Water-sediment Regulation Research Articles

Scenario-based multi-objective optimization of reservoirs in silt-laden rivers: A case study in the Lower Yellow River

Severe sedimentation often takes place in the river channel of silt-laden rivers, which is often mitigated through water-sediment regulation of the reservoirs. However, watersediment regulation is often competitive with other objectives of reservoirs, like water supply and hydropower generation; on the other hand, the reduction of channel sedimentation is often achieved at the expense of reservoir sedimentation, which reduces the service life of reservoirs. The Yellow River used to be the river with largest sediment transport over the world, but has experienced significant declination of runoff and sediment in recent years. This study presents a scenario-based multi-objective optimization operation model for the Xiaolangdi reservoir considering hydropower generation, reservoir sedimentation and channel sedimentation, with a generalized linear model coupled to calculate channel sedimentation based on runoff and sediment time series. A stochastic model that can reproduce both spatial correlations and low frequency attributes of the data series is adopted to generate two different scenarios based on different periods of observation and the performance of the multi-objective operation model under different scenarios is tested. The results indicate that: (1) the proposed optimization model can generate different schemes of reservoir operation and enhance operation performance; (2) the generalized linear model can well fit the relationship between daily channel sedimentation and runoff-sediment factors, but tends to overestimate the erosion efficiency after 2005; (3) the reservoir sedimentation and channel sedimentation show linear competitive relation, i.e., an average increase of 1 ton in reservoir sedimentation would result in declination of channel sedimentation from 0.455 to 0.488 tons, while the competitive relationship between hydropower generation and reservoir sedimentation is non-linear and weak; (4) the increase in the proportion of non-flood sediment load to the total sediment load makes it more difficult to prevent the reservoir from silting up.

Influence of Water–Sediment Regulation Scheme on Accretion and Erosion in a River Delta: a Case Study of the Yellow River Delta, China

Influence of Water–Sediment Regulation Scheme on Accretion and Erosion in a River Delta: a Case Study of the Yellow River Delta, China

Fluvial sediment source to sink transfer at the Yellow River Delta: Quantifications, causes, and environmental impacts

Intensified human interventions in river basins and deltas lead to more complexities of environmental changes during the Anthropocene. Changes in river regime especially a dramatic reduction in sediment delivery increase challenges of the morphological and ecological sustainability of river deltas. In evaluating deltaic risks and sustainable solutions, researches are often limited to single geomorphic units of the deltaic system, and investigations of sediment source to sink transfer at river deltas under recent river regimes are often missing. The Yellow River Delta (YRD) presents as a typical megadelta under stressors induced by changing environments. This study utilizes a period of 20-yr high-resolution topography data of the deltaic channel and its subaqueous delta to investigate sediment transport and source to sink process by integrated methods of field measurements and numerical simulations. The results indicate that the deltaic channel has transitioned from net accretion to erosion after the implementation of the Water-Sediment Regulation Scheme (WSRS) in 2002. The active river mouth experienced a slow accretion phase since the river channel diverted to Qing 8 channel, with a reduced vertical deposition rate of 0.15 m/yr, whilst its adjacent Gudong littoral zone had a −0.11 m/yr erosion rate. Under the new fluvial regime, the river-borne suspended sediment tends to transport southwards to the Laizhou Bay, followed by the river-derived sediment transport eastward and northward to the offshore delta. It is clear that with the continued human activities in the region, the YRD is at the potential state of deltaic transition both in the deltaic channel and its subaqueous delta. This transition is believed to be beneficial to the deltaic channel stability, but it could significantly impact on the geomorphic and ecologic sustainability of the entire deltaic system.

