Flood inundation mapping sensitivity to riverine spatial resolution and modelling approach

An innovative approach in the investigation of complex landscapes for hydraulic modelling applications is the use of terrestrial laser scanner (TLS) that can lead to a high-resolution digital elevation model (DEM). Another notable factor in flood modelling is the selection of the hydrodynamic model (1D, 2D and 1D/2D), especially in complex riverine topographies, that can influence the accuracy of flood inundation area and mapping. This paper uses different types of hydraulic–hydrodynamic modelling approaches and several types of river and riparian area spatial resolution for the implementation of a sensitivity analysis for floodplain mapping and flood inundation modelling process at ungauged watersheds. Four data sets have been used for the construction of the river and riparian areas: processed and unprocessed TLS data, topographic land survey data and typical digitized contours from 1:5000-scale topographic maps. Modelling approaches combinations consist of: one-dimensional hydraulic models (HEC-RAS, MIKE 11), two-dimensional hydraulic models (MIKE 21, MIKE 21 FM) and combinations of coupled hydraulic models (MIKE 11/MIKE 21) within the MIKE FLOOD platform. Historical flood records and estimated flooded area derived from an observed extreme flash-flood event have been used in the validation process using 2 × 2 contingency tables. Flood inundation maps have been generated for each modelling approach and landscape configuration at the lower part of Xerias River reach at Volos, Greece, and compared for assessing the sensitivity of input data and model structure uncertainty. Results provided from contingency table analysis indicate the sensitivity of floodplain modelling on the DEM spatial resolution and the hydraulic modelling approach.     

An Assessment of Structural Measures for Flood-prone Lowlands with High Population Density along the Keelung River in Taiwan

Midstream of the Keelung River Basin in Northern Taiwan has become highly urbanized and densely populated area. Flood inundation along riversides frequently occurred during typhoons or rainstorms. Three protection measures, including constructions of high-level protection levees, a diversion channel, and a detention reservoir, were proposed for flood mitigation. The main purpose of this study is to evaluate the flood mitigation performance of the three proposed structural measures by using combined hydrologic analyses and hydraulic routings. A semi-distributed parallel-type linear reservoirs rainfall-runoff model was used for estimating the surface runoff. Furthermore, a 1-D dynamic channel routing model was coupled with a two-dimensional inundation model to simulate the hydraulic characteristics of river flooding and overland flow. Simulation results of flood stages, runoff peak discharges, and inundation extent under design rainfall scenarios were chosen as the criteria for evaluation. The results showed a diversion channel is superior to the other two measures for flood mitigation of the study area. After the process of environmental impact assessment, a revised diversion channel approach has been approved for construction as the major structural measure.     

Tidal-Fluvial and Estuarine Processes in the Lower Columbia River: II. Water Level Models,Floodplain Wetland Inundation,and System Zones

Spatially varying water-level regimes are a factor controlling estuarine and tidal-fluvial wetland vegetation patterns. As described in Part I, water levels in the Lower Columbia River and estuary (LCRE) are influenced by tides, river flow, hydropower operations, and coastal processes. In Part II, regression models based on tidal theory are used to quantify the role of these processes in determining water levels in the mainstem river and floodplain wetlands, and to provide 21-year inundation hindcasts. Analyses are conducted at 19 LCRE mainstem channel stations and 23 tidally exposed floodplain wetland stations. Sum exceedance values (SEVs) are used to compare wetland hydrologic regimes at different locations on the river floodplain. A new predictive tool is introduced and validated, the potential SEV (pSEV), which can reduce the need for extensive new data collection in wetland restoration planning. Models of water levels and inundation frequency distinguish four zones encompassing eight reaches. The system zones are the wave- and current-dominated Entrance to river kilometer (rkm) 5; the Estuary (rkm-5 to 87), comprised of a lower reach with salinity, the energy minimum (where the turbidity maximum normally occurs), and an upper estuary reach without salinity; the Tidal River (rkm-87 to 229), with lower, middle, and upper reaches in which river flow becomes increasingly dominant over tides in determining water levels; and the steep and weakly tidal Cascade (rkm-229 to 234) immediately downstream from Bonneville Dam. The same zonation is seen in the water levels of floodplain stations, with considerable modification of tidal properties. The system zones and reaches defined here reflect geological features and their boundaries are congruent with five wetland vegetation zones.     

