Modeling disruptions causing domino effects in urban guided transport systems faced by flood hazards

Coastal cities are more vulnerable to floods due to the joint impact of rainfall and tide level. Quantitative risk assessment of disaster-causing factors is critical to urban flood management. This paper presents an integrated method to quantify the hazard degree of disaster-causing factors, rainfall and tide level, and to investigate the optimal management of flooding risk in different disaster-causing factor areas. First, an urban flood inundation model is used to simulate inundated extents in different drainage districts. Then, formulas are put forward to calculate the hazard degree of rainfall and tide level based on inundated extents in different combinations of rainfall and tide level. According to the hazard degree, the main disaster-causing factor could be identified in each drainage district. Finally, the optimal management of flooding risk in different disaster-causing factor areas is selected by disaster reduction analysis and cost–benefit analysis. Furthermore, the coastal city, Haikou of China, is taken as a case study. The results indicate that the hazard degree increases with the increasing distance between the drainage district and the Qiongzhou Strait or the Nandu River in the eastern of Haikou. Heavy rain is the main disaster-causing factor in inland areas, while high tide level is the main disaster-causing factor in island areas. For the area whose main disaster-causing factor is heavy rain, water storage projects could effectively reduce flooding. Meanwhile, pumps are economical choices for the area where tide level is the main disaster-causing factor. The results can provide reference for drainage planning in other coastal areas.     

Reconstruction and post-event analysis of a flash flood in a small ungauged basin: a case study in Slovak territory

Flash floods are one of the major natural hazards occurring in small streams with a negative effect on the country as well as on human lives. Heavy rainfall occurred on July 20, 2014 and July 21, 2014 and caused severe surface water flooding and a flash flood in the Malá Fatra National Park (Slovakia). The most affected was Vrátna Valley with the Varínka stream. This study presents a reconstruction and post-event analysis of a flash flood on small ungauged basin located in this protected area of Slovakia. The reconstruction included hydraulic terrain measurements on estimating the flood’s culmination and documenting the flood’s development. The measurements were taken at three cross sections of the Varínka stream. This paper is focused mainly on post-event analysis of the Varínka stream in two profiles: Strá?a (gauged profile) and Tiesňavy (ungauged cross section). Subsequently, the extremeness of the flash flood was preliminary evaluated. Results of the post-event analysis showed that the July 2014 flood was not the highest flood in this area despite its catastrophic consequences. By studying historical materials, we came to the conclusion that in the past (e.g. in 1848 or 1939) some devastating floods in this area had occurred, which had disastrous consequences for the population. The second part of the study is focused on comparing this flash flood with three major floods which have occurred in Slovak territory since 1998. The first flood occurred on the 20th of July, 1998 on the Malá Svinka stream, and the two others are floods which occurred on the 7th of June, 2011 in the Small Carpathian Mountains: on the Gidra stream in Píla village and on the Parná stream in Horné Ore?any village. Such comparison of flash floods from different geographical regions and different rainfall events can provide comprehensive information about their regimes, threats and disastrous effects.     

Variation of uncertainty of drainage density in flood hazard mapping assessment with coupled 1D–2D hydrodynamics model

Coupled 1D–2D hydrodynamic models are widely utilized in flood hazard mapping. Previous studies adopted conceptual hydrological models or 1D hydrodynamic models to evaluate the impact of drainage density on river flow. However, the drainage density affects not only river flow, but also the flooded area and location. Therefore, this work adopts the 1D–2D model SOBEK to investigate the impact of drainage density on river flow. The uncertainty of drainage density in flood hazard mapping is assessed by a designed case and a real case, Yanshuixi Drainage in Tainan, Taiwan. Analytical results indicate that under the same return period rainfall, reduction in tributary drainages in a model (indicating a lower drainage density) results in an underestimate of the flooded area in tributary drainages. This underestimate causes higher peak discharges and total volume of discharges in the drainages, leading to flooding in certain downstream reaches, thereby overestimating the flooded area. The uncertainty of drainage density decreases with increased rainfall. We suggest that modeling flood hazard mapping with low return period rainfalls requires tributary drainages. For extreme rainfall events, a lower drainage density could be selected, but the drainage density of local key areas should be raised.

