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Are your properties located far enough from rivers, sea shorelines or water bodies? If the answer is yes, this does not mean that they are fully safe from flooding.In an era governed by continuous climate instability and unstoppable expansion of cities, the exacerbation of hydrometeorological events is increasing the occurrence of pluvial floods. Pluvial flooding is induced by the combination of two factors: extreme precipitations and the incapability of the ground/drainage systems to effectively handle excessive rainwater.In an urban environment, the runoff generated by localized and intense rainstorms may quickly inundate streets and buildings undermining the safety of people and assets. The characteristic of being hardly predictable has inspired the definition of pluvial flood as an ‘invisible hazard’ and the related damages and losses are increasingly weighing on the budget of municipalities and private citizens.Looking at the upsetting climate projections, experts are resolute in developing comprehensive methodologies and strategies for flood risk assessment and management.In this work, we present the attempt of accomplishing a high-resolution pluvial flood risk assessment at the city scale. The city of Differdange (Luxembourg's third largest city) is used as case study in which the extreme rainfall-related impacts and hazards are analyzed through the implementation of a fully coupled 1D/2D dual drainage model. This type of hydrodynamic model closely mimics the complexity of an urban landscape allowing to simulate all hydraulic phenomena occurring both on the surface and through the sewer network. Despite the digital accuracy of these models, they are rarely implemented due to the vast amount of detailed information required; which are often unavailable.The implementation of the hydraulic model follows two main steps: the bi-dimensional discretization of the surface and the 1D modelling of the whole drainage network.Nowadays, many countries provide open-access high-resolution digital elevation models of their territories (50 cm for Luxembourg) and up-to-date cadastral planimetries from which essential information for the 2D component are extrapolated. Ground data is enriched by land use/cover and soil maps for the estimation of roughness and infiltration parameters.The drainage network contemplates all pipes carrying rainwater, meaning the newer storm-water system and the old combined sewer network. The geometric specifications required are size, shape, elevation, material of pipes, manholes and tanks. Important infrastructures, such as flooding barriers, have been systematically added to the model. The fully-distributed hydrological engine allows operating the rainfall-runoff transformation on each cell of the domain and the exchange of water between the surface and drainage network occurs through the nodes of the network (storm drains and manholes).The model’s outcomes allow for assessing the level of hazard to which each building is exposed, identifying the critical nodes within the drainage network, and proposing mitigation strategies.Furthermore, these insights may help authorities to improve their warning systems and emergency plans.

Traditionally, flood risk maps used by city officials and water resource managers for urban planning, by engineers for adequate flood defence infrastructure design, or by insurers and re-insurers for estimating financial risk exposure are the result of modelling flood hazard of rivers and their associated floodplain lands at different return periods. Often, any of these stakeholders would use the 1:100 return period of fluvial hazard to plan accordingly. However, with the climate crises signals clearly present during recent flood disasters, and especially with the 2021 Europe floods, water managers, cities and the financial risk sector are now starting to plan differently and are recognizing the need not only for better and more frequently updated flood risk analysis, particularly in urban areas, but also need to consider pluvial and flash floods that can happen in any part of a river basin and oftentimes take place in headwater areas or off the main river floodplains. Flash flooding greatly impacts urban areas where the storm drainage infrastructure is becoming largely insufficient due to the increasing duration and higher frequency of extreme intense rainstorms. Therefore, model simulations of flood hazard that account for these rather unprecedented types of extremely destructive events are required, and those need to be integrating the newest data from all types of sensors. At the same time, we observe that sustainable, nature-based solutions are now sought after because these solutions offer an inviting alternative to ever changing flood risk, particularly under the present and future climate crisis. It is stipulated that increasing healthy urban vegetation cover could reduce this risk and is a form of a nature-based solution for urban areas. Here we combine existing methods from the literature and develop a methodology relating time-series of satellite-based vegetation maps, topography and soil permeability to estimate excess runoff from intense precipitation. The runoff coefficient is mapped through the use of a composite curve number method.. The method of looking at the partition between rainfall and runoff is highly correlated to change in land use, and thus changes in vegetation cover. Relying on the NDVI index for green vegetation mapping, the methodology is able to capture the differences in the hydrological response even for seasonal or canopy integrity changes. Looking at different vegetation cover scenarios therefore allows the creation of different runoff responses, and therefore a possible reduction in flood risk.In this paper, we present initial results of this flood risk analysis, the goal of which is to produce runoff change maps at city, urban neighbourhood or city post code level using different scenarios in rainfall amounts from design storms coupled with existing or planned urban vegetation cover scenarios.

