Introduction

Sand spits are important relief forms of river mouths’ estuaries and delta (Bastos et al., 2012) and among the most forceful feature in the coastal zones (Dan et al., 2011). A spit is a long narrow extension of sand or shingle, which has one end attached to the coastline and the other projecting out to sea, or into an estuary (Hordern, 2003). The sand spits are dynamic in nature and show changes in their size, shape and position.

The natural formation of a spit near a river’s mouth: Sand spits near river outlet are formed by deposition and accumulation of sediment that are derived from alongshore drift and/or due to the river input flow (Rajasekaran et al., 2014). Spits can be formed at the river mouth under the influence of two main forces that control the deposition of the transported sediments by cross shore or longshore drift along the coastline (Leatherman, 1979; Jackson, 2014). The key factors of sedimentation and spit growth are the kinetic energy of the coastal drift and the flow of the river water. If the flow of the river water into the sea at an estuary is stronger than the drift, sediments are deposited at the outlet. The deposition of sediment forms a spit but its shape changes because of wave refraction (Jackson, 2014). Examples were found in different parts of the world such as the cases for Sahalin spit in the Danube Delta (Dan et al., 2009), La Banya spit in Ebro Delta (Jiménez and Sánchez-Arcilla, 2004), Goro spit in Po Delta (Simeoni et al., 2007) or Damietta spit in Nile Delta (Frihy et al., 2003). In addition, Rosetta Promontory is also faced this phenomena that spit formed at the end of the river mouth as described by Frihy and Lawrence (2004).

If the energy of the flow water discharged from the river is at lower than the energy of the alongshore drift, the deposition can occur inside the estuary. The spit formed in the River Nile Estuary at Rosetta Branch is after the establishment of Aswan High Dam. The aim of this work is to evaluate: (I) The spatial and temporal variation of the Rosetta Estuary spit. (II) The factors that enhance the spit formation inside the estuary and its migration. This data is important for the integrated coastal zone management (ICZM) and to predict the configuration of the spit and its environmental effects. The formation of the spit inside the estuary is indicator of the change of the estuarine ecology and its water salinity. They also impact on the stability of the shoreline, salt water intrusion, social and economic value of estuaries.

1 Materials and Methods

The skeleton of this work was designed to study the spatial and temporal variation of Rosetta Estuary spit through collection of historical data and recent information from field and laboratory work.

1.1 Site description

The Nile Delta coast in Egypt is an active system formed by the Nile River sediments discharged into the Mediterranean Sea through the Rosetta (Rashid) and Damietta promontories (Khafagy et al., 1992). Rosetta Branch promontory is located on the eastern side of Abu Qir Bay at about 60 km to the east of Alexandria City, Egypt (Figure 1). Since 1965, the water flow and sediments discharged by Rosetta Branch to the sea have been reduced due to the establishment of Aswan High Dam and the water control work along the River Nile (El-Sayed et al., 2007).

Rosetta Estuary is suffered from erosion due to elimination of sediments coming with the Nile water after the establishment of Aswan High Dam. The rate of erosion was 30 m/y until sea wall was built at the eastern and western entrance of the Rosetta Estuary. The horizontal erosion was stopped but the vertical erosion still work (Frihy, 1996).

1.2 Historical information

Nine profiles from successive sections of Rosetta Estuary have been determined by Coastal Research Institute team as shown in Figure 1 with code numbers 1, 2, 2.5, 3, 3.5, 4, 4.5, 5 and 5.2 during 2007. Historical Google Earth (GE) satellite images have been used to trace the locations of the spit at the Rosetta Estuary. Wind rose has been drown to check the dominant direction.

The amount of water discharged from Rosetta Branch of the Nile River according to the flow of water through Edfina Barrage during the period from 1956 to 2015 have been introduced. The source of the data is obtained from the Irrigation Department, the General Authority of Delta Barrages of the Egyptian ministry of Water Resources and Irrigation and from the published data of Said and Radwan (2009).

