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Near-bed hydrodynamics and sediment transport in the swash zone
Lanckriet, T. (2014). Near-bed hydrodynamics and sediment transport in the swash zone. PhD Thesis. University of Delaware: Newark. xx, 154 pp.
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Documenttype: Doctoraat/Thesis/Eindwerk
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| Abstract |
The swash zone, the section of the beach that is alternatingly inundated and exposed as a result of wave runup, is an important part of the nearshore coastal zone because of hydrodynamic and sediment transport processes that affect the morphology and ecology of the broader littoral area. The understanding of these swash-zone processes is poor and suffers from a lack of detailed measurements under natural conditions. A new comprehensive dataset of swash zone processes was collected during the Beach Sand Transport (BeST) field study, which was conducted in Perranporth (UK) in October 2012. Measurements were taken for approximately 3 hours around high tide during 10 consecutive tidal cycles, and included several innovative measurement techniques. First, a novel Conductivity Concentration Profiler (CCP) was developed that measures sediment concentration in the sheet flow layer using electrical conductivity as a proxy. The CCP measures a 29-point conductivity profile at 1 mm resolution by multiplexing through a vertical array of 32 plate electrodes. The relationship between conductivity and sediment concentration was calibrated during lab experiments with known masses of sediment neutrally suspended in a heavy liquid. The horizontal and vertical extent of the CCP measurement volume was analyzed using a numerical model of the electric field around the sensor, and indicated that sheet flow layers with a thickness greater than 5 mm are resolved accurately. CCP measurements during the BeST field study demonstrated that sheet flow occurs frequently in the swash zone during both the uprush and the backwash. A detailed analysis of sheet flow focusing on quasi-steady backwash events (when effects of phase lags, surface-generated turbulence and accelerations were small) showed that the sheet flow sediment concentration profile has a linear shape in the lower section of the sheet flow layer and a power-law shape in the upper section. The shape of the concentration profile is self-similar and can be described by a single curve for sheet flow layer thicknesses ranging from 6 to 18 mm. The sheet flow layer thickness and sheet load (the sediment mass mobilized in the sheet flow layer) are well-correlated with the hydrodynamic forcing represented by the mobility number. Secondly, high-resolution near-bed velocity profiles were measured by three profiling acoustic Doppler velocimeters and were used to estimate near-bed turbulence dissipation rates, derived from the structure function. Dissipation rates between 6 · 10-5 m2/s3 and 8 · 10-3 m2/s3 were observed. Temporal phasing of strong turbulence dissipation events agreed well with remotely sensed pixel intensity associated with wave breaking. Bore-generated turbulence decayed rapidly following bore arrival and followed a decay rate similar to grid turbulence. Vertical dissipation profiles demonstrate that turbulence was dominated by (advected) bore-generated turbulence during the uprush and initial stages of the backwash, and by bed-generated turbulence during the later stages of the backwash. A scaling analysis shows that near the bed, sediment-induced density stratification effects on the turbulent kinetic energy budget may have been an order of magnitude larger than turbulence dissipation. |
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