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With the extensive use of 2D flood models, the resolution and quality of Digital Elevation Models (DEMs) have come under greater focus especially in urban hydrology. One of the major research areas, in this regard, is the effect of DEM resolution on flood modelling. This study first investigates the root causes of the impact of DEM resolution on urban fluvial flood modelling outputs using DEMs with grid resolutions ranging from 1m to 50m. The study then investigates how DEM resolution affects the definition and characterisation of the river channel and the consequences of this for the modelled results. For this purpose, a separate set of merged DEMs was generated where the river channel as defined by the 1m resolution DEM is merged with coarser resolution DEMs. Data obtained during the flood event caused by Storm Desmond (2015) in Cockermouth (Cumbria, UK) was used for this study. The HEC-RAS 2D model was used for all of the simulations. The benchmark model obtained with the 1m resolution DEM was calibrated using measured water levels at two locations within the rivers. Results show that there is a 30% increase in flood extent from 58.9 ha to 79.0 ha and a 150% increase in mean flood depth from 1.74m to 4.30m when the resolution reduces from a 1m grid to a 50m grid. The main reason for this is the increasing lack of definition of the river channel with an associated reduction in the estimated depth of the river resulting in reduced river channel conveyance. This then leads to an increase in the flood extent and depth especially in the immediate vicinity of the river. This effect is amplified when the DEM grid size is greater than the river width. When the 1m resolution DEM for the river channel is used in conjunction with coarser resolution DEMs for the surrounding areas (merged DEMs), there is a significant improvement in the agreement between the modelled and the reference case (obtained from the benchmark model) flood extents and depths. The use of merged DEMs reduces the error in mean flood depth from 90% to 4% and reduces the overall RMSE in flood depths from 2.6m to 0.9m at 30m resolution. The 30m resolution DEM was tested because this is. The use of merged DEMs, where a higher resolution DEM is used to characterise the river channel in conjunction with a 30m resolution DEM (e.g., NASA Shuttle Radar Topography Mission DEMs) for the wider area could be a cost-effective solution for locations where higher resolution DEMs may not be available.

Vulnerability assessment is an essential tool in mitigating the impact of urban flooding. To date, most flood vulnerability research has focused on one type of flood, such as a pluvial or fluvial flood. However, cities can suffer from urban flooding for several reasons, such as precipitation and river levee overtopping. Therefore, a vulnerability assessment considering different types of floods (pluvial floods, fluvial floods, and compound flooding induced by both rainfall and river overtopping) was conducted in this study. First, a coupled urban flood model, considering both overland and sewer network flow, was developed using the storm water management model (SWMM) and LISFLOOD-FP model to simulate the different types of flood and applied to Lishui, China. Then, the results of the flood modeling were combined with a vulnerability curve to obtain the potential impact of flooding on different land-use classes. The results indicated that different types of floods could have different influence areas and result in various degrees of flood vulnerability for different land-use classes. The results also suggest that urban flood vulnerability can be underestimated due to a lack of consideration of the full flood-induced factors.

The integrative approach of water, energy, and food nexus (WEF nexus) is now widely accepted to offer better planning, development, and operation of these resources. This study presents a first attempt towards understanding the WEF nexus of urban environments in the Nile River Basin under conditions of hydrological droughts and fluvial floods. A case study was conducted for the capital of Sudan, Khartoum, at the confluence of the White Nile and the Blue Nile for illustration. The results were based on analyses of river flow and water turbidity data, field observations, a printed questionnaire and an interview of farmers practicing irrigated agriculture, and hydropower modeling. The study analyzes indicators for the association of the river water resources environment (intra-annual regime, quantity, and quality), the status of urban irrigated agriculture, water treatment for domestic use, and hydropower generation under hydrological extremes, i.e. droughts and fluvial floods. It additionally examines the consequent interactions between the impacts on three sectors. The present study shows how floods and droughts impose impacts on seasonal river water quality and quantity, water treatment for domestic use, irrigated agriculture, and hydro-energy supply in an urban environment. The results demonstrate how the two hydrological phenomena determine the state of hydropower generation from dams, i.e. high energy production during floods and vice versa during droughts. Hydropower dams, in turn, could induce cons in the form of low fertile soils in the downstream due to sediment retention by the reservoir. Finally, present and potential options to minimize the above risks are discussed. This study is hoped to offer good support for integrated decision making to increase the resource use efficiency over the urban environment within the Nile Basin.

