Determination of Flow Characteristics of Ohashi River through 3-D Hydrodynamic Model under Simplified and Detailed Bathymetric Conditions

The Ohashi River is a narrow water stream that connects two brackish lakes in Japan. Intermittent saline water intrusion often occurs in Lake Shinji from Lake Nakaumi through Ohashi River. In this study, two approaches were discussed to reproduce the hydrodynamic conditions of a morphologically complex river. In the first approach, the river sinuosity was straightened. The straightening of the river resulted in a higher flow velocity and water flux coefficient due to the reduction in the flow path and the resistance, and this approach was found to be appropriate for the reproduction of the flow velocity. However, the river shape was visually quite different from the actual river morphology. In the second approach, the prime focus was given to the shape and bathymetry to quantitively reproduce the flowrate of the saline water intrusion. This approach resulted in an underestimation of the flow velocity, which was compensated by increasing the cross-sectional area of the river. A slower flow velocity causes up to a 3-h time lag for the water mass to pass through the Ohashi River, which in principle should affect the temporal variations of the water temperature and salinity. Fortunately, as the typical time scale for water temperature and salinity fluctuations in the Ohashi River is a few days, a 3-h time lag did not cause any problems

Spatial–Temporal Distribution and Interrelationship of Sulfur and Iron Compounds in Seabed Sediments: A Case Study in the Closed Section of Mikawa Bay, Japan

Herein, the distribution of sulfur and iron compounds (dissolved sulfide: H2S and HS−, iron sulfide: FeS, and ionized iron: Fe2+ and Fe3+) in sediments (0–15 cm depth) at four stations in Mikawa Bay, Japan, was evaluated from April 2015 to March 2016. The maximum dissolved sulfide concentrations in the upper part of the sediment porewater (0–4 cm depth) (within 1.4–8.1 mmol·L−1) varied among stations located in a waterway of a large-scale port with a significant dead zone. Moreover, the iron sulfide and ionized iron concentrations in the upper part were highest at a station where the dissolved sulfide concentration was relatively low compared with that of the other sites. Analysis of the theoretical and hypothetical accumulation of particulate oxidized iron (FOOH) at the stations located in the dead zone revealed that the estimated particulate oxidized iron accumulation was higher (2303 mmol·m−2) at a station in which the dissolved sulfide concentration was low compared with the other stations (142–384 mmol·m−2). Altogether, these findings suggest that the sulfur–iron cycling can determine the amount of dissolved sulfides that accumulate in sediments. Hence, artificially adding iron compounds to the seabed may help mitigate free sulfides accumulation and prevent extreme hypoxia

Large-scale experiment to assess the collision impact force from a tsunami wave on a drifting castaway.

Although most fatalities in tsunami-related disasters are conjectured to be a result of drowning, injury risk owing to collision with other floating debris or fixed buildings has not been studied sufficiently. In this study, the impact force corresponding to the collision of a concrete block and drifting test body in a tsunami wave was experimentally investigated, and the injury risk was evaluated in terms of different biomechanical indexes; specifically, maximum acceleration, head injury criterion, and impact force. The injury risk indicated by the considered indexes was reasonably low. It was noted that if a healthy adult collided with a concrete wall under a velocity of 2.5 m s-1 and wave height of 0.59 m, the adult would likely not be critically injured. However, a similar collision impact poses considerable risk to infants and children, as well as the more sensitive regions of the adult body. Moreover, in the case of large tsunamis, such as that in the 2011 Great East Japan Earthquake, a drifting person may be at considerable risk for injuries. The collision impact occurring on the tip of a surge flow is notably significantly larger than that on a bore flow. This is because a surge flow, which arrives at the concrete block earlier than a bore flow, forms a certain water layer along the concrete wall and that layer acts as a cushion for any body drifting on the bore flow, indicating the importance of such a buffering effect. These findings can provide practical guidance regarding the formulation of effective tsunami-protection measures

Mass Mortality of Asari Clams (<i>Ruditapes philippinarum</i>) Triggered by Wind-Induced Upwelling of Hypoxic Water Masses

To investigate the mass mortality of the macrobenthos community, primarily asari clams, triggered by upwelling-driven hypoxia, we conducted continuous observations of temperature, salinity, and DO, and monthly macrobenthos monitoring on the Rokujo tidal flat in Mikawa Bay, central Japan, from 2014 to 2016. Additionally, laboratory experiments were conducted using sediments on a tidal flat containing macrobenthos to examine the possibility of hydrogen sulfide formation in tidal flats. The bottom layer at the offshore station was intermittently hypoxic, and the station of the tidal flat was occasionally hypoxic in August and September for three years. Hypoxia was mostly observed on the tidal flat when constant easterly winds were recorded offshore. The biomass of asari clams decreased considerably from September to October 2016 when hypoxia was intermittent. Hypoxia persisted for approximately one week from 20 September, which was associated with the calm weather and stagnation of tidal currents owing to the neap tide. Conversely, the hydrogen sulfide concentration in the water directly above the sediment exceeded 30 mg L−1 after 3 days of incubation in the laboratory experiment. Therefore, the possibility of oxygen consumption on tidal flats due to hydrogen sulfide formed by biological die-offs was considered in the long-term persistence of hypoxia