FLOW AND BEDFORM DYNAMICS IN AFLOOD-TIDE DELTA ENVIRONMENT

摘要

Dune dimensions and migration rate were measured daily over a spring-neaprntide cycle in a flood-tide delta environment. Two transects were monitored: one containedrnpersistently ebb-oriented dunes in a sub-tidal channel and the other contained persistentlyrnflood-oriented dunes on an inter-tidal shoal. In addition, water level, current velocity andrnturbulent kinetic energy were measured over a representative dune crest from eachrntransect. These measurements were obtained at 5 Hz for 10-minute bursts at half-hourlyrnintervals throughout a semi-diurnal tide cycle, once during springs and once during neaps.rnThe observed dune dynamics (changes in dune dimensions and migration rate) arernexplained in terms of the measured flows. In particular, the nature and relevance of thernturbulent component of the flow are discussed. It is inferred from the results of this study,rnand previous studies, that the dune migration rate is probably linked as much to turbulentrnkinetic energy production arising from flow separation over the dune crest as it is linked tornthe local bed shear stress. The measurements of dune dimensions and migration rate werernused as input to an existing geometric model to predict the internal structure of the dunes.rnThe model produces realistic results, and demonstrates the causal mechanism of at leastrntwo sedimentary structures that are diagnostic of inter-tidal dunes.