Destruction of Derna: why was flooding so bad in Libyan port city?

What do we know about the flooding?

Parts of eastern Libya have been hit by devastating flooding after Storm Daniel pounded the country’s Mediterranean coast. Confirmed death tolls given by officials so far have varied, but all are in the thousands, with thousands more on lists of the missing.

Apocalyptic footage of widespread damage has emerged from the port city of Derna, where officials estimate that as many as 20,000 people may have died:

The city with a population of 90,000 is bisected by the Wadi Derna, a seasonal river that flows from highlands to the south, and is normally protected from flooding by dams.

On Sunday night two dams collapsed, unleashing raging waters that swept away residential neighbourhoods on both banks of the river.