On the Causes of the Formation of Soft Mud at the Head of Tokyo Bay

Abstract

Accumulation of soft mud (muck) at the head of Tokyo Bay is one of the major causes of deterioration of water quality and ecosystems. We conducted field observation of sediment quality and numericalsimulation of grain size distribution to reveal the mechanism of its accumulation, which will provide a suggestion forthe future restoration of its estuarine environment. We revealed the detailed variation in sediment quality and one of the causes is attributed to the lower decomposition rate of organic matter due to the long-term exposure to hypoxic waters along with the higher rate of accumulation of very fine particles reflecting hydrodynamic properties.DeNiro, M. J. and S. Epstein (1978): Influence of diet on the distribution of carbon isotopes in animals, Geochim.

Modeling of Mud Accumulation and Bed Characteristics in Tokyo Bay

“…Moreover, regular high bed shear stresses around the shallows in front of the Obitsugawa River and around the bay mouth do not allow the fine sediment to deposit; thus only coarser sediments occupy these areas. Figure 13(b) shows recent information of particle size distribution inside the inner part of Tokyo Bay, investigated in situ by Sasaki et al [2007]. The model result is, at least, qualitatively consistent with the field data such that the bed sediment particles near the coasts of Urayasu City and Obitsugawa River Mouth are coarse while inside the bay head accumulated bed sediments are comparatively finer.…”

“…In this study the field data of accumulated mud in Tokyo Bay was applied instead of the data of particle sizes of sediment in river waters to assist in the particle classification. The suspended sediments flowing into the bay were classified into four classes based on the particle size distribution data of accumulated mud investigated by Sasaki et al [2007], who carried out sediment core sampling all over the inner part of Tokyo except tidal flats and shallows, at a total of 54 stations. At each station, a sediment core was taken and analyzed by dividing it into 4 layers; bed surface to 1 cm, 1 to 3 cm, 3 to 6 cm and deeper than 6 cm layers.…”

“…Sasaki and Igarashi [2005] investigated the spatial distribution of soft mud layer thickness all over the inner Tokyo Bay based on a conventional acoustic sounding method, whereas Okada and Furukawa [2005] presented a map of the spatial distribution of the sediment water content ratio, investigated based on a novel acoustic sounding system. Sasaki et al [2007] investigated the spatial distribution of particle size, water-content and nutrient-content ratios in accumulated mud all over the bay head by sediment sampling and laboratory analysis. However, numerical studies essential to analyze the mechanism of the formation of mud bed layer are still limited to the case of Tokyo Bay.…”