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River mouth bar formation, riverbed aggradation and channel migration in the modern Huanghe (Yellow) River delta, China
Geomorphology 74 (2006) 124 – 136 www.elsevier.com/locate/geomorph
River mouth bar formation, riverbed aggradation and channel migration in the modern Huanghe (Yellow) River delta, China Hui Fan a,b,c,*, Haijun Huang a, Thomas Q. Zeng d, Kairong Wang e a
b
Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Institute for Development Strategy of Science and Technology, Shandong Academy of Sciences, Jinan 250014, China c Graduate School of the Chinese Academy of Sciences, Beijing 100039, China d Division of Geography, School of Geosciences, University of Sydney, City Road, Sydney 2006, Australia e Institute of Hydraulic Research, Yellow River Conservancy Commission, Zhengzhou 450003, China Received 19 July 2004; received in revised form 8 August 2005; accepted 8 September 2005 Available online 25 October 2005
Abstract This paper addresses the recent (1970s–1990s) processes of river mouth bar formation, riverbed aggradation and distributary migration in the Huanghe River mouth area, in the light of station-based monitoring, field measurements and remote sensing interpretation. The results show that the morphological changes of the river mouth bar have been closely associated with the largely reduced fluvial discharge and sediment load. Landform development such as bar progradation occurred in two phases, i.e. before and after 1989, which correspond to faster and lower bar growth rates, respectively. Fast riverbed aggradation in the mouth channel was strongly related to river mouth bar progradation. During 1976–1996, about 2.8% of the total sediment loads were deposited in the river channel on the upper to middle delta. Therefore, the river water level rose by a few meters from 1984 to 1996. The frequent distributary channel migration, which switched the radial channel pattern into the SE-directed pattern in the mid-1980s, was linked with mouth bar formation. Marine conditions also constrain seaward bar progradation. Furthermore, the history of river mouth bar formation reflects human impacts, such as dredging and dyking in order to stabilize the coastal area. D 2005 Elsevier B.V. All rights reserved. Keywords: Huanghe (Yellow) River delta; River mouth bar; Sedimentation; Channel migration; Remote sensing; Human activity
1. Introduction Sediment transport from the source to the sink of the world large rivers, and its impact on the delta-coast morphology have been the major research topics in the past few decades (Milliman and Meade, 1983; Milliman and Syvitski, 1992; Chen et al., 2001a). The discharge of river water and sediment into the sea is vital for delta-coast geomorphological changes (Qian et * Corresponding author. Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China. E-mail address: [email protected] (H. Fan). 0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.08.015
al., 1993; Chen et al., 2001b). Recent changes in water and sediment of the Huanghe (Yellow) River, closely associated with intensifying human activity, have been widely noticed (Qin and Li, 1986; Aranuvachapun and Walling, 1988; Wright et al., 1988, 1990; Xue, 1993; Wang and Liang, 2000; Xu, 2003). The Huanghe River originates in Qinghai Province in western China, and it has a length of 5464 km and a catchment area of 752 000 km2 (Qian et al., 1993). Flowing through the Loess Plateau, it carries a huge amount of sediment, i.e. 1.4 109 t year1 on average (1919–1989) at the Sanmenxia Hydrological Station, about 1024 km from the river mouth (Chen, 1996).
