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Mediterranea

Journal of Environmental Management ] (]]]]) ]]]–]]]

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Beach impacts of shore-parallel breakwaters backing offshoresubmerged ridges, Western Mediterranean Coast of Egypt

Moheb M. Iskandera, Omran E. Frihya,�, Ahmed E. El Ansaryb,Mohamed M. Abd El Mootyb, Hossam M. Nagyb

aCoastal Research Institute, 15 El Pharaana Street, El Shallalat 21514, Alexandria, EgyptbFaculty of Engineering, Alexandria University, Egypt

Received 25 July 2005; received in revised form 3 November 2006; accepted 18 November 2006

Abstract

Seven breakwaters were constructed behind offshore submerged ridges to create a safe area for swimming and recreational activities

west of Alexandria on the Mediterranean coast of Egypt. Morphodynamic evaluation was based on the modified Perlin and Dean

numerical model (ImSedTran-2D) combined with successive shoreline and beach profile surveys conducted periodically between April

2001 and May 2005. Results reveal insignificant morphologic changes behind the detached breakwaters with slight coastline changes at

the down and up-drift beaches of the examined breakwaters (710m). These changes are associated with salient accretion (20–70m) in

the low-energy leeside of such structures. Concurrent with this sand accretion is the accumulation of a large amount of benthic algae

(Sargassum) in the coastal water of the shadow area of these structures, which in turn have adverse effects on swimmers. Practical

measures proposed in this study have successfully helped in mitigating such accumulation of algae in the recreation leeside of the

breakwaters. The accumulation of Sargassum, together with the virtual insignificant changes in the up-drift and down-drifts of these

structures, is a direct response to both coastal processes and the submerged carbonate ridges. Coastal processes encompass reversal of the

directions of long-shore sand transport versus shoreline orientation, the small littoral drift rate and sand deficiency of the littoral zone.

The beach response to the breakwaters together with the submerged ridges has also been confirmed by applying the ImSedTran-2D

model. Results indicate that submerged ridges play a principal role in the evolution of beach morphology along the west coast of

Alexandria. Although the study area is exposed to more than 70% wave exposures, the morphodynamic behavior of the beaches

indicates that the submerged ridges act in a similar way as an additional natural barrier together with the artificial detached structures.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Mediterranean Sea; Detached breakwaters; Submerged ridges; Beach management; Beach morphodynamics

1. Introduction

The study area of El Agami is located west of Alexandriaon the Mediterranean coast of Egypt and is considered as aprincipal public resort beach in this region. The 14-km-long coastline of El Agami is oriented SW–NE and isbordered by Abu Talat beach to the west and Agamiheadland to the east (Fig. 1C and B). To the west, thewestern Nobaria drain outlet with its pair of jetties limits

e front matter r 2006 Elsevier Ltd. All rights reserved.

nvman.2006.11.018

ng author. Tel.: +2035276126, mobile: +2030103078141;

14.

ess: Frihyomr@link.net (O.E. Frihy).

is article as: Iskander, M.M., et al., Beach impacts of sho

n Coast of Egypt. Journal of Environmental Management (200

Abu Talat beach. The coastline is almost straight andcomposed of wide beaches covered by white carbonateoolitic sand (Hilmy, 1951). The white sand covers the sub-aerial beach and the near-shore area reflects the waterclarity and the unique turquoise color that characterizesthis region. These conditions offer esthetic amenities thatattract residential construction. As a consequence, rapidand extensive recreation developments in the region havebeen increasingly undertaken over the last 20 years.However, the main problem facing the beaches of thisstretch is their unsuitability for swimming since they areclassified between reflective and moderately dissipativebeaches and are associated with hazardous rip currents(Nafaa and Frihy, 1993).

