Temporal changes in suspended sediment transport during the past five decades in a mountainous catchment, eastern China
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Analysis of sediment transport is an effective approach for identifying sediment sources and for catchment management. However, a long-term analysis of sediment variability at multiple time scales is less available, especially in mountainous catchments. This study aims to determine sediment sources and to identify sediment transport dynamics, as well as the influencing factors, through analyzing long-term sediment fluxes at different time scales.
Materials and methods
In this paper, 32 years of sediment flux in an instrumented mountainous catchment in eastern Zhenjiang Province, China, was investigated at multiple time scales (i.e., monthly, seasonal, annual, and event). Sediment yields (SYs) during three time periods (i.e., 1964–1977, 1978–1989, and 2010–2015) were first classified by the Mann-Kendall and cumulative anomaly tests, and then sediment fluxes for each period were investigated and compared at multiple time scales.
Results and discussion
Annual SYs ranged from 29 to 308 t year−1 during the recording period and were significantly influenced by several high magnitude flood events. The mean annual SYs decreased from 153.82 t year−1 in 1964–1977, to 97.79 t year−1 in 1978–1989, and to 91.08 t year−1 in 2010–2015 due to improved soil conservation measures and increased reservoirs. At the seasonal scale, over 92% of the sediment was transported in spring and summer over the recording period. Heterogeneous sediment sources, partial areal distribution of rainfalls, and other factors led to complex suspended sediment concentration versus water discharge hysteresis loop patterns at the event and monthly scales.
The improved soil conservation measures and increased reservoirs over the recent decades decreased sediment availability, and the number and the magnitude of flood events from 1964 onward. However, the flood sediment fluxes in a few months were still high due to extreme precipitation events in recent years. The work can provide guidance for addressing sediment problems in this and/or other similar catchments.
KeywordsHysteresis loop Mountainous catchment Multiple time scales Sediment dynamics
Information on sediment transport dynamics for a river catchment is vital to managing sediment-related issues such as river and reservoir siltation, and the transferring of nutrients and contaminant in river networks (Rickson 2014; Vercruysse et al. 2017). Suspended sediment is also linked to ecological degradation, flooding, and damage to infrastructure in an increasingly populated world (Taylor and Owens 2009; Kusimi et al. 2014). Sediment transport is highly variable and time dependent due to different erosion conditions, such as climate, topography, sediment availability, and catchment size (Ludwig and Probst 1998; Gay et al. 2014; Vercruysse et al. 2017). For example, sediment load is controlled by the hyperconcentrated flow on the Chinese Loess Plateau (Fang et al. 2008a, b), while alternatively snow-melt runoff in winter contributes a great deal of sediment in the glacierized Andean catchments (Mao and Carrillo 2017). Sediment load can also be affected by anthropogenic alterations. Human activities, such as soil and water conservation measures and reservoirs, can reduce sediment supply (Suif et al. 2016). On the contrary, human activities such as the intensification of land use, mining, and construction works can increase catchment sediment yields (SYs; Vercruysse et al. 2017).
