Flooding is an environmental problem that towns and cities all over the world contend with. It can be caused when high tides cause rivers or oceans to overflow their banks, or simply by heavy rainfall. There is higher vulnerability where natural drainage system is poor, or where the soil capacity to contain water is minimal. Risk is yet higher where there is poor urban planning, or where foresight is not shown, in cases where, for instance, development is allowed in places such as wetlands that may have faired better as buffers in times of flooding. Oyinloye M. and others at the Federal University of Technology Akure mentions in an article published in the Journal of Earth and Environmental Science that about one-third of the Nigerian land mass is at an elevation less than eight meters above sea level, and those areas are therefore covered often with flood waters. In their research, poor refuse disposal pratice, encroashment on flood plains, and lack of adequate setback from rivers were noted as the major cause of flooding especially in the Lagos area.
The big towns of Lagos and Ibadan have more than others experienced flooding. The Lagos flood of October 1886 was distressing enough to require efforts from the head of the Babalawo, Araba Agbaosi, the chief imam, Mohammed Nolla, and Bishop Tugwell of the Anglican church. The Ogunpa stream in Ibadan which snakes around the city bursts its banks in 1963 and again in 1979, at least on a large scale, recording loss of about a hundred and fifty lives and properties running into millions. Flood in 2012, ending in about 363 deaths, and with 2 million rendered homeless was said to be unprecedented. Though a yearly occurrence, flooding is becoming more severe due to adverse effect of climate change. As confirmed by Kolawole O.M. and others at the Microbiology department of UniIlorin, flooding affects more people on annual basis than any form of natural disaster, though not leading in number of lives claimed. The most deaths due to flood worldwide occurred in Central China with lost of 3.7 million lives in July 1931.
The two principal methods of flood control are the reservoir storage of part of flood water to mitigate the maximum intensity of the flood. The second method includes the increase of the capacity of the channel through which the flood water will pass. In addition to natural reservoirs like lakes and swamps, which have considerable storage capacity, there are artificial reservoirs. Man-made reservoirs, though rarely constructed solely for flood-control can be used damp-down a flood. A reservoir may act in storing a large portion of flood water as possible to release it after the flood has subsided. It may also provide, like the natural lake, an outlet which is constantly discharging based on the height of the water in the reservoir. Where there are no reservoirs, channels through which the flood water takes its course can be improved to increase the velocity of flow, and the cross-sectional area of channel. Ground drainage may take the form of open ditches and canals, or subsoil drains, depending on local conditions.