On the local dynamics of currents in the estuarine Marsdiep basin
de Vries, J.J. (2015). On the local dynamics of currents in the estuarine Marsdiep basin. PhD Thesis. Utrecht University: Utrecht. ISBN 978-94-6228-679-5. 175 pp.
dspace.library.uu.nl/handle/1874/317821
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| Abstract |
The estuarine dynamics of the periodically-stratified Marsdiep basin are investigatedthrough a broad range of observations and numerical model simulations.Spatial surveys and long-term point measurements of velocity, salinity, temperature,density and turbulence production and dissipation are complementedwith a 1-D water column model (GOTM) and simulations of the western DutchWadden Sea (GETM/GOTM). This thesis aims at providing a better understandingof the factors and mechanisms that determine the vertical structure of velocityin the Marsdiep basin.The spatial surveys show that the amplitude of the tidal currents in theMarsdiep basin is characterized by a great spatial variability as a result of thelarge variations in water depth. The tidal amplitude is proportional to the waterdepth, if smaller than approximately 15 m, due to the effect of bed frictionon the flow. The tidal amplitude remains relatively uniform for greaterwater depths, but the fortnightly modulation is markedly greater for these waterdepths. Therefore, the lateral shears in along-stream velocity increase fromneap to spring tide, and hence differential advection increases.In the Marsdiep basin, the tidal wave is distorted as a result of the tidebathymetryinteraction. In the ebb-dominant section of the Marsdiep basin,where most measurements are conducted, the tidal wave is characterized bygreater peak ebb than flood currents. Furthermore, the late flood phase consistsof a long period of small currents, whereas the late ebb remains subjectto large currents. The ebb current increases from neap to spring tide, whereasthe flood current remains relatively constant. The tidal distortion has a considerableimpact on the estuarine hydrodynamics.The vertical structure of the along-stream current in the main tidal channel,the Texelstroom, is modified by the ebb-flood asymmetry in bed frictionand by vertical stratification, which produce vertical profiles uncommon forstandard periodically-stratified estuaries. Generally, the superposition of thebarotropic and baroclinic pressure gradients creates vertical profiles with thenear-bed (near-surface) vertical shears in along-stream velocity greatest duringflood (ebb). In the Texelstroom, the near-bed shears are greatest duringebb due to the presence of asymmetric drag induced by the surrounding bathymetry.The large spatial variations in water depth and the variable distributionof bedforms implies that large spatial differences in drag coefficient are presentin the Marsdiep basin. A uniform profile is present in the upper part of the watercolumn during ebb, because the large ebb currents inhibit the generation ofxixii SUMMA RYvertical stratification by classical tidal straining, which generally enhances thevertical shears in the upper part of the water column.On the contrary, vertical stratification is observed during late flood, generatedby cross-stream tidal straining and differential advection. The vertical stratificationduring late flood persists because the small currents are not able to destroythe stratification. The vertical stratification dampens the vertical exchangeof momentum, which creates a mid-depth maximum in along-stream velocity.The mid-depth velocity maximum is characterized by negative vertical shearsin along-stream velocity in the upper part of the water column, which produceinternally-generated turbulence.A classical estuarine circulation is observed despite the absence of the classicaltidal straining circulation and the occurrence of flood vertical stratification,scaling well with the Simpson number. The observed estuarine circulationis characterized by great seasonal variability and is superimposed on a depthaveragedresidual current. The latter is forced by tide-bathymetry interactionand by remote wind effect. The strength of the estuarine circulation is primarilydetermined by the magnitude of the baroclinic pressure gradient and is lessdependent on tidal amplitude. The increase in tidal amplitude, and associatedmaximum vertical mixing, from neap to spring tide is less important for the stabilityof the water column, because well-mixed conditions are already presentduring peak ebb and flood of neap tide. The data suggest that the estuarine circulationis primarily generated by a non-steady gravitational circulation duringthe long period of small currents from late flood to slack before ebb, which isfurther enhanced by increased shears related to the presence of vertical stratification.The estuarine circulation increases with increased tidal mixing, i.e. fromneap to spring tide, which contrasts typical tidal straining estuaries. The increasein cross-stream tidal straining and differential advection towards springtide promotes the generation of vertical stratification during late flood, whichenhances the strength of the estuarine circulation during spring tide.Furthermore, an ebb-dominant asymmetry in lateral advection is observed inthe Marsdiep basin, which acts to reduce the strength of the estuarine circulationfor along-stream salinity gradients greater than 2.5*10?4 psu/m. The observationssuggest that the strength of the classical estuarine circulation in theperiodically-stratified Marsdiep basin is primarily determined by the durationof the period of weak vertical mixing during late flood, the flood stratificationand the strength of the baroclinic pressure gradient.Turbulence dynamics in the Marsdiep basin varies on an intra- and inter-tidaltimescale and influences the estuarine dynamics in a variety of ways. The magnitudeof bed-generated turbulence is proportional to the strength of the tidalcurrent. Therefore, the largest turbulence production is observed during peakebb. Consequently, the bottom boundary layer persists over the entire watercolumn during most of ebb, whereas it only covers the lower 8 to 12 m of theSUMMA RY xiiiwater column during late flood, only covering the entire water column duringpeak flood. Surprisingly, bed-generated turbulence is not the only source of turbulence.Internal shears in cross-stream velocity during late flood and peak ebbcontribute between 30 and 50 percent to the total turbulent kinetic energy production.The total production approximately balances turbulent kinetic energydissipation, but only when the cross-stream component is included.The presence of vertical stratification during late flood and early ebb createsa hysteresis effect in total production: values are smaller during late floodand early ebb, when the water column is weakly-stratified, than during theopposite phases of the tide, when the water column is well-mixed. Therefore,flood stratification during late flood acts as a sink for turbulent energy, which iscommonly only observed during ebb. However, vertical stratification indirectlystimulates internal turbulence production by creating a mid-depth maximum inalong-stream velocity, which enhances the vertical shears in the water column.Summarizing, the Marsdiep basin is an estuary where a wide variety of estuarineprocesses occur, which all contribute to the vertical structure of velocity.The estuarine circulation is characterized by a highly non-steady behavior asa result of the great temporal variability in current speed and in vertical stratification.The tidal current strength, bed friction and the density gradients remainthe most important underlying factors that drive the hydrodynamics inthe periodically-stratified Marsdiep basin, despite the plentiful deviations fromstandard textbook estuaries. |
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