Variations In Sediment Discharge Research Articles

Estimation of Potential Impact of Land Use Change on Sediment Yield From a Small Tropical Watershed Using the TREX Erosion Model

ABSTRACTAnalysis of the influence of changes in land use on the sediment yield from watersheds provides crucial inputs that aid the development of appropriate strategies for the sustainable management of consequent land degradation. Event‐based hydrologic models can be used for performing such analysis, as long‐term hydrological datasets are not available for most of the rivers in developing countries like India. In this study, an event‐based Two‐Dimensional Runoff, Erosion, and Export (TREX) model was used to simulate the soil erosion process in Moozhy, an ungauged watershed in the Karamana River basin in Thiruvananthapuram District, Kerala State, India. Rainfall, streamflow, and sediment concentration data corresponding to three isolated storm events were used to calibrate and validate the model. The performance of the model was assessed using four statistical measures, namely, Percent bias (PBIAS), Nash‐Sutcliffe Efficiency Coefficient (NSEC), Coefficient of determination (R2) and RMSE‐observations standard deviation ratio (RSR). The values of PBIAS varied between 46% and 49%, indicating satisfactory performance of the model. Also, from the values of NSEC and RSR, it can be concluded that the TREX model yields acceptable results. For a rainfall event that occurred on the 17 September 2017, the simulated values showed that about 1054 t of suspended sediments are transported from the watershed to the stream channels, and about 1020 t is carried out of the Moozhy watershed. Only a very small amount of sediment is left in the channel at the end of the simulation. About 34 t of sediment is deposited in the channel bed, about 0.04 t remains in suspension, and the balance 0.04 t remains in suspension as a suspended load. About 3% of the total sediment entering the reach of the stream considered in this study is deposited on the riverbed. The major component of the settled sediments is silt; about 12% of the total silt size fraction entering the stream settles on the river bed. Future scenarios of land use in the watershed were modelled using a hybrid Artificial Neural Network (ANN) and Cellular Automata (CA) model. These were used for simulating sediment discharge using the TREX erosion model. The predicted land use scenarios were used to investigate both short‐term (2025 and 2029) and long‐term (2037, 2045, and 2053) variations in sediment discharge. The sediment yield for the storm event that occurred on 17 September 2017 was used to benchmark the variation in sediment yield under the predicted land use scenarios. Results indicate that the peak runoff and the runoff volume are expected to increase by about 29% and 22% respectively in the period from 2005 to 2053. The expected increase in the volume of sediments in this period is about 50%; the peak sediment concentration is likely to increase by about 56%. The study highlights the threats of likely increase in soil erosion and consequent land degradation posed by unscientific changes in land use caused by urbanisation and calls for proper management interventions to address the problem.

Capturing the attenuating influence of a Three Gorges Dam on downstream sediment loads using artificial neural networks

ABSTRACT River sediment dynamics have been significantly altered by dam constructions, with the Three Gorges Dam (TGD) on the Yangtze River being a prominent example. This study aims to quantitatively assess the attenuating impact of TGD operations on downstream sediment discharge using artificial neural network (ANN) models. Seven ANN models were developed for stations along the middle and lower Yangtze River, trained on measured water and sediment data. The models effectively captured the relationships between upstream inputs and downstream outputs. Results revealed that variations in sediment discharge at each station were predominantly influenced by changes in sediment input from its immediate upstream section, rather than by changes in water discharge. This suggests that the TGD's impact on downstream sediment transport was primarily due to its sediment trapping efficiency, rather than alterations in the hydrological regime. The influence diminished gradually farther downstream, attributed to tributary contributions and sediment transport dynamics. ANNs, adept at handling input uncertainties, underscore the importance of boundary conditions; the integration of data-driven and physics-based models illuminates sediment dynamics in regulated rivers. This approach provides insight into dam management implications and sediment transport processes.

