Cascade Dams Research Articles

Dam cascade alters taxonomic composition of benthic macroinvertebrate community in upper Yangtze River

AbstractCascade dams segment river connectivity, modify the hydrological process, and change the hydraulic properties, thus influencing the river ecosystem significantly. Ecological effects of dams have been extensively studied, but investigations on zoobenthos in a large area covering tributaries and mainstream with cascade dams are lacking. Here, we studied the features of benthic macroinvertebrate assemblages in the lower Jinshajiang River, which is the upper reach of the Yangtze River. Over 15,000 macroinvertebrate individuals were collected in the mainstream and eight tributaries. Altogether, 105 taxa belonging to 19 orders and 55 families were identified. The structure of benthic macroinvertebrate assemblages was considerably different between the natural‐flow reach and the reservoir reach in the mainstream. The diversity and abundance of Ephemeroptera, Trichoptera, and Megaloptera decreased to almost zero as a result of dam impoundment. By contrast, the abundance of Diptera, Chironomidae, and Tubificida increased significantly in the mainstream reservoir reach. The diversity of macroinvertebrate species in the mainstream reservoir reach was only half of that in the natural‐flow mainstream. The difference between macroinvertebrate assemblages in natural‐flow mainstream and tributaries was insignificant. Therefore, dam construction was considered the main cause of macroinvertebrate assemblage changes. Our results highlighted the necessity of maintaining sufficient natural river sections between cascade reservoirs to conserve the biodiversity of benthic macroinvertebrate.

Regional scale hydrodynamic modeling of the river-floodplain-reservoir continuum

River floodplains and reservoirs interact throughout a basin drainage network, defining a coupled human-water system with multiple feedbacks. Recent modeling developments have aimed to improve the representation of such processes at regional to continental scales. However, most large-scale hydrological models adopt simplified lumped reservoir schemes, where an offline routine is run with inflows estimated by the model, with limited consideration of the complementarity between floodplains and reservoirs on altering the hydrological regime at regional scale. This paper presents a novel approach that fully couples river-floodplain-reservoir hydrodynamic and hydrological models, significantly improving the representation of reservoir dynamics and operation in the river-floodplain-reservoir continuum at large scale and across multiple dam cascades. The model is applied to the Paraná River Basin with explicit simulation of 31 large dams and river hydraulic variables at basin scale. Three types of reservoir bathymetry representation are compared, from lumped to distributed methods, combined with three reservoir operation schemes and varying degrees of input data requirement within two parameterization scenarios (global and regional setups). The operation schemes were more relevant than the reservoir bathymetry representation to estimate downstream flows and water levels. While the data-driven operation scheme, based on linear regressions between observed water levels and dam outflows, provided the best estimates of both active storage and discharges, the more generic operation reasonably estimated discharges and peak attenuation, albeit not as accurately for active storage. The global parameterization of reservoir operation resulted in poorer performance compared to the regional-based one, but it satisfactorily modeled discharge and peak attenuation. Regarding the reservoir bathymetry representation, a basin scale comparison of the lumped and distributed schemes indicated the inability of the former to represent backwater effects. This was further corroborated by validating the longitudinal water level profile of Itaipu dam with ICESat satellite altimetry data. Finally, the model was used to show the complementarity between floodplains and reservoirs on attenuating floods at regional scale. Large scale models should move beyond offline coupling strategies, and include regional-based, data-driven reservoir operation schemes together with a distributed representation of reservoir bathymetry into river-floodplain hydraulic schemes. This will largely improve the estimation of river discharges, water levels and flood storage, and thus the model ability to represent the regional scale river-floodplain-reservoir continuum.

