detached breakwaters were utilized to mitigate the erosion problem (Fanos et
al. [5]), they are not feasible in this study. This leads to the conclusion that the
prediction of the structure impact on the shore using numerical model is necessary to
suggest the reasonable countermeasure for the erosion problem.
Western dike
Western dike
Local scour
Figure 5: Morphological evolution of detached breakwaters (left)
and groins (right) for one year
Sedimentation Problem at Rosetta Promontory
The problem of Rosetta mouth sedimentation is severe as the Rosetta mouth is an
important waterway for the fishermen who do fishing on the Mediterranean Sea and
return to Rosetta village using the Rosetta mouth. Photo 3 shows the closure of Rosetta
mouth. Its closure severely affects their livelihood from one hand, and it endangers the
people due to releasing an emergency flood from the other hand. Therefore, the
solution of such problem is crucial by means of Delft 3D model.
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Photo 3: Snapshot taken from the river left bank in April 2003
showing 90% closure of Rosetta mouth
The conventional measure to mitigate such problem of sedimentation of inlets is the
two jetties as discussed in Fanos et al. [5] and Simeoni et al. [21]. This alternative was
simulated and the results are shown in Figure 6.
Figure 6: Morphological evolution after one year for the proposed two jetties
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The results showed that:
• The navigation opening is closed due to the sediment accumulation that
bypassed at the seaward of the eastern jetty.
• The dredging operation is necessary that may in turn interrupt the navigation
process. On the other hand, this solution is uneconomic as the dredging
operation cost will be added to the construction in addition to the maintenance
cost of the two jetties.
• The solution of such problem was recommended by Ahmed [16]. He proposed
only a dredging operation without any structure and the dredged sediment
should be bypassed to the regions where local scour takes places.
CONCLUSIONS
Based on the two case studies that were presented in this paper, the numerical
simulations showed that:
1- some conventional measures are not feasible although similar measures were
already constructed
2- the existing structures were proposed based on medium-term field investigation
3- neither the experience nor the field investigations are sufficient to propose a
countermeasure
Consequently, it was recommended to simulate numerically the Damietta promontory,
where sustainable projects are liable to be encompassed. This is to predict shoreline
changes and to apply a countermeasure that will maintain the shoreline and cease the
sedimentation in the access channel of the Damietta port.
ACKNOWLEDGMENTS
Hydraulics Research Institute (HRI), Coastal Research Institute (CoRI) and Egyptian
Authority for Shore Protection (SPA) are greatly acknowledged for providing field
data. The author would like to thank both Prof. Mustafa Gaweesh the deputy director
of National Water Research Center and Eng. Ibrahim El-Desouki the technical advisor
of HRI for their constructive comments and support. Also, Prof. S. El-Serafy is also
acknowledged for reviewing this paper.
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Tenth International Water Technology Conference, IWTC10 2006, Alexandria, Egypt
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