TSUNAMI INUNDATION MODELING FOR COASTALZONE OF ALEXANDRIA CITY
Mohamed Elsayed
MEE17720

Supervisor:

Yushiro FUJII
Bunichiro SHIBAZAKIABSTRACTThe Alexandria’s coastal zone suffered from disastrous tsunamigenic earthquakes; the 142, 365, 1303,
and 1222 earthquakes that were mainly produced from the Hellenic and Cyprian arcs. We focused on
the Alexandria middle coastal zone to investigate tsunami propagation and inundation by adopting theassumed fault models for the earthquakes. Using TUNAMI (Tohoku University’s Numerical Analysis
Model for Investigation) code, we performed numerical simulations and constructed inundation maps.
We downloaded bathymetry data of GEBCO 30 arc-sec and topographic data of SRTM 1 arc-sec. We
divided the computation domain into four grids, such as the finest grid represented inundated area
merging GEBCO and SRTM data. We set six coastal output points along the Egyptian shoreline
including the real Alexandria gauge to study tsunami height and arrival time. Moreover, we picked out
five important points to evaluate tsunami run-up height and inundated depth. Our computation results
showed that the first wave arrived at the coastline of the Alexandria after 62 min of the 142 earthquakewith the maximum tsunami run-up height of 6.48 m and inundated depth of 7.63 m. While, the 1222
source closest to Alexandria had the maximum tsunami run-up height of 0.55 m and inundated depth of
1.2 m. Computed inundation maps showed that the 142 event covered the largest inundation area thanother events. For all sources, the western inundated areas were much wider than the eastern inundated
areas.
Keywords: Alexandria, Tsunami Propagation, Tsunami Heights, Run-Up Heights, Inundation Depth.

1. INTRODUCTION
Recently, after the occurrence of two local tsunamis in the Eastern Mediterranean Sea (EMS): the one
in Algeria in 2003 (Mw = 6.8) and the other one in Turkey in 2017 (Mw = 6.6). These events verified
the capability of EMS to generate the tsunami in the future. Also, understanding the effect of tsunami
propagation and inundation is extremely important for the disaster preparedness in the planning andprotection of coastal areas. Alexandria city is located at the center of northern Egyptian shoreline along
the EMS. It is the oldest and the second largest city in Egypt regarding the population as well as a majorindustrial and economic centers. Historically, Alexandria was exposed to a few disastrous events: the365, 142, 1303 earthquakes along the Hellenic arc and 1222 earthquake along the Cyprian arc (Figure1) (Fokaefs and Papadopoulos, 2007; Valle et al., 2014). Nevertheless, there are no near-field faults
reported seriously to generate a tsunami in Alexandria. Such events do not comprise run-up height orinundation extension on land. Therefore, we must pay attention to this region for increasing theawareness of hazardous events and inundation.

National Research Institute of Astronomy and Geophysics (NRIAG), Seismology department, Egyptian National
Seismic Network (ENSN), Egypt.
International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.

12. DATA USED FOR TSUNAMI MODELING
2.1. Bathymetry and topography
dataFortsunamisimulationandinundation
modelings,
wedownloaded bathymetry data of 30
arc-sec grid in EMS from the General
Bathymetric Chart of the Ocean(GEBCO) 2014 (Wetherall et al.,2015) (http://www.gebco.net/). As
topography data is crucial to performtsunami inundation modeling, we also
downloaded data of 1 arc-sec gridfrom Shutter Radar Topographic
Mission(SRTM)
website(https://earthexplorer.usgs.gov/) toobtain accurate results.

Figure 1. Location of historical tsunamigenic earthquakes(blue circles). Six coastal output points are also shown byyellow triangle (existing tide gauge in Alexandria) and red
triangles (virtual tide gauges).

2.2. Computational regions and settings
We divided the computation area into four
regions that are Regions 1, 2, 3, and 4 tostudy tsunami inundation. Region 1 covers
EMS as the whole region of the computation
domain with the coarsest grid. For theinundation area, Region 4 covers smallerarea with the finest grid constructed fromGEBCO and SRTM data (Figure 2). Table 1
lists the computation parameters for eachregion with spatial resolutions of data.