Urban flooding is a severe and pervasive hazard caused by climate change, urbanization, and limitations of municipal drainage systems. Cities face risks from different types of floods, depending on various geographical, environmental, and hydrometeorological conditions. In response to the growing threat of urban flooding, a better understanding of urban flood vulnerability is needed. In this study, a comprehensive method was developed to evaluate the vulnerability of different types of urban floods. First, a coupled urban flood model was built to obtain the extent of influence of various flood scenarios caused by rainfall and river levee overtopping. Second, an assessment framework for urban flood vulnerability based on an indicator method was used to evaluate the vulnerability in different flood hazard scenarios. Finally, the method was applied to Lishui City, China, and the distribution and pattern of urban flood vulnerability were studied. The results highlight the spatial variability of flooding and the vulnerability distributions of different types of urban floods. Compound floods were identified to cause more severe effects in the urban areas.

Article publié e français (p. 33-45), avec un résumé étendu en anglais (p. 46-47) et 6 planches couleur hors-texte (p. 74-79); International audience; The analysis of the hydrographic network supplies several informations that one must know for better watershed management. The N’zi watershed is located in Ivory Coast between longitudes 3°49’ and 5°22’ West and latitudes 6°00’ and 9°26’ North. It extends to 35,500 km2 and its main river is the N’zi of a length of 725 km which is fed by many tributaries. Two different and complementary approaches are used to characterize the hydrographic network of the N’zi watershed. The first approach refers to the development of a methodology of Landsat 7 ETM+ image processing in order to highlight the basic drainage patterns of the N’zi watershed. First, Landsat 7 ETM+ images undergo preliminary treatment (mosaic of the 6 scenes of Landsat 7 ETM+, windows and extraction of the N’zi watershed). Then, resulting images are submitted to the digital processing (various operations of enhancement, combinations of bands, filtering specifically that carried out by the 7×7 matrix of Sobel). Finally, transformed images undergo a visual analysis for the morphological identification of the hydrographic network of the N’zi watershed. The second approach refers to the global hydrographic network processing extracted of the topographical map and which constitutes a reliable source of validated information of grounds. This one is used as network of reference for the recognition of the tracing of the rivers on the images. The hydrographic network is used for the statistical analysis and for mapping the drainage density and also the functioning zones of the N’zi watershed.The hydrographic network is firstly discretized in regular meshes of 5 km side using Linwin 2.0 software. In each mesh, the total number of drains and the cumulated lengths of the drains are determined. These two parameters are compiled in a circular histogram in order to discriminate them on the basis of their orientation. A correlation is determined between the number of drains and the cumulated lengths of the drains. From the cumulated lengths of the drains by mesh, an interpolation by kriging of these values is carried out under ArcGis 10 to generate the drainage density map. The global hydrographic network of the N’zi watershed is organized into a hierarchy according to the convention of STRAHLER (1968) because of its geomorphological significance. A counting of all the drains according to their order is also carried out. A graph showing the number of drains according to the orders of the rivers is established in order to highlight the mathematical law governing the distribution of the drains. Finally, the hierarchical hydrographic network is used for the establishment of the iso-orders curves. The junctions of the drains which have the same order (1, 2,….. n) on the N’zi watershed are joined by the curves of respective iso-orders 1, 2,……. n. On the basis of geomorphological analysis of the N’zi watershed, relatively three distinct functioning zones are mapped.The analysis of the satellite images and the interpretation of the hydrographic network allowed to highlight the basic drainage patterns: dendritic, subdendritic, parallel and radial. The compared analysis of the drains in number and cumulated lengths shows that major directional class N90-100 is the longest. The number of drains is moderately correlated with the cumulated lengths of the drains of the N’zi watershed. The essential of the drainage of the N’zi watershed is assured by tributaries of small orders (1 and 2) with the order 1 which accounts for 77%. The drains of the hydrographic network fit better to the exponential law. The drainage density is variable on the N’zi watershed. The N’zi watershed is order 6. Five iso-orders curves are highlighted on this one. It is possible to map three distinct functioning zones on the N’zi watershed.- the recharge zone: it corresponds to geographical place occupied generally by the rivers of order 1 and 2. It is generally the zone of plateau where the principal rivers and their tributaries begin. - the zone with potential flow: it is occupied by rivers of order 3 and 4 and represents the sloping ground of the watershed. Water is very turbulent there.- the zone of fluvial flood risk is the area of rivers of order 5 and 6 and the confluence zones from which they result. It is a zone of reception and accumulation of waters draining the watershed. Indeed, the contribution in water of the tributaries of the N’zi is not negligible and contributes to an increase in the flow of this one in the confluence zones. Thus, these areas are more susceptible to flood risks.The mapping of the flood area is an essential tool not only for the prevention of the flood risks, but also for the management, planning and management of hydro-systems and for the future programming of the intervention areas.; Les prises d’eau du N’zi entraînent des baisses drastiques de son régime, mettant ainsi en péril le processus de développement agricole. L’objectif de cette étude est de caractériser le réseau hydrographique du bassin versant du N’zi en vue de sa connaissance. La méthodologie consiste au développement de traitements numériques appropriés des images ETM+ de Landsat 7 et du réseau hydrographique extrait de la mosaïque des cartes topographiques. Le bassin versant du N’zi est caractérisé par des patrons de drainage de type dendritique à sub-dendritique, parallèle à radial. Il est d’ordre 6, avec les drains d’ordre 1 les plus représentatifs (77%). Ceux de direction N90-100 constitue la classe majoritaire et la plus dominante. Les drains s’ajustent mieux à la loi exponentielle. Le bassin versant du N’zi, avec un potentiel de drainage variable, est caractérisé par une zone d’alimentation, une zone à potentiel de flux et une zone à risque d’inondation fluviale. Cette dernière correspond aux cours d’eau d’ordre 5 et 6 et des confluences dont ils résultent.

