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Future sediment dynamics in the Mekong Delta floodplains: Impacts of hydropower development, climate change and sea level rise

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... These issues primarily stem from rising relative sea levels and a shortage of sediment supply from the upper of the Mekong River to the coastal regions [2,5]. In recent years, the effects of climate change combined with unsustainable anthropogenic activities in the Mekong has led to a concerning decline in the transport of alluvial sediment from the upstream to the downstream of the Mekong River [11][12][13]. This decline has become particularly noticeable since the 1990s when hydropower dams were constructed along the main branches of the Mekong River [12,14]. ...
... In recent years, the effects of climate change combined with unsustainable anthropogenic activities in the Mekong has led to a concerning decline in the transport of alluvial sediment from the upstream to the downstream of the Mekong River [11][12][13]. This decline has become particularly noticeable since the 1990s when hydropower dams were constructed along the main branches of the Mekong River [12,14]. Research conducted by the authors in [3] estimated a suspended sediment discharge of only 87. 4 Mt/yr at the Kratie crosssection. ...
... Numerous studies have highlighted the significant reduction in sediment supply to the Vietnamese Mekong Delta, attributing this phenomenon to dams' location of the Mekong River upstream [12] as well as sediment mining activities in the Mekong River downstream [12,16]. In the study investigating the effects of climate change and upstream hydropower dams on the Mekong Delta, [12] reported that during the flood season period (September-November), approximately 48 to 60% of sediment discharge from the Mekong River upstream will flow downstream of the Vietnamese Mekong Delta. ...
Article
Relevance. Annually, the coastal areas of the Mekong Delta receive approximately 50% of the total sediment load from the Mekong River, primarily during the southwest monsoon. During the northeast monsoon, this coastal sediment undergoes reworking and is transported southwestward along the coastal areas of the Mekong delta. Aim. Analyzing the grain-size distribution of sea-bed surface sediment helps improve our understanding of sediment redistribution along the coastal areas of the Mekong Delta. Methods. In order to illustrate changes in sea-bed surface sediment grain size between the two monsoons, we focused on the median grain size, a widely used parameter for sediment transport calculation, and created spatial maps of median grain size along the coastal areas of the Mekong Delta. The analysis was based on sediment samples collected along the coastal areas of the Mekong Delta during field excursions in the southwest and northeast seasons. Results. The results reveal that median grain size during northeast (mainly ranging from 0.005 to 0.01 mm) are larger than those during southwest (primarily ranging from 0.01 to 0.05 mm). These changes in median grain size and spatial distributions are most prominent along the east coast, specifically from Soc Trang to Bac Lieu. In contrast, median grain size along the west side does not exhibit significant differences between the southwest and northeast monsoons. These observations imply a more significant sediment transport along the east coastal areas of the Mekong Delta during the northeast monsoon and provide evidence for the transportation of sediment from the east coast to the west coast during the monsoons.
... Several natural and artificial factors control the effectiveness of bank erosion. These factors include the bank material (Simon and Darby 2002), cover vegetation (Vannoppen et al. 2015), suspended sediment (Marttila and Kløve 2010;Moran et al. 2013), social-economic activities (Das et al. 2014) such as ship traffic (Gharbi et al. 2010;Houser 2010), dam construction ( Van Manh et al. 2015;Wang et al. 2016;Kondolf et al. 2018;Yoshida et al. 2020;Phung et al. 2021), channel dredging (Van Maren et al. 2015), and various infrastructures and activities (Houser 2010), etc. ...
... This is a primary factor leading to the phenomenon of erosion in the Mekong Delta. In addition to natural factors and those influenced by human activities, which have been studied in previous research Jordan et al. 2019Jordan et al. , 2020Kantoush et al. 2017;Kondolf et al. 2018;Nga et al. 2019;Tha et al. 2022;Van Binh et al. 2020a, b;Van Manh et al. 2015), the construction of infrastructure and the increase in bank loads also contribute to erosion in this region. Survey results of erosion in An Giang province, for example, indicate that out of 7 severe erosion sites, 5 of them occurred in areas with residential or road construction (Hoai et al. 2020). ...
... However, one of the consequences of dam construction is the trapping of sediment, which leads to a significant reduction in sediment transport downstream. According to the Mekong River Commission (2011), there are currently 136 existing and planned dams in the Lower Mekong Basin (LMB), which have the potential to trap more than 90% of the sediment load in the lower river system ( Van Manh et al. 2015;Kondolf et al. 2018). This, combined with sand mining activities, contributes to the alteration of discharge patterns and a reduction in sediment load in the Vietnamese Mekong Delta (VMD), thereby complicating bank erosion in the area (Jordan et al. 2019; Van Binh et al. 2020a, b). ...
Article
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Sedimentation and erosion of riverbanks and canals in the Mekong Delta have been longstanding issues. However, the severity and frequency of these problems are on the rise due to multiple factors, including upstream influences, climate change, rising sea levels, and human development activities. In this study, mapping riverbank erosion potential in the Vietnamese Mekong Delta is proposed to be carried out using the MK-BEHI index, which has been modified from BEHI developed by Rosgen. The MK-BEHI assesses the potential riverbank erosion by considering various factors, including: near-bank stress, bank geology, vegetation cover, bank construction, accretion/erosion rate, and bank load. Two indicators added to the index, compared to the original BEHI index, are bank construction and bank load. The index is categorized into four levels: low, medium, high, and very high, using a semi-quantitative approach. It is calculated by summing the component hazard indexes multiplied by their respective weights. The findings reveal that segments with very high potential are concentrated in certain areas, such as the segment of the Hau River flowing National Highway 91, Vam Nao River passing through My Hoi Dong, An Giang province, and the Tien River segment flowing through Sa Dec. Furthermore, numerous high-potential erosion banks have been recorded along the Tien and Hau rivers, particularly at the beginnings of the islets, curved river sections, and the Ham Luong river area encompassing Cho Lach, Mo Cay Bac, Mo Cay Nam, Thanh Phu, and Ba Tri. The aim is to provide a foundation for disaster adaptation and sustainable management.
... The monsoon climate drives two clear-cut seasons: a dry season from December to May and a wet season from June to November (Tran & Tortajada, 2022). The rivers of the delta are estimated overall to discharge approximately 300-550 km 3 of water and between 40 and 167 million tons of sediment annually Kondolf et al., 2014; Van Binh, Kantoush, Saber, et al., 2020; Van, Popescu, et al., 2012) (Box 1). ...
... According statistics from the Ministry of Agriculture and Rural Development (MARD), the VMD loses about 500 hectares of land each year due to erosion (Tho, 2020;Vasilopoulos et al., 2021; . Sediment loads in the channels of the VMD have significantly reduced by upstream damming, and insitu sand mining (Lu et al 2006; Wang et al., 2011;Kondolf et al., 2014, Hackney et al., 2020; Van Binh, Kantoush, Saber, et al., 2020). Van Binh, Kantoush, Saber, et al. (2020) show that upstream dam hydroelectric dam construction and operation as well as irrigation expansion activities in the Greater Mekong basin had reduced suspended sediment loads in the rivers of VMD by 74%. ...
... Sediment loads in the channels of the VMD have significantly reduced by upstream damming, and insitu sand mining (Lu et al 2006; Wang et al., 2011;Kondolf et al., 2014, Hackney et al., 2020; Van Binh, Kantoush, Saber, et al., 2020). Van Binh, Kantoush, Saber, et al. (2020) show that upstream dam hydroelectric dam construction and operation as well as irrigation expansion activities in the Greater Mekong basin had reduced suspended sediment loads in the rivers of VMD by 74%. Sediment loads are expected to decrease drastically as more planned dams become active (Dunn & Minderhoud, 2022;Kondolf et al., 2014). ...
Article
Sinking and shrinking, the Vietnamese Mekong Delta is a materialization of dynamic river flows, sediment flows, and coastline processes. Past policy aspirations and extensive water infrastructures have shaped the delta into one the most significant food producing landscapes in Southeast Asia. Yet, these changes have also created new environmental risks by transforming the hydrological system. Research has produced a growing and increasingly diverse empirical literature on the delta's environmental context, without necessarily providing water resource managers, policymakers and practitioners with the information needed to galvanize more resilient development. This focus review presents a detailed overview of the recent scientific findings, exploring how the management of water resources is changing, as well as their inter‐relationship with land use, policy, socio‐economic transitions, and global environmental crises. Compound and systemic risks to the delta include climate change, hydrometeorological hazards, upstream developments and an unsustainable development trajectory. We outline scientific knowledge gaps, as well as the pressing need for sharable analysis‐ready data and innovations. Finally, we provide recommended future research avenues for multiscale actions toward a sustainable and resilient delta future. This article is categorized under: Human Water > Water Governance Science of Water > Water Extremes Science of Water > Water and Environmental Change
... The LMR originates from the northern foot of Tanggula Mountain on the Tibetan Plateau of China, spanning various physiographic regions of Myanmar, Laos, Thailand, Cambodia, and Vietnam. The LMR, with virtually 5000 km length (Liu et al., 2009a,b) and mean annual water discharge of around 470 km 3 (MRC, 2009), drains an area of 795000 km 2 from its source, i.e., the Tibetan Plateau, to the south China sea in Ho Chi Minh City, Vietnam (Manh et al., 2015). While the Lancang River runs through deep valleys in Hengduan Mountains, categorized as the north-south mountains, the downstream course, i.e., the Mekong River, flows through relatively flat terrains with broad ravines. ...
... The transported sediment flux from LMR to the lake and its surrounding area due to the reverse flow phenomenon (flow from LMR to the lake during flooding season) is, on average, 7 Mt.yr − 1 (Kummu et al., 2005;WUP-FIN, 2003), which brings vital nutrients fertilizing the floodplain for fish spawning and rice farming (Manh et al., 2013). During the non-reverse flow period, flow from the lake to the LMR has very low SSC (Manh et al., 2015). Mekong Delta, influenced by the transported fine-grained sediment from upstream regions , covers a large portion of Vietnam, the primary ricegrowing area for local communities (Grumbine et al., 2012;Manh et al., 2014). ...
... Extensive research has been conducted to address the sedimentation process along LMR, mainly using measured data (discontinuous and sporadic data), satellite images, and rating curves (Zhang et al., 2014;Unverricht et al., 2014;Dang et al., 2018a,b). Research into exploring spatio-temporal patterns of sediment load and driving factors in LMR before the operation of mega-dams is limited, and most researchers have focused on the lower Mekong Basin (Wang et al., 2011;Manh et al., 2015;DeMaster et al., 2017;Dang et al., 2018a,b;Shrestha et al., 2018), from Chiang Sean towards Mekong Delta, since related data was unavailable (Lu and Siew, 2006). This research provides additional insights into the role of the Chinese part on the Lower Mekong's sediment characteristics and sediment deposition in different areas. ...
Article
Sediment is of great importance for the Lancang-Mekong River (LMR) ecosystem, providing essential ecological services for local communities. Understanding sediment change and its driving factors are critical to the sustainable management of this large transboundary river system. Insufficient background knowledge of sediment dynamics driven by climate change and land cover change hinders our understanding of sediment change in the long term. In this study, a distributed hydrological model coupled with a sediment module was developed to address the sediment dynamics and its driving factors of the LMR basin from 1968 to 2002, i.e., the pre-dam period. A new index was also proposed to quantify the regional sediment composition by considering riverbed deposition. The major results are: 1) the leading factor influencing the suspended sediment concentration has changed over the study period, i.e., from rainfall before the 1980s to land cover change after the 1980s; 2) from 1968 to 2002, the land cover change had the greatest impact on the suspended sediment load followed by rainfall; 3) the contribution ratio of the upstream area could be overestimated without considering sediment deposition onto riverbed. The approach presented here provides additional insights into quantifying the regional sediment composition for the dynamic Lancang-Mekong River basin experiencing rapid anthropogenic stresses.
