3 Human Intervention and Erosion

River bank erosion is not a serious problem so far as no human settlement is present. But this natural hazard becomes a disaster when riparian buffers are not maintained, and human settlements are situated too close to eroding banks. Moreover, anthropogenic activities along the river stretch disturb the equilibrium of the river dynamics and accelerate the rate of bank erosion. Anthropogenic activities like deforestation, gravel mining, construction of dams and bridges, artificial cut offs, bank revetment and land use alterations change the morphology and natural dynamics of rivers (Kondolf, 1997). Human activities are stronger with respect to changing river dynamics than natural events as floods, droughts and landslides (Yamani et al., 2011).

3.1 Deforestation

It has been observed that bank erosion has increased by instability of the river behaviour due to deforestation and inadequate land use in the upper reach, which ultimately led to excessive sediment load into the rivers (Arohunsoro et al., 2014; Davinroy et al., 2003). Vegetation stabilizes banks primarily by increasing shear strength of the soil, reducing water velocity, and armoring the bank (Ott, 2000). Of course, the ability of vegetation to stabilize a bank is dependent upon factors such as plant vigor, density and rooting depth, etc. Some studies revealed that bank erosion in the upper reach was primarily due to destruction of riparian vegetation by people’s access and the effect of bridge constrictions on high flow, and secondarily to poorly installed channel revetments (Madej et al., 1994).

3.2 Dam and bridge construction

Dam construction (mainly in the middle reach) is another widespread impact on river systems around the world. More than fifty percent of the world’s largest river systems (172 of 292) have been affected to some extent by dams (Nilsson et al., 2005). Issues such as erosion and sedimentation have been carefully analysed for many dams around the world. Among them the High Aswan Dam in Egypt received considerable environmental and social attention. Like other dams, Aswan Dam contributes positive benefits to the country: generation of hydropower, increased industrial activities, increase in agricultural land, as well as crop intensification and diversification, etc. But at the same time it caused (a) physical impacts: changes in the level, velocity and discharge of the flow in the Nile River both upstream and downstream of the dam, changes in soil salinity and water logging, erosion of the river banks, beds and delta, sedimentation in the river and Lake Nasser, etc., and (b) biological impacts: changes in flora and fauna, implications for fish production, etc. (Biswas and Tortajada, 2012). However, it is claimed by some researchers that stable conditions have been achieved and erosion has almost stopped. This situation was achieved by limiting the amount of water flowing into Egypt to a maximum daily peak of 230 million cubic meters (Rushdi, 1996).

Erosion problem is also very significant in the case of Three Gorges Dam (TGD). The Three Gorges Dam has been constructed on the Yangtze River in China. It is one of the largest dams in the World. Like other rivers and dams, it has also been observed that severe erosion has occurred in the Yangtze River channels downstream of the dam in the post-TGD period. Since its operation in 2003, TGD has trapped nearly two-third of the sediment from upstream. As a result the downstream riverbed has been converted from deposition to erosion, in particular in the several hundreds of kilometres immediate to TGD (Yang et al., 2007). One major cause of increased river erosion after construction of dams is that all the sediment being carried by river into the reservoir retains there. Then the water released through the dam is very clean and has no sediment (i.e., dam-induced decrease in sediment load along the downstream reaches). Clean water causes more erosion than sediment laden water because it can carry more sediment.

Sometimes structures like bridges cause bank erosion, although bridges ensure and facilitate the communication across the rivers conveniently. These structures may have unfavourable effects on the hydrology and morphology of the rivers. A case study showed how bridge piers constrict the adjacent areas of the river and ultimately result in bank erosion (Biswas, 2010). Surma is a meandering river in Bangladesh. During the study period (monsoon 2008) a bridge at Kazir Bazaar was under construction. Aerial photographs of the bridge location revealed that both the upstream and downstream the river was affected by severe bank erosion. Erosion affected the local economy by engulfing a fish market, rice mills and others. So, it was predicted that if a river becomes constricted by some human intervention, bank erosion might happen, which is not generally expected at the time of planning.

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