A Nullah or Nulla (Hindi, also Nallah in Punjabi) is an arm of the sea, stream, or watercourse, a steep narrow valley. Like the wadi of the Arabs, the nullah is characteristic of mountainous or hilly country where there is little rainfall. In the drier parts of India and Pakistan, and in many parts of Australia there are small steep-sided valleys penetrating the hills, clothed with rough brushwood or small trees growing in the stony soil. During occasional heavy rains torrents rush down the nullahs and quickly disappear. There is little local action upon the sides, while the bed is lowered, and consequently these valleys are narrow and steep.
In Hong Kong, a nullah is typically a concrete lined channel designed to allow rapid run off of storm water from high ground, to prevent flooding of urbanised coastal areas.