Return Period Flood Research Articles

A Framework for Flood Risk Analysis and Benefit Assessment of Flood Control Measures in Urban Areas.

Flood risk analysis is more complex in urban areas than that in rural areas because of their closely packed buildings, different kinds of land uses, and large number of flood control works and drainage systems. The purpose of this paper is to propose a practical framework for flood risk analysis and benefit assessment of flood control measures in urban areas. Based on the concept of disaster risk triangle (hazard, vulnerability and exposure), a comprehensive analysis method and a general procedure were proposed for urban flood risk analysis. Urban Flood Simulation Model (UFSM) and Urban Flood Damage Assessment Model (UFDAM) were integrated to estimate the flood risk in the Pudong flood protection area (Shanghai, China). S-shaped functions were adopted to represent flood return period and damage (R-D) curves. The study results show that flood control works could significantly reduce the flood risk within the 66-year flood return period and the flood risk was reduced by 15.59%. However, the flood risk was only reduced by 7.06% when the flood return period exceeded 66-years. Hence, it is difficult to meet the increasing demands for flood control solely relying on structural measures. The R-D function is suitable to describe the changes of flood control capacity. This frame work can assess the flood risk reduction due to flood control measures, and provide crucial information for strategy development and planning adaptation.

The flow regulation services of wetlands

Wetlands potentially provide a range of ecological (or ecosystem) services including ground water recharge, nutrient retention, waste assimilation, shoreline stabilization, and carbon storage. One of the most cited and valuable services potentially provided by wetlands are their influence on flow regimes, especially flood attenuation and augmentation of low flows. Here we report the results of a meta-analysis of twenty-eight studies, including fifty-nine associated effect sizes, that have investigated the flow regulation services of wetlands. We found that, consistent with conventional wisdom, on average wetlands reduce the frequency and magnitude of floods and increase flood return period; augment low flows; and decrease runoff and streamflow. However, our results also indicate gross wetland characteristics have little predictive power with respect to the observed variation in the level of flow regulation services. This implies that in that in the absence of detailed site-specific information, estimates of flow regulation services provided by wetlands will generally have large uncertainty, as will any associated estimate of their economic value.

Flood Damage Analysis: First Floor Elevation Uncertainty Resulting from LiDAR-Derived Digital Surface Models

The use of high resolution ground-based light detection and ranging (LiDAR) datasets provides spatial density and vertical precision for obtaining highly accurate Digital Surface Models (DSMs). As a result, the reliability of flood damage analysis has improved significantly, owing to the increased accuracy of hydrodynamic models. In addition, considerable error reduction has been achieved in the estimation of first floor elevation, which is a critical parameter for determining structural and content damages in buildings. However, as with any discrete measurement technique, LiDAR data contain object space ambiguities, especially in urban areas where the presence of buildings and the floodplain gives rise to a highly complex landscape that is largely corrected by using ancillary information based on the addition of breaklines to a triangulated irregular network (TIN). The present study provides a methodological approach for assessing uncertainty regarding first floor elevation. This is based on: (i) generation an urban TIN from LiDAR data with a density of 0.5 points·m−2, complemented with the river bathymetry obtained from a field survey with a density of 0.3 points·m−2. The TIN was subsequently improved by adding breaklines and was finally transformed to a raster with a spatial resolution of 2 m; (ii) implementation of a two-dimensional (2D) hydrodynamic model based on the 500-year flood return period. The high resolution DSM obtained in the previous step, facilitated addressing the modelling, since it represented suitable urban features influencing hydraulics (e.g., streets and buildings); and (iii) determination of first floor elevation uncertainty within the 500-year flood zone by performing Monte Carlo simulations based on geostatistics and 1997 control elevation points in order to assess error. Deviations in first floor elevation (average: 0.56 m and standard deviation: 0.33 m) show that this parameter has to be neatly characterized in order to obtain reliable assessments of flood damage assessments and implement realistic risk management.

