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Eutrophication in an estuary occurs as an effect of the enrichment of nutrient inputs from rivers. This condition has become one of the most common environmental issues experienced around the globe and especially in Japan. Atsumi Bay is a eutrophic coastal area in Japan. The objective of this research was to analyze the influences of nutrient inputs from the Umeda River into Atsumi Bay on pre- and post-rainfall water quality conditions. This study was conducted from July to October 2010. The results showed a decrease of surface salinity after rainfall indicating that huge freshwater inputs had overlaid the surface layer of Atsumi Bay rather than the bottom layer. Moreover, post-rainfall conditions showed an increase of chlorophyll a as an effect of phytoplankton growth, followed by an increase of particulate nutrients. On the other hand, dissolved nutrients decreased due to uptake by phytoplankton and dilution by freshwater.

Yuk, J.-H.; Aoki, S., and Joh, M., 2016. Wind-induced water exchange between stratified basins. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 188–192. Coconut Creek (Florida), ISSN 0749-0208. Water exchange between Inohanako Estuary and Hamanako Bay, Japan with density stratification is investigated using the field data. The basins are connected by a narrow channel with length 200 m through which waters and nutrients are exchanged between them. The principal axis of channel between two basins is located nearly in the north-south direction, thus it is natural that the water flow between the estuary and bay is influenced more by the north-south component of wind than the east-west component of that. Although the relationship between water exchange and the east-west component of wind is relatively low compared with the north-south componen...

A series of field observation of turbidity and current velocity etc. were carried out at the sea bottom off a river mouth during flood periods. The measurement location was 1.2 km off the mouth of Tenryu River, Japan, where water depth was about 18m. Four flood events of which the maximum mean hourly discharge ranged from 1,570m3/s to 4,620 m3/s were selected and the time series of river discharge, turbidity, water temperature, salinity and current velocity were compared with each other and the relationships between them were discussed. Furthermore, vertical profiles of turbidity and salinity observed after a flood were used in the discussion. Bathymetric changes surveyed off the river mouth were also taken into account.

The sand dredging activities may cause various physical and environmental changes such as beach erosion, evolution of beach profile, change in current and wave field. To evaluate the relationship among the volume of sand dredging, method of sand dredging, time of sand dredging, shoreline retreat and evolution of beach profile, some experiments were conducted and a two-dimensional numerical model was used. The effect of sand dredging volume on shoreline retreat was investigated by dredging various amounts of sand at one time under the same regular wave condition. The results showed that the relationship between shoreline retreat and dredging volume is nonlinear. In the other considerations, sand bar was dredged as different methods i.e. periodic sand dredging method under the same regular wave condition as well as one time dredging under different regular wave conditions. The final shoreline retreat in the last two methods was nearly equal to the first method, however, the progress of shoreline retreat was slower. After sand dredging in all cases, the beach profiles were not translated to the same original form. Infill rate of the bar after sand dredging is also discussed in term of average absolute sediment transport rate.

Ishikawa, T.; Komine, T.; Aoki, S.I., and Okabe, T., 2014. Characteristics of rip current drowning on the shores of Japan. The characteristics of rip current drowning on the shores of Japan was investigated using the analysis of the lifesaver's rescue reports of 2013 and field observations of some beaches. Although the necessary rescue equipment and the water safety information such as signboards were mostly prepared, a large amount of rip current drowning occurred. On most re-occurring rip current drowning beaches, there are permanent rips and fixed rips associated with the characteristic of topography and the coastal structures at most beaches under the relatively high wave condition. Especially, in the case lifesavers cannot determine the swimming areas, swimming areas are generally determined by the local regulations from local governments and shop owners on most beaches. The most important outbreak factor regarding the rip current drowning is the human factor such as the management problems ...

The study examined the summertime behavior of phosphorus (P) and estimated P fluxes in the Inohana Lake Estuary, Shizuoka, Japan through a field observation conducted in the summer of 2007 and a bo...

Sandy beach shows gradual topographic change with long-term and short-term variations corresponding to the conditions of waves and tides. Because of the irregularity in the profile change, it is difficult to investigate the characteristics of shoreline change in detail. In this research, long-term survey of the beach profiles has been carried out on three different coasts almost every week for more than ten years. The topographic data were analyzed and the characteristics of the long-term and seasonal beach profile change were discussed using the statistical methods and the empirical orthogonal eigenfunction method (EOF).

