Research papersImpact assessment of reservoir desiltation measures for downstream riverbed migration in climate change: A case study in northern Taiwan
Introduction
Typhoon Soudelor, which struck in 2015, was the typhoon that had the greatest impact on northern Taiwan in recent years. It caused several disasters in the Greater Taipei Area, primarily in Wulai and Xindian. According to data from the Fushan Rainfall Station, the accumulated rainfall during Typhoon Soudelor was 792 mm. Similarly, the hourly rainfall (95 mm/h), 3-h rainfall (253 mm/3h), 6-h rainfall (442 mm/6h), and 12-h rainfall (655 mm/12h) were all the highest in the last 35 years. In Wulai, rainfall reached levels not seen in over 100 years. Such heavy rains caused the flood level of the mid- to upstream catchment areas of the Guishan Bridge (Chuchih gage), Hsiulung Bridge, and Sanxia to reach the Level 1 warning stage (Chang et al., 2015).
Lin and Chan (2015) found that the number of typhoons that form in the northwestern Pacific Ocean has substantially fallen since the 1990s, but the strength of the average individual typhoon has clearly increased. Knutson et al. (2010) conducted simulation analysis and found that, in the future, the general atmospheric circulation will be influenced by increases in greenhouse gases and the Earth’s temperature, resulting in stronger typhoons. Huang et al. (2015) also found that increased greenhouse gas levels cause stronger typhoons; according to their simulations, by 2100, the average strength of typhoons will increase by 2%–11%. Some studies have reported that tropical cyclones will occur less frequently, but strong tropical cyclones will occur more, leading to more frequent instances of stronger rainfall intensity (Emanuel, 2005, Tu et al., 2009, Webster et al., 2005). Su et al. (2014) used dynamical downscaling to study the rainfall characteristics of typhoon events in different climate change scenarios in Taiwan. The results showed that, in the future, the duration of rainfall during each typhoon event in Taiwan would decrease but the total rainfall would substantially increase. This indicates that Taiwan will experience more extreme rainfall intensities during typhoons in the future. The extreme rainfall will increase flooding, the magnitude of soil erosion, and sediment redistribution (Al-Safi and Sarukkalige, 2020, Foster et al., 2012, Gao et al., 2020, Hassan et al., 2019, Kang et al., 2007, Peizhen et al., 2001). In addition to the flash floods, soil erosion, and sediment redistribution occurring during extreme rainfall, degradation and aggradation of riverbeds will also increase in the future (Chao et al., 2016, Chao et al., 2018).
Typhoon Aere in 2004 caused severe sedimentation and loss of storage capacity in the Shihmen Reservoir in northern Taiwan. It dramatically increased the turbidity of water and seriously affected the water supply. To effectively maintain the stability of the water supply and the reservoir storage capacity, the Water Resources Agency of the Ministry of Economic Affairs (WRA), a major water resources department of the government of Taiwan, began in 2009 to plan and construct improvement measures, including a sediment flushing tunnel and the JhongJhuang Bank-Side Reservoir (JBSR) to strengthen siltation prevention in the Shihmen Reservoir. The sediment flushing tunnel was completed in 2012. It mainly connects a new sediment flushing pressure steel pipe to the original pressure steel pipe of the Shihmen Power Plant No. 2 to form a tunnel dedicated to effectively flushing the siltation at the bottom of the reservoir. The JBSR was completed in 2017, and its main function is to provide emergency backup water when the water turbidity of the Shihmen Reservoir increases during typhoons so that silt can be removed from the reservoir by means of hydraulic drainage without affecting the water supply in the downstream area. By early 2017, however, the amount of total siltation had reached 186 million m3, which is 32.6% of the designed storage capacity of the reservoir. Extreme hydraulic events due to climate change will increase both the risk to the stability of the water supply and the risk of flooding. To bolster the siltation prevention and flood storage capacities of the Shihmen Reservoir, the WRA has planned a desilting tunnel at the Amuping area within the Shihmen Reservoir storage range, the Amuping Desilting Tunnel (WRA, 2015a). Construction of the Amuping Desilting Tunnel began in 2018, and the tunnel is scheduled for completion and initial operation in 2022.
