Incoming Sediment Coefficient Research Articles

Cross-sectional geometrical characteristic for the bends along the lower Jingjiang reach

The evolution of the bar-pool configurations in response to the upstream damming has significant impacts on channel regulations, navigations, water intakes and protection projects. Herein, this paper reports and analyses the evolution of bar-pool configurations in the bends along the Lower Jingjiang Reach (LJR) after the impoundment of the Three Gorge Dam (TGD), which is distinguished from the natural evolution of the bends. The main factors to the different adjustments of bar-pool configurations are the changes in incoming flow and sediment regime during pre- and post-TGD periods. To capture the changes in the bar-pool configurations, we have presented a new cross-sectional geometrical characteristic - relative lateral distance of the centroid (RLDC). RLDC has close relations with incoming sediment coefficient (i.e. incoming discharge divided by suspended sediment concentration during flood season). RLDC is better than the conventional cross-sectional geometrical characteristic (e.g. width to depth ratio) to indicate the bar-pool configurations of the downstream of the large dam projects. Based on the delayed response model, the values of RLDC in the bends of the LJR are related to the previous 4-6 years’ incoming sediment coefficient, and the correlation coefficient is about 0.90. RLDC is expected to capture the variations of bar-pool configurations in the bends downstream of the large dam project.

Seasonal scour and siltation induced spatiotemporal variations in riverbed sediment leakage coefficients as measured via the thermal tracer method

Riverbed scour and siltation can change the hydraulic conductivity and thickness of sediments, leading to notable spatiotemporal variations of groundwater–surface water interaction. However, correlated quantitative research on the dynamic leakage coefficient (LC) remains lacking under riverbed scour and siltation. Based on dynamic monitoring and analysis of sediment medium properties, river-groundwater levels, and temperature profiles in the section of Liao River, this study compared the applicable conditions of different thermal analytical models and identified spatiotemporal variations in the LC values induced by riverbed scour and siltation. The results showed that: (1) The thermal tracer method combined with river-groundwater level monitoring can effectively quantify spatiotemporal variations in the LCs of riverbed sediment. Based on the sediment temperature presents diurnal fluctuations and gradient characteristics in different seasons, the LCs in summer, spring, and autumn compared to winter in seasonally frozen regions should be calculated using transient and steady-state analytical solution models, respectively. (2) The LCs ranged from 0.47 to 24.90 × 10-6 s−1, showed larger temporal variability than spatial, increased from the river bank to the center, and had larger values in autumn and winter. (3) Distinct from previous findings, the riverbed underwent a scouring process associated with reductions in the incoming sediment coefficient as the river stage shifted from high to low, eroding the surface 4–8 cm of sediment. During this process, the LC increased by 40 to 420%. Further, the influence of riverbed scour and siltation on the LC varied by river section. Near the riverbank, the sediment LC was primarily controlled by hydraulic conductivity related to the coarsening of sediment lithology; however, it was influenced by variations in both sediment thickness and hydraulic conductivity near the riverbed center. Collectively, these results further understand the mechanisms underlying changes in the sediment LC induced by riverbed scour and siltation, as well as provide a scientific basis for ensuring the sustainable development of riverside water sources under such changes.

Medium-sized channel shaping in the lower Yellow River due to water–sediment regulation

Abstract The Yellow River (YR) is famous for its ‘hanging river’ due to the high sedimentation rate in the lower reaches. Reservoir sediment retention (RSR) and water–sediment regulation (WSR) of the Xiaolangdi Reservoir (XLDR) contribute to mitigating the siltation and adjusting the bankfull channel geometry in the lower Yellow River (LYR). However, few studies have been conducted to quantify the contribution of RSR and WSR to medium-sized channel shaping. In this paper, we analyzed the temporal and spatial variations of bankfull discharge (BFD) in the LYR, established empirical expressions between the BFD and the previous five-year average discharge above 2,000 m3/s and the incoming sediment coefficient during flood seasons, and then constructed three different water and sediment scenarios to quantify the contribution of the WSR of the XLDR to medium-sized channel shaping in the LYR. Calculated results indicated that WSR accounted alone for 68%–94% of the medium-sized channel shaping, with RSR contributing 6%–32%. The contribution of WSR to the recovery of medium-sized channels increased with time and became stabilized eventually. Along the LYR, such contribution also increased from 68% to 97% and eventually became stabilized in narrow reaches, reaching over 90%. The results will lay a solid technical foundation for the WSR of the XLDR, river regulation downstream of the reservoir, and sediment disposal of the YR.