Analysis of sediment transport is an effective approach for quantifying sediment load and identifying sediment sources for river and catchment managements. In recent years, many papers on temporal dynamics of sediment transport have been published in different mountainous regions such as in the Mediterranean (e.g., Lenzi and Marchi 2000; Rovira and Batalla 2006; Béjar et al. 2018), the UK (e.g., Walling and Webb 1982; Haifa 1984; Williams 1989; Worrall et al. 2013), the USA (e.g., Gomez et al. 1997; Gao et al. 2013), Israel (Alexandrov et al. 2007), on the Chinese Loess Plateau (e.g., Fang et al. 2008a, b), in the Beibu Gulf region in southern China (Li et al. 2017), and in Taiwan, China (Kao et al. 2005), with a variety of results. For example, Alexandrov et al. (2007) found that convectively enhanced high intensity rainstorms produced a clockwise (CW) hysteresis in the suspended sediment concentration (SSC)-water discharge (Q) relation, while low intensity frontal storms led to either counter clockwise (CCW) or no hysteresis in the SSC–Q relation. Lenzi and Marchi (2000) demonstrated that when the sediment source was in channels, CW hysteresis occurred, CCW loops occurred when sediment is from slopes, and different SSC–Q relations can coexist in the same flood when sediment sources changed. Similarly, Nistor and Church (2005) and Fan et al. (2012) identified sediment sources through analyzing sediment dynamics at multiple time scales. However, although much work on sediment transport fluxes and their temporal variations have been conducted, there are at least three chief limitations. First, studies on long-term sediment variability at multiple time scales are less available, with several exceptions. For example, nearly 30 years (1986–2014) of sediment fluxes in an Alpine basin in Rio Cordon, Italy, was investigated by Rainato et al. (2017), and a good relationship between peak Q and sediment load at flood event scale was found, with assessing the sediment contributions of single floods and seasonal sediment flux to annual SYs. Gao et al. (2018) studied the changes of flow-sediment relationships of 14 catchments on the Chinese Loess Plateau during 1956–2014 and found that land use change greatly influenced these flow-sediment relations at daily, seasonal, and yearly scales. Second, information of sediment dynamics is lacking in river mountainous catchments such as those in Zhejiang Province, China, because high forest coverage and low SSCs draw less attention from scientists and local managers. However, due to pervasive human activities (e.g., deforestation, road construction, and mining) in past decades, sediment transported from upper catchments in Zhejiang Province has raised some river beds by 0.5 to 1. 0 m, decreasing the inland river course by more than 1000 km (Yu 1994). Finally, the impacts of human activities on sediment dynamics are still required although extensive studies have been conducted on some large rivers, such as in the Nile River Basin (Frihy et al. 1998) and in the Changjiang River basin (Mei et al. 2015; Li et al. 2017; Dai et al. 2018) where suspended sediment load decreased greatly due to human activities.
Against this background, there is a clear need for analyzing sediment fluxes and the influencing factors at multiple time scales for catchment management. Therefore, a catchment in the mountainous regions of eastern Zhejiang Province was selected for this study to analyze sediment dynamics over a long-term time period (1964–1989 and 2010–2015). The specific aims of this study were to (i) identify sediment dynamics at different time scales and (ii) explain their changing trends during this long-term period.
2 Materials and methods
2.1 The Quxian catchment
In the past decades, the landscape has experienced several changes since the establishment of the People’s Republic of China. From 1955 to 1958, some policies were installed to conserve the soil. Disorderly land reclamation was prohibited, and reforestation was advocated. However, during 1959–1961 and 1966–1976, large amounts of forests were destroyed, and steep lands were cultivated for grains. From 1980 onwards, local governments started to implement soil conservation measures again. These measures mainly include reforestation, contour cultivation, and planting pits. In 2000, a soil conservation plan was issued, and large-scale soil conservation measures were implemented. More agricultural lands on steep slopes, sparse forestlands, and some grasslands were implemented with conservation measures. Terraces were constructed in fields in combination with level ditches on sloping lands. Afforestation was conducted with fish-scale pits (a pit like a fish scale within which one tree is planted) in sparse forestlands. However, the lands with sparse grass and forest cover in regions over 500 m a.s.l were surrounded by fences for natural vegetation recovery. Land use maps indicated that until 2015, 72.50% of the catchment area was covered by forest lands, followed by 14.90% of dry lands. During 1985–2015, the area percentages of construction lands increased from 0.61 to 2.88% (Fig. S1 - ESM).
Reservoirs were built for the purpose of flood control in this region (Fig. 1c). Before 1966, a total of 12 reservoirs in three counties (i.e., Jiangshan, Kaihua, and Changshan) in the catchment were built, each having a capacity of less than one million cubic meters. From 1967 to 1977, 15 more reservoirs were constructed, including two large reservoirs. From 1980 to 1989, eight reservoirs were built. Up until 2005, there were a total of 35 reservoirs in the three counties, although data of constructed reservoirs in Chan County were not available before 1990 (Fig. S2 - ESM). Information regarding human activities was obtained from County Annals in the Quxian catchment.