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Fig. 1. (A) General map of the Arabs Gulf at the western Mediterranean coast of Egypt showing location of the shore-parallel limestone ridges partially

extended across the study area (modified after Butzer, 1960; Misdorp and Sestini, 1975). (B) Location map of El Agami coast and the 15 profiles lines

surveyed in this study. (C) Map showing the detached breakwaters constructed on the 6th of October beach west of Alexandria.

M.M. Iskander et al. / Journal of Environmental Management ] (]]]]) ]]]–]]]2

These circumstances have encouraged developers tobuild engineered structured measures in an effect to createsheltered recreational beaches without taking into con-

Please cite this article as: Iskander, M.M., et al., Beach impacts of sho

Mediterranean Coast of Egypt. Journal of Environmental Management (200

sideration their harmful impacts on the beach environment(Frihy, 2001). These sheltered beaches are of considerablerecreational importance to thousands of people who come

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from all over Egypt to use the west coast of Alexandriaduring summer holidays.

Although the coastline of this area is an open-seaenvironment, it has experienced long-term dynamic equili-brium through history due to its morphologic nature.The most prominent geomorphologic features are theexistence of a series of unconsolidated carbonate ridgeswhich are native to this area, Fig. 1A, the Arabs Gulfin particular (Fourtau, 1893; Shukri et al., 1956; Saidet al., 1956; Butzer, 1960; Lindell et al., 1991). The ridgesrun parallel to the coast in the backshore and in uplandareas from Alexandria to Sallum on the Egyptian/Libyanborder. They progressively increase in elevation from�10m along the coast to �100m some 40 km inland(Shukri et al., 1956). Other related ridges extend under-water down to a maximum depth of 20m across the innercontinental shelf of the Arabs Gulf between Alamienand Alexandria as reported by Lindell et al. (1991).The maximum crest height of these ridges is 6–11m abovethe seabed and they are generally found in certain waterareas of depths ranging between 10 and 15m. The originof the inland ridges ranges from marine, such as offshorebars and beach deposits, to eolian (Fourtau, 1893; Shukriet al., 1956; Said et al., 1956; Butzer, 1960). The inlandridges and submerged bedrock are the primary sources offormation of beaches and seabed sediments betweenAlexandria and Sallum (Hilmy, 1951; Misdorp and Sestini,1975). The tidal regime along the coast is microtidalsemi-diurnal with a mean range of approximately 0.40m(Debes, 2002).

2. Background

The present study is centered on the coastline backingthe detached breakwaters along the ‘‘6th of October’’

Fig. 2. Photograph (May 2004) showing the detached breakwaters along El A

harbor and the first detached breakwater (back).

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Mediterranean Coast of Egypt. Journal of Environmental Management (200

beach which were built between 1998 and 2003 west ofAlexandria (Fig. 1C and B). The primary function of thesebreakwaters was not to protect the beach but to provide asafe and secure area for swimming activities. To achievesuch a need, seven detached emerged breakwaters made ofdolos units were constructed in a water depth between 4and 5m and fronting a shoreline of about 1 km long. Eachindividual breakwater is 100m in length parallel to thebeach, 200m away from the shore and is spaced at 50mintervals. A crest level of +1.0m referenced to MSLwas adopted to force the wave energy of the storms.A small temporary harbor was built west of thesebreakwaters to facilitate construction activities of thebreakwaters (Figs. 1C and B and Fig. 2).It has been observed that sand accretion beyond the

detached breakwaters is also associated with accumulationof huge amounts of benthic algae (Sargassum) that imposeharmful constraints to swimmers using the protectedleeside of the breakwaters in summer holidays (Fig. 2).Problematic accumulations of algae (Sargassum) haveimposed adverse impacts on swimmers using the protectedleeside of the constructed breakwaters.The purpose of this study is to evaluate changes in the

morphology and coastal processes of the littoral zone of ElAgami in response of the shore-parallel detached break-waters and to discuss several factors that have contributedto these changes including the submerged ridges. Theanalysis presented is based primarily on field measurements(shoreline position, beach profiles, wave pattern, seabedgrain sizes and bottom slopes) and supplemented byapplying the modified Perlin and Dean numerical model(ImSedTran-2D). Another major objective is to propose anappropriate measure to mitigate the undesirable accumula-tion of Sargassum in the recreation area, i.e., leeside of theconstructed detached breakwaters.