Flood hazard assessment with multiparameter approach derived from coupled 1D and 2D hydrodynamic flow model

Hydrodynamic flow modeling is carried out using a coupled 1D and 2D hydrodynamic flow model in northern India where an industrial plant is proposed. Two flooding scenarios, one considering the flooding source at regional/catchment level and another considering all flooding sources at local level have been simulated. For simulating flooding scenario due to flooding of the upstream catchment, the probable maximum flood (PMF) in the main river is routed and its flooding impact at the plant site is studied, while at the local level flooding, in addition to PMF in the main river, the probable maximum precipitation at the plant site and breaches in the canals near the plant site have been considered. The flood extent, depth, level, duration and maximum flow velocity have been computed. Three parameters namely the flood depth, cross product of flood depth and velocity and flood duration have been used for assessing the flood hazard, and a flood hazard classification scheme has been proposed. Flood hazard assessment for flooding due to upstream catchment and study on local scale facilitates determination of plinth level for the plant site and helps in identifying the flood protection measures.     

Assessment of flood inundation mapping of Surat city by coupled 1D/2D hydrodynamic modeling: a case application of the new HEC-RAS 5

Surat city of India, situated 100 km downstream of Ukai Dam and 19.4 km upstream from the mouth of River Tapi, has experienced the largest flood in 2006. The peak discharge of about 25,770 m³ s^?1 released from the Ukai Dam was responsible for a disaster. To assess the flood and find inundation in low-lying areas, simulation work is carried out under the 1D/2D couple hydrodynamic modeling. Two hundred ninety-nine cross sections, two hydraulic structures and five major bridges across the river are considered for 1D modeling, whereas a topographic map at 0.5 m contour interval was used to produce a 5 m grid, and SRTM (30 and 90 m) grid has been considered for Surat and the Lower Tapi Basin. The tidal level at the river mouth and the release from the Ukai Dam during 2006 flood are considered as the downstream and upstream boundaries, respectively. The model is simulated under the unsteady flow condition and validated for the year 2006. The simulated result shows that 9th August was the worst day in terms of flooding for Surat city and a maximum 75–77% area was under inundation. Out of seven zones, the west zone had the deepest flood and inundated under 4–5 m. Furthermore, inundation is simulated under the bank protection work (i.e., levees, retaining wall) constructed after the 2006 flood. The simulated results show that the major zones are safe against the inundation under 14,430 m³ s^?1 water releases from Ukai Dam except for the west zone. The study shows the 2D capability of new HEC-RAS 5 for flood inundation mapping and management studies.     

基于EasyRiv1D模型的漳卫河流域河道洪水演进模拟研究

漳卫河中下游河道断面变化较大,该流域的洪水预报调度十分复杂。传统的水文学方法只是借助于历史洪水进行汇流参数率定,本文借助于水力学方法和河道实测断面,利用Easy Riv1D模型对河道洪水演进模拟研究,取得了较好的预报效果,可以为漳卫河流域的河道洪水演进提供技术支撑。     

Flood inundation mapping and hazard assessment of Baitarani River basin using hydrologic and hydraulic model

Frequent flood is a concern for most of the coastal regions of India. The importance of flood maps in governing strategies for flood risk management is of prime importance. Flood inundation maps are considered dependable output generated from simulation results from hydraulic models in evaluating flood risks. In the present work, a continuous hydrologic-hydraulic model has been implemented for mapping the flood, caused by the Baitarani River of Odisha, India. A rainfall time-series data were fed into the hydrologic model and the runoff generated from the model was given as an input into the hydraulic model. The study was performed using the HEC-HMS model and the FLO-2D model to map the extent of flooding in the area. Shuttle Radar Topographic Mission (SRTM) 90 m Digital Elevation Model (DEM) data, Land use/Land cover map (LULC), soil texture data of the basin area were used to compute the topographic and hydraulic parameters. Flood inundation was simulated using the FLO-2D model and based on the flow depth, hazard zones were specified using the MAPPER tool of the hydraulic model. Bhadrak District was found to be the most hazard-prone district affected by the flood of the Baitarani River. The result of the study exhibited the hydraulic model as a utile tool for generating inundation maps. An approach for assessing the risk of flooding and proper management could help in mitigating the flood. The automated procedure for mapping and the details of the study can be used for planning flood disaster preparedness in the worst affected area.

