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Bi-objective analysis of water–sediment regulation for channel scouring and delta maintenance: A study of the lower Yellow River
Global and Planetary Change 133 (2015) 27–34
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Bi-objective analysis of water–sediment regulation for channel scouring and delta maintenance: A study of the lower Yellow River Dongxian Kong, Chiyuan Miao ⁎, Jingwen Wu, Lin Jiang, Qingyun Duan State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, PR China Joint Center for Global Change Studies, Beijing 100875, PR China
a r t i c l e
i n f o
Article history: Received 24 May 2015 Received in revised form 16 July 2015 Accepted 29 July 2015 Available online 31 July 2015 Keywords: Channel scouring Delta maintenance Lower Yellow River Yellow River Delta
a b s t r a c t Long-term hydrological data and remotely-sensed satellite images were used to analyze the effects of the water– sediment regulation scheme (WSRS) implemented in the lower Yellow River (LYR), China, between 1983 and 2013. The WSRS aimed to control channel scouring in the LYR and maintain the Yellow River Delta (YRD). Channel erosion in the LYR has primarily depended on the incoming sediment concentration at Xiaolangdi, where the concentration must be lower than approximately 9.17 × 10−3 t m−3 to avoid rising of the riverbed. In 1996, an artificial diversion altered the evolution of the YRD. To maintain delta equilibrium, an average sediment load of about 441 × 106 t year−1 was required before 1996, after which this value decreased to 167 × 106 t year−1. We provide a preliminary estimate of the incoming water and sediment conditions required at the Xiaolangdi station to guarantee both LYR channel scouring and maintenance of the YRD. Our results show that it is feasible to transport sediment originally deposited in the LYR to the river mouth to maintain the delta, which is of great significance for the future management and environmental protection of the LYR. © 2015 Published by Elsevier B.V.
1. Introduction A river delta is a coastal feature created by the accumulation of sediment near the mouth of a river. Over the past few decades, river deltas have been a focus of research and an area of concern due to their socio-economic importance and unique ecological environment (Syvitski and Saito, 2007; Kong et al., 2015a). River deltas are an important agricultural area for the local residents because of the fertile soil. In addition, river delta wetlands are an important habitat and transfer area for many rare and endangered migrating birds. Unfortunately, almost all of the river deltas worldwide are at risk of sinking and shrinking (Syvitski et al., 2009; Syvitski and Higgins, 2012). Recent research shows that the trapping of sediment behind dams and subsequent reduction in sediment delivery to deltas, together with human control of river discharge routing across delta plains, have contributed to delta sinking (Syvitski, 2008; Blum and Roberts, 2009; Grill, 2015). In addition the tendency toward delta sinking has been further exacerbated by rising sea levels resulting from climate change (Blum and Roberts, 2009; Kirwan and Megonigal, 2013; Giosan et al., 2014; Ibáñez et al., 2014). The consequences include shoreline erosion, loss of swamps and wetlands, salinization of cultivated land, and the endangerment of
⁎ Corresponding author at: State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, PR China. E-mail address: [email protected] (C. Miao).
http://dx.doi.org/10.1016/j.gloplacha.2015.07.007 0921-8181/© 2015 Published by Elsevier B.V.
hundreds of millions of people (Syvitski, 2008; Restrepo, 2012; Temmerman et al., 2013). The Yellow River is the second largest river in the world in terms of sediment load, with an average of 1.1 × 109 t reaching the ocean annually (Milliman and Meade, 1983). In the last century, heavy soil erosion on the Loess Plateau led to intensive sedimentation in the lower Yellow River (LYR) channel (Miao et al., 2012). One effect of the heavy sedimentation in the LYR was an obvious shrinkage of the main channel accompanied by a sharp decrease in the flood discharge capacity, which markedly influenced the management of the river for flood control. The heavy sedimentation also led to the phenomenon of a secondary perched river in local reaches of the LYR, posing a huge flood risk to the local residents (Fig. 1). The heavy sedimentation in the river channel further influenced the evolution of the Yellow River Delta (YRD) because far less sediment was delivered to the river mouth. By the 2000s, the water discharge and sediment load delivered to the sea had decreased by 68% and 88%, respectively, compared with the 1950s (Miao et al., 2011). Since 2002, a water–sediment regulation scheme (WSRS) has been conducted annually by the Yellow River Conservancy Commission (YRCC) to regulate and control flow and sediment transport in the lower reaches of the river via reservoirs on the mainstream and tributaries, oriented toward flood control, ice control, and sediment reduction (Kong et al., 2015b). Most previous studies related to the WSRS focused on changes in the hydraulic characteristics (Zhang et al., 2009), sediment transportation (Xu and Si, 2009), and channel adjustment (Xu et al., 2005; Miao et al., 2010) in the LYR. It has been found that reducing the incoming
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D. Kong et al. / Global and Planetary Change 133 (2015) 27–34
Fig. 1. Location of the lower Yellow River (LYR) and the Yellow River Delta. In this study, the LYR is defined as the reach between Xiaolangdi Reservoir in Henan province and the river mouth in Shandong province, with a length of approximately 850 km flowing across the North China plain.