gami beach and accumulation of Algae (Sargassum). Note the temporal

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3. Methodology

3.1. Field survey

A total of 15 beach-nearshore profiles were surveyed to adepth of 12m, corresponding to �1.5 km offshore. Thesesurveys spanned the 14-km length of the study area andwere conducted three times over a period of 1 year; fromOctober 2002 to October 2003 (Fig. 1B). The profilesextend from a baseline fixed along the study area with anorientation profile approximately perpendicular to thecoastline (Fig. 1B). The distance between profiles rangedbetween 500 and 1000m. Seaward of each profile,soundings were taken every 10m from the baseline up toa depth of about �1.0m below MWL and then soundingswere taken every 3m up to the 12m contour depth.

In association with these profile surveys, the initial shorelineposition was measured in April 2001 following construction ofthe first three breakwaters. The profile surveys were carriedout during fair-weather using a rubber boat equipped with aDGPS (GBX-Pro) and echo sounder to determine theseasonal variability in shoreline and seabed morphology.The survey elevations, depth and land, are referenced to theEgyptian Survey Authority datum (mean sea level).

During the survey of October 2003, a total of 49 bottomsamples were collected at water depths of 2, 4, 6, 8 and 10malong the profile lines together with two beach samples takenat the swash zone (see Fig. 1B for the profile location). Grain-size analysis was completed in the laboratory by dry sievingusing standard ro-tap sieving at 1f sieve intervals. Thef-scale notation of Krumbein (1963) was used as a size scale.The mean grain size (Mz) for each sample was calculatedusing the formula of Folk and Ward (1957). The resultingvalues of Mz in f units were converted into millimetersaccording to the f-mm transformation: (mm) ¼ 1/2f.

Wave climate (wave height, period and incident waveangle) was analyzed based on available records measuredat Damietta Harbor located 220 km east of Alexandria.Although wave characteristics at both localities are muchsimilar waves starting at Abu Quir took about 8 h to reachRas El-Bar (Fanos et al., 1995). A pressure wave gauge(Inter Ocean System S4DW) was installed at about 12mdeep in the water. The wave gauge recorded the directionalwave and current spectrum for 20min every 4 h. Wave datawere compiled for a whole year period from November2002 to October 2003.

3.2. Model description

The numerical simulation of the coastal processes andsediment transport in the study area was conducted byusing the ImSedTran-2D model of Perlin and Dean (1983)with the following modification:

1.

P

M

The solution was modified to simulate the actual bedcontour instead of the ideal bed contour represented bythe equilibrium beach profile equation (h ¼ AY 2=3).

lease cite this article as: Iskander, M.M., et al., Beach impacts of sho

editerranean Coast of Egypt. Journal of Environmental Management (200

2.

re-

7),

The wave diffraction calculation based on the Kraus(1984) solution was added to simulate the bed morphol-ogy in the vicinity of the coastal structures. However,the governing equations used to determine the wavedirection and wave height distribution for refractioncalculations are summarized as follows:

qqx

K cos y�qqy

K sin y ¼ 0, (1)

qqx

rgH2

8CG sin y

� �þ

qqy

rgH2

8CG cos y

� �¼ 0, (2)

where x and y axes are in the long-shore and in theoffshore directions, respectively, K is the wave number,y the wave angle, r the mass density of water, g thegravitational acceleration, H the wave height, and CG

the group velocity.