The estuarine circulation and the material transport associated with that in shallow waters around the mouth of Toyogawa River was studied through the field measurement of currents and chemical analysis of waters, carried out in summer, 2008 and 2009. The estuarine circulation was observed throughout the measurement period. The circulation was driven mainly by the wind and was not strongly correlated with the river discharge. Nutrients released from the bottom sediment under the hypoxic condition of bottom waters are transported by the estuarine circulation into the shallow waters, which may contribute to the primary production around the river mouth.

Water exchange between Inohana and Hamana Lakes with density stratification is investigated based on the field data obtained in 2009. The basins are connected by a narrow channel with length of 200m through which waters, nutrients and other materials are exchanged between them. According to the authors' previous study, the water exchange was mainly caused by the wind, where even eastward wind induces significant exchange of waters. In this study, using the field data measured by ADCP and CTD, a mechanism of wind-induced water exchange is discussed. A simple theoretical approach suggests that the dynamic response of density surface in Hamana Lake to the wind force causes the circulating flow in the channel.

The area around tidal inlet is potentially dangerous for marine activities because of strong tidal currents and waves together with the complexity of topography. This study intends to collect useful information about the flow field around tidal inlet for prevention of marine accidents. Field observations of waves and currents have been conducted on two sites of Imakirekuchi Inlet at Lake Hamana in Shizuoka Prefecture and Daiichikoguchi Inlet at Saroma Lake in Hokkaido. The continuous 30 days observation of currents at both sites were successfully conducted. By analyzing the observation data, characteristics of temporal- and spatial-variation of currents in and around the inlet have been examined to clarify the relation among tide, current, and topography.

Bathymetric data in a wide coastal area were derived from frequent monitoring using fishfinder and GPS data logged during fishing operation in Enshu-nada Sea. The offshore bathymetric changes in near critical depth were evaluated by the annual changes of contour line positions along the cross-shore direction and these standard deviations. The analysis revealed long-term trend with short-term fluctuation in the sea bottom changes associate with the frequent monitoring data. We found that slow erosion appeared in the offshore area except around river mouth and inlet was indicated.

Mechanism of sand movement was investigated around of the Magome River mouth located on the Enshu-nada by the comparison of old geographical maps in about 200 years and by a color sand tracer experiment. The long term advancement of the shoreline as well as the rapid retreat in the last 20 years were identified. The movement of four color sands placed on the updrift and the downdrift sides of the river mouth jetty and in the jetty channel was monitored for three months. Entrapment of westward longshore transport by the jetty was found to be significant. A sand bypassing system was found to be feasible by utilizing an existing sewage discharge channel connecting the Tenryu River mouth to the Magome River.

An intensive field measurement was carried out in Inohana and Hamana Lakes including a narrow channel to investigate the phosphorus transport process in Inohana Lake during summer. Inohana Lake has a two-layer structure of the phosphorus distribution: the particulate phosphors (PP) in the surface layer, while the phosphate phosphorus (PO4-P) in the bottom layer. The phosphorus inflow from Hamana Lake through the bottom layer is dominant over the riverine loading and Inohana Lake becomes a sink of phosphorus from Hamana Lake. The settling and release fluxes of phosphorus are larger than the input and output of the phosphorus in Inohana Lake. The phosphorus budget suggests that the phosphorus accumulates in the sediment during summer.

Questionnaire surveys were conducted to investigate discrepancies between consciousness of citizens for tsunami disaster prevention assumed by administrations and actual consciousness of residents and visitors in tsunami vulnerable areas. It was found that the residents expect hardware measures (structures) against tsunami attacks, whereas visitors prefer software measures for evacuation. It is important to guide citizens toward software measures and give appropriatei nstructionsf or effectivee vacuation. Citizenws ho have experienceo ft sunamia ttack orlivingn ear the coastlineh ave clear images for threat of tsunamis.Goodc ommunicationb etween administrationsa nd citizensc an restrain the reduction of efficiencyf or tsunamip rotectionm easures.

In order to estimate sediment transport near a river mouth and a narrow inlet, it may be important to investigate the behavior of the long-period waves with periods of several minutes generated by wave groups in the sea. In this study, characteristics of the long-period waves and associated flows near Tenryu river mouth and the inlet of Hamana Lake are discussed based on the field data. Long-period waves are developed almost linearly to both significant wave heights and periods outside the inlet, but they are reduced about half in the inlet. The long-period fluctuation in the flow seems to consist of two components; one is associated with the long-period waves and the other is generated by tidal currents.

Beach changes around the mouth of the Magome River on the Enshu-nada coast and the long-term deformation of Nakatajima sand dune were investigated. The dune material originates from the Tenryu River. The supply of fine sand from this river has decreased owing to the construction of the dams in the upstream basin of the river, and the coastal forest was excessively planted close to the shoreline, resulting in decreasing sand supply from the shoreline to the sand dune. Thus, this sand dune has significantly been starving. To conserve this sand dune, supply of new sand or reuse of sand is required.