Morphological Evolution Characteristics of River Cross-Sections in the Lower Weihe River and Their Response to Streamflow and Sediment Changes

River cross-section morphology and water and sediment conditions are deeply connected. In recent years, the lower Wei River has experienced regular flooding and drastic changes in river channel shape, causing significant harm to the economy and development of the lower reaches. This research investigated the morphological evolution features based on annual extensive cross-section data and water and sediment data from the hydrological stations of Xianyang, Lintong, and Huaxian in the lower Weihe River from 2006 to 2018 of river cross-sections and the reaction to water and sediment variations. The findings indicated that the lower Wei River’s cross-sectional alterations between 2006 and 2018 exhibited a trend of “flushing at both ends and siltation in the middle” while continuing to exhibit “non-flood flushing and flood siltation” features. The incoming sediment coefficient in the lower Weihe River declined dramatically, whereas the median diameter of suspended sediment particles grew significantly at the Lintong station. The average elevation of the river channel was highly synchronized with the change in the coming sediment coefficient, and the impact of big floods dramatically influenced the shape of the river cross-section. Human activities such as river management have directly affected the morphology of the river cross-section at Lintong station and caused a significant increase in the median diameter of suspended sediment particles, resulting in siltation in the Lintong river. The study’s findings can serve as a theoretical foundation for water and sediment regulation and river training in the lower Weihe River, reducing flooding damage.

Lag response of groundwater to changes in water and sediment characteristics in the lower Yellow River, China

• XLD operation decreased groundwater level along the lower Yellow River. • The highest groundwater level was delayed after XLD operation. • Inter-annual changes in groundwater were mainly determined by WL, SSC, and ISC. • Response of groundwater to changes in water–sediment lagged by 2–5 months. The process of water exchange between rivers and groundwater is critical for the circulation of material and energy; however, this process is strongly affected by dam-regulated water and sediment. The lower Yellow River seeps downward throughout its coarse and suspended (perched) riverbed and feeds the surrounding floodplain groundwater. The Xiaolangdi Dam (XLD) and water–sediment regulation scheme (WSRS) have altered the exchange processes between river and groundwater along the lower Yellow River. However, few researchers have examined the response of this river-fed groundwater system to the operation of the XLD and implementation of the WSRS. As such, in this study, we analyzed the responses of groundwater to changes in flow and sediment characteristics based on statistical analysis using historical data, extremely randomized tree models, and distributed lag non-linear models. We found that, since the XLD began operating in 2002, the groundwater level near the lower Yellow River substantially decreased, and the groundwater level in June significantly decreased (at the 0.05 level). The highest groundwater level in the pre-dam period occurred in August, whereas the highest level in the post-dam period occurred in October. Changes in the annual groundwater level and in the mid- and post-WSRS seasons along the lower Yellow River were mainly determined by the water level and incoming sediment coefficient in the river, whereas those in the pre-WSRS season were mainly determined by the water level and suspended sediment concentration. Regarding the changes in groundwater level between the mid- and post-WSRS seasons, we found that suspended sediment concentration was the dominant factor in the pre-dam period, whereas the combined effect of water and sediment was the dominant factor in post-dam period. The response of the monthly groundwater level to changes in water and sediment characteristics lagged by 2–5 months. These results are essential to understanding the interaction between river water and groundwater.

