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.