Metodología para la estimación del riesgo aguas abajo de una presa en función de las políticas de operación a largo plazo

The common practice to assess the flooding risk downstream of a dam is to obtain design floods for different return periods and simulate their transit through the reservoir according to the operation rules of the spillway, assuming that those floods occur when the level in the reservoir is in the normal water level (NWL). This paper presents a methodology for assessing the flooding risk downstream of a dam considering that the level in the reservoir when the design flood is presented, is a random variable that depends on the long-term operation policy of the dam. For the application of the proposed methodology Cerro Prieto Dam is used, located in Nuevo Leon state in Mexico, the methods used for estimating floods associated to different return periods and for defining the long-term operation policy of the dam are presented first; therefore, simulating the dam operation based in that policy, the probabilities associated with the reservoir level during flood period are deducted. When simulating flood routing associated to each return period, conditional probabilities associated with the maximum discharge flow are obtained, and finally the flooding risk downstream of the dam can be estimated. The proposed methodology allows estimate accurate flood risk results without overestimating costs.

Using high‐resolution satellite precipitation for flood frequency analysis: case study over the Connecticut River Basin

This study evaluates the feasibility of using satellite precipitation datasets in flood frequency analysis based on the accuracy of different return period flows derived using a hydrological model driven with satellite and ground‐based reference rainfall fields over the Connecticut River Basin. Four quasi‐global satellite products (TRMM‐3B42V7, TRMM‐3B42RT, CMORPH, and PERSIANN) at 3‐h/0.25° resolution and the National Weather Service (Stage IV) gauge‐adjusted radar rainfall dataset (representing the reference rainfall) are integrated in this study, with the Coupled Routing and Excess Storage distributed hydrological model to simulate annual peak flows during warm season (May–November) months. The log‐Pearson type III frequency distribution applied to an 11‐year record of annual peak flow data is used to derive different return period flows. Evaluation against the Stage IV‐driven simulations shows that the TRMM‐3B42V7 product has the highest correlation and lowest bias in terms of the derived annual maxima flows compared to the other satellite products. In terms of the different return period flood frequency curves, the various satellite product‐based results well‐represent the variability across the different basins depicted in the reference precipitation‐driven simulations. With the increasing record length of high‐resolution satellite products, results from this paper can motivate future studies over basins lacking adequate ground‐based records to support flood frequency analyses.

FLOPROS: an evolving global database of flood protection standards

Abstract. With projected changes in climate, population and socioeconomic activity located in flood-prone areas, the global assessment of flood risk is essential to inform climate change policy and disaster risk management. Whilst global flood risk models exist for this purpose, the accuracy of their results is greatly limited by the lack of information on the current standard of protection to floods, with studies either neglecting this aspect or resorting to crude assumptions. Here we present a first global database of FLOod PROtection Standards, FLOPROS, which comprises information in the form of the flood return period associated with protection measures, at different spatial scales. FLOPROS comprises three layers of information, and combines them into one consistent database. The design layer contains empirical information about the actual standard of existing protection already in place; the policy layer contains information on protection standards from policy regulations; and the model layer uses a validated modelling approach to calculate protection standards. The policy layer and the model layer can be considered adequate proxies for actual protection standards included in the design layer, and serve to increase the spatial coverage of the database. Based on this first version of FLOPROS, we suggest a number of strategies to further extend and increase the resolution of the database. Moreover, as the database is intended to be continually updated, while flood protection standards are changing with new interventions, FLOPROS requires input from the flood risk community. We therefore invite researchers and practitioners to contribute information to this evolving database by corresponding to the authors.