The wave height at the location in question is simply theproduct of the specified partially refracted incident waveheight and diffraction coefficient. The angle of the wavecrest is computed assuming a circular wave front along anyradial; this angle is then refracted using Snell’s law.Throughout the refraction and diffraction schemes, thelocal wave heights were limited by the value 0.78 of waterdepth.Calculations of the wave distributions are based on

shoaling processing, refraction, diffraction, and depth-limited breaking. It is also designed to take the actual wavedata measurements at any point offshore of the breakingpoint.Then the continuity equation is used to simulate the

sediment transport and bathymetry changes:

qy

qtþ

qqx

qxþ

qqy

qy¼ 0, (3)

where qx and qy represent the long-shore and cross-shoresediment transport, respectively.

Qx is given by (Perlin and Dean, 1983)

Qxi;j¼ exp�

ðyi;j�1Þ þHbi

1:25ybi

� �3

� exp�ðyi;jÞ þHbi

1:25ybi

� �3" #(

�Kl

ðrs � rÞgð1� pÞEbi

CgbiSinð2a0i;j � 2aci;j Þ

�. ð4Þ

Qxi;jrepresents the long-shore sediment transport be-

tween depths di,j and d(i,j�1), a0i;j the averaged wave angle atthe location of Qxi;j

, and aci;j the local contour orientationangle, Hbi the breaking wave height at ‘‘i’’ grid-line alongthe x direction, and p the prosity.

Qy is given by (Bakker, 1968)

Qyi;j¼ DxKci;j ½yi;j�1 � yi;j þWEQi;j �. (5)

Qyi;jrepresents the cross-shore sediment transport, Kc is

an activity factor and WEQ(i,j) is the positive equilibriumprofile distance between yi,j and y(i,j�1).

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To solve the finite-difference form of the continuityequation boundary values (left side, right side, onshore,and offshore boundaries of the study area) are required.The ‘‘y’’ values along the left- and right-side boundary areassumed to be fixed at their initial locations. This meansthat the sediment transport quantity along these bound-aries is zero. The onshore boundary is treated by assumingthat the berm and beach face move in conjunction with theshoreline position. The offshore boundary is treated bykeeping the contour beyond the last simulated one fixeduntil the bed slope transcends the angle of repose thenresetting it to a position such that the slope equals the angleof repose. For more details on the model see Perlin andDean (1983), and for the modification see Iskander (2005).

4. Profile variability and sediment grain size

Beach profiles are used to understand and quantify thevariations in seabed levels and shoreline orientation, whichare undergoing continuous change in response to themarine processes such as waves and currents. Results of theprofile analysis reveal cross-shore marked variability in theirregularity of the seabed morphology, slope and grain sizedistribution. The most distinctive characteristic of thelittoral cell is its marked bathymetry due to the existence ofbedrock limestone features covered partially by carbonatesand. Visual examination of the constructed bathymetryand profile configurations revealed shore-parallel sub-merged ridges (Fig. 3a and b). The beach-face and surf-zone down to 10m depth are mostly covered by loosecarbonate sand, while the offshore bedrock is partiallyexposed. These bedrock outcrops form the most pro-nounced morphological feature in this region. Parts ofthese outcrops extend above the mean water level to formshore-parallel islets fronting the coastline. Profile #P58 hasbeen taken as an example to display the rough irregularitiesin the seabed of this area (Fig. 3a). The ridge crest is about1250m seaward and 500m wide at its base. The crest iselevated 6–11m from the seabed (Fig. 3a). This seabedconfiguration is more or less typical for the classical coastaltopography along the Alexandria littoral cell (Frihy et al.,2004).

