Nappe (water)
In hydraulic engineering, a nappe is a sheet or curtain of water that flows over a weir or dam. The upper and lower water surface have well-defined characteristics that are created by the crest of a dam or weir. Both structures have different features that characterize how a nappe might flow through or over impervious concrete structures. Hydraulic engineers distinguish these two water structures in characterizing and calculating the formation of a nappe. Engineers account for the bathymetry of standing bodies (like lakes) or moving bodies of water (like rivers or streams). An appropriate crest is built for the dam or weir so that dam failure is not caused by nappe vibration or air cavitation from free-overall structures. Weirs There are three types of nappe that form over the crest of a weir, depending on the air ventilation structure of a weir: free nappes, depressed nappes, and clinging nappes. A free nappe, which is ventilated to maintain atmospheric pressure below, does not come into contact with the underside of the weir. A depressed nappe is partially ventilated, which creates negative pressure beneath the nappe. The negative pressure leads to a 6% to 7% increase in discharged water compared to a