... In addition, sediment discharge in the Mississippi River decreased by 75% from 1940 to 2007, which was due to dams, bends, soil conservation, and other factors [5]. The suspended particulate matter in the Mekong River also decreased by 5% annually, which was mainly affected by the hydropower development project after dam construction [6,7]. What's more, the estuarine deltas in East and Southeast Asia were also at risk of erosion due to the reduction of sediment discharge [8]. ...
... In addition, the sediment of the Danube River had been reduced by 60-70% due to the interception of the Iron Gate Dam [4,62]. The sediment capacity of the Mekong River in Asia also decreased by 5% annually, which was mainly affected by hydropower development [6,7,[63][64][65][66]. ...
... In addition to climate-related factors, alterations in the hydrological regimes of the Mekong River are perceived to be linked to upstream hydropower development. Currently, there are more than 50 massive hydropower dams in operation and over 100 dams under construction or planned in the upper and middle stretches of the Mekong River and its tributaries in the riparian countries (Jordan et al., 2020;Manh et al., 2015;Ng and Park, 2021;Triet et al., 2020). Besides alterations of the flow regimes, there are many concerns over serious impacts induced by these hydropower dams on ecosystems (Loc et al., 2017), on sediment dynamics (Kondolf et al., 2018;Lu et al., 2014;Tamura et al., 2010), and on the livelihoods of millions of inhabitants living in the delta (Kuenzer et al., 2013;Nguyen et al., 2020;Tran et al., 2020aTran et al., , 2020bTran and Weger, 2018). ...
... Once all the major dams on the mainstream are completed, the situation in downstream areas such as the VMD will deteriorate further. The long-term consequences for the downstream region have been estimated by many studies but with high uncertainty (Fredrik, 2011;Hoang et al., 2019;Kuenzer et al., 2013;Kummu and Varis, 2007;Manh et al., 2015;Tran et al., 2020aTran et al., , 2020b. However, a general assessment of the impact of hydropower projects on the VMD was summarized in the report of the Ministry of Natural Resources and Environment dated March 31, 2020, citing the research results of the Danish international consultancy of DHI (DHI, 2015), indicating that given only cases for all 11 mainstream hydropower projects, the flow will decrease by 34%e41%; saltwater intrudes further into the Tien and Hau rivers from 7 to 13 km; the total amount of silt and sand decreased by 64%; the total amount of nutrients is reduced by up to 67%; fisheries production decreased by up to 51% and impacts on biodiversity; in case there are more tributary hydropower plants: in addition to the impacts as in the above scenario: sediments and sediments will decrease by 65%, aquatic products by up to 52%; In case there is more water transfer outside the basin of Thailand, in addition to the impacts like the above scenarios, it will also reduce the flow by 47% and saltwater intrusion up to 17 km deeper in the dry season. ...
Chapter
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Water management strategy for agriculture in the Vietnamese Mekong Delta (VMD) is undergoing radical changes in response to hydrological changes under the exogenous impacts of climate change, upstream developments, and in-situ water infrastructure development. There have been notable efforts in transitioning to sustainable agricultural development policies, aiming at increasing farmers’ income while protecting the environmental systems of the delta. However, realities suggest that rice famers’ livelihoods remain unstable and highly susceptible to environmental changes. In response to these challenges, farmers have decided to shift their farming practices by transforming from rice-based to more adaptive farming systems. These on-going processes of livelihood transformation raise important questions concerning how local efforts correspond to policymaking and planning toward adaptation. This chapter sheds light on the interface between state-led climate change adaptation and local dynamics of livelihood transformation and how they interact with each other. We first reviewed studies carried out in the VMD, focusing on the adaptation and adaptive capacity of farmers under climate change and environmental impacts. Then we used the data collected from our previous surveys to explore farmers’ perspectives and actions in response to environmental changes. Insights from this chapter contribute to the much-needed knowledge base about how farmers’ livelihoods and adaptive capacity can be enhanced to allow for effective climate change adaptation.
... The sediment regime of rivers is affected by various anthropogenic impacts (Cox et al., 2021;Gibling, 2018;Gregory, 2006;Maaß et al., 2021). Bedload retention measures in the catchment area (Guo et al., 2021;Kondolf et al., 2014), river regulation for flood protection and for the extraction of agricultural and forestry land (Shrestha et al., 2021), hydropower (Hauer et al., 2018;Klaver et al., 2007;Manh et al., 2015;Provansal et al., 2014) and also the use of rivers as waterways influence the sediment regime (Yang et al., 2017). Various issues and challenges occur on rivers that are related to the sediment regime and river morphology. ...
... Besides human impacts such as river engineering and water management measures, there are also natural discontinuities in the sediment continuum, such as lakes or animal obstacles, that prevent or interrupt sediment continuity (Frings and Maaß, 2018). Impacts to the river itself and climate change affect river regimes (de Vriend, 2015;Guo et al., 2020;Manh et al., 2015). Many studies therefore also deal with sea level rise caused by climate change, as well as measures on rivers and in their deltas (Akter et al., 2021;Cox et al., 2021;Frings et al., 2019;Tessler et al., 2018). ...
... These efforts drastically reshaped the flood infrastructure in the province. The dyke systems in An Giang are meant to prevent flooding and facilitate triple rice cropping; however, they also limit the supply of fluvial sediment reaching the floodplain (Manh et al., 2015;. Traditionally, farmers in An Giang have practiced annual single or double rice cropping because of seasonal flooding. ...
... All of these factors contributed to the justification of our selection of An Giang Province as a case study. While coastal provinces in the VMD might face climatic challenges such as higher sea-level rise and saline intrusion (Manh et al., 2015;Marchesiello et al., 2019), relatively little is known about how out-migration patterns have occurred in the upper part of the VMD within the context of coupled climatic and hydopower development challenges. Findings of this study could thus contribute to the extant literature on climate change and rural adaptive livelihoods in the VMD and the broader context of the Mekong region. ...
Article
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Climate change influences the adaptation responses and mobility patterns of smallholder farmers across multiple scales. This study employed an inductive approach to observe smallholder farmers in An Giang Province in the Vietnamese Mekong Delta to compare the effects of various environmental and climate-related stressors on households with and without contributing migrant household members and on households of different income levels in two rural communes. We looked into the roles that adaptation responses and human mobility patterns play in the daily livelihoods of (translocal) households. We adopted a mixed-methods approach, which involved the administration of a livelihood survey among households in two rural communes (N = 106) and, subsequently, two focus group discussions, unstructured in-depth interviews, and secondary data analysis. We discovered that human mobility, adaptation responses, and climate change are interwoven in a web of complex relationships. No clear differences in effects and climate adaptation responses were discovered between emigrant and nonemigrant households. Hence, paradigms that either portray migration as a failure to adapt or as a form of adaptation in the context of climate change do not adequately explain the findings of this study. Differences between income groups were, however, observed. Relative to other income groups, middle-income farmers were disproportionally affected by climate-related disasters. Additionally, out-migration, aging, upstream hydropower development, and COVID-19 lockdowns posed significant challenges to the livelihoods of smallholder farmers. The compound effects of these multiple stressors indicate that human mobility, climate change and adaptation patterns should be best approached as ‘wicked’ problems.
... As compared to pre-1992 records at the same site, sediment load had reduced by about 20 % there. However, recent estimates of annual sediment load reaching the VMD after 2010 have indicated a much larger decrease (Binh et al., 2020a;Darby et al., 2016;Ha et al., 2018;Manh et al., 2015). Because of differences in methodology and datasets, the estimated yearly load ranged from about 20 Mt. (Manh et al., 2015) to 43 Mt. ...
... However, recent estimates of annual sediment load reaching the VMD after 2010 have indicated a much larger decrease (Binh et al., 2020a;Darby et al., 2016;Ha et al., 2018;Manh et al., 2015). Because of differences in methodology and datasets, the estimated yearly load ranged from about 20 Mt. (Manh et al., 2015) to 43 Mt. (Binh et al., 2020a). ...
Article
The boom in water infrastructure in the Mekong Basin has raised concerns over the annual supply of water and sediment among its riparian communities. By consolidating various datasets, continuous series of sediment load data were estimated for several stations located within the Lower Mekong Basin. At Chiang Saen in Thailand, the nearest station to the Chinese dams, the average sediment load was 79 ± 32 Mt./yr during the pre-dam period of 1965–1991. However, from 2010 to 2019 – during which a series of mega-dams were built in China – the sediment load decreased drastically by 84 % to only 12.5 ± 4.6 Mt./yr. This phenomenon of reduced annual sediment load during the mega-dam era (2010–2019) as compared to during the pre-dam era (1965–1991) can be observed at stations downstream from Luang Prabang (−53 %) to Nong Khai (−62 %) to Khong Chiam (−33 %). One of the drivers of this sediment load crisis is the rapid development of upstream dams. Especially after 2003, Chinese dams have reduced sediment supply to the downstream Mekong Basin severely. Concurrently, there was an increase in sediment contribution from the stretch of the Mekong River from Chiang Saen to Khong Chiam. A positive outcome of this increased sediment contribution was its buffering effect against the reduction in sediment load from the Chinese part of the Mekong Basin. Although sediment load at Kratie – the gateway station before the alluvial stretch comprising the Cambodian floodplains and Vietnam Mekong Delta – decreased from 78 ± 22 Mt./yr (1995–2009) to only 60 ± 21 Mt./yr (2010–2019), the reduction would have been higher without the increased sediment from the Chiang Saen – Khong Chiam stretch. However, with upcoming planned dams in Laos and Cambodia, this buffering effect is likely to be temporary, implying that the sediment load crisis as already experienced by the downstream communities can only become more severe.
... Nhận thức được tầm quan trọng của KLN trong trầm tích, thời gian qua tại Việt Nam các nhà khoa học bắt đầu quan tâm với một số công trình nghiên cứu liên quan [3][4][5][6][7]. Tại vùng ĐBSCL, trước đây có một số công trình nghiên cứu của các tác giả trong nước [2,[8][9][10][11][12] và của các tác giả nước ngoài [13][14][15][16][17][18][19][20]. Tuy nhiên trước xu thế biến đổi khí hậu, ảnh hưởng của thủy điện trên dòng Mekong cũng như sự phát triển sôi động của nền kinh tế các quốc gia trong lưu vực cùng với đó là hoạt động sinh hoạt, sản xuất, nhiều khu đô thị, khu công nghiệp mộc lên thì rất cần công trình nghiên cứu, đánh giá hàm lượng KLN, trong đó đặc biệt là hàm lượng Zn, Cu vì đây là 2 nguyên tố có hàm lượng cao hơn các nguyên tố còn lại trong hầu hết công trình đã công bố. ...