Coarsening of sediments from the Huanghe (Yellow River) delta-coast and its environmental implications

We studied temporal-spatial variations in the grain size of sediments from the Huanghe (Yellow River) delta-coast. Four sediment push-cores and more than 200 surface sediment samples were collected. Key factors that control the sediment variability were revealed, as well as their environmental significance. Analytical results indicated that sediments of the Huanghe delta-coast have coarsened over the past decades, and that this coarsening has intensified in recent years. The average mean grain size (Mz) of two cores in the offshore area from the abandoned delta lobe increased by approximately 18%–32% in the 2010s compared to the 1990s. Surface sediments in the nearshore area were coarsened by about 25% on average from 28.6 μm in 1992 to 35.7 μm in 2000. The sediment coarsening of the abandoned delta lobe began in the 1990s due to the extensive erosion caused by the absence of riverine-sediment supply. Coarse-grained sediments that were resuspended and transported offshore also made the sediment of the abandoned delta lobe coarser. In contrast, sediments of two cores in the offshore area of the active delta lobe were coarsened by more than 30% in the 2010s compared to the 1990s. The average Mz of surface sediments in the nearshore area increased by more than 22% from 23.5 μm in 1992 to 28.7 μm in 2000. The sediment coarsening of the active delta lobe began in the 1990s and became more significant in the 2000s. This trend was due to the rapid decrease of the sediment load from the Huanghe, as well as the coarsening of river-delivered sediments (from 17.7 μm to 22.9 μm) caused by the Water-Sediment Regulation Scheme that has been in place since 2002. Increasing coastal hydrodynamics and the loss of riverine sediment also account for the coarsening of sediments of the delta lobe. Our data clearly indicate that the critical bed-shear stress of the Huanghe delta-coast increased significantly. As a result, it was more difficult to initiate motion of the seabed sediments, leading to the decline of resuspension and erosion. Moreover, since the content of sedimentary organic carbon is negatively correlated with sediment grain size, the coarsening sediment of the study area will have inevitably degraded the carbon-related biogeochemical processes of the Huanghe delta-coast. Our findings have significant implications for the evolution of large river deltas, which are now threatened by sediment starvation and erosion.

Hydro-geomorphological characteristics in response to the water-sediment regulation scheme of the Xiaolangdi Dam in the lower Yellow River

Water-sediment regulation schemes (WSRS) enacted by dams drive downstream hydrological and geomorphic processes. However, few studies have been conducted to reveal the seasonal variation in the relationship between hydrological and geomorphic processes and the main factors driving river channel geomorphology. In this study, the effects of the WSRS of Xiaolangdi Dam (XLD) on the hydro-morphological characteristics of the lower Yellow River were assessed using data from Huayuankou (HYK) and Jiahetan (JHT) hydrological stations for the years 1983–2019. The monitoring data included water levels, flow rates, suspended sediment concentrations (SSC), and cross-sectional profiles; data analysis was based on the methods of indicators of hydrologic alteration and range of variability approach (IHA-RVA) and random forest (RF) models. After its completion in 2001, the operation of the XLD greatly altered water levels in the lower Yellow River (90.40% at HYK; 44.04% at JHT). The WSRS of dam scoured the riverbed and lowered the water level, causing the main channel to be narrower and deeper than during the pre-dam period. However, dam operation had little effect on seasonal variation in water levels when considering the following three hydrological seasons: pre-WSRS (March-May), mid-WSRS (June-August), and post-WSRS (September-November). In contrast, the SSC varied among seasons during the pre-dam period, but not after dam construction. Additionally, in the post-WSRS season, the cross-sectional geomorphic coefficients (CSGC) was lower in the post-dam period, while the average depth of river cross-sections increased. RF models revealed that water levels and the SSC were the main factors influencing channel geomorphic evolution in the pre-WSRS season at HYK and JHT, respectively. The SSC was the most important factor at both stations in the post-WSRS season. These results provide basic theoretical support for river regulation under the influence of intense human activities.

