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•Sediment bypass tunnels (SBT) deliver sediment downstream of dams.•Benthic invertebrate communities after SBT installation were monitored for 16 years.•A dramatic recovery within a ...few years was suggested based on community similarity.•Filterer and large-sized taxa were replaced by scraper and small-sized taxa.•Additional actions are needed to maintain habitat heterogeneity in the long-term.
A sediment bypass tunnel (SBT) is a dam facility used to mitigate reservoir sedimentation and is expected to contribute to river ecosystem restoration by delivering sediment to the degraded downstream channel. We analyzed past monitoring data of benthic invertebrates sampled downstream of the Asahi Dam in Nara Prefecture Japan to understand the long-term changes in the invertebrate community after the installation of the SBT. Before the SBT installation, the reaches upstream of the dam (one Up site) and downstream of the dam (two Down sites) showed different invertebrate communities, dominated by invertebrates that preferred clean cobble beds, sand and gravel layers, or run habitats in the former and invertebrates that preferred stable or porous beds or pool habitats in the latter. A significant increase in the Up–Down site community similarity within a few years after the SBT installation indicated a rapid recovery of the invertebrate community at the Down sites. During the 16 years of the after-SBT period, the total density and taxon richness at the Down sites increased in the early half, while they decreased in the latter half due to bed changes associated with the continuous accumulation of sediments. The change in dominant invertebrates from taxa of the large sizes to those of the small sizes and the increase in the interhabitat community similarity suggest that the bed of the Down sites became finer and flatter by sand and gravel that filled interstitial spaces of cobbles in the late stage. While the invertebrate community at the Down sites recovered dramatically by the sediment supply through the SBT, additional actions (e.g., cobble augmentation, installation of flow guiding structures) to maintain a balanced sediment size distribution and developed pool-riffle structures would be necessary to maintain the potential habitat heterogeneity and biodiversity in the long term. Our study also demonstrated that invertebrates responded to the sediment supply according to their habitat preferences, which suggests that the classifications of invertebrates by habitat-related traits (e.g., bed-residence types and pool-riffle types in this study) would help to properly understand the sediment and bed status of river reaches under channel degradation or aggradation.
Human interventions at the river basin scale, such as sand mining and hydropower dam construction, have profoundly affected hydrological and hydraulic alteration regimes, sediment budgets, and ...morphological changes worldwide. Quantifying the consequences of unsustainable ongoing sand mining and hydropower is crucial for obtaining sediment load data and managing hydrogeomorphology. In this study, comprehensive long-term consecutive four-field monitoring, statistical methods, and hydrological models (SWAT) were applied to quantify the spatiotemporal changes in long-term discharge and sediment load from 1996 to 2020 for the tropical river of the Vu Gia Thu Bon (VGTB) in the central region of Vietnam. The SWAT model was calibrated from 1996 to 2010, validated from 2011 to 2020 and showed good performance for daily discharge and monthly sediment. The evolution of river bathymetric data (2010, 2015, 2018, and 2021) was analysed to clarify the upstream sediment supply trapped in the riverbed and how the sand mining volume was removed. The results showed that the mean annual sediment in the Vu Gia and Thu Bon Rivers decreased by 57.3% and 23.8%, respectively, in the postdam period compared with the predam period. The thalweg elevation decreased at the Ai Nghia and Giao Thuy stations from 2010 to 2021 by 1.8 m and 3.9 m, respectively. The water level decreased by 21.1% at Ai Nghia and 44.3% at Giao Thuy. Dam development, sand mining, and changes in land use are the main factors responsible for flow discharge and sediment morphodynamic alterations. Morphological change have increased the water transfer rate from the Vu Gia River to the Thu Bon River through the Quang Hue channel. Downstream of the Vu Gia River, water transfer and riverbed incision have decreased flow discharge and water level and increased saltwater intrusion in recent years. As a result, water shortages induced by saltwater intrusion during drought periods have emerged as a significant constraint in hindering the domestic water supply and agricultural production.
This open access book brings together research studies, developments, and application-related flash flood topics on wadi systems in arid regions. The major merit of this comprehensive book is its ...focus on research and technical papers as well as case study applications in different regions worldwide that cover many topics and answer several scientific questions. The book chapters comprehensively and significantly highlight different scientific research disciplines related to wadi flash floods, including climatology, hydrological models, new monitoring techniques, remote sensing techniques, field investigations, international collaboration projects, risk assessment and mitigation, sedimentation and sediment transport, and groundwater quality and quantity assessment and management. In this book, the contributing authors (engineers, researchers, and professionals) introduce their recent scientific findings to develop suitable, applicable, and innovative tools for forecasting, mitigation, and water management as well as society development under seven main research themes as follows: Part 1. Wadi Flash Flood Challenges and Strategies Part 2. Hydrometeorology and Climate Changes Part 3. Rainfall–Runoff Modeling and Approaches Part 4. Disaster Risk Reduction and Mitigation Part 5. Reservoir Sedimentation and Sediment Yield Part 6. Groundwater Management Part 7. Application and Case Studies The book includes selected high-quality papers from five series of the International Symposium on Flash Floods in Wadi Systems (ISFF) that were held in 2015, 2016, 2017, 2018, and 2020 in Japan, Egypt, Oman, Morocco, and Japan, respectively. These collections of chapters could provide valuable guidance and scientific content not only for academics, researchers, and students but also for decision-makers in the MENA region and worldwide.
