hydraulic engineering Hydraulic engineering as a sub-discipline of civil engineering is concerned with the flow and conveyance of fluids, principally water and sewage. One feature of these systems is the extensive use of gravity as the motive force to cause the mov ...

, a nappe is a sheet or curtain of water that flows over a

weir A weir or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. Weirs are also used to control the flow of water for outlets of l ...

dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, a ...

. 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 Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...

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 Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...

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 free nappe. 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. Engineers solve for the amount of discharge and the cross sectional area of a river/stream to calculate the adequate shape of the weir that should be implemented.

Dams

Many pathways of water can enter through a dam structure to produce a well-defined nappe. However, engineers classify dams as either overflow dams, where water consistently flows over or is blocked through a gate on top of crest, or non-overflow dams, which channel water through or around the dam with emergency flood gates. They both range in size. An overflow dam has a similar nappe typology to weirs (free, depressed and clinging nappes). Engineers usually construct an

ogee An ogee ( ) is the name given to objects, elements, and curves—often seen in architecture and building trades—that have been variously described as serpentine-, extended S-, or sigmoid-shaped. Ogees consist of a "double curve", the combinatio ...

crest, which forms a clinging nappe. This increases discharge, reduces atmospheric pressure and decreases the chances of air cavitation occurring.

Problems

Nappe vibration

Nappe vibration is classified in hydraulic literature as fluid dynamic excitation; vibrations are generated by the fluid and the flow characteristics at the point of detachment and impact are critical. This well known phenomenon occurs on free-overall structures (i.e. weirs, fountains or dams) and produce excessive noise on concrete structures. These are undesirable and dangerous on gates and further characterized by oscillations in the thin flow nappe cascading downstream of the crest. The vibrations send out a constant noise as water flows over structure, and may lead to cracks or air cavitation which cause catastrophic failure. The phenomenon results from

Kelvin–Helmholtz instability The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) is a fluid instability that occurs when there is velocity shear in a single continuous fluid or a velocity difference across the interface between two fluids. ...

, the shear forces that occur between two fluids of different velocities.

Cavitation

Cavitation is defined as the explosive growth of vapor bubbles within a liquid. These bubbles are formed and may be carried into areas of higher local pressures, which disappear before by collapse. Surface irregularities on hydraulic structures are prone to experience cavitation. Surface damage on this type of surface will start at the downstream end of the cloud of collapsing cavitation bubbles. Damage from cavitation has been reported in several hydraulic structures, including open channel spillways, bottom outlets in dams, high-head gates and gate slots, and energy dissipators with hydraulic-jump stilling basins. The velocity of water that impinges at the surface point is one of the causes of cavitation. Also, the increase height of spillways on high dams leads to an increase of cavitation caused by nappe flow.

References

{{reflist Hydrology