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International audience; Niamey, the capital of Niger, is particularly prone to floods, since it is on the banks of the Niger River, which in its middle basin has two flood peaks: one in summer (the red flood) and one in winter (the black flood). In 2020, the Niger River in Niamey reached its all-time highest levels following an abundant rainy season. On the other hand, the floods in Niamey have been particularly frequent in the last decade, a symptom of a change in hydroclimatic behaviour already observed since the end of the great droughts of the 1970s and 1980s and which is identified with the name of Sahelian Paradox. This study, starting from the analysis of the 2020 flood and from the update of the rating curve of the Niamey hydrometric station, analyses the rainfall–runoff relationship on the Sahelian basins of the Medium Niger River Basin (MNRB) that are at the origin of the local flood. The comparative analysis of runoffs, annual maximum flows (AMAX) and runoff coefficients with various rainfall indices calculated on gridded datasets allowed to hydroclimatically characterise the last decade as a different period from the wet one before the drought, the dry one and the post-drought one. Compared to the last one, the current period is characterised by a sustained increase in hydrological indicators (AMAX +27%) consistent with the increase in both the accumulation of precipitation (+11%) and the number (+51%) and magnitude (+54%) of extreme events in the MNRB. Furthermore, a greater concentration of rainfall and extremes (+78%) in August contributes to reinforcing the red flood’s positive anomalies (+2.23 st.dev in 2020). The study indicates that under these conditions the frequency of extreme hydrological events in Niamey will tend to increase further also because of the concurrence of drivers such as river-bed silting and levee effects. Consequently, the study concludes with the need for a comprehensive flood-risk assessment on the Niamey city that considers both recent hydroclimatic trends and urbanisation dynamics in flood zones hence defining the most appropriate risk-reduction strategies.

In the tropics, rural areas are still the place where many people live, despite ongoing urbanization [...]

Highlights ● Hydraulic modelling and satellite imagery enable hazard and exposure mapping. ● Local knowledge enables better understanding of hazards and their impacts. ● Benefit/cost and opportunity analysis support decision-making in risk treatment., In tropical regions, heavy precipitations may lead to catastrophic flooding due to the degradation of catchments and the expansion of settlements in flood prone zones. In the current situation, where information on rainfall and exposed assets is either scant, or requires significant time to be collected, pluvial flood risk assessments are conducted using participatory tools, without any scientific support. Another option is to use satellite precipitation products, digital terrain models and satellite images at high to moderate-resolution. However, these datasets do not reach the required accuracy at the local scale. Consequently, the potential damages and the evaluation component of risk assessment are often missing. Risk evaluation is pivotal for informed decision-making, with regards to the choice of treating or accepting the risk, implementing more effective measures, and for determining the safest areas for development. We proposed an improved method for assessing the risk of pluvial floods, which merges local and scientific knowledge and is consistent with the ISO 31010 standard. The method was successfully applied in five rural settlements in Niger and can be replicated in areas where information is scarce., Graphical abstract Improved method for assessing the risk of pluvial flooding locally. Its application in five rural settlements demonstrates its usefulness in decision-making regarding risk treatment and built-up expansion in safe areas.Image, graphical abstract

Flood risk assessments in the Global South have increased since the adoption of the United Nations Sendai Framework for Disaster Risk Reduction 2015–2030. However, they often fail to meet disaster risk reduction needs at the local scale, because they typically consider only one hazard (fluvial or pluvial floods). Furthermore, hazard and exposure are considered as stationary conditions, flood-prone assets are rarely identified, risk reduction measures are not identified in detail for specific locations, and the convenience of reducing or accepting risk is not evaluated. This paper describes a flood risk assessment method that is innovative in that it considers three hazard types (backwater, fluvial, and pluvial floods) and multiple risk scenarios; it uses orthophotos generated from images captured by an unmanned aerial vehicle and very high-resolution satellite images, and it involves communities in risk assessment. The method was applied to four rural settlements along the Sirba River, Niger. The assessment identifies the benefit of reducing risk in monetary terms, as well as the intangible benefits that reducing risk could generate, and it detects opportunities that flooding offers for rural development. The method can be replicated in all contexts where decision-making support is needed for flood risk assessment planning.•Risk analysis and evaluation is participatory.•Risk assessment is improved by combining local and technical knowledge.•Assets are identified using very-high-resolution satellite and drone images., Graphical abstract Improved method for assessing fluvial and pluvial flood risk. Application of this method in four rural settlements demonstrates its feasibility and potential.Image, graphical abstract

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.

