Participatory risk assessment of pluvial floods in four towns of Niger

Intense rainfalls in Sub-Saharan Africa are increasing in frequency. Land degradation, watercourses siltation, and flood defence failure turn these events into disastrous floods. Over the last decade flood risk assessments have been prepared to face these disasters. However, they have frequent limitations in design, accuracy, and completeness. The objectives of this study are (i) to integrate local and scientific knowledge into a participated pluvial flood risk assessment (ii) to identify assets and (iii) to estimate the potential impact and efficiency of risk-reduction measures. The assessment is developed in four rapidly expanding towns of Niger, flooded several times in recent years. Flood-prone areas and assets are identified according four flood scenarios using local knowledge, 2D hydraulic modelling, and visual photointerpretation of very-high-resolution satellite images. Risk-reduction measures are singled-out through public participation. The residual risk and benefit/cost analyses provide a decision-making tool to accept or treat risk. During the last decade the expansion of the four towns has been more rapid in flood-prone zones than in safe areas. Nowadays more than half of the housing stock could be flooded by rainfalls with 20 years return period. Catchment treatment and building retrofitting can reduce risk. from 100 to 29–82. Nevertheless, the benefit/cost of risk reduction is high for towns settled in small catchments only

Method for pluvial flood risk assessment in rural settlements characterised by scant information availability

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

Applicazioni geomatiche in aree ad elevata criticità climatica e infrastrutturale finalizzate allo sviluppo di un Early Warning System per le inondazioni del fiume Sirba nell’ambito del Progetto ANADIA II (Niger)

Realizzazione di un rilievo RTK senza stazioni master di base, lungo il Fiume Sirba (Niger) per una lunghezza di circa 100 km finalizzato alla realizzazione di un modello idraulico e di un sitema di allerta precoce lungo il fiume

Recent Changes in Hydroclimatic Patterns over Medium Niger River Basins at the Origin of the 2020 Flood in Niamey (Niger)

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 postdrought 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

Risque et adaptation climatique dans la région de Dosso au Niger

En Afrique tropicale la réduction du risque hydre-climatique peine à devenir une politique publique. La Région de Dosso au Niger (31 000 km2, 2 millions d’habitants) a été inondée et frappée par la sécheresse à plusieurs reprises durant les dernières 10 années. Alors, pour mieux comprendre et maîtriser ces aléas les changements climatiques d’ici au 2030 sont caractérisées à l’échelle locale. La base de données sur les inondations est transférée dans une archive ouverte. La dynamique spatiale des localités rurale est observée durant les 20 dernières années, L’analyse-évaluation du risque d’inondation et de sécheresse est développée à l’échelle régionale, communale et de localité rurale. Des services climatologiques sont offerts aux petits producteurs ruraux. Les méthodes proposées utilisent des informations en libre accès et sont donc aisément reproductibles dans d’autres régions du Pays et d’Afrique francophone. Ce livre présente des outils pour connaître et gérer le risque hydre-climatique à l’échelle locale

Community and Impact Based Early Warning System for Flood Risk Preparedness: The Experience of the Sirba River in Niger

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 topdown 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