Floodplain Settlement Dynamics in the Maouri Dallol at Guéchémé, Niger: A Multidisciplinary Approach

In Sahelian Africa, rural centers have been hit by catastrophic floods for many years. In order to prevent the impact of flooding, the flood-prone areas and the settlement dynamics within them must be identified. The aim of this study is to ascertain the floodplain settlement dynamics in the Maouri valley (135 km2) in the municipality of Guéchémé, Niger. Through hydraulic modeling, the analysis identified the flood-prone areas according to three return periods. The dynamics of the settlements in these areas between 2009 and 2019 were identified through the photointerpretation of high-resolution satellite images and compared with those in the adjacent non-flood-prone areas. Spatial planning was applied to extract the main dynamics. The synergic application of these disciplines in a rural context represents a novelty in the research field. Since 2009, the results have shown a 52% increase of the built-up area and a 12% increase in the number of buildings, though the increase was higher in the flood-prone areas. The factors that transform floods into catastrophes were identified through perceptions gathered from the local communities. Three dynamics of the expansion and consolidation of buildings were observed. Specific flood risk prevention and preparation actions are proposed for each type of dynamic

Detection of temporary surface water bodies in Niger using high resolution imagery

Temporary surface water bodies in sub-Saharan areas have important socio-cultural values, providing freshwater for population and many agro-pastoral services. Nevertheless, they can be the perfect habitat for insects and pests, thus endangering human health. Moreover, temporary water bodies can cover vast areas of cities and villages hindering the practicability of the roads networks. Addressing the problem within villages and cities requires not only the identification of the extension and position of the water bodies, but also of their seasonal maximum potential extension. Temporary surface water bodies are usually remote sensed from satellite imagery. This technique is very effective on large scale, although limited at local scale by temporal and spatial resolutions of satellites. Traditional surveys can be time-consuming and limited by the hard surveying condition of the area, a valuable alternative to collect punctual and high resolution data are the UAVs (Unmanned Aerial Vehicles). This contribution presents a semi-automatic method to detect temporary surface water bodies at local scale using UAV high resolution imagery. It was tested in two villages of the Tillaberì region, South-West Niger. A digital terrain model (DTM, 10 cm grid) generated from UAV imagery and analysed to localize the depressions of the area with fill sink algorithm. The depressed areas were classified based on their depth and extension. The areas presenting high depth and extension were considered as potentially interested by temporary surface water bodies. The method was validated by the comparison to radiometric information (6cm/pixel) collected from near infrared (IR) and visible (Red Green Blue) sensors mounted on UAV during the rainy season, in a period of minimum expansion of temporary surface water bodies. The radiometric data were elaborated in a Normalized Difference Water Index (NDWI); which information correspond to the one obtained from the DTM. The proposed methodology appears solid and effective, and allows the identification of those areas that may be interested by temporary stagnant water in case of abundant precipitations. The cross-reading of radiometric and digital elevation information provides a high resolution localization of present, and potentially present, temporary surface water bodies

Flood Assessment for Risk-informed Planning Along the Sirba River, Niger

South of the Sahara flood vulnerability and risk assessments at local level rarely identify the exposed areas according to the probability of flooding, the actions in place, localize the exposed items. They are, therefore, of little use for local development, risk prevention and contingency planning. The aim of this article is to assess the flood risk, providing useful information for local planning and an assessment methodology useful for other case studies. As a result, the first step involves identifying the information required by the local plans most used south of the Sahara. Four rural communities in Niger, frequently flooded by the Sirba River, are then considered. The risk is the product of the probability of a flood multiplied by the potential damage. Local knowledge and knowledge derived from a hydraulic numerical model, digital terrain model, very high resolution multispectral orthoimages and daily precipitation are used. The assessment identifies the probability of fluvial and pluvial flooding, the exposed areas, the position, quantity, type, replacement value of exposed items, and the risk level according to three flooding scenarios. Fifteen actions are suggested to reduce the risk and to turn adversity into opportunity

Climatological Analysis and Early Warning System in the Sirba basin

Climatological Analysis on the Sirba basin: evaluation of rain climate inde

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

Hydrological Web Services for Operational Flood Risk Monitoring and Forecasting at Local Scale in Niger

Emerging hydrological services provide stakeholders and political authorities with useful and reliable information to support the decision-making process and develop flood risk management strategies. Most of these services adopt the paradigm of open data and standard web services, paving the way to increase distributed hydrometeorological services’ interoperability. Moreover,sharing of data, models, information, and the use of open-source software, greatly contributes to expanding the knowledge on flood risk and to increasing flood preparedness. Nevertheless, services’ interoperability and open data are not common in local systems implemented in developing countries. This paper presents the web platform and related services developed for the Local Flood Early Warning System of the Sirba River in Niger (SLAPIS) to tailor hydroclimatic information to the user’s needs, both in content and format. Building upon open-source software components and interoperable web services, we created a software framework covering data capture and storage, data flow management procedures from several data providers, real-time web publication, and service-based information dissemination. The geospatial infrastructure and web services respond to the actual and local decision-making context to improve the usability and usefulness of information derived from hydrometeorological forecasts, hydraulic models, and real-time observations. This paper presents also the results of the three years of operational campaigns for flood early warning on the Sirba River in Niger. Semiautomatic flood warnings tailored and provided to end users bridge the gap between available technology and local users’ needs for adaptation, mitigation, and flood risk management, and make progress toward the sustainable development goals

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

Améliorations sur le système d’observation du bassin de la Rivière Sirba pour la gestion des risques naturels

Updating of the rating curves and of the historical series of Garbey Kourou (Sirba river - Niger

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