Engineers have shaped the environment across the centuries in order to improve the quality and safety of human life. The unrestrained invasion of nature led to significant environmental problems, for this reason nowadays engineering projects should be based on ecological concepts to protect our environment. This paper presents an integrated methodology that involves GIS tools, hydraulic numerical models, and landscape metrics to investigate ecological consequences caused by river restoration activities. The combined use of these different tools represents a bridge to connect the field of engineering with ecological techniques. The proposed method was tested to predict and assess the influence of a river restoration plan on a reach of the Orco river located in the northwest of Italy. Morphological alterations were simulated to reconnect remnant meanders and provide water to the floodplain, enhancing the ecological value of riparian ecosystems. The application of the hydraulic model permitted to evaluate the distribution of water inside the study area before and after the restoration plan. Thereafter, spatial configuration and temporal dynamics of the landscape structures were quantified using landscape metrics. The increase of patch density (PD) by 9% and edge density (ED) up to 10% highlights that restoration activities lead to a new configuration characterized by a higher level of fragmentation and heterogeneity. The characteristics of versatility, repeatability, and the possibility to predict the outcomes of a specific plan make the proposed method a useful tool that could help decision-makers to manage the territory while safeguarding natural ecosystems.

Humankind is currently living in an era governed by continuous climate warm-up and unstoppable urbanization, in which the ongoing climate change is leading to an exacerbation of hydrometeorological events. With an intensification of magnitude and frequency of extreme rainfall events, engineers and scientists are striving to develop methodologies and strategies to effectively defend people and assets from pluvial flooding. Pluvial floods produced by local, intense, and fast rainstorms cause the surcharge of urban drainage systems inducing the inundation of streets and buildings before the runoff reaches the receptor watercourse. Pluvial flood damage has been defined as an ‘invisible hazard’ but it increasingly weighs on the budget of direct flood losses, raising the costs incurred by flood damages. Besides the tangible losses, the costs may be even higher when the intangible share is considered, such as the potential loss of heritage held in ancient towns. For this reason, the inestimable cultural and artistic heritage preserved in historical buildings require a high-level of protection against hazards induced by natural calamities. The present study investigates extreme rainfall-related impacts and hazards threatening the cultural heritage situated in the most vulnerable areas of the Santa Croce district (Florence, Italy). The district hosts some of the most important buildings of the city: the National Central Library of Florence and the Opera di Santa Croce. The geographical location of this monumental complex makes the cultural heritage guarded inside of it dangerously exposed to multiple sources of flood hazard. Firstly, river flooding due to the proximity to the Arno River (this area has been already harshly damaged by the catastrophic flood in 1966). Secondly, flooding by sewage since that the internal drainage network is linked with one of the main sewer conduits of the city. Then, surface runoff flowing down from the headwater. Considering this framework, the pluvial flood hazard assessment is performed using a 1D/2D dual drainage model specifically implemented to simulate all hydraulic phenomena occurring both on the surface and through the sewer network. The analysis comprehends a series of scenarios designed to simulate the impact of hydrometeorological extremes on the study area and each possible concatenation of consequences or failures. The hydraulic model incorporates different layers of information: the high-resolution digital surface model of the area and buildings, the public sewer network, and the internal rainfall collection system of the district. Geometrical features and technical specifications of the sewer network have been retrieved from detailed field surveys and research in historical archives. Model’s outcomes allow identifying the critical nodes within the drainage network, delineating the most vulnerable areas, and prioritizing the rescue efforts in case of severe cloudbursts. Results may help site managers to improve the efficiency of their hazard management and emergency plans. Furthermore, the study intends to propose suitable technical solutions for safeguarding the cultural heritage where designing intrusive engineering works hardly fits within the historical urban context.