Unlike the Nile delta, microscopic examination of beachand seabed samples indicates that they are mainlycomprised of carbonate grains (95%), quartz grains (4%)and shell fragments (1%). Heavy minerals rarely occurredin beach and seabed samples. In general, the mean grainsize of the beach and bottom sediment increases in aseaward direction (Fig. 3b). The fine-grained sediments(0.11–0.51mm) mostly cover the beach and nearshore zonewhereas the coarse-grained sediments (0.51–0.74mm)occur mostly further offshore beyond the 8.0–10.0m depthcontour where the submerged ridges are located. The onlyexception is that sediment near the rocky ridges, especiallyalong profile 58, which is relatively coarse with a maximumpeak of 0.7mm, exists at 6.0m depth contour (Fig. 3b).A similar seaward coarsening trend is also observed along

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Mediterranean Coast of Egypt. Journal of Environmental Management (200

the littoral cell of Alexandria due to the existence of relictlate Pleistocene to mid-Holocene coarse-grained sandoffshore beyond the 8–10m water depth (Frihy et al.,2004).The cross-shore seabed slope of the study area generally

decreases from 0.20 in the beach-face to 0.02 in the surf-zone.In the along-shore direction, the beach-face between +1.0and �1.0m is relatively steeper averaging 0.10 (Fig. 3c).Generally, the beach-face slope systematically decreaseseastwards from �0.20 near Abu Talat to � 0.04 near ElAgami Headland. A relatively gentle seabed slope of 0.04 isobserved in the lee-side of the detached breakwaters due tothe effect of the salient accretionary formation. The swashzone is characterized by having coarser sediment(Mz ¼ 0.5–0.7mm) on a steeper slope surface (0.1) thanthe surf-zone which is relatively finer (Mz ¼ 0.2mm) andless steep (0.02).

5. Beach morphodynamics

By analyzing 1 year of actual wave measurement data,the average significant wave height and period are 1.04mand 6.2 s, respectively with a maximum wave height of4.45m blown from NW in winter. The N, NNW and NWwaves are important in inducing morphological changesbecause of their long duration particularly in winter. Theremaining NNE and NE components occasionally occurduring April and May.It has been established that when waves approach the

coast at an angle, they cause sediment to be transportedalong the shore i.e., long-shore sediment transport. Thedirection and magnitude of long-shore current and littoraldrift depend on the effective angle of incident waves andaverage shoreline orientation. The relationship betweenwave climate and the present shoreline orientation (571from the north) provides effective oblique wave exposures(Fig. 4a). Accordingly, two main wave exposures areresponsible for generating opposing SW and NE long-shore sediment transport (see the two arcs in Fig. 4a). Ingeneral, the predominant wave components propagatingfrom NW, WNW, and W sectors, 85% of the time, areresponsible for the generation of long-shore currentstowards the northeast, representing up to 70% of the totalwave distribution. Approximately 10% of the time wavesapproach from NNW, N, NNE, and NE, on the otherhand, and generate a reverse long-shore current towardsthe southwest, particularly during March and April, intotal contributing �19% of the wave distribution (Fig. 4a).The remaining component (11%) represents calm condi-tions generally with S and SE waves, i.e., from the inlanddirection, spanning �5% of the time. Evidence for thissmall net littoral drift has come from field observations ofsand accumulation patterns adjacent to the jetties con-structed to protect west Nobaria Drain and on both sidesof the small temporal harbor built in the study area.The model is applied to calculate the wave distributions

across the study area using the bathymetry map of October

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Fig. 3. (a) Pronounced seabed morphologic features along the study area as depicted from profile P #60. The surface of the submerged ridge with its high

relief accelerates wave breaking thus reducing wave energy reaching the breakwaters. (b) Spatial distribution of mean grain size (c) Beach slope and surf

zone of the study area.

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2002 for both the significant and maximum wave heights.Because of space limitations, the distribution of significantwave height and direction, resulting from Hs ¼ 1.04m,Ts ¼ 6.2 s and direction ¼ 2901 from the north, is shown inFig. 4b. Wave distribution in the sheltered zone of thebreakwater behaves in a refracted and diffracted pattern.