Impact of Large Reservoirs on Runoff and Sediment Load in the Jinsha River Basin

To develop clean energy hydropower, many dams were built in the Jinsha River Basin in the past thirty years and have significantly altered runoff and sediment transport processes. This study aims to evaluate the impacts of these reservoirs on runoff and sediment transport using data collected in the mainstream of the Jinsha River from the 1960s to 2020, for which the Mann–Kendall trend test method and double cumulative curve method are used to comprehensively judge the variation trends of annual runoff and suspended sediment load (SSL) and reveal the years in which there were credible sudden changes. The linear regression method is used to reveal the variation characteristics of the relationship between annual runoff and SSL before and after the years of abrupt change. The results show that the variations in runoff at Shigu and Panzhihua Stations have significant and relatively obvious increasing trends, respectively, and that 1985 was a sudden change year at Panzhihua Station. The runoff at Xiangjiaba Station increased slightly but not significantly. The variation in SSL shows temporal and spatial differentiation. The variation in sediment discharge at Shigu Station shows an increasing trend with a sudden change in the year 1997. Panzhihua Station shows a trend of increasing before 1998 but significantly decreasing after 1998. The fluctuation of sediment transport at Xiangjiaba Station was significant before 1998, but the trend is unclear. In the period between 1998 and 2020, a significant decreasing trend is observed, especially since 2013, when the mean annual SSL only accounted for 0.61% of its multi-year average. The variations in mean annual sediment concentration and coefficient of incoming sediment (CIS) at the hydrological stations are consistent with the variation trend of sediment transport. The correlation between water and sediment was strong before 2013 but extremely weak thereafter. The two sudden change points for the annual runoff and SSL in the years 1998 and 2013 are consistent with the years when large reservoirs were built in the river basin. The construction of large reservoirs and their large amount of sediment retention are the key reasons for the sudden changes in the water–sediment relationship and the sharp decrease in sediment transport in the downstream reach of the reservoir dam. The climate and underlying surface changes in the study area are not significant, and their impact on the water and sediment processes in the watershed is limited.

Spatial Variation in Specific Sediment Discharge Volume from First-Order Catchment due to Heavy Rainfall and its Factors

In this study, we evaluated the variations in sediment discharge between basins from the first- order catchment of the Akatani River watershed, where sediment discharges as a result of heavy rainfall. We tested the correlation between sediment discharge, topography, and rainfall magnitude using LiDAR datasets. The results showed that the specific eroded sediment volume in the first-order catchment was sufficiently greater than the specific deposited sediment volume, and the variation of the specific discharge sediment volume from the first-order catchment was determined by the variation of the specific eroded sediment volume. The spatial variations of specific discharged sediment volumes could not be evaluated solely by topography and rainfall indices; even in the catchment, where the topographic condition and rainfall magnitude were almost the same, the specific sediment discharge varied by more than one magnitude.

Evolution patterns and spatial sources of water and sediment discharge over the last 70 years in the Yellow River, China: A case study in the Ningxia Reach

The hydrology and sediment processes in large rivers play important roles in maintaining aquatic and coastal ecosystems and advancing civilization and production in human systems. Therefore, quantitatively analyzing the spatiotemporal variability and dynamics of water and sediment discharge in large rivers is essential for improving watershed management and sustainable development in the areas surrounding rivers, especially the Yellow River, which is one of the most sediment-laden rivers in the world. In this study, we analyzed the evolution patterns and spatial sources of water and sediment discharge in the Yellow River from 1951 to 2020 and determined the impacts of different factors on water and sediment discharge variations. The results showed that the annual water and sediment discharge significantly decreased (p < 0.05) over the past 70 years, with an abrupt change occurring in 1986. The first dominant periodicity of water discharge was approximately 29 years, while the first dominant periodicity of sediment discharge was approximately 28 years. In terms of the water and sediment discharge sources, the dominant factor affecting variations in water discharge was water diversion from 1951 to 2020, while the dominant factor affecting variations in sediment discharge was sediment aggradation from 1951 to 1985 and changed to tributary inflow sediment from 1986 to 2020. In addition, the water and sediment discharge changes were also affected by anthropogenic activities, such as water and sediment diversions, dams and reservoirs, and water and soil conservation measures. In particular, the water and sediment interception capabilities of the established soil and water conservation measures gradually became saturated over time. Specifically, the maximum water and sediment interception capabilities of the current soil and water conservation measures were 12.2 billion m3 and 1.9 Gt, respectively. Overall, the results of the present study can help tailor water and sediment regulation countermeasures in the future.

