The physical changes in the landscape created by urbanisation considerably alter the
hydrological response of an area to rainfall through higher river flow magnitudes, faster runoff
response to rainfall periods, lower levels of groundwater recharge, and reduced base flow. A
drainage system within an urban area has to deal with these difficulties so as to mitigate the
arising effects, particularly when the design flood occurs or is exceeded. Flood and
precipitation runoff prediction for design of stormwater controls is a highly statistical process,
which means there will be risks attached due to unpredictability of the exact outcome that will
happen in any flood event. Drains in most places, especially in Nairobi, have a lot of storm
debris which has largely reduced the capacity of drains. As a result, stormwater in many
sections of the city also moves towards residential areas instead of away from them. Therefore,
the level of exposure to flood hazard is heightened, causing damage either physically,
economically or socially. Urban development also catalyses the increase of these
vulnerabilities by concentrating human and material issues in areas previously dedicated to
other land uses but often subject to a flood hazard (Kovacs, Doussin, & Gaussens, 2017). More
accurate design that includes all quantitative and qualitative factors, needs to be carried out to
produce robust control systems.