THE DYNAMICS OF FINE FRACTION OF TIDAL FLAT SEDIMENT: AN ANALYSIS OF HYDRODYNAMICS AND ISOTOPES IN OHURA RIVER ESTUARY, OKINAWA, JAPAN

Abstract

 We examined relationships between sediment mud conten t and current velocity in a estuarine tidal flat of Ohura River, Okinawa, Japan. Sediment mud content was negatively related to the frequency of the occurrence of >15 cm/s current velocity. Particularly in the stations with <0.11-0.12 of the >15 cm/s frequency, sediment mud content was relatively higher and its variance was larger. Our analysis of the radio isotope, ⁷Be, showed that the averaged residence time of fine sediment fraction in the subsurface sediment was 36, 132 and 68 days for non-cohesive sandy sediment, cohesive sand-mud-mixed sediment, and cohesive muddy sediment, respectively. We suggest that sediment properties are controlled mainly by resuspension of sand fractions, sediment cohesiveness and settlement of fluid mud on sediment surface. Our results support the use of isotopic analysis for understanding of sediment dynamics, and also the non-linear response of sediment properties to variations of hydrodynamic conditions.