According to the Shihmen Reservoir Operational Regulations (WRA, 2017b), when the water level of the Shihmen Reservoir reaches the standard height of 227 m, the flood drainage tunnel will be opened; when the total discharge of the flood drainage tunnel is greater than 600 m3/s and the water level of the reservoir is higher than 237.5 m, the drainage tunnel will be closed and the spillway will be used instead.
Reservoir flood and sand drainage operations will cause siltation along the river course, and the particles on the riverbed may become finer, both of which may alter the original water flow and sediment transport conditions of the river. In addition to the impact of future changes in rainfall patterns, existing flood control capabilities will face severe challenges. Previous studies only focused on the impact of the sediment flushing tunnel and the Amuping Desilting Tunnel on the downstream riverbed (WRA, 2017a, WRA, 2015c), and did not consider the possibility of increasing sediment discharge after the completion of the JhongJhuang Bank-Side Reservoir. Therefore, this study used the numerical simulation to examine the flood control capabilities and changes in erosion and deposition of the main channels of the Tamsui and Dahan Rivers before and after the operation of the JhongJhuang Bank-Side Reservoir and the Amuping Desilting Tunnel under climate change. Based on the simulation results, suggestions were provided for the sediment flushing in the Shihmen Reservoir after the completion of the Amuping Desilting Tunnel in the future. The next (second) chapter, we will introduce the study area, the Shihmen Reservoir, climate change data, and related assessment models, such as rainfall-runoff model, and hydraulic and sediment transport model, and explain the verification results of the assessment models. The assumptions for the flood prevention and riverbed migration of four cases with different model settings under climate change are also explained in this chapter. The third chapter is the results and discussion, which can be divided into the changes of rainfall and runoff under the influence of climate change, and the simulation results of riverbed migration under a single extreme rainfall event and long-term influence. Finally, we summarize the important contributions of this research in the fourth chapter, especially the analysis of the benefits of the overall flood control and sediment desiltation of the Shihmen Reservoir after the completion of a series of improvement measures.
Section snippets
Tamsui River Basin
The Tamsui River Basin encompasses most of the Greater Taipei Area. The basin measures 2,726 km2, and the average slope is 20.83%. The basin can be roughly divided into the plain at the bottom (average elevation, 7.95 m) and the surrounding slopes (elevations of 135–3,500 m). The whole area is tilted from southeast to northwest, and it is thinner and longer along the south–north direction (Fig. 1).
In the Tamsui River Basin, the average rainfall is higher in summer than in winter. The average
Rainfall change under climate change
Fig. 6 presents a box and whisker plot of the analysis results for the baseline and late 21st century extreme typhoon rainfall characteristics under climate change; the results are the average regional results from the 13 rainfall stations examined in this study. Overall, the average total rainfall during the baseline extreme typhoons was 570 ± 120 mm (mean ± standard deviation), and the maximum value was 807 mm. For the late 21st century, the average total rainfall increased by 51% from the
Conclusions
This study applied WRF-MRI dynamical downscaling to climate change estimation data, considering the RCP 8.5 scenario, and analyzed the main river course flood-control ability for the main courses of the Tamsui River and Dahan River, finding that the scouring and deposition were influenced by climate change. Furthermore, the influence of the sediment flushing tunnel, JhongJhuang Bank-Side Reservoir, and Amuping Desilting Tunnel on the downstream river course was also investigated.
The estimated
Funding
This research was supported by the Ministry of Science and Technology, Taiwan [grant numbers MOST106-2621-M-865-001].
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to thank theme C of the Program for Risk Information on Climate Change of Japan (SOUSEI-C) for providing the MRI-AGCM data.
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