Human Factors of Flooding

Introduction

A flood refers to an overflow of water that causes hitherto dry land to become submerged. According to the EU (European Union) Floods Directive, a flood can be defined as a phenomenon in which land that is usually not covered by water becomes submerged by it (2). Floods could happen owing to an overflow of water contained in various water bodies, including lakes, oceans, or rivers. The water contained in such water bodies overflows, thus escaping its normal boundaries (Doocy et al., 2013). Alternatively, flowing could happen after rainwater has accumulated saturated grounds. Floods can be due to natural causes, including the saturation of soil owing to excessive precipitation that happens over an extended duration of time. Once the water table is at par with the ground surface, it leads to runoff or increased overland flow. However, human activities are also a cause of flooding, especially due to such human activities as deforestation, urbanisation, and river management, among others. The essay seeks to explore the extent to which human factors affect flooding.

How human factors affect flooding

Urbanisation

Increased urbanisation is associated with a rise in the incidents of flash floods, not to mention a rise in the number of higher flood levels. Urbanisation describes the changes made to the natural landscape through such human activities as the construction of buildings, roads, pavements, and drainage systems. Urbanisation has been triggered by a rapid rise in rural-urban population and in order to settle the increasing population, it has been necessary to reclaim even water catchment areas. This significantly affects the characteristics of river flooding. Urbanisation is an example of a human cause of flooding, especially in densely populated areas. Usually, urban flooding occurs due to the inability of the draining systems to handle the rainfall. The occurrence of floodplains in urban areas, coupled with their flat nature due to the need for housing, helps to escalate the effects of flooding. The situation is also exacerbated by the presence of tarmac and concrete, used for pavements and roads. Due to their impermeable nature, this hinders the infiltration of water into the ground, meaning that rivers get filled up much more quickly. Besides, the presence of paved roads and streets exacerbates floods by increasing the speed at which the flood water flows. Such flood flow presents a hazard to both the infrastructure and the population (White and Greer, 2006).  In addition, urbanisation leads to the removal of trees and other plant matter that would help to intercept rainwater. Accordingly, precipitation reaches the rivers at a much faster rate than would have been the case had there been interception of such water. Urban planning is such that surface water is often directed d directly into sewers and drains, which means that precipitation gets into the rivers at a much faster rate.

Construction of buildings and roads entails the removal of depressions, vegetation, and soil from the land surface. Permeable soi is lost and in its place, we have impermeable surfaces such as roofs, roads, sidewalks, and parking lots (Konrad, 2003). These not only store limited water, but also limit the rate at which water infiltrates the ground, not to mention their role in accelerating runoff to streams and ditches. There are various studies that have been conducted to determine the effect of urbanisation on flooding.  Hollis (1975) points towards an association between the rises in urbanisation on the one hand, and an increase in the percentage of the basin paved on the other hand, which is in turn linked to the increase in the reported cases of flowing. In his study on urbanisation and its effect on flooding, Hollis surmised that “small floods may be increased by 10 times by urbanisation and floods with a return period of 100 yr may be doubled in size by 30%” (Hallis, 1975, p.. 431).   A study by Bronstert et al. (2007) indicated that farming and urban changes acted as decisive factors in runoff responses. Their findings hinged on simulation tests conducted on different land cover conditions. Elsewhere, White and Greer (2006) concluded that peak flow and storm runoff increase with an increase in urban development. This is the case owing to the increase in the size of impervious land. Warburtan et al (2012) are of the view that partial irregularity makes it hard to address the runoff problems at local levels.

Deforestation

Deforestation has also been identified as playing a crucial role as far as flooding is concerned. This is because the availability of vegetation such as trees on land prevents sediment runoff caused by the rain. Moreover, forests tend to use and hold more water in comparison with grasslands or farms. Some rainwater is retained on the leaves and does not therefore reach the ground. Instead, it evaporates directly into the atmosphere, with the result that there is less rainwater that finds its way to the runoff. Besides, leaves tend to minimise the impact of raindrops, leading to gentler rains and by extension, less erosion. Trees are also known to absorb water from the soil. Consequently, the soil remains drier and can thus hold a lot more rainwater. Furthermore, trees and other vegetation hold the osil in place, thereby minimising the movement of sediments which has the potential to reduce river channels downstream. When trees are lost due to human activity, this plays a crucial role in exacerbating the flooding of rivers. Deforestation also affects flooding through the release of sediment. Deforestation causes erosion of vast amounts of soil which finds its way into river beds, thus leading to shrunken river channels, along with the river’s capability to hold water and not flood.

