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LONG-TERM CHANGE OF WATER QUALITY IN THE RESERVOIR OF THE ISAHAYA BAY RECLAMATION PROJECT Y. Mitsugi; N. Vongthanasunthorn; Y. Mishima; K. Koga; H. Araki; P. Ittisukananth
Lowland Technology International Vol 15 No 1, June (2013)
Publisher : International Association of Lowland Technology

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In 1997, the Isahaya Reservoir was constructed at the innermost part of Isahaya Bay, Japan to prevent natural disasters and to develop water resources for large-scale farm lands. The main purposes of this study were to analyze the mechanisms underlying the water quality changes observed and to collect significant information for water quality management at the reservoir. Observed water quality parameters on chlorophyll-a, suspended solids, total nitrogen, dissolved inorganic nitrogen, total phosporus, dissolved inorganic phosphorus and chloride ion were analyzed by using a water quality model. It was found taht the results obtained from the developed water quality model agreed with the observed data. After calibrating the observed data, a simple sensitivity analysis was conducted to demonstrate the mechanisms of the water quality changes. The major water quality problems were suspended solids that had been resuspended by strong wind and eutrophication due to the enrichment of nutrients. The major mechanisms of water quality changes in the Isahaya reservoir were a coagulation and flocculation by brackish water and the transformation process of phosphorus. The coagulation affected the settling velocity of suspended solids and dissolved phosphorus. It was also revealed that the resolution process of dissolved phosphorus from suspended solids was controlled by the salinity.

DEVELOPMENT OF AN EVALUATION METHOD FOR RIVER ENVIRONMENT H. Araki; K. Koga; Y. Nishi; Y. Kajiya; S. Imae; S. Kimura; K. Hirano; M. Sakanashi; S. Toyosaki; H. Fukuyama
Lowland Technology International Vol 1 No 2, Dec (1999)
Publisher : International Association of Lowland Technology

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Assessment in planning and designing process is necessary especially for nature-oriented river. The main purpose of this study is to develop an assessment method for river environment. At a workshop composed of experts on river environment, a viewpoint to identify characteristics of watershed and/or river environment was added to viewpoints of the River Council in Japan. Furthermore, the new concept of "intermediate nature" is proposed to develop a goal-oriented assessment method. In this study, the prototype of a new assessment method "EMOREK" is proposed, somewhat similar to the AMOEBA method developed in the Netherlands. A case study on the T-river in Japan was carried out to realize the capability of the assessment method through problem analysis on the river environment.

EFFECT OF SOIL STIFFNESS AND GROUND DISPLACEMENT IN EARTHQUAKE RESPONSE OF BRIDGES WITH PILE FOUNDATION R. Mahmudur; K. Izuka; K. Imamura; K. Koga; T. Ohtsuka; G. Aramaki
Lowland Technology International Vol 3 No 2, Dec (2001)
Publisher : International Association of Lowland Technology

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Dynamic analysis was carried for earthquake response of prototype bridge structure with pile foundation in clay soil with sand base. Analysis was carried out for different soil stiffness parameter, Vs (shear wave velocity)- value using "Single input model" which does not consider the ground displacement. It was found that with increasing soil stiffness, eefect of ground displacement on response becomes smaller and the two models yielded similar results. Consideration of ground displacement has great effect in earthquake response for clay with shear velocity, Vs less than 100 m/sec i.e. for soft clay. Vs-value of 150 m/sec may be considered the critical velocity after which consideration of ground displacement has much less effect and thus simpler "Single input model" may be used. This corresponds to cohesion, cu value of 60 kPa and N- value of 4 approximately. It was also shown that strain dependence of shear modulus and damping have significant effect on displacement of the soil deposit.

PREDICTION MODEL OF INUNDATION IN PUMPED-FIELD LOWLANDS AND SCENARIOS FOR IMPROVING ITS PERFOMANCE N. Cao Don; H. Araki; H. Yamanishi; K. Koga
Lowland Technology International Vol 5 No 1, June (2003)
Publisher : International Association of Lowland Technology

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During rainy season, frequent flooding by strom water is one of the most serious problems in lowland areas, causing heavy effect on transportation, agriculture, industry, and economic activities. As a result, the required drainage water levels in this area are generally lower than the water levels of boundary rivers. Under such circumstances, pumping system should be designed from a viewpoint of integrated control floods. The Nam Ha lowland, in Vietnam, bounded by four surrounding rivers, is selected as a case study. The operation scheme in this area is a key factor for drainage and flood protection. This developed mathematical model can be used as a tool to evaluate the present drainage system as well as flooding processes in the field as well as inundation areas and water levels along drainage channels. It is found that the proposed model can be applied to evaluate integrated flood control system for pumped-field lowland. Such an operating system provides an effective tool by means of which the drainage system can be operated appropriately taking into account of tidal effects, rainfall intensity, and reaching time of the rainwater.