International aid for climate change adaptation in West Africa is increasing exponentially, but our understanding of hydroclimatic risks is not keeping pace with that increase. The aim of this article is to develop a multihazard risk assessment on a regional scale based on existing information that can be repeated over time and space and that will be useful during decision-making processes. This assessment was conducted in Dosso (Niger), the region most hit by flooding in the country, with the highest hydroclimatic risk in West Africa. The assessment characterizes the climate, identifies hazards, and analyzes multihazard risk over the 2011&ndash, 2017 period for each of the region&rsquo, s 43 municipalities. Hazards and risk level are compared to the intervention areas and actions of 6 municipal development plans and 12 adaptation and resilience projects. Over the past seven years, heavy precipitation and dry spells in the Dosso region have been more frequent than during the previous 30-year period. As many as 606 settlements have been repeatedly hit and 15 municipalities are classified as being at elevated-to-severe multihazard risk. The geographical distribution of the adaptation and resilience projects does not reflect the risk level. A third of the local development plans examined propose actions that are inconsistent with the main hydroclimatic threats.

Although one-dimensional flow models have proved to perform adequately when flow is restricted between channel banks, two-dimensional flow modelling has shown to have the upper hand when assessing flows in topographically complex floodplains, where the flow is mostly attributed to be two-dimensional. The high accessibility of topographic data due to recent progresses in remote sensing techniques coupled with great advancements made in computing power have spurred the popularity of twodimensional hydraulic models. However, the high computation times still remain a drawback suffered by the majority of current two-dimensional flow models. This study aims to assess the effect of a newly developed modelling algorithm called high resolution sub-grid model on the recently released two-dimensional flow model from HEC-RAS 5.0. This algorithm intends to reduce the aforementioned high computation times of two-dimensional flow models by using larger mesh cell sizes while maintaining accurate results. The latter is achieved by integrating the details of the fine bathymetry from the underlying terrain model into the modelling process. Furthermore, this work thoroughly analyses the state of the art of two-dimensional hydraulic modelling, heavily focusing on the capabilities and hydrodynamics of the twodimensional HEC-RAS 5.0 module as well as on the implementation of the sub-grid model in the software. Subsequently, a hypothetical fluvial flooding event caused by a 500 year return period flow hydrograph in the lower part of the Ter river catchment is studied. Simulations are carried out for multiple computational mesh cell sizes with a computation interval that has been appropriately calibrated for each model beforehand. Finally, the results of the simulations for four relevant variables related to fluvial flooding events are considered. The sub-grid model seems to yield effective flooding extent, maximum water depth and flow velocity results for a mesh cell size of 25x25 m, whereas the 50x50 m model is only recommended for the maximum water depth analysis. These results are obtained in much lower computation times than the 10x10 m model. Specifically, the 25x25 and 50x50 m models do it 29 hours and 31 hours faster than the 10x10 m model respectively. As for the arrival time, the sub-grid model does not offer good predictions, at least for the mesh sizes studied. A