Impacts of Water-Sediment Regulation Scheme on Chromophoric Dissolved Organic Matter in the Lower Yellow River

Impacts of Water-Sediment Regulation Scheme on Chromophoric Dissolved Organic Matter in the Lower Yellow River

Water-sediment regulation scheme of the Xiaolangdi Dam influences redistribution and accumulation of heavy metals in sediments in the middle and lower reaches of the Yellow River

Water and sediment regulation by dams influence the erosion, deposition, and transport of sediments, which should cause the redistribution and accumulation of contaminants. In this study, we investigated the impacts of the water-sediment regulation scheme (WSRS) of the Xiaolangdi Dam (XLD) on the distribution and accumulation of six heavy metals (HMs) (Cr, Ni, Cu, Zn, Cd, and Pb) in sediments in the Yellow River. Our results showed that the concentrations of HMs were higher in sediments in the study area compared to background levels, and the accumulation of HMs mainly occurred in the downstream reaches during the WSRS. However, following the WSRS, the HMs were mainly distributed in the reservoir area. While not significantly different between the reservoir area and downstream reaches, HM concentrations varied spatially and were significantly different (p < 0.05) between seasons. Accumulation of HMs occurred in sediments in the study area, particularly Zn and Cd in the downstream reaches in post-WSRS seasons, and was driven by agricultural and urban industrial activities. The accumulation of Cd induced a very high contamination level and ecological risk of sediments, while Pb induced a considerable contamination and moderate ecological risk in the downstream reaches in the post-WSRS II season. In conclusion, the WSRS led to high HM concentrations in downstream reaches during the WSRS, whereas the XLD led to high HM concentrations in the reservoir area after the WSRS. These results are critical for better understanding the effects of the WSRS on contaminants and for informing the management and restoration of regulated rivers.

Impacts of water-sediment regulation on spatial-temporal variations of heavy metals in riparian sediments along the middle and lower reaches of the Yellow River

The water-sediment regulation scheme (WSRS) of dams influences the desorption, resuspension, and deposition processes of riparian sediments, which in turn affect the spatial-temporal variations of heavy metals (HMs) in riparian sediments and leads to severe degradation of soil and water quality. However, the difference between the trapping effect of dams and the redistribution effects of the WSRS on HMs in riparian sediments, as well as the consecutively seasonal change of HMs after the WSRS, are rarely reported. To fill this gap, the concentrations of six HMs including Cd, Cr, Cu, Ni, Pb, and Zn in riparian sediments along the Xiaolangdi Dam (XLD) Reservoir and its downstream reach were investigated, and the contamination level and potential ecological risk of HMs were assessed, to differentiate the effects of the XLD and its WSRS on the concentration, contamination level, and potential ecological risks of HMs. The results indicated that the mean HM concentrations in riparian sediments were higher than the background values in the study area and showed significant spatial and temporal variations. However, the regional differences of HM concentrations caused by the trapping effect of the XLD were less than the seasonal differences caused by the redistribution effects of the WSRS. The contamination and ecological risk assessment indicated that riparian sediments in the study area were contaminated by the six HMs, particularly by Cd and Pb, which overall exhibited a high and moderate ecological risk, respectively. The sources for Pb were likely agricultural inputs, while the sources for Cd should be attributed to both industrial and agricultural inputs. Overall, the trapping effect of the XLD led to the accumulation of HMs in riparian sediments along the reservoir area, while the regulation effects of the WSRS resulted in the redistribution of HMs in riparian sediments from the reservoir area to the downstream reach.

Full-stream erosion in the lower Yellow River: Feasibility, sustainability and opportunity

The issue of how to achieve removal of recently accumulated sediment is one of the largest unresolved puzzles in China's lower Yellow River. In this work, the feasibility and sustainability of achieving full-stream erosion in the lower Yellow River based on a water-sediment regulation scheme (WSRS) have been comprehensively investigated. The results indicate that the erosion–deposition state of the lower Yellow River is controlled by the incoming flow discharge level and the corresponding sediment concentration. The year 1987 is identified as an abrupt change point for streamflow in Xiaolangdi station. The relationships between channel erosion or deposition and the sediment concentration of the incoming discharge are constructed for the periods 1960–1987 and 1988–2017. Based on a constrained optimization approach, a sediment concentration lower than 19.96 kg/m3 in the discharge from Xiaolangdi Reservoir is determined to be the critical controlling condition for achieving full-stream erosion in the lower Yellow River under the average discharge level observed during 1960–1987; however, it is challenging to achieve full-stream erosion under the average discharge level of 1988–2017. Although this erosion status could potentially be achieved through continued implementation of the WSRS, its sustainability is restricted by the declining storage capacity of Xiaolangdi Reservoir, decreasing water discharge levels and riverbed coarsening in the Yellow River. It is necessary to design cross-basin collaboration measures for the upper, middle and lower Yellow River to achieve sustainable sediment reduction and healthy development of the Yellow River basin as a whole.