1.3 Field and laboratory work

1.3.1 Salinity

During this work in March 2015, salinity profile values within the Rosetta Estuary water were measured in the field using Aquaread CTD AP-5000 connected with GPS at sites R1, R2, R3 and RB as shown in Figure 1. Inflatable rubber boat (Zodiac) with engine 30 horse was used for CTD field measurements.

1.3.2 Grain size analysis

Grab sampler has been used to collect surface sediment samples from R1, R2, R3 and RB sites. Mechanical analysis was carried out for the collecting sediments using electrical shakers with series of thieves. Sediments mean grain size, sorting, kurtosis and skewness were estimated and calculated using the grain size statistics program GSSTAT version 2.04 manufacture by USGS science for a changing world.

2 Results and Discussion

The construction of Aswan High Dam was completed in 1968. It resulted the low flow released from Edfina Barrage to the end of Rosetta branch, which is not able to push the sediments away from the estuary (Frihy, 1996). As a result, the sediment deposits inside the estuary as spit. The case where the discharge from the estuary is negligible, the deposited material succeeds to close the outlet. This case is already happened at Slapton Ley and Low Pool in the South West Peninsula of England (Kidson, 1972).

To understand the temporal and spatial variation of the spit, satellite images presented by Google Earth program have been used. Six successive images of Rosetta Estuary during different periods starting from February 2007 to September 2015 were investigated. Figure 2 (A-F) illustrates that the spit was formed already inside the Rosetta Estuary. It can be observed that the Rosetta Estuarine spit migrated toward the riverside. This indicates that Rosetta Estuarine environment is an extremely dynamic. Similar observations are also made by Weitkamp (1994).

Rate of spit migration is estimated from the analysis of historical images and field measurements (Figure 2). The Google earth ruler option has been used to measure the distance between the locations of the successive spits. Regression analysis has been carried out for estimation of the spit migration rate and it revealed positive linear trend with 0.0005 slope. The calculated value (0.9587) of the coefficient of determination (R²) indicated high correlation between the displacements of the spit with the time. The analysis indicated that the displacement of the spit was about 1.5 km during 8 years with rate of about 187 m/y (Figure 3). Actually, during the 8 years study, that rate varied inter annually depending on the variations in wave characteristics, river discharge. This is in agreement with the finding of Anthony (2015).

2.1 Evolution of Rosetta Estuary spit

Two spits at the Rosetta mouth were identified during 1986-1993 (Frihy and Lawrence, 2004). The two small spits protrude toward the main entrance of the Nile from both sides of the Rosetta inlet attached to the shore at one end, and extend parallel to the coastline for 0.6 km and relief of 0.5m above sea level. The platform of the spits indicated that the western spit was formed from the prevailing longshore current to the east, whereas the eastern spit was associated with the local reversal of the prevailing current to the west.

The obstruction of the spit between the period from 2003-2007 and due to the low discharges released downstream Edfina Barrages to the sea after construction of High Aswan Dam (HAD), spit was formed inside the end part of the estuary. It formed and grew perpendicular to the eastern shoreline of the estuary and enlarged in size with time. It nearly enclosed the outlet as shown in Figure 2 (A). Different profiles of Rosetta Estuary descriped the shrinkage of the navigational channel of the estuary at profile P4.5 due to the spit formation during 2007 (Figure 4). This problem causes navigation hazards, and nearly closure of the estuarine outlet. In addition, it causes the economical bad effect due to the decrease the cross sections of Rosetta outlet region. It also control the emergency discharges of the flood river water to the sea. This problem was solved by dredging. Dredging works region of Rosetta Promontory increased the capacity of Rosetta promontory to pass high amount of discharge as discussed by Mahmoud et al. (2006). Mohrig and his group discussed the case of closing the outlet due to increasing sedimentation into the estuary inlet and harbor channels, which caused navigation hazards, and closure of the lagoon inlets (Mohrig et al., 2000).

