Plans of economic growth in Bhutan, a small developing country in the eastern Himalayas, include a large number of hydropower plants, to exploit the potential of water resources and high topographic gradients. The economic importance of such projects is very high, given that hydropower is estimated to provide about 25% of Bhutan’s GDP and almost half of government revenue (expected to soar to 75% in coming years). Emerging environmental problems related to the construction of these dams include increased landslide activity in the impounded reservoir area, widespread disruption to forests, river systems and habitats of endangered species, flooding of agricultural land, obliteration of paddy land and drying of underground springs. Here we address an even more fundamental and safety related problem of the construction of hydropower plants, which results from insufficient geological investigations and geotechnical understanding of the selected dam site location. Among the new power plants under construction, the 1200 MW Punatsangchhu I Hydroelectric Project (located 80 km east of the capital Thimphu and 20 km south of Punakha) is discussed in this contribution. An autonomous body has been set up for the implementation of this mega project by the two governments of Bhutan and India. Most of the underground excavations of this project have been completed, consisting of 4 intake tunnels each of 6.4 m diameter and total length of 1926.78 m, 4 underground desilting chambers each of 420 m (L) x 17/19 m (W) x 24.74 m (H) and an underground power house located 15 km downstream the diversion dam at Kamechu, Dagar Gewog. Currently under construction with long delays is a concrete gravity diversion dam of 91 m height and 223.8 m length. During excavation works for this gravity dam, which began in November 2008, a 1 million m3 rockslide was triggered at the right abutment in July 2013, causing severe safety and operational issues. The failure caused the construction to come to a stop in favour of stabilisation measures and led to an estimated threefold increase in the expected costs and raised public concern. We have investigated the right side slope in the area of the diversion dam with an InSAR time series analysis between February 2007 and July 2017 obtained with ALOS-1, ALOS-2 and Sentinel-1 data, and complemented these results with field observations collected in October 2017. Our results indicate that the dam site is not only confronted with excavation induced slope failures, but that the dam site is located within a very large and active landslide body encompassing the entire east facing valley slope. The instability extends for more than 5 km along the valley axis and 4 km upslope, covers an area or about 14 km2 and the relief between the toe of the slope (at 1200 m a.s.l.) and the head scarp (at around 2300 m a.s.l.) is 1200 m. During the observation period (2007-2017) LOS displacements rates have increased on average from 3 cm/year to almost 10 cm/year over different sectors of the slope. The area over which displacements are seen in the observation period covers at least 10 km2, this suggesting potential total volumes between 0.5 and 1 km3, for average depths between 50 and 100 m. The fastest displacements are observed immediately upslope of the slope failure that occurred in 2013, where large scarps are visible. No signs of decelerations are observed neither in the InSAR time series nor in the local displacements at the dam site recorded by a total station installed after the failure (data from the latter kindly provided by construction company). This indicates that the stabilisation measures that were put in place have not been effective and that stabilisation may be impossible, due to the depth of the large instability. This is confirmed by boreholes drilled after the 2013 event, which have not reached the basal shear zone. Although the satellite based time series are unable to resolve the sharp accelerations observed in the local in situ data, the overall trends and cumulative displacements at the dam site match very well. The area currently monitored by the project is only within the secondary slide at the slope toe and covers an area much smaller than the landslide inferred from displacements observed in the InSAR velocity maps, morphological features on ALOS DSM and outcrop data. The main concerns are related to the feasibility of project completion, to the likely decrease of slope stability during reservoir impoundment, to the long term dam stability and resulting final capacity of the reservoir. A notable case in the Italian Alps, the Beauregard dam site, appears similar to Punatsangchhu-I in terms of size, surface displacement patterns and poor rock conditions. A maximum reservoir capacity at Beauregard was imposed at 6.8 million m3 as opposed to the 70 million m3 for which it had been designed and this may be a likely scenario for Punatsangchhu, for safety to be ensured. Our results show how EO data and methods can play a vital role in providing information for decision making prior and during the construction of large infrastructures, can monitor the efficacy of stabilisation measures, and also shed light on future hazard potential.
Dini, B., Manconi, A. (2019). Surface displacements over the Punatsangchhu dam site, Bhutan, measured with InSAR time series. In Living Planet Symposium ESA 2019.
