(本文)(Abstract)

Hanoi, the 1000-year-old capital of Vietnam, had a population of 3.4 million in 2007. The old sewerage system and the quite flat floodplain made Hanoi more sensitive to urban flood. Frequent inundation caused the damages to infrastructure, transport, environment and human health. Sewerage system constructed between the years 1905-1945 and only covered an area of 1000 ha in the central part of the old city. The effluents drain to the South of Hanoi to the settling ponds in Thanh Tri district before eventually discharging to the Red River.

Drainage and sewerage form a combined system that flows by gravity into lakes, ponds and rivers. Inner parts of the city have an underground drainage network, while outer parts still rely entirely on open drains. From an engineering point of view, the system has major limitations. The total length of the central sewerage and drainage system, placed under maintenance and management of Hanoi Sewerage and Drainage Company (HSDC), is about 318 km, approximately the 60% of total road. It is estimated that the total coverage of the drainage system just accounts for 40% of the city's total area. The ratio of sewer length per capita in Hanoi city is about 0.3 m/person, much less than average ratio in other developing countries, i. e. 2m/person.

Inundation takes place quite often in Hanoi downtown, 5-7 times per year. When inundation occurs, solid waste, stormwater, wastewater from domestic, industrial, hospital sources without any treatment were mixed in the sewerage system, lake. This situation not only affects to the local areas during the inundation but also brings about environmental pollution and human health risk to downstream areas.

Several local studies mentioned about fluvial flooding, however, the urban inundation caused by internal runoff was not paid much attention. Inundation simulation and risk assessment of inundation in urban areas were also studied but mainly focused on flooding due to rivers and not discussed about water quality such as pathogenic indicators. This study was aimed to answer some questions as follows:

- What are the main reasons of inundation in Hanoi, what could affect to inundation situation in Hanoi?

- What is about characteristics of inundation in Hanoi? Which areas are more sensitive to inundation?

- How is about environmental impact and potential of human health impact when inundation occurs?

- Hanoi is city of lakes. How could we include these lakes as storage facility for contributing to inundation mitigation?

The overall objective of this study is (1) to characterize inundation situation and (2) to evaluate potential of environmental impact and human health impact regarding to pathogenic indicators under wet weather condition in Hanoi downtown.

The specific objectives were (1) to collect input data and elaborate for fitting requirement of inundation simulation and calibration, (2) to clarify which factors which could affect to inundation in Hanoi downtown, (3) to make inundation hazard map using simulation results and water quality monitoring for evaluation impact of inundation to human health and human activities, (4) to evaluate the role of lakes in inundation mitigation for Hanoi. Outcomes from this study may contribute to several aspects, such as:

- Providing useful suggestions for input data collection used to simulate inundation. Proposals of modified parameters for calibration model when applied to Hanoi.

- Getting deeper understanding about inundation characteristics in Hanoi (e. g. due to backwater, water clogging, deposit sediment in sewer system etc.), about the role of rivers and lakes system in urban drainage operation and management for Hanoi.

- Providing concrete evidences about potential impact of inundation to environment and human health in downtown of Hanoi

- Results could be applied to Hanoi drainage project Phase II (2008-2012) with the aim to integrate environmental aspect into urban drainage management

Dissertation was divided into 8 chapters; the main content focused on Chapter 4, 5, 6, and 7.

Chapter 1, sewerage system including sewer pipe, lakes, rivers are introduced to understand more about inundation condition in Hanoi and its impact to environment and human health. In this Chapter the main objectives, scope of research and content of dissertation are also presented to get an overview of the research scheme.

Chapter 2 discussed about related studies and research in Vietnam and other countries. The originality and new ideas also were introduced to make clear the strong points of this study.

Chapter 3 introduces the methodology and materials used to reach final objectives. Study area was introduced together with the detail methodology to collect and to elaborate input data for inundation simulation, in which, ground elevation data were described more detail. Water sampling procedures in Hanoi also were mentioned together with inundation simulation scenarios to characterization inundation conditions.

Chapter 4 provides results from input data collection and elaboration. Input data for inundation simulation in Hanoi were not available or quality was too rough. For example, rainfall data existed only in 1-hour interval, DEM data, landuse data were not available, sewer system was in a hard copy form. Other input data for model simulation such as infiltration parameter, wastewater profile, relation of area and water level in lake etc. also were not applicable. Output from this chapter will be used for inundation simulation in Chapter 6 and inundation hazard mapping in Chapter 7.

Chapter 5 shows water quality sampling and results on surface water environment, at inflows and outflows of lakes, road runoff and inundated water to get more concrete evident of inundation impact to surface water environment. The monitoring data showed that surface water quality exceeded the Vietnamese standard for surface waters in downtown area of Hanoi and polluted by discharging wastewater from sewerage system. The levels of COD, T-N, and pathogen indicators such as total coliform were also considerably high and comparative to the influent of sewage treatment plant. High values of Coliforms also indicated strong potential of human health impact when inundation occurs.

Chapter 6 introduces inundation simulation results using distributed model, XP-SWMM. Inundation conditions with different cases were applied to the selected areas to investigate the dependence of sewerage system under wet weather condition and characterizing inundation in Hanoi. Inundation characterization was investigated using different cases (different water level kept in lakes, boundary conditions, and structural measure to mitigate inundation etc.). Using some scenarios we also realized the role of lakes in inundation mitigation, inundation could be reduced by better operation and management of sewerage system.