Coastal sand barrier spits close to river mouth are often extremely dynamic, which involves transport of sediments in both the long shore and cross-shore wave directions with rapid migration rates (Simeoni et al., 2007; Dan et al., 2009; 2011). The investigation of the spit formed in Rosetta Estuary revealed that its formation is a result from the coastal currents induced by waves that carry the coastal sediments to inside the estuary.

Considering the Rosetta Estuary sand spit shape and position evolution, three major scenarios can be recognized. (I) Spit encroaching the inlet mouth seaward before establishment of Aswan High Dam and between 1986 and 1993 (Frihy and Lawrence, 2004; Kaiser, 2014). (II) No spit formation, just after undertaken the dredging of the spit. (III) Spit forming towards river side after the construction of Aswan High Dam and as consequences of the limitation of of water discharge.

2.2 Factors affecting the formation of the spit inside the Rosetta Estuary

Different human intervention like building of dams, dredging in river mouth and deforestation in the upstream side often decrease or increase the amount of sediments reaching the coastal zone (Stanica et al., 2007; Nagarajan et al., 2015). After the sediment budget of the spit inside the Rosetta Estuary is the human interference by construction of Aswan High Dam and the hydraulic control structures. The factors affecting the formation of Rosetta spit can be summarized as the following.

2.2.1 Reduction in water discharge to Mediterranean Sea through Rosetta Estuary

Limitation of water discharge from River Nile is one of the main source that enhanced accumulation of sediments to form spits originated from the coastal area. Figure 5 shows the annual discharge of Nile River water from Edfina barrage through Rosetta Estuary during the period 1956-2015. The original source of these data is from the General Authority of Irrigation from Delta Barrages of the recent values from 2008 to 2015 and the rest values are reported by Said and Radwan (2009).

Before the construction of the Aswan High Dam, Nile River discharged to the Mediterranean 62 km³ during 1912-1942 (Gerges, 1976) and 46.93 km³ during 1956-1965 through its branches. After 1966 with the establishment of the High Dam the discharge to the sea remarkably decreased to be 3.92 km³ during 42 years (1966-2007) through Rosetta estuary (Said and Radwan, 2009).

2.2.2 Change in salinity distribution

Temporal increase of the salinity values was found in Rosetta Estuary. During 1986-1987 the salinity range in Rosetta Estuary was 0.2-11.0 ppt with average 4.06 ppt (Samaan et al., 1996) and increased to 2.8-38.3 ppt with average 18.4 ppt during 2004-2005 (Gharib, 2006). Recent measurements of the salinity during this study in 2015 indicated that the salinity of last part of Rashid Estuary until 1.5 km turned to marine water with average 39.54 ppt. Within the estuarine water column, the measurement of salinity revealed variation that the upper water layers have relatively lower salinity than the middle and the bottom layers but still all are recognized as saline water as shown in Figure 6. This indicated that the seawater entered to Rosetta Estuary instead of the river fresh water due to the reduction of the flow of river water after the construction of Aswan High Dam and the river hydraulic structures such as barrages.

Increase of salinity of the Rosetta Estuary my affect the adjacent agricultural area. Salt-water could be introduce as leakage into the agricultural land around. Saline resistant plant only can tolerate and grow under the increase of salinity.

One of the adverse effects of the entrance of the marine water to the Rosetta Estuary in conjunction with the wave attack, the rocks of the side border of the estuary at Borg Meghazil site was collapsed. An project was set to repair the side border of the estuary by the Authority of Shore Protection with consultation of Coastal Research Institute.

2.2.3 Limitation of sediments flow with river water to Rosetta Estuary

Seawater with loss of its kinetic energy to transport sediment can deposit and build up layers of sediment via gravity. Deposition occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of particle weight and friction, creating a resistance to motion. The main direction of sedimentation is obvious as in Figure 7 from North West to inside the Rosetta Estuary. It shows the submerged sedimentation north of the spit during 2014.