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Mediterranean Coast of Egypt. Journal of Environmental Management (200

Wave heights notably decrease beyond the western break-waters due to the double sheltering effect of these break-waters and the temporal harbor.To quantify the changes in the shoreline positions and

bottom contours observed in the study area, the initialsurveys of April 2001and May 2005 are graphically plotted

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Fig. 4. (a) Wave climate (wave rose and average orientation of the coastline) and the two main wave exposures inducing northeasterly and southwesterly

sediment transport pathways. (b) Modeled spatial wave distribution along the study area. (c) Along-shore changes in shoreline positions from baseline (m).

(d) Long-shore shoreline changes and position of the four profiles (I–IV) used in the model calibration and verification.

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in Fig. 4c. The along-shore changes in all survey periodsare examined in more detail using the bivariate plot in Fig.4d. The variations in shoreline positions on both sides ofthe detached breakwaters along both the up-coast anddown-coast are relatively small, fluctuating between710m. During the period from April 2001 to May 2005,the average retreat of the shoreline up-coast of thebreakwaters reached 8.0m while it reached 10m in the

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Mediterranean Coast of Egypt. Journal of Environmental Management (200

down-coast. These two coastal stretches extend about1.5 km on both sides of the breakwaters. It is likely that thiserosion/accretion fluctuation is changeable, rather thancontinuous, due to the seasonal variation in wave climateand reversibility in the direction of long-shore sandtransport, to the NE and to the SW (Fig. 4a). In contrast,the shoreline of June 2003 beyond the detached breakersadvanced seaward by �35m since the initial 2001 survey

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(Fig. 4d). This accretion occurred following construction ofthe first three breakwaters in June 2003 and graduallyincreased afterwards. In the same locality, the shoreline ofMay 2005 accreted seaward ranging between 20 and 70m,with an average value of 40m, spanning 4 years from theinitial survey of April 2001.

Comparison between the survey conducted immediatelybefore May 2004 and after removal of the temporal harborin May 2005 indicates that the beach width diminishedeastward and the shoreline tends to be parallel to thebreakwaters (Fig. 4d). The accretion/erosion patternsduring this period ranged between 725m with a generaltrend of sand accumulation at an average of 6.0m. Thisdegree of sand accretion beyond the detached breakwatersis also associated with accumulation of huge amounts ofSargassum, which disrupts swimming activities during thesheltered areas in summer holidays (Fig. 2). Considerableefforts were made to mechanically remove this algae butaccumulation has reoccurred. An accumulation of Sargas-sum has been also detected in this region using remotesensing in depressions confined between submerged ridgesin the Arabs Gulf (Lindell et al., 1991). This problem wastotally solved following dismantling of the temporaryharbor in May 2005.

In general, the observed morphology of shoreline andseabed contours is unlike that previously established incommon responses to detached breakwaters. Commonly,erosion occurs in the gaps, tombolos or salients in the waveshadows of each breakwater and significant changes mightbe expected on both sides of the breakwater system. Within5 years (1998–2003), during all stages of the breakwatersconstruction, the configuration pattern of the shoreline andbottom contours seemed to be more or less linear with nopronounced undulating features that may exist behind thebreakwaters (Fig. 1C and B). In comparison, the observedbeach morphology along the study area does not resemblethe sedimentation pattern observed at Marabella break-waters, about 62 km west of El Agami, where no ridgeshave been observed. Unlike the El Agami breakwaters, therapidly formed tombolos in the shadow zone of thosebreakwaters have blocked the sediment flow to the east,thus contributing to beach erosion at the downdrift side ofthese structures (Frihy, 2001).

