Benthic and Intermediate Nepheloid Layers in Lake Biwa

Abstract

Benthic nepheloid layers and intermediate nepheloid layers in Lake Biwa were observed between 1991 and 1993, and the characteristics of both nepheloid layers are discussed using field data from Pre-BITEX in 1991-1993 and BITEX'93 in 1993.The benthic nepheloid layer in Lake Biwa is formed during a stratified period from May to October. The top edge of the benthic nepheloid layer is very sharp as if it is a step function, and turbidity is higher in the benthic nepheloid layer than outside. The vertical distribution of turbidity in the benthic boundary nepheloid layer does not fit to the Rouse distribution. We showed that internal waves induce the oscillating flows in the benthic boundary layer, and it makes such distributions of suspended sediments as a step function.The intermediate nepheloid layer in Lake Biwa was also discovered during Pre-BITEX and BITEX'93. The intermediate nepheloid layer observed during BITEX moved in one direction with a continuously horizontal velocity of 2.0 to 4.0 cm⋅sec-1, for almost 8 hrs. The intermediate nepheloid layer was about 800 m long and 5 m thick, and it brings suspended sediments rich in Mn.By comparing these physical data with the results of chemical analyses, we can verify that the material in nepheloid layers originated from bottom sediments.