Patterns of Fish Assemblage, Growth, and Diet Composition in a Tropical River between Two Cascading Hydropower Dams

Construction of cascade dams has been shown to have impacts on fish assemblages and biodiversity. Yet, there is no literature on fish assemblages in the Murum River that connects the cascading Bakun and Murum dams in Sarawak, Malaysia. Hence, study on this modified ecosystem is necessitated to better understand the effects of the cascade dam construction on fish fauna. For this, fish samples were caught at five stations located along the river during both dry and wet seasons. Environmental parameters were taken concurrently with fish sampling. Length-weight relationship, condition factors, and diet composition of selected fish species in the river were also determined. The present study demonstrated that there are indications of the impact of cascading dams on the formation of a complex ecosystem in the Murum River, that is, changing from the shallow downstream of the Murum Dam to the deep transitional and inundated zone of the Bakun reservoir. The transitional zone in the Murum River exhibited the lowest fish species diversity, richness, and evenness during the dry season due to low pH and DO coupled with high turbidity. The biological indices improved when the water quality improved during the wet season. On the contrary, the diversity and evenness indices at the inundated tributary station decreased remarkably during the wet season, likely due to the migration of fish during the onset of the rainy season. This study showed that Barbonymus schwanenfeldii has a wider feeding habit which contributes to its higher distribution and abundance in the Murum River. The growth patterns of B. schwanenfeldii, Cyclocheilichthys apogon, Hampala macrolepidota, Lobocheilos ovalis, and Osteochilus enneaporos were better during wet than dry season. Overall, the condition factor of all native fish species in the Murum River was in poor to fair condition, whereas the exotic species, Oreochromis mossambicus, exhibited excellent condition (K value > 2) for both seasons. The increase in the number of O. mossambicus coupled with its high condition factor indicates biological intrusion and a potential threat to the native fish species in the Murum River. Continuous monitoring is essential to detect in-time risk issues associated with environmental degradation and biological invasion in this regulated and inundated river ecosystem.

Effects of upstream cascade dams and longitudinal environmental gradients on variations in fish assemblages of the Three Gorges Reservoir

AbstractThe construction of dams creates a series of reservoirs, causing severe environmental changes and a biodiversity crisis. We aimed to investigate the relative roles of the construction of upstream cascade dams and the longitudinal gradient in environmental conditions in the spatial and temporal variations in fish assemblages in a downstream reservoir. Fish assemblage and abiotic data from four longitudinally distributed reaches in the Three Gorges Reservoir (TGR), China—one of the largest reservoirs worldwide—were collected over 8 years (2011–2018) and analysed. The results showed that the blockage to fish passage, as well as the flow modification, caused by the upstream dams and reservoirs can significantly affect the variations in fish assemblages downstream of the TGR; fish assemblages in the upper area of the TGR were more vulnerable to upstream dam construction than those in the lower area. The longitudinal environmental gradient was more important in determining the spatial and temporal distributions of fish assemblages in the reservoir than the combined effects caused by the upstream dams and reservoirs. The spatial distribution pattern of fish assemblage structure in the TGR was mainly determined by the gradient difference in the flow regime among stations, whereas the construction of two upstream dams mainly affected the temporal variations in fish assemblages of the TGR by altering the downstream flow and thermal regimes and blocking fish movement down to the TGR.

Effects of cascade reservoirs on the transformation of nitrogen in pore water of sediments in the Lancang River

AbstractIn the mainstream of the Lancang River, eight cascade dams have been constructed. In order to find the effects of the cascade reservoirs on the transformation of nitrate (NO3‐N) and ammonium (NH4‐N) in pore water of sediments, a field survey was carried out both in the upper natural river reach (NRR) and cascade reservoirs reach (CRR) of the Lancang River from January 10 to 20, 2018. The results indicated that (1) in the NRR, NO3‐N concentrations in sediment pore water were higher than those in the CRR, whereas the total nitrogen (TN) and NH4‐N were significantly lower; the diffusion fluxes of NH4‐N from sediment to overlying water varied from −0.180 to 0.049 mg·(m2·d)‐1, and the diffusion fluxes of NO3‐N varied from 0.012 to 0.098 mg·(m2·d)‐1. (2) in the CRR, the NH4‐N concentrations in sediment pore water were much higher than those in the overlying water, but the NO3‐N concentrations in the overlying water were much higher; the diffusion fluxes of NH4‐N from the sediments to the overlying water varied from 0.161 to 1.561 mg·(m2·d)‐1, but the NO3‐N diffused downward from the overlying water to the sediments and the diffusion fluxes varied from −0.011 to −0.223 mg·(m2·d)‐1. It was concluded that the cascade reservoirs could increase the NH4‐N concentration and inversely decrease the NO3‐N concentration in the sediment pore water, which meant that the risk of the nitrogen release from the sediment in the cascade reservoirs was much higher than that in the natural river.