Recent variation in channel erosion efficiency of the Lower Yellow river with different channel patterns

• Both temporal and spatial declines existed in the channel erosion efficiency of the Lower Yellow River. • An excessive decrease in sediment transport capacity and a deficit of riverbed supply led to the temporal decline in channel erosion efficiency. • Cross-sectional geometry and bed material composition were the key factors to cause the differences in channel erosion efficiency of different reaches in the LYR. The operation of the Xiaolangdi Reservoir has exerted prominent effects on the processes of sediment transport and channel evolution in the Lower Yellow River (LYR) since 2000. The investigation into the spatio-temporal adjustment characteristics of channel evolution is of significance to the management of the LYR covering different channel-pattern reaches of braided, transitional and meandering. An indicator of channel erosion efficiency was firstly defined as the ratio of annual channel evolution volume to annual incoming water volume in a specified study reach. Furthermore, the values of channel erosion efficiency in different reaches of the LYR were calculated using the measured hydrological data and cross-sectional profiles during the post-dam stage. Calculated results show that the channel erosion efficiency in the LYR was exhibiting a decline tendency at both temporal and spatial scales, with the mean value of the whole reach decreasing from 6.0 (2000–2007) to 2.7 (2008–2020) and that of the braided reach four times higher than the meandering reach. Main factors influencing the channel erosion efficiency were investigated, covering the incoming flow-sediment regime, riverbed composition and cross-sectional geometry. The temporal decreases in these factors, including incoming sediment coefficient, bed material supply and geomorphic coefficient, resulted in the temporal decline in channel erosion efficiency. The discrepancies in channel erosion efficiency of different reaches were mainly ascribable to the variations in cross-sectional geometry and riverbed composition. The geomorphic coefficient in the braided reach decreased from 19.7 to 9.0 m −1/2 , and meanwhile the percentages of fine and medium grain sizes in the riverbed reduced by around 50% from 2000 to 2020, which led to the most dramatic variation in channel erosion efficiency and the different trajectory of channel evolution. In addition, the seldom varied channel slope played a minor role in the temporal variation of channel erosion efficiency, but partly accounted for the spatial reduction along the LYR. This study provides an illustrative example for the response of a large alluvial river with different channel patterns to the varying boundary conditions caused by upstream damming.

Transport characteristics of non-uniform suspended sediment in the Jingjiang Reach after the Three Gorges Project operation

三峡工程运用后，坝下游荆江段来沙量大幅度减小，处于严重的不平衡输沙状态，次饱和水流冲刷河床使悬沙量沿程恢复.基于实测水沙资料，分析了三峡工程运用后荆江段非均匀悬沙恢复特点.提出了恢复效率的概念用以表征悬移质沿程恢复的程度，并根据实测水沙资料计算了荆江段1994-2017年非均匀悬沙的恢复效率.结果表明：三峡工程运用前，荆江段各粒径组悬沙恢复效率绝对值均接近0，故该时期内各粒径组泥沙冲淤幅度不大；三峡工程运用后，各粒径组悬沙恢复效率绝对值均明显增大，且粗沙（d>0.125 mm）恢复效率绝对值远大于细沙（d<0.125 mm），故粗沙恢复程度更高.这主要是由于荆江段床沙组成中粗沙部分含量大，而细沙含量小.最后建立了三峡水库蓄水后非均匀悬沙恢复效率与来水来沙条件（来沙系数）的关系，结果表明：各粒径组悬移质恢复效率均与来沙系数呈正相关关系，全沙、细沙和中沙的决定系数（R²）分别为0.89、0.67和0.69，相关性较高，故荆江段各粒径组悬移质泥沙恢复效率较大程度上受到来水来沙条件的影响.;The amount of suspended sediment entering the Jingjiang Reach of the middle Yangtze River has been greatly reduced owing to the operation of the Three Gorges Project (TGP), which caused the sediment transport of the reach at a severe non-equilibrium state, and the suspended sediment concentration recovered during the channel scour process. Therefore, the characteristics of non-equilibrium transport of non-uniform suspended sediment were analyzed based on the measured flow and sediment data. In addition, the recovery efficiency was proposed to represent the recovery speed for suspended sediment load, and the corresponding values were calculated annually from 1994 to 2017 based on the measured data of the Jingjiang Reach. Calculated results show that the suspended sediment recovery efficiency of all particle-size groups was very close to zero before the TGP operation, which means that the channel deformation was small during this period. However, the recovery efficiency of each particle-size group increased greatly after the operation of TGP, and the recovery efficiency of coarse sediment (d>0.125 mm) was much higher than that of fine sediment (d<0.125 mm). This is mainly because the content of coarse sediment in bed-material of Jingjiang Reach is large, while the content of fine sediment is relatively small. Finally, empirical relationship was developed between the recovery efficiency and incoming flow-sediment regime, which shows that the recovery efficiency of each group was negatively related to incoming sediment coefficient, with the correlation coefficient for the total, fine and coarse sediment being 0.89, 0.67 and 0.69, respectively. Therefore, it can be concluded that the recovery efficiency of suspended sediment was significantly affected by the incoming flow and sediment conditions.