Satellite Observed Spatial and Temporal Variabilities of Particulate Organic Carbon in the East China Sea

In this study, we investigate spatial and temporal variabilities of particulate organic carbon (POC) in the East China Sea (ECS) and explore the dominant influencing factors for its three subregions—coastal, continental shelf and open sea—by analyzing satellite derived POC from 2001 to 2011 after calibration against in situ samplings. The analysis reveals that the largest POC occurs in spring at the coastal region with a value of 297.5 mg m−3, and the smallest in summer at the open sea region with a value of 108.9 mg m−3. POC in the coastal region is dominated by biological activity related to river discharge of nutrients and organic matter, and is regulated by sediment discharge variation due to the Three Gorge Project and chemical fertilizer application along the Changjiang River watershed; the open sea region is dominated by water exchange with the Kuroshio current, which results in the highest sea surface temperature and the lowest chlorophyll and POC among the three subregions; POC in the continental shelf region is driven by the seasonal competition between the influence of the coastal ocean and the Kuroshio current through biological activity and water exchange. A method to estimate the organic matter reserve in the ECS was developed based on satellite data and in situ samplings. Organic carbon storage was estimated at 4.08 × 1013 g, which could be used as a baseline for future estimates. An improved estimation could be found with additional in situ data and with the use of a more sophisticated algorithm for satellite POC analysis.

Using four approaches to separate the effects of climate change and human activities on sediment discharge in karst watersheds

Quantifying the relative contributions of climate change and human activities to changes in sediment discharge is closely related to regional water resources, soil management, and healthy ecosystem function. However, few studies have used different methods to decompose influences of climatic variability and human activities on changes to sediment discharge, especially in karst regions of southwest China which are experiencing severe soil erosion. This study systematically reviewed four methods (simple linear regression, double mass curve, sediment identify factor analysis, and elastic coefficient method) to separate the contribution of climate change and human activities to sediment discharge variation in four karst watersheds of southwest China during 1955 to 2015. The trend and abrupt change year of sediment discharge in the four watersheds were obtained using a nonparametric Mann-Kendall trend test and sequential test method, respectively. Result showed that the sediment discharge in all four watersheds all decreased significantly (P < 0.05), and the abrupt change years were 2003 or 2004 for these watersheds. The simple linear regression method, double mass curve method, and elastic coefficient method all indicated that the sediment discharge was mainly influenced by human activities, which contributed more than 73% of the observed change. For the sediment identify factor analysis method, sediment concentration was the principal factor influencing sediment discharge variation. The results provide a reference for evaluating the influence of climate and human activities on sediment discharge, and can aid in preparing regional soil conservation policy in karst watersheds.

Spatiotemporal Variations of Sediment Discharge and In‐Reach Sediment Budget in the Yellow River From the Headwater to the Delta

AbstractStrong anthropogenic activities and climate change have caused distinct hydrology and sediment dynamics in river systems worldwide. Yet, the integrated picture of sediment discharge changes and quantitative attribution of these changes from a basin‐wide perspective were limitedly understood. In this study, we quantified the variations of sediment discharge and in‐reach sediment budget in the Yellow River basin (YRB) during 1956–2019 and examined the underlying reasons. Both streamflow and sediment load in the YRB have substantially decreased during 1956–2000 except the headwater, and the reduction rate increased along the mainstem. However, the streamflow has significantly increased (p &lt; 0.05) since 2000 due to increased precipitation, and the sediment load at most stations tended to stabilize. The contribution of soil conservation measures (SCMs, including check‐dams, terraces, grass plantation, and afforestation) and reservoirs to sediment load reduction was more than 50% in the headwater and middle reaches and over 90% in the upper and lower reaches. The sediment yield coefficient decreased linearly with reservoir capacity and exponentially with the proportional area of SCMs (p &lt; 0.01). With continuous dam construction, the channel erosion in the headwater decreased to nearly zero, and the sediment deposition in the upper and middle reaches increased linearly with storage capacity of reservoirs. In contrast, the sediment deposition in the lower reaches displayed a linear correlation with the concentration of sediment derived from the middle reaches (p &lt; 0.05). This study provides quantitative results for sediment dynamics of entire river system, which facilitates development of sustainable watershed management strategies.