Integrating hydrological, landscape ecological, and economic assessment during hydropower exploitation in the upper Yangtze River

Comprehensive investigation of hydrological processes associated with landscape ecology and economic development plays a key role in watershed management, and is less developed in watersheds with large-scale cascade dams. With the abundant hydropower resources and its unprecedented advantages, hydropower exploitation in the upper Yangtze River (Jinsha River) is critical to energy structure adjustment in China. Therefore, we integrated hydrological modeling, landscape ecology analysis, and economic analysis in the dammed Jinsha River. With climate variations in the Jinsha River Basin, the average flow near the uppermost dams in the mainstream grew from 796 m3 s−1 (1990s), to 918 m3 s−1 (2000s), and further to 1025 m3 s−1 (2010s). During 1991 to 2017, the source power in the headwater region grew slightly, but varied little in the downstream area. In the lower dammed Jinsha River, analysis of landscape indicators showed that the landscape was enriched, while the landscape type distribution was more uniform. Moreover, hydropower exploitation brought benefits to regional economic development. Principal component analysis further highlighted the landscape ecological and economic variations with high loadings in the first principal component. With the non-significant temporal variations and normal spatial fluctuations in flow discharge, the landscape pattern was basically stable, and the utilization of hydropower can be sustainable in the Jinsha River. In addition, hydropower development drove local economic development. Based on the integrated analysis of hydrological, landscape ecological, and economic assessment at the watershed scale, our results stressed the significance of hydropower exploitation in the Jinsha River. However, more attention should be paid to the warming climate during hydropower exploitation. These findings are valuable for the scientific planning of hydropower bases in watersheds with large-scale cascade dams, and have substantial implications for sustainable hydropower development.

Analisa Keruntuhan Bendungan Batujai dan Pengga dengan Aplikasi HEC-RAS 5.0.7

ABSTRACT: Batujai and Pengga Dam are cascade dams with homogeneous types, are authorized in 1982 and 1994. These dams are located at the same mainstream that ends at Alas Strait. Batujai and Pengga Dam break may occur due to overtopping and piping. According to HEC-RAS 5.0.7 simulation suggested that the collapsed of each or both dams has the biggest impact when conditioned with design flood of QPMF are due to overtopping, when the peak of discharge are at 2036.151 m3/second and 3204.089 m3/second, which caused 34.127 km2 and 107.025 km2 area to be flooded, with maximum inundation of 19.656 meters and 19.837 meters. According to InaSAFE, if the collapse of these dams were merged, reported that there are 34 villages and 29,100 people are exposed to the risk, classified with high disaster risk. The cost that has to be paid by the dam’s owner due to the collapse is estimated to be  Rp 201,584,808.36/Ha or Rp. 2,845,409,886,907 in total.

A cascade of dams affects fish spatial distributions and functional groups of local assemblages in a subtropical river

Abstract Dams reduce the longitudinal connectivity of rivers and thereby disrupt fish migration and the spatial distribution of species, impacts that remain poorly studied for some Neotropical rivers from mega-diverse basins. We investigated the spatial distribution of fish species with different trophic and movement/reproductive/size characteristics to assess how functional groups have responded to a cascade of dams on the Uruguai River in southern Brazil. Fish abundance, biomass, and species composition were evaluated at eight locations along the longitudinal gradient. The fish assemblage in the upper stretch was mainly characterized by small and medium-sized species at higher trophic levels, whereas the sites located furthest downstream displayed more medium and large-sized species, including many carnivorous species. Species with high fecundity, seasonal migrants, and catfishes with internal fertilization were common in the river´s middle and lower reaches. Detritivorous species dominated areas distant from the dams. Overall, functional diversity of local fish assemblages was greater in lower reaches. The cascade of dams has impacted the distribution of functional groups of local fish assemblages of Uruguai River. The alteration of functional groups in upper reaches of the river has potential consequences for ecosystem processes and services, such as nutrient cycling and fisheries.

