Wednesday, May 29, 2024

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More details regarding the Wadi Derna dams.


Google Earth image of the lower dam and the city of Derna.

It is estimated that the dam collapse in Wadi Derna, Libya resulted in the death of approximately 20,000 individuals.

The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.

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As the devastating effects of the Derna flood in Libya unfold, new details are coming to light about the two dams located on Wadi Derna that caused the disaster. The most significant source of information is a website called Hidrotehnika, but I am unable to link to it due to security concerns. This site contains data on the two dams.

There appears to be uncertainty surrounding the names of the two dams, therefore I will refer to them as upper and lower.

According to the website, the top dam measured 45 meters in height and could hold 1.5 million cubic meters of water. The lower dam was taller at 75 meters and had a storage capacity of 18 million cubic meters. I find this information questionable based on the data from Google Earth’s digital elevation model.

Hidroprojekt, a company from Beograd in the former Yugoslavia, built both structures for the Ministry of Agriculture in Libya. The construction took place between 1973 and 1977.

New footage on Twitter has surfaced depicting the collapsed top dam.

The structure has clearly experienced a total failure. On Google Maps, there is a photograph of the dam before it collapsed.

The upper dam on Wadi Derna prior to failure.

The image on Google Earth depicts the upper dam on Wadi Derna before it collapsed.

The presence of shrubs growing on the dam crest is a clear indication of neglect in maintenance. Additionally, I have observed that there is no emergency spillway present. It is probable that the dam failed due to the spillway’s inadequate size in handling the large amount of water entering the catchment, resulting in overflow.

Dams are remarkable structures with a good (but imperfect) safety record. However, like all infrastructure they need to be maintained and it is essential to consider regularly whether they are fit for purpose. Climate change is an existential threat as rainfall totals start to exceed the design capacity. In such cases, new engineering is needed. In properly functioning societies this can happen (although it can be argued that recent events suggest that some dams are being inadequately maintained in both the US and the UK).

There are numerous takeaways to be gleaned from Derna. Regrettably, there will be countless other locations, particularly in nations where societal systems are not operating properly, where similar dangers exist.

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