Impact of Human Activities on Natural Hazards
July 25, 2008
Natural hazards are naturally occurring phenomena that have disastrous impact on humanity. These phenomena had been in existence even before the advent of humanity. The hazardous dimension of these natural phenomena are in the context of the impact that such a phenomenon would have on human population in the area affected by that phenomenon. In this essay, the effect that human activity has on these natural hazards would be analyzed. Some human activities may be exacerbating the factors that cause the natural hazard, like the impact of excessive and unplanned logging on floods and droughts. In certain other cases the human activities may cause subsequent or supplementary hazards to a primary hazard event, like building dams in earthquake prone zones may lead to flash floods and landslides in the event of a rupture.
A hazard can be defined as an event that has the potential to cause harm. This potential may be on account of its unexpected timing of occurrence or the actual intensity of the event itself. Human societies can withstand these events within a normal scale of occurrence. However, human societies become vulnerable when these events occur unexpectedly or are of an intensity or duration that falls beyond that normal scale (O’Hare and Rivas, 2005). Natural hazards can be broadly classified under the heads of geological, hydrological, climatic and diseases. This essay would limit its scope to analyzing causal relationships, if any, of human activities on landslides, floods and drought and the secondary hazards triggered by those activities in the event of an earthquake.
Of all human activities that have a direct or indirect impact on natural hazards, deforestation is by far the most significant. Deforestation is the removal or destruction of forest cover of an area. It may occur due to unscientific logging practices without regeneration and may be accompanied by subsequent conversion to non-forest usage like agriculture, pasture, urban, mining or industrial development, fallow or wetland.
At a very broad level, it has been argued that deforestation is a major cause of global climatic changes. It has been predicted that removal of forest cover will lead to violent and unpredictable environmental fluctuations. At a smaller landscape, deforestation has a direct bearing upon the climatic, hydrological, edaphic and biological aspects of that area. Deforestation is associated with higher levels of soil erosion and landslides, sedimentation in river beds and changes in fluvial geomorphology (Haigh, 1984). Quite a few of these effects of deforestation have a direct bearing on the natural hazards that will be covered in this essay.
One of the major functions of a forest is to maintain the humidity level in the atmosphere. Trees withdraw groundwater through their roots and transpire the excess water through their leaves. Forests return a major part of the rainfall received by them through evapotranspiration. Annual evapotranspiration in tropical moist lowland forests ranges up to 1500 mm per year, with transpiration accounting for a maximum of 1045 mm per year (Bruijnzeel, 1990). This process of evapotranspiration in the leaves of trees takes the latent heat of evaporation from the surrounding atmosphere. Thus evapotranspiration has a cooling effect on the atmosphere that aids precipitation. Deforestation denies the atmosphere of this cooling effect and is thus a contributing factor to lowering of annual rainfall in an area.
Further, the effects of deforestation generally compound the severity of drought. Lack of trees translates to the lack of root fibers that hold the topsoil. In the event of a drought, the topsoil flakes and gets blown by the wind, leading to severe dust storms. This phenomenon had devastated the American Great Plains for close to a decade in 1930s. The dust bowl covered farming areas in Colorado, Kansas, north west Oklahoma, north Texas and north east New Mexico. The fertile soil of the plains was exposed due to lack of vegetation cover and actions of the plow. These farming techniques that led to severe soil erosion, coupled with prolonged periods of extremely low rainfall, led to a series of severe dust storms that ranged up to the Atlantic coast. Much of the fertile topsoil was lost in the Atlantic (Cartensen et al., 1999).
Direct causal relationship between human activity and drought is yet to be conclusively established. However, there are studies available that point to a positive correlation between the two. For example, climate-modeling studies have indicated that the 20th century Sahel drought was caused by changing sea surface temperatures. These changes were due to a combination of natural variability and human induced atmospheric changes. The anthropogenic factors in this case were rise in greenhouse gas levels and aerosols (GFDL Climate Modeling Research Highlights, 2007).
The effect of human activities like deforestation is rather more direct and pronounced in case of hydrological hazards like fluvial floods. Fluvial floods occur when the discharge of a river exceeds its bankfull capacity. Forests create deep, open textured soils that can hold large quantities of water. When the forest cover is removed through logging, the soil becomes compacted. More rainwater is converted to runoff or near surface flow and less proportion percolates as groundwater. Research has shown significant increase in monthly runoff following logging activities (Rahim and Harding, 1993).
The runoff rainwater carries with it considerable amounts of loose soil particles. Removal of vegetation cover through excessive logging activities or overgrazing leaves the soil bare. In such a situation, the upper layer of the soils becomes susceptible to erosion by surface runoff. These suspended soil particles are deposited on the riverbeds. The effect of this type of soil erosion by surface runoff is even more pronounced when the deforestation happens in the riparian zones as well.
