Nappe (water)
{{Short description|Sheet of water that flows over an edge}}
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.{{cite web|title=Water and Wastewater Terms|url=http://www.owp.csus.edu/glossary/nappe.php|website=Sacramento State (Office of Water Programs)|accessdate=21 April 2018}} Both structures have different features that characterize how a nappe might flow through or over impervious concrete structures.{{cite web|title=What is the difference between a dam, a weir and a barrage?|url=http://greenbugenergy.com/sp_faq/what-is-the-difference-between-a-dam-a-weir-and-a-barrage|website=GreenBug Energy Inc.|accessdate=21 April 2018}} Hydraulic engineers distinguish these two water structures in characterizing and calculating the formation of a nappe.{{cite journal|last1=Chanson|first1=Hurbert|title=Hydraulics of Nappe Flow Regime above Stepped Chutes and Spillways|date=1 January 1994|volume=CE36|issue=1|pages=69–76|url=http://staff.civil.uq.edu.au/h.chanson/reprints/acset_94.pdf|accessdate=21 April 2018}} 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{{cite journal|last1=Lodomez|first1=Maurine|title=Frequencies of Nappe Vibration for Free-overfall Structures|date=June 1, 2016|url=https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1029&context=ewhs|accessdate=21 April 2018}} or air cavitation from free-overall structures.{{cite web|last1=Chanson|first1=Hurbert|title=Design of Spillway Aeration Devices to prevent Cavitation Damage on Chutes and Spillways|url=http://staff.civil.uq.edu.au/h.chanson/aer_dev.html|publisher=The University of Queensland, School of Civil Engineering|accessdate=21 April 2018}}
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.{{cite web|title=Weirs|url=http://www.codecogs.com/library/engineering/fluid_mechanics/weirs/index.php|website=Code Cogs|accessdate=21 April 2018}} A free nappe, which is ventilated to maintain atmospheric pressure below, does not come into contact with the underside of the weir.{{cite web|title=Free Nappe|url=http://glossary.ametsoc.org/wiki/Free_nappe|website=Meteorology Glossary American Meteorological Society|accessdate=21 April 2018}} 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.{{cite book|last1=Sawhney|first1=G.S.|title=Fundamentals of Fluid Mechanics|date=2011|publisher=IK International Pvt Ltd.|page=411|edition=2|url=https://books.google.com/books?id=_fiYDj_p1KQC&pg=PA412|accessdate=21 April 2018|isbn=9789380578859}} 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.{{cite journal|last1=Thandaveswara|first1=B.S.|title=Flow Over Weirs|journal=Hydraulics|url=http://nptel.ac.in/courses/105106114/pdfs/Unit14/14_4.pdf|accessdate=21 April 2018}} 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.
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 floodgates. They both range in size.{{cite journal|title=Energy of Moving Water|journal=The Need Project|date=2013–2014|issue=Student Guide|page=14|url=http://www.switchenergyproject.com/education/CurriculaPDFs/SwitchCurricula-Intermediate-Hydro/SwitchCurricula-Intermediate-EnergyOfMovingWaterStudent.pdf|accessdate=21 April 2018}} An overflow dam has a similar nappe typology to weirs (free, depressed, and clinging nappes).{{cite journal|last1=Kharagpur|title=Spillways and Energy Dissipators Version 2|journal=Hydraulic Structures for Flow Diversion and Storage|issue=2|pages=4–11|url=http://nptel.ac.in/courses/105105110/pdf/m4l08.pdf|accessdate=21 April 2018}} Engineers usually construct an ogee crest, which forms a clinging nappe. This increases discharge, reduces atmospheric pressure, and decreases the chances of air cavitation occurring.{{cite journal|title=Energy of Moving Water|journal=The Need Project|date=2013–2014|issue=Student Guide|page=14|url=http://www.switchenergyproject.com/education/CurriculaPDFs/SwitchCurricula-Intermediate-Hydro/SwitchCurricula-Intermediate-EnergyOfMovingWaterStudent.pdf|accessdate=21 April 2018}}{{cite journal|last1=Kharagpur|title=Spillways and Energy Dissipators Version 2|journal=Hydraulic Structures for Flow Diversion and Storage|issue=2|pages=4–11|url=http://nptel.ac.in/courses/105105110/pdf/m4l08.pdf|accessdate=21 April 2018}}
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.{{cite book|last1=Naudascher|first1=Eduard|last2=Rockwell|first2=Donald|url=https://books.google.com/books?id=hmQ-DwAAQBAJ|title=Flow-induced Vibrations: an Engineering Guide|date=2012|publisher=Courier Corporation|isbn=978-9054101314|pages=2–21|accessdate=21 April 2018}} This well-known phenomenon occurs on free-overall structures (i.e. weirs, fountains, or dams) and produces excessive noise on concrete structures.{{cite journal|last1=Crookston|first1=B.M.|last2=Tullis|first2=B.P.|title=Downloaded 435 times Technical Papers Hydraulic Design and Analysis of Labyrinth Weirs. II: Nappe Aeration, Instability, and Vibration|journal=Journal of Irrigation and Drainage Engineering|date=9 October 2012|volume=139|issue=5|pages=371–377|doi=10.1061/(asce)ir.1943-4774.0000553}} 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 shear forces that occur between two fluids of different velocities.{{cite journal|last1=Helmholtz|first1=Hermann von|title=Hydraulic design and analysis of labyrinth weirs. II: Nappe aeration, instability, and vibration|journal=Journal of Irrigation and Drainage Engineering|volume=36|issue=4|pages=371–377}}
= Cavitation =
Cavitation is defined as the explosive growth of vapor bubbles within a liquid.{{cite journal|last1=Falvey|first1=Henry|title=Cavitation in chutes and spillways|journal=US Department of the Interior, Bureau of Reclamation|date=1990|issue=42|pages=2–8|url=https://www.researchgate.net/publication/236013942|accessdate=21 April 2018}} These bubbles are formed in, and may be carried into, areas of higher local pressures, which disappear before by collapse. Surface irregularities on hydraulic structures are prone to experiencing cavitation. Damage on this type of surface will start at the downstream end of the cloud of collapsing cavitation bubbles.{{cite journal|last1=Falvey|first1=Henry|title=Cavitation in chutes and spillways|journal=US Department of the Interior, Bureau of Reclamation|date=April 1990|issue=42|pages=29–35|url=https://www.researchgate.net/publication/236013942|accessdate=21 April 2018}} 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 increased height of spillways on high dams leads to an increase of cavitation caused by nappe flow.{{cite journal|last1=Chanson|first1=Hubert|title=Hydraulics of Nappe Flow Regime above Stepped Chutes and Spillways|journal=Australian Civil/Structural Engineering Transactions|date=1994-01-01|volume=CE36|issue=1|pages=69–76|url=https://espace.library.uq.edu.au/view/UQ:9376|accessdate=21 April 2018}}
References
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