... Consequently, the sediment load decreased by 64%, dropping from 160Mt/yr before the dam construction to 57Mt/yr (1993-2015) [17]. In addition to dam construction upstream, sand mining activities are another factor contributing to the reduction in sediment and the increase in erosion in the VMD [18][19][20][21]. Approximately 27Mm 3 (around 57 Mm 3 ) of sediment are extracted annually, with 86% of it being sand, mainly occurring between 2008 and 2012 [22]. ...
Article
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The segment of the Tien River flowing through Tan Chau town is a meandering section with complex flow patterns, influenced by the flow dynamics and the lack of upstream sediment supply. This has resulted in the formation of deep erosional channels that affect the bank stability. This study focuses on evaluating the bed change of the Tien River within Tan Chau town through the analysis of measurement data and some scenarios from numerical modeling. The findings reveal that the Tan Chau - Hong Ngu curved section experiences severe channel erosion, particularly concentrated in the topographic channel and inclined towards the concave bank of Tan Chau (the inner bank). An analysis of causative factors indicates that reduced sediment supply due to dam construction and sand mining activities have led to changes in the riverbed, deeper erosion, and bank erosion near the sand mining area. The results quantifies the sediment deficit in the area at 0.86%, while the impact of sand mining in this area is 0.26%. The findings from this research provide a database to support local planning for bank protection projects and disaster mitigation measures due to bank erosion.
... A hydro dam can significantly change the sediment dynamics of its surroundings due to the reservoir's ability to trap a considerable amount of sediments upstream of the dam (Van Manh et al., 2015). According toKondolf et al. (2014), the amount of sediments trapped in a dam is directly proportional to the reservoir's size. ...
Book
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Proceedings of International Conference on Life Sciences, Engineering and Technology 2023 Editors Valarie Akerson, Ali Bicer, Sabri Turgut Volume 1, Pages 1-172 Proceedings of International Conference on Life Sciences, Engineering and Technology © 2023 Published by the ISTES Organization ISBN: 978-1-952092-46-6 Editors: Valarie Akerson, Ali Bicer, & Sabri Turgut Articles: 1-13 Conference: International Conference on Life Sciences, Engineering and Technology (iLSET) Dates: April 13-16, 2023 Location: Denver, CO, USA Conference Chair(s): Mack Shelley, Iowa State University, United States Stephen Jackowicz, University of Bridgeport, United States
... Ngoài tác động xây dựng đập ở thượng nguồn, hoạt động khai thác cát cũng là một nguyên nhân làm giảm lượng bùn cát, gia tăng xói lở ở khu vực ĐBSCL [13][14][15][16]. Lượng cát hàng năm bị khai thác khoảng 57 triệu tấn, trong đó 86% là cát chủ yếu diễn ra mạnh trong giai đoạn từ 2008 đến 2012 [17]. ...
... A diverse strategy which does not rely on concrete structures and which combines appropriate site-specific elements can respond in a flexible way to future scenarios about flow regimes and sediment patterns. The dynamic coastline of the Mekong Delta, for example, is largely influenced by sediment transport from the Mekong River which is predicted to diminish by 50% in 2050-2060 mainly due to hydropower development in the catchment area [87]. The need for a coastal defence strategy which is viable over time has also been identified as the solution for the dynamic mudbank mangrove system along the coast of Guyana [88]. ...
Chapter
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Development and the unsustainable use of natural resources in the coastal zone of the Mekong Delta, Vietnam, as well as erosion are threatening the protection function of the mangrove forests which protect the land behind the dyke from flooding and storms and provide co-benefits and livelihood for people in the coastal zone. These threats will be exacerbated by the impacts of climate change. Tidal flats and mangrove forests are an energy conversion system that provides ecosystem-based coastal protection. In sites where the mangrove belt has been destroyed and the tidal flat eroded, restoration of the tidal flats is a precondition for mangrove rehabilitation. Permeable bamboo fences, arranged in a T-shape, are effective for reducing erosion, stimulating sedimentation and thereby restoring tidal flats and re-creating conditions for mangrove regeneration. This cost-effective approach is only feasible within specific boundary conditions. Mangroves need to be protected from future anthropogenic destruction. This can best be achieved though co-management with the local people.
... We use the operation and maintenance costs reported in previous analyses (Danh and Khai, 2014;Hillen et al., 2010;Mai et al., 2008). Regarding the farm's net return by producing rice, we extract the rice yield and its price from GSO (GSO, 2023), production costs, and benefits of reducing rice intensification from the literature (Berg et al., 2017;Danh and Khai, 2014;Hung et al., 2014aHung et al., , 2014bManh et al., 2015;Tong, 2017;Tong and Clarke, 2020;Tran et al., 2018). We use annual discount rates of 0-3 % Danh and Khai, 2014) for all the calculations. ...
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The Mekong Delta is one of the most productive rice-producing regions in the world, exporting approximately one-fifth of the global rice traded annually. Previous studies note that saltwater intrusion is a serious concern, and the intensity of saltwater intrusion is primarily driven by sea level rise, land subsidence, anthropogenic sediment starvation, and upstream hydro-infrastructure developments. However, these studies often rely on scenario-based approaches instead of an integrated approach to assess the possible impacts of saltwater intrusion. Using an integrated hydrodynamic-statistical-economic model, we investigate how and the extent to which these drivers may impact the saltwater intrusion. We also examine the costs and returns of two popular saltwater intrusion control policies, i.e., hard-engineering structural and soft-land use planning. When comparing the baseline scenarios, the findings indicate that anthropogenic forces lead to a four times greater saltwater intrusion intensity than the climate change-induced sea level rise. The results further reveal a 50 % or less chance that annual saltwater-affected areas would exceed 1.93 million ha for the baseline, but the likelihood is highly likely to be 100 % with a sea level rising of 22 cm. Under the combined effects of sea-level rise, land subsidence, and riverbed incision, our model shows that the probability of annual saltwater-affected areas staying above 2.30 million ha is almost equal to one. This finding implies that a large share of the current rice-planted areas of the Delta could be wiped out of production for at least one season a year. The findings show that a combination of hard and soft policies would be a more sustainable and cost-effective strategy to lower the intensity and risks of saltwater intrusion. Therefore, there is an urgent need for better coordination of governance and investments among regions within the Delta and counties in the whole Mekong River Basin.
... For example, Cambodia's economically vital Tonle Sap Lake has experienced negative alterations in flood dynamics linked to the Mekong, resulting in a steep decrease in flood pulse, as reported by Chua et al. (2022). Downstream countries are thus expressing growing concerns about the changes in water patterns attributed to ongoing human activities (Räsänen et al., 2012;Van Manh et al., 2015;Räsänen et al., 2017;Wang et al., 2017;Pokhrel et al., 2018b). As a result, the LMR has gained significant attention and has become a subject of contention, not only for the six riparian countries but also for the international community . ...
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Study region: The Lancang-Mekong River (LMR) in Southeast Asia. Study focus: The LMR provides unparalleled economic and ecological resources for six Asian countries. However, the intensified impact of human activities, coupled with the effects of climate change, has led to alterations in its natural flow, thereby disrupting the river's flow regime. Ensuring the future sustainable management and downstream ecological integrity of this trans-boundary river requires understanding the regional contributions to the river's altered flow at the sub-basin level. We conducted an analysis for the last four decades using a hydrological model, which divided the entire basin into eight sub-basins, each associated with one of the eight main hydrological stations. New hydrological insights for the region: Our analysis showed significant regional differences in precipitation variation across the LMR, from its headwaters to the lower regions, during both periods. Furthermore, a spatial variation was observed for the impacts of human activities on the natural runoff in both seasons, indicating a lack of effective coordinated management efforts among riparian countries. Notably, our findings indicated that agricultural expansion and the construction of tributary dams in certain sub-basins had a more significant impact on the reduction of the mainstream flow compared to the mainstream dams. These findings underscore the enormous potential of regional water governance to alleviate the impacts of changing climate, offering valuable insights for addressing this critical issue.
... Tidal pumping can significantly influence the ocean circulation, land-ocean interaction, and further sediment mixing in the tidal reaches (Geyer et al., 2001;Van Manh et al., 2015). The strong sediment mixing from multiple sources in tidal reaches has been noticed in tideinfluenced rivers, e.g., Salmon River, Gironde River, and Pearl River (Dalrymple et al., 2012;Tessier, 2012;Savoye et al., 2012;Liu et al., 2015). ...
... Therefore, it is essential to consider all four components together to develop a comprehensive understanding of channel change in a fluvial system. This understanding can then be used to make accurate predictions about future changes, such as the effects of environmental or human-induced disturbances [9][10][12][13][14]. ...
... Therefore, it is essential to consider all four components together to develop a comprehensive understanding of channel change in a fluvial system. This understanding can then be used to make accurate predictions about future changes, such as the effects of environmental or human-induced disturbances [9][10][12][13][14]. ...
... We use the operation and maintenance costs reported in previous analyses (Danh and Khai, 2014;Hillen et al., 2010;Mai et al., 2008). Regarding the farm's net return by producing rice, we extract the rice yield and its price from GSO (GSO, 2023), production costs, and benefits of reducing rice intensification from the literature (Berg et al., 2017;Danh and Khai, 2014;Hung et al., 2014aHung et al., , 2014bManh et al., 2015;Tong, 2017;Tong and Clarke, 2020;Tran et al., 2018). We use annual discount rates of 0-3 % Danh and Khai, 2014) for all the calculations. ...
Preprint
This manuscript was accepted for publication in Science of The Total Environment.
... A direct consequence of the presence of dams is the reduction of river flow downstream, as well as the retention of sediments upstream (Alcérreca-Huerta et al., 2019;Le et al., 2021;Liro et al., 2020) and reduction of biodiversity (Ferrazzi and Botter, 2019;Poff et al., 2007;Ziv et al., 2012). . Such effects may be further exacerbated by climate change and rising sea levels (Arias et al., 2014;Bussi et al., 2021;Manh et al., 2015). The decrease in freshwater discharge and increase in sea level alter the estuary's hydrological conditions (Xu et al., 2016;Zhao et al., 2017), reducing the role of the river in controlling the hydrodynamic processes in the estuary, and increases the importance of marine effects like the tidal and saline intrusion into the estuarine basin. ...
Article
The São Francisco River is the fourth largest river in Latin America, and particularly important since it crosses the Brazilian semi-arid region. During the 1960s-90s, a series of dams for hydropower were built in cascade, changing drastically the flow regime. To evaluate the influence of changes in the flow on hydrodynamic processes and saline intrusion in the estuary, a three-dimensional hydrodynamic model was applied to the system. The model was calibrated and validated from hydrodynamic data collected in the estuary. The simulated scenarios of river discharges were Q min , Q 10 , Q 80 and Q max (corresponding to 600, 5000, 1200, and 12,000 m 3 s − 1 , respectively), determined from historical river discharge observations. There was an increase in the mixing and a decrease in the river contribution after the installation of dams. The average salinity in the estuarine domain ranged from 0.7 g kg − 1 for intermediate pre-dam discharge to 15.6 g kg − 1 in the lowest discharges after the dam installations. The results of the simulations were used to establish an exponential relationship between fluvial discharge and saline intrusion, which can be used by management agencies to optimize the water quality in catchments for human consumption.
... T.C. Nguyen et al. dates, which are often used as a benchmark. Dunn and Minderhoud (2022) summarized and discussed different projections about the future VMD development until 2050 related to anthropogenic impacts (Binh et al., 2020a;Bussi et al., 2021;Darby et al., 2016;Kondolf et al., 2014;Kummu et al., 2010;Manh et al., 2015;Nowacki et al., 2015;Schmitt et al., 2017). In all these projections the influence of short-term water level fluctuations, as they are presented in this study, are not considered. ...