2.2.4 Erosion state

Erosion of the headland of Rosetta is the main source of sediments in Rashid coastal area. After the construction of the eastern and western seawalls, the behaviour of the erosion was changed to be vertical in front of the seawalls and in the coastal zones west of the western seawall as shown in Figure 2 and east of the eastern seawall. These sediments can transport either long-shore or cross-shore and enter the Rosetta Estuary.

2.2.5 Wave action

The wave actions can be demonstrated by the wave rose of Rosetta coast as shown in Figure 8. According to the averaged wave climate of five years of actual measurements between 1986 and 1990 in Rosetta, the majority of wave directions are originated from NW WNW, NNW. Small portions of waves arrived from N, W, NNE, SSW and NE especially in March and April were monitored (Frihy and Dewidar, 2003; El Sayed et al., 2007). The dominant wave is from NW direction which perpendicular to the mouth of Rosetta branch. The satellite image in Figure 9 shows the propagation of the waves inside the estuary. The marine origin waves attack the eastern border of Rosetta Estuary and destroy some parts of this border at Borg Meghazil site.

The lower rate of discharge from the river is in coincidence with high energy of sea wave, the spit can be developed in the river side direction. These driving forces enhances the migration of the spit toward riverside. The incoming wave action towards the shore plays a major role in shaping the sand spit. Spits increase in height above the water only when the waves and the shore drift move from the direction of their land connection. Relations between the variables of current direction, wave direction, wave energy, the amount of sediment available, and depth of water determine the direction the spit formed (Evans, 1972).

2.2.6 The water (sea) level fluctuation

The evolution of the sand spit may be attributed to sea level fluctuations as described by Bernardes and Rocha (2007). The high tide permit the marine water to enter the estuary. In addition the sea level rise due to the climate change increase the possiblity to enter the saline water into the estuary.

2.2.7 Other environmental forces

Natural disturbances caused by the environmental forces include winds and tidal currents (Miththapala, 2013). Short-term natural events causing significant stress to estuarine environments are primarily weather related: hurricanes, tropical storms or northeasters.

2.3 Grain size analysis of Rosetta Estuary sediments

Sediments samples were collected from the middle of the stream of Rosetta Estuary at four sites (R1, R2, R3 and RB) at the interval of 1/2 km distance as shown in Figure 1. The dominant mean grain size of the sediments under investigation is fine sand with 98% with no gravel or clay. Table 1 lists the percentages of sediment gradient components. Table 2 categorized the results of sediment grain size analysis as mean grain size, sorting, skewness and kurtosis and their description. The sorting of the surface sediments indicated that there is similarity of the sorting category of the sediments in front of the estuary and inside it as moderately well sorted or moderately sorted. This indicated that the estuarine sediments contains allochthonous sediments, which is probably from the coastal area. The investigation of the grain size analysis of the sediments revealed that due to diminish of alluvial river sediments the sand detected in the Rosetta Estuary is originally transported from the coastal area.

The results of skewness indicated that the distribution of the grain size is near symmetrical and it skewed slightly toward the coarse size at the end of the estuary (R3) as shown in Table 2. This is in agreement with the finding of King (1972).

3 Conclusions

Estuaries are highly dynamic ecosystems that are very vulnerable both to natural and anthropogenic disturbances. In order to study the morphology of Rosetta Estuary especially its depositional land forms such as spit, this work has been done. The investigation of Rosetta spit revealed that through the period of study from 2007 to 2015 its shape and location were changed with time. Spit was formed inside the Rosetta Estuary. Different factors were discussed, which affecting the formation of the spit inside the Rosetta Estuary. The first factor is the Reduction in water discharge to Mediterranean Sea through Rosetta Estuary, which mainly occurred after the establishment of the Aswan high Dam. The difference in salinity between the river water and the recent estuarine water is considered as the second factor. Limitation of sediments flow with river water to Rosetta Estuary is one of the important driving force enabling the formation of the spit inside the estuary by allochthonous sediments originally from the coastal area. Mechanical analysis of the geological samples confirm the transportation of the coastal sand to inside the Rosetta Estuary to form the spit. Erosion state and wave action played an important rule for the displacement of the spit toward the riverside. The water (sea) level fluctuation can enhance the flow of the seawater toward the riverside.

