The coastal city of Alexandria in Egypt, which has survived invasions, fires and earthquakes since it was founded by Alexander the Great more than 2,000 years ago, now faces a new threat of climate change.
Sea level rise threatens to inundate archaeological sites, prompting authorities to erect concrete barriers in the sea to break the tide. A severe storm in 2015 flooded large parts of the city, killing at least six people and the collapse of about twenty homes, see Figure (1).
Alexandria, the country's second largest city, is surrounded on three sides by the Mediterranean and backed by a lake, making it uniquely vulnerable to sea level rise caused by global warming and melting polar ice caps.
In the late 1940's and 1950's, it was a haven for writers and artists who attracted wealthy Egyptian and foreign tourists for its beauty and charm. Today, more than 60 kilometers (40 miles) of the waterfront makes it a prime summer destination for Egyptians, but many of its most famous beaches are already showing signs of erosion.
The United Nations' Intergovernmental Panel on Climate Change has warned that global sea levels could rise by 0.28 to 0.98 meters (1–3 feet) by 2100, with "serious repercussions for coastal cities, deltas and low-lying countries."
Experts acknowledge that regional differences in sea level rise and its impacts remain poorly understood. But in Alexandria, a port city of more than 5 million people and about 40% of Egypt's industrial capacity, there are already signs of change.
The Egyptian Ministry of Water Resources and Irrigation says that the sea level rose at an average rate of 1.8 mm per year until 1993. Over the next two decades, the water level rose to 2.1 mm per year, and since 2012 it reached 3.2 mm per year, which is enough to threaten building foundations.
The land on which Alexandria is built, along with the surrounding Nile Delta, is sinking at roughly the same rate, in part due to upstream dams that prevent the regeneration of silt and the extraction of natural gas. This is expected to exacerbate the effects of sea level rise, with potentially dire consequences.
A 2018 study predicted that as many as 734 square kilometers (more than 283 square miles) of the Nile Delta could be submerged by 2050 and 2,660 square kilometers (more than 1,020 square miles) by the end of the century, affecting 5.7 million people .
In El-Max neighborhood, hundreds of people were forced to leave their homes after severe floods in 2015. The Ministry of Housing built nine housing complexes to house them after the area was declared unsafe.
The archeological sites in the city are those that have survived its turbulent history are truly threatened.
The Pharos Lighthouse, once among the tallest buildings ever built and one of the Seven Wonders of the Ancient World, was hit by an earthquake in the 14th century. The famous Library of Alexandria was completely burned down when Julius Caesar set fire to a hostile fleet in 48 BC.
But Citadel of Qaitbay, a medieval fortress built on the ruins of the lighthouse at the end of a narrow peninsula jutting out into the sea, still looms over the sprawling central port of the city, on the other side of the modern Library of Alexandria, a research center has opened. In 2002.
The castle is particularly vulnerable. Increasingly strong waves and currents pushed to the foundations, forcing authorities to install a long series of concrete sea barriers visible from the downtown waterfront, known as the Corniche.
Inland sites are also at risk, including the Catacombs of Kom al-Shuqafa and the Cemetery of Shatby, the sites have the lowest terrain, dating back to the second century AD with architectural styles inspired by ancient Egypt. The catacombs and other sites such as the Shatby cemetery were flooded in 2015, as shown in Figs. 2 and 3).
Prophet Daniel Street in the city center is one of the oldest streets in the world, and today it passes in front of a mosque, synagogue and St. Mark's Church, the seat of the Coptic Christian Patriarchate.
We realize that this street, which has survived for hundreds of years, could be underwater in the years to come, in our lives.
"Every year the waves are stronger than they were in the previous year. The winters are harsher and the summers are hotter."
Groundwater infiltration into pores or fissures of rock and soil has a great influence on the engineering mechanical properties of rocks and soil. For example, groundwater intrusion will deform rock and soil seepage, which will directly affect the stability and integrity of structures inside, buildings and foundations; The change of groundwater level will alter the effective stress field in soil / rocks, resulting in soil recovery or stabilization; Changing the groundwater level will also change the soil moisture content, which will also change the mechanical properties of the soil and cause the bearing capacity of the soil.
Rising groundwater levels due to sea level rise and heavy rains will inevitably lead to a decrease in the carrying capacity of sandy soils as catacombs are excavated. The bearing capacity of sandy soils under water saturation conditions is less than that of unsaturated soils, and the average reduction range is 26 − 25%, [1–2].
This study provides a comprehensive analysis of the safety of underground antiquities. The safety analysis not only includes failure analysis but the effect of groundwater level rise around underground structures on the differential settlement has been thoroughly investigated.
Since about 1975, a series of industrial and agricultural changes have led to a remarkable increase in the groundwater level in the city of Alexandria, especially Lake Mariout (400 meters from the catacombs), which is the main source of this groundwater inside the catacombs along with the heavy rains in the winter. The ice surface gradually reaches a height higher than the height of many underground "catacombs" monuments. As a result, some intense water leakage occurred at various locations within these underground structures.
The rise in groundwater levels can be in dramatic places due to subterranean development and this reduces the bearing capacity of rocks. The rise or fall of the water level may have dire consequences for the stability (structural effect) in terms of flooding of the subsurface parts. In some cases, changes in the groundwater level have had significant impacts on stability.
After the groundwater level decreased in 1995, the second level of the catacombs was opened to visitors but the lowest level is still submerged with some groundwater so far.
All the underground ruins (catacombs) in Alexandria suffer from a common problem: water leakage. Over time, the infiltration may lead to increased humidity to excessive levels within the void created by the structure. This moisture and water seepage within the structure will cause cracking and peeling of rock surface layers and the formation of salt blooms and sub-fluorescences that can be dangerous to wall paint layers. The interaction of moisture with carbon dioxide may further degrade, resulting in the potential leakage of unwanted gases or dangerous chemicals from the surrounding soil.
The analysis of the integrity of surface and subterranean excavations in the Shore cemetery not only includes failure analysis but also the effect of high water level around underground structures on the differential settlement was investigated.
Analysis of water samples collected from El Shatibi cemetery indicates that the water that floods all the cemetery grounds is sea water and sewage water from surrounding buildings. The rise in groundwater levels can be in dramatic places due to subterranean development and this reduces the bearing capacity of rocks. The rise or fall of the water level may have dire consequences for the stability (structural effect) in terms of flooding of the subsurface parts. In some cases, changes in the groundwater level have had significant impacts on stability.
Surface and underground excavations at El Shatbi suffer from a common problem of water leakage. Over time, the infiltration may lead to increased humidity to excessive levels within the void created by the structure. This moisture and water seepage within the structure will cause cracking and peeling of rock surface layers and the formation of salt blooms and sub-fluorescences that can be dangerous to wall paint layers. The interaction of moisture with carbon dioxide may further degrade, resulting in the potential leakage of unwanted gases or dangerous chemicals from the surrounding soil.