2.2 Data source and treatment
The Quxian hydrologic station was built in the upper reaches of Qiantang River, and Q and SSC have been measured since 1964. At the hydrologic station, two or three water samples were collected for each sampling time. A horizontal sediment sampler was used to collect samples at the center of the river where the mean flow velocity was usually reached. The sampling intervals ranged from 5 min to 12 h, depending on the size of the flood. A total of 8149 samples were collected during the recording period (1964–1989, 2010–2015).
The samples were analyzed in the laboratory to determine their SSCs (kg m−3). Water level was observed at the staff gauge, and Q was obtained using previously established Q–water level curve. The curve has been calibrated with the discharges using current meters. The sampling method was kept unchanged during the past decades. The event and daily water yields (WYs; m3) and sediment yields (SYs; kg) were calculated using the measurements and time intervals. Monthly and yearly Q, SSC, WY, and SY were derived from daily data. In the catchment, rainfalls were recorded at five sites. Areal precipitation for the catchment was obtained by using Thiessen Method with the point station data.
All measurements of water stage, Q, and SSC at the Quxian hydrologic station, and rainfall data at six meteorological stations (i.e., Changfeng, Misai, Xujiacun, Shuangtadi, Xiakou, and Quxian) between 1964 and 1989 and 2000 and 2015 were compiled by the Qiantang River Water Resources Commission and printed for internal use. Because the SSC data in 1990–2009 were not available, sediment dynamics in this period were not analyzed in this paper. The TM images in 1985 and 2015 were available from Geospatial Data Cloud, Computer Network Information Center of Chinese Academy of Sciences (http://www.gscloud.cn), and several land use types were obtained using supervised classification (Fig. S1 - ESM).
2.3 Abrupt temporal change point test
2.3.2 Cumulative anomaly method
2.4 SSC–Q hysteresis loops and categorization
The hysteresis loop between SSC and Q can usually be used to investigate sediment sources (Williams 1989; Fang et al. 2008a, b). A CW hysteresis loop indicates a rapid delivery of sediment from channels, while CCW loops suggest the delay of sediment transport. The relations of SSC–Q at multiple time scales were categorized by using the method in Williams (1989).
3.1 Variations of annual sediment load
Statistical characteristics of annual precipitation amount, annual sediment load and water yield for different time periods
WY (× 108 m3)
SY (× 104 t)
SSC (kg m−3)
Similarly, a statistically significant change point of annual WY was also detected in 1977 through those two tests, while no significant changes were detected for annual precipitation amounts. The yearly mean WYs first declined from 65 × 108 m3 in 1964–1977 to 55 × 108 m3 in 1978–1989, and then increased by 72 × 108 m3 in 2010–2015. Annual SSCs also varied greatly, ranging from 0.11 to 0.38 kg m−3 during the recording period. For the three time periods, annual SSCs decreased from 0.24 kg m−3 in 1964–1977, 0.17 kg m−3 in 1978–1989, to 0.14 kg m−3 in 2010–2015 (Table 1).
3.2 Monthly and seasonal sediment dynamics
3.3 Sediment variability at the event scale
Discharge-sediment hysteresis loops and flood types of the recorded 201 flood events taking place during the recording period
Number of events
Flood characteristics, including SSCp, mean SSC (SSCm), SY, Qp, mean Q (Qm), WY, and flood duration, influence the flood type. All of these variables presented extraordinary fluctuations (Table S1 - ESM). Long duration floods frequently occurred. During the recording period, the mean duration of the floods was 124.8 h. The minimum and maximum durations of the floods lasted 1 day and 20 days, respectively. The Qm was 781.2 m3 s−1, ranging from 281.0 to 2290.4 m3 s−1. The Qp was above 7000 m3 s−1. A long duration and high Q yielded higher WY and SY values. For example, the flood from July 8–16, 1966, lasted 9 days and transported 94.1 tons of sediment, occupying over 60% of the total in 1966. A flood event in 1971 lasted 14 days and transported 143.0 million tons of sediment, which accounted for over 82% of the annual SY. For the three time periods, the mean event WY was the largest in 2010–2015 (i.e., 748.4 million m3). However, the SSCm of 0.1 kg m−3 in 2010–2015 was the lowest. For the recording periods, the mean event SY was 15.5 × 104 t, ranging from less than 0.1 × 104 to 143.0 × 104 t. Approximately 6.0 floods occurred each year during the recording period, and the annual mean numbers of flood events decreased from 6.9 floods in 1964–1977, 6.8 floods in 1978–1989, to 3.8 floods in 2010–2015.