     

Spatio-temporal dynamics in the flood exposure due to land use changes in the Alpine Lech Valley in Tyrol (Austria)

Flood risk is expected to increase in many regions of the world in the next decades with rising flood losses as a consequence. First and foremost, it can be attributed to the expansion of settlement and industrial areas into flood plains and the resulting accumulation of assets. For a future-oriented and a more robust flood risk management, it is therefore of importance not only to estimate potential impacts of climate change on the flood hazard, but also to analyze the spatio-temporal dynamics of flood exposure due to land use changes. In this study, carried out in the Alpine Lech Valley in Tyrol (Austria), various land use scenarios until 2030 were developed by means of a spatially explicit land use model, national spatial planning scenarios and current spatial policies. The combination of the simulated land use patterns with different inundation scenarios enabled us to derive statements about possible future changes in flood-exposed built-up areas. The results indicate that the potential assets at risk depend very much on the selected socioeconomic scenario. The important conditions affecting the potential assets at risk that differ between the scenarios are the demand for new built-up areas as well as on the types of conversions allowed to provide the necessary areas at certain locations. The range of potential changes in flood-exposed residential areas varies from no further change in the most moderate scenario ‘Overall Risk’ to 119 % increase in the most extreme scenario ‘Overall Growth’ (under current spatial policy) and 159 % increase when disregarding current building restrictions.     

基于改进填洼模型的城市洪涝灾害计算方法

城市水文模型广泛应用于城市雨洪管理研究中,但由于它无法直接给出流域的淹没情况,制约了其在城市洪涝灾害评价中的应用,因此,有必要对其进行改进。本研究开发了考虑下游影响的填洼模型,并将其与较为成熟的城市水文模型相耦合,使改进后的模型可以模拟流域的淹没状况,并在北京市天堂河流域进行了试验研究。结果表明:改进后的模型可以快速模拟流域的淹没水深及淹没范围,其淹没水深结果与相关水动力学方法模拟结果较为一致。天堂河流域河道行洪能力低于20年一遇设计标准,流域洪水风险较大的区域集中在天堂河下游区域和田营沟与永北干渠的交汇处。若在下游区域进行开发,洪水风险还可能会继续增加。     

长江中下游河道冲淤演变的防洪效应

近年来长江中下游来沙量持续减少，河道面临长距离、长历时的冲淤调整，河道蓄泄关系发生变化，对防洪造成影响。在长江中下游河道冲淤及其蓄泄能力变化预测成果的基础上，对比计算了现状和未来河道蓄泄能力条件下，遇1954年洪水，长江上游水库防洪调度和中下游地区超额洪量的变化情况。结果表明，未来随着长江中下游河道进一步冲刷，河道槽蓄容积增加，相同防洪控制水位下的河道安全泄量增大，三峡水库在进行防洪调度时可下泄流量增大，总拦蓄洪量减小，长江中下游地区总超额洪量减小，但超额洪量在地区分布上存在从上游向下游转移的情况。     

长江中下游河道冲淤演变的防洪效应

近年来长江中下游来沙量持续减少,河道面临长距离、长历时的冲淤调整,河道蓄泄关系发生变化,对防洪造成影响。在长江中下游河道冲淤及其蓄泄能力变化预测成果的基础上,对比计算了现状和未来河道蓄泄能力条件下,遇1954年洪水,长江上游水库防洪调度和中下游地区超额洪量的变化情况。结果表明,未来随着长江中下游河道进一步冲刷,河道槽蓄容积增加,相同防洪控制水位下的河道安全泄量增大,三峡水库在进行防洪调度时可下泄流量增大,总拦蓄洪量减小,长江中下游地区总超额洪量减小,但超额洪量在地区分布上存在从上游向下游转移的情况。     

National Floodplain Mapping: Datasets and Methods – 160,000 km in 12 months

This paper describes two projects requiring production of national floodplain maps for England and Wales – some 80,000 km of river. The novel solutions developed have brought together a national Digital Elevation Model (DEM), automatically-generated peak flow estimates at intervals along the watercourses and two alternative methods of calculating the outlines: normal depth calculation; and a purpose-built 2-dimensional raster-based floodplain model, JFLOW. The DEM was derived using Interferometric Synthetic Aperture Radar (IFSAR) techniques and has a vertical precision of ±0.5 m–1.0 m (RMSE) and a 5 m horizontal resolution. The flow estimates were derived by automating Flood Estimation Handbook (FEH) techniques. The normal depth calculations are applied at a number of discrete cross-sections with linear interpolation between to form a 3-dimensional water surface. This is overlain on the DEM to produce the flood outline. Careful manual checking is required at a number of stages. The JFLOW model is based on a discretised form of the 2-dimensional diffusive wave equation and directly simulates the flood outline in a series of overlapping short (1 km) reaches. Flood outlines from the overlapping reaches are merged to produce the overall flood envelope. The model has been written to work as a screen-saver, allowing distributed processing across all computers in an office and manual intervention is minimal. In simple valley situations both methods give similar results, but show differences in more complex areas. Each has advantages and disadvantages, but both have been shown to be a practicable solution to allow production of 160,000 km of flood outline in 12 months.     