Establishing a rainfall threshold for flash flood warnings in China’s mountainous areas based on a distributed hydrological model

Summary Flash flooding is one of the most common natural hazards in China, particularly in mountainous areas, and usually causes heavy damage and casualties. However, the forecasting of flash flooding in mountainous regions remains challenging because of the short response time and limited monitoring capacity. This paper aims to establish a strategy for flash flood warnings in mountainous ungauged catchments across humid, semi-humid and semi-arid regions of China. First, we implement a geomorphology-based hydrological model (GBHM) in four mountainous catchments with drainage areas that ranges from 493 to 1601 km2. The results show that the GBHM can simulate flash floods appropriately in these four study catchments. We propose a method to determine the rainfall threshold for flood warning by using frequency analysis and binary classification based on long-term GBHM simulations that are forced by historical rainfall data to create a practically easy and straightforward approach for flash flood forecasting in ungauged mountainous catchments with drainage areas from tens to hundreds of square kilometers. The results show that the rainfall threshold value decreases significantly with increasing antecedent soil moisture in humid regions, while this value decreases slightly with increasing soil moisture in semi-humid and semi-arid regions. We also find that accumulative rainfall over a certain time span (or rainfall over a long time span) is an appropriate threshold for flash flood warnings in humid regions because the runoff is dominated by excess saturation. However, the rainfall intensity (or rainfall over a short time span) is more suitable in semi-humid and semi-arid regions because excess infiltration dominates the runoff in these regions. We conduct a comprehensive evaluation of the rainfall threshold and find that the proposed method produces reasonably accurate flash flood warnings in the study catchments. An evaluation of the performance at uncalibrated interior points in the four gauged catchments provides results that are indicative of the expected performance at ungauged locations. We also find that insufficient historical data lengths (13 years with a 5-year flood return period in this study) may introduce uncertainty in the estimation of the flood/rainfall threshold because of the small number of flood events that are used in binary classification. A data sample that contains enough flood events (10 events suggested in the present study) that exceed the threshold value is necessary to obtain acceptable results from binary classification.

Detection of Change in Flood Return Levels under Global Warming

Using recent advancements in the statistical extreme value theory, this study proposes a methodology for detection of change in flood return levels under climate change. Nonstationary scaling of regional projected peak flows with global warming is first tested by a likelihood ratio test. For nonstationary possible future realizations, the authors then investigate how long the stationary historical design magnitudes or return levels of floods will remain valid, taking into account the uncertainties in the estimation of observed and projected return levels. Although some flood projections are found to be nonstationary, many are stationary in nature. No coherent change in flood return level across the projections is detected in the case study of floods in the Columbia River using available streamflow projections. Most projections yield flood quantiles that are not likely to be critical in the coming century. However, for some simulations detection is achieved, with earlier detection in design magnitudes of lower return periods. A possible worst-case scenario considering the maximum of all the projections shows detection of change in floods of higher return periods in the 21st century.

Prediction of Return Periods of Sewer Flooding Due to Climate Change in Major Cities

In this study, rainfall characteristics with stationary and non-stationary perspectives were analyzed using generalized extreme value (GEV) distribution and Gumbel distribution models with rainfall data collected in major cities of Korea to reevaluate the return period of sewer flooding in those cities. As a result, the probable rainfall for GEV and Gumbel distribution in non-stationary state both increased with time(t), compared to the stationary probable rainfall. Considering the reliability of , a variable reflecting the increase of storm events due to climate change, the reliability of the rainfall duration for Seoul, Daegu, and Gwangju in the GEV distribution was over 90%, indicating that the probability of rainfall increase was high. As for the Gumbel distribution, Wonju, Daegu, and Gwangju showed the higher reliability while Daejeon showed the lower reliability than the other cities. In addition, application of the maximum annual rainfall change rate () to the location parameter made possible the prediction of return period by time, therefore leading to the evaluation of design recurrence interval.

