Much of your reporting of the Derna flood disaster (Libyans call for inquiry as fury grows over death toll from catastrophic floods, 14 September) has focused on the state of the dams and the political situation, together with the extreme weather event, but there is another factor that makes the town highly vulnerable. It appears to be built on an alluvial fan – a cone-shaped feature of the landscape, formed over millennia by sediment deposition from the river. These features form where steep rivers exit mountain upland, spread out and split into a series of distributary channels down the fan (like a delta).
In many cases around the world, as in Derna, the water flow has been engineered into a single, embanked channel on one flank of the fan and building allowed right up to its margins. In an extreme event, as has happened here, the enormous water flow has reverted to its natural pathways, overflowing the constrained channel and flowing in multiple pathways down the fan feature on which most of the town is built.
Such locations are particularly vulnerable to the effects of extreme flows, and demonstrate the dangers of trying to control nature by large-scale channel diversion and embanking. It is essential, especially with climate change, that natural settings and situations are recognised and that we work with nature much more to manage floods and rivers sustainably.
Prof Janet Hooke
University of Liverpool
• On 2 November 1925, the Eigiau dam in north Wales broke, releasing 70m tons of water down the valley. Ten adults and six children were killed. Since then, no deaths have resulted from UK dam breaks. There are more than 2,000 reservoirs in the UK that contain over 25,000 cubic metres of water. It is shocking to see how many buildings today are at risk from dam breach inundation on the flood-risk maps on the government’s website.
Planning permissions are still routinely given for new developments on floodplains and in dam breach shadow zones. This should surely be illegal because of the growing number of lives destroyed for those living in flooding homes. The Libyan dam break demonstrated how catastrophic the effects of a major dam break can be. It may be only a matter of time before UK dams are similarly challenged by the increasingly extreme weather events we are experiencing here.
Susan Roaf
Oxford
• The flooding in Derna is no doubt the result of the climate crisis and may well have been prevented by competent governance (The Guardian view on Libya’s floods: humans, not just nature, caused this disaster, 13 September). But few people know that during Muammar Gaddafi’s tenure there was a strategic plan to develop all of Libya’s vast coastline from Tripoli to the Egyptian border for tourism, creating new towns, improving conservation and modernising a thousand miles of infrastructure.
A coastal pipeline was being built prior to the war to bring Saharan fossil water to irrigate what once was the Roman empire’s breadbasket. In my job as a landscape architect I designed four national parks, including Al Guarsha in Benghazi.
Numerous new hospitals and universities were under construction, all of which came to a crashing halt at the onset of the “green revolution” that was crushed by the regime, before Gaddafi in turn was justly defeated in the ensuing conflict. After David Cameron’s bold promises of democracy and freedom made in Benghazi at the end of the war, what part has Britain played in the rebuilding of post-conflict Libya?
David O’Brien
Director, Dobla Landscape Architects
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