Adjusting the water-sediment matching relationship to reduce sedimentation in the flow rate constraint reach of the Lower Yellow River, China

The bank-full discharge along the Dongbatou-Aishan river reach of the Lower Yellow River is significantly lower than that of the upper reach and is the flow rate constraint reach that dictates the flood safety of the Lower Yellow River. The water and sediment matching relationship is the important factor that affects scour and siltation along this reach. It is necessary to control the relationship between water and sediment by retaining a portion of sediments in the upper reach in order to increase the overflow capacity in this reach. We first analyse the relationship between the water and sediment processes of the Lower Yellow River and the scouring and siltation of the constraint reach and verify the water and sediment conditions that require sediment retention in the upper reach. By analysing the characteristics of the river, we determined the area of sediment retention to be the reach that stretches from Xiaolangdi to Huayuankou. The relationship between the incoming sediment coefficient at Huayuankou Station and the percentage of sediment discharged and deposited in the lower reach is analysed. The results showed that when the incoming sediment coefficient is more than 0.04 kg.s/m6, sediment should be retain in the Xiaolangdi-Huayuankou river reach to maintain non-siltation in the constraint reach. Using bank-full discharge in the river reach from Xiaolangdi to Huayuankou and the channel stability as constraints we propose that the sediment retention in this river reach should be less than 0.144 billion m3.

Changes and implications of the relationship between rainfall, runoff and sediment load in the Wuding River basin on the Chinese Loess Plateau

The interactions between precipitation, runoff and sedimentation could reveal information about the processes of runoff and sediment production and the environmental evolution mechanisms of basins. Based on the hydrometeorological data collected over a long-term time series (1955–2012) which has been divided into five periods (P1 (1956–1969), P2 (1970–1979), P3 (1980–1989), P4 (1990–1999) and P5 (2000−2012)), changes and implications of the relationships between precipitation, runoff and sediment load (precipitation-runoff-sediment relationships) were studied in the Wuding River basin, China. Conclusions are as follows. (1) There is no obvious change occurs on the distribution of annual precipitation during 1955 to 2012. However, the annual runoff and sediment load are significantly decreased by 51.9% and 86.4%, respectively, from P1 to P5. (2) Under the same precipitation conditions, the runoff and sediment present overall decreasing trends. Decrease in the runoff was greater than that in the sediment load. The incoming sediment coefficient (ISC) reduced by 69.2%, indicating that the sediment transport ability per unit runoff was weakened. The significant reduction of ISC along with the decrease of the runoff amount caused the sediment brought into the Yellow River (STWY) decreased from P1 to P5. (3) Soil conservation measures played a critical role for the changes in precipitation-runoff-sediment relationships. Due to the rapid increases in the available storage of check dams in P2 and P5 and the areas of forest and grassland in P2, relationships among the precipitation, runoff and sediment has drastically changed. Decrease of the effects on sediment interception in check dams caused the sediment increased from P3 to P4. (4) Decreases of runoff and sediment load, and changes in their relationships would continue with the sustainable management of slopes and gullies. The potentially detrimental changes in the storage of check dams and those in runoff and sediment influenced by extreme weather conditions should be monitored and managed.