sediment concentration is conducive to scouring the riverbed and reducing the risk of flooding (Kong et al., 2015b). In contrast, the YRD will shrink if the incoming sediment load is inadequate (Kong et al., 2015a). Although the effects of the WSRS on water and sediment delivery to the sea have been addressed previously (Yu et al., 2013), few studies have quantified the effects on both the YRD and the LYR simultaneously. In this study, we review and quantify the influence of the WSRS on channel erosion in the LYR, quantitatively analyze the evolution of the YRD region, and conduct a preliminary bi-objective analysis for channel scouring and delta maintenance. The insights gained into the behavior of the LYR over time will be useful to those making
decisions on such issues as further implementations of the WSRS, large-scale construction in the region, and environmental-protection measures that affect the LYR. 2. Dataset and methodology 2.1. Data sources We utilized multi-temporal remotely-sensed Landsat data from a Multispectral Scanner (MSS), a Thematic Mapper (TM), and an Enhanced Thematic Mapper (ETM+) for the period from 1983 to 2013 (Table 1),
Table 1 List of satellite images used in the present work. Acquisition data
Image type
Resolution (m)
Bands
Acquisition date
Image type
Resolution (m)
Bands
07/07/1983 07/06/1984 06/09/1985 08/08/1986 08/06/1987 10/06/1988 15/07/1989 16/06/1990 06/08/1991 07/07/1992 08/06/1993 30/08/1994 18/09/1995 02/07/1996 06/08/1997 09/08/1998
MSS MSS TM TM TM TM TM TM TM TM TM TM TM TM TM TM
80 80 30 30 30 30 30 30 30 30 30 30 30 30 30 30
4 4 7 7 7 7 7 7 7 7 7 7 7 7 7 7
06/04/1999 08/04/2000 06/06/2001 29/09/2002 07/08/2003 05/05/2004 03/07/2005 04/06/2006 07/06/2007 12/08/2008 07/08/2009 11/09/2010 02/06/2011 04/06/2012 26/08/2013
TM TM ETM+ ETM+ TM TM ETM+ ETM+ ETM+ ETM+ TM TM ETM+ ETM+ ETM+
30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
7 7 8 8 7 7 8 8 8 8 7 7 8 8 8
D. Kong et al. / Global and Planetary Change 133 (2015) 27–34
totaling thirty-one scenes archived by the Earth Resources Observation and Science (EROS) Center (http://glovis.usgs.gov/). All data accounted for the impact of cloud cover. According to the Worldwide Reference System, one full MSS (path 130, row 34) or TM (path 121, row 34) scene fully covered the Yellow River Delta in this study. Data on annual runoff and suspended sediment load at selected hydrological stations (Xiaolangdi and Lijin) from 1983 to 2013 were obtained from the Yellow River Conservancy Commission (YRCC). Data for the annual sediment load and runoff at the Xiaolangdi station are missing for 2001 and 2002, when the Xiaolangdi Reservoir was under impoundment. Consequently, we took this period out of consideration when analyzing data for this study. 2.2. Methodology Values for annual channel erosion/deposition were calculated from the annual sediment load at the Xiaolangdi station minus the annual sediment load at the Lijin station, with positive values indicating deposition and negative values indicating erosion. This calculation method is based on the fact that there are few tributaries of the LYR which may introduce additional or export partial sediment load. It should be noted that the values for annual channel erosion/deposition represent only the overall performance of the river channel over the year, without concern for any intermediate changes during the year. Linear regression was used to quantify the relationship between incoming sediment concentration at Xiaolangdi and the amount of erosion/deposition between Xiaolangdi and Lijin. In order to explore the evolution of the YRD, we applied an interactive interpretation technique combining an automatic boundarydetection algorithm with human supervision to detect the land–ocean shoreline boundaries from satellite images and calculate the delta area. The automatic boundary-detection procedure consisted of five steps. First, we calculated the background trend to remove specular reflection of solar radiation on non-flat water surfaces. Second, we applied a noise-removing algorithm to reduce scattered noise contaminating the satellite images. Third, we chose an adaptive threshold that
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differentiated the land from the ocean, and then we transformed the data into black–white (BW) binary form. Fourth, we employed an automatic boundary-detection algorithm to locate boundaries in the BW images, and verified or adjusted them with reference to a combined Thematic Mapper (TM) 432 pseudocolor image. The Matlab Image Processing Toolbox (Mathworks, Natick, MA, USA) was used to implement the algorithm and batch-process all the Landsat images. Finally, we used the ArcGIS 10 Image Processing Toolbox (Esri, Redlands, CA, USA) to obtain the delta area in each satellite image (Fig. 2). The exact position of the Yellow River Delta coastline varies with time and tide, so errors are introduced to the analysis when comparing coastlines extracted from satellite images acquired at different times in any given day. To evaluate the maximum error caused by tidal effects, we calculated the error in delta area as the product of the maximum coastline distance and coastline length, under the assumption that the entire coastline remains parallel under different tidal conditions. It was found that the maximum relative error is about 1.1% of the total delta area, indicating that our calculation results are relatively accurate. We used doublemass curve analysis to explore the influence of an artificial river diversion on the evolution of the YRD, and we used linear regression to establish the relationship between changes in the delta area and sediment feed. 3. Results 3.1. Quantification of the relationship between incoming sediment concentration and channel erosion in the lower Yellow River Previous studies have shown that channel erosion and deposition in the LYR are not dominated by either incoming runoff or sediment load individually: the dominant factor is the incoming sediment concentration (Kong et al., 2015b). Fig. 3 shows the variations in average annual sediment concentration at Xiaolangdi and annual channel erosion between Xiaolangdi and Lijin from 1983 to 2013. The levels of sediment concentration prior to 2000 were high, with an average value of 27.15 × 10− 3 t m−3. However, sediment concentrations plunged to
Fig. 2. Satellite images showing changes to the Yellow River Delta over the years.
