Flood Hazard Mapping Using Two Digital Elevation Models: Application in a Semi-Arid Environment of Morocco

The High Atlas of Morocco is a semi-arid mountainous environment that frequently suffers from natural hazards. For example, the watersheds upstream of Marrakech city are subject to extreme floods, caused by heavy rains. These episodes are frequent and often devastating, as was the August 1995 event that caused hundreds of deaths in the Ourika Valley. The purpose of this work is to characterize the risk of flooding in this valley, by simulating the water levels and the floodplain extension. This watershed of the Ourika is characterized by a high relief, a rugged topography and a low permeability substratum. To perform this hydraulic simulation, the resolution and accuracy of Digital Elevation Models (DEM) can strongly impact the results in terms of water levels and flow velocities during floods. Two digital elevation models (DEM) were compared: a DEM ASTER with a spatial resolution of 30 m and a DEM derived from stereoscopic images of Pleiades with a resolution of 4 m. Using a hydraulic model (HEC-RAS) and the two DEM resolutions, flood areas corresponding to different return periods are simulated and compared. For the assessment of the two DEM, many areas are selected that are characterized by different types of exposure: highly frequented tourist areas near a regional road and agricultural areas on alluvial terraces, where cultivated fields and infrastructure are vulnerable. The results showed that the high-resolution Pleiades DEM allows for accurate mapping of floodplains in complex terrain, as it realistically representsthe topography and allows correct simulation of observed water levels. This study highlights the added value of a high-resolution remote sensing for flood modeling in areas where data are scarce

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Modélisation hydraulique et vulnérabilité territoriale aux inondations en milieu montagneux semi-aride : cas des bassins versants de l’Ourika et la Rheraya (Maroc)

As for various regions of Morocco, watersheds in the south-western of the country are sensitive to extreme hydro-climatic events, including floods linked to intense rainfall events. Such episodes are frequent and can be devastating, such as the cases of the Ourika flood in August 1995 and in the Tensift watershed in November 2014 and more recently on the High and the Anti-Atlas during the summer of 2019. Thus, modeling these extreme events is a scientific issue to improve flood forecasting systems and to protect people and infrastructure. To this end, this work aims to couple a physical modeling of flood hazard for some sites with high stakes, such as the Ourika and Rheraya watersheds, with a parametric approach to mapping flood zones. It aims also to a territorial diagnosis of human vulnerability and exposed infrastructures, in order to ensure a good flood risk management. Physiographic and hydro-pluviometric study of the watersheds made it possible to understand their flood dynamics. A frequency analysis of these floods was conducted in order to estimate their probabilities and return periods. In addition, on certain sections presenting a high vulnerability; and using a hydraulic simulation model, a modeling was carried out to analyze the relationship between water level and riverbed flows and to determine the flood zones and the corresponding flood extent at different return periods. This is done by comparing two Digital Elevation Models; an Aster DEM with a spatial resolution of 30 m and a DEM derived from Pleiades stereoscopic imagery with a resolution of 4 m. A second approach, unprecedented in Morocco, has been used to spatialize the risk of flooding at the scale of the watershed. It is a geo-morpho-climatic approach low cost and adapted to the specificities of the watershed studied. This approach allowed us to have a mapping of the flood zones over the whole of the Rheraya watershed. This result validates, in part, that of the hydraulic modeling previously carried out. Finally, a territorial diagnosis made it possible to analyze the issues exposed at the level of the Ourika valley and to identify the areas vulnerable to flood hazards. This diagnosis led to propose evacuation plans, allowing easy access to potential refuge areas in the flood event. The reproducible, inexpensive and relevant nature of this approach would facilitate decision-making and preventative measures.Comme pour diverses régions du Maroc, les bassins versants du Sud-Ouest du pays sont sensibles aux évènements hydro-climatiques extrêmes, notamment les crues liées à des épisodes de précipitations intenses. De tels épisodes sont fréquents et peuvent être dévastateurs, comme les cas des crues de l’Ourika en aout 1995 et du bassin de Tensift en novembre 2014. La modélisation de ces événements extrêmes est ainsi un enjeu scientifique essentiel pour améliorer les systèmes de prévision et protéger les populations et les infrastructures. A cet effet, ce travail vise à coupler une modélisation physique de l’aléa inondation pour certains sites à forts enjeux des bassins versants de l’Ourika et de la Rheraya avec une approche paramétrique de cartographie des zones inondables ainsi qu’un diagnostic territorial de vulnérabilité humaine et des infrastructures exposées. Ceci afin de cerner les degrés du risque et assurer une bonne gestion des inondations. L’étude physiographique et hydro-pluviométrique a permis de comprendre la dynamique des crues et le rôle du milieu physique et du climat dans leur déclenchement. Par ailleurs, une analyse fréquentielle de ces crues a permis d’estimer leurs probabilités d’occurrence et leurs périodes de retours. Les résultats sont ensuite utilisés dans un modèle de simulation hydraulique afin de déduire les hauteurs d’eau correspondantes à différentes pointes de débits. Ceci a permis de déterminer les zones inondables et l’étendue des crues de différentes périodes de retour sur certains tronçons présentant une forte vulnérabilité. Cette modélisation hydraulique est effectuée en comparant deux Modèles Numériques de Terrain, un MNT Aster avec une résolution spatiale de 30m et un MNT dérivé de l’imagerie stéréoscopique de Pléiades avec une résolution de 4m. Une seconde approche, inédite au Maroc, a été utilisée pour spatialiser le risque d’inondation à l’échelle du bassin versant de la Rheraya. Il s’agit d’une approche géo-morpho-climatique qui nous a permis d’avoir une cartographie des zones inondables sur l’ensemble du bassin. Ce résultat valide en partie celui de la modélisation hydraulique précédemment effectué. Finalement un diagnostic territorial a permis d’analyser les enjeux exposés au niveau de la vallée de l’Ourika et d’identifier les zones vulnérables aux risques d’inondation. Ce diagnostic a abouti à la proposition de plans d’évacuations, permettant un accès facile aux zones de refuges potentiels en cas d’inondation. Le caractère reproductible, peu coûteux et pertinent de cette approche faciliterait sa transposition sur d’autres bassins et la prise des meilleures décisions de prévention