Cumulative impact of dam constructions on streamflow and sediment regime in lower reaches of the Jinsha River, China

The cumulative effect of cascade hydropower stations on river ecological environment has been widely concerned because of the significant streamflow hydrology change induced by dam constructions. The characteristics of the change in the lower reaches of the Jinsha River, China are analyzed based on long-term (1952–2015) hydrological and sedimentological data. The averaging coefficient, reservoir regulation coefficient (RRC), incoming sediment coefficient (ISC), and sediment transport modulus (STM), which reflect the variation of streamflow and sediment regimes, are defined and calculated. The results show that the construction and regulation of reservoirs reduces flow in flood season, increases flow in dry season, significantly altering the monthly discharge regimes. These alterations also led directly to changes in the timing of extreme flows at Pingshan Station. The monthly flow records at the basin outlet are reconstructed using stepwise regression, to reduce reservoir impacts. Comparisons of observed and reconstructed monthly flows demonstrate that the previous studies overestimated the cumulative effects of cascade reservoirs on flow processes. Furthermore, this study clearly illustrates that the reduction in sediment trapping and sediment transportation capacity together lead to the sharp reduction in annual sediment yield at the Pingshan Station. The earlier constructed reservoirs have more obvious effects on the ISC and STM than the more recent reservoirs and the effect of sediment trapping is related to reservoir location, on the main stream versus tributaries.

The influence of human activity on variations in basin erosion and runoff-sediment relationship of the Yangtze River

ABSTRACTIn this article, the effect of reservoir sediment retention is studied by introducing the incoming sediment coefficient, and sediment reduction by reservoir and soil-water conservation is quantitatively calculated in the Yangtze River. Results demonstrate that different human activities, including soil and water conservation, hydro-project construction and over construction, have different impact on the runoff and sediment variation. Moreover, sediment trapping by reservoirs is the main factor, soil and water conservation is an important factor and other human activities such as river sand mining and over construction are secondary factors for the variation of basin erosion and sediment transport in the Yangtze River. In addition, the total annual sediment reduction is 33.64 million tons in typical tributary of the upper Yangtze River, of which the contribution of the reservoir is 21.54 million tons, accounting for 64%, and the contribution of soil-water conservation is nearly 12.1 million tons accounting for 36%.

Small-scale sediment scouring and siltation laws in the evolution trends of fluvial facies in the Ningxia Plain Reaches of the Yellow River (NPRYR)

This study presents an analysis of variable sediment erosion and deposition, and their the complex mechanism in natural heavy silt-carrying rivers in the Ningxia Plain Reaches of the Yellow River (NPRYR). The aims of this study are to examine the sediment scouring and siltation laws, along with the evolution trend of fluvial facies in the NPRYR. Four small-scale typical reaches (R1, R2, R3, and R4) were divided firstly based on river regimes and geological conditions of the river beds. And then with ground-truth data from 1993 to 2015, a cross-section morphology method was used to calculate the average fluvial facies coefficient ζa of the typical river reaches, and also the sediment siltation weight T during the different time periods. In this study, with application of mathematical statistics and regression analysis, the main channel, beach, and whole reach were used as the basic units in order to identify the relationships of the responses among the suspended sediment concentration per unit discharge (ξ), sediment deposition per unit water weight (W), and variations of the fluvial facies coefficient (△ζa) in different units. This analysis is for establishing the prediction method for the sediment erosion and deposition, along with the evolution trends of fluvial facies. Our results show a logarithmic mapping relationship between the ξ and W, and significant correlation between T and △ζa. Therefore, a quick analysis and prediction of the silt sedimentation, and the trends of fluvial facies variations in the small-scale river reaches under a single water and sediment condition are feasible with our approaches presented here. The critical incoming sediment coefficient ξcs for the scouring and siltation balance in the R1 to R3 reaches were 0.0051 kg·s·m−6, 0.0055 kg·s·m−6, and 0.0049 kg·s·m−6, respectively, while the R4 reach shows an overall continuous siltation trend. The fluvial facies coefficient of the R1 to R4 reaches show an overall decreasing trend. Our results provided robust theoretical support for analyzing the sediment erosion and siltation, and the river regime evolution in the NPRYR. Our study also indicates a need for future research with an aim to further understand the evolution of erosion and siltation in heavy silt-carrying rivers.