Self-purification of water in the Soła River dam cascade and the possibility of supporting activities

Self-purification of water in the Soła River dam cascade and the possibility of supporting activities

River damming and drought affect water cycle dynamics in an ephemeral river based on stable isotopes: The Dagu River of North China

The flow regime and biogeochemical cycles are greatly affected by river damming and drought, especially in ephemeral rivers. However, the combined effects have been rarely considered. This study, taking the Dagu River in Jiaodong Peninsula of North China as an example, investigated the dynamic changes in water cycle related to river damming and drought using stable water isotopes for the period 2018–2019. The results indicated that river water isotopes significantly varied temporally and spatially. The temporal variations in river water isotopes appeared to be linked with those in precipitation, but the relationship between river water and precipitation isotopes was greatly affected by river damming, river water-groundwater exchange and potential water pollution. Spatially, a single dam exhibited no significant effect on river water isotopes, but the accumulative impacts of cascade dams resulted in the enrichment of heavy isotopes in river water towards the downstream through increasing hydraulic residence time and water evaporation largely. The inter-annual variations in river water isotopes with increased evaporative fractionation were highlighted by their strong response to the drought in 2019. The combined effects of cascade dams and drought greatly changed water cycle dynamics and further exacerbated water shortage, which should thus be fully considered for water resource management, especially for regions with water-limited but heavily-regulated rivers.

Mismatch between critical and accumulated temperature following river damming impacts fish spawning

Dam operations considerably influence water temperature regimes in rivers, which affects fish spawning activities. Previous studies have focused on the effects of critical temperature (CT) alterations during the spawning period, and largely ignored the effects of accumulated temperature (AT) alterations on gonadal development. Successful spawning relies on the simultaneous achievement of the two thermal requirements at appropriate times. River damming may cause a mismatch between the times of achieving CT and AT thresholds, and in turn influence fish reproduction. In the present study, spawning events of Coreius heterodon (C. heterodon) from 2009 to 2015 in the upper reaches of the Yangtze River, which are under the influence of cascade dams, were analysed based on the times of achievement of CT and AT thresholds. The CT and AT thresholds for C. heterodon spawning were 18.4 °C and 1324.9 °C·d, respectively. Under pre-impoundment conditions, the time of achievement of the AT threshold was 23 d on average later than that under post-impoundment conditions; however, the time of achievement of the CT threshold was similar under both conditions. The time of achievement of the AT threshold was 10 d earlier than that of achievement of the CT threshold in post-impoundment conditions. Earlier achievement of AT thresholds was followed by reduced spawning. The alteration of temperature rhythm caused by reservoir operations could be the major factor decreasing spawning abundance after river damming. The results of the present study could facilitate sustainable reservoir operations with regards to water temperature management, and thereby improve the conservation of fish resources.

RATIONALIZATION OF DISCHARGE DATA FLUCTUATIONS USING MINIMUM RESIDUALS MODEL OF MOVING AVERAGE METHODS IN CITARUM CASCADE RESERVOIRS

Since 1981, a cascade of dams consisting of the Saguling, Cirata, and Jatiluhur reservoirs has been constructed along the Citarum River in order to fulfill demands for flood control, water supply, and electrical power generation, and this requires the implementation of an appropriate hydrological approach. The efficient operation of a water resource system requires an accurate water flow analysis that approximates the actual fact as closely as possible to minimize risks in decision-making. Discharge is a random and stochastic component in hydrology, influenced by rainfall variations with a maximum in the tropical wet season and a minimum in the dry season. Thus, discharge analysis requires a statistical approach especially minimum residuals. Values for average annual inflows has constructed through four different calculation techniques for the moving average method to rationalize the fluctuations in discharge data Citarum cascade reservoirs by reducing randomness in the data, with the eventual aim of obtaining more rational trend lines. An analysis of minimum residuals between the results of the calculation and those of model regressions has demonstrated that the cumulative moving average calculation technique provides the best fit among all the models with the smallest residual value, as shown in its lowest mean squared error values of 0.83, 2.04, and 1.77 for the three reservoirs.

