nappe
Geological nappe whose underside is not in contact with the overflow structure and is at ambient atmospheric pressure.
nappe: the sheet of water that flies off a spillway
A nappe is the continuous sheet of water that leaves the crest of a spillway, weir, or dam and travels through the air before hitting the tailwater below. The term comes from the French word for tablecloth, which captures the image well: a thin, coherent layer of fluid suspended in space. The nappe is characterized by having atmospheric pressure on both its upper and lower surfaces, which distinguishes it from other flow conditions where water remains in contact with the structure.
Nappes form on spillways when flow velocity and crest geometry align to create separation between the water surface and the underlying concrete or masonry. The water accelerates over the crest lip, drops in pressure, and peels away cleanly. The thickness of the nappe varies with discharge: at low flows it may be thin and unstable, while at design discharge it becomes a thick, stable sheet. The shape and behavior of the nappe directly affects the impact zone and erosion patterns in the stilling basin below.
Nappe failure and air demand
If a nappe collapses back onto the spillway surface, the space beneath it becomes a partial vacuum. Water rushes inward to fill this void, creating violent oscillation and cavitation damage to the concrete. This is why properly designed spillways include aeration slots or steps along the face: they allow air to flow into the space under the nappe and maintain atmospheric pressure. Without adequate aeration, even well-constructed spillways fail rapidly under sustained high flows.
The term nappe is sometimes confused with the geological nappe (a large sheet-like body of rock), but in hydraulic engineering it refers exclusively to the water sheet. Designers must account for nappe characteristics when modeling stilling basins, calculating impact velocities, and setting elevation of the tailwater. The nappe's width, depth, and trajectory determine whether the basin geometry will diffuse energy effectively or create dangerous jet action.
Free nappes contrast with clinging nappes, where water remains pressed against the spillway face under certain flow conditions. The transition between these states depends on discharge, surface roughness, and air pressure conditions. Operational stability and structural durability depend on understanding which state is present at any given flow rate.