Characteristics and influencing factors of sediment deposition-scour in the Sanhuhekou-Toudaoguai Reach of the upper Yellow River, China

The annual changes of sediment deposition-scour on the riverbed in the Sanhuhekou-Toudaoguai Reach of the upper Yellow River during the years 1952–2010 were investigated based on runoff and sediment transport observations from the Sanhuhekou and Toudaoguai hydrological stations. Multiple influencing factors such as reservoir operations, tributary inflows, as well as runoff and sediment loads from the Shidakongdui area were analyzed. The results show that even though the sediment loads from the major sources, the Shidakongdui area as well as the upstream tributaries such as the Qingshui River and the Zuli River have reduced especially since the 2000s as a result of enhanced water-soil conservation measures and improvement of vegetation cover, the study reach was still generally in a status of cumulative aggradation. This is mainly due to the joint operations of the Liujiaxia Reservoir and the Longyangxia Reservoir, which significantly reduced the annual runoff and sediment loads at the Sanhuhekou Crosssection. The reservoirs also remarkably altered the summer flood characteristics of the study reach, inducing the shape of the annual flow curve changing from a ‘single-peak’ into a ‘double-peak’. These alternations sharply decreased the sediment transport capacity of flooding in the summer flood season which yields more than 90% of the sediment loads, leading to an unbalanced relation between the water and sediment. In addition, the estimated incoming sediment coefficient of the Sanhuhekou Crosssection ranged from 0.003 to 0.014 kg s/m6, of which 0.004 kg s/m6 was suggested as a rough critical value to determine the scour or deposition status of the study reach.

Factors driving riverbed scouring and sedimentation in the Bayangaole to Toudaoguai reaches of the Upper Yellow River

The Inner Mongolia reaches of the Yellow River face problems of severe sedimentation caused by a variety of complex factors. The sedimentation process in those reaches has been characterized using the sediment balance method, and the key factors affecting the process have been analyzed using the correlation analysis method. The results show that during the period 1952–2012 the Bayangaole (Bayan Gol) to Toudaoguai reaches in Inner Mongolia have undergone successive processes of accumulative sedimentation, then relative balance, and then accumulative sedimentation once again. The total annual sedimentation is 12.0341×108 m3, of which accumulations from July to October account for 95.1% and the reaches from Sanhuhekou to Toudaoguai account for 98.5%. The main factor affecting scouring and sedimentation of the Bayangaole to Sanhuhekou reaches is the combined water and sediment condition. The critical conditions for equilibrium are an incoming sediment coefficient 700 m3·s–1. The main factor affecting scouring and sedimentation of the Sanhuhekou to Toudaoguai reaches is the incoming sediment from the tributaries on the south bank and the combined water and sediment condition of the main stream. The critical conditions of the main stream for maintaining equilibrium status are a flow discharge of the main stream exceeding 800 m3·s–1 and a comprehensive incoming sediment coefficient < 0.005 kg·s·m–6. The incoming sediment from the tributaries has little impact on the main stream when the annual sediment load is less than 0.1×108 t. The incoming sediment coefficient of the main stream and the incoming sediment from the tributaries both play vital roles in the riverbed evolution of the Inner Mongolia reaches, but the latter contributes the most.

