Tadashi Suetsugi

Impact of climate and land-use changes on hydrological processes and sediment yield—a case study of the Be River catchment, Vietnam

Abstract The impact of climate and land-use changes on hydrological processes and sediment yield is investigated in the Be River catchment, Vietnam, using the Soil and Water Assessment Tool (SWAT) hydrological model. The sensitivity analysis, model calibration and validation indicated that the SWAT model could reasonably simulate the hydrology and sediment yield in the catchment. From this, the responses of the hydrology and sediment to climate change and land-use changes were considered. The results indicate that deforestation had increased the annual flow (by 1.2%) and sediment load (by 11.3%), and that climate change had also significantly increased the annual streamflow (by 26.3%) and sediment load (by 31.7%). Under the impact of coupled climate and land-use changes, the annual streamflow and sediment load increased by 28.0% and 46.4%, respectively. In general, during the 1978–2000 period, climate change influenced the hydrological processes in the Be River catchment more strongly than the land-use change. Editor Z.W. Kundzewicz; Associate editor Q. Zhang Citation Khoi, D.N. and Suetsugi, T., 2014. Impact of climate and land-use changes on hydrological processes and sediment yield—a case study of the Be River catchment, Vietnam. Hydrological Sciences Journal, 59 (5), 1095–1108.

METHODS FOR MEASURING BEDLOAD DISCHARGE BY TRACKING DUNE MOVEMENT AND BEDLOAD PARTICLES BY USING ACOUSTIC SENSING

河道の流砂量を精度良く計測することは重要な技 術課題である.浮遊砂・ウォッシュロードに関して は採水分析,光学式濁度計 ,差圧式濃度計 ,超音 波流速計 3 など様々な技術が実用化されており,当 該河川の特徴や洪水の水理量などに応じて適切な方 法を採用すればよいところまできている. 一方,河床変動等を予測する上で最も重要な掃流 砂に関しては,バケット型採取機,トレンチ方式な どが古くから試みられ,最近では衝突音式 ,転倒 マス式 ,超音波計測 などユニークな方法が提案 されている.しかし,バケット型採取機は洪水中に 沈める作業が相当危険であり,トレンチ方式は小規 模だと洪水初期に埋没してしまい,大規模だと施工 費等の面で困難である.また,最近提案されている 各種方法は精度や操作性などの面からまだ実用の域 に達していないと思われる. これらの方法のうち,超音波を利用する技術は水 面に装置を浸すことで水中の様子をリモートセンシ ングできるため,安全性や簡便性などの面から将来 性が期待される.そこで本研究では,横山ら が提案 した音響式SS 測定装置を用いて掃流砂を計測するこ とを目的として,水路実験により精度を検証する. すでに開発済みのSS 測定装置を活用することで,掃 流砂専用の装置を作成せずに済むため,ひとつの装 置で水中の全流砂を把握することが可能になり,さ らにコストを下げられる利点を目指している. A hydraulic experiment was conducted on a moving bed channel to develop a method for measuring bedload discharge using Acoustic sensing. Sounds at a frequency of 960 kHz are simultaneously emitted at the upstream and downstream beds from two transducers; the detected sound is expected to reveal some information about the moving bed. The bed forms were surveyed continuously, and the dune shape and rate of progress were analyzed. The time series of the cross-sectional dune shape was clearly understood. The bed load discharge was calculated using the dune height and rate of progress based the assumption that the dune is triangular in shape; the estimated discharge from Acoustic sensing agreed well with the measured sand discharge. The signal strength propagation on the bed surface from upstream to downstream was analyzed as the velocity of moving particles on the bed. The estimated particle velocity of the bottom layer was explained by a logarithmic velocity distribution. 超音波による河床波・転動粒子の追跡と 掃流砂量の推定方法

STUDY OF GRAVEL-BAR SUSTAINABILITY AFTER RESTORATION WORK ON THE NAGATA DISTRICT IN THE TAMA RIVER

On the Nagata district in the Tama River, gravel-bar restoration project started in 2001. In such a project, it is important to monitor and estimate the condition of river environment before and after the project. Some of authors have proposed an index, which expresses the sustainability of gravel bar based on the mechanism sustaining it, related to the gravel-bar restoration project. In this study, hydraulic model test was conducted to predict the riverbed change after the project, and the condition of gravel bar was estimated with the index. Though it only expresses the condition at a point of gravel bar, it was recognized that the index is efficient and practical to roughly estimate it. On the other hand, to estimate the distribution of the vegetated area and bared land area in some reaches of the river, a two-dimensional flow calculation model was developed and the condition of gravel bar was estimated applying the index to the calculated result, which explains the recent distribution of them in a gravel bar of Nagata district. From these results, it was shown that this extended index is efficient to estimate the gravel-bar sustainability.

BANK EROSION AND BED TOPOGRAPHY NEAR NATIONAL HIGHWAY NO.4 BRIDGE BY THE 1998 FLOOD IN YOSASA RIVER

In the downstream basin, risk of inundation is lower. Bank erosion often occurs in the upstream basin of rivers due to rapid flood flow. Predicting weak points of bank erosion is also important to improve river efficiently. At a present stage, It is important that machanism of bank erosion is investigated by the use of field data, model experiments, numerical analysis.In this paper, it is made clear that causes of bank erosion and bed topography near national highway No.4 bridge by the 1998 flood in Yosasa river.

THE INFLUENCE OF WATER HEAT ENERGY USE ON STREAM TEMPERATURE IN URBAN RIVERS UNDER TIDAL VARIATION

To predict the water temperature in lower reaches of urban rivers, field measurements and model development were performed for simultaneous transport of water and heat in a stream. Detailed information on river flow and heat transport under tidal motion was obtained through an intensive measurement in the lower reach of the Ara River. Based on the knowledge of time variations and cross-sectional profiles of the flow and the stream temperature, a onedimensional dynamic model is developed to predict flow and water temperature in tidally affected urban streams. The model takes into account of tidal motion, longitudinal dispersion, water exchange with tributaries, wastewater effluent and the heat exchange between the atmosphere and the stream water. Using the calibrated model, effects of heated water discharge on the stream temperatures due to the district heating and cooling system were estimated for a possible scenario in which the river thermal energy is used through the water heat pump technology.

Development of a simplified Water Balance Model in an Urban watershed

Urban surfaces covered with asphalt and concrete have been changing hydrological cycle in urban river basin. To assess the effects of various measures for conserving natural water balance, its appropriate quantification must be carried out.As a detailed model for estimating hydrological cycle requires a lot of hydrologic data and computational resource. We tried to buildup a simple model which utilize existent data source to grasp monthly water balance. The model was applied to the Tsurumi river basin to verify the accuracy in estimating flow rate and other hydrological variables. It is found that the model is likely to be useful in assessing monthly water balance both in rainfall and non-rainfall periods.