Masato Sekine

INTEGRATED NUMERICAL PREDICTION OF URBAN INUNDATION AND RIVER FLOOD IN THE SHAKUJII RIVER BASIN

近年,気象の極端化が顕在化する中で,時間雨量 50mm を超える豪雨が稀ではなくなってきている.東 京では 50mm/h を設計強度とする「雨水排除システム」 が整備されている.しかし,この強度をはるかに超え る豪雨に見舞われると,都市域(堤内地)では深刻な 内水氾濫が生じ,都市河川では洪水が発生することに なる.この場合,河川水位の上昇に伴い,雨水吐を介し た下水道からの水の流れや下水道内の流れそのものにも 顕著な影響が現れるようになる.状況によっては,河川 から下水道内への逆流が生じるような事態にもなり得 る.そして,さらに降雨の規模が増大すると,河川から の越水が発生して,外水氾濫にまで到ることも懸念され る.東京都 23 区などのような高度に都市化されたエリ アには,浸水から街を守るため,下水道と都市河川とが 一体となって機能する「雨水排除システム」がすでに整 備されている.このようなエリアにおいて発生する浸水 ならびに洪水事象を,これまで以上に的確に捉えるため には,現状に即した数値予測手法が必要である.本研 究で用いた数値予測手法 1) は,都市内の「地上-下水 道-都市河川」の雨水の流れを一体的に解くものであ る.この手法には補正係数はもちろんのこと不必要な 仮定や仮説は持ち込まれておらず,現象を力学法則の みに基づいて可能な限り精緻に解くことができる.ま た,実在する都市インフラについての情報や,土地利用 状況に関わる情報がすべて忠実に考慮され計算される. 本研究は,この手法を東京都 23 区の北西部を流れる 石神井川流域に適用して検討を行ったものである.こ の流域ではこれまでに幾度となく氾濫被害が発生して おり,最近では 2005 年と 2010 年の二度にわたり下流 域で外水氾濫被害が生じている.著者らのこれまでの 検討と同様に,本研究でも 2005 年に東京都杉並区で発 生した豪雨(ここではこれを「杉並豪雨」と呼ぶ)時 のデータを対象降雨とし,流域内の浸水の数値予測を 行った.本論文では,対象エリアの浸水危険度を明ら かにするとともに,下水道と都市河川とを結ぶ雨水 吐を介した水の流出入に特に注目して考察を加えた. 本研究に関連する研究のうち主なものを挙げると,名 古屋市堀川流域を対象とした武田ら 2) の研究,遠賀川流 域を対象とした秋山ら 3) のもの,神田川流域を対象とし た福岡 4) ら,ならびに関根 5) らのものなどがある.詳 しくは元論文を参照されたい. 石神井川流域を対象とした豪雨時の 内水氾濫と河川洪水流の一体予測計算

NUMERICAL ANALYSIS OF INUNDATION IN UNDERGROUND SPACE AND SUBWAY TUNNEL IN TOKYO

Recently, localized torrential rain occurs frequently. In order to clarify inundation risk of Tokyo metropolitan area, numerical simulation of urban inundation was conducted. In this study, following three investigations were conducted. First, numerical simulation of inundation event which occurred on June 29, 2014 was carried out by using the actual rain data which was obtained by X-band MP radar. Based understand the expanding process of water in the subway tunnel by another numerical computation.

EFFECT OF PERMEABILITY AND ADHESIVENESS OF MATERIALS ON CHANNEL NETWORK CAUSED BY SLOPE EROSION

In this study, series of experiments were conducted in order to investigate the effect of permeability of sediment on slope erosion by artificial rainfall. In this study, the slope was mainly composed of sand whose grain size was different in each case. In addition to such experiments, another investigation was carried out to find out the effect of adhesiveness on the erosion of slope which contains some amount of clay. As a result, it was found that overland flow affects surface erosion, and therefore the slope permeability governs the slope erosion considerably. And it was also confirmed that adhesiveness affects channel network formation due to the surface erosion considerably.

