Sediment suspension under groupy waves

grantor: University of TorontoBoth field and laboratory experiments were performed to investigate the suspension of sand under shoaling grouped waves above a rippled sandy bed. The temporal and spatial variations in concentration resulting from the trajectories of nonhomogeneous, sediment-laden, separation vortices and bedform migration were found to be extremely large. The functional dependency between concentration and velocity was improved after accounting for the time lag of concentration with respect to velocity, the threshold condition for sediment suspension and the occurrence of more than one concentration peak within a half-wave cycle. The frequency dependence of concentration was affected by shifts in the spectral density of concentration from the primary wave frequency to both lower and higher frequency bands. During the passage of wave groups suitable for the development of steep vortex ripples, suspended sediment concentration was found to increase consistently with respect to velocity. Under certain conditions, however, sediment concentration actually decreased with respect to velocity at some point; this occurred when several consecutive velocity maxima exceeded the transition velocity for the change from steep-vortex ripples to less steep vortex ripples. Sediment suspension under wave groups was found to be complicated by a number of parameters including not only the absolute magnitude of velocity and the size of sediment, but also the velocity sequence within a group, the run-length of a group and the boundary roughness changes. A large auto-regressive component of concentration is introduced by the movement of sediment-laden vortices and the inheritance of suspended sediment from wave to wave during the passage of wave groups. This auto-regressive component explained on average 40 percent of the total concentration variance, compared with an average of only 21 percent explained by the instantaneous velocity. Comparison of the auto-regressive components of concentration and velocity in the explanation of the instantaneous concentration demonstrated that the preceding velocities were much less important. The accuracy of concentration modeling depends upon how these temporal and spatial variations in concentration can be incorporated efficiently in a model.Ph.D