Surface displacements over the Punatsangchhu dam site, Bhutan, measured with InSAR time series
Dini, B;
2019
Abstract
Plans of economic growth in Bhutan, a small developing country in the eastern Himalayas, include a large number of hydropower plants, to exploit the potential of water resources and high topographic gradients. The economic importance of such projects is very high, given that hydropower is estimated to provide about 25% of Bhutan’s GDP and almost half of government revenue (expected to soar to 75% in coming years). Emerging environmental problems related to the construction of these dams include increased landslide activity in the impounded reservoir area, widespread disruption to forests, river systems and habitats of endangered species, flooding of agricultural land, obliteration of paddy land and drying of underground springs. Here we address an even more fundamental and safety related problem of the construction of hydropower plants, which results from insufficient geological investigations and geotechnical understanding of the selected dam site location. Among the new power plants under construction, the 1200 MW Punatsangchhu I Hydroelectric Project (located 80 km east of the capital Thimphu and 20 km south of Punakha) is discussed in this contribution. An autonomous body has been set up for the implementation of this mega project by the two governments of Bhutan and India. Most of the underground excavations of this project have been completed, consisting of 4 intake tunnels each of 6.4 m diameter and total length of 1926.78 m, 4 underground desilting chambers each of 420 m (L) x 17/19 m (W) x 24.74 m (H) and an underground power house located 15 km downstream the diversion dam at Kamechu, Dagar Gewog. Currently under construction with long delays is a concrete gravity diversion dam of 91 m height and 223.8 m length. During excavation works for this gravity dam, which began in November 2008, a 1 million m3 rockslide was triggered at the right abutment in July 2013, causing severe safety and operational issues. The failure caused the construction to come to a stop in favour of stabilisation measures and led to an estimated threefold increase in the expected costs and raised public concern. We have investigated the right side slope in the area of the diversion dam with an InSAR time series analysis between February 2007 and July 2017 obtained with ALOS-1, ALOS-2 and Sentinel-1 data, and complemented these results with field observations collected in October 2017. Our results indicate that the dam site is not only confronted with excavation induced slope failures, but that the dam site is located within a very large and active landslide body encompassing the entire east facing valley slope. The instability extends for more than 5 km along the valley axis and 4 km upslope, covers an area or about 14 km2 and the relief between the toe of the slope (at 1200 m a.s.l.) and the head scarp (at around 2300 m a.s.l.) is 1200 m. During the observation period (2007-2017) LOS displacements rates have increased on average from 3 cm/year to almost 10 cm/year over different sectors of the slope. The area over which displacements are seen in the observation period covers at least 10 km2, this suggesting potential total volumes between 0.5 and 1 km3, for average depths between 50 and 100 m. The fastest displacements are observed immediately upslope of the slope failure that occurred in 2013, where large scarps are visible. No signs of decelerations are observed neither in the InSAR time series nor in the local displacements at the dam site recorded by a total station installed after the failure (data from the latter kindly provided by construction company). This indicates that the stabilisation measures that were put in place have not been effective and that stabilisation may be impossible, due to the depth of the large instability. This is confirmed by boreholes drilled after the 2013 event, which have not reached the basal shear zone. Although the satellite based time series are unable to resolve the sharp accelerations observed in the local in situ data, the overall trends and cumulative displacements at the dam site match very well. The area currently monitored by the project is only within the secondary slide at the slope toe and covers an area much smaller than the landslide inferred from displacements observed in the InSAR velocity maps, morphological features on ALOS DSM and outcrop data. The main concerns are related to the feasibility of project completion, to the likely decrease of slope stability during reservoir impoundment, to the long term dam stability and resulting final capacity of the reservoir. A notable case in the Italian Alps, the Beauregard dam site, appears similar to Punatsangchhu-I in terms of size, surface displacement patterns and poor rock conditions. A maximum reservoir capacity at Beauregard was imposed at 6.8 million m3 as opposed to the 70 million m3 for which it had been designed and this may be a likely scenario for Punatsangchhu, for safety to be ensured. Our results show how EO data and methods can play a vital role in providing information for decision making prior and during the construction of large infrastructures, can monitor the efficacy of stabilisation measures, and also shed light on future hazard potential.
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