Impacts of ENSO on multi-scale variations in sediment discharge from the Pearl River to the South China Sea

Sediment load delivered by rivers is an important terrestrial factor in the evolution and productivity of coastal ecosystems and coastal morphology. As the strongest interannual climate signal, the El Niño Southern Oscillation (ENSO) is closely related to variations in the hydrological cycle at global and regional scales. However, the influence of ENSO on temporal variations in sediment discharge is poorly understood. In this paper, we examine periodic variations in sediment discharge to the South China Sea from the Pearl River since the 1950s using wavelet transform analysis (WT). Furthermore, we apply cross wavelet spectrum (XWT) and wavelet coherence (WTC) to investigate the linkages between ENSO and sediment variability. The WT results revealed that periodic oscillations in sediment discharge in the Pearl River occurred annually (1yr) before the 2000s, interannually (2–8yr) from 1960–2002, and decadally (10–16yr) from 1975–1995. These periodic variations in the sediment load series had common spectrum power with the water discharge and precipitation series, indicating an important climatic control. The XWT and WTC results revealed significant impacts of ENSO on precipitation, water discharge and sediment load at interannual time scales of 2–4.6yr from 1960–2002 with a shift of patterns of ENSO on sediment variability after the 1970s. In addition, an in-phase relation between sediment discharge and ENSO at time scales of 10–16yr from 1975–1995 was detected, indicating that variations at decadal scales could be related to other climatic teleconnections such as the Pacific Decadal Oscillation. Compared with the spectrum structures of periodic variations in precipitation and water discharge and their relationship with ENSO, there was a loss of energy in the sediment load at annual time scales after 2002 that can be attributed to dam construction in the river basin. Our study provides perspectives on the connections between ENSO and sediment variability at different time scales, which is of practical and scientific importance for forecasting global hydrological processes and for river management.

Impacts of topography on sediment discharge in Loess Plateau, China

Geomorphologic characteristics of a basin affect mass movement, water and sediment transport. The purpose of this study is to examine coupling relationship between topography and soil erosion in Loess Plateau during past 50 years (1960–2011). We firstly describe the distribution patterns of topographic characteristics and sediment discharge in specific river basins. By using mathematical methods (PCA method, linear regression and ANOVA analysis) and topographic variables, we build a comprehensive index – Erosion Topography Index (ETI). According to the research, the following findings are obtained. Firstly, average sediment discharge varies from 0.13 × 108 t to 7.56 × 108 t along the Yellow River from the upper-stream to down-stream. The sediment discharge of this region decreased 60%–90% since the year of 1980, and decrease tendency is more obvious after the year 2000 compared to the period of 1980–2011. Secondly, topographic characteristics and the Erosion Topography Index has similar distribution pattern. Thirdly, there is no significant linear correlation between ETI and all four sediment discharge variables (average sediment, total sediment, sediment of 1980–2011, and sediment of 2000–2011), but there are significant positive linear correlations (p < 0.05) between ETI and each sediment discharge variable. Furthermore, the correlation between ETI and sediment of 2000–2011 is stronger (p = 0.035) than other variables, and the gradient coefficient of them is higher (k = 0.667) than other variables too. Except the topographic characteristics, the vegetation coverage and human activities should be other factors that can influence the variations of sediment discharge in Loss Plateau.