The morphologic performance in the vicinity of thebreakwaters gives an indication of the influence of thesubmerged existing seaward ridges of such structures. Aremnant submerged ridge, possibly the inner one (coast-ward), was detected during all sounding surveys (Fig. 1B).The sheltering effect of such ridges provides a naturalprotection system for this coastal stretch. Incoming wavesbreak and their energy dissipates on the ridge crest, therebysheltering the littoral zone hosting the breakwaters. Fromfield observation, we noticed that the breakwater providesmuch less wave energy at the beach-face when the ridge ispresent. Acting together with this hydrodynamic responseis the very limited littoral drift. The net littoral driftpotential in the area appears to be relatively moderate at an

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Mediterranean Coast of Egypt. Journal of Environmental Management (200

approximate rate of 0.42� 106m3/year (El-Fishawi, 1994).Another factor is that the littoral cell faces a substantialdeficit in sand supply due to its self-contained source.Beach and seabed sediment of the surf-zone mainlyoriginate from eroded carbonate ridges along the coastand their contiguous bedrock.

6. Modeling

The modified Perlin and Dean numerical model (Im-SedTran-2D) was used to examine the variability of thebeach profiles under two conditions: firstly, dealing withthe existing condition in which both the detached break-waters and submerged ridges exist, and secondly, applyingthe model with the assumption that the temporal harborbuilt to serve breakwater construction works had beenremoved. Prior to dealing with these two conditions, themodel was initially calibrated.

6.1. Model calibration

The constructed contour maps obtained from thesurveyed profiles along with the wave data and mean grainsize of the seabed sediments were used to calibrate theImSedTran-2D model to be applied in the vicinity of thedetached breakwaters. The field data used spanned the 12months between October 2002 and October 2003. Startingwith the initial survey of October 2002, a series ofsimulations/runs were carried out in order to reproducethe true seabed contours of October 2003 (Fig. 5a). Ineach run, the calibration parameters of the model includingthe activity factor, the long-shore equation factorand the breaking wave energy parameter were carefullyadjusted to simulate bottom contours comparable with theoriginal field data surveyed in October 2003. The activityfactor adjusts the cross-shore sediment transport. Thelong-shore transport equation factor controls the long-shore transport. The breaking wave energy parameterrepresents the effect of the breaking wave energy, whichoccurs primarily near the water surface, on the sedimenttransport near the bed. This calibration procedure sug-gested values of these parameters to be 0.00001, 0.0035,1.0, respectively.A comparison between the measured and calculated

bathymetric survey of October 2003 is depicted alongfour selected cross-shore profiles shown in Fig. 5.Profiles #I and #IV are positioned, respectively, in theup-drift and lee sides of the breakwaters, whereas Profiles#II and #III are located within the breakwater zone(Fig. 4c). These profiles served as the basis to quantifybottom changes as a function of existing conditions i.e., thepresence of both the detached breakwaters and ridges.The seabed variability in these profiles shows a goodagreement between the measured and calculated seabedmorphology. The relative similarity of seabed configura-tion in each profile confirms the high compatibility ofboth the simulated seabed and the field survey, i.e.,

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Fig. 5. Model calibration (a) and model verification (b), showing comparison between predicted seabed topography and field measurements across four

selected profiles. Profile positions are shown in Fig. 4c.

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the model predictions correspond well with the fieldobservations.

6.2. Model verification

The next step was to verify the model. The bathymetricsurvey of June 2003 together with the available wavedata measured between October 2002 and June 2003 wasused in the verification processes using the justifiedcalibration parameters. Results of such verification aredepicted as seabed configuration along the same profilelines used above in the processes of calibration (Fig. 5b).The small difference between the field data and themodel results is perhaps related to inaccurate simulationof the submerged ridges in the model. The changes in thespatial variability of the seabed along the examined profileswere a bit different due to dissipation of incident waves

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Mediterranean Coast of Egypt. Journal of Environmental Management (200

crossing the ridges and the breakwaters. Insignificantseabed accretion appeared just west of the detachedbreakwaters whereas relative erosion occurred to the leeside east of the breakwaters. This slight erosion/accretionpattern reflects the limited effect of the breakwaters due tothe existing ridges. In contrast, marked accretion exists inthe shadow area of the breakwaters associated with localerosion near the deeper part of the surf-zone due to theeffect of winter storms. Of importance is that the resultingbathymetry shows relatively straight and simple seabedcontours.