Hydraulic modeling and territorial vulnerability to flooding in a semi-arid mountain environment : the case of the Ourika and Rheraya watersheds (Morocco)

Comme pour diverses régions du Maroc, les bassins versants du Sud-Ouest du pays sont sensibles aux évènements hydro-climatiques extrêmes, notamment les crues liées à des épisodes de précipitations intenses. De tels épisodes sont fréquents et peuvent être dévastateurs, comme les cas des crues de l’Ourika en aout 1995 et du bassin de Tensift en novembre 2014. La modélisation de ces événements extrêmes est ainsi un enjeu scientifique essentiel pour améliorer les systèmes de prévision et protéger les populations et les infrastructures. A cet effet, ce travail vise à coupler une modélisation physique de l’aléa inondation pour certains sites à forts enjeux des bassins versants de l’Ourika et de la Rheraya avec une approche paramétrique de cartographie des zones inondables ainsi qu’un diagnostic territorial de vulnérabilité humaine et des infrastructures exposées. Ceci afin de cerner les degrés du risque et assurer une bonne gestion des inondations. L’étude physiographique et hydro-pluviométrique a permis de comprendre la dynamique des crues et le rôle du milieu physique et du climat dans leur déclenchement. Par ailleurs, une analyse fréquentielle de ces crues a permis d’estimer leurs probabilités d’occurrence et leurs périodes de retours. Les résultats sont ensuite utilisés dans un modèle de simulation hydraulique afin de déduire les hauteurs d’eau correspondantes à différentes pointes de débits. Ceci a permis de déterminer les zones inondables et l’étendue des crues de différentes périodes de retour sur certains tronçons présentant une forte vulnérabilité. Cette modélisation hydraulique est effectuée en comparant deux Modèles Numériques de Terrain, un MNT Aster avec une résolution spatiale de 30m et un MNT dérivé de l’imagerie stéréoscopique de Pléiades avec une résolution de 4m. Une seconde approche, inédite au Maroc, a été utilisée pour spatialiser le risque d’inondation à l’échelle du bassin versant de la Rheraya. Il s’agit d’une approche géo-morpho-climatique qui nous a permis d’avoir une cartographie des zones inondables sur l’ensemble du bassin. Ce résultat valide en partie celui de la modélisation hydraulique précédemment effectué. Finalement un diagnostic territorial a permis d’analyser les enjeux exposés au niveau de la vallée de l’Ourika et d’identifier les zones vulnérables aux risques d’inondation. Ce diagnostic a abouti à la proposition de plans d’évacuations, permettant un accès facile aux zones de refuges potentiels en cas d’inondation. Le caractère reproductible, peu coûteux et pertinent de cette approche faciliterait sa transposition sur d’autres bassins et la prise des meilleures décisions de prévention.As for various regions of Morocco, watersheds in the south-western of the country are sensitive to extreme hydro-climatic events, including floods linked to intense rainfall events. Such episodes are frequent and can be devastating, such as the cases of the Ourika flood in August 1995 and in the Tensift watershed in November 2014 and more recently on the High and the Anti-Atlas during the summer of 2019. Thus, modeling these extreme events is a scientific issue to improve flood forecasting systems and to protect people and infrastructure. To this end, this work aims to couple a physical modeling of flood hazard for some sites with high stakes, such as the Ourika and Rheraya watersheds, with a parametric approach to mapping flood zones. It aims also to a territorial diagnosis of human vulnerability and exposed infrastructures, in order to ensure a good flood risk management. Physiographic and hydro-pluviometric study of the watersheds made it possible to understand their flood dynamics. A frequency analysis of these floods was conducted in order to estimate their probabilities and return periods. In addition, on certain sections presenting a high vulnerability; and using a hydraulic simulation model, a modeling was carried out to analyze the relationship between water level and riverbed flows and to determine the flood zones and the corresponding flood extent at different return periods. This is done by comparing two Digital Elevation Models; an Aster DEM with a spatial resolution of 30 m and a DEM derived from Pleiades stereoscopic imagery with a resolution of 4 m. A second approach, unprecedented in Morocco, has been used to spatialize the risk of flooding at the scale of the watershed. It is a geo-morpho-climatic approach low cost and adapted to the specificities of the watershed studied. This approach allowed us to have a mapping of the flood zones over the whole of the Rheraya watershed. This result validates, in part, that of the hydraulic modeling previously carried out. Finally, a territorial diagnosis made it possible to analyze the issues exposed at the level of the Ourika valley and to identify the areas vulnerable to flood hazards. This diagnosis led to propose evacuation plans, allowing easy access to potential refuge areas in the flood event. The reproducible, inexpensive and relevant nature of this approach would facilitate decision-making and preventative measures

