Toshihiko Yamashita

Evaluation of the impact of water dilution within the hypereutrophic Lake Barato, Japan

Abstract Field data and three-dimensional numerical simulations were used to evaluate the impact of water dilution on eutrophication. To dilute eutrophic water in Lake Barato, Japan, an Inlet Project was carried out during the summer of 2005 via a 1 m3/sec inlet. River water was discharged from the margin of the upper basin of the lake to dilute the water. We undertook a numerical simulation of total nitrogen (TN) and total phosphorus (TP) distribution considering five inlet cases by using Princeton Ocean Model (POM). Total N and TP concentrations of inlet water were TN: 1.3 mg/l and TP: 0.08 mg/l. Model results suggest the most effective and feasible way to achieve dilution is via a 5 m3/sec inlet (78% related to the total volume in the lake) and opening the Shinko Gate, connecting the lake with Ishikari Bay. Following this scheme, TN and TP concentrations would be reduced by 28% following 30 days of discharge according to the simulation results.

Influence of Wind-induced Currents to Sand Drift around Bakkai Fishing Port

北海道北部の利尻水道に面した抜海岬の先端に建設さ れた抜海漁港(図-1)では,小規模な港ながら大量の漂 砂が発生し,港口等の激しい地形変化や港内埋没が長年 問題となってきた.しかし,漂砂外力としての強い流れ の発生は現地観測等では確認されているがその発生機構 は不明な点が多い.潮汐の小さい日本海の港周辺の地形 変化予測の際,流れは海浜流のみを考慮する場合が多い. しかし抜海漁港は前面に礼文島,利尻島があり,西~南 西方向の波の場合は遮蔽され弱い海浜流しか発生しな い.一方,稚内周辺は風力発電も盛んな強風地域であり, 強い吹送流が発生し,かつ狭窄部である利尻水道や抜海 岬で加速されることが考えられる. 佐藤(1995)は,日本海側の深い水深域(水深15m程 度)での流動は継続する強風による吹送流が主要因であ ることを現地観測と数値解析で明らかにした.また,吉 永ら(2008)は新潟西海岸に設置された潜堤周辺の流 動・地形変化特性が海浜流にアメダス地点の風による吹 送流を考慮する事で予測精度が向上することを報告して いる. 抜海漁港は,上述のとおり波浪と風況等が特異な環境 であることからその漂砂のメカニズムは十分な解明には 至っていない.そこで,本研究では,まず深浅測量デー タから漁港周辺の地形変化特性を明らかにする.次に流 動の現地観測結果より流動の時空間特性を把握し,海浜 流と吹送流の計算値と比較検討することにより,流動の 発生機構を考察する.さらに,高波浪時の波向き毎の波 高と広域海上風の相関を把握し,流れとして吹送流を付 加した地形変化予測を行い,実測値と比較検討する.

Ground Deformation Characteristics and Influences on Coastal Area in East Hokkaido

Characteristics of ground deformation are surveyed from the data of triangulation point, bench mark, permanent GPS monument and tide level, in east area of Hokkaido. The rate of subsidence in east area of Hokkaido is high, especially, is 1.36cm/year in Notsuke peninsula. The change velocity of tide level is almost constant in the long term. In the area that has high velocity of tide level change, the effect of sea level rise is relatively small, and change velocity of tide level is almost equal to that of subsidence. In Notsuke peninsula, the erosion of coastal area is a serious problem. About quarter of coastal erosion in Notsuke peninsula is caused by subsidence.

FIELD OBSERVATION OF FRAZIL ICE RIVER IN COLD REGION

In cold region, many rivers and lakes are freeze up in winter season. Frazil ice is a small particle that generated in supercooled and turbulent water surface. They are casual factor of freeze up the water surface of rives. In addition, the water intakes that located at riverside are often clogged by accretion of frazil ice at midwinter. To overcome about these types of ice problem, it is important to know about the property of the frazil ice flow down in a cold stream. In this study, in order to collect frazil ice, field observations were carried out at the Teshio River in winter season. The distribution of frazil ice in the stream and the volume of generated frazil ice were measured.

