Boundary Layer Turbulence and Sediment Transport in an Inter-Tidal Mudflat Channel.

Abstract

Mudflats along the west coast of South Korea are influenced by high tidal ranges and strong winter monsoonal storms, and are drained by a network of incised channels. In a selected inter-tidal mudflat channel of Namyang Bay, seven levels of 3-D turbulent velocity data, five levels of water temperature data, and two levels of sediment concentration data were collected to analyze turbulent boundary layer flow with stratification. Using time series analysis techniques, the basic turbulent parameters such as turbulent intensity (variance of u$\sp\prime$), vertical momentum flux, $\rm\langle w\sp\prime u\sp\prime\rangle $, and variance of w$\sp\prime$ were obtained. The Reynolds stress distribution obtained from the "eddy correlation" technique clearly shows the presence of a constant stress layer, the thickness of which is less than in previous results. Three methods of calculating turbulent energy dissipation rate (i.e., "Kolmogorov law", "turbulent scale", and "integration") are summarized, and three basic methods for estimating bottom shear stress (i.e., "velocity shear", "eddy correlation", and "inertial dissipation") are evaluated. The final results indicate that: (1) the shear stress estimated from the "eddy correlation" technique is less than from the other two methods; (2) the results from the "velocity shear" method are strongly related to mean flow velocities; and (3) the "inertial dissipation" method can give satisfactory results when the turbulent velocity fluctuations are measured within the constant stress layer (lower 100-150 cm) and the corresponding Reynolds number is greater than the critical Reynolds number (Re$\rm\sb{c}$ = 3000). In the meantime, sediment transport processes on mudflats have been of concern, which lead to formation of a new conceptual processing model. Three types of roughness length variations with bottom shear stress: proportional, constant, and reverse proportional are summarized. The sediment composition changes could exert an important influence on roughness length. Finally, a modified "inertial dissipation" method has been derived in which non-neutral stratification caused by sediment resuspension is considered. The results of this method indicate that bottom shear stress can be reduced by about 6-35% in the presence of non-neutral stratification induced by sediment resuspension.