4.1 Flood events and SY
The event SSCs in the study catchment were not very high. The largest instantaneous SSC was less than 4.50 kg m−3 (Fig. 6). Although these SSC values varied greatly, the SSCs were much smaller than those in other regions. For example, the event SSC in the Chinese Loess Plateau is hundreds of kilograms per cubic meter (Xu 2002; Fang et al. 2008a, b; Zhang et al. 2016). However, the flood duration in Quxian catchment was quite long due to frontal storms that usually have a high intensity in summer (Feng 1990; Xu et al. 2013). For example, a rainstorm at Misai station started on May 28 and ended on June 26, 1964, with a maximum daily rainfall amount of 73.3 mm. For the recorded 201 events, the flood events lasting over 120 h (i.e., 5 days) and over 72 h (i.e., 3 days) at Quxian station occupied over 38% and 70% of the total, respectively (Table S1 - ESM). Due to similar climate condition, long durations of rainfalls and floods also occurred in the Changjiang River (Dai et al. 2016; Mei et al. 2018). A single flood event could thus transport a large amount of sediment, and greatly influenced the annual SY. During the recording period, the SY in the flood year 1983 (defined as the annual WY exceeding the 90th percentile threshold; Mei et al. 2018) was the highest, and as much as 304.4 million tons of sediment was transported out of catchment outlet during this year. The instantaneous Q larger than 1500 m3 s−1 continued from July 7 to 15 with water level higher than 5 m or even above 8 m. The five flood events during this year exported nearly 80% of the annual SY. This is consistent with studies in other regions (i.e., Smith et al. 2003; Fang et al. 2008a, b). For example, Turowski et al. (2009) observed that approximately 41% of the annual SY in 1982–2008 was transported out of the catchment by three flood events in the Erlenbach Basin, Switzerland. In the Chabagou catchment of the Chinese Loess Plateau, over 95% sediment of the total in 1966 was transported by four flood events (Fang et al. 2008a, b). Similarly, in the Loushui river catchment, Jianxi Province of China, the contributions of individual flood events to annual SY ranged from 36 to 91%, with an average of 78% (Sun et al. 2016).
4.2 Sediment source and SSC–Q relations
The water flow velocity on hillslopes is several orders of magnitude lower than that in channels. When a sediment source area is a channel, SSCp precedes Qp, or they occur simultaneously. On the other hand, when sediment source areas are on a catchment’s slope, or in the upper part of a catchment, SSCp lags behind Qp (Haifa 1984; Rovira and Batalla 2006). Thus, analysis of SSC–Q relations for individual events can be of assistance in identifying sediment source areas (Williams 1989). For the study catchment, the percentage of type I and II floods occupied 87% of the total events (Table 2). The sediment could come from slopes and be deposited into the channel network and/or be directly eroded from the channel bank. The available sediment in the channels was flushed away when intensive floods occurred.
The SSC–Q relations were generally shown as CW and CCW hysteresis loops (Table S1 - ESM). A single CW or CCW loop usually had a pair of SSC and Q peaks (Fig. S5b,d - ESM). The fast response of sediment for type I and/or II floods led to a higher SSC on its rising limb than that on the falling limb for a given Q, resulting in a CW hysteresis loop. For example, the SSC of the flood on April 3–7, 1982, was 3.2 kg m−3 on the rising limb, while it was only 0.046 kg m−3 on the recession limb, given a Q value of 395 m3 s−1 (Fig. S5b - ESM). Due to progressive exhaustion of sediment availability, CW hysteresis loops dominated the study area which is in agreement with the observations by Walling and Webb (1982), Asselman (1999), and Rovira and Batalla (2006). In contrast, only three floods had CCW hysteresis loops for the 201 floods. This finding was also similar to those in large catchments (Williams 1989). This can be explained by increased sediment supply from slopes or the upper catchment and a downstream increase in lag time between SSC and Q peaks during long rainfall durations (Klein 1984; De Girolamo et al. 2015). For example, the lag time of the example shown in Fig. S5d (ESM) was up to 8 h.