... T.C. Nguyen et al. dates, which are often used as a benchmark. Dunn and Minderhoud (2022) summarized and discussed different projections about the future VMD development until 2050 related to anthropogenic impacts (Binh et al., 2020a;Bussi et al., 2021;Darby et al., 2016;Kondolf et al., 2014;Kummu et al., 2010;Manh et al., 2015;Nowacki et al., 2015;Schmitt et al., 2017). In all these projections the influence of short-term water level fluctuations, as they are presented in this study, are not considered. ...
Article
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Southeast Asian deltas are highly threatened areas for flooding as a response to the combined effects of natural compaction and subsidence exacerbated by human impacts, e.g. oil, gas and water extraction, retention of sediment discharge due to river damming and sand mining, land use changes, sea-level rise and storm-induced water-level setup. Tide-induced water-level fluctuations on different time- and spatial scales, seasonal variations of freshwater runoff and sea-level setup can amplify the impact of sea-level rise and of storm surges on the coastal environment and its inhabitants. Moreover, increasing populations accompanied by growing societal demands can lead to further pressure on delta areas. For the Vietnamese Mekong Delta (VMD) and the Saigon-Dong Nai River (SGDNR), rates of subsidence of several cm/yr have already been determined, but for the coastal area, which is most vulnerable to sea-level rise and storm surges, a lack of detailed spatial and temporal information of this parameter exists. To assess the influence of tide- and water-level fluctuations, records from 11 stations, from the tectonically stable position Vung Tau north of the SGDNR entry to Ganh Hao in the southern VMD, were analyzed. The results reveal a relative sea-level rise at Vung Tau reaching 2.2 ± 0.3 mm/yr for the period from 1987 to 2015, while along the VMD those rates show a spatially variation from 5.6 ± 0.3 mm/yr at Ben Trai to 13.5 ± 0.7 mm/yr at My Thanh. Considering Vung Tau as tectonically stable, these numbers indicate subsidence rates of the coastal sections along the VMD ranging from 3.4 ± 0.3 mm/yr to 11.3 ± 0.7 mm/yr. It is likely that the rates of subsidence along the coast of VMD have accelerated since 2005. They are not only up to 4 times higher than the mean sea-level rise but are variable along the whole SGDNR estuary and the VMD East Sea coastline. Additionally, the importance of daily, fortnightly and seasonal water level fluctuations due to tides and atmospheric influence, which are acting on top of the long-term relative sea-level rise, is shown. Especially the identified regional variations make these data valuable for setting regional priorities for protection strategies to mitigate riverine and marine flooding, especially when both coincide.
... Sediment reduction in the Vietnamese Mekong Delta due to upstream dam struction has also been studied extensively, and constitutes a process that also contri to shoreline erosion in the Mekong delta [1][2][3][4][5]. Moreover, other driving forces, inclu changing wind wave climatology with climate change, sea level rise, sand mining land subsidence due to the over-extraction of groundwater, have exacerbated sho erosion in the region [6][7][8]. Waves coming from the water's surface and traveling fro open sea to coastal areas can have a strong influence on coastal topography [9]. ...
Article
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The coastline of the Ca Mau and Kien Giang provinces in the Vietnamese Mekong Delta has been severely eroded in recent decades. Pile–Rock Breakwaters (PRBWs) are among the most widely adopted structures for controlling shoreline erosion in this region. These structures are effective for wave energy dissipation, stimulating sediment accumulation, and facilitating the restoration of mangrove forests. These breakwaters are generally considered to be the best-engineering practice; however, there is currently insufficient scientific evidence with regard to specific structural design aspects. This can lead to PRBW structures being compromised when deployed in the field. This study used a physical model of a PRBW in a laboratory to investigate several design parameters, including crest width and working states (i.e., submerged, transition, and emerged), and investigated their relationship with the wave transmission coefficient, wave reflection coefficient, and wave energy dissipation. To investigate these relationships further, empirical formulas were derived for PRBWs under different sea states and crest widths to aid the design process. The results showed that the PRBW width had a significant influence on the wave energy coefficients. The findings revealed that the crest width of the breakwater was inversely proportional to the wave transmission coefficient (Kt) under the emerged state. The crest width was also proportional to the wave reduction efficiency and wave energy dissipation in both working states (i.e., the submerged and emerged states). The front wave disturbance coefficient (Kf) was found to be proportional to the wave reflection coefficient, and the wave height in front of the structure was found to increase by up to 1.4 times in the emerged state. The wave reflection coefficient requires special consideration to reduce the toe erosion in the structure. Lastly, empirical equations including linear and non-linear formulas were compared with previous studies for different classes of breakwaters. These empirical equations will be useful for understanding the wave transmission efficiency of PRBWs. The findings of this study provide important guidance for PRBW design in the coastal area of the Mekong Delta.
... Sediment reduction in the Vietnamese Mekong Delta due to upstream dam construction has also been studied extensively, which also contributes to shoreline erosion in the Mekong delta [1][2][3][4][5]. Moreover, other driving forces, including changing wind wave climatology with climate change, sea level rise, sand mining, and land subsidence due to over-extraction of groundwater, have exacerbated shoreline erosion in the region [6][7][8]. Waves coming from the water surface and traveling from the open sea to coastal areas can have a strong influence on coastal topography [9]. ...
Preprint
Full-text available
The coastline in the Ca Mau and the Kien Giang provinces of the Vietnamese Mekong Delta has been severely eroded in recent decades. Pile-Rock Breakwaters (PRBW) are one of the most widely adopted structures for controlling shoreline erosion in this region. These structures are effective for wave energy dissipation, stimulating sediment accumulation, and facilitating the restoration of mangrove forests. These breakwaters are generally considered to be best-engineering practice however there is currently insufficient scientific evidence with regard to specific structural design aspects. This can lead to PRBW structures being compromised when deployed in the field. This study uses a physical model of a PRBW in a laboratory to investigate several design parameters, including crest width and working states (i.e. submerged, transition, and emerged), and investigates their relationship with the wave transmission coefficient, wave reflection coefficient, and wave energy dissipation. To investigate these relationships further, empirical formulas were derived for PRBWs under different sea states and crest widths to aid the design process. The results showed that PRBW width had a significant influence on the wave energy coefficients. The findings revealed that the crest width of the breakwater is inversely proportional to the wave transmission coefficient (Kt) under the emerged state. The crest width is also proportional to the wave reduction efficiency and wave energy dissipation in both working states (i.e., submerged and emerged states). The front wave disturbance coefficient (Kf) was found to be proportional to the wave reflection coefficient, and the wave height in front of the structure was found to increase by up to 1.4 times in the emerged state. The wave reflection coefficient requires special consideration to reduce the toe erosion in the structure. Lastly, empirical equations including linear and non-linear formulas were compared with previous studies for different classes of breakwaters. These empirical equations will be useful for understanding the wave transmission efficiency of PRBWs. The findings of this study provide important guidance for PRBW design in the coastal area of the Mekong Delta.
... Manh's 2014 results indicate that hydropower development is the dominant driver of the change to the floodplain sediment dynamics of the Mekong Delta [16]. If these planned developments are realised, overall floodplain sedimentation in the whole Mekong Delta would decrease by approximately 40%, and the sediment load to the South China Sea would be reduced to half of the current rates [17]. ...
Article
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This paper presents the result of suspended and deposited sediment measurements on the floodplain of Long Xuyen Quadrangle-An Giang (LXQ-AG) along the Cambodia-Vietnam border during a flood event in 2018. GIS and Remote sensing techniques were used to build maps for the flooded areas. Water level and discharge were also measured on main channels and on the floodplain. The two correlation equations between deposition sediment, SSC and water discharge were build. The results of Landsat interpretting showed that inundation area of the floodplain was 35,765 ha. SSC and deposited sediment decreased with distance from the main channels. Total sediment load (suspended and deposited sediment load) brought to the floodplain during a single flood event in 2018 was 5.09 million tons (14.24kg/m 2), of which total suspended sediment load was 70.58 thousand tons (0.2kg/m 2), total deposited sediment mass was 5.023 million tons (14.04 kg/m 2).
... Manh's 2014 results indicate that hydropower development is the dominant driver of the change to the floodplain sediment dynamics of the Mekong Delta (Manh, NV, 2014). If these planned developments are realised, overall floodplain sedimentation in the whole Mekong Delta would decrease by approximately 40%, and the sediment load to the South China Sea would be reduced to half of the current rates (Manh, NV, 2015). The silt shortage is not only related to losses of soil nutrient, but also a shortage of material to help protect the southern Mekong delta, the long-term process of adaptation to land loss by rising sea levels. ...
... Our case study is Vietnam, which as a one-party state, provides the opportunity to examine a political context with different understandings of citizen participation to those in the global North whose histories have shaped contemporary debates around citizenship. Additionally, Vietnam is one of the countries most affected by climate change and environmental degradation (Manh et al., 2015;Whitehead et al., 2019) and the government has taken steps to build a policy framework which recognises the need for environmental protection, while acknowledging the need to involve larger segments of the population in policy processes, including young people (Ministry of Home Affairs, 2012). Domestic and international NGOs and international organisations also work with young people outside of the party-state apparatus, often underpinned by discourses of empowerment and democracy. ...
Article
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Youth participation has become an important element of environmental governance and is also a way that young people learn about the expectations of citizenship. In the Global South, young people are confronted with multiple understandings of citizenship as international development organisations may introduce citizenship in a liberal, democratic framing which may differ from national citizenship norms. In Vietnam, state agencies have a history of supporting youth participation linked to nation building and community service. These activities create an imaginary which highlights citizenship as a status and the national scale as being central. More recently, liberalisation policies have opened the door for activities of international organisations which extend imaginaries of citizenship to the global scale and beyond status to a process centred on creating a sense of belonging. Other forms of participation are also flourishing, thanks to the increasing reach of social media. This paper explores how the diversity of this landscape creates liminal spaces of citizenship which young people navigate, working within and between different scales and imaginaries.
Chapter
Riverine floodplains are characterized by highly productive habitats, rich in biodiversity, and provide a wide array of ecosystem services.
Article
The Vietnamese Mekong River Delta (VMD) is one of the largest and lowest elevated deltas on Earth, shaped over the past thousands of years following delta progradation and sediment deposition. The geologically young delta sediments have high porosity and compressibility, resulting in high natural sediment consolidation (also known as autocompaction). Autocompaction is a natural intrinsic process that governs the spatio‐temporal morphological evolution and shallow compaction (i.e., land subsidence) in a delta. As a delta aggrades and progrades, the weight of accumulated sediments increases the effective stress experienced by underlying sediments, driving internal shallow compaction processes. Compaction of shallow sediments considerably contributes to land subsidence in the VMD, influencing the morphology and elevation of the delta plain and increasing the deltas exposure to natural hazards like flooding and relative sea‐level rise. In this study, we introduce a novel methodology to quantify sediment accumulation and autocompaction while taking into account the depositional history and heterogeneous nature of subsurface sediments in deltas like the VMD. We derived the depositional history, spatial heterogeneity and palaeo‐sedimentation rates by combining extensive datasets with lithological borelogs, sediment datings and geomechanical characterization of the delta's most representative lithologies. To simulate the spatio‐temporal formation and evolution of the delta over the last 4000 years, we employ the NATSUB3D finite element model to simulate sediment deposition and consolidation over time using an adaptive three‐dimensional mesh. The resulting 3D hydro‐stratigraphical and geomechanical characterization provides unique insights on past Holocene spatio‐temporal evolution of the VMD and current autocompaction dynamics. The model enables the prediction of shallow compaction rates under future sediment deposition and can facilitate process‐based quantification of delta elevation evolution under natural and human‐engineered sedimentation. This unlocks new opportunities to evaluate the effectiveness of nature‐based solutions and sediment enhancing strategies aimed to prevent elevation loss and combat relative sea‐level rise in the Mekong delta and similar lowly elevated coastal‐deltaic landforms elsewhere.
