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Dam It, Why So Much Erosion?

Updated: Dec 27, 2020

ABOUT | Riverbank erosion naturally occurs, but anthropogenic activities like dam constructions could also cause geomorphological changes of rivers. This may render the erosion at a faster rate and induce degradations of riverine habitats and inland farmlands. This article examples the erosional process and impacts, using a case study from Preah Romkel in the lower Mekong.


Team members: 
Xu Shiying, Liew Jing Yee, Lab Sot Ny

INTRODUCTION


Riverbank erosion occurs both naturally and through anthropogenic impacts. The natural process takes place at a relatively slow rate over the long period of time. In the process, materials are removed from a river channel due to certain actions of water. On the other hand, human activities like hydropower dam building could also configure the geomorphological process of rivers[1] like river channel formations. Human-induced channel changes may render riverbank erosion at a faster rate. This may cause negative environmental and socio-economic impacts.


Riverbank erosion and its resulting fallen trees observed in Lngo Island


Preah Romkel is a Cambodian village situated in the the Lower Mekong Basin (LMB) and Lngo Island is located near Preah Romkel (see the map below). The large area of LMB is made up of alluvial channels, and riverbank erosions have been increasingly observed in many sites of the lower Mekong[2].


Grappling with the erosion in LMB is a transboundary issue, as dam development upstream could be one of the factors that affect the downstream erosional processes. A study shows that the construction of a dam on an upstream tributary causes wide-ranging impacts on downstream rivers, including hydrologic changes in water and sediment discharges, that ultimately lead to the riverbank erosion[3].


A location map of the riverbank in Preah Romkel (highlighted in red) and Lngo Island (described as Don Langa)[5]


EROSION INDUCED BY DAM CONSTRUCTIONS


In Preah Romkel and Lgno island, local communities stated that the rate of riverbank erosion had become much faster compared to before, especially since the commence of the Don Sahong Dam construction in Laos in 2016.

The construction of Don Sahong Dam in Laos, as seen from Preah Romkel Village


Our interviewees on Lngo Island emphasised that the sudden increase in the erosion rate caused a significant impact on their lives. A 55-year-old shop owner, Ms. Si, had to move further inland by about 20m, as erosion rendered the land unusable. Furthermore, some portions of of her farmland near the riverbank were eroded, and this caused her an income loss of about US$ 200.


A photo of a big tree at the head of Lngo Island, which was taken one month before our site visit


A photo of the same area with the tree fallen, which was taken during our site visit


One interviewee noticed that the rate of erosion between July 2018 and Sep 2018 was unprecedented. It is highly likely that this rapid erosion was induced by unusually high level of water, which was not only brought about by the wet-season increase in rainfall, but also the collapse of Saddle Dam D in Champasak Province of Laos on 23 July 2018[4]. Five billion cubic metres of water was released, and this triggered massive floods downstream (ibid).


IMPACTS OF CHANGING HYDROLOGICAL FEATURES


The construction of the Don Sahong Dam has not been managed effectively[5]. The same sentiment was also expressed by both Cambodian and Laotian villagers living in the affected region.


Dam building requires not only explosions on sites but also sand dredging in downstream for the supply of construction materials. Both actions forcibly remove sediments from the riverbank and river bed respectively, while accumulating suspended sediments inside the river. This results in rapid erosion, while decreasing water quality. The combination of these two factors leads to the destruction of natural habitats of wild life, for example, fishes and endangered Irrawaddy dolphins that inhabit in the LMB. Consequently, this change in the sediment regime has indeed caused the decline in fish catches in the region.


Furthermore, the rapid increase in water level caused by the upstream dam collapse also rendered several riverbanks inundated or destroyed. This erosion of the embankments further put protected croplands situated near the riverbanks in a risk to flooding, especially in the rainy season.

Damages observed in a corn and eggplant farm on Lngo Island


Several efforts have been made to tackle this problem. The 60-year-old village chief of Preah Rumkel, Mr. Hout Seng, has attempted to reduce the erosional impacts of the Don Sahong dam, in collaboration with NGOs. They also submitted a petition calling for the cancellation of the dam construction, but it only managed to slow down the dam building process.


CONCLUSION: WAY FORWARD


A greater action has to be taken in order to aid the affected communities. As the dam construction continues, the most immediate action that can be implemented is the mitigation of the erosional impacts by the residents of Lngo Island. For example, the planting of certain plant species like bamboos may help retain the structure of the riverbank. Such long-term solution, as opposed to short ones like house relocations, requires technical and scientific understandings of erosional and planting processes. Therefore, educational supports should be provided. We hope that the communities will be able to retain their way of life and allow future generations to continue building their homes on the island.

 

REFERENCES

  1. Kummu, M., Lu X.X., Rasphone, A., Sarkkula, J., Koponen, J. (2008) Riverbank changes along the Mekong River: Remote sensing detection in the Vientiane–Nong Khai area. Quaternary International 186, pp 100–112

  2. Gupta, A., Liew, S.C., (2007). The Mekong from satellite imagery: a quick look at a large river. Geomorphology 85 (3–4), pp 259–274

  3. Merritt, D.M., Wohl, E.E., (2003). Downstream hydraulic geometry and channel adjustment during a flood along an ephemeral, arid-region drainage. Geomorphology 52, pp 165–180

  4. Olarn, K., Sandi, S., Westcott, B. (2018) ‘Substandard construction’ caused Laos dam collapse, minister says. CNN. Accessed on 10 Oct 18 from: https://edition.cnn.com/2018/07/27/asia/laos-dam-collapse-construction-intl/index.html

  5. International Rivers. (2018). Don Sahong Dam. International Rivers. Accessed on 29 Sep 18 from: https://www.internationalrivers.org/campaigns/don-sahong-dam