长江中下游江湖关系演变趋势数值模拟

以长江中下游防洪系统为对象,概述了在大型复杂防洪系统洪水行为数值模拟基础上,成功地将长江中下游洪水演进数学模型转化为面向长江防洪系统防洪规划方案评估需求的长江中下游江湖水沙演变的数学模型.为适应防洪规划方案论证涉及江湖水沙相互制衡相互关联客观情况,建立了面向江湖水沙关系及其演变的数学模型.针对长江中下游江湖水沙运动特点,在水沙数值模拟的范围内侧重对下荆江河道冲刷、荆江三口分流分沙模式、洞庭湖泥沙淤积、江湖耦合等环节进行了讨论,提出了合理可行的数值处理方法.模拟结果较好反映了江湖水沙演变规律,主要成果已应用于长江中下游防洪规划和防汛调度方案中.     

The Indus flood of 2010 in Pakistan: a perspective analysis using remote sensing data

The Indus flood in 2010 was one of the greatest river disasters in recent history, which affected more than 14 million people in Pakistan. Although excessive rainfall between July and September 2010 has been cited as the major causative factor for this disaster, the human interventions in the river system over the years made this disaster a catastrophe. Geomorphic analysis suggests that the Indus River has had a very dynamic regime in the past. However, the river has now been constrained by embankments on both sides, and several barrages have been constructed along the river. As a result, the river has been aggrading rapidly during the last few decades due to its exceptionally high sediment load particularly in reaches upstream of the barrages. This in turn has caused significant increase in cross-valley gradient leading to breaches upstream of the barrages and inundation of large areas. Our flow accumulation analysis using SRTM data not only supports this interpretation but also points out that there are several reaches along the Indus River, which are still vulnerable to such breaches and flooding. Even though the Indus flood in 2010 was characterized by exceptionally high discharges, our experience in working on Himalayan rivers and similar recent events in rivers in Nepal and India suggest that such events can occur at relatively low discharges. It is therefore of utmost importance to identify such areas and plan mitigation measures as soon as possible. We emphasize the role of geomorphology in flood analysis and management and urge the river managers to take urgent steps to incorporate the geomorphic understanding of Himalayan rivers in river management plans.     

长江中下游现代河漫滩土层物理力学性质及工程地质评价

长江出三峡南津关口自宜昌而下, 由于河谷增宽, 水力梯度减小, 沉积物的颗粒由上游至下游逐渐变细、各类土层的工程地质性质也逐次变差。本文以长江中下游河谷两岸漫滩土层的物理力学性质和基本地质环境条件予以工程评价, 欲望能对长江中下游河谷两岸的各类土木工程建筑提供基础资料。     

The reconstruction of a glacial lake outburst flood using HEC-RAS and its significance for future hazard assessments: an example from Lake 513 in the Cordillera Blanca,Peru

In April 2010, an ice/rockfall into Lake 513 triggered a glacial lake outburst flood (GLOF) along the Chucchun River in the Cordillera Blanca of Peru. This paper reconstructs the hydrological characteristics of this as yet undocumented event using a 1D flood model prepared with HEC-RAS. The principle model inputs were obtained during detailed field surveys of surface characteristics and topography within the river and across the adjacent floodplain; a total of 120 cross-sections were surveyed. These inputs were refined further by eyewitness accounts and additional geomorphological observations. The flood modelling has enabled us to constrain the extent of the water surface and its elevation at each cross-section in addition to defining the peak discharge (580 m³ s^?1). These modelling results show good agreement with other information about the flood including: flood marks and minimum flood levels; the lake displacement wave height; the extent of the flooded area; and the travel time from Lake 513 to the confluence with the Santa River. This demonstrates that the model offers a reliable reconstruction of the basic hydrological characteristics of the GLOF. It provides important information about the flood intensity and significantly improves our ability to model future flood scenarios along both the studied river and within neighbouring catchments. The flood hazard, defined by the flood depth during peak discharge, shows that the majority of the damaged infrastructure (houses, bridges, and a drinking water treatment plant) was only subjected to low or medium flood intensities (defined by a maximum water depth of less than 2 m). These low flood intensities help to explain why the flooding caused comparatively minor damage despite the significant public attention it attracted.     

