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 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 in a short time, are usually characterised by raging torrents, which is a large amount of water moving quickly. It rips through river beds, urban streets or mountain canyons and sweeps everything that stands in the way. Flash floods can happen within minutes of the excessive downpour. It should also be noted that flash floods 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 other things. For instance, global climate change is resulting in changing weather patterns and increased cases of high-intensity rainfall events occurring within a shorter time. 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.
It is also important to note that improper and inadequate drainage system, improper waste management, lack of awareness and lack of management measures by government agencies are among the contributing factors to urban flooding. Likewise, encroachment of flood plains, land allocation and acquisition challenges contribute to the occupation of marginal lands and are among 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 eliminates natural watercourses necessary to drain excess water; and storm water drainage systems, whose capacities are overwhelmed when rainfall of high intensity happens; and rapid urbanisation are some of the key factors that exacerbate urban flooding.
Encroachment continues to distort if not wiping out natural water courses, reducing the capacity of 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 also significantly decreases the capacity of natural drains.
The rapid surge in population, leading to rapid urbanisation, increases demand for living space. This is against the background of lengthy land acquisition processes, which could be forcing some citizens to resort to encroaching land that may, unfortunately, include flood plains, natural water courses, among other things, without regard for 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 systems, no proper sanitation facilities, among other things, leading to health-related risks that include disease outbreaks such as cholera, among others.
In conclusion, just like the architects and engineers design buildings— be it roads and bridges— just like bridges tolerate specific weights as designed; in the same way, city planners design storm water management systems meant to accommodate expected environmental behaviour, such as river bank 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.