pesar de que los modelos unidimensionales de flujo hayan demostrado ser capaces de rendir adecuadamente cuando el flujo se encuentra encauzado, los modelos bidimensionales de flujo han demostrado ser superiores cuando se trataba de flujos en planicies de inundación con una topografía compleja, donde el flujo tiende a ser mayoritariamente bidimensional. La popularidad de los modelos bidimensionales hidráulicos viene a raíz de la elevada accesibilidad a información topográfica debido al progreso hecho en el campo de la detección remota por satélite junto a los avances tecnológicos realizados en el campo de la computación. De todas formas, los altos tiempos computacionales continúan siendo una de las desventajas comunes entre los modelos bidimensionales de flujo. Este trabajo tiene como objetivo principal evaluar el efecto de un algoritmo de modelización recientemente desarrollado llamado geometría sub-malla en el nuevo modelo de flujo bidimensional que incluye HEC-RAS 5.0. El algoritmo aspira reducir los altos tiempos computacionales característicos de los modelos bidimensionales de flujo utilizando tamaños de malla más grandes pero conservando resultados rigurosos. Lo último se consigue mediante la integración de los detalles de la batimetría provenientes del modelo del terreno en el proceso de modelización. Además, el trabajo analiza con gran profundidad el estado del arte de la modelización hidráulica, centrándose en su gran parte en las capacidades y en el esquema hidrodinámico del módulo bidimensional de HEC-RAS 5.0, así como en la implementación explícita de la geometría sub-malla en el software. Posteriormente, se estudia una hipotética inundación fluvial causada por un hidrograma correspondiente a un período de retorno de 500 años en el tramo bajo del río Ter. Se llevan a cabo varias simulaciones para diversos tamaños de malla con el correspondiente tiempo de paso que ha sido debidamente calibrado para cada modelo anteriormente. Finalmente se consideran los resultados de las simulaciones para cuatro variables relacionadas con inundaciones fluviales. La geometría sub-malla parece dar buenos resultados para la mancha de inundación, calado máximo y velocidad de flujo máxima para un tamaño de malla de 25x25 m, mientras que el uso del modelo de 50x50 m se recomienda exclusivamente para el análisis de calados máximos. Concretamente, los tiempos computacionales de los modelos de 25x25 m y de 50x50 m tardan 29 y 31 horas menos que el modelo de 10x10 m respectivamente. Para el tamaño de mallas estudiado, la geometría sub-malla no logra predecir con precisión el tiempo de llegada del agua. Palabras clave: modelos bidimensionales de flujo, HEC-RAS, inundación fluvial, geometría sub-malla, malla, tiempo computacional.

High aggradation and degradation in a river induced by last flood event will respectively raise and decrease the risk of an upcoming flood event. However, this point is likely to be ignored or is not fully considered in some 2D fluvial flood models. To address this problem, this work develops a 2D high-resolution hydrodynamic model coupled with the sediment transport and the river bed evolution models. The modelling system is within the framework of a Godunov-type finite volume scheme. GPU technique is applied to accelerate the computation by more than 10 times, comparing to the CPU counterpart. After being validated against an experimental benchmark test, the model is applied to simulate the effects of the morphological change on flood patterns for the Bayangaole Reach of Yellow River, China. The results indicate that the effect of perturbed bed could be of significance for the fluvial flood over movable bed. It is therefore suggested to take into account when evaluating the flood risk in this case.