Barely a year after major works were done on the winding Aburi-Ayi Mensah stretch of the N4, motorists using the south-bound lane have been stranded on two occasions after a heavy downpour caused landslides.

Whether these were rock-falls or landslides, the Accra Mining Network is sending her team of Geoscience professionals and Mining Engineers to make an independent assessment of the situation and thereafter proffer organic and custom-made made solution for the prevailing challenge.

The area dovetails into a fault zone and theoretically, the network believes this consideration must be made to arrive at a lasting solution instead of importing what others have done at other places, since this may not necessarily fit our peculiar situation at the Aburi Hills. But what causes landslides in the first place?

There are two primary causes of landslides: (1). human causes and (2). natural causes. Sometimes, landslides are caused, or made worse, by a combination of these two factors.

How can Human Activities cause Landslides?

Population explosion creating neighbourhoods, towns, and cities is the primary means by which humans contribute to the occurrence of landslides. For some time now, the Akwapim Hills has been colloquially referred to as Little London because of the sweet breeze and the low associated morning temperatures. This has undoubtedly created an exotic demand for real estate developments on the Akwapim Hills. Human activities disturb or change the existing drainage patterns, destabilize existing slopes, and invariably remove vegetation. These are therefore the common human-induced factors that may initiate landslides. Other examples include oversteepening of slopes by undercutting the bottom and loading the top of a slope to exceed the bearing strength of the soil or other component material. However, landslides may also occur in once-stable areas due to other human activities such as irrigation, lawn watering, draining of reservoirs (or creating them), leaking pipes, and improper excavating or grading on slopes. New construction on landslide-prone land can be improved through proper engineering (for example, grading, excavating) by first identifying the site’s susceptibility to slope failures and by creating appropriate landslide zoning.  Though theoretical, the network believes that a cross pollination of ideas between the network’s geoscientists and the network’s mining engineers can provide the requisite solution at Aburi.

Natural Occurrences of Landslides

This category has three major triggering mechanisms that can occur either singly or in combination — (1) water, (2) seismic activity, and (3) volcanic activity. Effects of all of these causes vary widely and depend on factors such as steepness of slope, morphology or shape of terrain, soil type, underlying geology, and whether there are people or structures on the affected areas.

Landslides and Water

Slope saturation by water is a primary cause of landslides. Could this explain why there had been landslides recorded after the heavy downpours? Well, saturation can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and surface-water level changes along coastlines, earth dams, and in the banks of lakes, reservoirs, canals, and rivers. Landslides and flooding are closely associated because both are related to precipitation, runoff, and the saturation of ground by water. Flooding may cause landslides by undercutting banks of streams and rivers and by saturation of slopes by surface water (overland flow). In addition, debris flows, and mudflows usually occur in small, steep stream channels and commonly are mistaken for floods; in fact, these two events often occur simultaneously in the same area.

Conversely, landslides also can cause flooding when sliding rock and debris block stream channels and other waterways, allowing large volumes of water to back up behind such dams.

This causes backwater flooding and, if the dam fails, subsequent downstream flooding. Moreover, solid landslide debris can “bulk” or add volume and density to otherwise normal streamflow or cause channel blockages and diversions, creating flood conditions or localized erosion.

Steep wildfire-burned slopes often are landslide-prone. Due to a combination of the burning and resultant denudation of vegetation on slopes, a change in soil chemistry due to burning, and subsequent saturation of slopes by water from various sources, such as rainfall make such areas susceptible to landslides. Debris flows are the most common type of landslide on burned slopes. Wildfires, of course, maybe the result of natural or human causes.

Landslides and Volcanic Activity

The West African Craton is stable and so volcanic activities are not a common occurrence. Nonetheless, landslides due to volcanic activity in other places in the world represent some of the most devastating types of failures. Volcanic lava may melt snow rapidly, which can form a deluge of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volcanoes, devastating anything in its path. These volcanic debris flows can reach great distances after they leave the flanks of the volcano and can damage structures in flat areas surrounding the volcanoes. Volcanic edifices are young, unconsolidated, and geologically weak structures that in many cases can collapse and cause rockslides, landslides, and debris avalanches. Many islands of volcanic origin experience periodic failure of their perimeter areas (due to the weak volcanic surface deposits), and masses of soil and rockslide into the ocean or other water bodies, such as inlets. Such collapses may create massive sub-marine landslides that may also rapidly displace water, subsequently creating deadly tsunamis that can travel and do damage at great distances, as well as locally.

Conclusion

From the above, landslides are common but preventable phenomena which can be triggered by numerous activities by humans and by nature. What exactly is happening at the Aburi Hills can only be ascertained from a field study which AMN intends to explore as her contribution.

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Credits: Lynn Highland, Peter Bobrowsky and www.geologyengineering.com