The flood in Libya left tons of silt filling streets and homes in the eastern city of Derna, and as rescue teams race against time to remove the devastating effects of this rare event, experts say that the removal of silt, as happened in the recent flood in Pakistan, must receive great attention.

The term "silt" is generally used to refer to sediments washed away by floodwaters, but experts believe that the best description is "sedimentary soil", as this description allows them to distinguish between 3 types of sediments, namely silt, sand and gravel, the first indicates a certain degree of size of sediment ranging between 0.002 mm and 0.063 mm, while the second refers to a size ranging between 0.063 mm and 2 mm, and the latter ranges between 2 and 60 mm.

Threatened areas are keen to know the type of sediment that floods bring, to take appropriate engineering measures (Reuters)

Know the type of sediment

Silt is the most common among flood deposits, and its density exceeds the density of water two and a half times, so its accumulation after floods may cause pressure on buildings, and prevent the safe re-paving of roads, so it must be expedited to remove it, according to Abbas Sharaki, professor of geology and water resources at Cairo University, in telephone statements to Al Jazeera Net.

All threatened areas are keen to know what kind of sediment the floods bring, to take appropriate engineering measures, as the study published in the May issue of the Journal of Hydrology shows, which tracked sediment quality during flooding events in the Huangfu River basin in the north of China's Loess Plateau, finding that coarse sand (56.89%) was higher than silt (41.81%) and clay (1.3%).

In the Libyan case, the images indicate that silt is the largest component, and the most appropriate place after lifting it is to use it as a fertilizer for desert lands, according to Sharaki, who warned against returning it back to the riverbed, because this would reduce the carrying capacity of the river, leading to a flood of water with any heavy rains.

Libya's desert lands may be the most suitable place to transport silt (Reuters)

Risk of liquefaction

Despite the presence of 3 types of sediments, attention is always directed to silt, which is attributed by Zakaria Hamimi, vice president of the International Union of Geosciences Ethics in telephone statements to "Al Jazeera Net", to the role played by the size of the silt.

"Silt size is very important, because it controls how well soil drains during future rainfall (i.e. permeability), engineering properties such as strength and endurance, and since fine sediment such as silt drainage for water, with minimal pores, is poor, compared to coarse deposits of sand or gravel, its presence can cause a hazard to buildings, especially in the event of an earthquake, as silt has the ability to liquefy," he says.

"Liquefaction" occurs when the soil significantly loses its hardness and hardness under the influence of applied stress such as vibration during earthquakes or any sudden change in the stress state, in which case the material that is usually solid behaves like a liquid.

Hamimi agrees with Sharaki's view that the desert lands in Libya may be the most suitable place to transport silt to them, warning against dealing lightly with this danger, but at the same time, he points to a situation in which silt is not dangerous, which is when its accumulation is minimal (less than 15 mm), and in an open area, in which case there is no need to remove it, because it will erode and disappear over time.

Rebuilding inside the valley in Derna must be reconsidered (Reuters)

Pakistani Experience

"Until the reconstruction of Derna begins, the level of silt accumulation in the city must be assessed, removed from areas that have witnessed a large accumulation, and reconstruction within the valley must be reconsidered, and construction should be limited to the Delta region, with the necessary safety measures taken when rebuilding Derna dams, similar to the measures taken by Pakistan after the summer flood of 2022," Sharaki said.

While Pakistan has not yet fully recovered from the fallout from this flood, and the restoration of infrastructure from roads and bridges is still underway, it has significantly addressed its dam systems to reduce future risks, and this Libya must start immediately, Sharaki explains.

The famous province of Khyber Pakhtunkhwa in Pakistan runs through 8 major rivers, in addition to mountain ranges, hills, flat green plains and arid plateaus, making it vulnerable to earthquakes, landslides, flash floods, glacier lake floods and melting glaciers.

After last summer's flood, Pakistani authorities took some steps to prepare, installing an early warning system on 7 major rivers to monitor water levels, strengthening dams, and repairing and strengthening the banks of rivers breached by the floods.

New dams in Derna must be equipped with an early warning system (Reuters)

Different construction of dams

"Libya needs to implement similar measures that include rebuilding the two dams in Derna with concrete, because they were built about half a century ago, between 1973 and 1977, by a Yugoslav construction company, using rocks and silt, and this type is known as cumulus dams, which can be acceptable in uninhabited areas, but they are completely rejected in the case of Derna, where under the dam there is a large city, with a population of at least 100,<> people," Sharaki said.

"These new dams must be equipped with an early warning system, and they must be equipped with a natural spillway to drain excess water when the river level rises, and in addition, attention must be paid to periodic maintenance and monitoring of the condition of the dam," he added.

If the financial capabilities are available, Sharaki believes that "the construction of the two dams in Derna can take place within one year, because they are low-rise dams with limited storage capacity, as one of them is 75 meters high, while the other is 45 meters high, they are not compared to a huge dam such as the High Dam in Egypt, which has a height of 3800,<> meters."