Carbon biogeochemical processes in a subtropical karst river–reservoir system

Cascade damming has seriously affected the hydrologic regimes and solutes transport of the river systems. However, the effect of damming on carbon cycle and the underlying mechanisms are still unclear. In this study, to reveal the effects of damming on riverine dissolved inorganic carbon (DIC) transport, the water chemistry and stable isotopic signature of dissolved inorganic carbon (δ13CDIC) were analyzed in the tributaries and four subtropical karst reservoirs on the Pearl River in southern China. From reservoir epilimnion to hypolimnion, DIC concentrations increased from 183.2 to 229.4 mg L−1, 167.6 to 192.0 mg L−1, 179.1 to 181.5 mg L−1, and 182.4 to 183.8 mg L−1 while δ13CDIC decreased from −6.6‰ to −9.7‰, −7.8‰ to −10.4‰, −9.7‰ to −10.0‰ and −9.9‰ to −10.0‰ in the Chaishitan (CST), Longtan (LT), Yantan (YT), and Dahua (DH) reservoirs, respectively. Our analyses suggest that primary production and organic matter degradation were two dominant processes in carbon evolution in the epilimnion and hypolimnion, respectively. However, there was no obvious spatial heterogeneity of solutes’ concentration in the DH reservoir. Our results also suggested that cascade reservoirs had a regional damming effect and could be significantly weakened without damming or in daily regulated reservoirs. Furthermore, δ13CDIC had a significant positive relationship with the reservoir effect index (Ri), which was affected by hydraulic retention time and water temperature in the water column of reservoirs and is useful to evaluate the thermal stratification strength. This study highlights that dam constructions affected carbon transport and transformation significantly, and the degree of the damming effect on the carbon cycle can be expressed by Ri.

Do bacterioplankton respond equally to different river regulations? A quantitative study in the single-dammed Yarlung Tsangpo River and the cascade-dammed Lancang River

River damming has raised controversial concerns as it simultaneously contributes to socioeconomic development but may jeopardize aquatic ecology. Since bacterioplankton catalyze vital biogeochemical reactions and play important roles in aquatic ecosystems, more attention has been paid to their responses in dammed rivers. Here, a comparative study was conducted between single-dammed (the Yarlung Tsangpo River) and cascade-dammed (the Lancang River) rivers in Southwest China to investigate whether bacterioplankton respond equally to different river regulations. Our results showed that the decreased bacterioplankton abundance and the increased α-diversity always co-occurred in reservoirs of the Yarlung Tsangpo River and the Lancang River. However, the impact of damming on bacterioplankton abundance and α-diversity were resilient in the Lancang River, which can be attributed to the repeated alterations of environmental heterogeneity in cascade damming reaches. Meanwhile, a generalized additive model (GAM) was applied to identify the important drivers affecting bacterioplankton variation. The abundance was influenced by trophic conditions, such as dissolved silicon, while α-diversity was closely related to the microbial dispersal process, such as elevation and distance-from source. And it is also noted that the bacterioplankton dispersal process was interrupted in cascade damming reaches. In addition, based on their important drivers, variations in abundance and α-diversity were also predicted by GAM. As revealed by the quantitative mutual validation between the two rivers, abundance and α-diversity in the cascade-dammed river can be predicted by their response to single-dammed river, suggesting that the impact of cascade damming on bacterioplankton can be pre-assessed by referring to the single stage damming effect. Therefore, our study provides the first trial of quantitative evidence that bacterioplankton do not respond equally to different river regulations, and the impact of cascade damming on bacterioplankton can be predicted based on single stage damming effect, which can contribute to the protection of aquatic ecology in the cascade hydropower development.