     

Modelling coincidence and dependence of flood hazard phenomena in a Probabilistic Flood Hazard Assessment (PFHA) framework: case study in Le Havre

Many coastal urban areas and many coastal facilities must be protected against pluvial and marine floods, as their location near the sea is necessary. As part of the development of a Probabilistic Flood Hazard Approach (PFHA), several flood phenomena have to be modelled at the same time (or with an offset time) to estimate the contribution of each one. Modelling the combination and the dependence of several flooding sources is a key issue in the context of a PFHA. As coastal zones in France are densely populated, marine flooding represents a natural hazard threatening the coastal populations and facilities in several areas along the shore. Indeed, marine flooding is the most important source of coastal lowlands inundations. It is mainly generated by storm action that makes sea level rise above the tide. Furthermore, when combined with rainfall, coastal flooding can be more consequent. While there are several approaches to analyse and characterize marine flooding hazard with either extreme sea levels or intense rainfall, only few studies combine these two phenomena in a PFHA framework. Thus this study aims to develop a method for the analysis of a combined action of rainfall and sea level. This analysis is performed on the city of Le Havre, a French urban city on the English Channel coast, as a case study. In this work, we have used deterministic materials for rainfall and sea level modelling and proposed a new approach for estimating the probabilities of flooding.

     

Karst flash flooding in a Mediterranean karst, the example of Fontaine de Nîmes

Karst flash flooding, identified as one of the hazards in karst terrains, is directly linked to the structure and hydraulic properties of karst aquifers. Due to the characteristics of flow within karst aquifers, characterized by a dual flow – diffuse flow within fissured limestone and conduit flow within karst conduits networks – flash flooding may be important in volume and dynamics. Such phenomenon may cause serious damages including loss of lives, as it occurred on 3rd October 1988 in Nîmes (Gard, South France). Flash floods there have been considered to be the result of very intensive rainfall events conjugated to runoff due to the geomorphologic context of the city located down hill. However, preliminary results of recent studies of the hydrologic behaviour of groundwater and surface water for a specific event (September 2005) show that the karst plays an important role in the flood genesis. The main characteristics of the Nîmes karst system leading to karst flash flooding are presented in this paper. A methodology comprising modelling of the karst system allowed proposing simple warning thresholds for various part of the karst (water level threshold for the karst conduits and cumulative rainfall threshold for the overflowing fissured karst). These thresholds can be included in the flash flood warning system of the Nîmes city.     

Flood hazard in Hungary: a re-assessment

Some decades ago the concept of flood hazard in the Carpathian Basin was interpreted solely as riverine flood hazard, mostly restricted to the Tisza and Danube Rivers, and was closely associated with the impacts of river flow regulation in the second half of the 19th century. Recent assessments, however, allow us to outline a more diverse picture. Climate change is predicted to bring about both an increase in the frequency of droughts and excessive rainfall events, resulting in irregulaties in the water regimes of rivers in Hungary. Excess water hazard from raised groundwater levels is found to affect much larger areas than previously thought. Recent strongly localized cloudbursts, point to the increasing significance of flash floods.Riverine flooding and excess water hazard are more common in lowlands, whereas flash flood hazards are primarily, but not exclusively, affect the mountainous and hilly regions of the country. This paper intends to assess the relative importance of the three types of inundation hazard analyzed and to illustrate their overall spatial occurrences by microregions on a map series.     

Flash flood hazard mapping based on quantitative hydrology,geomorphology and GIS techniques (case study of Wadi Al Lith,Saudi Arabia)