Impact of water-sediment regulation on the concentration and transport of dissolved heavy metals in the middle and lower reaches of the Yellow River

Anthropogenic activities in river basins, especially large-scale water conservancy projects, have notably impacted the physical, chemical and ecological environments of estuaries and coastal areas. In this paper, the effects of water and sediment regulation (WSR) on the concentration and transport of heavy metals in the Yellow River were studied based on a continuous daily heavy metal survey in both the middle reaches (Xiaolangdi station) and lower reaches (Lijin station) of the Yellow River during the WSR period in 2019. The results indicated that the variation in the water oxidation-reduction environment of the Xiaolangdi reservoir during the WSR process exerted an important impact on the concentrations of dissolved Cu, Cd, Pb, Cr and As at the Xiaolangdi station but exerted almost no influence on the concentration of dissolved Ni. At Lijin station, the dissolved heavy metal content first increased and then decreased in the first stage, which mainly depended on the release of heavy metals from resuspended sediments. In the second stage, the heavy metal content gradually decreased due to adsorption onto fine particles discharged from the reservoir. The dissolved heavy metal flux during the water-sediment regulation scheme (WSRS) period accounted for 16.9–33.4% of the annual total dissolved heavy metal flux. WSRS changed transport of water and sediment. The dissolved heavy metal concentrations at the Xiaolangdi station were mainly controlled by the discharge of water and sediments from the Xiaolangdi reservoir, while the dissolved heavy metal concentration at the Lijin station was largely affected by the sediments resuspended from downstream riverbeds and the water and sediment scheduling mode of the Xiaolangdi reservoir. Dissolved heavy metal transportation was highly influenced by the WSR process within a short time. Human intervention, especially WSRS operation, apparently alters the natural states of both the mainstream and estuarine environments of the Yellow River.

Environmental magnetic properties of core sediments in the Yellow River subaqueous delta and their chronologicalapplications

