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Beach Erosion - Sediment Motion with Wave-induced High Orbital Velocity in the Big Wave Flume Hannover

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Sprachtranskript
The Big Wave Flume in Hanover is the largest facility of this kind in the world. The
powerful 900 KW wave maker generates the waves.
Maximum wave heights of 2.5 m can be generated both as regular waves and as wave spectra.
The wave motion is computer controlled.
The waves propagate within a channel with a length of 320 m, a width of 5 m and a depth of 7 m. The single water particles of the wave seem to move in the direction of the wave propagation. The truth is that in deep water the particles keep moving in orbits without a resultant forward motion. The
near surface elliptic orbits are flattened out near the bed and so cause an oscillatory flow and water particles are moving in the direction of wave propagation and the opposite direction respectively. By this means the sediment transport is initiated.
You can observe this process in the wave flume through this window
and you can see the turbulence and vortex motion near the sand bed directly, here near the lee of a sand ripple with a length of about 2 feet.
Within projekt A6 the movement of beach material on a horizontal bed is investigated at wave heights ranging from 0.5 to 1.8 m and wave periods between 4 and 9 seconds. The water depth is 3.2 and 4.5 m.
The following test is carried out at a water depth of 4.5 meters and a wave height of 1.8 meters. The wave period is 6 seconds and the wave length is 35 meters. The macro shots show us a field of view of about 3 cm in height and 4 cm in width. They depict the near bottom boundary layer close above the sand bed. The movements of sediments were shot
at 500 frames per second, that means a slow motion factor of 20. The propagation of a wave crest induces a current from the left to the right. From Stokes' wave theory 2nd order the orbital velocity above the bed is calculated to 1.3 m/s at its maximum. The following wave trough induces a current reversal. In this case the calculated orbital velocity is 0.9 m/s. The movement of the single grains is assumed to be the same as the orbital velocity. Which is approximately the same as the actual orbital velocity near the bed. So it is possible to make a comparision with the calculated orbital velocity. Current reversal. With quickly increasing velocity the current becomes turbulent and from single grains protruding from the bottom layer vortices take of. The first jump of a sand particle is effected by turbulence, shear stresses suction and form drag forces. The wave induced oscillatoary flow generates an instationary boundary layer near the bed. In that narrow layer the sediment movement is observed.
With the linear wave theory the orbital velocity is calculated to 1.1 m/s in both directions.
The actual orbital velocities are much smaller. The red area shows the analysed maximum orbital velocities as a function of height above bed for a propagating wave crest on the right and a wave trough on the left. The measured velocities seem to be fairly low. But it has to be considered, that the object field was mostly oriented inside the boundary layer. This has a calculated thickness of 1.4 cm and a characteristic exponential velocity profile. This shot shows the orbital velocity increasing from 0 at the bottom to the orbital velocity in the undisturbed outer zone of the boundary layer above the bed.
The transport as bed load occurs in a layer with a thickness of only 1mm. This is much smaller than the predicted thickness on the order of some centimeters.
In the next test the water depth is decreased to 3.2 meters. The wave height is only 1.4 meters and the wave period is 8 seconds, according to a wave length of 45 meters.
During passage of the wave crest the maximum horizontal orbital velocity above the bed is 1.9 m/s calculated with the Stokes' 2nd order theory. The linear wave theory gives a horizontal orbital velocity of only 1.1 m/s in both directions. Directly above the boundary layer the lineartheory seems to give better results than the theories of higher order. When a certain velocity is reached, grains in the upper layer of the bed start to move in the direction of current. If the orbital velocity decreases below the defined threshold the sediment movement stops and the particles rain out towards the bottom. The sediment movement starts again, when the orbital velocity after reversal of direction increases above a certain threshold. At the maximum orbital velocity so-called sheet flow conditions are predominant at the bed. In the case of very highly turbulent current conditions, the grains are moved to and fro in a very thin layer carried by dilatation stresses. During this phase there is an interchange of momentum between the moving and resting material.
The linear wave theory gives a horizontal component of the orbital velocity of 1.1 m/s at its maximum in both directions.
The red area again shows the measured horizontal velocity in the darker brown boundary layer with a calculated thickness of approx. 2 cm.
The layer of highly concentrated bed load movement is within 1.2 mm, i.e. considerably lower than that on the order of several cm as cited in the references.
In the following test the wave height it decreased to 1.1 meter and the wave period of 4s is only one half of that in the previous test. Water depth is 3.2 meters and the wave length is 20 meters.
The essential changes due to different wave parameters are obvious by comparison of the both wave shapes.
With these wave parameters the Stokes' second order wave theory gives a maximum horizontal orbital velocity of 0.75 m/s above the bed. Such a low velocity means that only very low activity accours at the sand bed. Only very little material is suspension and is transported. Under such circumstances sheet flow conditions near the bed are not observed.
In this case the linear wave theory gives a maximum velocity of 0.7 m/s. By comparsion the measured maximum velocities are marked in red for the wave
crest and the wave trough. Within the dark brown boundary layer with a calculated thickness of 7 mm this profile is typical exponential. Above this layer there is a fairly good agreement between calculated and measured velocity.
As in the other tests with longer waves bottom thickness changes only by 1 mm.
This film was produced by project A6 within the Special Interdisciplinary Research Project 205 - Coastal Engineering - established at the University of Hanover. The Research Project is funded by the Deutsche Forschungsgemeinschaft (DFG).
Fahrgeschwindigkeit
Nassdampfturbine
Schiffsrumpf
Einzylindermotor
Nassdampfturbine
Bett
Förderleistung
Nassdampfturbine
Schiffsrumpf
Nähmaschine
Bett
Material
Uhrwerk
Schubumkehr
Kaltumformen
Nassdampfturbine
Schiffsrumpf
Motorschiff
Biegen
Bett
Fahrgeschwindigkeit
Kümpeln
Förderleistung
Übungsmunition
Uhrwerk
Motorschiff
Biegen
Bett
Fahrgeschwindigkeit
Nassdampfturbine
Schiffsrumpf
Bett
Schubumkehr
Feinstblech
Motorschiff
Biegen
Bett
Fahrgeschwindigkeit
Material
Förderleistung
Übungsmunition
Uhrwerk
Trenntechnik
Bett
Fahrgeschwindigkeit
Uhrwerk
Nassdampfturbine
Schiffsrumpf
Feinstblech
Motorschiff
Bett
Material
Fahrgeschwindigkeit
Förderleistung
Übungsmunition
Biegen
Fahrgeschwindigkeit
Greiffinger