流域土地利用变化对不同重现期洪水的影响——以奉化江皎口水库流域为例

PDF HTML阅读 XML下载 导出引用 引用提醒 流域土地利用变化对不同重现期洪水的影响——以奉化江皎口水库流域为例 DOI: 10.5846/stxb201501220183 作者: 作者单位: 南京大学,南京大学,南京大学,南京大学,南京大学 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金面上项目（41371046，41371044）；水利部水利公益专项项目（201201072）；江苏省自然科学基金项目（BK20131276）；江苏水利科技基金重大项目（2015003） Impacts of land-use change on flood process and frequency of various return periods: a case study of Jiaokou Reservoir watershed in Fenghua River Author: Affiliation: Nanjing University,Nanjing University,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土地利用变化对流域洪水过程产生显著影响，并导致设计洪水发生变化。为进一步指导流域防洪及水库洪水设计，以浙东沿海奉化江皎口水库流域为例，应用HEC-HMS水文模型模拟分析土地利用/覆被变化（LUCC）对不同重现期暴雨洪水事件的影响。结果表明，流域内1985-2003年土地利用变化引起不同重现期洪水过程与洪水量级发生改变，其中，洪量和洪峰均增加，洪量较洪峰变化明显。LUCC对小洪水过程影响更明显，5年一遇以上洪水的洪峰和洪量分别平均增加3%和7.6%，而小于2年一遇洪水的洪峰和洪量分别平均增加5.41%和11.91%。同时，LUCC使100年、50年和25年一遇洪水重现期分别提前了15、6a和2a，即其对量级最高的特大洪水的重现期影响最大。此外，不同的土地利用转变方式对洪水的影响程度不一，其中，林地向裸地转变对洪水影响最大，林地向灌草地转变次之，林地向耕地变化对洪水影响最小，且这种差异性在低重现期洪水表现更明显。 Abstract:The effects of land-use/land-cover change (LUCC) on the watershed flood process have become one of the major hydrological concerns in the world. A change in the flood process leads to the alteration in flood design and control system, which has caused serious problems to human life and urban security. In this study, we take Jiaokou Reservoir watershed as a sample area to analyze the LUCC effects on storm floods of different return periods based on the hydrological model (HEC-HMS) and GIS-related support. Jiaokou Reservior is located in the west of Ningbo city, Zhejiang Province, and is controlled by the plum rains and typhoon storms with various return periods during rainy seasons. Due to rapid economic development and intensive human activity, the land utilization pattern has changed in recent years. The spatial data of land use of the study area during 1985-2003 was obtained from Landsat TM (Thematic Mapper) image data, and the images from different periods were interpreted and overlapped to setup the transfer matrix of the land-use types. The land-use types were dominated by forest land, bush grassland, and cultivated land, which covered 92% of the watershed. The main LUCC in this area is the conversion of forest to bare land, grassland, and agricultural land. The analysis of the LUCC impacts on flood shows that (1) during 1985-2003, 3%-30% of the forest land was changed to bare land, bush grass, or agricultural land, and the LUCC has different effects on floods of different return periods. Both flood runoff and peak increased for all flood magnitudes, but the runoff changed at a higher range as compared to the flood peak. (2) Low magnitude flood was found to be more sensitive to the LUCC, the increase of the total runoff and flood peak of a 5-year flood increased by 3% and 7.6%, respectively, while those of less than 2-year return period floods increased by 5.41% and 11.91%, respectively. From the perceptive of flood classification, the flood return periods of 100, 50, and 25 years were reduced by 15, 6, and 2 years, respectively, during the study period. (3) Different land-use change patterns lead to flood variations. Along with the forest being characterized by a change ratio of 10%-50% since 1985, forest-bare land-use change pattern impacts flooding most intensively, and the forest-grassland change pattern has stronger effects than the forest-agricultural change pattern. Compared with a simulated flood in 1985, when forest was converted into bare land with a ratio of 10%-50%, the flood peak increased by 0.88%-4.3% and the total runoff increased by 1.61%-7.91% in the 100-year-return-period flood. While, the widened range of the flood peak and total runoff under the same condition were 2.18%-10.67% and 4.15%-20.62%, respectively, in the 5-year-return-period flood. As the change ratio gets bigger, the difference in impacts of different patterns of forest transformation to other lands on floods became more distinct, especially to the low-magnitude flood. The results of this study can provide scientific supports for the flood control and water resource management. 参考文献 相似文献 引证文献