During the last two decades, the sub-Saharan region has experienced unusual floods that have differentially impacted the region. No official and precise data regarding flood damage and impacts on the population are available, and the magnitude of events are not easily evaluated. Most previous studies have investigated this new threat using data derived from local media sources or world disaster databases. The aim of this study was to provide the scientific community and policy makers with an updated and reliable referenced data source concerning floods in Niger between 1998 and 2015, at national, regional and sub-regional scales. Reliable information regarding floods was derived from the national official flood damage database (ANADIA DB) showing their impact on the country. During the investigated period, considerable numbers regarding flood impacts were found (about 4000 settlements and 1.7 million people were affected by floods). The analysis also indicates a sudden increase in flood impacts since 2010. Regions in the south-west (Tillabery, Dosso and Niamey district) are the most affected, however, this kind of risk involves the whole country, and some particularly vulnerable areas have been identified. A data modeling comprehensive framework based on remotely sensed rainfall (climate hazards group infrared precipitation with stations (CHIRPS)) and vegetation index (moderate resolution imagery spectroradiometer normalized difference vegetation index (MODIS NDVI)) datasets data along with census data were used to investigate which variables are most able to explain the recent and sudden Niger flood vulnerability detected at the departmental scale. Only a few statistically significant flood damage models were found (61 out of 297), due essentially to the non-linearity of the increase in damage time series compared to environmental and climatic trends. The population increase is the most significant variable at national level, however, at regional and sub-regional scales, different patterns provided evidence to identify local triggers for vulnerability.

This book aims to inspire decision makers and practitioners to change their approach to climate planning in the tropics through the application of modern technologies for characterizing local climate and tracking vulnerability and risk, and using decision-making tools. Drawing on 16 case studies conducted mainly in the Caribbean, Central America, Western and Eastern Africa, and South East Asia it is shown how successful integration of traditional and modern knowledge can enhance disaster risk reduction and adaptation to climate change in the tropics. The case studies encompass both rural and urban settings and cover different scales: rural communities, cities, and regions. In addition, the book looks to the future of planning by addressing topics of major importance, including residual risk integration in local development plans, damage insurance and the potential role of climate vulnerability reduction credits. In many regions of the tropics, climate planning is growing but has still very low quality. This book identifies the weaknesses and proposes effective solutions.

In the Tropics, a significant increase in the number of cities provided with climate plans by 2020, as announced in the 11th Sustainable development goal of the United Nations, requires an unprecedented effort. To achieve it, we have to simplify the planning process and improve the quality of the plans. The aim of this book was to collect methods and experiences to inspire the simplification of the planning process and increase the quality of climate planning. We focused attention on the three critical phases of the planning process: analysis, decision making in planning, climate measures. Sixteen case studies from Ethiopia, Haiti, Malawi, Mexico, Niger, Senegal, Tanzania and Thailand cover automatic weather stations in remote areas, rainfall estimation gridded datasets, open data for vulnerability index to climate change, early warning systems, quality of climate plans index, multi-risk local assessment, flooding risk evaluation method, backcasting, spatial dimension in disaster risk reduction and resilience, gasification stoves, index-based insurance and vulnerability risk credit. After indicating the possible analyses, 19 recommendations were supplied to the United Nations SDGs monitoring system, the national weather services and those responsible for natural risks, to the Development banks, Official development aid and the research institutions.