Pluvial floods occur when heavy rainstorms cause the surcharge of the sewer network drainage, representing one of the most impacting natural hazard in urban watersheds. Pluvial flood hazard is usually assessed considering the effect of annual maxima rainfall of short duration, comparable with the typical concentration times of small urban watersheds. However, short duration annual maxima can be affected by an error of underestimation due to the time resolution as well as the aggregation time of the rainfall time series. This study aims at determining the impact of rainfall data aggregation on pluvial flood hazard assessment. Tuscany region (Central Italy) is selected as a case study to perform the assessment of the annual maxima rainfall underestimation error, since the entire region has the same temporal aggregation of rainfall data. Pluvial flood hazard is then evaluated for an urban watershed in the city of Florence (Tuscany) comparing the results obtained using observed (uncorrected) and corrected annual maxima rainfall as meteorological forcing. The results show how the design of rainfall events with a duration of 30 min or shorter is significantly affected by the temporal aggregation, highlighting the importance of correcting annual maxima rainfall for a proper pluvial flood hazard evaluation.

During the last 20 years, floods have become a major hazard in West Africa, particularly in the Sahelian belt, affecting livelihoods, infrastructure and production systems, and hence heavily impacting on sustainable development. The Sendai Framework for Disaster Risk Reduction 2015–2030 recognized Climate Services (CS) as a powerful tool for more effective disaster preparedness. The European research and innovation Roadmap for CS expands their contribution, particularly “hydrometeorological services”, to the Sendai Framework. From this convergence, Hydrometeorological Early Warning Systems (EWS) become a strategic target and a building block of preparedness to hydrometeorological risks in developed and developing countries. In West Africa, EWS for floods are in place only for the main rivers and are conceived mainly top-down and hazard centered, lacking links with exposed communities and expected impacts. These gaps reduce the effectiveness of a flood EWS, while engaging local communities since de beginning through a co-production process can improve the effectiveness and ensure better response in case of alert.Our study aims to present the lessons learnt from the set-up of a Community and Impact Based Flood EWS on the Sirba River in Niger. The service was developed with stakeholders at different levels, leveraging on existing resources and knowledge, using simple but effective approaches and integrating state-of-the-art hydro-meteorological science in a decisional scheme of Sahelian rural areas. This mechanism can be replicable in different contexts characterized by knowledge and structural deficits, by creating a better capacity to exchange data and information and by directly connecting available technical capabilities with the local level. The participatory approach allowed the beneficiaries to define the rules of the system, which, in any case, needed to be consistent with the national legislation and internationally recognized best practices.The study suggests that it is not necessary to develop complex forecasting tools, while it can be preferable to enhance those already operating and calibrate them on the local scale through risk thresholds, field observations and potential impacts using flood scenarios. The strength of simplicity also lies in not having to spread complex messages, but simply the reference risk scenario, and finally its color-code (according to the international standards of ISO 22324:2015), which already embeds all other information including potential impacts. The simple and integrated approach illustrated in this case study, bridging the gap between top-down and bottom-up approaches, can inspire Governments, local administrations and development partners to invest in the improvement of existing tools and knowledge and in strengthening cooperation, collaboration and coordination to reduce hazards’ impacts and sustain the development of rural and urban areas.

Floods have recently become a major hazard in West Africa (WA) in terms of both their magnitude and frequency. They affect livelihoods, infrastructure and production systems, hence impacting on Sustainable Development (SD). Early Warning Systems (EWS) for floods that properly address all four EWS components, while also being community and impact-based, do not yet exist in WA. Existing systems address only the main rivers, are conceived in a top-down manner and are hazard-centered. This study on the Sirba river in Niger aims to demonstrate that an operational community and impact-based EWS for floods can be set up by leveraging the existing tools, local stakeholders and knowledge. The main finding of the study is that bridging the gap between top-down and bottom-up approaches is possible by directly connecting the available technical capabilities with the local level through a participatory approach. This allows the beneficiaries to define the rules that will develop the whole system, strengthening their ability to understand the information and take action. Moreover, the integration of hydrological forecasts and observations with the community monitoring and preparedness system provides a lead time suitable for operational decision-making at national and local levels. The study points out the need for the commitment of governments to the transboundary sharing of flood information for EWS and SD.

