Flood risk preliminary mapping in Niamey, Niger

Flood mapping is still rare in the large cities South of Sahara. The lack of information on the characteristics of floods, the orography of the sites and the receptors hampers its production. However, even with scant information, it is possible to create preliminary risk mapping. This tool can be used by local administrations in decision making on emergency plans or on climate change (CC) action plans. From 2010 onwards the River Niger at Niamey (1.1 million inhabitants, 123 km2 in 2014) swelled at unseasonal times. That new river flood pattern can be linked to CC. Each flooding event affected thousands of people and homes. The unceasing development of areas that did not seem to be flood prone in the past is the main cause of these impacts. These areas require special measures if further impact is to be avoided in the future. This chapter presents the flood risk preliminary map of Niamey 1:20,000. The map is built up using an historic approach (flooded area derived from satellite images) and considering risk (R) as the result of hazard (H) and damage (D), R = H * D. Risk is measured according to two scenarios: medium and high probability of flooding. The inverse of the return period of river and pluvial flooding (H) and the potential damage to buildings and crops according the water depth are used. Information to measure risk components is sourced by daily rainfall and daily discharge of the River Niger from 1946 to 2014 and from high-resolution satellite images (2014). The risk map identifies hot spots for emergency and CC action planning. The fifth district alone contains 52% of the potential damage. 99% of the potential damage is concentrated within 225 hectars. Reinforcing existing embankments and constructing new ones to protect these areas seems more appropriate than resettlement. The cost of the works would equal the potential damage if it remains within 2,580 euro/ml.