Refurbishment and Installation of a Monitoring and Early Warning System on the Sierroz Embankments in Aix-les-Bains, France

Sierroz embankments are located in an urban area, in the city of Aix-les-Bains (Savoie, France). Their linear is 400m on both rims. These embankments protect about 900 people against Sierroz river floods. Hydraulic studies performed in the frame of the Aix-les-Bains Inundation Risk Prevention Plan (PPRI in French) have shown that these embankments have a high risk of failure by overtopping during a 100 year return period flood of the Sierroz River. These studies have defined inundation maps with High Danger areas. From these results, the city of Aix-les- Bains asked EDF to perform a safety assessment study, in order to assess the safety margins of these embankments for each of their potential failure modes: overtopping erosion, internal erosion, slope sliding and liquefaction. This study confirmed that the main risk is breaching by overtopping erosion during flood. This diagnosis led the city of Aix-les-Bains to launch two parades: (i) the embankment refurbishment in order to provide them satisfactory safety margins during a 100 year return period flood and (ii) a monitoring and early-warning system able to alert population at risk as early as possible when breaching is imminent. These two parades are presented including environmental constraints and cost estimates.

宁夏暴雨洪水特点及峰量关系分析

宁夏局地暴雨较多，具有暴雨强度大，历时短的特点。区内大多河道流域面积小，受暴雨时程分布、空间分布变化较大的影响，暴雨与其产生洪水的重现期往往不同，且同一站点产生的洪水其洪峰与洪量的重现期也不同。这一特点与设计洪水计算中暴雨所产生的洪水(洪峰与洪量)同频率的假定有较大差异。通过用代表水文站实测资料分析，本区域洪峰与洪量相关性差，并通过典型实例分析暴雨与洪水重现期的一致性，影响洪峰与洪量的主要因素为降雨时空分布、降雨强度以及其产生的洪水过程等，通过典型实例分析暴雨与洪水重现期的一致性，为掌握本区域暴雨与洪水特性以及设计洪水计算提供参考。 Local rainstorm frequently occurs in Ningxia, with the characteristics of big intensity and short duration. River basin area is mostly small in this region. Affected by great variation of rainstorm’s schedule and spatial distribution change, rainstorm and the flood produced often have different return periods, and also do the flood peak and volume of the same site. There is a large difference between this characteristic and hypothesis that flood peak and volume produced by rainstorm have the same co-occurrence frequency in the design flood estimation. By analyzing the observed data of representative hydrological stations, the regional flood peak has a poor correlation with the flood volume. The main factors affecting the flood peak and volume are the schedule and spatial distribution of rainfall, rainfall intensity, the process that the flood produces and so on. The consistency of the return period of rainstorm and flood is analyzed through typical cases, providing reference to grasp the regional rainstorm and flood characteristics and design flood estimation.

Impact of climate change on floodplain inundation and hydrological connectivity between wetlands and rivers in a tropical river catchment