6.3. Model application

The calibrated model was also used to answer thefollowing question: What would happen if the smalltemporary harbor built west of the structure were to be

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dismantled? The results obtained, as anticipated, followingapplication of the model indicated that the sedimentationwill be enhanced in a short time without major disturbanceto beach morphology. A landward shift between depthcontours varying between 1.0 and 2.0m would occur. Onthe other hand, harbor dismantling would gently increasewater flow between the shoreline and breakwaters and thuswould virtually eliminate accumulation of dead algae in theswimming water.

6.4. The empirical equations

Offshore breakwaters have been used successfully tocontrol shoreline evolution when the dimensions of suchstructures are correctly defined. These dimensions includethe breakwater crest height (H), effect of the offshoredistance of the breakwater (X), the length (B) and thebreakwater gap (G). In this study, we applied the empiricaldesign equations for the detached breakwaters developedby Herbich (1991), Hallermeier (1983), Seiji et al. (1987),Hsu and Silvester (1990) in an attempt to test the validity ofthese equations. Results indicate that the dimensions usedin designing the El Agami breakwaters are comparablewith cases producing minimum morphologic changes. Inour case, morphologic responses appeared in the form of asalient with a maximum length of 40m. In addition,associated beach erosion facing breakwater gaps seems tobe minimal with a nearly straight shoreline and bottomcontours.

7. Summary and conclusions

This study dealt with the effect of submerged ridges incontrolling profile evolution in the vicinity of detachedbreakwaters built to stabilize the recreation beach at ElAgami on the west coast of Alexandria. Of particularimportance is the role of the carbonate ridges in dissipatingincoming waves, which influence the hydrodynamic fieldthat in turn affects sediment transport processes in thevicinity of such structures.

Field observations spanning 4 years of shoreline positionand beach profile data indicated that there has been nosignificant effect of the breakwaters on the adjacentshoreline and seabed contours. In the along-shore direc-tion, both the down- and upcoast of the breakwaters showsmall changes (710m), with a considerable salient accre-tion (20–70m) in the lee side of these structures. Anotherchange observed was an accumulation of Sargassum in theswimming area between the shore and the detachedbreakwaters. The considerably smaller spatial morphologicvariability documented in this coastal stretch doesn’tconform to the ‘‘classic’’ forms described elsewhere behindbreakwaters due to the effect of a combination of severalfactors. Of particular importance is the existence of theoffshore submerged ridges which effectively dissipate theenergy of the deep water incident waves before reaching thebreakwaters and thus act as additional natural shore-

Please cite this article as: Iskander, M.M., et al., Beach impacts of sho

Mediterranean Coast of Egypt. Journal of Environmental Management (200

parallel barriers and perform the same functions as thosedescribed for offshore breakwaters. Other factors, are theeffect of shoreline orientation on the sediment transportdirection due to the action of coastal processes, and thesmaller littoral drift rate and sediment deficiency within thesurf zone.Although the examined beach was classified as reflective

to moderately dissipative, the beach morphology tends toquickly shift to a more fully dissipative stage, effectivelybreaking the wave energy on the ridge surface beforereaching the breakwaters.Lessons learned from this study are relevant to

numerous engineering endeavors such as the assessmentof beach stability, design of coastal structures, as well aspredictions of the dynamic response of coastal protectionstructures. Results of this study indicate that raisedbedrock systems that allow wave dissipation, can beutilized in artificial protection works. For example, raisedbedrock features can be used as a toe or as a foundationbase for protective structures such as detached breakwatersor as offshore shelters for beach nourishment projects.Another lesson is that detailed bathymetric surveys over amuch wider area of a structure and surrounding seabed arenecessary for engineering purposes where knowledge of thesubmerged morphologic features and coastal processeswithin the study area is required. Understanding thecontribution of these ridges in coastal protection is animportant topic for further research.

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