EROSION RATE OF COHESIVE SEDIMENT WITH DIFFERENT CLAY-CONTENT-RATIO

Erosion rate formula of cohesive sediment had been derived by the first author, but the there was very few information about it in the range that clay-content-ratio Rcc was less than 0.3. A series of erosion experiments were conducted in order to understand the effect of Rcc on the erosion rate. As a result, it was confirmed that the formula was valid in the range of Rcc between 0.1 and 1.0. The influence of watercontent-ratio on the test sample was discussed on the basis of experimental results. A deformation process and a stable profile of cohesive straight channel with a different value of Rcc were also investigated in order to understand the erosion process of cohesive sediment from a different point of view.

NUMERICAL ANALYSIS OF INUNDATION CAUSED BY A HEAVY RAINFALL AND AN OVERFLOW FROM THE SHAKUJII-GAWA RIVER IN 2005

Recently, we often experience the heavy rainfall whose intensity exceeds beyond the designed magnitude, because of the influence of a heat island phenomenon in urban area as well as a global climate change. On September, 2005, such a heavy rainfall caused the flood damage in Tokyo Kita Ward due to an overflow from the Shakujii-gawa River. In this study, numerical analysis was conducted to investigate how this inundation occurred. As a result of this analysis, the inundation which caused only by the heavy rainfall also happened simultaneously, and therefore the damage became more serious than that only caused by an overflow water of the river.

NUMERICAL SIMULATION OF EVACUATION BEHAVIOR FROM THE INUNDATED UNDERGROUND SPACE

Numerical model of evacuation behavior of people from the inundated underground space was investigated in this study. Information about a human behavior was taken into account as much as possible, a walking velocity, for example, was evaluated on the basis of relation between it and the congestion density of crowd. It was considered that the movement of each people was affected by both the movement by others and the approching flow of water on the floor. Numerical simulation was conducted in the relatively large scale model of underground space. It can be concluded that the present model works well and the reasonable simulation of evacuation behavior is possible to be realized.

EXPERIMENTAL STUDY ON EROSION RATE OF COHESIVE SEDIMENT BY USING A MOVABLE APPARATUS OF EROSION TEST

Erosion rate of soft cohesive sediment was investigated experimentally in the present paper. In this study, we developed a movable apparatus of erosion test, which was designed to conduct a field measurement of erosion rate. In order to check the validity and utility of this apparatus, the several series of measurements were carried out outside in large-scale experimental flume. It was verified that the results by this new measuring system has almost same order of accuracy as that by our standard test in laboratory. Effect of water temperature on the erosion rate were also evaluated quantitatively.

CHANNEL DEFORMATION DURING A FLOOD IN KINU RIVER AND NUMERICAL PREDICTION

The channel deformation in Kinu River during a flood period was investigated in the present study. Field measurement and film analysis were conducted to quantify the temporal and spatial changes in channel profile in the reach of 54.5-56.5 km from the confluence point to Tone River in the term between 2001 and 2003. Considerable amount of bank erosion and channel shift were observed, and the alternative channel was created just after the final flood during this term. As a result of this analysis, it was found that the magnitude of channel deformation depends on an individual flood discharge and also on the history of the former flood events. Numerical analysis was also conducted to understand more deeply the mechanism of this channel deformation. It was confirmed that the numerical model worked fairly well.

NUMERICAL ANALYSIS OF INUNDATION PROCESS IN UNDERGROUND SPACE CONNECTING TO A SUBWAY STATION

Inundation process in underground space which connecting to a subway station in urban area was studied numerically. Actual data sets of (1) road network, (2) drainage network and (3) underground space in the area around Shibuya Station were considered here, and a simplified numerical model was constructed. Based on the results of numerical computation, the characteristics and the weak points of the drainage system in this area were quantitatively understood. It was revealed that (1) the local area whose ground elevation is relatively lower and (2) the undergournd space are the possible area where extensive damage will result if there happenes to be the rainfall whose intensity exceeds the designed one.

NUMERICAL SIMULATION OF SURFACE EROSION AND SEDIMENT YIELD FROM BARE HILLSIDE

Erosion process of bare hillside was investigated experimentally and numerically in the present study. Experiments were conducted on the model hillside with the artificial rainfall in the experimental flume. The numerical simulation model has been developed to predict the erosion process and to understand it more deeply. The numerical model was applied to the phenomenon which occurred on the slope in the experimental flume. Based on the comparison between the experimental results and the numerical ones, the validity of the present numerical model was verified. The process of channel network evolution and the head migration of the gully were investigated on the basis of this numerical analysis. Temporal variation of sediment yield from the hillside was also studied.