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Fig. 3. Variations in average annual sediment concentration at Xiaolangdi (top) and annual channel erosion between Xiaolangdi and Lijin (bottom) from 1983 to 2013. The values for annual channel erosion/deposition were calculated as the annual sediment load at the Xiaolangdi station minus the annual sediment load at the Lijin station, with positive values indicating deposition and negative values indicating erosion. Data for annual sediment load and runoff at the Xiaolangdi station are missing for 2001 and 2002.
low levels after 2002, with an average value of 3.11 × 10−3 t m−3. The changes in channel erosion are almost the mirror image of the changes in sediment concentration. Thus, there must be a threshold value at which suspended sediment concentration influences sediment movement. Excessive sediment will be deposited on the riverbed when the suspended sediment concentration exceeds this threshold value. Correspondingly, when the suspended sediment concentration is below the threshold value, the sediment deposited on the riverbed will be rolled and washed away. Fig. 4 shows the relationship between the incoming sediment concentration at Xiaolangdi and annual channel erosion between Xiaolangdi and Lijin. Despite the large degree of scatter in the data, there is a significant positive correlation between the incoming sediment concentration and annual channel deposition. A linear regression fits the data well during the period 1983–2013: y ¼ 15:9 103 x−145 R ¼ 0:92
Pb0:001
ð1Þ
Fig. 4. Relationship between the incoming sediment concentration at Xiaolangdi and annual channel erosion between Xiaolangdi and Lijin. Data points are from 1983 to 1999 and 2002 to 2013.
where x and y are the incoming sediment concentration (t m−3) and annual channel erosion (106 t), respectively. In Eq. (1), the incoming sediment concentration (x) equals 9.17 × 10−3 t m−3 when the annual channel erosion (y) is 0. This indicates that the incoming sediment concentration at the Xiaolangdi station must be lower than approximately 9.17 × 10−3 t m−3 to ensure that channel erosion takes place between Xiaolangdi and Lijin. Data from most years are consistent with this value.
3.2. Quantification of the relationship between the evolution of the Yellow River Delta and incoming sediment load Generally, interactions among soil, fluvial, and coastal dynamics are responsible for the relative roles played by sedimentation and erosion at delta lobes, thus controlling the growth and shape of the overall delta. For the YRD to be in equilibrium and its area to remain constant, the rate of sediment accumulation from the river must equal the rate of erosion from near-shore coastal flows. Over the past thirty years, the near-shore coastal dynamics have hardly varied at the mouth of the Yellow River (Hu and Cao, 2003; Wang et al., 2010) and so changes to the river flux appear to be the key factor controlling the evolution of the delta (Fan et al., 2006). Previous studies have shown that the total sediment feed is the dominant factor influencing the evolution of the YRD rather than the combination of sediment and water feeds (Wang et al., 2006; Kong et al., 2015a). This is mainly attributed to a significant linear correlation between sediment load and water discharge. Since the longitudinal slope of the coastal area in the YRD is almost small to horizontal (less than 0.01 o) (Wang et al., 2010), the sediment required to maintain the delta shoreline can be regarded as constant in short period. Riverbed features and coastal dynamics are unique to each particular channel, so we performed double-mass curve analysis to explore the influence of an artificial river diversion on the evolution of the YRD as shown in Fig. 5. We found that the slope of the relationship between sediment feed and delta area was different before and after the artificial diversion in 1996, indicating that the threshold value of sediment feed required to keep the YRD stable varies for different channels. This is consistent with previous studies (Wang et al., 2006).
D. Kong et al. / Global and Planetary Change 133 (2015) 27–34
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3.3. Estimate of water and sediment conditions for channel scouring and delta maintenance From the above analysis, we can see that the total sediment feed at the Lijin station is the dominant factor influencing the evolution of the YRD, and the sediment load at the Lijin station is related to the incoming sediment concentration at the Xiaolangdi station. Thus, channel scouring and delta maintenance can be achieved concurrently by regulating the incoming water and sediment load at the Xiaolangdi station. From Eq. (1), SL J ¼ SXLD −15:9
Fig. 5. Double-mass curve analysis of the Yellow River Delta area and sediment feed. The slope of the relationship between sediment feed and delta area was different before and after the artificial diversion in 1996.
Noting that the runoff and sediment reached the sea through different channels before and after the artificial diversion was introduced in 1996, we conducted linear regression analysis over two phases, 1983–1995 and 1997–2013. Here, we define the annual area change in the i + 1th year as the arithmetic difference between the area of the YRD in the i + 1th year minus that in the ith year. The linear regression functions obtained for the annual sediment load (x, 109 t) and the annual change in YRD area (y, km2) for the two phases are: R ¼ 0:59 P ¼ 0:033 before 1996
ð2Þ
y ¼ 0:347x−57:8 R ¼ 0:66 P ¼ 0:005 after 1996 :
ð3Þ
y ¼ 0:176x−77:5
Fig. 6 plots the change in YRD area against sediment load together with the regression lines, for both phases. Although scatter is evident, in both cases the trend is for an increase in delta area with an increase in sediment load. From Eqs. (2) and (3), the annual change in YRD area was zero and hence equilibrium was achieved, when the average annual sediment load equaled 441 × 106 t for the period 1983–1995 and 167 × 106 t for the period 1997–2013. The two values for threshold sediment load are substantially different, demonstrating again the important influence of river flux characteristics on the evolution of the delta.
SXLD þ 145 W XLD
ð4Þ
where SLJ is the sediment load at the Lijin station (106 t), SXLD is the incoming sediment load at the Xiaolangdi station (106 t), and WXLD is the water discharge at the Xiaolangdi station (109 m3). From Eqs. (2) and (3), sediment loads of about 441 × 106 t year−1 and 167 × 106 t year−1 were required to maintain the YRD equilibrium before and after the artificial diversion in 1996. Thus, SXLD −15:9
SXLD þ 145≥441 W XLD
before 1996
ð5Þ
SXLD −15:9
SXLD þ 145≥167 W XLD
after 1996 :
ð6Þ
In order to achieve channel scouring in the LYR, the incoming sediment concentration should be no more than 9.17 × 10− 3 t m− 3, according to Eq. (1). Thus, SXLD ≤9:17: W XLD
ð7Þ
Eqs. (5) and (6) can therefore be simplified as follows: SXLD ≥
296W XLD W XLD −15:9
before 1996
ð8Þ
SXLD ≥
21:6W XLD W XLD −15:9
after 1996 :
ð9Þ
Eq. (7) represents the threshold condition for channel scouring in the LYR, Eq. (8) represents the threshold condition for area maintenance of the YRD before 1996, and Eq. (9) represents the threshold condition for area maintenance of the YRD after 1996. The water and sediment conditions for channel scouring and delta maintenance are
Fig. 6. Relationship between annual sediment load and annual change in delta area before (a) and after (b) the artificial diversion in 1996.