Spatio-temporal variation of water flow and sediment discharge in the Mahanadi River, India

The transport of sediments by rivers to the oceans represents an important link between the terrestrial and marine ecosystem. Therefore, this work aims to study spatio-temporal variation of the sediment discharge and erosion rate in the Mahanadi river, one of the biggest rivers in India, over past three decades vis-à-vis their controlling factors. To understand the sediment load variation, the trend analysis in the time series data of rainfall, water and sediment discharge of the Mahanadi river were also attempted. The non-parametric Mann-Kendall and Sen's methods were used to determine whether there was a positive or negative trend in the time series data with their statistical significance. The occurrence of abrupt changes was detected using Pettitt test. The trend test result represents that sediment load delivered from the Mahanadi river to the global ocean has decreased sharply at the rate of 0.515×106tons/year between 1980 and 2010. Water discharge and rainfall in the basin showed no significant decreasing trend except at only one tributary. The decline in sediment discharge from the basin to the Bay of Bengal is mainly due to the increase in the number of dams, which has recorded the increase from 70 to 253 during the period of 1980 to 2010. Over the past 30years the Mahanadi river has discharged about 49.0±20.5km3 of water and 17.4±12.7×106tons of sediment annually to the Bay of Bengal whereas the mean erosional rate is 265±125tons/km2/year over the period of 30years in the basin. Based on the current data (2000–2001 to 2009–2010), sediment flux and water discharge to the ocean are 12±5×106tons/year and 49±16km3/year respectively; and ranking Mahanadi river second in terms of water discharge and sediment flux to the ocean among the peninsular rivers in India.

Spatial–temporal variation and periodic change in streamflow and suspended sediment discharge along the mainstream of the Yellow River during 1950‐2013

ObjectivesUnder the background of global climate change, variations in streamflow and sediment discharge in the Yellow River would continue with intensified human activities and changes in the ecological environment. To harness the Yellow River, analyzing the degrees of change and the reasons for the streamflow and sediment discharge, as well as predicting the future trends, are urgently needed. MethodsIn this paper, the non-parametric Mann–Kendall test, Sen's estimator of slope, Pettitt's test and wavelet transform were applied to detect the trends, the magnitude of the trends and the abrupt changes and periodic variation in streamflow and suspended sediment discharge at eight hydrological stations from 1950 to 2013 along the mainstream of the Yellow River. ResultsOver the past 64years (1950–2013), the spatial distribution of the annual average streamflow was of the “M” type, and the annual average suspended sediment discharge had a parabolic curve shape along the mainstream of the Yellow River. The temporal variation showed a distinct decreasing trend in streamflow since the 1990s and in suspended sediment discharge since the 1980s. Many cycles of oscillations occurred in the streamflow and suspended sediment discharge variation, leading to an alternate change in the wet/dry periods and the high/low sediment discharge periods. ConclusionsIn various regions of the Yellow River Basin since the 1970s, climate changes contributed reductions of 17.0–45.0% and 12.2–50.3% for streamflow and suspended sediment discharge, respectively, whereas human activities contributed reductions of 55.0–83.0% and 49.7–87.8%. Therefore, human activities were the major causes of the significant decline in the streamflow and the suspended sediment discharge of the Yellow River over the past six decades. Reasonable and possible measures should be taken to achieve water resources optimization and configuration and to promote watershed management and sustainability in the regions of the Yellow River Basin.

Spatiotemporal Variations in Annual Sediment Yield from the Middle Yellow River, China, 1950–2010

AbstractThe Yellow River is an important source of sediment flux to oceans in the world, and approximately 90% of the sediment originates from the middle Yellow River, an area with known high rates of soil erosion and sediment delivery. Trends and interannual variations in sediment discharge have considerable implications for reservoir operation and river management of the lower Yellow River. The annual sediment discharge of four main stream stations and ten tributary stations was used to show the spatiotemporal variation in sediment yield and to quantify the relative contribution of precipitation change and human activities, especially the influence of vegetation restoration with implementation of the Grain for Green Project (GGP) in 1999. The Mann-Kendall and Pettitt tests were used to detect the abrupt change in sediment discharge since the 1950s. The annual sediment yield of the middle Yellow River has shown a significant downward trend, decreasing by an order of magnitude in the last decade (2000–201...