Channel management

Channelisation refers to an intentional attempt to change the natural course of the river. Channelisation might involve widening and deepening of the river in a bid to enhance the capacity of the channel. Consequently, there is an increase in the river’s hydraulic efficiency, thereby permitting the channel to hold larger discharge, thereby aiding in the prevention of flooding. However, channel management is also associated with such inevitable consequences as elevated discharges, as well as higher flood magnitude downstream (Konrad, 2003). Channel management also alters or moves channels such as the construction of culverts and bridges are other examples of the impact of human activity on flooding. Construction, followed by the filling in of floodplains has been shown to hinder flood conveyance, thereby minimising existing floodplain storage, and hence escalating flood heights. Filling also hinders flood flows, leading to increased volumes of floodwaters upstream in addition to adjacent properties. Besides, filling minimises the floodplain’s capacity to store excess water (Bronstert et al., 2007). Consequently, larger volumes of water tend to flow downstream, effectively leading to flooding. There is also a resultant rise in floodwater velocity. Furthermore, flooding causes a reduction in land available to facilitate the recharging and infiltration of groundwater levels.

Increasing natural wetlands, and reducing levee systems limits water levels along river channels, and more so during heavy rains. The ensuing floodwater tends to spread across a larger area, while water levels are slower and lower in comparison with the manmade, levee-based systems. The use of levees as a technique to reclaim land is thus associated with several drawbacks, including the possibility of increasing river crest levels above the levels of human settlements. In the event of a levee failing, it could lead to considerable flooding, leading to the destruction of farms, and homes and possible loss of lives. A good example is the levee failure in New Orleans triggered by Hurricane Katrina. It caused levee failure, which in turn caused flooding of the city, leaving in its wake the destruction of property and the deaths of hundreds of people (Ehrhard, 2015).

Hydroelectric dams

The construction of hydroelectric dams is aimed at trapping water flowing through a river, thus facilitating the construction of a reservoir at a height above the dam. This is meant to facilitate a careful regulation of the water that flows through the dam. Nonetheless, there could be a failure in terms of the structural development of the reservoir, and the dam could burst and turn catastrophic. A good example is the Banqiao Dam which collapsed in 1975. The effects of the dam burst were catastrophic, as it led to the deaths of 171,000 people (Kazmeyer, 2017). Channelisation and the construction of hydroelectric dams could also turn catastrophic, leading to the loss of property and lives.

Conclusion

Flooding can be a result of both physical and human factors. Physical causes are due to an interaction between atmospheric conditions, vegetation, drainage basin geology, and size, as well as the variations in channel courses over space and time. However, flooding can also be due to human activities. Humans affect the association between drainage basin response and precipitation via agricultural development, deforestation, urbanisation, as well as channel management. Deforestation limits the transpiration and interception feedback, leading to enhanced rates and quantities of surface run-off. Urbanisation results in increased build-up of impermeable surfaces like parking lots and roads which escalates the rate of surface run-off.

References

Bronstert, A., Bardossy, A., Bismuth, C., Buiteveld, H., Disse, M., Engel, H., Fritsch, U., Hundecha, Y., Lammersen, R., Niehoff, D., and Ritter, N., 2007.  Multi-scale modeling of land-use change and river training effects on floods in the Rhine basin. River Res. Appl., 23, 1102-1125.

Doocy, S., Daniels, A., Murray, S., and Kirsch, T.D., 2013. The Human Impact of Floods: a Historical Review of Events 1980-2009 and Systematic Literature Review. PloS Curr., 5. [Online].

Ehrhard, P., 2015. New Orleans’ levee system failure after Katrina has mistaken culprit. [Online].

European Union Floods Directive 2007. DIRECTIVE 2007/60/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL. [Online].

Hollis, G.E., 1975. The effect of urbanization on floods of different recurrence interval. AGU Journal, 11(3), 431-435.

Kazmeyer, M., 2017. Two Activities Humans Do That Increase the Chance of Flooding. [Online].

Konrad, C.P., 2003. Effects of Urban Development on Floods. [Online].

Warburton, M. L., Schulze, R. E., and Jewitt, G. P. W., 2012. Hydrological impacts of land use change in three diverse South African catchments. J. Hydrol., 414-415.

White, M. D., and Greer, K. A. 2006. The effects of watershed urban- ization on the stream hydrology and riparian vegetation of Los Peñasquitos Creek, California, Land. Urb. Plan., 74, 125-138.