Daisuke Komori

Assessing the impacts of reservoir operation to floodplain inundation by combining hydrological, reservoir management, and hydrodynamic models

A catastrophic flood event which caused massive economic losses occurred in Thailand, in 2011. Several studies have already been conducted to analyze the Thai floods, but none of them have assessed the impacts of reservoir operation on flood inundation. This study addresses this gap by combining physically based hydrological models to explicitly simulate the impacts of reservoir operation on flooding in the Chao Phraya River Basin, Thailand. H08, an integrated water resources model with a reservoir operation module, was combined with CaMa-Flood, a river routing model with representation of flood dynamics. The combined H08-CaMa model was applied to simulate and assess the historical and alternative reservoir operation rules in the two largest reservoirs in the basin. The combined H08-CaMa model effectively simulated the 2011 flood: regulated flows at a major gauging station have high daily NSE-coefficient of 92% as compared with observed discharge; spatiotemporal extent of simulated flood inundation match well with those of satellite observations. Simulation results show that through the operation of reservoirs in 2011, flood volume was reduced by 8.6 billion m3 and both depth and area of flooding were reduced by 40% on the average. Nonetheless, simple modifications in reservoir operation proved to further reduce the flood volume by 2.4 million m3 and the depth and area of flooding by 20% on the average. By modeling reservoir operation with a hydrodynamic model, a more realistic simulation of the 2011 Thai flood was made possible, and the potential of reducing flood inundation through improved reservoir management was quantified.

Numerical Study on Tsunami Propagation into a River

ABSTRACT Aoyama, Y.; Adityawan, M.B., Widiyanto, W., Mitobe, Y., Komori, D., and Tanaka, H., 2016. Numerical Study on Tsunami Propagation into a River. 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. 1017 - 1021. Coconut Creek (Florida), ISSN 0749-0208. The Tohoku Earthquake and Tsunami in 2011 caused serious damage not only to coastal structures, but also to riverine infrastructure due to long distance of wave propagation into rivers located on the coast facing the Pacific Ocean. Although there have been numerous investigations on tsunami, limited number of studies have been made for tsunami propagation into a river channel. This study investigates tsunami propagation into a river numerically by comparing with laboratory experiment data. The present numerical simulation is based on shallow water equations, which are solved with the MacCormack scheme. Difference between calculated and experimental results are evaluated in terms of root mean square error. It is concluded that the present numerical simulation yields good agreement with experimental data in a wave flume. In addition it is observed that geographical characteristics in the river channel, such as sandbars and estuarine sand spits, highly affect tsunami propagation process in a river, causing lowering water level along with late arrival of tsunami peak.

Opportunities and constraints in adapting to flood and drought conditions in the Upper Chao Phraya River basin in Thailand

ABSTRACT To assess alterations of the rice-growing season as a sustainable option to overcome temporary groundwater storage shortages resulting from flood and drought conditions in the Upper Chao Phraya River basin in Thailand, rice was planted in May (1.5 months earlier) to harvest before seasonal flooding occurred (normally in October). The assessment showed that a cropping intensity (CI) of approximately 1.4 (100% and 40% of the total agricultural area for the wet and dry seasons, respectively) could be supported by the total available water (surface runoff and renewable groundwater storage). Shifting the growing season did not have a negative impact in terms of rice production area, but it did ensure that the area of rice cultivation avoided flooding or surface water shortage conditions. Expanding the rice production area was also assessed; however, this required the implementation of artificial groundwater recharge (AGR). For example, 1.0 km3 of AGR annually was enough to support CI equal to 1.5 without experiencing groundwater depletion.

Estimating woody debris recruitment in a stream caused by a typhoon-induced landslide: a case study of Typhoon Lionrock in Iwaizumi, Iwate prefecture, Japan

Abstract A landslide can generate large amounts of debris in the form of boulders, cobbles, soil, and wood. The woody debris produced by a landslide flows into a downstream river or village; it can form obstructions in the stream and destroy houses. In this study, we aimed to develop a procedure for estimating woody debris recruitment into streams following a landslide. Understanding the volume of woody debris can help predict and prevent hazards from this debris. The proposed procedure combines a shallow landslide model, tree density data, and observational data following landslide occurrence. The study site is a sub-watershed of the Omoto River watershed in the town of Iwaizumi in Iwate prefecture in Japan; this town was affected by Typhoon Lionrock in 2016. Typhoon Lionrock delivered over 200 mm of rainfall in 24 h and induced many landslides. Based on field surveys, we found that approximately 524 m3 of woody debris jammed the narrow section under a railway bridge (including voids) and approximately 178 m3 of woody debris to formed a dam in the stream channel of the target watershed (including voids). Using the proposed protocol, we estimate that woody debris recruitment to the stream was approximately 638 m3.