Flash floods are considered as catastrophic phenomena possessing major hazardous threat to the coastal cities, towns, villages and infrastructures. This study deals with the evaluation of flash flood hazard in the ungauged Wadi Al Lith basin depending on detailed morphometric characteristics of Al Lith basin and its sub-basins. For the detailed study, ASTER data were used for preparing digital elevation model (DEM), and geographical information system (GIS) was used in the evaluation of linear, areal and relief aspects of morphometric parameters. The major parameters such as watershed boundary, flow accumulation, flow direction, flow length and stream ordering are prepared using the ArcHydro Tool. Surface Tool in ArcGIS-10 software, and ASTER (DEM) was used to create different thematic maps such as DEM, contour, slope aspect and hill shade maps. Twenty-five morphometric parameters were measured, calculated and interlinked to produce nine effective parameters for evaluation of the flash flood hazard degree of the study area. Based on nine morphometric parameters which affect the hydrologic behaviour of the Wadi, by influence on time of concentration which has a direct influence on flooding prone area. The flash flood hazard of the Al Lith basin and its sub-basins was identified and classified into three groups (high, medium and low hazard degree). The study provides details on the flash flood-prone area (Wadi Al Lith) and the mitigation measures. This study also helps to plan rainwater harvesting and watershed management in the flash flood alert zones.     

Influence of hydrate exploitation on stability of submarine slopes

Urban pluvial flash floods have become a matter of widespread concern, as they severely impact people’s lives in urban areas. Hydrological and hydraulic models have been widely used for urban flood management and urban planning. Traditionally, to reduce the complexity of urban flood modelling and simulations, simplification or generalization methods have been used; for example, some models focus on the simulation of overland water flow, and some models focus on the simulation of the water flow in sewer systems. However, the water flow of urban floods includes both overland flow and sewer system flow. The overland flow processes are impacted by many different geographical features in what is an extremely spatially heterogeneous environment. Therefore, this article is based on two widely used models (SWMM and ANUGA) that are coupled to develop a bi-directional method of simulating water flow processes in urban areas. The open source overland flow model uses the unstructured triangular as the spatial discretization scheme. The unstructured triangular-based hydraulic model can be better used to capture the spatial heterogeneity of the urban surfaces. So, the unstructured triangular-based model is an essential condition for heterogeneous feature-based urban flood simulation. The experiments indicate that the proposed coupled model in this article can accurately depict surface waterlogged areas and that the heterogeneous feature-based urban flood model can be used to determine different types of urban flow processes.

     

Evaluation of real-time flash flood forecasts for Haiti during the passage of Hurricane Tomas, November 4–6, 2010

The January 2010 earthquake that devastated Haiti left its population ever more vulnerable to rainfall-induced flash floods. A flash flood guidance system has been implemented to provide real-time information on the potential of small (~70 km²) basins for flash flooding throughout Haiti. This system has components for satellite rainfall ingest and adjustment on the basis of rain gauge information, dynamic soil water deficit estimation, ingest of operational mesoscale model quantitative precipitation forecasts, and estimation of the times of channel flow at bankfull. The result of the system integration is the estimation of the flash flood guidance (FFG) for a given basin and for a given duration. FFG is the amount of rain of a given duration over a small basin that causes minor flooding in the outlet of the basin. Amounts predicted or nowcasted that are higher than the FFG indicate basins with potential for flash flooding. In preparation for Hurricane Tomas’ landfall in early November 2010, the FFG system was used to generate 36-h forecasts of flash flood occurrence based on rainfall forecasts of the nested high-resolution North American Model of the National Centers for Environmental Prediction. Assessment of the forecast flood maps and forecast precipitation indicates the utility and value of the forecasts in understanding the spatial distribution of the expected flooding for mitigation and disaster management. It also highlights the need for explicit uncertainty characterization of forecast risk products due to large uncertainties in quantitative precipitation forecasts on hydrologic basin scales.     

Pedestrian hazard criteria for flooded urban areas

Urban floods are sudden phenomena normally characterized by short flooding durations and devastating effects due to the high concentrations of persons, goods, and properties located in urban areas. In these cases, uncontrolled runoff can flow on urban streets, creating a significant hazard for pedestrians and vehicles. The safety of people can be compromised when they are exposed to flows that exceed their ability to remain standing or to traverse flow paths. With the aim of studying the stability of pedestrians in flooded street during storm events, a model representing an urban street in real scale was designed and built. Following a specific protocol that considering several flow rates (up to 500 l/s) and different longitudinal slopes (up to 10 %), 834 tests, using 23 human subjects, were carried out in good and poor light conditions. Hazard conditions were classified into three groups (low, medium, and high), and the parameters for which these conditions occurred were recorded. Results showed that for subjects weighing 50–60 daN (50–60 kg mass) and flow depths between 9 and 16 cm, velocities of 1.5–2 m/s can generate loss of stability due to phenomena of dragging or overturning. Empirical expressions were proposed to relate human subject characteristics (weight and height) and limiting flow conditions at which loss of stability occurs.     