Article
Natural and socio‐economic development under climate change has affected the distribution of surface water resources (SWRs) in many deltas worldwide. The dynamics in the SWRs has negatively impacted agricultural production, which is one of the growing issues in the Vietnamese Mekong Delta (VMD) as the national rice bowl. Our study aims to assess SWR dynamics related to its effects on rice production in the upper VMD. We applied Mann–Kendall, linear regression and Pettit tests to statistically determine trends of SWRs in terms of water level, total water volume and rainfall in 1996–2020. The findings show that high water level trends decrease significantly after 2011, and this reduction has been due to climate variability and dam development upstream of the Mekong River. Moreover, rice yield data analysis from 1996 to 2020 indicated a decreasing trend in the yield of the winter–spring rice crop in the An Phu district of An Giang Province after 2011. Our interviews with 33 water experts and 90 local farmers revealed 80% agreement that the decline in water resources has affected rice yield. This study provides empirical evidence and a platform for knowledge sharing and learning across different deltas and initiatives worldwide.
Chapter
Major alterations have been caused to river systems by human activities. In this chapter the following are considered: dam and reservoir construction, interbasin water transfers, channelisation, changes in river channels as a result of land use and land cover changes, sedimentation of floodplains, deltas, and changes in flooding regimes.
Article
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Climate change, hydro-power dams and socio-economic development in the upper Mekong River, have caused changes in surface water resources and agricultural transformation in the Vietnamese Mekong Delta (VMD). It is unpredictable and influences decision-making processes on water resource management. This study aims at applying the DAPP (Dynamic Adaptive Policy Pathways) approach to develop solution roadmaps supporting rice production in the Mekong Delta projection to 2030 under the context of uncertain changes on surface water resources. Data were collected from local officers, farmers, experts at Can Tho University, Technical University Delft, and Utrecht University using semi-structured interview techniques including workshops, group discussions, and in-depth interviews. The consolidated solutions from stakeholder’s opinions were evaluated at different socio-economic and environmental dimensions, specially focused on the time of tipping point of each solution. The study results show that two solution roadmaps were developed as sample scenarios to promote rice production in the upper area of the VMD. The determination of the solutions roadmaps based on the agreement of the stakeholders, which is a new feature of this approach in linking many stakeholders for the general development of the region. The DAPP approach could be suitable to support decision-making on surface water resources management in the context of uncertain changes. Therefore, further studies can apply this approach to other regions in the VMD which will support to reinforce and demonstrate the applicability of the DAPP approach on supporting decision-making in the face of increasingly uncertain changes in the VMD.
Article
450 million people live on river deltas and thus on land that is precariously low above the sea level and sinking because of human activities and natural processes. Although global debates around coastal risk typically focus on sea level rise, it is sinking lands and rising seas that together endanger lives and livelihoods in river deltas. However, the ability to quantify and address those risks in and integrated manner remains limited. Herein, we identify four priority areas where a lack of data, models, and knowledge are limiting sustainable delta management, namely (1) developing practical models for delta-scale processes and nature-based solutions, (2) coupling models for basin and delta processes, (3) closing knowledge disparities between river deltas, and (4) integrating deltas in assessments of global change and vice versa. Addressing those challenges through global scientific efforts is instrumental to identify local-to-global levers to design adaptation and mitigation measures for resilient river deltas.
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Considering the dynamics of soil quality and pollution by potentially toxic elements (PTEs) in the soils due to sand mining in Nigeria, the study focused on the effect of upland sand mining on the agricultural land in Obowo, Southeastern Nigeria. Soil samples collected from soil depths (0-15, 15-30 cm) in eight traverse points and control, with points designated soil sampling point (SSP) ranging between SSP 1-SSP 8 and control sampling point (CSP 1). The results showed significant differences (p<0.05) among points. Results of the particle size distribution were in order of Sand ≥ Clay ≥ Silt, with Textural class ranging from sandy loams to loamy sandy. The mean soil pH ranged from 4.8-5.7 signifying moderate to strong acidity. The available P, Total N, SOC,SOM and BS ≤ control and FAO/World Reference Base. Effective cation exchange capacity (ECEC) (cmol/kg) (Ca 2+ , Mg 2+ , K + and Na +), with mean Ca 2+ 3.02, Mg 2+, 0.64, K + 0.09, Na + 0.10 ≤ control respectively. Exchangeable acidity recorded 1.04 ≥ control, and CEC:6.22 ≤ the control. With the nutrient ratings , the soil nutrients status of the sand mined sites ranged from very low to low status for total N(0.02-0.04),avail.P (6.55-9.96), exchangeable K, (0.07-0.14), exchangeable Ca (2.25-3.55), and exchangeable Mg (0.25-0.85). Except Chromium with 2.65, Copper recorded mean value of 15.21, Lead: 12.20 and Cadmium (1.80) ≥ control (zero) above the FAO/WHO permissible levels respectively. The potentially toxic heavy metals are in order of abundance in sand-mined soil: Cu ≥ Pb ≥ Cr ≥ Cd. Therefore, the area requires ecological restoration and regeneration of degraded mining site and the prohibition of indiscriminate mining activities as well as strict local control and enforcement of mining laws.
Thesis
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This thesis provides insights into the hydrological processes in the Cambodian Mekong delta, as well as their relationship to ecosystem services from which the local population benefits. In particular, the role of the Prek channels is examined. First constructed during the period of the French protectorate in the early20th century, these channels diverge perpendicularly from the Mekong and its deltaic distributary, the Bassac, and provide a connection between the river mainstream and its low-lying floodplains by breaching the elevated river banks. The aim of this thesis is to assess how large-scale changes in the Mekong basin,such as climate and land use change, as well as hydropower development, are reflected at a local scale in its delta. It also aims to elucidate how local infrastructure such as Preks catalyzes these processes and could help mitigate negative effects of future scenarios.To address all of these research questions, a multi-method approach was chosen. First, a thorough literature review of the complex challenges facing the Mekong basin and its delta was carried out. Subsequently, an analysis of long-term trends in water levels, precipitation patterns, and inundation extents and dynamics at the scale of the Cambodian Mekong delta is provided. Water level time series at four hydrological stations maintained by the Mekong River Commission (MRC) are subjected to a trend and break point analysis. Furthermore, precipitation data from three in-situ stations, as well as remotely sensed CHIRPS data are compared. Inundation extents are assessed on the basis of a new model developed in the course of this thesis: WAFL - Water Level Flood Link. WAFL uses in-situ water level time series and inundation maps derived from Sentinel-1 and -2 images taken between 2017 and 2020, and establishes a correlative link between them, accounting for flood propagation delays and attenuation. It can then be used to reconstruct past inundation extents based on water levels, overcoming spatial data sparsity. It is able to reconstruct earlier events (floods in the 1990’s and 2000s) with good accuracy, in comparison to historical Landsat-derived maps. This indicates that long-term changes of flood patterns are mostly due to shifts in boundary inflow, rather than in the local configuration of the link between the riverand its floodplains. Overall, average inundation durations have declined by 19 days and the incidence of inundations in the early wet season (July and August) has decreased by an average of to 21 Furthermore, fieldwork was carried out in a case study area on the banks of the Bassac river, which encompasses ten Preks. Local stakeholders, such as farmers and village chiefs, were interviewed to gain a better understanding of the ecosystem services linked to local hydrological processes, and the capacity in which the Preks catalyze these processes. The interviews also served as a basis to characterize typical cropping patterns and practices, from irrigation to average and maximum yield. To further enhance the understanding of the role of the Preks for local communities and agricultural practices archival research was undertaken. In addition, local land use and its variations depending on the spatial configuration of hydrological processes and Preks was analyzed using a land use - land cover (LULC) classification based on Sentinel imagery and a supervised Gradient Tree Boosting machine learning approach.Finally, to formalize the understanding of the role of Preks for local hydrological processes and ecosystem services in the Cambodian Mekong delta, and to simulate future scenarios, a numerical model was constructed in Python. It uses the data generated through interviews, literature research, and remote sensing analyses as input and simulates four future scenarios with respect to local climate variables, the configuration of Prek channels and river water levels, the latter depending on basin-wide climate change and hydropower development. Flood-related and non-flood related ecosystem services were simulated separately, to central role of annual monsoon inundations. The results show that under likely future precipitation, temperature, and water level patterns, agricultural production is set to decrease, but that this decrease could be compensated by Prek rehabilitation.
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Rice is the essential food crop of An Giang Province. Vietnam and the whole world are facing several problems hindering climate change, such as increased temperature and CO2 concentration that many manufacturers’ companies and managers need to estimate output to make production plans or adjust policies. In this study, the model known as SIMPLE was applied to simulate the biomass and yield of rice in 2 crop seasons Autumn - Winter 2020 (AW) and Winter - Spring 2020 - 2021 (WS), in Cho Moi district, An Giang province, Vietnam (10° 23' 47"N, 105° 27' 41"E) and analyzed the effects of climate variabilities and scenarios on simulation results. Heat stress showed a relatively negative impact on the growth and development of rice in AW more seriously than WS due to climate variabilities. Climate change scenario RCP8.5 (RCP - Representative Concentration Pathway) has predicted that atmosphere temperature may increase above 4 °C and CO2 concentration to reach 900 ppm by the end of the 21st century. As a result, from the model, for every 100 ppm CO2 concentration increase, the cumulative rice biomass increased by 8 and 10 % in AW and WS, respectively. Moreover, conditions assumed from the model that increased 5 °C caused a decrease in cumulative biomass up to 7.2 % in AW season compared to 3.1 % in WS season. However, with responses of 5 °C increasing in the model, rice yield decreased relatively rapidly from 8.5 % in AW and 7 % WS. HIGHLIGHTS The model known as SIMPLE has been used in this study RMSE of our model differs from the observed yield from 4.2 % (Winter-Spring crop-WS) to 5.5 % (Autumn-Winter crop-AW) For every 100-ppm CO2 concentration increased, the cumulative rice biomass increased by 8 and 10 % in AW and WS, respectively Increasing 5 °C, rice yield decreased 8.5 % in AW and 7 % WS Sensitivity analysis showed that RUE (Radiation Use Efficiency) has the most influencing factor on rice yield GRAPHICAL ABSTRACT
Article
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Trade-offs between socio-economic growth and environmental protection have remained a critical issue of sustainable development, especially in the Global South. In the floodplains of the Vietnamese Mekong Delta (VMD), the development of high-dike polders for intensive rice production has degraded ecosystems and changed socio-economic patterns. Sustainable development pathways must be considered during policy formulations to keep pace with such transformations. In this study, the interwoven socio-economic development and rice-based agricultural production processes were assessed based on mixed data sources, including 550 interviews with farmers in two major delta floodplain provinces – An Giang and Dong Thap. It highlights the pros and cons of the triple-rice farming systems under high-dike protections compared to low-dike farming systems. Results showed that the environmental degradation due to the overuse of agrochemicals (e.g., fertilizer and pesticides) costs approximately US $565 per hectare per crop season, resulting in the lower marginal benefits for the triple-rice production compared to the double-rice production pattern. This includes higher costs borne by local farmers/communities, given the adverse effects of agrochemicals on their health. The study urgently calls for local governments to consider relevant drivers of environmental degradation in agricultural production, especially in rice cultivation. Future policy needs to consider whether the intensification in agriculture, such as triple-rice production, would be an appropriate development pathway for the rural economy. Our study conveys to central and local governments and associated stakeholders that the agriculture-driven development policies would not be a sustainable development pathway under new environmental complexities in the delta.
