Why can heavy rain cause dangerous floods in Arabian Desert wadis?

In the Arabian Desert, most drainage channels are ephemeral, meaning they remain dry for long periods and only flow when rainfall is heavy. Because these wadis are typically dry, they are often not prepared to handle sudden, large volumes of water.1

When torrential rains occur, they trigger infrequent but highly destructive floods that move rapidly through these drainage basins. These events are dangerous because the water arrives suddenly and with significant force, catching those in the area off guard. While these floods are powerful enough to shape the landscape more effectively than wind, they are largely contained within the wadi systems, except in areas where the channels have been dammed.12