Biogeography of eukaryotic plankton communities along the upper Yangtze River: The potential impact of cascade dams and reservoirs

The effects of large cascade dams on eukaryotic plankton communities in freshwater river-reservoir ecosystems are probably uncharted in previous studies. The purpose of this research was to investigate the large-scale genetic diversity and biogeography patterns within large cascade dams and to reveal the key drivers shaping eukaryotic plankton community. Eukaryotic plankton communities of 24 sampling sites along the upper Yangtze River were assessed based on Illumina Miseq sequencing. Generally, the mainly dominant eukaryotic plankton communities were Cryptophyta, Fungi and Alveolata, with these three clades encompassing 52.36%, 24.34% and 15.26% of all sequences. Generally, the Shannon diversity was higher in the dry season than that in the flood season, varying from 0.60 to 3.83 in the flood season and 1.41–4.53 in the dry season. Moreover, the α-diversity index decreased significantly from the upstream to the downstream. Principal coordinate analysis of the eukaryotic plankton taxa yielded a distinct biogeographical distribution pattern between different seasons and locations. Redundancy analysis results suggested that total organic carbon, DO, Chl a and PC have significant influence on the community compositions. Variation partitioning analysis indicated that the purely environmental variables, pure spatial variables, pure hydrological variables explained 17.5%, 5.1% and 10.0% of the community. The distance decay analysis (eukaryotic plankton community dissimilarity and site location) also suggests that geographical distance was important in structuring the eukaryotic plankton community and determining the spatial dissimilarity in the river network. Our findings show that eukaryotic plankton community across the upper Yangtze River exhibit an ecological succession along the river continuum, and this is primarily driven by the combination of environmental, spatial and hydrological variables in the river-reservoir system.

Hydrologic impacts of cascade dams in a small headwater watershed under climate variability

Cascade damming poses a threat to watershed health; however, little is known about the hydrologic impacts of cascade dams in the context of simultaneous disturbance from climate variability. A comparison modeling approach that integrates Soil and Water Assessment Tool (SWAT) and Indicators of Hydrologic Alteration (IHA) was proposed in this study to evaluate the hydrologic impacts of cascade dams under climate variability in a small headwater watershed in Southeast China. The results showed that the verified SWAT with the reservoir module had higher Nash-Sutcliffe Efficiency values than those derived without the reservoir module. Cascade dams significantly regulated water in small flood events and reduced streamflow by enhancing evapotranspiration. Climate variability influenced the dam operation on streamflow regime. It was found that dams might cushion the pulses in wet years or fragment the pulses in dry years. Dams amplified the effects of climate variability, represented by increasing baseflow index in wet years. This research demonstrated that the comparison modeling approach proposed could provide useful insights into the hydrologic impacts of cascade dams under climate variability.

Dam Break Modeling in a Cascade of Small Earthen Dams: Case Study of the Čižina River in the Czech Republic

Failures of small dams can pose a serious threat to people and property even if the size of the schemes is relatively low. In many cases, small dams are situated in a cascade along streams, meaning that the failure of the uppermost dam may cause the dams downstream to fail. In this paper, a cascade of three small reservoirs, Lichnov II (14.6 m high), Lichnov III (10 m high), and Pocheň (8.5 m high), is the subject of the dam break analyses carried out via various methods such as empirical formulae, analogy, and hydraulic modeling. The dam-break flood routing was simulated using a shallow water flow hydraulic model. The simulations confirm that the attenuation effect of the peak discharge is governed by the flood volume, slope, and morphology of the floodplain and increases with the distance from the breached dam following an approximately exponential trend. When estimating peak discharge, empirical formulae derived for a single dam break should be applied carefully as they may underestimate the peak outflow by up to 10% in the case of a dam cascade. The attenuation volume of small reservoirs is small when compared to the flood volume, meaning that the attenuation of the peak discharge usually varies between 5–10%.