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Fig. 7. Incoming water (WXLD) and sediment (SXLD) conditions at the Xiaolangdi station for channel scouring in the lower Yellow River (a) and maintenance of the Yellow River Delta (b), and channel scouring and delta maintenance together (c). Each subplot is divided into several numbered regions, relating to the target conditions (see Table 2).
shown in Fig. 7 and Table 2. It can be seen that there is a specific set of conditions relating to incoming water and sediment load at the Xiaolangdi station that provides both channel scouring in the LYR and area maintenance of the YRD (Fig. 7c). Data from 1983 to 2013 fit well with the results of our analysis.
Table 2 Summary of the graphical regions from Fig. 7. Region
1 2 3 4 5 6 7 8 9 10 11
Period (1983–1996)
Period (1996–2013)
Channel scouring
Channel scouring
Delta maintaining
× √
√ √ √ × × ×
Delta maintaining
× √ × × √ × × √ √ × ×
√ √ √ × × ×
× √ √ × √ √ √ √ ×
“√” indicates that the water and sediment conditions in the correspondingly numbered region satisfy the target requirements; “×” indicates that the water and sediment conditions do not satisfy the target requirements. A black space indicates that a target was not specified for that numbered region.
4. Discussion As is well known, adequate sediment feed is necessary to maintain the YRD. The sediment in the Yellow River comes mainly from the middle Yellow River (MYR) basin where soil erosion is very serious, especially on the Loess Plateau. This leads directly to the high sediment concentration in the LYR. Due to the flat terrain surrounding the LYR, a substantial portion of the sediment load is deposited in the river channel, resulting in uplift of the riverbed. Therefore, the sediment concentration is relatively low at the point where the Yellow River flows into the sea. This process is shown in a simplified form in Fig. 8a. The growth or shrinkage of the YRD depends on the amount of sediment present at the mouth of the river. In recent decades, many soil and water conservation measures including afforestation, grass plantation, and land terracing have been implemented in the MYR basin. Since the 1970s, sediment flux from the Yellow River to the sea has shown a marked tendency to decrease, which is unfavorable for protection of wetlands in the YRD (Xu, 2003). Prior to this, water flux with a low sediment concentration and high kinetic energy flowed into the LYR. This scoured the riverbed and transported the sediment downstream. The river therefore had high sediment flux when it reached the estuary. In this case, the growth or shrinkage of the YRD depends on the amount of MYR sediment remaining, plus the transported sediment from the LYR. This process is shown in simplified form in Fig. 8b. Our analysis shows that it is feasible to transport the sediment deposited in the LYR to the river mouth (Fig. 7 and Table 2, region 2). However, the
D. Kong et al. / Global and Planetary Change 133 (2015) 27–34
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Fig. 8. Schematic diagram illustrating two different programs affecting maintenance of the Yellow River Delta.
sediment transported from the LYR alone is insufficient to maintain the YRD, as shown by Eq. (4), and an appropriate inflowing sediment load from the MYR basin is required. Thus, appropriate water and sediment conditions need to be adjusted via the WSRS. For any particular river system, the features of the river will not change greatly in the short term without human intervention. Therefore, for the LYR, a specific regulation scheme is required to produce the appropriate water flow and sediment concentrations, and then to achieve both channel scouring and delta maintenance. However, it is noteworthy that the sediment requirement for YRD maintenance is not immutable. In particular, artificial river diversion can significantly alter the sedimentation features in the Yellow River estuary, leading to a change in the minimum sediment requirement (for example, 441 × 106 t from 1983–1995 and 167 × 106 t from 1997–2013). Thus, the regulation scheme needs to be adjusted to satisfy new circumstances. This can be clearly observed in Fig. 7. We found that the acceptable bounds for regulation widened after the artificial diversion in 1996, due to the lower concentration of sediment required to maintain the delta. This is attributed to reduced erosion by the tides and waveinduced long shore currents in the new estuary after creation of the artificial diversion (Cui and Li, 2011). The reality is that the WSRS is a huge project with many missions. Various objectives – such as reservoir sediment flushing, domestic water consumption, and ecological water requirements of the LYR and estuary – also need to be considered alongside the requirements for channel scouring and delta maintenance. Thus, the error or uncertainty of our calculation results cannot be technically estimated. Nevertheless, our analysis results are still of great credibility because of the high degree of consistency with the actual situation.
