Hydrological modeling with reservoir operation in the Chao Phraya River Basin for flood mitigation

Flood disasters are the leading cause of deaths and economic losses among natural catastrophes. The extent of damages due to flood disasters is more pronounced in Asia [Berz, 2000]. In 2011, the worst flooding worldwide in terms of economic losses which happened from 1900 to 2012 occurred in Thailand [EM-DAT, 2012], particularly in the Chao Phraya (CP) River Basin. The geographic location and characteristics of the CP River Basin make it prone to flooding. The disaster is said to be mainly caused by occurrences of several intense rainfall [Komori, et al., 2012]. Although the flood disaster was mainly caused by extreme precipitation, other dimensions of the flood disaster such as reservoir operation should not be ignored. Around October 2011, for example, several local news [Bangkok Post, 2011] pointed out that too much water was being released from the big dams upstream while flooding is already occurring downstream. The main issue revolves around not reducing the dam storage to a low level because of fears of running out of water for the next dry season. It is important to note that Thailand is an agricultural country which makes it sensitive to droughts. The difficulty in flood risk management in this case arises from conflicting storage of water for agricultural purposes for the upcoming dry season. This study aims to understand and resolve several issues in mitigating flood risks in the CP River Basin with the aid of hydrological models. It particularly focuses on three key factors which significantly affect flood mitigation in the CP River Basin: (1) rainfall-runoff relationships, (2) inundation, and (3) reservoir operation. The two biggest dams, Bhumibol and Sirikit, were chosen for the reservoir operation studies. Drought risk mitigation was taken into consideration due to its importance to the economy of Thailand. The results of this study are deemed to be useful for implementation or further improvement of the Chao Phraya River Master Plan which is currently under revision. The methodology and results of this study were presented in three parts. In the first part, the naturalized discharge was simulated using H08, an integrated global water resources model. H08 was calibrated at C2 Station, then simulations were conducted from 1981-2004. The results showed good correspondence with the naturalized observed discharge and yielded high Nash-Sutcliffe Efficiency coefficients in the daily, monthly, and annual scales. The results were then validated in four other stations. After which, simulations were conducted from 2010-2011. The simulation at 2010-2011 showed good results, attesting to the capability of the calibrated model to adequately predict river discharge given a good precipitation forecast. The second part of the study deals with simulating both the naturalized discharge and inundation. The runoff output of H08 was used as input to CaMa-Flood, a global river routing model that incorporates floodplain inundation dynamics. Again, parameters of H08 and CaMa-Flood were calibrated to suit the local conditions in the CP River Basin. Simulations of naturalized discharge were again conducted from 1981-2004 and 2010-2011. The results of the coupled H08 and CaMa-Flood (H08-CaMa) models showed an improved simulation of naturalized river discharge as well as good simulation of the inundated area. The general shape of the simulated inundated areas showed good correspondence with satellite images taken during the 2011 flood event. However, the percent of flooded area was overestimated. The third part deals with incorporating the effects of reservoir operation into the simulation of discharge and inundation in the CP Basin. The reservoir operation module of H08 was used to simulate the reservoir operation rules in the two big dams, Bhumibol and Sirikit. Two types of simplified reservoir operation schemes were explored: one using a 2-season, constant seasonal dam release and the other using a 5-season, varying seasonal dam release. The dam operations in the past were taken into account in creating these schemes. Using the 5-season, varying seasonal dam release type of operation, the storage levels that should be reached by the end of April and which could either mitigate flooding or drought were found. To simulate both discharge and inundation, the discharge at Bhumibol and Sirikit Dams were used as forcing input discharge to the coupled H08-CaMa model. The analysis showed that the model could simulate both the historical discharge and inundation with dam operation. Further analysis of several proposed dam operation schemes showed that lowering the dam storage to a certain level by the end of April and maintaining this low storage until the end of July would reduce the occurrence of dam overflows. Controlling the dam release as a function of storage during the dry season would prevent the occurrence of dam dry ups. This kind of operation was shown to reduce the dam discharge downstream as well as inundation during the wet season. This part of the study proved that reservoir operation could be modeled and its effects downstream could be simulated. As a whole, this study proved that the current research framework is effective in simulating both the naturalized and dam-influenced discharge in the CP River Basin. The results of this study have numerous applications. First, it could be used to predict the discharge and inundation given a good precipitation forecast. Second, the long-term analysis of the discharge and inundation could help in revising the design of flood structures. Third, it could be used to analyze the effects of several planned reservoir operation to the downstream discharge and inundation. In summary, this research has developed a framework and a tool which could be used for decision and policy making towards flood mitigation in the Chao Phraya River Basin.報告番号: ; 学位授与年月日: 2012-09-27 ; 学位の種別: 修士 ; 学位の種類: 修士（工学） ; 学位記番号: ; 研究科・専攻: 工学系研究科社会基盤学専