Poor drainage design is the cause of flooded roads. The construction system meant to drain the river-like roads from storm water is defective.
Most drains have soil surfaces which are easily eroded and clogged. In addition, some drains are not wide enough to serve their purpose.
Drains should be done according to the predictable volume and velocity of storm water. Deep enough concrete is the solution as it not only drains better while self-cleaning, but is also easier to unclog.
Underground channels should have enough and correctly-sized ground inlets to siphon ground water. Roads with kerbs require enough wide surface inlets. The side to collect water should be designed lower to hold water as it flows to the drainage systems. The absence of such is the reason why many roads are turning into small rivers even during light rains.
Examples of such defective designs include the highly acclaimed Thika superhighway, which contains spacious underground channels but has very narrow inlets. This causes rain water to remain on its surface for long thereby causing the flooding effect.
At drifts such as Muthaiga, there is no outlet for the river. The surface overflows while underground channels are dry.
Countryside roadside are left to silt and grow plants, which makes water flow along the roads damaging the surface of the tarmac. Culverts are clogged, which makes water to run on the road.
The Ministry of Roads don’t seem to inspect roads for regular maintenance.
Allowing buildings to be constructed on river channels and road reserves has worsened the drainage mess.
In Kenya, smooth road surfaces remain till the next rains. Rebuilding roads every time is so expensive. The money wasted on maintenance could be put to better use.