5. Conclusions Previous studies have identified the climate change and human activities as two primary factors that impact the water and sediment load delivered to the river mouth. This study has focused on recent changes taking place to the LYR and its influences to the YRD. Longterm data from hydrological monitoring stations and remotely-sensed satellite images were used to explore water and sediment conditions for scouring the channel and maintaining the delta of the LYR from 1983 to 2013. We found that the degree of channel deposition in the LYR depends mainly on the incoming sediment concentration at Xiaolangdi station, which must be lower than approximately 9.17 × 10−3 t m−3 to ensure that channel erosion takes place. The artificial diversion in 1996 altered the evolution of Yellow River Delta (YRD), and the YRD would have required average sediment loads of about 441 × 106 t year−1 before 1996 and 167 × 106 t year− 1 after 1996 to maintain equilibrium. Besides, the incoming water and sediment condition at Xiaolangdi station for collectively guaranteeing scouring channel and maintaining delta in LYR is also preliminarily estimated. We found that it is feasible to transport the sediment deposited at LYR to the river mouth in order to maintain the YRD. This study provides a preliminary basis for future multi-objective analyses of the WSRS in the LYR, and also offers a scientific reference for the optimization of water–sediment regulation in other rivers across the world. Acknowledgments Funding for this research was provided by the National Natural Science Foundation of China (no. 41440003), the Open Foundation of
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Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha
Bi-objective analysis of water–sediment regulation for channel scouring and delta maintenance: A study of the lower Yellow River Dongxian Kong, Chiyuan Miao ⁎, Jingwen Wu, Lin Jiang, Qingyun Duan State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, PR China Joint Center for Global Change Studies, Beijing 100875, PR China
a r t i c l e
i n f o
Article history: Received 24 May 2015 Received in revised form 16 July 2015 Accepted 29 July 2015 Available online 31 July 2015 Keywords: Channel scouring Delta maintenance Lower Yellow River Yellow River Delta
a b s t r a c t Long-term hydrological data and remotely-sensed satellite images were used to analyze the effects of the water– sediment regulation scheme (WSRS) implemented in the lower Yellow River (LYR), China, between 1983 and 2013. The WSRS aimed to control channel scouring in the LYR and maintain the Yellow River Delta (YRD). Channel erosion in the LYR has primarily depended on the incoming sediment concentration at Xiaolangdi, where the concentration must be lower than approximately 9.17 × 10−3 t m−3 to avoid rising of the riverbed. In 1996, an artificial diversion altered the evolution of the YRD. To maintain delta equilibrium, an average sediment load of about 441 × 106 t year−1 was required before 1996, after which this value decreased to 167 × 106 t year−1. We provide a preliminary estimate of the incoming water and sediment conditions required at the Xiaolangdi station to guarantee both LYR channel scouring and maintenance of the YRD. Our results show that it is feasible to transport sediment originally deposited in the LYR to the river mouth to maintain the delta, which is of great significance for the future management and environmental protection of the LYR. © 2015 Published by Elsevier B.V.
1. Introduction A river delta is a coastal feature created by the accumulation of sediment near the mouth of a river. Over the past few decades, river deltas have been a focus of research and an area of concern due to their socio-economic importance and unique ecological environment (Syvitski and Saito, 2007; Kong et al., 2015a). River deltas are an important agricultural area for the local residents because of the fertile soil. In addition, river delta wetlands are an important habitat and transfer area for many rare and endangered migrating birds. Unfortunately, almost all of the river deltas worldwide are at risk of sinking and shrinking (Syvitski et al., 2009; Syvitski and Higgins, 2012). Recent research shows that the trapping of sediment behind dams and subsequent reduction in sediment delivery to deltas, together with human control of river discharge routing across delta plains, have contributed to delta sinking (Syvitski, 2008; Blum and Roberts, 2009; Grill, 2015). In addition the tendency toward delta sinking has been further exacerbated by rising sea levels resulting from climate change (Blum and Roberts, 2009; Kirwan and Megonigal, 2013; Giosan et al., 2014; Ibáñez et al., 2014). The consequences include shoreline erosion, loss of swamps and wetlands, salinization of cultivated land, and the endangerment of
⁎ Corresponding author at: State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, PR China. E-mail address: [email protected] (C. Miao).
http://dx.doi.org/10.1016/j.gloplacha.2015.07.007 0921-8181/© 2015 Published by Elsevier B.V.
hundreds of millions of people (Syvitski, 2008; Restrepo, 2012; Temmerman et al., 2013). The Yellow River is the second largest river in the world in terms of sediment load, with an average of 1.1 × 109 t reaching the ocean annually (Milliman and Meade, 1983). In the last century, heavy soil erosion on the Loess Plateau led to intensive sedimentation in the lower Yellow River (LYR) channel (Miao et al., 2012). One effect of the heavy sedimentation in the LYR was an obvious shrinkage of the main channel accompanied by a sharp decrease in the flood discharge capacity, which markedly influenced the management of the river for flood control. The heavy sedimentation also led to the phenomenon of a secondary perched river in local reaches of the LYR, posing a huge flood risk to the local residents (Fig. 1). The heavy sedimentation in the river channel further influenced the evolution of the Yellow River Delta (YRD) because far less sediment was delivered to the river mouth. By the 2000s, the water discharge and sediment load delivered to the sea had decreased by 68% and 88%, respectively, compared with the 1950s (Miao et al., 2011). Since 2002, a water–sediment regulation scheme (WSRS) has been conducted annually by the Yellow River Conservancy Commission (YRCC) to regulate and control flow and sediment transport in the lower reaches of the river via reservoirs on the mainstream and tributaries, oriented toward flood control, ice control, and sediment reduction (Kong et al., 2015b). Most previous studies related to the WSRS focused on changes in the hydraulic characteristics (Zhang et al., 2009), sediment transportation (Xu and Si, 2009), and channel adjustment (Xu et al., 2005; Miao et al., 2010) in the LYR. It has been found that reducing the incoming
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Fig. 1. Location of the lower Yellow River (LYR) and the Yellow River Delta. In this study, the LYR is defined as the reach between Xiaolangdi Reservoir in Henan province and the river mouth in Shandong province, with a length of approximately 850 km flowing across the North China plain.