EFFECTS OF SUSPENDED SEDIMENT ON BEHAVIOR OF DISSOLVED PHOSPHATE IN ESTUARY OF THE ISHIKARI RIVER

Field observations were carried to investigate effects of suspended sediment on the behavior of dissolved phosphate in estuary of the Ishikari river during the snowmelt season in 2004. Suspended sediment was sampled during the snowmelt flood at Sapporo Oh-hashi, at the same time water quality was observed in the coastal area near the river mouth. The phosphorus chemical combined forms of the samples were analyzed, and experiments were conducted to study variation of the release of phosphate to salinity and pH change. Behavior of the phosphate adsorption from the suspended sediment was followed the Langmuir relation in each salinity and pH condition. At the flood dissolved phosphate concentration changed at the estuary, and was highest at lkm from the river mouth. And the behavior of the dissolved phosphate in estuary was explained by the Langmuir relation.

CHARACTERISTICS OF THE FLUCTUATIONS OF CURRENTS AND THE WATER TEMPERATURE AT THE ISHIKARI BAY IN THE SUMMER SEASON

Field observation of currents and water temperature was carried out at the sites of water depth of 700m and 40m of the Ishikari bay in the summer season. At the site of water depth of 700m, the change of flow and the water temprature at the cycle of 15-20 days according to change of Tsushima warm current is dominant.The vertical distribution of the Tsushima warm current was clarified.The change of flow at the cycle of 16 hours exists. At the site of water depth of 40m, flow is mainly driven by the wind, and the change of flow at the cycle of about four days is dominant. The compensation flow by the land breeze is shown in the decrease in the water temperature. This shows upwelling of the deep water. The changes of flow and water temperatureat the cycle of 16-24 hours are exist.

INVESTIGATION ON CONTENT OF PHOSPHOROUS ABSORBED IN SUSPENDED SEDIMENT TRANSPORTED TO THE COAST

Conditions of watershed and rivers are closely connected with coastal environments. Suspended sediment is important to water quality on the coast. In particular, at times of flooding a large quantity of suspended sediment is transported to the coast, and the majority of phosphorus is transported in particle form rather than dissolved form. But not all sediment phosphorus circulates to the coast, for example apatite phosphorus. For this reason, a quantitative analysis on the chemical form of phosphorus was conducted. Suspended sediment was sampled during flooding of the Ishikari, Shiribetsu and Mukawa rivers, and the phosphorus chemical combined forms of the samples were analyzed. The results show that more than 60% of the phosphorous in the suspended sediment is circulating phosphorous, and that the total phosphorous content is related to the suspended sediment diameter and is different for each river according to land use.

EXPERIMENTAL STUDY ON ABRASION OF SAND PARTICLES AND VERTICAL DISTRIBUTION OF GRAIN SIZE DUE TO DRIFT SAND

Abrasion of sand particles due to littoral drift was studied experimentally by large scale oscillatory flow apparatus. Abrasion of sand of 1.5mm in median diameter generates fine-grained sand with a wide range (2μm-1000μm) diameter. Fine-grained sand accumulates below the layer of sheet-flow sediment motion. The characteristics of vertical distribution of grain size are studied.

CHARA CTERISTICS OF COASTAL CURRENTS IN TOMAKOMAI COAST

To understand the environments of water quality, sediments and ecology in a coastal sea area, the characteristics of coastal currents must be clarified, In this study, we analyzed observation data of flow velocity and wind velocity in Tomakomai Coast including the area of Mukawa River mouth.Along Tomakomai Coast, a strong south-west or south-east wind often cased eastward or westward currents parallel to the shoreline in winter. Flow velocity in offshore of Siraoi Coast was strong and more than 30cm/s at a depth of 50m. In both summer and winter wind occurred currents parallel to the shoreline.In summer, fiow fluctuations with the period of about one day were occured by sea and land wind.

EXPERIMENTAL STUDY ON THE EFFECTS OF DRIFT SAND ON THE CONSUMPTION OF ALGAE BY SEA URCHIN

Communities of algae have disappeared from the shallow waters along the Japan Sea coastline in southwest Hokkaido. A large number of studies have been made on coralline flats, it has been shown that the consumption of algae by sea urchins is an important factor affecting the formation of coralline flats, and is closely related to the physical environment of coastal areas. Recent investigations have shown that algae communities have formed at the region surrounded by sand and gravel in coralline flats. The effects of drift sand on the consumption of algae by sea urchin are experimentally studied. The smaller sand diameter is, and the larger fluid velocity is, the harder sea urchin moves to algae due to drift sand. Therefore consumption of algae by sea urchin decreases under these conditions. Strongylocentrotus intermedius dislikes drift sand more than Strongylocentrotus nudus, so consumption of algae by Strongylocentrotus intermedius decreases.