The hysteresis loops can thus be used to identify sediment source areas (Klein 1984; Williams 1989; Mao and Carrillo 2017). In the case of CW hysteresis loops, the sediment source is the channels or banks, whereas CCW hysteresis loops occur when the sediment source is in relatively faraway places (Oeurng et al. 2010). In this study, single CW loops occupied over 50% of the total, whereas CCW loops were less than 2%. Complex and/or figure-eight loops combined several single CW and/or CCW loops together. This phenomenon indicated that sediment in channels was initially activated during floods, and then the sediment from slopes or in the upper catchment came later.
Approximately 95% of the 201 floods occurred during late spring and summer (i.e., from April to August). This implies that the sediment was accumulated in other months and flushed away during this time period. Physical weathering is intense in the study area, and large amounts of sediment are produced due to the interaction of the freezing-thawing process (Liao 2008; Chen 2016). During autumn and spring, cultivation action on slopes also provides more available sediment. Some sediment may even fall into streams due to steep slopes. Therefore, sediment is relatively rich before flood comes. As a result, SSCs were high in drought years or in the initial stage of a flood year. This implies that in the future, soil conservation measures should be implemented both in channels and on slope surface areas.
4.3 Impact of soil conservation measures and reservoirs
The numbers of flood types and SSC–Q hysteresis loops changed over the three time stages, probably resulting from continuously improved soil conservation measures and reservoirs. The percentages of type I floods decreased from 76% in 1964–1977, 74% in 1978–1989, to 52% in 2010–2015. The percentages of single CW loops at flood event scale also decreased from some 54.5% in 1964–1977, 55.6% in 1978–1989, to 13% in 2010–2015. This implies that less sediment was available from channels over time. During the past decades, many agricultural fields along the rivers were replaced by construction lands (Fig. S1b - ESM). This land use change decreased soil erodibility and also prohibited sediment from directly entering into rivers. Furthermore, more soil conservation measures have been implemented since 1977. The built reservoirs in the catchment can also trap much sediment from upstream mountainous areas (Fig. 1c). In the Chinese Yellow River, the operation of large dams together with land cover change in the middle reaches, has caused stepwise decrease in sediment delivery to the sea (Wang et al. 2017). Therefore, some smaller floods were controlled by the implemented soil conservation measures and reservoirs. In contrast, larger floods were less influenced or not easily controlled and still occurred from May to July 2010–2015 (Fig. S4 - ESM). The shape and extent were the other two characteristics of an SSC–Q loop, and their quantifications have been given in literatures (e.g., Fang et al. 2011; Zuecco et al. 2016). Quantification analysis of these indices in the future can reflect more information of sediment flux. However, although this study did not analyze these indices, it can be estimated that the extents of the SSC–Q loops can become narrower with time due to lower SSC at a given Q in recent years (Fig. 6). In addition, the time duration for future flood monitoring can be shortened to a few months (Fig. S4 - ESM).
4.4 Explanation of SSC–Q relations
The catchment has an area of over 5000 km2, and there are multiple factors influencing sediment behavior, including sediment sources, rainstorm characteristics, spatial and temporal distributions of water and sediment, runoff amounts and rates, travel rates, and the distances of flood water in the main channel to the catchment outlet (Klein 1984; de Boer and Edmonton 1989; Seeger et al. 2004; De Girolamo et al. 2015; Pietroń et al. 2015; Vercruysse et al. 2017; Sadeghi et al. 2018). Due to fast response of sediment in channels, CW hysteresis loops dominated (Fig. S5b - ESM). However, this does not mean that sediment mainly came from channel networks. Due to the large catchment area, the sediment from slopes could first be deposited in channels and then was transported out of the catchment. This means that the SSC–Q hysteresis becomes more complex in the large catchment depending on storage-removal of sediments in rivers. Sediment source and SSC–Q relations could be altered by land use change. The conversion of cultivated fields to construction lands during 1985–2015 could directly reduce sediment delivery to rivers (Fig. S1 - ESM), resulting in less percentage of CW loops (Table 2). Furthermore, the asynchronicity between sediment production and flood areas in different subcatchments can also induce complex SSC–Q patterns. For example, the rainfall center of mass usually changes over time. As a result, the relative travel times of the flood waves and the sediment fluxes changed in different streams, inducing multiple SS–Q hysteresis loops (Fig. S5e,f - ESM). The figure-eight loop on April 30 to May 4, 2013, can be explained by different sediment source areas.