With time, this sedimentation decreases the depth of the riverbed and, thereby, the water carrying capacity of that river. When the flow of water in the river increases due to a variety of reasons like rainfall, seasonal melting of ice etc, that river can no longer contain the flow within its channel due to reduced drainage efficiency. This excess water inundates adjoining areas causing floods. The effect of soil erosion and subsequent sedimentation of the riverbeds enhances both the occurrences of floods and the area affected by floods.
The impact of deforestation driven soil erosion is particularly severe in mountainous terrain. In the western Himalayas, comparison of bedload sediments trapped from parallel streams found that the sediment loads from undisturbed forest were five-seven times smaller than from deforested areas covered by grass and scrub. Deforested areas had a much smaller depth of soil and in many places large patches of underlying bedrock had become exposed ( Haigh et al., 1998).
The result of this type of soil erosion on floods is amply demonstrated in the river systems in peninsular Malaysia. Malaysia is located in the equatorial belt and receives very heavy rainfall throughout the year. Peninsular Malaysia has a dense river network. The largest of around one hundred river systems is the Pahang. The runoff along exposed hillsides on the upper courses of the rivers lead to heavy soil erosion and major silting in the lower courses. Peninsular Malaysia has a major tin mining industry and disposal of unwanted tin mining tailings in watercourses has greatly accentuated the silting process. This has majorly worsened the flood situation, both in terms of inundation area and duration of flooding. The effect is particularly severe in Perak and Selangor (Chan and Parker, 1996).
The type of natural hazard that is most closely linked to human activity is the landslide. A landslide can be defined as the movement of a mass of rock, soil or debris downward a slope. These occur on steep slopes of hilly terrains that demonstrate certain inherent factors like susceptible rock structure, weak material or slope form. The preparatory factors actively produce the changes that make slopes more vulnerable to a slide, without actually causing it. Some preparatory factor may ultimately become the triggering factor and start the landslide. In some other cases, geological or climatic events like earthquakes or rainstorms initiate the movement.
Human activities cause some of the more widespread preparatory factors. Removal of forest cover from mountain slopes deprives the soil of the binding force of the roots of vegetation, thereby making it more vulnerable. Removal of the toe of the slope renders the mass above, unstable. This is because the lateral buttressing support for the bulk of the slope that lies on top of the excavated area is removed. Human activities like building of roads or quarrying of minerals are responsible for this type of preparatory factor. In many cases, human settlement on the slopes alters the original surface drainage of that area, eventually rendering it hydrologically unstable.
The effects of human activity in the preparatory factors of a landslide were demonstrated in the landslide at Abbotsford, South Island, New Zealand on 8th August 1979. Deforestation, quarrying and modification of surface drainage further endangered the geologically unstable slope. Heavy rainfall and leakage from city water supply pipeline finally triggered the massive landslide (Pacione, 1999).
Human activity like construction of roads can have a major impact on the vulnerability of a mountain slope. For example, the Indian Central Himalayas have seen major increase in road construction activities after the war with China in 1962. Many of these roads are poorly designed and constructed. This has dramatically increased in the occurrences of landslides in the region (Ives, 2004).
Human intervention on the natural drainage of a slope as a major contributing factor to a landslide is amply demonstrated in the multiple occurrences of landslides on the hill slopes of La Paz city in Bolivia. La Paz region has considerable human settlements that are on unconsolidated slopes. These slopes are frequently wetted to saturation and forced to move. Some of these factors are natural, like seasonal convective showers of high intensity, flooding of lowers slopes by the rivers and streams draining that rainfall and water seepage from fluctuating water tables of adjoining lake Titicaca basin. However, the water saturation of the slopes is exacerbated by the human settlement on the slopes. Much of such settlements are unplanned, self-help housing that have no access to public sanitation and drainage systems. Waste water from such houses are drained directly on to the slopes. These factors have led to repeated landslides in the La Paz region in the past (O’Hare and Rivas, 2005).
Another type of mass movement that is seen in plain areas is subsidence. Subsidence is vertical sinking of materials. In many unplanned cities, the city has sprawled into areas not covered by municipal water distribution network and indiscriminate ground water usage through bore wells have severely depleted water tables. In some cities like Mexico and Bangkok, the drained soil has compacted, leading to subsidence. In some other regions like the Raniganj – Jharia coal belt in India, improper mining excavations and inappropriate filling of excavated tunnels have led to widespread subsidence.
In certain cases, though human activity does not cause a natural hazard, it may lead to secondary hazard events subsequent to the occurrence of a primary hazard. A case in point is the building of a very high multipurpose dam at Tehri in the Himalayan region in India that may be susceptible to seismic activity. In the eventuality of an earthquake and subsequent rupture of such a high dam, a tremendous flash flood is inevitable on the lower courses of the river, accompanied by major landslides as well (Ives, 2004).
Thus, it can be concluded that human activities have some impact on occurrence of natural hazards. For some hazards like landslides and subsidence, there is direct, causal relationship between human activity and hazard occurrence. In some other cases like drought and floods, unscientific and unplanned exploitation of natural resources exacerbate natural hazards. In yet other cases, human activity compounds the effect of a natural hazard by triggering other hazard events that follow.
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