Migration of the Rosetta spit toward riverside was recognized. The rate of spit migration was estimated as 187 m/y through 8 years from 2007 to 2015. During 2007, spit was nearly close the outlet of Rosetta Estuary and this problem was solved by dredging.

Factors enhance the formation of spit inside the Rosetta Estuary and its migration were diminish in water discharge, change of water column salinity, limitation of sediments transported from river, erosion state of Rosetta promontory, wave action, water level fluctuation, and other environmental factors. The forecasting of the new location of the spit is important. The control of the water flow from Edfina Barrage is important to balance the high salinity at the end of the Rosetta Estuary to reduce its environmental effects.

3.1 Added value of the paper

Spit is an example of harmonious coexistence between the nature and humans with unique abundance of natural and cultural properties. Spit cultural landscape reflects changes in the natural environment and local community, the need for deliberate action and ability to choose the appropriate forms of activity in order to survive in the hanging environment. The updating geomorphological changes is very important prerequisite for the protection of the Rosetta Estuary. Its study is important for the overall and sustainable strategy for integrated coastal zone management and forecasting of the future coastal changes. Spit formation and migration inside the Rosetta Estuary is considered as indicator to test environmental changes in the study area, which not only affect the estuarine ecology, they also affect the stability of the shoreline, water salinity change, social and economic value of estuaries. Increase of salinity of the Rosetta Estuary may be effect on adjacent agricultural area due to salt-water intrusion.

The factors that enhance the spit formation inside the estuary and its migration are listed. This data is important for the ICZM approach and sustainable development of the study area, and also to predict the new site for the spit and its environmental effects.

Previous study that has been discussed the Rosetta spit was published since 14 years ago and includes data series lasted in year 2000. In order to update this document, this work was planned and prepared for study the spatial and temporal variation of the spit with its new form.

3.2 Main differences between two studies

The previous work study the Rosetta promontory during the period 1922-2000. This period characterized by severe erosion for the Rosetta headland. This study focused on the shoreline retreat. During this period, all the formed spits were found at the tip of the promontory and it has different nature than the recent spits. Its movements were due to the erosion of the Rosetta headland. During this work, the period of study was from 2007 to 2015. The behavior of the spits are different from the old period (1922-2000). It formed and migrated inside the Rosetta Estuary with different source of sands.

The previous work calculated the rate of the retreat of the Rosetta promontory land to estimate the amount of lost land. This information was important during the period of study of this previous work but after the establishment of the Rosetta seawalls the shoreline is fixed in front of the seawall but the erosion is still happen but vertically. In this work, the rate of the spit movement toward riverside was estimated and simple mathematical model deduced to estimate the future migration.

The previous work did not mention the salinity differences inside the estuary. In this work, field measures of salinity through the water columns indicated the present of saline water replaced the freshwater inside the Rosetta estuary. The presence of saline water inside the estuary result salt-water intrusion into the agricultural land around. This could alter the regime of selected plants agriculture in this area. In addition, the saline water played an important role in destroy part of eastern border of the estuary at Borg Meghazil with the attack of the sea waves, which can enter the estuary. A project was set to repair the side border of the estuary by the Authority of Shore Protection with consultation of Coastal Research Institute.

The previous work discussed the factors, which affect the land retreat during erosion of the promontory. In this work, the factors possibly case the spit migration were discussed.

The previous work discussed the land variation using different forms of types of satellite images, which are very expensive. In this work, illustrate that the this kind of work deals with big objects such as spit can be study temporally using the new feature of the Google Earth program images.

Acknowledgments

The author would like to acknowledge the staff and marine teams of the Coastal Research Institute for the historical information about the investigated area. In addition, I thank the General Authority of Irrigation from Delta Barrages for the data of the water discharge through Edfina barrage.

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