Investigation and Strategy Study of Agricultural Soil and Water Conservation in the South of Dabie Mountains

The runoff, sediment concentration and sediment load of Wan River hydrological station in south Dabie Mountain were used as correlation indicators to investigate the regularity and possible cause of soil erosion which further provided the strategies and methods for agricultural soil and water conservation. The sediment discharge variation of Wan River was positive related with the fluctuation of runoff and sediment concentration, indicating that rainstorm in June and July is the high time of soil erosion and the storm-type of precipitation is the root cause. The key strategies for agricultural soil and water conservation should be directly focused on the heavy rainfall in June and July.

Recent Changes of Water Discharge and Sediment Load in the Buyuan River Basin, China

Changes in stream flow and sediment load in the Buyuan River Basin is a focus due to its vital ecological niche in supply feeding and spawning habitats for upstream migrant fish species of Mekong River. Based on the precipitation and water discharge series data (19592008), and sediment load series data (19932008), the runoff and sediment discharge variation and its response to precipitation has been analyzed. The results shown that, during the study period of 19592008 at the Manan gauging station, annual runoff of Buyuan River has generally decreased, but not significant at the 95% confidence level. However, the decline trend of sediment load was obvious for gauged period from 1993 to 2008. The seasonal runoff distribution in the Buyuan River was uneven and demonstrated an observably unimodal distribution. Both water discharge and sediment load are largely derived from wet season (from Jun to Nov), especially in flood season from July to September. During the study period 1993-2008, both annual and monthly observed value of water discharge and sediment load had coincident changing processes. Double mass curves indicated decreasing precipitation was the main reason for runoff reduction in the Buyuan River Basin; declining rainfall and water discharge were two key factors for sediment load reduction. In 2002, however, sediment discharge undergone abrupt change due to huge peak discharge and two consecutive years strong precipitation in the Buyuan River Basin.

Changes in streamflow and sediment discharge and the response to human activities in the middle reaches of the Yellow River

Abstract. The changes in streamflow and sediment discharge in the middle reaches of the Yellow River are a focus. In this paper, based on the precipitation, streamflow and sediment discharge series data (1950–2008), the streamflow and sediment discharge variation and its impact on precipitation/response to human activities have been analysis. The results show that significant decreasing trends in annual streamflow and sediment discharge have existed since the late 1950s in the middle reaches of the Yellow River (P = 0.01). Change-point analyses further revealed that transition years existed and that abrupt decline in streamflow and sediment discharge began in 1985 and 1981, respectively, in the middle reaches of the Yellow River (P = 0.05). Adoption of conservation measures in the 1980s and 1990s corroborates the identified transition years. Double-mass curves of precipitation vs. streamflow (sediment) for the periods before and after the transition year show remarkable decreases in proportionality of streamflow (sediment) generation. Compared with the period before the transition year, cumulative streamflow and cumulative sediment discharge reduced respectively by 17.8% and 28% during 1985–2008, which was caused by human intervention, in the middle reaches of the Yellow River. It is, therefore, concluded that human activities occupied a dominant position and played a major role in the streamflow and sediment discharge reduction in the middle reaches of the Yellow River.

Long-term variations in sediment discharge from mountain basins

Long-term variations in sediment discharge from mountain basins

The influence of shallow landslides on sediment supply: A flume-based investigation using sandy soil

