Why can a flash flood hit Kenyan ground that has barely had rain?

Why can a flash flood strike a place where it has barely rained? Because the water that floods it did not fall there. Rain that lands on hillsides and roads further up the slope gathers in streams and reaches the lower ground in a rush, so a river can rise where little rain has fallen. The Kenya Meteorological Department says as much in its current heavy rainfall advisory: "flood waters may occur in areas where it has not rained heavily, particularly downstream."

The advisory was issued at 18:47 on 24 October and expires at 09:00 on 30 October. It expects moderate to heavy rain, above 20 mm within 24 hours, over the counties of the Coast, Northeastern Kenya and the Highlands west and east of the Rift Valley. The rain is expected from 24 to 28 October, peaking on 26 to 28 October, and to ease by 30 October. The department rates the severity as moderate, the urgency as expected and the certainty as likely.

Capital FM reported on 25 October that the warning also covers the Central Highlands, including parts of Nairobi, and sections of the Southeastern Lowlands. Its list of counties runs from Mombasa, Kilifi and Lamu on the coast to Kisumu, Nakuru and Kakamega in the west. It adds southerly to southeasterly winds above 25 knots (about 46 km/h) over coastal, eastern and northern regions.

A flash flood is a flood that follows heavy rain quickly. The US National Oceanic and Atmospheric Administration's National Severe Storms Laboratory (NSSL) says flash floods occur "when heavy rainfall exceeds the ability of the ground to absorb it" and "can happen within minutes of the causative rainfall". Think of a roof and its downpipe. Rain falls across the whole roof, but the water leaves through one pipe, which can overflow while the ground beside it is only damp. The roof is the catchment, the area that drains to one channel; the downpipe is the stream.

Which ground sends water to the channel fastest? The NSSL names three kinds. "Mountains and steep hills produce rapid runoff, which causes streams to rise quickly." "Saturated soils can also lead to rapid flash flooding." And "the construction of buildings, highways, driveways, and parking lots increases runoff." Runoff is the rain that runs over the surface instead of soaking in. The laboratory also describes the upstream case: the rain can fall above a canyon while hikers below are trapped by rapidly rising water.

In a brief on flooding in Nairobi's informal settlements, published on 27 July, the World Resources Institute (WRI) used hydrodynamic flood modelling and exploratory household surveys with 98 households in Kibera, Mukuru Viwandani and Mathare. It found that many informal settlements lie along the Nairobi, Ngong and Mathare river corridors, and that degraded wetlands, expanding impervious surfaces, blocked drains and poor solid waste management worsen flood impacts.

Nairobi's March–May 2024 floods brought 400–660 mm of rain, 200 mm above the seasonal average. They displaced 20,968 families and caused at least 11 deaths, and Mathare, Mukuru and Kibera were among the hardest-hit settlements.

Time to react is short. That speed, the NSSL says, limits "the time available to warn and protect the public". Most flash-flood deaths in the US are of people in vehicles driving into flooded roadways.

Kenya Met's advisory tells residents in all the areas to stay vigilant for flash floods and poor visibility. They should not cross flooded roads or swollen rivers, should stay away from drainage channels, take care when travelling and follow official weather updates. If flooding occurs, they should move to safer ground. Because of lightning, they should not shelter under trees or near grilled windows. Capital FM adds that motorists should drive cautiously in poor visibility, that people should not cross streams either, and that they should secure loose objects in windy areas.