Development of a comprehensive framework for assessing the impacts of climate change and dam construction on flow regimes

The undeniable fact of climate change and dam construction alters the hydrological systems and threatens aquatic ecosystems of rivers around the world. Quantifying the impact of climate change and dam construction on flow regime is essential for water resources management and environmental protection. However, previous studies have rarely separated the effects of two factors on flow regimes, and the individual impact of each remains insufficient to date. Moreover, the existing methods have not fully captured the characteristics and temporal changes of flow regimes. In this study, we propose a new framework that integrates a streamflow reconstruction technique, indicators of hydrologic alteration (IHA), indicators of intra-annual flow variations, a revised range of variability approach to assessing the climate change and dam-induced impacts on the flow regime. The Upper Yellow River Basin (UYRB) was selected as the study area. The results show that climate change significantly reduces the monthly flow, extreme water conditions and the intra-annual indicators, and advances the Julian date of annual maximum flow. Dam construction delays the Julian date of annual maximum flow that opposes climate change impact, reduces the monthly flow in the flood season and the annual maximum flow, but increases the monthly flow in the dry season and annual minimum flow. Changes in flow regimes in the UYRB are found to be mainly due to climate change albeit there exists a cascade of dams. Based on the range of variability approach (RVA), fifteen of forty indicators are moderately changed. While the number increases to thirty-seven when considering the temporal order alteration of flow regime. The case study demonstrates that the developed framework can serve as a useful tool to evaluate the impact of climate change and dam construction on flow regimes.

Bacterial community structure upstream and downstream of cascade dams along the Lancang River in southwestern China

Extensive construction of dams by humans has caused alterations in flow regimes and concomitant alterations in river ecosystems. Even so, bacterioplankton diversity in large rivers influenced by cascade dams has been largely ignored. In this study, bacterial community diversity and profiles of seven cascade dams along the720 km of the Lancang River were studied using Illumina sequencing of the V3-V4 hypervariable region of the 16S rRNA gene. Spatiotemporal variations of bacterial communities in sediment and water of the Gongguoqiao hydroelectric dam and factors affecting these variations were also examined. Microbial diversity and richness in surface water increased slightly from upstream toward downstream along the river. A significant positive correlation between spatial distance and dissimilarities in bacterial community structure was confirmed (Mantel test, r = 0.4826, p = 0.001). At the Gongguoqiao hydroelectric dam, temporal differences in water overwhelmed spatial variability in bacterial communities. Temperature, precipitation, and nutrient levels were major drivers of seasonal microbial changes. Most functional groups associated with carbon cycling in sediment samples decreased from winter to summer. Our findings improve our understanding of associations, compositions, and predicted functional profiles of microbial communities in a large riverine ecosystem influenced by multiple cascade dams.

A quantitative risk analysis model for cascade reservoirs overtopping: principle and application

Quantitative and effective flood control operation of cascade reservoirs under emergency conditions can guarantee the safety of dams, reduce the social loss caused by flood disaster. A quantitative and practical risk analysis model of cascade dams overtopping breach (DAMSBREACH) has been developed based on the improvements to the analytical method of single dam breach. A single dam breach module, a flood routing module, a cascade dam overtopping and warning module have been assembled in the DAMSBREACH for predicting cascade dams overtopping flood and risk warning. The main improvements include a hyperbolic soil erosion model, the circular slip surface used in lateral enlargement modeling, and a numerical algorithm that uses velocity increment and allows straight forward calculation for the breach flood hydrograph. The DAMSBREACH is applied to Xiaerxia–Dawei–Shuangjiangkou of three cascade dams, which located in the upper reaches of Dadu river in southwest of China. The results show that only under the condition to open all release structures of Shuangjiangkou reservoir while Dawei dam starts to break, Shuangjiangkou reservoir will not be overtopped and the highest water level reaches 2507.56 m that is lower than the elevation of dam crest 2510.00 m. The integrated risk of cascade reservoirs has a direct relationship with the initial water level, uncertainty flood from the upstream, the discharge capacity and warning time of the reservoir. The risk analysis model of cascade dams overtopping break can help the decision-makers choose the best operation mode according to the characteristic of the reservoir under different emergency operating conditions.