A study on future land use considering flood disaster in Osato district in Kaiyo Town, Tokushima

本研究では，徳島県海陽町大里地区の善蔵川流域を対象とし，外水氾濫，内水氾濫の浸水深と人口を用いて水害リスク評価値を，生活施設の位置を用いて住環境評価値を計算した。これらのデータと土地利用データを用いてクラスター分析により地域を分類し，その結果から，今後の土地利用方針を示した

An Experimental Study on Bed Degradation Due to Falling Works Improvement

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Turbulent structures in the flow through compound meandering channels

River engineeringNumerical modelling in river engineerin

Reliability Analysis of River Bridge against Scours and Earthquakes

This study proposes a bridge safety evaluation process against seismic and flood hazards. Because uncertainties in the scours, seismic hazard, and structural performance for a given seismic excitation are inevitable and important, reliability analysis is adopted. A scour prediction equation for a bridge with a complicated foundation system is developed and a probabilistic scour curve is constructed to measure the risk of scours using the Monte Carlo simulation. The seismic hazard is measured using the probabilistic seismic-hazard analysis. A series of nonlinear time-history analyses are performed to determine the structural performance under different peak-ground-acceleration values. SAP2000 is used to build the finite-element model wherein the soil is modeled using a bilinear link. A plastic hinge is predefined to simulate the nonlinear behaviors of the pier and caisson of the bridge. The displacement ductility is used to measure the structural performance and to construct the fragility curve for various limit states. The Nanyun Bridge located in central Taiwan is selected as an example to demonstrate the proposed safety-evaluation procedure. The results show that the probable scour depth of the Nanyun bridge is from 3 to 5 m. The failure probability considering the floods and earthquakes is insignificant. A deterministic design value, considering both the hazards, is provided for a given reliability target (e.g., β=3) to help engineers in their present design processes

IMPROVEMENT OF FLOOD REDUCTION FUNCTION OF FORESTED BASIN BY REINFORCEMENT OF EVAPORATION RATE AND GOUUND ROUGHNESS

流域治水における活用を念頭に，遮断蒸発率と地表面抵抗の増強による森林の洪水低減機能の向上について洪水ピーク流量の視点から検討した．徳島県内に位置する２つの林地（一斉植林・皆伐型施業のスギ人工林，弱間伐・択伐型施業のスギ・ヒノキを中心とする針広混交複層林）を対象に水文観測を実施し，地表面流分離直列二段タンクモデルと将来予測降雨を用いた流出シミュレーションを実施した．遮断蒸発率と地表面抵抗の洪水ピークの流量低減と遅延効果について検討した結果，対象としたスギ人工林を針広混交複層林・弱間伐に変更すると20％程度の洪水ピーク流量低減効果を得られる可能性が示唆された．Two forest operation techniques with an flood peak reduction effect were discussed using a tank-model and two small artificial forested basins where forest physiognomy was different. One operation was the way to strengthen the insulation evaporative rate by changing its physiognomy, other was the way to make forest floor vegetation and ground roughness increased. The effect when artificial cedar forest was replaced with a mixed conifer-hardwood forest was discussed. As a result, it was estimated that the flood peak flow decreased around 20% before the replacement