Metadaten

Formale Metadaten

Titel Beach Erosion - Sediment Motion with Wave-induced High Orbital Velocity in the Big Wave Flume Hannover
Alternativer Titel Stranderosion - Sedimentbewegung bei wellenerzeugter hoher Orbitalgeschwindigkeit im großen Wellenkanal Hannover
Autor Dette, H. H.
Uliczka, K.
Rahlf, H.
Lizenz Keine Open-Access-Lizenz:
Es gilt deutsches Urheberrecht. Der Film darf zum eigenen Gebrauch kostenfrei genutzt, aber nicht im Internet bereitgestellt oder an Außenstehende weitergegeben werden.
GEMA Dieser Film enthält Musik, für die die Verwertungsgesellschaft GEMA die Rechte wahr nimmt.
DOI 10.3203/IWF/B-1791eng
IWF-Signatur B 1791
Herausgeber IWF (Göttingen)
Erscheinungsjahr 1992
Sprache Englisch
Produzent IWF (Göttingen)
Produktionsjahr 1991

Technische Metadaten

IWF-Filmdaten Film, 16 mm, LT, 166 m ; F, 15 1/2 min

Inhaltliche Metadaten

Fachgebiet Technik
Abstract The sedimentation movement is observed in a sand basin with waves of high orbital speed in the Big Wave Flume. Films of sand grain trajectory in slow-motion, with wavelengths 20.35 and 45 m, water depths 3.20 and 4.4 m, wave heights from 0.5 to 1.8 m, periodicity 6 to 9 s. Speed profiles shown in graphics. Sheet-Flow-Status.
Schlagwörter engineering simulation
coast protection
beach erosion
sedimentation
sheet-Flow
elliptical orbits
waves
turbulence
border layer
wave theory
beach dynamics

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