Transportation of Suspended Sediment Slowly Settling in a Caldera Lake

Abstract

Dynamic behaviors of suspended sediment settling in volcanic Lake Kuttara, Hokkaido were examined by measuring the amount of trapped deposits and analyzing their particle size, biology, chemistry and mineralogy during a period from 14 September to 14 October 1989. The lake then was thermally stratified under conditions of the autumnal epilimnion cooling. The mean daily settling flux (g⋅m^-2⋅d^-1) of suspended sediment was numerically obtained from the amount of deposits trapped at five stations, and its horizontal distribution was successively described at depths of 15 m, 60 m and 100 m or 2 m above the bottom. About 50 to 60 vol.% of trapped sediment was occupied by endogenic phytoplankton (diatoms and dinoflagellates) and zooplankton (mainly, Bosmina longirostris), ranging over 11-125 μm in size. Its representative settling velocity is given as 1.26×10^-5 m⋅sec^-1, using the mean size of 32.6 μm and the mean particle density of 1, 034 kg⋅m^-3. The spatial distributions of settling flux of sediment and a scale analysis of the sediment transport equation suggest that from the water surface to near the thermocline (15 m depth), the transport of settling sediment is controlled by vertically circulating flows due to a wind-forced surface wave, while below the thermocline (depths more than 15 m), it is decided by a secondary, vertically circulating flow due to rotating internal waves.