Figure 2. Merged bathymetry and topography data withlocation of coastal output points (P1 – P5) in Region 4.
Table 1. Computational dimensions and data used for tsunami simulation.
RegionLongitude(°E)

Latitude (°N)

Region 1

18 - 36.5

30 - 37.5Region 2

28.5- 31.5

30.75 -32

Region 3

29.5 - 30.5

30.91 - 31.66

Region 4

29.7- 30

31.05 - 31.25

Bathymetry&Topography data

GEBCO
30 arc - sec
GEBCO
30 arc-sec +SRTM
1 arc - sec

2

Grid Sizedx (m) × dy (m)1 arc-min1540 × 1848
20 arc-sec
514 × 616
6.66667 arc-sec
171 × 205
2.22222 arc-sec
57 × 68

GridDimension
nx × ny1110 × 450
540 × 225
540 × 405
486 × 324

2.3. Fault parameters of tsunami sources
We adopted the fault models that were assumed by Valle et al. (2014) for the 142 earthquake andHamouda (2010) for the 1222 earthquake (Table 2) in order to achieve the main purpose of studying
tsunami propagation and inundation.
Table 2. Fault parameters for the 142 and 1222 earthquakes.
Year1421222

SourceFault location

LengthWidth

Strike

Dip

Rake

Slip

Top depth

Lon(°E)lat(°N)

km

km

deg

deg

deg

mkm

8.6

28.6

35.5

190

90

235

2090

10

7.5

7.8

32.5

34.5

80

30305

35110

315Mw

Valle et al.
(2014)
Hamouda(2010)

3. THEORETICAL CONCEPTS AND METHODOLOGY
To simulate the tsunami propagation and inundation, we applied the theory of nonlinear shallow waterin the spherical coordinate system based on the following governing equations namely, continuity Eq. )1) and momentum Eqs. (2) and (3) (Yanagisawa, 2018).
𝜕𝜂1𝜕𝑀𝜕(𝑁𝑐𝑜𝑠𝜃)] = 0+[+𝜕𝑡 𝑅𝑐𝑜𝑠𝜃 𝜕𝜆 𝜕𝜃

(1)

𝜕𝑀𝑔𝐷 𝜕𝜂
1 𝜕 𝑀21𝜕𝑀𝑁
𝜏𝑏𝑥++( )+(𝑐𝑜𝑠𝜃)+=0
𝜕𝑡 𝑅𝑐𝑜𝑠𝜃 𝜕𝜆 𝑅𝑐𝑜𝑠𝜃 𝜕𝜆 𝐷𝑅𝑐𝑜𝑠𝜃 𝜕𝜃𝐷𝑃(2)𝜏𝑏𝑦𝜕𝑁𝑔𝐷 𝜕
1 𝜕𝑁2
1 𝜕 𝑀𝑁(𝑐𝑜𝑠𝜃𝜂) ++(𝑐𝑜𝑠𝜃 ) +()+=0𝜕𝑡 𝑅𝑐𝑜𝑠𝜃 𝜕𝜃𝑅𝑐𝑜𝑠𝜃 𝜕𝜆𝐷𝑅𝑐𝑜𝑠𝜃 𝜕𝜆 𝐷𝑃(3)where 𝜆 and 𝜃 are the latitude and longitude, 𝜂 is the elevation of water surface, D is total water
depth of 𝜂 + h (still water depth), and R is the earth radius. M and N are the component of discharge
along 𝜃 and 𝜆.

𝜏𝑏𝑥
𝑔𝑛2= 7 𝑀√𝑀2 + 𝑁 2
𝑃
𝐷3and

𝜏𝑏𝑦
𝑃=𝑔𝑛2
7𝐷3

𝑁√𝑀2 + 𝑁 2 are the bottom frictional terms.

3.1. Tsunami numerical stability
We can eschew unexpected instability errors by allocating the suitable time step to satisfying the Courant
Fredric Lewy (CFL) stability conditions (Imamura et al., 2006), which is determined by∆𝑡 ≤ ∆𝑥/√2𝑔hmax
where Δt and Δx are temporal and spatial grid Table 3. Temporal grid size for tsunami simulation.
size, g is gravity acceleration (9.8 ms-2) and hmax
Region123
4
is still maximum water depth in the computation
1.00
1.00
1.00
∆𝑡 (sec) 1.00
domain. We set the temporal time step (∆𝑡) to1540
513.3
171.1
57.0
∆𝑥 (𝑚)
be 1 sec. The total calculation time for tsunami
hmax(m)
5128
2644
1218
82.7
propagation is 21,600 sec. Table 3 shows the∆𝑥
4.85
2.25
1.10
1.4
∆𝑡 values for each region in the computation√2𝑔hmax
domain.

4. RESULTS AND DISCUSSION

34.1. Seafloor deformation
The initial step for the tsunami generationand propagation is to calculate the sea
bottom displacement using the elastic
theory (Okada, 1985). Figure 3 shows theseafloor displacement for the 142 Mw 8.6
and the 1222 Mw 7.8 earthquakes. For the142 event, the uplift and subsidence are
4.73 m and 1.38 m, but for the 1222 eventthey are 0.99 m and 0.13 m.
4.2. Tsunami propagation
Figure 4 shows snapshots of tsunami
propagation at 0, 60, 90 min for the 142
and 1222 earthquakes to study thetsunami wave behaviors. For the 142
earthquake, the first wave reached to theAlexandria’s coastal zone with positive
amplitude. We selected five points P1 toP5 as shown in Figure 2 to study tsunami
waveforms in Region 4 including the real
existing Alexandria tide gauge (next toP3) as coastal point in Region 1. The time
history of the calculated tsunami
waveforms at the six coastal output points
are shown in Figure 5. For the case of the1222 earthquake, the first wave has also
positive amplitude, which it is the closest
event to Alexandria.