At the monthly scale, CW loops were first formed due to abundant previously deposited sediment in channels. On the contrary, when sediment was limited in channels, the eroded soils from slopes can supplement sediment again. As a result, monthly CW plus CCW loops formed. Areal and temporal distributions of precipitation and water flow can activate multiple peaks of monthly sediment and runoff flow, inducing complex loops. Monthly precipitations in 1972 and 1988 were shown as examples here (Fig. S6 - ESM). In 1972, the precipitation amounts ranged from 1570 mm at the Quxian station to 1854 mm at the Changfeng station. Multiple peaks of monthly precipitation occurred at the six meteorological stations, with inconsistent changed characteristics in this year. This can thus cause complex monthly SSC–Q loops. In contrast, the monthly patterns of precipitation in 1988 at these stations were simple and consistent, resulting in relatively simple loops (Fig. 5b). Therefore, sediment availability and rainfall patterns could be the main causes inducing complex monthly SSC–Q loops.
This study was to identify long-term sediment dynamics and to explain their changing trends in a catchment. These analyses at one site could be challenging to understand what is going in this large catchment and to discriminate the contributions of individual factors to sediment flux change. This information can be solved in the future study using spatially distributed soil erosion models such as the famous Soil and Water Assessment Tool SWAT. However, in this study, the detailed analyses and discussion of sediment fluxes at different time scales can still identify the changes of sediment flux over a long-time period, identify the general effect of the influencing factors, and give some implications for river and catchment managements.
The analysis of sediment dynamics at different time scales (monthly, seasonal, annual, and event) at Quxian catchment can provide insight into the characteristics of sediment fluxes in eastern China. In unraveling the temporal dynamics of sediment issue in the mountainous catchments, the present work contributes to an increase in the knowledge regarding the long-term (1964–2015) and multiple-scale sediment transport processes of river sediment transport in eastern Zhejiang Province, China.
Sediment transport varied greatly at different time scales, depending on water flow, sediment availability, and other factors. Abrupt temporal change points in the annual SY were first identified, and then three stages were classified. Due to improved soil conservation measures and reservoirs, mean annual SY and SSC continuously decreased over the recording period. For each individual time period, over 92% of the sediment was transported during the spring and summer periods due to high Q and long flood durations. Impacted by human activities (e.g., contour cultivation, reforestation, and constructed reservoirs), a large percentage of sediment was transported during few months (i.e., May to July) in 2010–2015. Water supply, sediment availability, and human activities exerted complex effects on monthly SSC–Q relations, inducing multiple hysteresis loops. Annual sediment transport was greatly influenced by several floods. From 1964 onwards, the numbers of annual flood events decreased. More CCW and complex loops occurred, due to changed sediment source areas. This study indicated that improved soil conservation measures and reservoirs were responsible for the decrease in the number and magnitude of flood events. However, the SYs in a few months (i.e., May to July) were still high due to increased precipitation amounts and extreme events in recent years. These analyses at one site could be challenging to understand what is going in the large catchment, and to discriminate their individual contribution. However, the findings of this study can contribute to providing guidance for addressing sediment problems for this and/or other similar catchments around the world by identifying potential sediment sources for further implementing soil conservation measures in rivers and on slopes, and by detecting flood behavior conditions in rivers in advance. In this manner, the sediment peaks in the few months could be mitigated or controlled.
This work was financially supported by projects of the National Natural Science Foundation of China (grant number 41571271).
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