The Yellow River has shifted frequently in the downstream region since 1885; it has flowed into the sea through the Qingshuigou channel from 1976 onward, thus forming the latest delta lobe (1976–present). During the past ~40 years, the sediment load of the Yellow River has been greatly reduced because of human and natural activities, such as reservoir construction, soil and water conservation, and river avulsion. Beginning in 2002, the water-sediment regulation scheme (WSRS) has also changed the sediment loading patterns of the Yellow River. Several studies have reported on the erosion and deposition characteristics of the latest lobe of the Yellow River Delta over the past several decades, based on remote sensing methods and water-depth measurements. However, continuous sedimentary records of delta erosion and deposition and their responses to natural and human activities in the watershed are lacking. Here, we obtained three sediments cores, YD01, YD02, and YD03, from the subaqueous delta of the latest lobe of the Yellow River Delta. These cores contain sediments deposited from 1976 to 2018. We analyzed the environmental magnetism and lithological characteristics of these cores, and investigated their responses to natural and human activities in the watershed of the Yellow River, as well as possible applications to sediment dating in the Yellow River Delta. The results show that frequency-dependent susceptibility ( χ fd%) and anhysteretic remanent magnetization susceptibility ( χ ARM) values were high in core sediments deposited from 1976 to 1996, indicating abundant contents of fine single-domain (SD) and superparamagnetic (SP) magnetic particles, and therefore a dominant sediment supply from the Chinese Loess Plateau. Small decreases in χ fd% and χ ARM in sediments deposited after 1986 revealed slight reduction of the sediment load from the Yellow River, which was likely caused by sediment retention related to the construction of the Longyangxia Reservoir. Rapid rises in χ fd% and χ ARM occurred in core YD01 sediments deposited during 2002 and ~2007, likely in response to the implementation of the WSRS, which resulted in an increase in sediment transportation. Increases in hard isothermal remanent magnetization (HIRM) values, an indicator of high-coercivity magnetic minerals such as goethite and hematite, in cores YD02 and YD03 from 2002 to ~2007 may also represent the start of the WSRS. This is because the downstream riverbed of the Xiaolangdi Reservoir underwent intense erosion during the implementation of the WSRS, leading to large-scale transportation of coarse sediments enriched in goethite and hematite, which were deposited on the riverbed before the implementation of the WSRS. The lack of increased contents of fine SD and SP magnetic minerals in cores YD02 and YD03 compared with YD01 in the interval deposited between 2002 and ~2007 was attributed to erosion and redeposition in the areas around cores YD02 and YD03. Large drops in SD and SP contents after ~2007 were detected in all three cores, revealing intense erosion near the river mouth likely caused by river channel shift and reduced capability of the WSRS to remove sediments. Contents of SD and SP particles increased again in sediments deposited after 2014 in cores YD02 and YD03, likely related to erosion of the present (after 1996) Yellow River mouth and transportation of fine particles to the region near cores YD02 and YD03. In summary, the magnetic parameters χ fd% and χ ARM and the lithological characteristics of cores YD01, YD02, and YD03 recorded sediment delivery changes caused by four events in the Yellow River watershed: The main river channel shift in 1976, construction of the Longyangxia Reservoir in 1986, the beginning of the WSRS in 2002, and weakened capability of the WSRS to remove sediments after ~2007. The notable changes of χ fd%, χ ARM, and lithology at the 1976, 1986, 2002, and ~2007 horizons may be used as dating points in future research of sediment cores from the Yellow River Mouth.

Variability of heavy metal transport during the water–sediment regulation period of the Yellow River in 2018

To understand the impacts of the human-induced flood event on heavy metal (HM) transport, spatiotemporal variations in contents and fluxes of metals (Cr, Ni, Cu, Zn, As, Pb, Cd), Pb stable isotopes and characteristics of water and sediment transport into the sea during Water-Sediment Regulation Scheme (WSRS) in Yellow River (YR) were studied based on field investigation at Xiaolangdi Reservoir (XLD), Lijin Station and Yellow River estuary (YRE). The HM transport was significantly controlled by hydrological process and dominated by particulate form with strong associations with particle size and suspended sediment concentration (SSC). In first stage, dissolved heavy metal (DHM) and particulate heavy metal (PHM) contents both increased significantly as coarser sediment with a mixed source of downstream river channel and XLD, while that maintained higher value for stable source of fine-grained XLD sediment in second stage. The HMs into the sea were mainly originated from upper and middle reaches but also contributed by human emissions from downstream area. As the source of HMs into the sea, the downstream area also acted as an important sink, especially in first stage, playing a role of buffering and filtration. During WSRS, the YR discharged 49%–60% of annual HM flux into the sea, and the second stage is the main transport period, leading to a great alternation in geochemical composition of the YRE sediment.

Cd absorption characteristics of Suaeda salsa under different sediment burial and exogenous Cd input conditions in the Yellow River estuary, China.