The impact of rainfall-induced shallow landslides on hillslope sediment discharge is not well understood. This paper reports experimental measurements of sediment discharge after water-induced shallow landslides are triggered on sandy soil in a flume under simulated rainfall. The principal aim of the research was to investigate how varying soil depth affects the location and occurrence of shallow slope failures, as well as how it affects sediment yields downslope. Four experiments were conducted using the same sandy soil and a 30° and 10° compound slope configuration under average rainfall intensity of 50 mm h − 1 for up to 390 min. Soil depths were set to 200, 300, 400 and 500 mm. Engineering and geotechnical properties of the soil were examined. Sediment discharge and runoff were collected from the flume outlet at 15 minute intervals. Changes in the soil slope profiles after landslides and soil physical properties resulted from soil armouring, under continuous rainfall were also recorded. Results showed that sediment yields at the flume outlet, before landslides occurred, were very low and limited to the finer soil particles as would be expected for a sandy soil. However subsequent variations in sediment discharge were strongly related to failure events and their proximity to the outlet. Sediment yield was also affected by the original soil depth; the greater the depth, the higher the sediment yields. Post-failure reductions in sediment discharge were observed and attributed to post-failure slope stabilization under continuing rainfall and extensive soil armouring near the flume outlet. The results provide a clear linkage between landslides and sediment discharge due to hydrological processes occurring in the hillslope. This knowledge is being used to develop a model to predict sediment discharges from hillslopes following shallow landslide events.

Bimodal analysis of sediment transport in mountain torrents using hydrophones and sediment traps

Measurement of sediment transport in mountain torrents is important to elucidate the nature of the process. Many obstacles have prevented long-term continuous measurement of natural river basins with a substantial scale, so only a few sporadic observations have been collected over short durations. Direct sampling, therefore, has recently been augmented by more indirect but stable methods (hereafter ‘indirect methods’) including a hydrophone pipe-microphone acoustic sediment-discharge measuring system (hereafter ‘hydrophone system’). Hydrophone systems count the times (hereafter ‘pulses’) at which bedload sediments strike a steel pipe-microphone acoustic sensor. A statistical analytical method can be used to calibrate and estimate bedload amounts sampled by a sediment pit with a linear combination of pulses and flow discharges. Sediment discharges exhibit striking variability in orders of magnitude during flood events. However, researchers have not explicitly taken into account the volatile nature of sediment transport phenomena, as most quantitative studies have applied unitary analytical forms. In this study, three hydrophone systems were installed in 100- and 200-km2-scale river basins together with sediment trap pits. During floods, most sediment (approximately 80%) occurred within the first one-quarter of sediment sequences reordered in a descending manner. Therefore, sediment-hydraulic quantities were dissected into two sets: the upper quarter and the lower three-quarters. Frequency distributions of sediment-hydraulic quantities for each set were compared closely to differentiate the two sets based on a set of cutoffs using only direct methods. Bimodal analytical fitting forms were drawn for each set. The results revealed that at the time scale of flood events, the quantitative accuracy of sediment discharge estimates can be improved by dissecting sediment discharge phenomena into periods of concentrated and of minor occurrences, incorporating sediment discharge variation during the duration of floods.

Seasonal variations of sediment discharge from the Yangtze River before and after impoundment of the Three Gorges Dam

Over the past decades, > 50,000 dams and reforestation on the Yangtze River (Changjiang) have had little impact on water discharge but have drastically altered annual and particularly seasonal sediment discharge. Before impoundment of the Three Gorges Dam (TGD) in June 2003, annual sediment discharge had decreased by 60%, and the hysteresis of seasonal rating curves in the upper reaches at Yichang station had shifted from clockwise to counterclockwise. In addition, the river channel in middle-lower reaches had changed from depositional to erosional in 2002. During the four years (2003–2006) after TGD impoundment, ~ 60% of sediment entering the Three Gorges Reservoir was trapped, primarily during the high-discharge months (June–September). Although periodic sediment deposition continues downstream of the TGD, during most months substantial erosion has occurred, supplying ~ 70 million tons per year (Mt/y) of channel-derived sediment to the lower reaches of the river. If sand extraction (~ 40 Mt/y) is taken into consideration, the river channel loses a total of 110 Mt/y. During the extreme drought year 2006, sediment discharge in the upper reaches drastically decreased to 9 Mt (only 2% of its 1950–1960s level) because of decreased water discharge and TGD trapping. In addition, Dongting Lake in the middle reaches, for the first time, changed from trapping net sediment from the mainstem to supplying 14 Mt net sediment to the mainstem. Severe channel erosion and drastic sediment decline have put considerable pressure on the Yangtze coastal areas and East China Sea.