Apport de modélisation hydraulique à la cartographie des zones inondables. Cas du bassin versant de la Gheraya (Haut Atlas, Maroc)

International audienceLes oueds du Haut Atlas de Marrakech ont la triste réputation d’être imprévisibles et redoutables parleurs crues éclairs et leurs inondations répétées des dernières décennies (1989, 1995, 1999, 2006,2009, 2014). Le bassin versant de la Gheraya (225 km²) ne sort pas de la règle. Ses fortes cruespeuvent inonder des zones agricoles, des zones de fréquentation touristique et les voies decommunication. A cet effet, ce travail consiste à caractériser l’aléa de crue en simulant les hauteursdes eaux de l’oued et les extensions latérales des crues. Après une analyse fréquentielle des débitsde pointe, et à l’aide d’un modèle numérique de terrain (MNT) à bonne résolution (4 mètres) et desoutils de modélisation hydraulique, nous avons simulé l’étendue des crues de différentes périodes deretour sur les berges de l’oued. Le choix des secteurs d’application est motivé par deux types d’enjeu: une zone touristique habituellement fréquentée par les visiteurs (Rha de Moulay Brahim) où sontexposés des bâtiments de restauration, la route régionale 203 et évidemment les vies humaines. Laseconde zone est un secteur d’activité agricole sur les terrasses alluviales au niveau de la ville deTahannaout où les champs et quelques aménagements anthropiques sont vulnérables. Lacartographie des zones inondables attire l’attention sur le risque des débordements de la Gherayasur ces zones d’activités humaines ; notamment par l’inondabilité des champs cultivés, de certainsbâtiments et de la route qui désenclave la haute vallée d’Imlil et d’Asni

A spatial and integrated flood risk diagnosis

International audiencePurpose-The purpose of this paper is to identify vulnerable areas for flood hazard and to analyze stakes exposed in touristic valley of Morrocan mountains. The three goals are: methodological (low-cost diagnosis without starting data), operational (to show the risk and identify avenues of prevention in Ourika) and incentive (to reproduce this on other sectors in Morocco). Design/methodology/approach-The vulnerability of three areas of the Ourika valley (the most frequented) was assessed by a hydro-geomorphological study, human frequentation surveys and risk indices at the building scale. Findings-Surveys carried out in the field allowed the identification of areas with high risk, the evaluation of the buildings' vulnerability and the frequentation of the valley. Evacuation plans, allowing easy access to potential refuges in case of flood, were finally proposed. Originality/value-The reproducible, inexpensive and relevant nature of the approach (integrated and spatialized) helps in decision making and facilitating dialogue for prevention