Hamanako is an economically and culturally important tidal lake on the Pacific coast of Honshu, Japan. Between 1954 and 1973, entrance works were constructed to regulate the channel to the lake. In...

A new numerical model of regional sediment transport and barrier elongation, supplied by sediment coming from longshore sediment transport (LST), was developed. The spit growth and barrier elongation model was based on the model proposed by Kraus (1999). The LST rate used as input for the spit growth and barrier elongation model was simulated by the one-line model of shoreline change (Hanson, 1987) combined with the inlet reservoir model (Kraus, 2000; Larson et al., 2006). The model was applied to field data from the south coast of Long Island, United States. The simulation results were compared with measurements of the annual net LST rate and the barrier elongation. Overall, the model simulations were in good agreement with the measured data. © 2011 ASCE. (Less)

Three events of storm surge which yielded different tidal departures in the head of Mikawa Bay, Japan are compared and discussed from the viewpoint of response to the wind forcing. The mechanism of sudden sea level rise is investigated based on the data of wind, sea level and velocity obtained in the period of three different Typhoons which passed near the bay. The magnitude of preceding east wind which gives rise to the sea level in Ise Bay and quick inversion in wind direction from east to west are key factors to the development of large tidal departure in the head of Mikawa Bay. The propagation of surge waves as a free long wave is confirmed through the velocity profile data and also tested using a 2D simple numerical model.

Long period waterlevel variations, from 14 d to 1 yr, are examined for Hamanako, a tidal inlet on the Pacific coast of Honshu, Japan. Eleven years of records, prior to the regulation of the entrance to the inlet, are analyzed. At all the frequencies examined there were substantial fluctuations, some periodic and some irregular. Dominant features were abrupt changes in mean waterlevel and tidal range, and regular 14 d and 28 d cycles in most years. These cycles have been related to the role of the entrance constriction in generating spring-tidal pumping (STP). The rapid changes in tidal amplitude and mean waterlevel are explained in terms of changes in the hydraulic resistance of the entrance and have been correlated with outflows from the tidal lagoon caused by typhoons and catchment runoff. An annual cycle of amplitude 12.5 cm has been explained in terms of the annual cycle of the Kuroshio. Novel features of this study are the extensions to conventional time series analysis, and showing that the 14 d periodic fluctuations were caused by STP and that this process also contributed to the 28 d fluctuations. STP is invariably present in estuaries with a shallow inlet but is rarely identified; this paper shows methods to evaluate it and other long period components of the waterlevel record.

A post-2011 Tohoku earthquake tsunami survey was conducted at the Nakoso Coast, Fukushima Prefecture, Japan, on Mar 24-25 and Apr 1-2, 2011. Spatial distribution of the tsunami inundation height, together with the seawall height and damages, was investigated. We found a sharp contrast of tsunami damages within the target area; tsunami damage is the most serious in the north region (north of the Samegawa River) where seawalls with different heights played diverse roles for protection of local community. A relatively high seawall in this region kept intact after tsunami attack, and protected the lee side human properties; whereas, 71% of the low seawalls were broken during the tsunami and the disaster developed in the corresponding sheltered area. A sharp contrast at the boundary of these two types of seawalls was observed. Behind the high seawall, the tsunami intensity and velocity were significantly reduced and the inundation depth is only 0.6 m. Nevertheless, tsunami intensity was enhanced in the seawall-broken area with an inland inundation depth being 4.7 m, which ruins away the local houses. In the middle region between the Samegawa River and the Bindagawa River, the high seawall survived from the tsunami attack. We confirmed that the coastal pine tree forest, serving as a buffer zone, played a significant role in reducing the tsunami height approximately by 4 m within a cross-shore distance of 230 m. To the south of the Bindagawa River, the tsunami damage is the mildest due to the protection by the robust high seawall as well as high elevation of land topography.

Intensive field measurements of water quality and current were carried out in summer 2010 off the Toyogawa river mouth located at the head of Mikawa Bay. Structures of water density and dissolved oxygen concentration and their variations were discussed based on the data. The residual currents were compared with wind data and freshwater discharge from the river. The estuarine circulation observed in the measurement seemed to be caused mainly by wind action, but large amount of freshwater discharge may cause current toward the river mouth near the bottom. Dissolved nutrients especially phosphorus may be transported upstream by the bottom flow and used in the primary production in shallow waters.