Flash flood: the seasonal flood in a wadi.
A wadi is a river in an arid region. It is usually dry, but in the winter, there can be heavy rainfall that can cause unexpected and violent “flash floods” in the areas downstream. One of the armies of Alexander the Great suffered serious losses in the Gedrosian Desert because it was caught in a flash flood.
It so happened that the army bivouacked by a small stream, for the sake of the water it afforded, and about the second watch of the night it was suddenly swollen by rain. The actual rain was falling far away out of sight, but the stream nevertheless grew into such a torrent that it drowned most of the camp-followers' women and children and swept away the royal tent with everything it contained, and all the surviving animals, while the troops themselves barely managed to escape, saving nothing but their weapons - and not even all of those.note[Arrian, Anabasis 6.25.1.]
The problem for ancient farmers was not the lack of water, but the fact that it was available for only a short period in the winter. Typically, agriculturalists tried to divert the waters of flash floods by building dams, sluices, and cisterns. They have been found in Libya (e.g., at Suq al-Awty) and Saudi Arabia (in the Al-‘Ula oasis).