Article
Sediment trapping by hydropower dams is one major cause of sediment starvation in global river deltas, but how climate change would influence the damming-induced sediment starvation remains unexplored. The Mekong River is the most important international river in Asia with rich biodiversity and massive hydropower potential. Here, we model the combined effect of climate change and damming on the Mekong River's sediment budget. Our study shows that six mega-dams in China currently reduce the sediment load into the Mekong Delta by 12.1%, while major downstream dams quadruple (49.7%) this reduction. The projected warmer and wetter climate may fully neutralize the impact of China's dams (>60% likelihood) and partially offset the impact of downstream dams. The neutralization effect comes with a notable shift in the sediment regime. In a rapid-change scenario, the basin-wide soil erosion, reservoir sedimentation and river sediment settling have changes of +9%, +36%, -135%, respectively, which degrades regional sustainability, particularly in the lower basin. Novel solutions from the perspective of the water-food-energy-climate nexus are urgently needed to maintain healthy sediment budgets in global mega-rivers including the Mekong River.
Article
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The Vu Gia Thu Bon basin constitutes the primary water supply in Central Vietnam. While climate change disturbs stream discharges and affects flood extremes, upstream dam development may intensify or mitigate such impacts. Therefore, this study provides a quantitative evaluation of long-term alterations in the flow regimes of the VGTB rivers from 1977 to 2020 resulting from the impacts of upstream anthropogenic developments. The datasets are divided into two periods, pre-2000 (1977-2000) and post-2000 (2001-2020), using different indices and analytical methods. The analyses show that since 2011, reservoir operations have reduced the maximum and high-flow discharges downstream in excess of climate change and land-use effects. However, due to the impact of water transfer by the Dak Mi 4 hydropower dam from the Vu Gia River to the Thu Bon River through a diversion channel, the minimum and low-flow discharges decreased in the pre-dam period and increased in the post-dam period.
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We adopted a spectral clustering algorithm to divide the document co-citation network of 1,776 papers in the field of Lancang-Mekong water, and 14 clusters were identified. For each cluster, the top-cited references construct the knowledge base, and the most-coverage cities are taken as the research frontier. Three indicators, namely betweenness centrality, citation burstness strength, and Sigma, were used to identify the research outputs with pioneering and transformative value. The changes in the research topics and hotspots are closely related to the planning, construction, and operation progress of hydropower engineering, that affected by the gaming results of all parties. The 2009–2010 is an important time boundary, with the original research hotspots including the impact of upstream reservoirs on the hydrological regime and sediment (Clu#3) and arsenic contamination of groundwater in the Lower Mekong (Clu#4) that obtained periodical achievements and reached consensus to some extent around 2008, and the new research boom turns to the Tonle Sap Lake and flood pulse (Clu#2) in short-term characterized literatures with the highest burstness strength mainly concentrated around 2012. HIGHLIGHTS The intellectual bases and research frontiers of 14 identified topics were analyzed based on Document Co-citation Network analysis.; Vietnamese Mekong Delta is the topic with the largest size and arsenic contamination is the cluster with the longest time span.; Evolution of research hotspots is related to the widespread controversy over the planned dams in the Lower Mekong mainstream and the progress of the multi-party games.;
Article
Extreme coastline erosion has become a major issue in Vietnam's Mekong Delta. Many coastal protection measures have been assessed based on their ability to dissipate incoming waves rather than their ability to facilitate environmental exchange and sedimentation processes. The goal of this study is to evaluate the ability of hollow and porous types of breakwaters to exchange suspended sediment on the deltaic coast. The experiments are set considering the hydrodynamic conditions of the Mekong Delta. We thought of four different types of breakwaters, including pile-rock porous breakwaters (CMD), two-sided perforated hollow breakwaters (TC1 & DRT/VTC), and curtain breakwaters (CWB45). Physical modeling in a wave flume was engaged to investigate the relationship between breakwater shape and sediment-capturing ability. All different breakwaters were placed on the wave flume with identical boundary conditions. Besides the physical modeling in the laboratory, Computational Fluid Dynamics (CFD) techniques using the FLOW3D model were applied. This provided an opportunity to understand the current distribution and the interactions between the fluid and the breakwater, as well as the ability to extract the vertical velocity profile and further additional insights from the interactions of waves and currents with the breakwaters. The wave parameters collected in the lab were further used to evaluate the numerical simulations in FLOW3D, and model validation demonstrated a good agreement with R² = 0.74–0.98 and NSE = 0.74–0.96 for FLOW3D calibration and validation. It was found that fine sand and mud silt deposition on the shoreline of the breakwater alignment were significantly reduced in the case of the perforated hollow breakwaters. Therefore, using these hollow breakwaters for wave energy dissipation and sediment trapping presents a dual purpose in creating an environmentally friendly solution to recover mudflats. Hollow breakwaters also provide numerous advantages for supporting the creation of favorable conditions for the revival of mangrove forests, development of regional biodiversity, and overall improvement of coastal ecosystems on the deltaic coast.
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The coastline in Ca Mau and Kien Giang provinces on Mekong Delta has been severely eroded in recent years. The Pile-Rock Breakwater (PRBW) was selected as predominant structure to construct widely more than 30 km on the west coast of Ca Mau. This structure shows the effectiveness of wave reduction, stimulating sediment accumulation and facilitating restoration of mangrove forest. However, this breakwater had been designed on the basis of best-engineering practice, lacking sufficient scientific background with regard to the structural design, capacity of wave reduction, working conditions. This study is to investigate the interaction of crest width, working states (submerged, transition and emerged) and the wave transmission coefficient, wave reflection coefficient and wave energy dissipation of PRBW by experiment in the laboratory and derive the empirical formulas for this construction under different sea states and crest widths. The results show a significant influence of width dimensions on the above coefficients. The findings revealed that the crest width of the breakwater is inversely proportional to the wave transmission coefficient (Kt) under emerged state. The crest width is also proportional to the wave reduction efficiency and wave energy dissipation in both working states (submerged and emerged states). The front wave disturbance coefficient is proportional to the reflected wave coefficient and the wave height in front of the structure can increase by 1.4 times in the emerged state. It is especially important to note when designing this structure to reduce the erosion in the structure toe due to the reflected waves. The empirical equations including linear and non-linear formulas have also developed and compared with previous studies for different breakwaters. This is necessary to be considered to calculate the structure and stability of breakwater. The results of this study serve as the basis for the design of pile-rock breakwater to reduce the waves under different natural conditions in the coastal area of the Mekong Delta.
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In this study, we used Landsat satellite imagery to monitor coastline changes in critical estuaries of the lower Mekong River basin from 1989 to 2017. To delineate the shoreline dynamics, we used the automated water extraction index (AWEI) and the digital shoreline analysis system (DSAS) to calculate the rate-of-change statistics from multiple historical shoreline positions. The rate of shoreline change was assessed using linear regression (LRR) and end point rate (EPR) methods. The erosion and accretion observed in the estuaries were intermingled, with total areas of 4,732.46 and 13,471.7 ha, respectively. Erosion and accretion impacted the shoreline rates considerably at a fast pace and the erosion and accretion speeds depended on each of the five periods analyzed in this study. From 2015 to 2017, the highest erosion rate (507.78 m y⁻¹) was recorded in the village of An Thuy in Ben Tre Province. From 2005 to 2009, the highest erosion rate (312.11 m y⁻¹) was recorded in the village of Thoi Thuan in Ben Tre Province. The accuracy (T) of the largest erosion/accretion areas during 2005-2009 was estimated to be 87.9%, with a kappa coefficient (K) of 0.76, while the lowest overall T (78.5%) and K (0.57) were obtained for the 1989-2001 period. We calculated the total estimated erosion and accretion areas from 2017 to 2019 (estimated to be 166.14 and 1,432.29 ha, respectively), and forecasted values for 2019-2025 (235.93 and 1,126.51 ha, respectively). This study confirms that satellite imagery and statistical methods can be reliably used to analyze the shoreline changes in coastal areas, which can then support policies addressing coastal planning and development.
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The transboundary Mekong River is facing two on-going changes that are estimated to significantly impact its hydrology and the characteristics of its exceptional flood pulse. The rapid economic development of the riparian countries has led to massive plans for hydropower construction, and the projected climate change is expected to alter the monsoon patterns and increase temperature in the basin. The aim of this study is to assess the cumulative impact of these factors on the hydrology of the Mekong within next 20–30 yr. We downscaled output of five General Circulation Models (GCMs) that were found to perform well in the Mekong region. For the simulation of reservoir operation, we used an optimisation approach to estimate the operation of multiple reservoirs, including both existing and planned hydropower reservoirs. For hydrological assessment, we used a distributed hydrological model, VMod, with a grid resolution of 5 km × 5 km. In terms of climate change's impact to hydrology, we found a high variation in the discharge results depending on which of the GCMs is used as input. The simulated change in discharge at Kratie (Cambodia) between the baseline (1982–1992) and projected time period (2032–2042) ranges from −11% to +15% for the wet season and −10% to +13% for the dry season. Our analysis also shows that the changes in discharge due to planned reservoir operations are clearly larger than those simulated due to climate change: 25–160% higher dry season flows and 5–24% lower flood peaks in Kratie. The projected cumulative impacts follow rather closely the reservoir operation impacts, with an envelope around them induced by the different GCMs. Our results thus indicate that within the coming 20–30 yr, the operation of planned hydropower reservoirs is likely to have a larger impact on the Mekong hydrograph than the impacts of climate change, particularly during the dry season. On the other hand, climate change will increase the uncertainty of the estimated hydropower impacts. Consequently, both dam planners and dam operators should pay better attention to the cumulative impacts of climate change and reservoir operation to the aquatic ecosystems, including the multibillion-dollar Mekong fisheries.
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Calibration of hydrodynamic models is – compared to other disciplines like e.g. hydrology – still underdeveloped. This has mainly two reasons: the lack of appropriate data and the large computational demand in terms of CPU-time. Both aspects are aggravated in large-scale applications. However, there are recent developments that improve the situation on both the data and computing side. Remote sensing, especially radar-based techniques proved to provide highly valuable information on flood extents, and in case high precision DEMs are present, also on spatially distributed inundation depths. On the computing side the use of parallelization techniques brought significant performance gains. In the presented study we build on these developments by calibrating a large-scale 1-D hydrodynamic model of the whole Mekong Delta downstream of Kratie in Cambodia: we combined in-situ data from a network of river gauging stations, i.e. data with high temporal but low spatial resolution, with a series of inundation maps derived from ENVISAT Advanced Synthetic Aperture Radar (ASAR) satellite images, i.e. data with low temporal but high spatial resolution, in an multi-objective automatic calibration process. It is shown that an automatic, multi-objective calibration of hydrodynamic models, even of such complexity and on a large scale and complex as a model for the Mekong Delta, is possible. Furthermore, the calibration process revealed model deficiencies in the model structure, i.e. the representation of the dike system in Vietnam, which would have been difficult to detect by a standard manual calibration procedure.