Hydraulic infrastructures, such as reservoirs and water diversion channels, cause altered streamflow worldwide. Therefore, this study aimed to assess the coupled impacts of reservoir operations and ...water transfer on downstream streamflow over 42 years (1979–2020) for a tropical river in Vietnam, the Vu Gia Thu Bon (VGTB). We also quantified variations in the multi‐sub‐basin contributions to the water budget associated with hydraulic structure development. Therefore, a semi‐distributed hydrological model, SWAT (Soil and Water Assessment Tool), was developed for the entire VGTB basin considering two plausible scenarios: with a dam and without a dam. In this study, reservoirs substantially affected the streamflow during the 2011–2020 period when 12 cascading hydropower dams were constructed in the Vu Gia sub‐basins. The cascading reservoirs across the Vu Gia River reduced the annual average streamflow by 28.1% during this period, whereas their influence was augmented by 13.9% at reaches further downstream. In contrast, the local reservoir and flow diversions created on the Thu Bon River resulted in a 6.5% increase in streamflow. The upstream reservoir operation significantly increased streamflow at the midstream stations by 27.8% compared to the no‐dam period. The streamflow decreased in the dry season by 5.6% in the Vu Gia sub‐basins and increased by 61.7% in the Thu Bon sub‐basins. However, the impacts decreased in the wet season by 41.3% due to the operation of reservoirs, in which Dak Mi 4 had the most significant influence. It was found that the water diverted to the Thu Bon River was governed and reduced by the cascading hydropower dams. Therefore, the operation of 11 reservoirs has partially compensated for the lost water in the Vu Gia sub‐basins, to which the Dak Mi 4 plant has transferred 19.7 m3/s (14%). Our findings classify the impact of cascading dams and diversion structures and their interaction with climate change.
Impacts of hydraulic infrastructures and contributions of the sub‐basins on streamflow in the Vu Gia Thu Bon River basin, Vietnam
This study presents two machine learning models, namely, the light gradient boosting machine (LightGBM) and categorical boosting (CatBoost), for the first time for predicting flash flood ...susceptibility (FFS) in the Wadi System (Hurghada, Egypt). A flood inventory map with 445 flash flood sites was produced and randomly divided into two groups for training (70%) and testing (30%). Fourteen flood controlling factors were selected and evaluated for their relative importance in flood occurrence prediction. The performance of the two models was assessed using various indexes in comparison to the common random forest (RF) method. The results show areas under the receiver operating characteristic curves (AUROC) of above 97% for all models and that LightGBM outperforms other models in terms of classification metrics and processing time. The developed FFS maps demonstrate that highly populated areas are the most susceptible to flash floods. The present study proves that the employed algorithms (LightGBM and CatBoost) can be efficiently used for FFS mapping.
Comprehensive restoration programs are expected to influence sediment‐associated microbial community structure and functional diversity following changes attributed to the restoration of habitat ...characteristics in the dam‐impacted channel.
To address if the construction or recreation of in‐channel structures, i.e., gravel bars, by implementing gravel augmentation and ecological flow restoration, resulted in habitat restoration and enhanced environmental heterogeneity, we profiled the community composition, estimated diversity, and annotated putative metabolic functions of the sediment microbial communities of the dam‐regulated Trinity River in northern California.
Our results provided supporting evidence on the positive impact of habitat restoration conducted in the Trinity River with the non‐dam influenced, undisturbed tributaries as the basis of comparison. In addition, gravel bar recreation and restoration contributed to the increased microbial beta diversity, possibly through the increased environmental heterogeneity at the river scale.
The significant positive correlation between the taxonomic and functional diversity of the identified microbial taxa suggests that differences in the detected putative metabolic functions were closely related to dissimilarities in community composition. We also provided valuable insights into the potential microbial processes in the sediment that might be contributing to the biogeochemical processes carried out by the microbial communities in the river.
The results of this study have implications on the impact of construction and restoration of gravel bars in a dam‐impacted river on environmental heterogeneity and how this influences the taxonomic and functional diversities of sediment‐associated microbial communities.