Figure 3. Seafloor displacement computed for the 142(top) and 1222 (bottom) events. The red and blue colors
are uplift and subsidence (right). Red and blue contour
lines interval of the 142 are 0.5 m, but for the 1222 they
are 0.2 m and 0.02 m. Vertical displacement is shown
along cross section AB (left).

4.3. Tsunami height and tsunami travel
time
Figure 4. Snapshots of tsunami propagation for the 142
Figure 6 shows the computed Maximum(left) and 1222 (right) earthquakes.
Tsunami Height (MTH) at
Alexandria. For the 142
event, the MTH was 5.41
m after 62 min (60-65min). It could be risky atAlexandria city, especiallyfor the western side as it ischaracterized by lowlands
andpossesses
moreupgradingcoastalbuildings. For the 1222
event, the MTH did notexceed 1 meter (0.55 m)
after 73 min (70-75 min).
Figure 7 shows theFigure 5. Output tsunami waveforms in Region 1 (blue colors) and fivetsunami arrival time at thepoints in Region 4 (red colors) for the 142 (left) and 1222 (right) events.
six output points.

4Figure 6. Tsunami height for the 142 (left) and 1222 (right) earthquakes.
4.4. Tsunami run-up height
Figure 8 shows the location of five significant
points that were selected nearly the coastline
while Figure 9 represents the tsunami evaluatedrun-up heights. The maximum computed
tsunami run-up height of 6.4 m was recorded atBibliotheca Alexandrina as it is topographicallymore elevated than other selected points. Thesevalues were gradually increased in the eastern
coastline except for Alexandria port as it isprotected by seawall and the Citadel of Qaitbay
as it is mainly constructed so near to theshoreline.
Figure 7. Tsunami arrival time.

Figure 8. Location of evaluation points (Source:google earth).
Figure 9. Evaluated tsunami run-up heights.
4.5. Tsunami inundation
The case of the 142 earthquake covered larger inundated areas than other events due to probably thelongest length of tsunami source, the shallow top depth of the fault and the directivity of tsunami wave
energy towards Alexandria (Figure 10). The maximum elevation of water on land for this event isestimated to be 7.63 m. Conversely, the shape of the inundated area for the 1222 earthquake represented
the smallest and limited inundated areas with the calculated maximum inundation depth of 1.2 m (Figure11).

5Figure 10. Inundated water depth for the 142 Figure 11. Same as Figure 10, but for the 1222
event.
event.

5. CONCLUSIONS
Using TUNAMI code, tsunami simulation and inundation results show that the maximum run-up heightand inundated depth were 6.48 m and 7.63 m for the 142 event with inundated horizontal distance ofabout 1,550 m from a coastline. While, the 1222 event as the less risky source had the maximum tsunami
height and run-up height of less than 1 m. It would be hazardous particularly for people swimming orstanding on a beach with the inundation distance of about 650 m. For all earthquake sources, themaximum run-up height was recorded at Bibliotheca Alexandrina as its highly elevated, whilst theAlexandria port represented the least computed run-up height and inundated depth as it is protected bythe breakwater. The Ras el Tin area was highly inundated. Western coastline areas were more exposedand vulnerable to inundation than the eastern coastline as it is topographically more lowland areas.

6. RECOMMENDATIONS
1. A precise local bathymetry and topography data are required for accurate results in tsunami modeling.
2. Preserving of barriers, large boulders and some ponds and channels are considered as one of theinitial steps for disaster preparedness and mitigations in future tsunami risk in Alexandria.
3. Besides, early warning systems and evacuation planning are a key to minimize tsunami casualties
and economic losses.

ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to my supervisor Dr. FUJII and my supervisor and advisor
Dr. SHIBAZAKI for their continuous support, valuable guidance and suggestions during my study.

REFERENCES
Fokaefs, A., and Papadopoulos, G.A., 2007, Natural Hazards 40.3, 503-526.
Hamouda, A.Z., 2010, Marine Geophysical Researches, 31.3, 197-214.
Imamura, F., Yalciner, A.C., and Ozyurt, G., 2006, DCRC, Tohoku University, Japan.
Okada, Y., 1985, Bulletin of the Seismological Society of America, 75(4), 1135-1154.
Valle, B. L., et al., 2014, Proceedings of the Institution of Civil Engineers-Engineering andComputational Mechanics, 167(3), 99-105.
Weatherall, P., et al., 2015, Earth and Space Science, 2, 331–345.
Yanagisawa, H., 2018, IISEE, BRI, Japan.

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