HYDRAULICS OF GROUNDWATER FLOW AND MECHANICAL PROPERTIES AFFECTING AQUIFER SYSTEM COMPRESSION IN SHIROISHI, SAGA PLAIN N. Cao Don; H. Araki; H. Yamanishi; K. Koga
Lowland Technology International Vol 5 No 2, Dec (2003)
Publisher : International Association of Lowland Technology

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Land subsidence in Shiroishi area in the Saga plain, Japan, has become noticeable since 1960s. Water supplied to agriculture has traditionally been a high priority for water managers in this region. Intense withdrawals of groundwater in excess of natural recharge have resulted in land subsidence in this region. Land subsidence can have several negative economic and social implications. Many problems associated with land subsidence can be observed in this area. In this paper, a numerical model that incorporated 3-D groundwater flow and I-D ground consolidation was applied to simulate groundwater level and ground consolidation. It is found that subsidence rapidly occurs throughout the area with the central prone in Shiroishi basin. Certain hydrogeologic parameters such as inelastic storage coefficient, elastic storage coefficient, and hydraulic conductivity of soil layers contribute significantly effects to both the rate and magnitude of consolidation. Moreover, a considerable reduction in discharge is supposed necessary for future development of the region to alleviate the effects of groundwater overdraft.

ESTIMATING THE POTENTIAL FOR CONJUNCTIVE WATER MANAGEMENT IN COASTAL PLAINS N. Cao Don; H. Araki; N. T. Minh Hang; H. Yamanishi; K. Koga
Lowland Technology International Vol 8 No 1, June (2006)
Publisher : International Association of Lowland Technology

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This paper presents the development and application of a conjunctive water management model for lowland catchments. The model incorporates a simulation model and a management model to simulate groundwater movement, ground consolidation and to search for the potential pumping amount of groundwater without violating physical and environmental constraints. The results reveal that groundwater levels in a coastal aquifer greatly vary in response to pumping. Consequently subsidence rapidly occurs throughout the area. The study also suggests that conjunctive water management can be used to improve water supply reliability, to reduce groundwater overdraft and land subsidence and to improve environmental conditions.

SEGREGATION OF MICROPHYTOBENTHIC CHLOROPHYLL-A FROM SUSPENDED PARTICLES DURING TIDAL CYCLE IN NANAURA MUDFLAT, ARIAKE SEA, JAPAN C.-H. Koh; J. S. Khim; H. Araki; H. Yamanishi; K. Koga
Lowland Technology International Vol 9 No 2, Dec (2007)
Publisher : International Association of Lowland Technology

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The fluxes of microphytobenthic chlorophyll a (Chl-a) and suspended particulate matter (SPM) in water column and their relationship against tidal current have been examined during 15-days of full spring-neap tidal periods in the upper intertidal flat of Nanaura, Saga, Ariake Sea, Japan. The flood-ebb and spring-neap tidal conditions influenced within-day and daily variability in the fluxes of Chl-a and SPM, in terms of tidal energy. The Chl-a flux decreased relatively at slower pace than corresponding SPM flux at stage of decelerating incoming tide, which indicated the segregation of Chl-a from SPM followed by the slower settlement at depositional period. This ‘decoupling’ trend was consistently observed during the high tidal energy period of spring, particularly during the lower wind period (0-1 m s-1), but this was no more prevalent during the low tidal energy of neap and/or higher wind period (3-5 m s-1).

STUDY ON ALGAL GROWTH IN ISAHAYA RESERVOIR P. Ittisukananth; K. Koga; N. Vongthanasunthorn
Lowland Technology International Vol 10 No 2, Dec (2008)
Publisher : International Association of Lowland Technology

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Isahaya reservoir was constructed at the innermost sea of Isahaya Bay, Japan, in 1997 for preventing natural disasters and developing water resources. Algal growth in this reservoir has been changed since the beginning of the Isahaya-Bay Sea Reclamation Project. As the result, the main purpose of this study is to investigate the effect of algae growth in the Isahaya reservoir using a water quality model. In mass balance equation of this model, several interactions among water quality parameters, namely chlorophyll-a (Chl-a), chemical oxygen demand (COD), suspended solid (SS), total nitrogen (TN), dissolved inorganic nitrogen (DIN), total phosphorus (TP), dissolved inorganic phosphorus (DIP) and chloride (Cl-), are incorporated. Sensitivity analysis reveals that nutrients in the Isahaya reservoir are contributed from land area, resuspension-release from mud bed, and coagulation-flocculation-precipitation by seawater. This study found that, before fiscal year 2000, lime (calcium hydroxide, Ca(OH)2) which is used for soil improvement was one of algal growth inhibition in the Isahaya reservoir. Since fiscal year 2000, diatom is the most dominant algal species in this reservoir.

SOIL IMPROVEMENT OF SOFT GROUND AROUND PILE FOUNDATION IN EARTHQUAKE-RESISTANT DESIGN T. Ohsutka; G Aramaki; K. Koga
Lowland Technology International Vol 6 No 1, June (2004)
Publisher : International Association of Lowland Technology

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When a pile foundation structure is design in soft ground with Japanese road bridge design spesifications, there are many cases in which the cross section, the amount of steel reinforcement, and the number of piles are decided according to the displacement limiting value. In such soft ground, it is possible to expect that displacement magnitude of the base structure is suppressed by improving the soft ground around the piles. This study clarified that there was a depression effect by improving the peripheral grounf of piles, for both the displacement of the base structure and the bending moment in piles. The calculation was carried out variously, by change of improvement depth and improvement width around the pile. Additionally, the effect of soil improvement for nonliniear response of the bridge pier was also examined. In a series of calculations, a two-dimensional finite element analysis program (FLUSH) often used in earthquake-resistant design was used.

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