Suaeda salsa (L.) Pall., a typical halophyte plant in the Yellow River estuary, has high enrichment capacity for heavy metals. However, few studies have investigated the Cd absorption characteristics of S. salsa under different sediment burial and exogenous Cd input conditions, especially following the water-sediment regulation scheme (WSRS), which brought sediment burial and exogenous substances to the estuary. So, we established a greenhouse pot culture experiment with four sediment burial depths (0 cm, 3 cm, 6 cm, and 12 cm) and exogenous Cd input levels (0 mg·kg-1, 0.5 mg·kg-1, 1.0 mg·kg-1, and 1.5 mg·kg-1) and analyzed the leaf, stem, root, and total biomass; leaf, stem, and root Cd content; and storage, sediment Cd content, accumulation factor, root/leaf (R/L), root/stem (R/S), and stem/leaf (S/L) ratios to study the Cd absorption characteristics of S. salsa under the different sediment burial and exogenous Cd input. Results showed that high Cd content in roots, stems, and leaves was harmful to S. salsa growth, and then led to a decrease in biomass (characterized by stem, leaf, and total biomass). Suaeda salsa exhibited a survival strategy to deal with Cd toxicity, which involved the roots absorbing Cd from the sediment and storing it in stems and leaves (stem and leaf Cd content peaked at 0.5 mg·kg-1 Cd input) at low Cd input, whereas roots stored more Cd and reduced Cd transport to stems and leaves at high Cd input. Therefore, we observed the maximum value of leaf (500.63 ± 19.15 g·m-2), stem (648.22 ± 50.08 g·m-2), and total biomass (1246.92 ± 55.49 g·m-2) in the treatment with 1.5 mg·kg-1 Cd input and 3-cm sedimentdepth due to the Cd content in leaves and stems being relatively low. The accumulation factors of leaves, stems, and roots varied (0.39-0.99, 0.19-2.58, and 0.80-20.45, respectively), and most of the accumulation factors for roots and leaves and the R/L and R/S ratios were >1, which indicated that S. salsa had high enrichment levels of Cd, which mostly accumulated in the roots. Shallow or moderate burial depth was beneficial to S. salsa growth, but sediment burial was not beneficial to Cd absorption because the sum of leaf, stem, and root Cd storage was higher at 0-cm depth compared with the other depths. Variance analysis showed that the influence of Cd input on leaf, stem, root, and sediment Cd content and stem and root Cd storage was significant (P < 0.05), whereas sediment burial, interaction of sediment burial and Cd input on Cd content, storage, and biomass were not significant (P > 0.05). Therefore, we concluded that more attention should be paid to the control of sediment burial and heavy metal input, especially during the WSRS, in the Yellow River estuary.

Effects of the Water-Sediment Regulation Scheme on the Expansion of Spartina alterniflora at the Yellow River Estuary, China

In recent decades, the invasion of saltmarsh plant Spartina alterniflora (S. alterniflora) over a large part of coastal wetlands in China, including the Yellow River Estuary (YRE) as a regional economic hub and global ecosystem services hotspot, has caused increasing concern because of its serious threats to native ecosystems. During the same period, local authorities have implemented a Water-Sediment Regulation Scheme (WSRS) in the Yellow River for flood mitigation and delta restoration purposes. The altered hydrological regime has resulted in unintended changes to estuarine ecosystem. However, the direct consequence of the WSRS on the expansion of S. alterniflora remains unclear. In this study, quantitative relationship between the inter- and intra-annual expansion patterns of S. alterniflora represented by relevant landscape metrics and indicators that quantify the concurrent variations of river and sediment discharges as the proxy of the WSRS impacts were analysed over the period of Year 2011–2018, and the analyses were performed on the YRE as a whole and on five different zones subdivided based on the invasion sequence. The results showed that there was no significant difference in the inter-annual area variation of S. alterniflora between the years with and without WSRS. Compared with the years without WSRS (2016–2017), the intra-annual (monthly) increment of the various landscape metrics [i.e., NP (number of patches), CA (class area), LPI (largest patch index) and AI (aggregation index] were found to be significantly higher in the initial stage of peak growing season (June-July) than in the mid- and late stages (July-September) in the years with WSRS (2011–2015, 2018) in the subregion located close to the south bank of YRE as the most prominent impact zone. In addition, F (mean flow), Ff (number of high flow pulses), Tf (Julian date of maximum flow) and D (duration of WSRS) were identified as the explanatory variables for the intra-annual vegetation landscape pattern changes, and their relative contributions to resultant changes were also assessed. Our results broaden the understanding of estuarine hydrological disturbance as a potential driver regulating the saltmarsh vegetation, and also have implications for S. alterniflora invasion control at estuaries under changing environment.