sediment concentration is conducive to scouring the riverbed and reducing the risk of flooding (Kong et al., 2015b). In contrast, the YRD will shrink if the incoming sediment load is inadequate (Kong et al., 2015a). Although the effects of the WSRS on water and sediment delivery to the sea have been addressed previously (Yu et al., 2013), few studies have quantified the effects on both the YRD and the LYR simultaneously. In this study, we review and quantify the influence of the WSRS on channel erosion in the LYR, quantitatively analyze the evolution of the YRD region, and conduct a preliminary bi-objective analysis for channel scouring and delta maintenance. The insights gained into the behavior of the LYR over time will be useful to those making
decisions on such issues as further implementations of the WSRS, large-scale construction in the region, and environmental-protection measures that affect the LYR. 2. Dataset and methodology 2.1. Data sources We utilized multi-temporal remotely-sensed Landsat data from a Multispectral Scanner (MSS), a Thematic Mapper (TM), and an Enhanced Thematic Mapper (ETM+) for the period from 1983 to 2013 (Table 1),
Table 1 List of satellite images used in the present work. Acquisition data
Image type
Resolution (m)
Bands
Acquisition date
Image type
Resolution (m)
Bands
07/07/1983 07/06/1984 06/09/1985 08/08/1986 08/06/1987 10/06/1988 15/07/1989 16/06/1990 06/08/1991 07/07/1992 08/06/1993 30/08/1994 18/09/1995 02/07/1996 06/08/1997 09/08/1998
MSS MSS TM TM TM TM TM TM TM TM TM TM TM TM TM TM
80 80 30 30 30 30 30 30 30 30 30 30 30 30 30 30
4 4 7 7 7 7 7 7 7 7 7 7 7 7 7 7
06/04/1999 08/04/2000 06/06/2001 29/09/2002 07/08/2003 05/05/2004 03/07/2005 04/06/2006 07/06/2007 12/08/2008 07/08/2009 11/09/2010 02/06/2011 04/06/2012 26/08/2013
TM TM ETM+ ETM+ TM TM ETM+ ETM+ ETM+ ETM+ TM TM ETM+ ETM+ ETM+
30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
7 7 8 8 7 7 8 8 8 8 7 7 8 8 8
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totaling thirty-one scenes archived by the Earth Resources Observation and Science (EROS) Center (http://glovis.usgs.gov/). All data accounted for the impact of cloud cover. According to the Worldwide Reference System, one full MSS (path 130, row 34) or TM (path 121, row 34) scene fully covered the Yellow River Delta in this study. Data on annual runoff and suspended sediment load at selected hydrological stations (Xiaolangdi and Lijin) from 1983 to 2013 were obtained from the Yellow River Conservancy Commission (YRCC). Data for the annual sediment load and runoff at the Xiaolangdi station are missing for 2001 and 2002, when the Xiaolangdi Reservoir was under impoundment. Consequently, we took this period out of consideration when analyzing data for this study. 2.2. Methodology Values for annual channel erosion/deposition were calculated from the annual sediment load at the Xiaolangdi station minus the annual sediment load at the Lijin station, with positive values indicating deposition and negative values indicating erosion. This calculation method is based on the fact that there are few tributaries of the LYR which may introduce additional or export partial sediment load. It should be noted that the values for annual channel erosion/deposition represent only the overall performance of the river channel over the year, without concern for any intermediate changes during the year. Linear regression was used to quantify the relationship between incoming sediment concentration at Xiaolangdi and the amount of erosion/deposition between Xiaolangdi and Lijin. In order to explore the evolution of the YRD, we applied an interactive interpretation technique combining an automatic boundarydetection algorithm with human supervision to detect the land–ocean shoreline boundaries from satellite images and calculate the delta area. The automatic boundary-detection procedure consisted of five steps. First, we calculated the background trend to remove specular reflection of solar radiation on non-flat water surfaces. Second, we applied a noise-removing algorithm to reduce scattered noise contaminating the satellite images. Third, we chose an adaptive threshold that
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differentiated the land from the ocean, and then we transformed the data into black–white (BW) binary form. Fourth, we employed an automatic boundary-detection algorithm to locate boundaries in the BW images, and verified or adjusted them with reference to a combined Thematic Mapper (TM) 432 pseudocolor image. The Matlab Image Processing Toolbox (Mathworks, Natick, MA, USA) was used to implement the algorithm and batch-process all the Landsat images. Finally, we used the ArcGIS 10 Image Processing Toolbox (Esri, Redlands, CA, USA) to obtain the delta area in each satellite image (Fig. 2). The exact position of the Yellow River Delta coastline varies with time and tide, so errors are introduced to the analysis when comparing coastlines extracted from satellite images acquired at different times in any given day. To evaluate the maximum error caused by tidal effects, we calculated the error in delta area as the product of the maximum coastline distance and coastline length, under the assumption that the entire coastline remains parallel under different tidal conditions. It was found that the maximum relative error is about 1.1% of the total delta area, indicating that our calculation results are relatively accurate. We used doublemass curve analysis to explore the influence of an artificial river diversion on the evolution of the YRD, and we used linear regression to establish the relationship between changes in the delta area and sediment feed. 3. Results 3.1. Quantification of the relationship between incoming sediment concentration and channel erosion in the lower Yellow River Previous studies have shown that channel erosion and deposition in the LYR are not dominated by either incoming runoff or sediment load individually: the dominant factor is the incoming sediment concentration (Kong et al., 2015b). Fig. 3 shows the variations in average annual sediment concentration at Xiaolangdi and annual channel erosion between Xiaolangdi and Lijin from 1983 to 2013. The levels of sediment concentration prior to 2000 were high, with an average value of 27.15 × 10− 3 t m−3. However, sediment concentrations plunged to
Fig. 2. Satellite images showing changes to the Yellow River Delta over the years.
