Flooding is basically an overflow of water onto normally dry land. Urban flooding is an inundation of land in a constructed setting, particularly in densely populated areas. It is mainly due to heavy or excessive rainfall resulting into rising water in an existing waterway such as a river, stream or drainage systems. Put simply, it happens when heavy and excessive rainfall overwhelms the capacity of drainage systems.
Flash floods, which are also caused by heavy or excessive rainfall within a short time, are usually characterised by raging torrents, which is a large amount of water moving quickly that rips through river beds, urban streets or mountain canyons and sweep everything before them.
Flashfloods can happen within minutes of the excessive downpour. It should also be noted that flashfloods can occur without rainfall and these could be due to failure of levees or dams.
Urban flooding is becoming a major issue in cities in many parts of the world. This is attributed to urbanisation, climate change, higher land density usage, rise in sea level as a result of global warming, among others. For instance, global climate change is resulting in changing weather patterns and increased cases of high-intensity rainfall events occurring in shorter periods of time.
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Then the threat of sea-level rise is also looming large, threatening all the coastal cities. Further, in urban areas, impervious surfaces prevent rainfall from infiltrating into the ground, thereby causing a higher surface run-off that may be in excess of local drainage capacity.
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It is also important to note that improper and inadequate drainage system, improper waste management, lack of awareness and lack of, or inadequate, management measures by government agencies are among the contributing factors to urban flooding.
Likewise, encroachment of flood plains, land allocation and acquisition challenges contributing to occupation of marginal lands are some of the factors that trigger urban flooding.
Urban flooding is a serious problem, which must be effectively managed, considering that urban areas host vital economic infrastructures that need to be protected at all times. Therefore, the damage or loss of these vital infrastructures not only lead to huge economic losses but can also lead to loss of life and property.
Haphazard and irregular planning of cities, compounded by problems of illegal encroachment which eliminate natural watercourses necessary to drain excess water, storm water drainage systems whose capacities are overwhelmed when rainfall of high intensity happens and rapid urbanisation are some of the factors that exacerbate urban flooding.
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Encroachment continues to distort natural water courses, reducing the capacity of the natural drains, hence frequent urban flooding. This is also true with poor waste disposal, especially solid waste that includes domestic, commercial and industrial waste.
Likewise, dumping of construction waste or debris into drains significantly decreases the capacity of natural drains.
And, again, the rapid surge in population is leading to rapid urbanisation and increased demand for living space. This, against the background of lengthy land acquisition processes, could be forcing some citizens to resort to encroaching land that may, unfortunately, include floodplains, natural water courses, among others, without regard to proper urban planning.
The weak and uncoordinated enforcement of regulatory measures or provisions can also contribute to encroachment and the proliferation of substandard structures that are characterised by poor or no drainage system, no proper sanitation facilities, among others, leading to health-related risks that include disease outbreaks such as cholera, among others.
Just like architects and engineers design infrastructure such as roads, bridges, among others, and just like bridges tolerate given or specific weights as designed, this is also true with city planners who design storm water management systems meant to accommodate expected environmental behaviour— such as riverbank overruns and the amount of precipitation falling over a given area in a given period of time.
This is in view of the fact that when the amount of water becomes higher than what is typical for an area, flooding happens. It must be noted though, that, much as the development of urban plans and designs, just like the development of regulatory framework and promulgation of regulations, is important, the key lies in the implementation and effective enforcement of the plans and regulations to address urban flooding.