Multi-objective synergetic reservoir operation in a sediment-laden river

Multi-objective requirements including water supply, hydropower generation, water–sediment regulation (WSR), and inner-river ecosystem maintenance should be fully valued in the operation of reservoirs located on a sediment-laden river. In order to balance the satisfaction of multiple objectives with intense competition and guarantee the primary requirements preferentially, this paper presents a novel framework that incorporates the recognition and classification of primary and secondary requirements of multiple operation targets, a set of principles for multi-objective synergetic regulation of reservoirs (MOSRR) following the theory of synergetics, and the optimization model construction of MOSRR considering the maximization synergetic degree of multi-objective. The Analytic Hierarchy Process and Particle Swarm Optimization algorithm are employed to determine the weight and solve the MOSRR model, respectively. MOSRR is applied to a system of cascade reservoirs along the Upper Yellow River in China considering climatic and hydrologic variability. The results from MOSRR are compared to those from a traditional multi-objective optimal operation model. It is found that the guarantee of multi-objective requirements from MOSRR have been improved visibly compare with the traditional model at the expense of a slight reduction in the hydropower generation (especially the secondary requirement of the energy output in the dry season). Also, MOSRR can mitigate the effects of runoff reduction through minimizing the damage of primary requirements, which is more appropriate for multi-objective reservoir operation with limited water in sediment-laden river. The proposed MOSRR framework can be applied to other large-scale reservoir systems with stiff competition in water resources.

Evaluating the Effectiveness of Regulation Schemes in Improving Navigation Condition of Reservoir Channels

Sedimentation has become a world-wide issue for almost all types of dam reservoirs. The impact of sedimentation on navigation is mainly reflected in the obstruction of navigation channels, usually at reservoir ends. The issue can be partially solved by joint water-sediment regulation during the drawdown period, called “sedimentation-reduction (SR) regulation”. It aims to enhance scour at shoals by increasing the outflow of the reservoir and accelerates the water level decline. In general, the more inflow discharge or lower dam water level, the better scouring effects, although the waste of water for hydropower generation also needs to be considered. In this work, we propose a model to evaluate the effectiveness of SR regulation schemes in improving navigation condition of reservoirs channels. The constraints for successful regulation operation are analysed in details. The navigation benefits are estimated by the increment of sediment transport volume, and the power generation impacts are estimated by the loss of effective water head. The model gives optimal starting dam water level and corresponding inflow discharge. The results were compared with actual regulation parameters of the Three Gorge reservoir. This work can provide guidelines for reservoir sediment management.

Metals in Yellow River estuary sediments during the 2018 water-sediment regulation scheme period

The Water-Sediment Regulation Scheme (WSRS) in the Yellow River has greatly changed the natural state of water and sediment discharges into the sea, which is likely to have a significant impact on the ecological environment in the estuary and even Bohai Sea. Based on a total of 33 surface sediment samples collected in the Yellow River estuary during the WSRS in 2018, analyses of grain size, contents of heavy metals, major elements, and clay minerals were conducted to evaluate sources and pollution status of heavy metals. The results indicated that sediment compositions were significantly impacted by the process of the WSRS. Most metals were from natural sources, but As may be affected by human activities. Contamination assessments of heavy metals revealed that the WSRS had limited impact on the ecology environment, but As could be a threat to the environment.

