Like day follows night, it floods whenever Nairobi receives moderate to heavy rainfall. A combination of factors such as the city's geographical location and its poor drainage system are mostly to blame. On 12th May 2015, the city in the sun received unusually heavy precipitation and the results were disastrous. More than ten people were reported dead in different parts of the city and losses worth millions were incurred. The flooding affected most parts of Nairobi but I will focus on the areas that were arguably worst hit - the areas along the Ngong River valley – and attempt to explain how and why this was so.

The Ngong River flows from the Ngong and Kikuyu area, passing through Kibera, Nairobi Dam, Nairobi West, South C, South B, Industrial Area and onward through Eastlands up to Ruai. It is one of the rivers in the Nairobi river basin and it drains the higher, western side of the city (including Kikuyu, Dagoretti and Ngong), flowing east to join the Athi River. The river is noticeable when it crosses Mbagathi Road near Highrise, Langata Road near T-Mall, Nairobi West near Nairobi West Mall, Mombasa Road next to the ICC church and South B near Mater Hospital. It has the distinction of being the most polluted river in Nairobi by virtue of the areas it flows through.

On this particular day, the different weather stations in Nairobi recorded over 100mm of rainfall in a matter of hours. Weather stations in the catchment areas of the Ngong River (the higher areas drained by the river) including Ngong, Wilson airport and Dagoretti registered 102mm, 128mm and 189mm of rainfall respectively. To put that into context, the average monthly rainfall during the most wet month in Nairobi (April) is approximately 130mm. Even if we had adequate storm drainage systems, it was bound to flood given such amounts of precipitation. The city’s geography only served to complicate matters further.

Listening to residents of Nairobi West describe how the flooding occurred, I concluded that Nairobi dam must have overflowed on this particular night. There were reports that the volume of the Ngong River rose such that it burst its banks and flowed over the bridge on Langata road near TMall. This might explain the amount of flooding that occurred at the NHC apartments nearby. In Nairobi West, the river similarly burst its banks and flowed over the road at the bridge near West Mall. Given the gradient of the area, the river water then flowed down Muhoho Avenue and headed towards South C.

Residents in areas near West Mall described how flood waters flowed into their living rooms in scarcely believable speed. At about 9pm, they were sat at home and the flooding was restricted to Muhoho Avenue outside their houses as is usually the case when it rains. But this was no ordinary rain. Flood waters started to trickle into the houses and in about 15 minutes, the water level rose to waist height. They quickly moved to higher ground before part of the perimeter walls fell from the pressure of the water. Past West Mall, the river takes a sharp turn and flows towards South C. The water was therefore trying to find its way back to the river and whatever stood in its way faced the wrath of Mother Nature. This was the area where the Makini School bus was stuck as were many other cars. Three bodies were discovered in this area the next morning.

The flooding was so intense that the school near the mosque had its entire field flooded. Behind the school flows the river and once again, the flood waters brought down the wall and rejoined the river. Further down the river near Mombasa road, the KPLC station opposite ICC church was also submerged. As was the case in Nairobi West, flood waters appeared suddenly and with such speed that the families of KPLC staff who live next to the station had to climb to their ceilings to escape the raging waters. The station was submerged and power had to be switched off in Nairobi West and South C (areas served by the station) for almost three days. The wisdom of having the station next to a low lying area such as a river has to be questioned but that’s a story for another day.

In South B, residents of Mukuru slums faced the wrath of the flood waters and the shanty houses were swept away. The wall of the South B mosque had collapsed on part of the slum the previous day from heavy rain, killing 10 people. All these areas were hit hard probably due their proximity to Nairobi dam and the huge volume of water flowing with high velocity as the gradient of the area dips suddenly from the dam near Highrise.

Looking at the river, it is noticeable that certain parts are narrow and shallow while others are wide and deep. The narrow, shallow areas saw the river burst its banks and flow over bridges. To mitigate future disasters, the county government can consider dredging the river bed in such areas to increase its depth and allow for a higher volume of water to flow through the channel. This is a middle ground as far as solutions go since it is obvious that we are not ready to clear developments that have been built in riparian areas. Muhoho Avenue also needs enhanced drainage system to address the perennial flooding that cuts off Nairobi West from South C whenever it rains. With the river closely located, it should not prove too difficult to help the water find its way to it. Failure to do this will mean more lives lost and property destroyed when it rains heavily in the future.