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Sediment dynamics play a major role in the agricultural and fishery productivity of the Mekong Delta. However, the understanding of sediment dynamics in the delta, one of the most complex river deltas in the world, is very limited. This is a consequence of its large extent, the intricate system of rivers, channels and floodplains, and the scarcity of observations. This study quantifies, for the first time, the suspended sediment transport and sediment deposition in the whole Mekong Delta. To this end, a quasi-2D hydrodynamic model is combined with a cohesive sediment transport model. The combined model is calibrated using six objective functions to represent the different aspects of the hydraulic and sediment transport components. The model is calibrated for the extreme flood season in 2011 and shows good performance for 2 validation years with very different flood characteristics. It is shown how sediment transport and sediment deposition is differentiated from Kratie at the entrance of the delta on its way to the coast. The main factors influencing the spatial sediment dynamics are the river and channel system, dike rings, sluice gate operations, the magnitude of the floods, and tidal influences. The superposition of these factors leads to high spatial variability of sediment transport, in particular in the Vietnamese floodplains. Depending on the flood magnitude, annual sediment loads reaching the coast vary from 48 to 60% of the sediment load at Kratie. Deposited sediment varies from 19 to 23% of the annual load at Kratie in Cambodian floodplains, and from 1 to 6% in the compartmented and diked floodplains in Vietnam. Annual deposited nutrients (N, P, K), which are associated with the sediment deposition, provide on average more than 50% of mineral fertilizers typically applied for rice crops in non-flooded ring dike floodplains in Vietnam. Through the quantification of sediment and related nutrient input, the presented study provides a quantitative basis for estimating the benefits of annual Mekong floods for agriculture and fishery, and is an important piece of information with regard to the assessment of the impacts of deltaic subsidence and climate-change-related sea level rise on delta morphology.
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Quantity and quality of sediment deposition in complex floodplains are affected by many processes that are typically highly spatially and temporally variable and hard to quantify exactly. The main processes in this context are suspended sediment transport dynamics in rivers, floodplain channel interactions, and internal floodplain processes. In consequence, any point measurement of sedimentation in floodplains contains a high degree of uncertainty, both stemming from measurement errors and from the lack of representativeness for a larger area. However, up to now, uncertainty analyses have not been performed as part of publications on floodplain sedimentation data. Therefore the present work illustrates a field sampling strategy aiming at the monitoring of floodplain deposition and spatial variability on a large scale and at the quantification of uncertainties associated to sediment deposition data. The study was performed in the Mekong Delta, being an example for a large and complex floodplain with a high degree of anthropogenic disturbances. We present a procedure for the quantification of the uncertainty associated to the data, based on the design of the monitoring campaign, sampling procedures, and floodplain characteristics. Sediment traps were distributed strategically over the floodplain in clusters of three mat traps representing one monitoring point. The uncertainty originating from collection of the traps from still ponding water is quantified by lab experiments. The overall uncertainty of the deposition samples and the associated nutrient content is quantified in a Monte Carlo simulation and illustrated by uncertainty bounds. For the study area the results show a very high variability of the annual floodplain deposition (2.2–60 kg m−2) with uncertainty bounds ranging from −61 to +129% relative to overall mean deposition of 11.4 kg m−2. No correlations in the spatial distribution of sedimentation in the floodplains could be found. This is caused by the highly complex channel and dike system and the high number of hydraulic structures. Also, no differences in deposition between floodplain compartments protected with high and low dikes could be detected. However, it can be shown that within single floodplain compartments the spatial deposition variability depends on the dike levels and operation and location of hydraulic structures.
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Rising sea level poses critical ecological and economical consequences for the low-lying megadeltas of the world where dependent populations and agriculture are at risk. The Mekong Delta of Vietnam is one of many deltas that are especially vulnerable because much of the land surface is below mean sea level and because there is a lack of coastal barrier protection. Food security related to rice and shrimp farming in the Mekong Delta is currently under threat from saltwater intrusion, relative sea level rise, and storm surge potential. Understanding the degree of potential change in sea level under climate change is needed to undertake regional assessments of potential impacts and to formulate adaptation strategies. This report provides constructed time series of potential sea level rise scenarios for the Mekong Delta region by incorporating (1) aspects of observed intra- and inter-annual sea level variability from tide records and (2) projected estimates for different rates of regional subsidence and accelerated eustacy through the year 2100 corresponding with the Intergovernmental Panel on Climate Change (IPCC) climate models and emission scenarios.
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The downstream impact of dams is a complex problem in watershed management. In the upper Mekong River watershed and its main channel, dam construct ion projects were started in the 1950s to meet increasing demands for energy and food production. Dams called the Mekong Cascade were completed on the Mekong River in China, the Manwan Dam in 1996 and the Dachaoshan Dam in 2003. We evaluated the impact of the Manwan Dam and its related watershed development on seasonal water discharge and suspended sediment transportation using hydrologica l simulations of target years 1991 (before dam construction) and 2002 (after dam completion). Our study area was the main channel of the Mekong River in northern Thailand extending about 100 km downstream from the intersection of Myanmar, Thailand and Laos. We used the MIKE SHE and MIKE11 (Enterprise) models to calculate seasonal changes of water discharge and sediment transport at five points 15- 35-km apart in this interval. Sediment load was calculated from a regression equation between sediment load and water discharge, using suspended sediment concentrations in monthly river water samp les taken between November 2007 and November 2008. Finally we estimated annual sediment load alo ng the study reach using from both of simulated ann ual hydrograph and the regression equation. Our simulat ions showed that after construction of the dam, the re was a moderate decrease in peak discharge volume and during the rainy season in August and September and a corresponding increase in the subsequent mont hs. Accordingly, sediment transportation budgets we re increased in months after the rainy season. The sus pended sediment transportation in Chiang Sean was increased from 21.13 to 27.90 (M ton/year) in our m odel condition.
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The Mekong River, largely undeveloped prior to 1990, is undergoing rapid dam construction. Seven dams are under construction on the mainstem in China and 133 proposed for the Lower Mekong River and tributaries. We delineated nine distinct geomorphic regions, for which we estimated sediment yields based on geomorphic characteristics, tectonic history, and the limited sediment transport data available. We then applied the 3W model [Minear and Kondolf, 2009] to calculate cumulative sediment trapping by these dams, accounting for changing trap efficiency over time and multiple dams on a single river system. Under a 'definite future' scenario of 38 dams (built or under construction), cumulative sediment reduction to the Delta would be 51%. Under full build-out of all planned dams, cumulative sediment trapping will be 96%. That is, once in-channel stored sediment is exhausted, only 4% of the pre-dam sediment load would be expected to reach the Delta. This scenario would have profound consequences on productivity of the river and persistence of the Delta landform itself, and suggests that strategies to pass sediment through/around dams should be explored to prevent the consequences of downstream sediment starvation.
Article
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Sediment dynamics play a major role for the agricultural and fishery productivity of the Mekong Delta. However, the understanding of sediment dynamics in the Mekong Delta, one of the most complex river deltas in the world, is very limited. This is a consequence of its large extent, the intricate system of rivers, channels and floodplains and the scarcity of observations. This study quantifies, for the first time, the suspended sediment transport and sediment-nutrient deposition in the whole Mekong Delta. To this end, a quasi-2-D hydrodynamic model is combined with a cohesive sediment transport model. The combined model is calibrated automatically using six objective functions to represent the different aspects of the hydraulic and sediment transport components. The model is calibrated for the extreme flood season in 2011 and shows good performance for the two validation years with very different flood characteristics. It is shown how sediment transport and sediment deposition vary from Kratie at the entrance of the Delta to the coast. The main factors influencing the spatial sediment dynamics are the setup of rivers, channels and dike-rings, the sluice gate operations, the magnitude of the floods and tidal influences. The superposition of these factors leads to high spatial variability of sediment transport, in particular in the Vietnamese floodplains. Depending on the flood magnitude, the annual sedimentation rate averaged over the Vietnamese floodplains varies from 0.3 to 2.1 kg m-2 yr-1, and the ring dike floodplains trap between 1 and 6% of the total sediment load at Kratie. This is equivalent to 29 × 103-440 × 103 t of nutrients (N, P, K, TOC) deposited in the Vietnamese floodplains. This large-scale quantification provides a basis for estimating the benefits of the annual Mekong floods for agriculture and fishery, and is important information for assessing the effects of deltaic subsidence and climate change related sea level rise.
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The Mekong Basin in Southeast Asia is facing rapid development, impacting its hydrology and sediment dynamics. Although the understanding of the sediment transport rates in the Mekong is gradually growing, the sediment dynamics in the lower Mekong floodplains (downstream from Kratie) are poorly understood. The aim of this study is to conduct an analysis to increase the understanding of the sediment dynamics at the Chaktomuk confluence of the Mekong River, and the Tonle Sap River in the Lower Mekong River in Cambodia. This study is based on the data from a detailed field survey over the three hydrological years (May 2008 - April 2011) at the two sites (the Mekong mainstream and the Tonle Sap River) at the Chaktomuk confluence. We further compared the sediment fluxes at Chaktomuk to an upstream station (i.e. Mukdahan) with longer time series. Inflow sediment load towards the lake was lower than that of the outflow, with a ratio on average of 84%. Although annually only a small amount of sediment load from the Tonle Sap contributes to the delta (less than 15%), its share is substantial during the February-April period. The annual sediment load transport from the confluence to the delta in 2009 and 2010 accounted for 54 and 50 Mt, respectively. This was on average only 55% of the sediment fluxes measured at Mukdahan, more upstream station. And when compared to sediment loads further downstream at Cambodia-Vietnam border, we found that the suspended sediment flux continued to decline towards the South China Sea. Our findings thus indicate that the sediment load to the South China Sea is much lower than the previous estimate 150-160 Mt/yr. This article is protected by copyright. All rights reserved.
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The Mekong is one of the world's great rivers. It has the greatest mean annual flow in the world for a river basin of comparable size. The flow regime, with very distinct wet and dry seasons, supports a rich biodiversity and the world's largest freshwater fishery. Given that at the present time the hydrological regime of the Mekong remains in its natural state, the accelerating pace of water resources development will induce hydrological change. The natural productivity of the system is therefore potentially jeopardized. This paper reports the findings of simulation studies of the potential hydrological impacts of water resource development scenarios over future planning horizons. In the Definite Future scenario (next 5 years), the seasonal redistribution of water by on-going hydropower development will increase the dry season flow by 40–60% in the upper portion of the basin and by 20–30% in the Mekong Delta. The Foreseeable Future scenario (next 20 years) and Long-Term Future scenario (next 50 years) will result in relatively small changes to the flow regime as further increases in dry season reservoir releases will be offset by planned increases in irrigation and other consumptive water demands. All scenarios were predicted to reduce the average wet season flows by 4–14%, flow reversal to the Tonle Sap Lake by 7–16%, flooded areas by 5–8% and salinity intrusion areas in the Viet Nam Delta by 15–17%. Predicted changes in Definite Future scenario will be irreversible, necessitating improved coordination between the LMB countries and cooperation with China in order to manage the risks and maximize the regional benefits. The scenario assessments highlighted the areas where research is necessary to mitigate and manage impacts in order to ensure the reasonable and equitable use of the Mekong basin's water resources. Copyright © 2013 John Wiley & Sons, Ltd.
