Nappe (water)
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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.[1] Both structures have different features that characterize how a nappe might flow through or over impervious concrete structures.[2] Hydraulic engineers distinguish these two water structures in characterizing and calculating the formation of a nappe.[3] 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[4] or air cavitation from free-overall structures.[5]
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.[6] A free nappe, which is ventilated to maintain atmospheric pressure below, does not come into contact with the underside of the weir.[7] 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 free nappe.[8] Clinging nappes have no air beneath, and the stream flows along the face of the weir. The shape that fills in this area is called an ogee. Discharge for these weirs is approximately 25% to 30% more than free nappes. The geometry of a weir dictates the coefficient of discharge that passes through the crest, which is proportional to the nappe formation.[9] Engineers solve for the amount of discharge and the cross sectional area of a river to calculate the adequate shape of the weir that should be implemented.