Estimation of the upper bound of seismic hazard curve by using the generalised extreme value distribution

Flash floods are the most common type of natural hazards that cause loss of life and massive damage to economic activities. During the last few decades, their impact increased due to rapid urbanization and settlement in downstream areas, which are desirable place for development. Wadi Asyuti, much like other wadis in the Eastern Desert of Egypt, is prone to flash flood problems. Analysis and interpretation of microwave remotely sensed data obtained from the Shuttle Radar Topography Mission (SRTM) and Tropical Rainfall Measuring Mission (TRMM) data using GIS techniques provided information on physical characteristics of catchments and rainfall zones. These data play a crucial role in mapping flash flood potentials and predicting hydrologic conditions in space and time. In order to delineate flash flood potentials in Wadi Asyuti basin, several morphometric parameters that tend to promote higher flood peak and runoff, including drainage characteristics, basin relief, texture, and geometry were computed, ranked, and combined using several approaches. The resulting flash flood potential maps, categorized the sub-basins into five classes, ranging from very low to very high flood potentials. In addition, integrating the spatially distributed drainage density, rainfall intensity, and slope gradient further highlighted areas of potential flooding within the Wadi Asyuti basin. Processing of recent Landsat-8 imagery acquired on March 15, 2014, validated the flood potential maps and offered an opportunity to measure the extent (200–900 m in width) of the flooding zone within the flash flood event on March 9, 2014, as well as revealed vulnerable areas of social and economic activities. These results demonstrated that excessive rainfall intensity in areas of higher topographic relief, steep slope, and drainage density are the major causes of flash floods. Furthermore, integration of remote sensing data and GIS techniques allowed mapping flood-prone areas in a fast and cost-effective to help decision makers in preventing flood hazards in the future.     

Flash flood mitigation as a positive consequence of anthropogenic forcing on the groundwater resource in a karst catchment

The Mediterranean coastal region is prone to high-intensity rainfall events that are frequently associated with devastating flash floods. This paper discusses the role of a karst aquifer system in the flash floods of a Mediterranean river, the Lez river. Most of the Lez river watershed is located on karst terrains where interactions between surface water and groundwater take place. During extreme rainfall events, the presence of fractures and well-developed karst features in carbonate terrains enhances the infiltration processes and involves the concentration of the recharge into highly organized and permeable flow paths. The groundwater, therefore, quickly moves towards the natural outlets of the karst system. The influence of the Lez karst aquifer system on the associated river floods dynamics is analysed while considering the spatially distributed rainfall, as well as the time series of the groundwater level within the aquifer and of the Lez river discharge measured at various gauging stations. Special attention is given to the relative importance of the surface and underground processes involved in flash flood genesis. It is shown that the karst groundwater contributes to flash floods under certain conditions, while high-rate pumping within the karst aquifer, which generates significant drawdown, may mitigate flash floods under other conditions.     

Analysis of soil boundary conditions of flash floods in a small basin in SW Hungary

Flash floods are one of the most significant natural hazards of today. Due to the complexity of flash flood triggering factors, to prevent or mitigate flood triggered losses, numeric model based flood forecasting models are capable tools to predict stream water levels. The main goal of the current research was to reproduce two flow peaks with the HEC-HMS rainfall-runoff model and test the model sensitivity for various input parameters. To obtain sufficient input data, we monitored soil depth, maximum infiltration rate, soil moisture content, rainfall, time of concentration and flow. To obtain input data, parameters were calculated, measured in the Sás Valley experimental watershed (SW Hungary) or optimized with the built in function of the HEC-HMS. Soil moisture was monitored in the 1.7 km2 pilot catchment over the period between September 2008 and September 2009. HEC-HMS had a good performance reproducing the two events, however simulated flow time series are highly influenced by the antecedent soil moisture, infiltration rate and canopy storage. Outflow modeled data were verified for two flood events (June 4, 2008 and July 9, 2009). The HEC-HMS was over-sensitive for input soil moisture and with increasing input rainfall and increasing outflow, larger simulation errors were observed.     