The research of mechanism of constructing riverbed and index of flow and sediment of floodwater in the Lower Yellow River

Using the methods of field data analyses and theoretical study, the mechanism of constructing riverbed and index of flow and sediment of floodwater is researched in the Lower Yellow River. The results show that floodwater exert transporting sediment and constructing riverbed adequately, it can lower flood peak and store floodwater and aggrade floodplain and scour channel. The overflowing floodwater can be divided into three phases. In earlier phase, the floodplain aggradation and channel scour has not been exhibited; in metaphase, flow and sediment exchange of the floodplain and channel is sufficient, the floodplain aggradation and channel scour effect is good; in anaphase, the floodplain aggradation and channel scour slower. Synthesis considering riverbed evolvement and the security of floodplain, the controlling indexes are the overflowing parameter 1.4 γ p <0.028. At present, the bankfull discharge of the Lower Yellow River resume 4000 m3/s, in favor of figuring and maintaining logical size of channel, through the controlling of Xiaolangdi reservoir, the peak discharge of ejective floodwater should be 5600-6000 m3/s, the incoming sediment coefficient should be less than 0.028. If controlling floodwater can not overflowing, the peak discharge of ejective floodwater should be 3500-4000 m3/s approaching the bankfull discharge.

Response of reach-scale bankfull channel geometry to the altered flow and sediment regime in the lower Yellow River

The channel geometry in an alluvial river usually varies in response to the altered flow and sediment regime from upstream damming. Bankfull channel dimensions have significantly adjusted along the lower Yellow River (LYR) because of recent channel degradation caused by the operation of the Xiaolangdi Reservoir, which has led to longitudinal variability in the cross-sectional geometry. Therefore, describing the bankfull channel geometry of the LYR is more representative using reach-averaged variables. In this study, an integrated approach is proposed for calculating reach-scale bankfull channel dimensions based on the measured section-scale bankfull geometry in the LYR, and the proposed approach integrates a geometric mean based on the log-transformation with a weighted average based on the spacing between two consecutive sections. The post-flood reach-scale bankfull channel dimensions in three different channel-pattern reaches of the LYR were then calculated annually during the period from 1999 to 2012 using the proposed approach and the surveyed post-flood profiles at 91 sedimentation sections. Calculated results indicate that the channel geometry changed in the components of bankfull width and depth in the braided reach, with an increase of about 400m in the reach-scale bankfull width over the past 13years; and the channel evolution occurred mainly in the component of bankfull depth in the transitional and meandering reaches, with the increased reach-scale bankfull depths of 2.4 and 1.8m, respectively. Finally, empirical relationships for the three reaches of the LYR were developed between the reach-scale bankfull channel dimensions and the previous four-year average discharge and incoming sediment coefficient during flood seasons, with higher correlations being obtained when calibrated by the measurements from 1999 to 2010. Furthermore, the developed relations were also verified by the observed data in 2011 and 2012, with encouraging results.

Recent variation in reach‐scale bankfull discharge in the Lower Yellow River

ABSTRACTBankfull discharge is a key parameter in the context of river engineering and geomorphology, as an indicator of flood discharge capacity in alluvial rivers, and varying in response to the incoming flow and sediment regimes. Bankfull channel dimensions have significantly adjusted along the Lower Yellow River (LYR) due to recent channel degradation, caused by the operation of the Xiaolangdi Reservoir, which has led to longitudinal variability in cross‐sectional bankfull discharges. Therefore, it is more representative to describe the flood discharge capacity of the LYR, using the concept of reach‐averaged bankfull discharge. Previous simple mean methods to estimate reach‐scale bankfull discharge cannot meet the condition of flow continuity or account for the effect of different spacing between two sections. In this study, a general method to calculate cross‐sectional bankfull discharge using the simulated stage‐discharge relation is outlined briefly, and an integrated method is then proposed for estimating reach‐scale bankfull discharge. The proposed method integrates a geometric mean based on the log‐transformation with a weighted average based on the spacing between two consecutive sections, which avoids the shortcomings of previous methods. The post‐flood reach‐scale bankfull discharges in three different channel‐pattern reaches of the LYR were estimated annually during the period from 1999 to 2010 using the proposed method, based on surveyed post‐flood profiles at 91 sedimentation sections and the measured hydrological data at seven hydrometric sections. The calculated results indicate that: (i) the estimated reach‐scale bankfull discharges can effectively represent the flood discharge capacity of different reaches, with their ranges of variation being less than those of typical cross‐sectional bankfull discharges; and (ii) the magnitude of the reach‐scale bankfull discharge in each reach can respond well to the accumulative effect of incoming flow and sediment conditions. Finally, empirical relationships for different reaches in the LYR were developed between the reach‐scale bankfull discharge and the previous four‐year average discharge and incoming sediment coefficient during flood seasons, with relatively high correlation coefficients between them being obtained, and the reach‐scale bankfull discharges in different reaches predicted by the delayed response model were also presented for a comparison. These relations for the prediction of reach‐scale bankfull discharges were validated using the cross‐sectional profiles and hydrological data measured in 2011. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Shaping and maintaining a medium-sized main channel in the Lower Yellow River