The sub-Saharan climate is experiencing a marked increase in temperature and intensification of precipitation intensity and variability. Besides, longer dry spells are compromising the reliability of local agricultural practices. The present study provides a comprehensive investigation about the benefits induced by using indigenous rainwater harvesting techniques (RWHT) against hydrometeorological threats affecting the Sahelian areas. Different RWHT have been tested in term of runoff retention, infiltration increase into the root zone, and soil water stress mitigation. To achieve these purposes, hydrological processes at the field scale have been investigated using a two-dimensional distributed hydrological model. To make the study representative of the whole Sahelian areas, several simulations were carried out adopting a wide range of input parameters based on conventional values of those areas. The results reveal that RWHT may lead to a runoff retention up to 87% and to double the infiltration. Intercepting and storing runoff, RWHT increase the water content in the root zone and the right design can diminish the crop water stress. Furthermore, the results show that adopting RWHT makes it possible to extend the growing season up to 20 days, enhancing the yield. These benefits contribute to the reduction of the climate-related water stress and the prevention of crop failure.

The original version of this paper was unfortunately published with an error. The online visualization of the paper is correct; instead, the printed version (page 5) reports strange lines into the vectors of Eq. (2), and the G is divided from its expression (see Fig. 1).

In Sahelian countries, a vast number of people are still affected every year by flood despite the efforts to prevent or mitigate these catastrophic events. This phenomenon is exacerbated by the incessant population growth and the increase of extreme natural events. Hence, the development of flood management strategies such as flood hazard mapping and Early Warning Systems has become a crucial objective for the affected nations. This study presents a comprehensive hazard assessment of the Nigerien reach of the Sirba River, the main tributary Middle Niger River. Hazard thresholds were defined both on hydrological analysis and field effects, according to national guidelines. Non-stationary analyses were carried out to consider changes in the hydrological behavior of the Sirba basin over time. Data from topographical land surveys and discharge gauges collected during the 2018 dry and wet seasons were used to implement the hydraulic numerical model of the analyzed reach. The use of the proposed hydraulic model allowed the delineation of flood hazard maps as well the calculation of the flood propagation time from the upstream hydrometric station and the validation of the rating curves of the two gauging sites. These significative outcomes will allow the implementation of the Early Warning System for the river flood hazard and risk reduction plans preparation for each settlement.

The Sahelian regions are affected by an increasing number of catastrophic floods in recent years as a consequence of climate and land use/land cover changes. River flow data is key to understanding river behavior and develop flood mitigation and prevention strategies. The present study provides a revision and an update of the existing discharge dataset of the Sirba River with the aim of enhancing the reliability of these data. The revision also includes the recalibration of the Garbey Kourou rating curves. The analysis of the revised discharge time series strengthens the previous findings, evidencing a positive trend in flood frequency and intensity over the entire analyzed period of 1956–2018. This positive trend is more pronounced for the last 40 years due to a significant underestimation of the rating curves used. A relevant finding is a new changepoint in the time series, detected for 2008, which represents the beginning of the period in which the highest flood magnitudes were registered. The effect of land use/land cover changes and climate changes on the water resource is depicted using flow duration curves. This research produces a revised and more reliable discharge time series that will be a new starting point for future hydrological analyses.

The development of adaptation and mitigation strategies to tackle anthropic and climate changes impacts is becoming a priority in drought-prone areas. This study examines the capabilities of indigenous rainwater harvesting techniques (RWHT) to be used as a viable solution for flood mitigation. The study analyses the hydraulic performance of the most used micro-catchment RWHT in sub-Saharan regions, in terms of flow peak reduction (FPR) and volume reduction (VR) at the field and basin scale. Parametrized hyetographs were built to replicate the extreme precipitations that strike Sahelian countries during rainy seasons. 2D hydrodynamic simulations showed that half-moons placed with a staggered configuration (S-HM) have the best performances in reducing runoff. At the field scale, S-HM showed a remarkable FPR of 77% and a VR of 70% in case of extreme rainfall. Instead at the basin scale, in which only 5% of the surface was treated, 13% and 8% respectively for FPR and VR were obtained. In addition, the reduction of the runoff coefficient (Rc) between the different configuration was analyzed. The study critically evaluates hydraulic performances of the different techniques and shows how pitting practices cannot guarantee high performance in case of extreme precipitations. These results will enrich the knowledge of the hydraulic behavior of RWHT; aspect marginally investigated in the scientific literature. Moreover, this study presents the first scientific application of HEC-RAS as a rainfall-runoff model. Despite some limitations, this model has the effective feature of using very high-resolution topography as input for hydraulic simulations. The results presented in this study should encourage stakeholders to upscale the use of RWHT in order to lessen the flood hazard and land degradation that oppresses arid and semi-arid areas.