Lagos does not have a drainage problem. It has a catchment problem that shows up as drainage.
Almost every flooding complaint in Lagos is reported at the point where water appears — a junction, an underpass, a stretch of road. That is almost never where the problem is.
The unit of analysis is the catchment
Water arriving at a junction has come from somewhere upstream that used to absorb it and now does not. Designing a bigger drain at the point of complaint moves the problem downstream, usually onto someone with less political weight.
A drainage master plan starts by delineating the catchment properly, modelling runoff under current and projected development, and then deciding where in the system to intervene — attenuation upstream, conveyance in the middle, outfall capacity at the end.
Development consumes attenuation faster than plans account for
The most common failure in a Lagos drainage design is a runoff coefficient set from land use at the time of survey. By the time the scheme is built the catchment has hardened further, and the design is already undersized.
We now model a development horizon rather than a snapshot, and we size for it explicitly. It costs more at construction and considerably less over the life of the asset.
Outfall is the binding constraint
In a city at this elevation relative to the lagoon, tidal conditions at the outfall govern what the whole upstream system can discharge. Any drainage plan that does not begin at the outfall and work backwards is describing a system that cannot empty.
