Muddy sea-floor response to wave action, Atchafalaya Shelf, Louisiana, USA
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- Tez No: 400384
- Danışmanlar: DR. ALEX SHEREMET
- Tez Türü: Doktora
- Konular: Coğrafya, Mühendislik Bilimleri, Geography, Engineering Sciences
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2012
- Dil: İngilizce
- Üniversite: University of Florida
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Oşinografi Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 91
Özet
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Özet (Çeviri)
The interaction between muddy sea-floor and the hydrodynamics is investigatedbased on wave, current, sediment and acoustic backscatter observations made in 2006and 2008 on the muddy Atchafalaya Shelf, Louisiana.A method to estimate the vertical structure of suspended sediment concentrationfrom acoustic backscatter observations is developed using 2006 data. Acousticestimates of suspended sediment concentration are used to re-examine the relationbetween sediment stratification and floc size in a cohesive sedimentary environment.Concentration measurements by optical backscatterance sensors at two verticallevels are used to calibrate the backscatter intensity of the acoustic profiler. In spite ofthe complexities due to the rapidly changing flow and cohesive sediment properties,the acoustic estimates of sediment concentrations agree well with the observations.Estimated vertical suspended sediment concentration profiles and the measuredcurrent profiles are used to calibrate a one-dimensional-vertical (1DV) boundary layernumerical model for combined wave-current flow on muddy beds. The effect of thefloc size on the vertical structure of the suspended sediment concentration profileis investigated using the acoustic and numerical estimates of concentrations, andthe floc size measurements. For similar flow conditions, smaller flocs result in moremixed profiles with higher concentration in the upper water column and lower near-bedconcentration. This observation based on field measurements supports the previouslypublished numerical results.Wave, current, acoustic backscatter and suspended sediment concentrationmeasurements (both single-point and vertical profiles estimated by conversion ofacoustic backscatter data) collected in 2008 are used to investigate wave-current-cohesivesediment interaction on the muddy Atchafalaya inner shelf. During an energetic storm,bed state follows a cycle of dilation due to fluidization, erosion, deposition with fluid mudformation and consolidation. A one-dimensional-vertical cohesive sediment transportmodel is calibrated using current and concentration profiles to estimate the physicalparameters that could not be measured directly, e.g., bottom stresses. Estimated bedposition and computed bottom stresses suggest that the critical erosion threshold isin the range of 0.3 Pa to 0.5 Pa. The study site is impacted by a sediment-laden freshwater plume coming from the Atchafalaya River mouth. Bed density evolution duringthe storm is estimated from vertical sediment exchange between the water columnand the bed excluding the duration of passage of a sediment-carrying water front. Thevalues are in the range of 1,030 kg/m3 to 1,200 kg/m3 and indicate that the bed densityincreases during the erosion phase and decreases during deposition. At the end ofthe storm, it shows a steady increasing trend during hindered settling and exceeds thespace-filling value during consolidation. Both the critical erosion shear stress and beddensity values are consistent with the results of laboratory tests on samples from theexperimental site. The applicability of these results for a larger population of stormsis also investigated. The results generalize the estimates of bed yield stress and beddensity evolution, allow for a statistical model for the bed reworking cycle and representa first step towards a forecasting model for wave-bed coupling in muddy environments.
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