AbstractSurface water connectivity between waterbodies in a river–floodplain system is considered one of the key determinants of habitat quality, biodiversity and ecological integrity. This manuscript presents results from an investigation into the potential changes in floodplain inundation and connectivity between wetlands and rivers under projected future climates, in a large river catchment in Western Australia. The study was conducted using a two‐dimensional hydrodynamic model (MIKE 21), and the modelling domain included the floodplain reaches encompassing the ecologically important wetlands. A lumped rainfall–runoff model (SIMHYD) was used to estimate local runoff and inflows from ungauged catchments. A SRTM derived 30‐m elevation data was used to parameterize land topography and stream networks in the hydrodynamic model. Hydraulic roughness parameters were estimated using a land cover map, which was developed using a combination of aerial photography, topographic maps and Google Earth imagery. The hydrodynamic model was calibrated using stream gauge data and flood inundation maps derived from Moderate Resolution Imaging Spectroradiometer imagery. Model simulated stage heights were combined with land topography to identify floodplain pathways that connect wetlands with rivers.The connectivity of 30 off‐stream wetlands was evaluated under present and future climates. The duration of connection of the individual wetlands to the main river channel varied from 1 to 40 days depending on flood magnitude and duration. Topographic relief, location on the floodplain and magnitude and duration of the flood were found to be key factors governing the level of connectivity, and the relationship between return period of flood and inundated area was found to be non‐linear. Modelling under a drier future climate indicated that the duration of connectivity of wetlands could be up to 20% less than under the current climate, whilst under a wetter climate the connectivity could be 5% longer. The results of this study provide potential use for future studies on movement and recruitment patterns of aquatic biota, wetland habitat characteristics and water quality and wetland biodiversity assessment. Copyright © 2015 John Wiley & Sons, Ltd.

Statistical approach to flood disaster management and risks reduction - in IBU River Basin

In the past four decades, economic losses due to flood have increased tremendously and resulted in major loss of human lives and livelihoods, the destruction of economic and social infrastructure, as well as environmental damage. This study focuses on flood disaster management through the establishment of a flood prediction model for Ibu River, a major tributary of the Ona River using the statistical approach. The Hazen plotting position was used to calculate the flood return periods while the floods were fitted into log-Pearson Type III distribution to obtain extreme flood estimate. Estimates showed that extreme floods are expected to occur between the months of June and October when rainfall is prominent within the basin area. In addition, flood magnitudes were found to increase with an increase in return periods in the Hazen plotting position. The model Q = 27.89Tr 0.548 (R 2 = 0.97) generated from fitting the extreme values (peaks) of the River Ibu into the log-Pearson type III distribution, can reduce flood risks within the River Ibu basin, particularly in Sagamu, and this information is highly helpful to hydrologists and engineers in planning for flood regulation and protection measures. Keywords : Ibu River, Hazen Plotting Position, Log-Pearson Type III Distribution, Peaks, Flood Regulation

Statistical approach to flood disaster management and risks reduction - in IBU River Basin

In the past four decades, economic losses due to flood have increased tremendously and resulted in major loss of human lives and livelihoods, the destruction of economic and social infrastructure, as well as environmental damage. This study focuses on flood disaster management through the establishment of a flood prediction model for Ibu River, a major tributary of the Ona River using the statistical approach. The Hazen plotting position was used to calculate the flood return periods while the floods were fitted into log-Pearson Type III distribution to obtain extreme flood estimate. Estimates showed that extreme floods are expected to occur between the months of June and October when rainfall is prominent within the basin area. In addition, flood magnitudes were found to increase with an increase in return periods in the Hazen plotting position. The model Q = 27.89Tr0.548 (R2 = 0.97) generated from fitting the extreme values (peaks) of the River Ibu into the log-Pearson type III distribution, can reduce flood risks within the River Ibu basin, particularly in Sagamu, and this information is highly helpful to hydrologists and engineers in planning for flood regulation and protection measures.Keywords: Ibu River, Hazen Plotting Position, Log-Pearson Type III Distribution, Peaks, Flood Regulation