Impact of the Water–Sediment Regulation Scheme on the phytoplankton community in the Yellow River estuary

The Water–Sediment Regulation Scheme (WSRS) transports large amounts of water, sediment, and nutrients to the Yellow River estuary (YRE) in a very short time, which significantly changes the ecological environment of the estuary area in summer (wet season). The influence of these changes on the distribution and abundance of phytoplankton communities in the YRE are still inadequately understood. We studied the spatial distribution of environmental factors, zooplankton abundance and biomass, and phytoplankton abundance and community in the YRE at different WSRS stages during 2018 (WSRS year) and compared with corresponding durations in 2017 (non–WSRS year). Our results demonstrate that the WSRS (1) promoted phytoplankton growth in estuarine areas at the pre-stage and decreased phytoplankton abundances in the entire study area during the inter-stage, (2) decreased phytoplankton biodiversity and increased abundance and proportion of Synedra sp., and (3) led to a temporary rapid increase in zooplankton abundance and biomass. Bottom–up effects dominated phytoplankton variability at the pre- and end-stages of the WSRS, while top–down effects dominated the inter-stage. Redundancy analysis indicated that phytoplankton assemblages were primarily influenced by nitrate and the ratio of nitrogen to phosphorus at the pre-stage, by zooplankton abundance and biomass at the inter-stage, and by ammonium and dissolved silica at the end-stage. Our findings provide essential information for understanding and predicting the changes in the phytoplankton community due to increases in hydrodynamics, nutrients, and turbidity conditions in river estuaries.

Accretion–Erosion Dynamics of the Yellow River Delta and the Relationships with Runoff and Sediment from 1976 to 2018

Timely understanding of the coastal accretion–erosion dynamics of the Yellow River Delta (YRD) can not only deepen the understanding of the evolution of the delta but also provide scientific support for water-sediment regulation (WSR) in the lower reaches of Yellow River and the implementation of a protection strategy for the Yellow River Estuary. In this long-term study, Landsat images from 1976 to 2018 were acquired, and the cloud processing platform of the Google Earth Engine was used for extraction of coastlines. On the basis of these coastlines, the area and accretion–erosion dynamics were analyzed. Then, after statistical analysis of the interannual and intra-annual variations in runoff and sediment, we discuss the relationship between the accretion–erosion dynamics and the annual runoff and sediment. The results show that (1) the coastline of the YRD lengthened first and then shortened, and the average annual growth rate was 1.48 km/a. (2) The land area of the YRD showed a significant accretionary trend before 1996, with an average annual growth rate of 28.60 km2/a. Then, the area gradually decreased from 1997 to 2001. After WSR was implemented in 2002, the accretion–erosion dynamics gradually became smooth, with an annual growth rate of 0.31 km2/a. (3) After WSR, the maximum annual sedimentation decreased by 79.70%. The average annual sediment discharge accounted for only 6.69% from November to March of the following year during the non-flood season. (4) With the continuous decrease in sediment discharge, the determination coefficient (R2) between the cumulative accretion–erosion area of the estuary and the annual sedimentation decreased from 0.98 in 1976–1996 to 0.77 after 2002. Overall, although WSR has maintained a steady increase in delta land area, it cannot change the long-term decrease in the land area of the delta. The insights gained from our study can provide some references for related coastline research, and will be useful to science community and decision makers for coastal environmental monitoring, management, protection, and sustainable development of the YRD.

The influence of water-sediment regulation on macrobenthic community structures in the Huanghe River (Yellow River) Estuary during 2012–2016

To determine if water-sediment regulation has affected macrobenthic community structure in the Huanghe River Estuary, China, macrobenthic samples were collected following regulation events from 2012 to 2016. We identify seven phyla and 138 macrobenthic species from within samples throughout the survey area, over time. Species richness and abundance in 2012 were significantly higher than in 2016. Biomass did not differ significantly during 2012–2016. Dominant species were mostly small polychaetes, with mollusks, arthropods, and echinoderms all being relatively rare. In 2016, dominant species were small polychaetes. MDS reveals macrobenthic communities at all surveyed distances from the estuary to have become the same community structure over time. ShannonWiener diversity and Margalef richness indexes trended down over time. CCA reveals the most dominant sediment-dwelling species to prefer lower dissolved oxygen, sulfides, and pH, and sediments with high D50 and low clay content. We speculate that water-sediment regulation has affected seabed communities, particularly Region A in our survey area.