Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
This dissertation aims at utilizing the acoustic approach to measure cohesive sediment behaviors including (1) suspension, (2) settling, (3) deposition and (4) consolidation. The first two processes were attempted to interpret by means of backscattered signal analysis, while the last two processes were done by echo signal analysis. The acoustic instruments used in this study include Acoustic Doppler Velocimeter (ADV), Pulse Coherent Acoustic Doppler Profiler (PC-ADP) and Micro-Chirp system. Used sediments are pure kaolinite and in-situ sediments collected from Mai Po and Clay Bank. 5-MHz ADV was used to estimate the suspended sediment concentration (SSC) and settling velocity (ws). For a limited range of SSC, the time-averaged backscatter wave strength can be well correlated with the SSC. Backscattered signals would be sometimes too noisy due to high amplification ratio, high sampling rate and small sampling volume, and thus, a moving average was used to yield the instantaneous changes of SSC. The measurement of ws with Clay Bank sediment showed that turbulence can increase ws, up to one order larger than that for calm water. When turbulence is stronger than a limit, however, it contributes to the decrease in ws. For the measurement of SSC profile, the performance of 1.5 MHz PC-ADP was evaluated. Clay Bank sediment showed a higher correlation coefficient between range-corrected volume scattering (SSC) and backscattered signal within a limited SSC range (ca. < 10 g/L). On the other hand, kaolinite showed a much smaller range of SSC for linear correlation. This different response might be attributed to the fact that the acoustic response is primarily controlled by the SSC and particle size in suspension at a given frequency. This study suggests that PC-ADP is a potential instrument to reveal the high-resolution (about 1.6 cm) SSC profiles near the bed, if the sediment is sufficiently large. Annular flume experiments with Mai Po sediment were conducted to address a debatable issue regarding the critical shear stress for deposition (tau cd). The direct observation from the flume bottom suggests that tau cd does exist, and that the deposition only occurs when the local bed shear stress (taub) is less than taucd. The changes of deposit length and SSC under the simulated tidal cycles demonstrate that deposition can happen only at tidal decelerating phases with a recognizable tau cd. This study further proves that both taub (a hydrodynamic parameter) and taucd (a sediment parameter) are the main controlling parameters for determining cohesive sediment deposition. A non-intrusive acoustic technique and a signal-processing protocol were developed to estimate the bulk density at consolidating sediment interface. Using high-frequency (300-700 KHz) Chirp acoustic waves, laboratory measurements were carried out in a consolidation tank filled with clay-water mixtures. Because the acoustic echo strength is proportional to the difference in acoustic impedance, and the sound speed in water is close to that in clay, the approximation of bulk density could be successfully presented. The acoustic wave reflectivity increased with increasing the bulk density at the water-sediment interface, which are well correlated with the consolidation status.
© The Author
Ha, Ho Kyung, "Acoustic measurements of cohesive sediment transport: Suspension to consolidation" (2008). Dissertations, Theses, and Masters Projects. Paper 1539616680.