An empirical assessment of which inland floods can be managed

Riverine flooding is a significant global issue. Although it is well documented that the influence of landscape structure on floods decreases as flood size increases, studies that define a threshold flood-return period, above which landscape features such as topography, land cover and impoundments can curtail floods, are lacking. Further, the relative influences of natural versus built features on floods is poorly understood. Assumptions about the types of floods that can be managed have considerable implications for the cost-effectiveness of decisions to invest in transforming land cover (e.g., reforestation) and in constructing structures (e.g., storm-water ponds) to control floods. This study defines parameters of floods for which changes in landscape structure can have an impact. We compare nine flood-return periods across 31 watersheds with widely varying topography and land cover in the southeastern United States, using long-term hydrologic records (≥20 years). We also assess the effects of built flow-regulating features (best management practices and artificial water bodies) on selected flood metrics across urban watersheds. We show that landscape features affect magnitude and duration of only those floods with return periods ≤10 years, which suggests that larger floods cannot be managed effectively by manipulating landscape structure. Overall, urban watersheds exhibited larger (270 m3/s) but quicker (0.41 days) floods than non-urban watersheds (50 m3/s and 1.5 days). However, urban watersheds with more flow-regulating features had lower flood magnitudes (154 m3/s), but similar flood durations (0.55 days), compared to urban watersheds with fewer flow-regulating features (360 m3/s and 0.23 days). Our analysis provides insight into the magnitude, duration and count of floods that can be curtailed by landscape structure and its management. Our findings are relevant to other areas with similar climate, topography, and land use, and can help ensure that investments in flood management are made wisely after considering the limitations of landscape features to regulate floods.

Optimal Operation of Multi-reservoir Systems Considering Time-lags of Flood Routing

Operations of multi-reservoir systems are nonlinear and high-dimensional problems, which are difficult to find the optimal or near-optimal solution owing to the heavy computation burden. This study focuses on flood control operation of multi-reservoir systems considering time-lags caused by Muskingum flood routing of river channels. An optimal model is established to jointly minimize the flood peak on the downstream flood control station for the multi-reservoir systems. A hybrid algorithm, Progressive Optimality Algorithm and Successive Approximation (POA-SA), is improved to solve the multi-reservoir operation model by modifying the POA. The POA-SA uses the DPSA to reduce the spatial dimensionality due to the multiple reservoirs, and adopts an improved POA to alleviate the temporal dimensionality caused by the time-lags of the Muskingum flood routing. Linear programming is then implemented to verify the solution of the POA-SA method with a linear approximation of the discharge capacity curve. The multi-reservoir systems of China’s Xijiang River is selected for a case study. Results show that the flood peak of Wuzhou station can be averagely decreased by 6730 m3/s (12.8 %) for the 100-year return period floods, indicating that the proposed method is efficient to operate the multi-reservoir systems and resolve the time-lags issues.

A high-resolution global flood hazard model.

Floods are a natural hazard that affect communities worldwide, but to date the vast majority of flood hazard research and mapping has been undertaken by wealthy developed nations. As populations and economies have grown across the developing world, so too has demand from governments, businesses, and NGOs for modeled flood hazard data in these data‐scarce regions. We identify six key challenges faced when developing a flood hazard model that can be applied globally and present a framework methodology that leverages recent cross‐disciplinary advances to tackle each challenge. The model produces return period flood hazard maps at ∼90 m resolution for the whole terrestrial land surface between 56°S and 60°N, and results are validated against high‐resolution government flood hazard data sets from the UK and Canada. The global model is shown to capture between two thirds and three quarters of the area determined to be at risk in the benchmark data without generating excessive false positive predictions. When aggregated to ∼1 km, mean absolute error in flooded fraction falls to ∼5%. The full complexity global model contains an automatically parameterized subgrid channel network, and comparison to both a simplified 2‐D only variant and an independently developed pan‐European model shows the explicit inclusion of channels to be a critical contributor to improved model performance. While careful processing of existing global terrain data sets enables reasonable model performance in urban areas, adoption of forthcoming next‐generation global terrain data sets will offer the best prospect for a step‐change improvement in model performance.

