Depressed nappe over a weir: pressure condition below the nappe A depressed nappe forms when air cannot freely circulate beneath the falling sheet of water (nappe). Identify the correct pressure condition beneath the nappe.

Concept/ApproachA depressed nappe occurs when air is not properly ventilated below the nappe, creating sub-atmospheric (negative gauge) pressure beneath it. If vented, the pressure would be atmospheric and the nappe would be free, not depressed.

Final Answerthe pressure below the nappe is negative