Authors:
Ousmane Seidou
Dr. Ousmane Seidou is an associate professor of water resources engineering at the University of Ottawa, and an adjunct professor at the United Nations University, Centre for Water, Environmental and Health (UNU-INWEH). He is involved in research, teaching and capacity development activities related to water resources management, hydrological risk and adaptation to climate variability and change, with a focus on North America and West Africa.
Fatiman Alher
Fatiman Alher is a Master 2 student in the Department of Geography of the Abdou Moumouni University of Niamey. She is a key member of the OpenStreetMap community in Niger and is the representative of the International Women and IT Network in Niger. She is leading a socio-economic and flood vulnerability data assessment in Niamey, Niger. Fatiman is passionate about geographical information systems, open data, and information technology. She is the local representative of the International Women and ICT Network in Niger.
A city plagued by recurrent floods
The Niger River is part of the daily life of the city of Niamey, capital of the Republic of Niger. Lined by fancy restaurants, luxurious gardens and rice fields that extend to the city center, it provides transportation, food and recreation to the local population. Unfortunately, almost every year in the rainy season, the capital is mourned by sudden floods which are caused either by rising water levels in the Niger River or by intense precipitations. These floods cause the collapse not only of houses built with dried clay, but also of modern houses built within the flow of runoff due to unplanned urbanization.
From June to August 2017, floods due to heavy rains have reportedly killed 43 people and affected 68,000 people in the country. Nigeriens remember clearly the catastrophic floods in 2012 which claimed the lives of 87 individuals and affected more than 525,000 people nationwide. In these and other events, the majority of the affected persons live in the poorest parts of the city; most casualties are due to mud houses collapsing after the walls are weakened by a long rainfall event or accumulated runoff. Once water recedes, however, people rebuild in the the exact same spaces and the stage is set for the next disaster.
Figure 1A
Figure 1B
Figure 1: a) collapsed house after an intense rainfall; b) houses threatened by the rising water level (courtesy of Fatiman Alher)
The anarchic expansion of the city without the proper drainage infrastructure contributes to rainfall-generated flooding that can occur anywhere in the city. Part of the recent extensions of the city happened directly in some tributaries of the Niger River that have become dry because of the decrease in rainfall that occurred in the last three decades. These tributaries will eventually start flowing if the region recovers from the recent drought conditions – and there are hints that the recovery is underway.
Figure 2
Figure 2: Flooded neighborhood in a usually dry tributary of the Niger River in Niamey (Courtesy of Aziz Kountché, Director, Drone Africa Services)
A desire for change, but a lack of actionable data
Despite these recurring losses, the flood response in Niger is primarily a post-disaster response that consists of providing temporary shelter and food to the affected populations. In 2012, the Niger Basin Authority issued a warning that water levels in the river have reached critical levels, but the warning was not followed by action until the dikes protecting the lower parts of the city breached. In reaction to the 2012 floods, Niger Government with the support of the World Bank launched the PGCR-DU (Projet de Gestion des Risques de Catastrophes et de Développement Urbain – Disaster Risk Management and Urban Development Project) to reduce the vulnerability of populations at risk of flooding, while taking into account the requirements of community development and capacity building of national structures both at central and local level.
So far, the PGRC-DU has launched various flood risk mitigation actions including the retrofitting of construction of new facilities for the Nigerien civil protection services, the retrofitting of three flood protection dikes in Niamey, and the pavement of selected streets in Niamey functioning as drainage systems during intense storms. Without a thorough understanding of flooding dynamics in the area, and a comprehensive inventory of exposed assets and people, it is hard to objectively prioritize the interventions and make sure resources are used efficiently for an optimal relief. With the support of the World Bank, the PGRC-DU is developing state-of the art tools to be able to identify flooding hot-spots and evaluate the added value of potential flood mitigation measure. These tools will lead to a better knowledge of exposed assets and people, and a better knowledge of flood dynamic in the city of Niamey.
A geodatabase of exposed assets and people.
Led by Fatiman Alher, a key member of OSM (Open Street Map) in Niger, a team of 20 survey crew were trained on digital cartography using OpenStreetMap. They are using a mobile application called GeoODK (Geographical Open Data Kit) to collect critical socio-economic information in all areas in Niamey that are deemed vulnerable to floods. The collected information include the geographic coordinates and the characteristics of each building in the household, the number of persons living there, as well as the preferred contact channel in case of emergency. They also collect the characteristics of key infrastructure elements such as culverts and streetlights.
Figure 3A
Figure 3B
Figure 3: Survey crew collecting data on the field, and analyzing it when back at their office.
The information is uploaded to a server which generates a digital map of the spatial distribution of exposed assets and people as shown on the photo below.
Figure 4
Figure 4: Surveyed buildings as of 30/9/2017
High-resolution imagery and elevation data
Understand flood dynamics in urban areas where both the built environment and natural slopes control water movements is extremely challenging. While satellite imagery and topographic maps can easily be downloaded from the Internet for any area in the world, their coarse resolution and quality are insufficient for urban runoff characterization. A young Nigerien start-up called Drone Africa Services has been hired to acquire high-resolution images of potentially flooded areas that would help better identify buildings characteristics, and develop a Digital Terrain Model or DTM (a fancy word for topography) with 10cm vertical resolution to better predict water movements in the area.
Figure 5A
Figure 5B
Figure 5: Drones on display before a field data collection in Niamey (Courtesy of Aziz Kountché, Director, Drone Africa Services)
Drone Africa Services will also provide a training to up to 40 persons from the academia, government services and the OSM community on the operation and data acquisition using drones.
Simulating flood risk under various intervention scenarios
Future floods will not necessarily be similar to past ones, because both the natural and built environment are in constant evolution. Provided high quality data is available, computer models can simulate flood extent and water velocity in the city even if major changes have occurred (e.g. higher dike protection dikes, new urbanized areas, etc.). In collaboration with the Dr. Ousmane Seidou (University of Ottawa, Canada), a 2D hydrodynamic model is being built to dynamically simulate flood propagation in the city of Niamey under anticipated flow and precipitation condition. The model will use the DTM to be developed by Drone Africa for an improved accuracy. The simulated data can be used in conjunction with the geodatabase of exposed assets and people to identify the number of persons potentially affected by an imminent flood, of the value of properties at stake.
Figure 6
Figure 6: preliminary simulation of the 2012 floods in Niamey. The accuracy of the simulation will be improved with the DTM being collected by Drone Africa Services
Conclusions and perspectives
The tools and data products under development will give, for the first time, a quantitative portrait of flood risk in Niamey in the current situation, but also under various intervention scenarios. If adequately communicated to stakeholders, the knowledge generated by these tools should help the decision makers objectively choose between alternative flood protection measures. Ultimately, there will be a better use of each dollar invested in flood risk reduction in the city of Niamey.