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Fig. 3. Variations in average annual sediment concentration at Xiaolangdi (top) and annual channel erosion between Xiaolangdi and Lijin (bottom) from 1983 to 2013. The values for annual channel erosion/deposition were calculated as the annual sediment load at the Xiaolangdi station minus the annual sediment load at the Lijin station, with positive values indicating deposition and negative values indicating erosion. Data for annual sediment load and runoff at the Xiaolangdi station are missing for 2001 and 2002.
low levels after 2002, with an average value of 3.11 × 10−3 t m−3. The changes in channel erosion are almost the mirror image of the changes in sediment concentration. Thus, there must be a threshold value at which suspended sediment concentration influences sediment movement. Excessive sediment will be deposited on the riverbed when the suspended sediment concentration exceeds this threshold value. Correspondingly, when the suspended sediment concentration is below the threshold value, the sediment deposited on the riverbed will be rolled and washed away. Fig. 4 shows the relationship between the incoming sediment concentration at Xiaolangdi and annual channel erosion between Xiaolangdi and Lijin. Despite the large degree of scatter in the data, there is a significant positive correlation between the incoming sediment concentration and annual channel deposition. A linear regression fits the data well during the period 1983–2013: y ¼ 15:9 103 x−145 R ¼ 0:92
Pb0:001
ð1Þ
Fig. 4. Relationship between the incoming sediment concentration at Xiaolangdi and annual channel erosion between Xiaolangdi and Lijin. Data points are from 1983 to 1999 and 2002 to 2013.
where x and y are the incoming sediment concentration (t m−3) and annual channel erosion (106 t), respectively. In Eq. (1), the incoming sediment concentration (x) equals 9.17 × 10−3 t m−3 when the annual channel erosion (y) is 0. This indicates that the incoming sediment concentration at the Xiaolangdi station must be lower than approximately 9.17 × 10−3 t m−3 to ensure that channel erosion takes place between Xiaolangdi and Lijin. Data from most years are consistent with this value.
3.2. Quantification of the relationship between the evolution of the Yellow River Delta and incoming sediment load Generally, interactions among soil, fluvial, and coastal dynamics are responsible for the relative roles played by sedimentation and erosion at delta lobes, thus controlling the growth and shape of the overall delta. For the YRD to be in equilibrium and its area to remain constant, the rate of sediment accumulation from the river must equal the rate of erosion from near-shore coastal flows. Over the past thirty years, the near-shore coastal dynamics have hardly varied at the mouth of the Yellow River (Hu and Cao, 2003; Wang et al., 2010) and so changes to the river flux appear to be the key factor controlling the evolution of the delta (Fan et al., 2006). Previous studies have shown that the total sediment feed is the dominant factor influencing the evolution of the YRD rather than the combination of sediment and water feeds (Wang et al., 2006; Kong et al., 2015a). This is mainly attributed to a significant linear correlation between sediment load and water discharge. Since the longitudinal slope of the coastal area in the YRD is almost small to horizontal (less than 0.01 o) (Wang et al., 2010), the sediment required to maintain the delta shoreline can be regarded as constant in short period. Riverbed features and coastal dynamics are unique to each particular channel, so we performed double-mass curve analysis to explore the influence of an artificial river diversion on the evolution of the YRD as shown in Fig. 5. We found that the slope of the relationship between sediment feed and delta area was different before and after the artificial diversion in 1996, indicating that the threshold value of sediment feed required to keep the YRD stable varies for different channels. This is consistent with previous studies (Wang et al., 2006).
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3.3. Estimate of water and sediment conditions for channel scouring and delta maintenance From the above analysis, we can see that the total sediment feed at the Lijin station is the dominant factor influencing the evolution of the YRD, and the sediment load at the Lijin station is related to the incoming sediment concentration at the Xiaolangdi station. Thus, channel scouring and delta maintenance can be achieved concurrently by regulating the incoming water and sediment load at the Xiaolangdi station. From Eq. (1), SL J ¼ SXLD −15:9
Fig. 5. Double-mass curve analysis of the Yellow River Delta area and sediment feed. The slope of the relationship between sediment feed and delta area was different before and after the artificial diversion in 1996.
Noting that the runoff and sediment reached the sea through different channels before and after the artificial diversion was introduced in 1996, we conducted linear regression analysis over two phases, 1983–1995 and 1997–2013. Here, we define the annual area change in the i + 1th year as the arithmetic difference between the area of the YRD in the i + 1th year minus that in the ith year. The linear regression functions obtained for the annual sediment load (x, 109 t) and the annual change in YRD area (y, km2) for the two phases are: R ¼ 0:59 P ¼ 0:033 before 1996
ð2Þ
y ¼ 0:347x−57:8 R ¼ 0:66 P ¼ 0:005 after 1996 :
ð3Þ
y ¼ 0:176x−77:5
Fig. 6 plots the change in YRD area against sediment load together with the regression lines, for both phases. Although scatter is evident, in both cases the trend is for an increase in delta area with an increase in sediment load. From Eqs. (2) and (3), the annual change in YRD area was zero and hence equilibrium was achieved, when the average annual sediment load equaled 441 × 106 t for the period 1983–1995 and 167 × 106 t for the period 1997–2013. The two values for threshold sediment load are substantially different, demonstrating again the important influence of river flux characteristics on the evolution of the delta.
SXLD þ 145 W XLD
ð4Þ
where SLJ is the sediment load at the Lijin station (106 t), SXLD is the incoming sediment load at the Xiaolangdi station (106 t), and WXLD is the water discharge at the Xiaolangdi station (109 m3). From Eqs. (2) and (3), sediment loads of about 441 × 106 t year−1 and 167 × 106 t year−1 were required to maintain the YRD equilibrium before and after the artificial diversion in 1996. Thus, SXLD −15:9
SXLD þ 145≥441 W XLD
before 1996
ð5Þ
SXLD −15:9
SXLD þ 145≥167 W XLD
after 1996 :
ð6Þ
In order to achieve channel scouring in the LYR, the incoming sediment concentration should be no more than 9.17 × 10− 3 t m− 3, according to Eq. (1). Thus, SXLD ≤9:17: W XLD
ð7Þ
Eqs. (5) and (6) can therefore be simplified as follows: SXLD ≥
296W XLD W XLD −15:9
before 1996
ð8Þ
SXLD ≥
21:6W XLD W XLD −15:9
after 1996 :
ð9Þ
Eq. (7) represents the threshold condition for channel scouring in the LYR, Eq. (8) represents the threshold condition for area maintenance of the YRD before 1996, and Eq. (9) represents the threshold condition for area maintenance of the YRD after 1996. The water and sediment conditions for channel scouring and delta maintenance are
Fig. 6. Relationship between annual sediment load and annual change in delta area before (a) and after (b) the artificial diversion in 1996.
