Council officials have told people living on the riverbank to leave their homes until the rainy season has passed. Shikoli, who sells groundnuts and fruit from a small stall next to the river, makes just $2 (£1.5) a day.
Forecasters have warned that this year's El Niño could be one of the strongest on record, bringing heavy rain and flooding to Kenya and extreme weather to many other parts of the world.
In Nairobi, the risk falls hardest on informal settlements near rivers, including Kibera and Mathare. Kibera, and Kenya more widely, are on the front line of preparations for the potential impact.
The heavy rains predicted so far have not occurred. Experts remain concerned, and there are fears that El Niño will worsen problems the city has already failed to contain.
"These houses aren't made of stone,"a 62-year-old resident said. The resident keeps one photograph out of sight."When I see a picture of her, I feel uneasy, that's why I keep it far from me, so I don't see it.".
The scars of floods in the long rainy season earlier this year are still visible, including a broken television left in a corner after it was immersed in water.
Blocked drains and inadequate sewage systems were widely blamed for the high number of deaths after those floods. The excess rainfall had nowhere to escape.
Simiyu said the existing drainage system was developed 30 years ago to cater for a population of about 2.5 million residents."We're dealing with about 7 million in the daytime and 5.5 million night-time,"Simiyu said.
That gap, Simiyu said, cannot be closed cheaply. Investment of up to $192 million (£143 million) is needed to upgrade the sewerage and drainage system entirely.
On the rains ahead, Simiyu said a moderate projection is that about 5,000 households, or about 20,000 people, may be at risk.
The upper estimate is far larger."Our worst case scenario looking at low-income settlements is that up to about 100,000 people are at risk,"Simiyu said.
Anyone forced to leave their home will be relocated to 167 designated holding grounds, where a stockpile of relief food and supplies will be distributed, Simiyu said.
Kenya is not the only African country worried about El Niño. In neighbouring Somalia, the disaster management agency has warned that 1.2 million people are at risk of displacement caused by extreme weather, including both floods and drought.
National governments in many countries are doing what they can to prepare. The extent of El Niño's impact remains impossible to predict.