Nairobi Dam Spillway: How the new flood-protection structure is designed to work

Water is now flowing through the new emergency spillway at Nairobi Dam in Lang'ata, according to the Nairobi Rivers Commission (NRC).

The commission says the structure is part of urgent flood-prevention work undertaken with Nairobi City County and other government agencies.

Construction and associated flood-mitigation repairs began on October 13.

The new channel is intended to provide a controlled route for releasing excess water, helping regulate the reservoir level when heavy rainfall increases inflow.

A spillway is an engineered channel or structure that allows water to leave a reservoir safely once it approaches a level that needs to be controlled.

Without an adequate outlet, rising water can overtop a dam's embankment, eroding the structure and increasing the risk of damage.

At Nairobi Dam, the immediate objective is to reduce that pressure and direct water into the Ngong River system rather than allow uncontrolled overflow.

The concern around the dam

Nairobi Dam was built in the 1950s along the Ngong River as part of a flood-attenuation system intended to help regulate water moving through the city.

It was also designed to offer recreational space for Nairobi residents.

Over decades, however, pollution, sediment accumulation, damaged sewer infrastructure and encroachment around the reservoir have raised concerns about its condition.

The Nairobi Rivers Commission has said the dam had not received major maintenance since 1998.

The danger became more visible during the March 2026 rains, when the dam overflowed and raised concerns for communities downstream.

The incident prompted renewed calls to address the dam's structural condition, water management and the surrounding drainage system.

The new spillway design

The new channel provides a route for releasing excess water and can help protect the dam's embankment from erosion caused by uncontrolled overtopping.

Lowering the reservoir level before major rainfall can also create additional capacity to accommodate incoming water.

But the spillway is not a guarantee against flooding.

Its effectiveness depends on rainfall intensity, the amount of water entering the reservoir, the condition of the dam, and the ability of downstream channels to carry released water safely.

If rivers and drains become overwhelmed or are obstructed, flooding can still occur in nearby neighbourhoods.

That is why authorities have urged residents below the dam to remain alert, avoid flowing water and river channels, and follow instructions from emergency agencies.

Which neighbourhoods are at risk?

Warnings have been directed at communities around Nyayo Highrise, Dam Estate, parts of Kibera, Nairobi West and Madaraka.

Previous county briefings have also highlighted Lang'ata, South C, South B, the Industrial Area and parts of Mukuru as locations that could be affected by changing river conditions during heavy rainfall.

These locations are not all equally exposed, and the opening of the spillway does not mean they are expected to flood.

The advice is precautionary because water released from the dam travels into a wider urban drainage network, where local conditions and rainfall can change the level of risk.

Residents should keep children away from the spillway and river channels, avoid crossing flowing water and heed any evacuation or safety instructions issued by authorities.

Future rehabilitation

The spillway is an emergency intervention, not the full rehabilitation of Nairobi Dam.

Nairobi Governor Johnson Sakaja has said comprehensive rehabilitation of the reservoir is planned after the rainy season.

That longer programme is expected to address the dam's condition and persistent environmental problems, including pollution, sediment and encroachment.

The NRC has also been coordinating wider work along the Nairobi, Mathare and Ngong rivers.

These interventions include clearing drainage channels, removing waste, rehabilitating waterways and reclaiming riparian areas where necessary.

The broader Nairobi River Basin Regeneration Programme aims to improve the condition of the river system across Nairobi and neighbouring counties.

The commission has said it wants to complete major rehabilitation work by January 2027, although land disputes, relocation issues and infrastructure works remain challenges.