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Fig. 7. Incoming water (WXLD) and sediment (SXLD) conditions at the Xiaolangdi station for channel scouring in the lower Yellow River (a) and maintenance of the Yellow River Delta (b), and channel scouring and delta maintenance together (c). Each subplot is divided into several numbered regions, relating to the target conditions (see Table 2).
shown in Fig. 7 and Table 2. It can be seen that there is a specific set of conditions relating to incoming water and sediment load at the Xiaolangdi station that provides both channel scouring in the LYR and area maintenance of the YRD (Fig. 7c). Data from 1983 to 2013 fit well with the results of our analysis.
Table 2 Summary of the graphical regions from Fig. 7. Region
1 2 3 4 5 6 7 8 9 10 11
Period (1983–1996)
Period (1996–2013)
Channel scouring
Channel scouring
Delta maintaining
× √
√ √ √ × × ×
Delta maintaining
× √ × × √ × × √ √ × ×
√ √ √ × × ×
× √ √ × √ √ √ √ ×
“√” indicates that the water and sediment conditions in the correspondingly numbered region satisfy the target requirements; “×” indicates that the water and sediment conditions do not satisfy the target requirements. A black space indicates that a target was not specified for that numbered region.
4. Discussion As is well known, adequate sediment feed is necessary to maintain the YRD. The sediment in the Yellow River comes mainly from the middle Yellow River (MYR) basin where soil erosion is very serious, especially on the Loess Plateau. This leads directly to the high sediment concentration in the LYR. Due to the flat terrain surrounding the LYR, a substantial portion of the sediment load is deposited in the river channel, resulting in uplift of the riverbed. Therefore, the sediment concentration is relatively low at the point where the Yellow River flows into the sea. This process is shown in a simplified form in Fig. 8a. The growth or shrinkage of the YRD depends on the amount of sediment present at the mouth of the river. In recent decades, many soil and water conservation measures including afforestation, grass plantation, and land terracing have been implemented in the MYR basin. Since the 1970s, sediment flux from the Yellow River to the sea has shown a marked tendency to decrease, which is unfavorable for protection of wetlands in the YRD (Xu, 2003). Prior to this, water flux with a low sediment concentration and high kinetic energy flowed into the LYR. This scoured the riverbed and transported the sediment downstream. The river therefore had high sediment flux when it reached the estuary. In this case, the growth or shrinkage of the YRD depends on the amount of MYR sediment remaining, plus the transported sediment from the LYR. This process is shown in simplified form in Fig. 8b. Our analysis shows that it is feasible to transport the sediment deposited in the LYR to the river mouth (Fig. 7 and Table 2, region 2). However, the
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Fig. 8. Schematic diagram illustrating two different programs affecting maintenance of the Yellow River Delta.
sediment transported from the LYR alone is insufficient to maintain the YRD, as shown by Eq. (4), and an appropriate inflowing sediment load from the MYR basin is required. Thus, appropriate water and sediment conditions need to be adjusted via the WSRS. For any particular river system, the features of the river will not change greatly in the short term without human intervention. Therefore, for the LYR, a specific regulation scheme is required to produce the appropriate water flow and sediment concentrations, and then to achieve both channel scouring and delta maintenance. However, it is noteworthy that the sediment requirement for YRD maintenance is not immutable. In particular, artificial river diversion can significantly alter the sedimentation features in the Yellow River estuary, leading to a change in the minimum sediment requirement (for example, 441 × 106 t from 1983–1995 and 167 × 106 t from 1997–2013). Thus, the regulation scheme needs to be adjusted to satisfy new circumstances. This can be clearly observed in Fig. 7. We found that the acceptable bounds for regulation widened after the artificial diversion in 1996, due to the lower concentration of sediment required to maintain the delta. This is attributed to reduced erosion by the tides and waveinduced long shore currents in the new estuary after creation of the artificial diversion (Cui and Li, 2011). The reality is that the WSRS is a huge project with many missions. Various objectives – such as reservoir sediment flushing, domestic water consumption, and ecological water requirements of the LYR and estuary – also need to be considered alongside the requirements for channel scouring and delta maintenance. Thus, the error or uncertainty of our calculation results cannot be technically estimated. Nevertheless, our analysis results are still of great credibility because of the high degree of consistency with the actual situation.
5. Conclusions Previous studies have identified the climate change and human activities as two primary factors that impact the water and sediment load delivered to the river mouth. This study has focused on recent changes taking place to the LYR and its influences to the YRD. Longterm data from hydrological monitoring stations and remotely-sensed satellite images were used to explore water and sediment conditions for scouring the channel and maintaining the delta of the LYR from 1983 to 2013. We found that the degree of channel deposition in the LYR depends mainly on the incoming sediment concentration at Xiaolangdi station, which must be lower than approximately 9.17 × 10−3 t m−3 to ensure that channel erosion takes place. The artificial diversion in 1996 altered the evolution of Yellow River Delta (YRD), and the YRD would have required average sediment loads of about 441 × 106 t year−1 before 1996 and 167 × 106 t year− 1 after 1996 to maintain equilibrium. Besides, the incoming water and sediment condition at Xiaolangdi station for collectively guaranteeing scouring channel and maintaining delta in LYR is also preliminarily estimated. We found that it is feasible to transport the sediment deposited at LYR to the river mouth in order to maintain the YRD. This study provides a preliminary basis for future multi-objective analyses of the WSRS in the LYR, and also offers a scientific reference for the optimization of water–sediment regulation in other rivers across the world. Acknowledgments Funding for this research was provided by the National Natural Science Foundation of China (no. 41440003), the Open Foundation of
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