Riverine ponds, which are formed and sustained through sediment erosion and deposition, are key habitats for enhancing biodiversity in river reaches. The objective of this study was to understand the ...roles of traditional river-training wooden structures called “seigyu” on the formation of ponds on nonvegetated bars. Here, the spatial and temporal patterns of the flow and bedform coupled with pond formations for several flood events were assessed. The surface flow patterns were monitored by an unmanned aerial vehicle (UAV) and evaluated by large-scale particle image velocimetry (LSPIV); the maximum flow velocities were 1.3 m s−1 and 1.9 m s−1 during floods when seigyu units were partially and fully submerged, respectively. Although the overall mean flow velocity was greater for the latter events, the spatial variation in flow velocity and dissipation rate of turbulent kinetic energy (TKE) around seigyu was greater for the former events. Such flow patterns affected both bed formation and ecological habitats; ponds were formed at locations beside and behind seigyu, where the flow converged and bed scouring occurred during floods. The frequency and size of ponds around seigyu increased in the early half of the season, and they decreased in the other half when floods were greater in magnitude, which suggests that the bed scouring effect of seigyu was greater in flood stages with partial than fully submergence. Although the bar ponds lack shade to temper the effects of incident light and atmospheric conditions, the ponds displayed smaller daily oscillations in temperature than did the main river, probably due to hyporheic water supply to the ponds. Because many aquatic species cannot tolerate extremely high temperatures in summer, the generation and maintenance of deeper ponds by scouring with sufficient water exchange with the hyporheic zone can be key to enhancing colonization by various aquatic species.
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•Flow behavior around river training structures were monitored by UAV during floods.•Flow was diverted and converged due to the structures in partially submerged floods.•The ponds developed by bed sours around the structures on a bar after the floods.•The ponds were filled by sand and dewatered after fully submerged floods.•Water temperature fluctuation was smaller for the ponds than river main flow.
•Flow regime alterations in 2009–2015 are more pronounced than in 1993–2008.•Dams in 1993–2008 are unable to increase dry flows and reduce flood flows.•Dam effects on flow regimes in 2009–2015 are ...greater than climate change effects.•Riverbed incision causes decreased dry water levels in the Vietnamese Mekong Delta.•Decreased dry water levels has increased salinity intrusion.
The Mekong basin, where climate change and anthropogenic interventions (e.g., dams, sand mining, and sluice gates) have intensified in the recent decades affecting the pristine flow regime and salinity intrusion.
This paper aims at quantifying the flow regime alterations in the entire Mekong from 1980 to 2015 and linking with the controlling drivers of alterations. In this regard, various indicators, analytical methods, and a semi two-dimensional hydrodynamic and advection-dispersion model were used.
The flow regime alterations in the high-dam development period (2009–2015) are more pronounced than in the low-dam development period (1993–2008), compared to the no-dam development period (1980–1992), based on most of the indicators analyzed. In the high-dam development period all existing dams with large reservoir capacity seemed to have cumulatively reduced the flood pulses and frequency and increased the low-flow discharge along the entire Mekong through reservoir operations, exceeding climate change effect. In the recent years the water levels in the low-flow season in the Vietnamese Mekong Delta (VMD) have decreased, possibly because of increased riverbed incision caused by reduced sediment supply and increased sand mining. The reduced water levels together with the increased number of the sluice gates constructed seemed to have increased salinity intrusion in the VMD which may be partly reduced by early emergency water release from upstream dams.
The hydrogeomorphology of the Vietnamese Mekong Delta (VMD) has been significantly altered by natural and anthropogenic drivers. In this study, the spatiotemporal changes of the flow regime were ...examined by analysing the long‐term daily, monthly, annual and extreme discharges and water levels from 1980 to 2018, supported by further investigation of the long‐term annual sediment load (from the 1960s to 2015), river bathymetric data (in 1998, 2014 and 2017) and daily salinity concentration (from the 1990s to 2015) using various statistical methods and a coupled numerical model. Then, the effects of riverbed incision on the hydrology were investigated. The results show that the dry season discharge (i.e., in March–June) of the Tien River increased by up to 23% from the predam period (1980–1992) to the postdam period (1993–2018) but that the dry season water level at My Thuan decreased by up to −46%. The annual mean and monthly water levels in June at Tan Chau and in January and June–October at My Thuan in the Tien River decreased statistically, even though the respective discharges increased significantly. These decreased water levels instead of the increased discharges were attributed to the accelerated riverbed incision upstream from My Thuan, which increased by more than three times, from a mean rate of −0.16 m/year (−16.7 Mm3/year) in 1998–2014 to −0.5 m/year (−52.5 Mm3/year) in 2014–2017. This accelerated riverbed incision was likely caused by the reduction in the sediment load of the VMD (from 166.7 Mt/year in the predam period to 57.6 Mt/year in the postdam period) and increase in sand mining (from 3.9 Mm3 in 2012 to 13.43 Mm3 in 2018). Collectively, the decreased dry season water level in the Tien River is likely one of the main causes of the enhanced salinity intrusion.
The hydrogeomorphology of the Vietnamese Mekong Delta has been significantly altered by natural and anthropogenic drivers. By analysing the long‐term discharge, water level, sediment, and bathymetric data, supported by a coupled 2D hydro‐sediment‐morpho dynamics model, we observed that riverbed incision was the main cause of the decreased dry season water levels in the Tien River, although the respective discharges increased. Consequently, the decreased water level was likely one of the main causes of the enhanced salinity intrusion.
The Vu Gia Thu Bon (VGTB) 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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