Remote sensing digital image analysis has been applied to monitor land clearing and degradation processes on a plateau covered by tiger bush near Niamey in South West Niger, where signs of severe landscape degradation due to fuelwood supply have been observed in the last decades. A MODIS NDVI dataset (2000–2015) and five LANDSAT images (1986–2012) were used to identify spatial and temporal dynamics and to emphasize areas of greater degradation. The study indicates that the land clearing found by previous investigations in the second part of the 20 th century is still ongoing, with a decreasing trend of MODIS NDVI values recorded in the period 2000–2015. This trend appeared to be linked to an increase in bare soil areas that was demonstrated by analysis of LANDSAT SAVI images. The investigation also indicated that rates of degradation are stronger in more deteriorated areas like those located nearer Niamey; degradation patterns also tend to increase from the inner areas to the edges of the plateau. These results attest to the urgency to develop effective environmental preservation policies and find alternative solutions for domestic energy supply.

In the last seven years, tropical cities with a climate plan have tripled compared to the previous seven years. According to the 11th United Nations’ Sustainable Development Goal, climate planning should significantly increase by 2030. The Sendai framework for disaster risk reduction (2015) and the New urban agenda signed in Quito (2016) indicate how to achieve this goal through analysis, categories of plans and specific measures. This chapter identifies the main obstacles to the significant increase in tropical human settlements with a climate plan and the possible solutions. First of all, the distribution and trend at 2030 of tropical human settlements are ascertained. Then local access to information on damage, hazard, exposure, vulnerability and risk, and the consideration of these aspects in the national guides to local climate planning are verified. Lastly, the categories of plans and climate measures recommended by the United Nations are compared with those that are most common today, using a database of 401 climate plans for 338 tropical cities relating to 41 countries. The chapter highlights the fact that the prescription for treating tropical cities affected by climate change has been prepared without an accurate diagnosis. Significantly increasing climate planning must consider that small-medium human settlements in the Tropics will prevail at least until 2030. And most effort will be required from Developing and Least Developed Countries. The recommendations of the United Nations concerning the preliminary analyses ignore the fact that local authorities usually do not have access to the necessary information. Climate plans and recommended measures are not those currently in use. We propose three areas of action to facilitate the mainstreaming of the recommendations in the tropical context. They require a renewal of the local planning process if we intend to reach the 11th SDG by 2030.

Acacia trees are a key element in Sahelian semiarid ecosystems, being some of the few tree species surviving in such extreme environments. Understanding their growth dynamics represents a fundamental step to improve forest sustainable management. Dendroclimatology, the study of tree rings and their relationships with climate, could help in understanding how changing climatic conditions may impact these ecosystems. We studied Acacia seyal Delile trees growth in Keita Valley, in the Sahelian zone of Niger. Wood from 8 trees of known age was sampled to study the relationships between growth zones, i.e. ring patterns, and climatic conditions. We present herein, a study based on cross sections instead of cores. Wood-anatomical and dendrochronological analyses enabled us to evaluate the relationships between age and ring formation, and to confirm that one tree ring for rainy season is formed. Moreover, surprisingly for a species growing at such latitudes, acacia tree rings have a climatic signal, mainly due to the precipitation falling during the rainy season but also influenced by the average seasonal temperature.

Nel Sahel, a fronte del progressivo degrado delle risorse forestali dovuto al fenomeno dei cambiamenti climatici e alla crescente pressione antropica, sono stati realizzati negli ultimi 30 anni importanti interventi di rimboschimento. La quantificazione della biomassa legnosa e il sistema piu opportuno per valutarne l’efficacia. In questo lavoro viene proposto un approccio a basso impatto ambientale per stime correnti di biomassa. Sono stati sviluppati e analizzati modelli allometrici in piantagioni di Acacia spp. nella valle di Keita, in Niger e si e valutata la possibilita di utilizzare un modello generalizzato per le specie dominati: Acacia seyal Del., A. raddiana Savi (syn. A. tortilis Hayne), A. nilotica (Guill. et Perrott.). Le funzioni sono state costruite adottando come variabili esplicative il diametro a 0,80 m e l’altezza totale delle piante modello. L’analisi della regressione e stata realizzata con il metodo dei minimi quadrati ordinari dopo aver eseguito la trasformazione logaritmica delle osservazioni. In seguito e stata confrontata l’efficacia perequativa del modello multi-specifico con quella dei modelli specie-specifici. L’analisi dell’entita degli scostamenti tra valori osservati e attesi ha mostrato un’ottima capacita interpretativa in entrambi gli approcci (R2>0,98, P