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Asano Y.,University of Tokyo | Uchida T.,Japan National Institute for Land and Infrastructure Management
Water Resources Research | Year: 2012

Transit time of discharge is a hydrological characteristic used in water resource management. Previous studies have demonstrated large spatial variation in the mean transit time (MTT) of stream base flow in meso-scale catchments. Various relationships between topography and MTT have been reported. Although it is generally assumed that base flow MTT is controlled by the depth of the hydrologically active layer that recharges a stream, this hypothesis has not been tested in field studies. This study confirmed that the depth of hydrologically active soil and bedrock controls spatial variation in MTT. The study used isotopic and geochemical tracer data gathered in the 4.27 km 2 Fudoji catchment, central Japan. The results, together with previously documented relationships between topography and MTT, indicate that the depth of the hydrologically active layer is sometimes, but not always, related to topography. A comprehensive understanding of the factors that control base flow production in mountainous catchments will require further study of the water flow path depths that recharge streams. Copyright © 2012 by the American Geophysical Union.

Tebakari T.,Toyama Prefectural University | Yoshitani J.,Japan National Institute for Land and Infrastructure Management | Suvanpimol P.,Office of Hydrology and Water Management
Hydrological Processes | Year: 2012

The Chao Phraya River basin, the largest basin in Thailand, is located in the centre of the northern part of the country. This basin has two large-scale reservoirs: the Bhumibol Reservoir on the Ping River and the Sirikit Reservoir on the Nan River. A comparison of the annual and monthly flow regimes downstream from the reservoirs before and after reservoir development showed a constant increase in low flow and a drastic decrease in high flow. The spectrum of the daily discharge was analysed using the fast Fourier transform on data collected in the area of Nakhon Sawan and immediately downstream from the Bhumibol Reservoir after the reservoir was constructed. The flow at Nakhon Sawan had a periodic characteristic of 7 days. The water released from the Bhumibol Reservoir at hydrological station P.12 also had a periodic characteristic of 7days. Reservoir operations have a significant impact on the hydrological cycles. The effect of human activities is evident in the spectrum analysis of recorded historical discharge data. © 2012 John Wiley & Sons, Ltd.

Furukawa K.,Japan National Institute for Land and Infrastructure Management
Ocean and Coastal Management | Year: 2013

To effectively restore coastal habitats in urban areas, the surrounding dynamic coastal ecosystems, such as wetlands must be restored. In well-developed urban areas, most aquatic environments may be degraded in terms of water and sediment quality, as well as the essential hydraulic circulation. Restoring coastal habitats in urban areas is therefore a challenging task. Environmental status and its effort to restoration summarized for Tokyo Bay, Japan followed by two case studies of urban wetland restoration. At Shibaura Island, Tokyo, tide pool construction is shown as hardware type restoration case study. One of the means to ensure success in restoring habitats in an urban area is to use a sound ecosystem-based approach to guide the selection of restoration sites and the application of appropriate engineering and management methodologies. At Yokohama, software type restoration by terrace type habitat creation is shown. The development and management of the site with public participation, undertaken in an adaptive manner, is a key element of successful habitat restoration. This paper presents lessons learned from urban habitat restoration projects in Japan as examples. © 2012 Elsevier Ltd.

Ashie Y.,Japan National Institute for Land and Infrastructure Management | Kono T.,Japan Building Research Institute
International Journal of Climatology | Year: 2011

Recently, countermeasures against the urban heat island effect have become increasingly important in Tokyo. Such countermeasures include reduction of anthropogenic heat release and enhancement of urban ventilation. Evaluations of urban ventilation require construction of a high-resolution computational fluid dynamics (CFD) model, which takes into account complex urban morphology. The morphological complexity arises from multi-scale geometry consisting of buildings, forests, and rivers, which is superimposed on varying topography. Given this background, airflow and temperature fields over the 23 wards of Tokyo were simulated with a CFD technique using a total of approximately 5 billion computational grid cells with a horizontal grid spacing of 5 m. The root mean square (RMS) error of the air temperature between the simulation and observation results at 127 points was 1.1 °C. Using the developed model, an urban redevelopment plan for two districts in metropolitan Tokyo was examined from the viewpoint of air temperature mitigation. Numerical results showed that a reduction by 1 ha in the area covered by buildings increases the area with temperatures below 30 °C by 12 ha. Copyright © 2010 Royal Meteorological Society.

Sato C.,Tokyo Kyuei Co. | Nakayama K.,Kitami Institute of Technology | Furukawa K.,Japan National Institute for Land and Infrastructure Management
Estuarine, Coastal and Shelf Science | Year: 2012

This paper proposes a method of evaluating the occurrence of hypoxia or anoxia in bottom water of Tokyo Bay, which is typical of enclosed bays in Japan. In Tokyo Bay, the exchange of seawater with the ocean has previously been found to be dominantly controlled by estuarine circulation. Thus, one would expect the concentrations of dissolved oxygen (DO) in the bay head to be influenced mainly by estuarine circulation, which changes according to wind strength and river discharges. We investigated the effects of wind and river discharge on DO concentration around the bay head using a three-dimensional ecological model. To evaluate the occurrence of hypoxic or anoxic water, we developed a conceptual DO model, which was verified through the good agreement with the results from a three-dimensional ecological model. We conclude that the conceptual DO model has good potential for evaluating the factors leading to hypoxic or anoxic water around the bay head. Modeling suggested that, on average, wind effects were the dominant factor in the variation of bottom DO concentrations. However, we found that the contribution of extreme events, such as floods and strong winds exceeding 10 m s -1, was 50% or more. This suggests that extreme events play an important role in controlling the variation in DO concentration at the bottom of Tokyo Bay. © 2012 Elsevier Ltd.

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