Paving Drainage and Falls in Nairobi: Deciding Where Water Goes

Paving does not create water, it concentrates it

A lawn absorbs rain. Two hundred square metres of cabro does not, and all of that water now arrives somewhere in a hurry. During the Nairobi long rains that is a serious volume.

So the first question on any paving job is not what the blocks look like. It is where is the water going, and the second is whether that place can take it.

Falls

Surfaces are laid to fall, typically 1:60 to 1:80, so water runs deliberately rather than ponding. Flat paving holds puddles, and standing water works jointing sand out of the joints, which starts the same interlock failure as missing edge restraint.

Falls are set at survey with levels taken across the whole area, not judged by eye on the day. On a sloping plot the existing ground frequently falls towards the house, and correcting that is a design decision made before excavation.

Channels and gullies

Where a fall cannot take water clear on its own, a channel across the head of a driveway or along its low edge collects it and carries it to a gully.

A gully then needs an outfall: a soakaway, a storm drain or a discharge point that will actually accept the volume. A gully connected to nothing is a hole that fills up.

Where there is no outfall

On many Nairobi plots there simply is nowhere to send it. Then the options are a properly sized soakaway, or permeable paving with an open-graded sub-base that stores water and lets it infiltrate.

Permeable construction costs more and it is the honest answer on a plot with no discharge point. It is also the answer where a client wants to pave a large area on heavy soil and has been told by somebody else that it will be fine.

Black cotton

Black cotton neither drains nor stays still. Soakaways in it underperform badly, and saturated subgrade loses bearing capacity, which is how a driveway that was fine in January is rutted in May.

Where we find it, the drainage and the sub-base are designed together rather than separately.