An integrated approach of flood risk assessment in the eastern part of Dhaka City

The flood risk is a function of the flood hazard, the exposed values, and their vulnerability. In addition to extreme hydrological events, different anthropogenic activities such as extensive urbanization and land use play an important role in producing catastrophic floods. Considerations of both physical and social dimensions are therefore equally important in flood risk assessment. However, very often the risk assessment studies focus either on physical or social dimensions. In addition, the available studies often focus on economic valuation of only direct tangible costs. In this study, we provide an integrated flood risk assessment approach that goes beyond the valuation of direct tangible costs, through incorporating physical dimensions in hazard and exposure and social dimensions in vulnerability. The method has been implemented in the Dhaka City, Bangladesh, an area internationally recognized as hot spot for flood risk. In this study, flood hazards for different return periods are calculated in spatial environment using a hydrologic model, HEC-RAS. Vulnerability is assessed through aggregation of various social dimensions, i.e., coping and adaptive capacities, and susceptibility. We assess vulnerability for both baseline and improved scenarios. In the baseline scenario, current early warning for study area is considered. In the alternative scenario, the warning system is expected to improve. Aggregating hazard, exposure and vulnerability, risk maps (in terms of both tangible and intangible costs) of several return period floods are produced for both baseline and improved scenarios. Compared to traditional assessments, the integrated assessment approach used in this study generates more information about the flood risk. Consequently, the results are useful in evaluating policy alternatives and minimizing property loss in the study area.

The 2011 flood event in the Assiniboine River Basin: causes, assessment and damages

In spring and summer of 2011, the Assiniboine River and its tributaries underwent a flood of unprecedented proportions. It was, by several measures, including the computed return period and duration of flooding, a more extreme event than the 1997 Red River “Flood of the Century,” and may have been the most severe flood experienced in the history of Canada. The 2011 flood was the largest recorded in the over 100 years that flow records have been kept on the Assiniboine River. As reported in the 2013 Manitoba 2011 Flood Review Task Force Report, antecedent conditions, winter snowpack and summer rains combined to produce one of the largest floods ever experienced in Manitoba. The spring of 2011 was preceded by a very wet fall in 2010 when precipitation averaged between 150 and 200% of normal over a wide area in Manitoba and Saskatchewan This produced extremely high regional antecedent moisture conditions whereby soil moisture levels were 100 to 250% of the long-term average. Added to this, the winter snowpack was relatively high, varying geographically in the range of 90 to 130% of normal snow water content and lower-than-normal winter temperatures, causing significant frost penetration. The combination of high soil moisture in the previous summer/fall and deep frost penetration reduced the soil’s ability to absorb spring meltwater and increased runoff volumes contributing to the magnitude of the spring peak. The severe winter was followed by a series of rainfall events that resulted in unprecedented rainfall volumes in the region throughout the months of May, June and July, when some areas received rainfall amounts that were 350% of normal and produced up to two additional flood peaks at some locations. Flooding on the Assiniboine River near Brandon lasted for 120 days. Large volumes of floodwater were diverted to Lake Manitoba due to the use of the Portage Diversion to mitigate flooding downstream of the City of Portage La Prairie. This large volume of inflow resulted in a wind-effect-eliminated Lake Manitoba level that peaked at 249.1 m in late July of 2011, which was close to 1 m higher than flood stage and almost 1.5 m higher than the top of the desirable range. Peak outflow from Lake Manitoba into the Fairford River was almost double historic maxima and resulted in Lake St. Martin levels that peaked in July of 2011 at 245.55 m, which was more than 0.6 m higher than the historic peak that occurred under natural conditions in 1955, and 1.4 m above flood stage. The damage estimate for this flood event has not been finalized but is likely in excess of CAD $1 billion. The majority of flood damages were incurred by communities and infrastructure along major tributaries like the Souris and Qu’Appelle Rivers, and the communities on Lake Manitoba and Lake St. Martin. The Lake St. Martin First Nations community was virtually destroyed, and as of 2014 some First Nations people from the communities along Lake St. Martin are still evacuated and waiting for their homes and communities to be rebuilt. Temporary flood control works were largely effective in preventing infrastructure damage to communities along the Assiniboine River, such as Brandon, although the agricultural damage and damage to individual properties were extensive.