Extreme hydrological events in karst areas of Slovenia,the case of the Unica River basin

The study presents natural hazards in Slovenia's karst, focusing on flooding in karst poljes. A specific study was done on the flood dynamics of two typical and connected karst poljes (Cerknica and Planina) of the Classical Karst region. In the case of particularly extreme hydrological conditions in the autumn of 2008, detailed analyses of the recharge-discharge regime and the interrelationship of flooding on the two poljes were done. Daily precipitation, discharge, and water level values from several monitoring sites were analyzed and cross-correlated, and additional hydrological analyses were done using a digital elevation model in order to acquire water level increase and decrease intensity, flood water volumes, and the extent of flooding and to understand the conditions controlling karst flooding. The results reveal that the hydrological functioning of the studied karst poljes is influenced by the hydrogeological and temporary hydrological conditions in the catchment area. The response of the binary karst system (i.e., the influence of autogenic and allogenic recharge) is especially distinct. The study shows that during extremely intense recharge, the reactions of karst aquifer systems to precipitation are as rapid as the response of surface waters (the water level of Cerknica Lake increased with an intensity of 38-63 cm/day or 55 m3/s respectively) while retention capacities are negligible. In contrast to flash floods, floods in karst areas may last from several weeks to several months. For the observed period a three-dimensional simulation of the flooding was made. At the maximum recorded water level, the volume of water on the Cerknica polje was 51 million m³, and 26 million m³ on the Planina polje. The maximum extent of flooding on the Cerknica polje was 23 km² and on the Planina polje 9.5 km². On the basis of the study, information was provided regarding future hazard mitigation. However, the study demonstrated that a sufficiently dense monitoring network is necessary to predict the occurrence and duration of floods with greater certainty.     

淮河干流洪水预报系统及其在1991年大洪水的应用

综述了淮河流域概况，淮河干流洪水预报系统物采用的预报方法。１９９１年淮河干流洪水预报采用了降雨径流预报与上，下游站相应流量预防方法相配合，并注重实时水情分析；在行洪区多，行洪后水面宽广而比降又极小的河段的汇流计算，采用了以实测洪水资料绘制的经验蓄曲线为充分发挥的湖泊洪水演算方法。     

山区小流域洪水分布模式模拟与危险性评估

流域暴雨山洪过程时空异质性强,准确评估雨洪变化特性和洪水危险性对山洪灾害防治具有重要意义。以7个降雨特征指标和6个洪水特征指标刻画流域场次雨洪特性,采用中国山洪水文模型和洪水频率指标相结合,模拟和评估口前流域洪水过程及其危险性。结果表明：场次洪水洪峰模数、洪峰时间偏度、高脉冲历时占比、涨落洪速率与降雨总量、平均雨量、最大雨强、雨峰位置系数、基尼系数等降雨特征指标显著相关,三场致灾洪水过程的降雨均呈现量级大、强度大、历时短、暴雨中心偏中下游的特点;率定期和验证期的平均径流深相对误差均在9%以内,平均洪峰流量相对误差均在11%以内,平均峰现时间误差均在1.7 h以内,平均Nash-Sutcliffe系数为0.80和0.76;各场次洪水有0.0%～93.3%的河段流量达到一般危险及以上等级,三场致灾洪水过程的危险性等级最高,分别有80.0%、35.0%和1.7%的小流域河段流量达到高危险及以上等级。研究可为山区小流域暴雨洪水危险性评估、灾害响应和复盘等提供技术支撑。     

Rainfall-runoff modeling of ungauged Wadis in arid environments (case study Wadi Rabigh—Saudi Arabia)