Large-scale rainfall–runoff–inundation modeling for upper Citarum River watershed,Indonesia

The Citarum River is one of the strategic rivers in West Java, Indonesia. Its total watershed area is approximately 1800 km². Almost every year, the overflow from the Citarum River causes the inundation of most of the upper Citarum River watershed. To prevent and mitigate flood damage, it is necessary to understand the flooding characteristics. The region, however, suffers from a lack of observational data. Therefore, to analyze the inundation caused by flooding in the upper Citarum River watershed, a rainfall–runoff–inundation (RRI) model was employed. It used the following multiple satellite-derived datasets as input data as well as for model verification: Global Satellite Mapping of Precipitation, Hydrological data and maps based on Shuttle elevation Derivatives at multiple scales, Global Mosaics of the standard MODIS land cover type data product, and Landsat 7 satellite images. Parameter calibration was performed using a Monte Carlo simulation. The simulation was performed for February 2010. The results of this study show that the RRI model identifies inundation areas in large-scale river watersheds more effectively when using multiple satellite-derived datasets compared with the observed inundation map obtained from JICA in 2010 and Landsat 7 images. The model results can be improved if high-quality observed rainfall data, topographic data, and river cross-sectional data are available.     

Assessment of potential outburst floods from the Tsho Rolpa glacial lake in Nepal

Flood and sediment disasters caused by glacial lake outbursts have occurred frequently in recent years in the Himalayas of Nepal. Glacial lake outburst floods (GLOFs) can cause catastrophic flooding in downstream areas with serious damage to lives and property. It is thus important to investigate outburst floods from potentially dangerous glacial lakes. In this study, the characteristics of potential outburst floods from the Tsho Rolpa glacial lake due to two types of moraine dam failure caused by seepage flow or water overtopping were analyzed with various scenarios by using integrated modeling system of three numerical models: (1) the flow and bed-surface erosion model, (2) the seepage model and (3) the slope stability model. Flood inundation areas were also identified by using the numerical model of the flow and moraine dam failure and geographical information system tools. Possible threats and damages due to the potential GLOF events from the lake were also analyzed based on numerical results, flood inundation maps and field investigations.     

Impact of forecasted land use changes on flood risk in the Polish Carpathians

Flooding is a major environmental hazard in Poland with risks that are likely to increase in the future. Land use and land cover (LULC) have a strong influencing on flood risk. In the Polish Carpathians, the two main projected land use change processes are forest expansion and urbanization. These processes have a contradictory impact on flood risk, which makes the future impact of LULC changes on flooding in the Carpathians hard to estimate. In this paper, we investigate the impact of the projected LULC changes on future flood risk in the Polish Carpathians for the test area of Ropa river basin. We used three models of spatially explicit future LULC scenarios for the year 2060. We conduct hydrological simulations for the current state and for the three projected land use scenarios (trend extrapolation, ‘liberalization’ and ‘self-sufficiency’). In addition, we calculated the amount of flood-related monetary losses, based on the current flood plain area and both actual and projected land use maps under each of the three scenarios. The results show that in the Ropa river, depending on scenario, either peak discharge decreases due to the forest expansion or the peak discharge remains constant—the impact of LULC changes on the hydrology of such mountainous basins is relatively low. However, the peak discharges are very diverse across sub-catchments within the modeling area. Despite the overall decrease of peak discharge, there are areas of flow increase and there is a substantial projected increase in flood-related monetary losses within the already flood-prone areas, related to the projected degree of urbanization.     

基于水动力学模型的沿海城市洪水实时演进模拟

为了准确分析洪涝灾害对防洪体系现状的影响，做出相应的防洪减灾措施，以浙江省台州市灵江下游流域为研究区域，构建了基于Saint-Venant方程的水动力学耦合模型，对河道溃决洪水过程进行实时仿真模拟。综合考虑研究区域地形、气象、水文资料、水利工程、下垫面条件等因素，在一维河网模型和二维水动力学模型耦合衔接中，最大程度还原真实地形中河槽内外的水流交互淹没，借助研究区域内典型台风暴雨资料，率定验证本文建立的一维-二维耦合水动力学模型，检验后的模型可实现灵江下游沿岸城市不同量级设计洪水及历史洪水的实时淹没过程模拟。模型计算结果表明，该模型模拟复杂地形条件情况下流域洪水实时演进过程达到了较高精度，在水系沿程典型断面水位计算值与实测值误差不超过0.1 m。