Chapter 7 introduces the results of inundation hazard mapping and inundation impact evaluation including water quality monitoring in Chapter 5 and simulation results in Chapter 6. Simulation results with different rainfall return period (0.2, 0.5, 1, 2, 5, 10 years) were integrated with water quality monitoring on streets, on lakes and during inundation to make inundation hazard map. This map could provide information about depth, area of inundation and water quality at that point that mainly focused on pathogenic indicators. When overlapping inundation hazard map with population density, streets and communes having more potential of human health impact under wet weather condition were indicated.

Chapter 8 shows conclusions and recommendations Hanoi sewerage management and operation. Proposals to improve inundation condition in this area and other suggestions for further study also were provided.

東南アジアの発展途上国の大都市では,都市排水や下水処理の施設整備が遅れており,市街地の浸水常襲地域における健康関連微生物に着目した水質汚濁状況の評価を行うことは,浸水時における衛生状態や健康リスクを検討する上でも重要である.

そこで,本論文は,ベトナム社会主義共和国の首都ハノイ市を対象とし,密集市街地において晴天時だけでなく,雨天時や浸水時においても,健康リスクに関わる病原微生物指標を含めた水質モニタリングを現地で実施している.また,市街地に点在する多くの湖沼は,豪雨時における流出抑制効果が期待される一方,生活汚水の排除先でもあるために大規模な浸水時には汚濁物質の負荷源として作用する可能性がある.そこで,分布型都市流出解析モデルを利用して,都市排水施設の機能や湖沼の流出雨水の貯留機能を評価するとともに,浸水解析を通じた影響評価を検討した成果をまとめている.

本研究は,「Characterization of urban inundation impacts by distributed model simulation and pathogenic pollution monitoring in the downtown of Hanoi:分布型モデル解析及び健康関連微生物調査に基づくハノイ市街地における浸水インパクトの評価)」と題して,8つの章から構成されている.

第1章では,研究の背景と目的,および論文の構成を述べている.

第2章では,ハノイ市における水質汚濁状況や汚染源の整理,都市排水整備状況,浸水実態に加えて,水質調査に基づく汚濁状況に関する既存の研究をとりまとめて示している.また,都市の浸水解析に関する研究事例や洪水や浸水のリスクに関する文献の整理を行っている.

第3章では,調査研究の対象地域であるハノイ市街地の概況説明に加えて,分布型モデル解析手法や解析に必要なデータをリスト化して,求められる精度やデータ間の関連性を整理している.また,水質汚濁調査に関する調査方法や水質分析方法についても記載されている.

第4章では,まず,モデル解析に必要な降雨データとして,酸性雨調査用の降雨観測データを活用して,雨水流出解析に必要となる5分間隔の降雨データセットの作成を行っている.また,分布型雨水流出モデル解析に必要となる土地利用,下水道施設ネットワークデータ,汚水水質データなどの入力データ,浸水状況評価に必要となる地表標高データについて示されている.

第5章では,晴天時における広域的な河川や湖沼の水質汚濁調査,雨天時における道路排水や湖沼に排出されている雨天時下水の水質調査に関して結果がまとめられている.未処理下水の流入に伴う水質汚染が,河川や湖沼で進行していること,ベトナムの水質環境基準のBクラスも満足できていないことだけでなく,クラスター解析を利用して流入下水や処理水の水質との類似性から,水質汚染レベルの類型化を行っている.この整理手法や検討成果は,発展途上国における水質汚染実態を把握する上で有効であると考えられる.また,湖沼への流入下水と流出水の濃度変化から水質浄化機能についても検討をしている.さらに,2008年10月31日からの浸水時において,健康リスクに関わる病原微生物指標を含めて水質モニタリングを体系立て実施して,汚染レベルの高さを示した成果は非常に貴重なものである.

第6章では,新たに収集した都市流出解析用データを入力して,分布型都市流出解析モデルの一つであるXP-SWMMを用いて市街地の浸水解析を行っている.そして,浸水状況を定量的に評価している.特に,湖沼群の浸水抑制効果を評価するため,湖沼の有無,仮想的な排水ネットワーク,放流先である河川の水位などの状況を様々に設定して,浸水解析を行い,その機能や影響度合いを考察している.これらのシナリオ解析により,浸水リスクからみた排水路の特徴づけや改善手法を検討するために有意義な成果である.

第7章では,1年から20年までの確率年の計画降雨に対して,浸水状況を表示した地図を作成している.さらに,浸水時での採水・水質調査から得られた大腸菌群数やコミューン単位の人口密度などを考慮して浸水のハザードマップを作成している.そして,人の健康リスクや都市活動へのインパクト評価について考察を加えている.

第8章では,上記の研究成果から導かれる結論と今後の課題,浸水対策への提案や展望が述べられている.

以上の成果では,ハノイ市街地における都市河川や湖沼群の水質汚濁状況や水質浄化機能を調査した結果が示されている.また,雨天時都市排水の採水調査や浸水状況における水質汚濁調査も精力的に行い,貴重な健康関連微生物の汚染実態を把握している.さらに,過去の浸水実績に対応させながら,分布型流出モデルの検定を行い,想定される豪雨時における浸水解析を実施して,浸水状況の定量的な把握,浸水に伴う影響度合いなどをマッピングすることでわかり易い形で浸水インパクトを表示するなど,有用な知見や成果を得ている.これらの知見は,ハノイ市街地の浸水リスクを把握するのに役立つだけでなく,浸水に伴う健康リスクを検討する上で非常に有用なデータや知見を提供しており,都市環境工学の学術の進展に大きく寄与するものである.

よって,本論文は博士(工学)の学位請求論文として合格と認められる.