5.Research on Improving Water and Sediment Qualities in Enclosed Bays

Background and Objectives

The water quality and ecosystem in an enclosed marine area are affected by terrestrial inputs and the marine atmospheric boundary layer. They also varying depending on tidal exchange with the open sea and material exchange processes at the sediment-water interface as well as at the sea surface. For example, exchange of water mass at the bay entrance and oxygen consumption processes by sediments play important roles in formation of hypoxia, which is a typical water quality deterioration phenomenon in semi-enclosed coastal seas. Therefore, understanding the processes across these boundaries is necessary.
Following the ratification of the 1996 London Convention Protocol to regulate waste dumping into the sea, there is a need for control methods for siltation and technologies to promote beneficial use of dredged material from ports and harbors. To do this, fundamental knowledge on the physicochemical processes at and around sediment surface is indispensable to understand sediment movements as well as the fate and transport of chemicals. Such fundamental information will be utilized to assess adverse effects of chemicals on benthic fauna and to develop countermeasures
for contaminated sediments.

Research topics

Exchange processes at bay entrance have been investigated through continuous measurements of flow field at the cross section and surface water quality at the mouth of Tokyo and Ise Bays by using instruments mounted on ferries. Differences in the tidal water exchange system at the mouth
of the bays and variations of water quality such as formation of hypoxic water in the inner part of the bays have been compared.
The Coastal Bottom Boundary Eco-hydro Dynamics Experimental Flume was developed in FY 2006 to investigate transport processes of fine sediment under external forces. Experiments with intact sediments collected in Tokyo and Ariake bays by using a special container device were conducted. Intensive field observations in the two bays as well as laboratory experimental results have been utilized to develop a mathematical model on the re-suspension processes. Prediction of movements of sediments with a broader particle size distribution on the seashore has also been investigated by a model analysis.
In a research on the material cycling processes inside the sediments, analytical models on the dynamics of dissolved oxygen and nutrients in the benthic boundary layers have been developed. Field surveys of chemicals and benthic fauna in port and harbor sediments have been conducted to understand the fate and transport of chemicals and to evaluate adverse effects of chemicals on living organisms. Such fundamental knowledge has been further utilized to develop a scheme for countermeasures against sediment pollution as well as to promote the beneficial use of dredged material.

Activities in FY 2010

Analysis of the monitoring data of flow fields collected by ferryboats at the cross section of the mouth of Tokyo and Ise Bays in FY 2010 revealed that variations of a mixing intensity at the bay entrances caused a different formation of hypoxic water mass in respective bays. Topographic changes off Haneda monitored with video cameras for a long period also showed that tidal flats geometries were gradually restored after large-scale flooding of the river.
The dominant bio-chemical process on the formation of hypoxic water mass in the bottom layers of an enclosed bay has been identified as the oxygen consumption process by sediments. Analytical models on dynamics of dissolved oxygen and phosphorus in sediments as well as in the benthic boundary layers were formulated. For hydraulically smooth surfaces, in which modeling had been traditionally advanced, and also for rough surfaces, modeling has been successfully reproduced, showing the microstructure of oxygen distribution and sedimentary oxygen consumption processes with reasonable accuracy. We successfully performed field observations of sediment physical transport processes in the benthic boundary layer, and advective transport of fluid mud with high turbidity was also detected above the sea floor, which might be responsible for the sediment focusing phenomena in Tokyo Bay. In respect to this phenomenon, we analyzed the dynamics of fluid mud with a one-dimensional model in FY 2010.
We proposed an analytical method to predict variations of currents, tides and drifting sands caused by constructed coastal structures, such as piers and submerged levees, as well as of a grain sizes in sediment qualities along with these changes to estimate the amount of drifting sands at specific ports. This type of model can be employed to predict changes in coastal topography and variations in size distributions of sediment under a wide range of condition.
Finally, we also analyzed chemical survey results, which are already known as toxic substances for the coastal environment, in seawater, sediments and benthic fauna as well as fish to reveal that the bio-concentrations in organisms of higher trophic level highly differentiate depending on chemicals.
In the current environment in which new materials are increasingly manufactured and used, a preventative perspective requires estimating the adverse effects of chemicals already regulated and highly persistent chemical substances that may be subject to future measures.
In this regard, we have initiated research to develop a method of screening these emerging chemicals and also conducted a survey on several selected substances in Tokyo Bay and the Port of Nagoya.

Monitored blue tide in Tokyo Bay (near Makuhari at the Port of Chiba)