Flash flood forecasting of catchment systems is one of the challenges especially in the arid ungauged basins. This study is attempted to estimate the relationship between rainfall and runoff and also to provide flash flood hazard warnings for ungauged basins based on the hydrological characteristics using geographic information system (GIS). Morphometric characteristics of drainage basins provide a means for describing the hydrological behavior of a basin. The study examined the morphometric parameters of Wadi Rabigh with emphasis on its implication for hydrologic processes through the integration analysis between morphometric parameters and GIS techniques. Data for this study were obtained from ASTER data for digital elevation model (DEM) with 30-m resolution, topographic map (1:50,000), and geological maps (1,250,000) which were subject to field confirmation. About 36 morphometric parameters were measured and calculated, and interlinked to produce nine effective parameters for the evaluation of the flash flood hazard degree of the study area. Based on nine effective morphometric parameters that directly influence on the hydrologic behavior of the Wadi through time of concentration, the flash flood hazard of the Rabigh basin and its subbasins was identified and classified into three groups (High, medium, and low hazard degree). The present work proved that the physiographic features of drainage basin contribute to the possibility of a flash flood hazard evaluation for any particular drainage area. The study provides details on the flash flood prone subbasins and the mitigation measures. This study also helps to plan rainwater harvesting and watershed management in the flash flood alert zones. Based on two historical data events of rainfall and the corresponding maximum flow rate, morphometric parameters and Stormwater Management and Design Aid software (SMADA 6), it could be to generate the hydrograph of Wadi Rabigh basin. As a result of the model applied to Wadi Rabigh basin, a rainfall event of a total of 22 mm with a duration of 5 h at the station nearby the study area, which has an exceedance probability of 50 % and return period around 2 years, produces a discharge volume of 15.2?×?10⁶ m³ at the delta, outlet of the basin, as 12.5 mm of the rainfall infiltrates (recharge).     

Flooding trends and their impacts on coastal communities of Western Cape Province,South Africa

Climate change-induced extreme weather events have been at their worst increase in the past decade (2010–2020) across Africa and globally. This has proved disruptive to global socio-economic activities. One of the challenges that has been faced in this regard is the increased coastal flooding of cities. This study examined the trends and impacts of coastal flooding in the Western Cape province of South Africa. Making use of archival climate data and primary data from key informants and field observations, it emerged that there is a statistically significant increase in the frequency of flooding and consequent human and economic losses from such in the coastal cities of the province. Flooding in urban areas of the Western Cape is a factor of human and natural factors ranging from extreme rainfall, usually caused by persistent cut off-lows, midlatitude cyclones, cold fronts and intense storms. Such floods become compounded by poor drainage caused by vegetative overgrowth on waterways and land pollution that can be traced to poor drainage maintenance. Clogging of waterways and drainage systems enhances the risk of flooding. Increased urbanisation, overpopulation in some areas and non-adherence to environmental laws results in both the affluent and poor settling on vulnerable ecosystems. These include coastal areas, estuaries, and waterways, and this worsens the risk of flooding. The study recommends a comprehensive approach to deal with factors that increase the risk of flooding as informed by the provisions of both the Sustainable Development Goals framework and the Sendai Framework for Disaster Risk Reduction 2015–2030 in a bid to de-risking human settlement in South Africa.

     

Flash flood routing modeling for levee-breaks and overbank flows due to typhoon events in a complicated river system

There has been a yearly increase in precipitation in Taiwan, consistent with trends seen across the world. In the summer and fall, typhoons or tropical cyclones with torrential rainfall frequently occur as a result of Taiwan’s subtropical climate. Flash floods may cause a levee-break and/or the overtopping of banks at narrow neck locations in a river system, which may in turn produce inundation in urban areas. Therefore, a model that predicts flash floods is of vital importance for river management. The present study is based on a flash flood routing model, which incorporates levee-break and overbank functions to calculate the discharge hydrographs in the complicated Danshuei River system of northern Taiwan. The numerical model was calibrated and verified against observed water stages using three typhoon events. The results indicate reasonable agreement between the model simulations and the observed data. The model was then used to calculate the levee-break and overbank flow hydrographs due to Typhoon Talim (2005) and Typhoon Nari (2001), respectively. The simulated results indicate that several parameters significantly affect the flow hydrograph during a levee-break and should be carefully monitored when levee-break events occur in the river system. The simulated water stages at several stations are consistent with observed data from Typhoon Nari. The simulated overbank flow results quantitatively agree with reported information. The data also confirm that most of the overbank events occurred at the upper reaches of the Keelung River, consistent with the low levee height protection.