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Many of the world’s largest deltas are densely populated and heavily farmed. Yet many of their inhabitants are becoming increasingly vulnerable to flooding and conversions of their land to open ocean. The vulnerability is a result of sediment compaction from the removal of oil, gas and water from the delta’s underlying sediments, the trapping of sediment in reservoirs upstream and floodplain engineering in combination with rising global sea level. Here we present an assessment of 33 deltas chosen to represent the world’s deltas. We find that in the past decade, 85% of the deltas experienced severe flooding, resulting in the temporary submergence of 260,000 km2. We conservatively estimate that the delta surface area vulnerable to flooding could increase by 50% under the current projected values for sea-level rise in the twenty-first century. This figure could increase if the capture of sediment upstream persists and continues to prevent the growth and buffering of the deltas.
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Quantity and quality of sediment deposition in complex floodplains are affected by many uncertain factors, ranging from suspended sediment transport dynamics in rivers and floodplain channel interactions to internal floodplain processes. In consequence, any point measurement of sedimentation in floodplains contains a high degree of uncertainty calling for a careful analysis of the measured data. However, uncertainty analyses are not documented in publications on floodplain sedimentation data. Therefore the presented work illustrates a field sampling strategy aiming at the quantification of uncertainties associated to sediment deposition data, as well as the spatial variability of sediments deposition on floodplains. The study was performed in the Mekong Delta (MD), being an example for a large and complex floodplain with a high degree of anthropogenic disturbances. We present a procedure for the quantification of the uncertainty associated to the data, based on the design of the monitoring campaign and floodplain characteristics. Sediment traps were distributed strategically over the floodplain in clusters of three mat traps representing one monitoring point. The uncertainty originating from collection of the traps in ponding water is quantified by lab experiments. The uncertainty of a single monitoring point is then quantified in a Monte Carlo simulation, propagating the uncertainty from the different uncertainty sources to final uncertainty bounds of the monitored sediment data. For the case study area, it is shown that there are no correlations in the spatial distribution of sedimentation in floodplains. This can be explained by the highly complex channel and dike system and the high number of hydraulic structures. However, it can be shown that within single floodplain compartments the spatial deposition variability depends on the dike levels and operation and location of hydraulic structures.
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Changes in the sediment load of a river can have important impacts on river channel evolution, nutrient fluxes, aquatic ecology and delta erosion and sedimentation, and the possibility of changes in the sediment load of the Lancang-Mekong River has attracted increasing concern in recent years. Existing studies present conflicting findings regarding the nature and magnitude of recent changes in the sediment load of the Lancang-Meking River and the authors have attempted to assemble the most reliable data on annual sediment loads for the period 1965–2003, to assess recent trends in the sediment load of the river. The changes in annual sediment load at 7 stations on the river are analyzed. Important sediment contributing areas are found in the reaches between Gajiu and Yunjinghong, Chiang Saen and Luang Prabang and downstream of Nong Khai. The sediment load increased at Gajiu, Yunjinghong and Chiang Saen over the period 1985–1992 because of serious soil erosion caused by the expansion of cultivation, the replacement of natural forest by plantations and land disturbance associated with hydropower dam construction. A marked reduction in sediment load occurred at Gajiu after the impoundment of the Manwan Hydropower dam on the Lancang River, but this reduction was not evident downstream at Yunjinghong and the stations further downstream. Significant increases in sediment load appeared at Mukdahan and Khong Chiam. These contrasting patterns of change reflect the influence of sediment contributions from the intervening catchment areas and channel systems as well as storage and remobilization of sediment from the channel system and the impact of hydraulic works such as irrigation systems. The long term mean annual sediment load of the Mekong River at its mouth is estimated to be ca. 145×106 t a−1, which is lower than previously reported values and it seems likely that this will be reduced in the foreseeable future.
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Identification of spatial gradients in ecosystem vulnerability to global climate change and local stressors is an important step in the formulation and implementation of appropriate countermeasures. Here we build on recent work to map ecoregional exposure to future climate, using an envelope-based gauge of future climate stability—defined as a measure of how similar the future climate of a region will be to the present climate. We incorporate an assessment of each ecoregion’s adaptive capacity, based on spatial analysis of its natural integrity—the proportion of intact natural vegetation—to present a measure of global ecosystem vulnerability. The relationship between intactness (adaptive capacity) and stability (exposure) varies widely across ecoregions, with some of the most vulnerable, according to this measure, located in southern and southeastern Asia, western and central Europe, eastern South America and southern Australia. To ensure the applicability of these findings to conservation, we provide a matrix that highlights the potential implications of this vulnerability assessment for adaptation planning and offers a spatially explicit management guide.
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The Tonle Sap Lake of Cambodia is the largest freshwater body of Southeast Asia, forming an important part of the Mekong River system. The lake has an extremely productive ecosystem and operates as a natural floodwater reservoir for the lower Mekong Basin, offering flood protection and assuring the dry season flow to the Mekong Delta. In light of the accelerating pace of water resources development within the Mekong Basin and the anticipation of potentially significant hydrological impacts, it is critical to understand the overall hydrologic regime of Tonle Sap Lake. We present here a detailed water balance model based on observed data of discharges from the lake's tributaries, discharge between Mekong and the lake through the Tonle Sap River, precipitation, and evaporation. The overland flow between the Mekong and lake was modelled with the EIA 3D hydrodynamic model. We found that majority (53.5%) of the water originates from the Mekong mainstream, but the lake's tributaries also play an important role contributing 34% of the annual flow, while 12.5% is derived from precipitation. The water level in the lake is mainly controlled by the water level in the Mekong mainstream. The Tonle Sap system is hence very vulnerable, from a water quantity point of view, to possible changes in the Mekong mainstream and thus, development activities in the whole Mekong basin. From a biogeochemical point of view, the possible changes in the lake's own catchment are equally important, together with the changes in the whole Mekong Basin. Based on our findings, we recommend of continuing the monitoring programmes in lake's tributaries and urgently starting of groundwater measurement campaign within the floodplain, and including the groundwater modelling to be part of the hydrodynamic models applied for the lake.
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The flood pulse is a key element characterizing the hydrology of the Mekong River and driving the high ecosystem productivity in the Lower Mekong floodplains, both in the Cambodian lowlands and the Mekong Delta in Vietnam. This paper assesses the impacts of climate change, both in terms of changed basin water balance and sea level rise, on the Lower Mekong flood pulse. The impacts were simulated by a three-dimensional hydrodynamic model using the projected changes in sea level and the Mekong mainstream discharge under the influence of climate change as boundary conditions. The model simulations projected that average and maximum water levels and flood duration increase in 2010–2049. The most consistent and notable changes occurred in the average and dry hydrological years. Sea level rise had the greatest effects in the Mekong Delta, whereas the impacts of changed basin water balance were more notable in the upper areas of the Mekong floodplains. The projected impacts were mostly opposite to those resulting from regional water infrastructure development. Higher and longer flooding could cause damage to crops, infrastructure and floodplain vegetation, and decrease the fertile land area. On the other hand, it might boost ecosystem productivity and enhance dry season water availability.
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The Mekong River Basin in Southeast Asia is experiencing extensive hydropower development. Concerns have been raised about the consequences of the development for the ecosystems, livelihoods and food security in the region. The largest planned hydropower dam cascade in the basin, the Lancang-Jiang cascade, is currently under construction and already partly built into the Upper Mekong Basin, China. In this paper we assess the impact of the Lancang-Jiang cascade on downstream hydrology by using a combination of a hydrological model and a reservoir cascade optimization model. The hydrological changes were quantified in detail at the Chiang Saen gauging station in Thailand, the first gauge station downstream from the cascade, and in lesser detail at four other downstream locations in the Mekong mainstream. We found that on average the Lancang-Jiang cascade increased the December–May discharge by 34–155 % and decreased the July–September discharge by 29–36 % at Chiang Saen. Furthermore, the Lancang-Jiang cascade reduced (increased) the range of hydrological variability during the wet season (dry season) months. The dry season hydrological changes were significant also in all downstream gauging stations, even as far as Kratie in Cambodia. Thus the Mekong’s hydrological regime has been significantly altered by the Lancang-Jiang cascade, but what the consequences are for ecosystems and livelihoods, needs further study.
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The transboundary Mekong River is facing two ongoing changes that are expected to significantly impact its hydrology and the characteristics of its exceptional flood pulse. The rapid economic development of the riparian countries has led to massive plans for hydropower construction, and projected climate change is expected to alter the monsoon patterns and increase temperature in the basin. The aim of this study is to assess the cumulative impact of these factors on the hydrology of the Mekong within next 20–30 yr. We downscaled the output of five general circulation models (GCMs) that were found to perform well in the Mekong region. For the simulation of reservoir operation, we used an optimisation approach to estimate the operation of multiple reservoirs, including both existing and planned hydropower reservoirs. For the hydrological assessment, we used a distributed hydrological model, VMod, with a grid resolution of 5 km × 5 km. In terms of climate change's impact on hydrology, we found a high variation in the discharge results depending on which of the GCMs is used as input. The simulated change in discharge at Kratie (Cambodia) between the baseline (1982–1992) and projected time period (2032–2042) ranges from −11% to +15% for the wet season and −10% to +13% for the dry season. Our analysis also shows that the changes in discharge due to planned reservoir operations are clearly larger than those simulated due to climate change: 25–160% higher dry season flows and 5–24% lower flood peaks in Kratie. The projected cumulative impacts follow rather closely the reservoir operation impacts, with an envelope around them induced by the different GCMs. Our results thus indicate that within the coming 20–30 yr, the operation of planned hydropower reservoirs is likely to have a larger impact on the Mekong hydrograph than the impacts of climate change, particularly during the dry season. On the other hand, climate change will increase the uncertainty of the estimated reservoir operation impacts: our results indicate that even the direction of the flow-related changes induced by climate change is partly unclear. Consequently, both dam planners and dam operators should pay closer attention to the cumulative impacts of climate change and reservoir operation on aquatic ecosystems, including the multibillion-dollar Mekong fisheries.
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The Mekong Fish Database developed by the Mekong River Commission provides the most comprehensive data source on Mekong fishes, although it does not include any information on species from areas upstream of Yunnan and in Myanmar. Current speed and channel morphology change as the Mekong River flows downstream affect the species that favor specific parts of the river, and the upper and lower reaches of the Mekong harbor different fish faunas. The Mekong River mainstream and the lowland sections of tributaries are highly turbid during the flood season because of the heavy sediment load. Catfishes are well adapted to this environment with their sensitive barbels that allow them to locate food without the help of their eyes. The threadfins use their filamentous pectoral fins in a similar way and both groups are well adapted to an environment with almost no light penetration. Some species that show preference for certain parts of the mainstream are the endemic Mekongina erythrospila and Labeo dyocheilus that have a preference for fast-flowing water and are rare or absent downstream of Kratie where there are no rapids.
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