This paper studies relations between bankfull discharge, lateral cross section variation and the incoming flow and sediment condition in the Lower Yellow River using measured data from 1950 to 2003. Since 1950 the bankfull discharge has obviously decreased and the ratio of channel width to flow depth has increased. The critical annual average incoming sediment coefficient (defined as the ratio of sediment concentration to discharge) and discharge at the Huayuankou station are approximately 0.012 and 1,850 m3 s−1, respectively, for no accumulative deposition occurring in the reach from Huayuankou to Lijin. On this basis, a mathematical model is used to study the scale of the main channel in the Lower Yellow River and its corresponding bankfull discharge under possible incoming flow and sediment conditions in the near future. The main factors influencing the scale of the main channel are analyzed, and measures to shape and maintain a medium-sized channel are discussed. The results show the effect of various water and sediment combinations released from the Xiaolangdi Reservoir on the shaping of the main channel and suggest that under recent incoming flow and sediment conditions, it is possible to shape and maintain a medium-sized channel with a bankfull discharge of approximate 4,000 m3 s−1.

Sediment delivery ratio and its uncertainties on flood event scale: Quantification for the Lower Yellow River

Sediment delivery ratio (SDR) for fluvial rivers was formulated with sediment rating curve. The observed data of SDR on flood event scale of the Lower Yellow River (LYR) were adopted to examine the formulation and to calibrate the model parameters. A regression formula of SDR was then established and its 95% prediction interval was accordingly quantified to represent its overall uncertainties. Three types of factors including diversity of the incoming flow conditions, river self-regulation processes, and human activities were ascribed to the uncertainties. The following were shown: (1) With the incoming sediment coefficient (ISC) being a variable, it was not necessary to adopt the incoming flow discharge as the second variable in the formulation of SDR; and (2) ISC=0.003 and therefore SDR=2 might be a threshold for distinguishing the characteristics of sediment transport within the LYR. These findings would highlight sediment transport characteristics on the scale of flood event and contribute to uncertainty based analysis of water volume required for sediment transport and channel maintenance of the LYR.

A study of sediment delivery by floods in the lower Yellow River, China

The concept of sediment delivery ratio (SDR) is used at the scale of channel reach to define the efficiency of sediment transport in a given river reach. Based on the data from 144 flood events during the periods 1950–1960 and 1969–1985, sediment delivery by floods in the lower Yellow River has been studied. The results show that SDR decreases with the increases in the event-averaged sediment concentration and the “incoming sediment coefficient” expressed as sediment concentration divided by water discharge. At a flood discharge of 4000m3 s−1, the SDR reaches its maximum. The SDR of flood events strongly influenced by hyperconcentrated flows is low. Channel shape has some influence on sediment delivery ratio. A relatively deep and narrow channel may enhance the sediment carrying capacity of the river and leads to a higher SDR. Significant difference exists in the event SDR between the floods that come from different source areas. The SDR of a given flood event is found to be related with the combination of sediment from different source areas; when the sediment from the coarse sediment producing area accounts for 50% of the event total, or the sediment from the fine sediment producing area accounts for 40% of the event total, the event SDR reached its maximum.