THE RELATION OF BETWEEN RAINFALL AND SEDIMENT DISCHARGE IN THE NISHIDANI AND THE KUMANOGAWADANI RIVER BASIN

我が国では，近年，森林の荒廃により，降水時に土砂生産・流入や崩壊危険度が増大している流域が存在するため，降水や森林荒廃が土砂生産等に影響を及ぼす程度を明確にする必要がある．本研究では，和歌山県の二級河川富田川支川の西谷川および熊野川谷川流域で，降水，河川水位，流砂量の観測を行うとともに，富田川流域に隣接する一級河川熊野川右支川の音無川流域における流出土砂推計値の検討を行った結果，降水量と流砂量の間に，雨量の閾値，土砂流出に有効な降雨強度および一次関数の関係式を見出せた．また，得られた関係式に基づく流出土砂量の予測により効率的な河川管理を行い，治水安全度の向上に資する可能性について考察を行った．Due to forest dilapidations in recent years, sediment yields from a slope and slope collapsing risk are increased. In addition to devastating sediment disasters such as happened in Kii Peninsula flooding in 2011, continuous sediment runoff following to such disasters causes riverbed aggradation and revetment damage under normal flood period,which seriously threatens safety level of flood control. Thus it is important to clarify the relationship between precipitation and sediment discharge. In this study, we analysed the relationship in the Otonashi River basin, a tributary of the Kumano River in Kii Peninsula, and tried to derive a critical hourly precipitation, that is effective to yield sediment discharge. From the analyses, the critical hourly precipitation can be found around 15mm/h or 20mm/h. Moreover, by applying the critical hourly precipitation as a cut-off value to the accumulated precipitation, a linear function between the adjusted precipitation and sediment discharge becomes clearer. These concepts were also applied to the observational results in the Nishidani and the Kumanogawadani river Basin, tributaries of the Tonda River adjacent to the Kumano River, then we showed the proposed linear function can estimate sediment discharge in accordance with a rainfall event reasonably well. Although the coefficients of the function vary from one river basin to another, the concept can be useful to estimate sediment deposition within a river section after normal flood roughly but quite easily

Scour Depth Evaluation of a Bridge with a Complex Pier Foundation

A scour depth prediction formula for a river bridge is established using experimental data in which the effects of the pier, pile-cap and pile group are considered. More than 170 experimental data entries, including different pier structural sizes, flow depths and soil covering depths, are collected and verified by existing formulae, which failed to deliver a promising prediction. A machine learning prediction model was then developed to enhance the accuracy. For application purpose, a sequential quadratic programming optimization was adopted to construct an explicit prediction formula. The MAPE was significantly improved from 102.8 to 28.9. The results indicate that the proposed formula can simultaneously satisfy the requirements of accuracy and simplicity. The proposed formula has the advantages of being conceptually consistent with observed scour behaviors and provides a solid scour depth prediction, which is an important and critical step in the bridge safety evaluation if floods are considered

FLOOD CONTROL FUNCTION OF FARMLAND AS GREEN INFRASTRUCTURE USING INUNDATION ANALYSIS MODEL

本研究では,徳島県にある海部川支川善蔵川流域を対象とし,農地や湿地をグリーンインフラとし,洪水調節機能を定量的に評価する内水氾濫解析モデルを構築し,人口減少が進む当該流域における将来の土地利用の在り方を考察した.平成26年台風12号を元に,Hazen法によって確率雨量を評価し,降雨外力の違いと土地利用の違いによる洪水調節機能に与える影響を評価した.前者では,床上浸水の被害が20年確率降雨以上で顕著に増加したことから,水害リスクの高い場所と農地による洪水調節機能の発揮限界について検討した.後者では,宅地転換が進む前の過去の土地利用では,湛水量の増減の応答が早く,とりわけ集中的に浸水する領域を特定した.以上から,浸水リスクがある場所での開発を控え,社会状況に連動した優先的な退避の施策の成立が今後重要視される.In this paper, flood control function of agricultural land and wetlands were considered using an inundation analysis model, expecting to work as green infrastructure against flood. The study area is located in Zenzo River basin, a tributary of the Kaifu River, Tokushima Prefecture, where population decline is severe in recent years. Based on the rainfall by Typhoon 1412 several probable rainfalls were evaluated, and the flood control function was examined with the different rainfalls, as well as the difference of land use. The results show that, since the flood damage due to inundation above floor level remarkably increase at and over 20-year period probable rainfall, the flood control function has a limit at some stage against extreme rainfall events. On the other hand, in the past land use where more farmlands existed, temporal variation of inundation volume reduces more rapidly compared with the current land use. The results indicate it will be necessary to suppress development in high risk areas, and the past land use will be a good reference for a wise land use