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About 0.3 109 t year1 of the sediment have been deposited in the river channel below Sanmenxia (Xu et al., 1990; Qian et al., 1993). This has resulted in a rise of the riverbed by more than 10 m above the surrounding floodplains (Li and Brian, 1993; Qian et al., 1993; Yu, 2002). Therefore, the lower Huanghe river channel is called dHanging RiverT (Xu and Cheng, 2002; Yu, 2002). Furthermore, about 23% of supplied sediment is entrapped in the delta coast, about 40–55% is sequestrated in the nearshore, and only 21–37% is transported to the outer ocean (SPSTC, 1991). The Huanghe River serves as world’s largest contributor of fluvial sediment load to the ocean (Ren and Shi, 1986; Wang and Aubrey, 1987), accounting for ca. 5% of the global river sediment budget (Milliman and Meade, 1983; Milliman and Syvitski, 1992). During the Holocene, about 8.0 1012 t sediment was delivered into the Yellow Sea and the Bohai Sea (Shi et al., 2002) to alter the coastal morphology (Qin and Li, 1986; Wang and Aubrey, 1987; Ren, 1989; Wang and Ke, 1989; Cauwet and Mackenzie, 1993). Dramatic sediment accumulation on the delta-coast has caused the large-scale avulsion of the river channel in the last few thousand years (Pang and Si, 1979; Xue, 1993; Ren and Walker, 1998). The latest one occurred in 1855 when the river swung back from the southern Yellow Sea Coast (Jiangsu Coast) to the Bohai Sea of northern China, leading to the formation of the modern Huanghe River delta (Fig. 1). Since 1855, the Huanghe River has changed its course more than 50 times, and 8–12 of them were treated as the major historical events (Pang and Si, 1979; Xue, 1993, 1994). On average, the major shifts of the river course occurred about every 10 years (Fig. 1) (Pang and Si, 1979; Ren and Shi, 1986; Qian et al., 1993; Xue, 1994; Wang and Liang, 2000). The Qingshuigou distributary, which resulted from artificial geoengineering in 1976, is the current prime outlet of the Huanghe River to the sea. A mean discharge of 1330 m3 s1 and a mean suspended sediment concentration (SSC) of 25.5 kg m3, recorded at the Lijin hydrological station about 100 km from the coast (Fig. 1), are also applicable to the delta-coast (SPSTC, 1991). Although the river discharge has decreased significantly since the Qingshuigou distributary formed, high SSC (N100 kg m3) has often been measured in the channel during the flood period from July to October (Li et al., 1998a; Xu, 2002). The Huanghe River mouth can be classified as a low to mesotidal estuary based on Hayes (1979). The tidal range is about 1.2–1.8 m (SPSTC, 1991; Li et al., 1998b; Chen, 2001). Tidal currents around the river
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mouth are irregular, semi-diurnal and bidirectional. The flood current generally flows SSE and the ebb current directs NNW (Chen, 1988; Wang and Su, 1989; SPSTC, 1991). The depth-averaged velocity of the flood current is usually larger than that of the ebb current, and the maximum flood velocity in the delta front reaches 2.2 m s1 (SPSTC, 1991; Zeng et al., 1997). The maximum tidal current intrusion into the distributary system is about 20 km inland when the recent river discharge is extremely low, whereas it is only 2–3 km when the discharge is 1000 m3 s1, and no intrusion takes place when the discharge is more than 2000 m3 s1 (Zeng et al., 1997; Li et al., 1998a; Chen, 2001). The residual current in the Huanghe River mouth is mainly driven by wind, with a mean velocity of 0.10–0.25 m s1. The surface residual current and local waves are subject to the monsoon, and generally direct southward in the winter and northward in the summer (SPSTC, 1991). The wave height in the river mouth is about 0.8 m under the normal weather conditions (Chen, 2001). Weak tidal and wave sediment dynamics, and a large riverine sediment supply has resulted in deposition of more than two-thirds of the annual sediment load in the Qingshuigou distributary delta lobe (Fig. 1; Wright et al., 1990; Qian et al., 1993; Shi and Zhang, 2003). The average sediment accumulation rate for 1976–1992 is about 1.2 m year1 and the river mouth area has been an estuarine depocenter (Wang et al., 1992; Li et al., 1998b). As a consequence, the active river-mouth bar prograded with a rate of 1–4 km year1 and the delta lobe extended with a rate of 20–25 km2 year1 (SPSTC, 1991; Van Gelder et al., 1994; Li et al., 1998b; Yang et al., 1999; Wang and Liang, 2000; Li et al., 2002; Yu, 2002). Although the active rivermouth bar is not large in size, it has a profound effect on the rapid growth of the Huanghe River delta (Li et al., 1998b). The rapid deposition on the river-mouth bar causes its seaward progradation that has been an important control on the upstream siltation in the lower channel of the Huanghe River, serving as a stimulus to the river channel migration (Qian et al., 1993; Shi and Zhang, 2003; Shi, 2005). Heavy sedimentation in the lower reaches of the river channel causes the riverbed to aggrade at a rate of several centimeters per year (Qian et al., 1993; Van Gelder et al., 1994; Xu and Cheng, 2002). This aggradation increases the flood risk on the floodplain, making the river channel avulsionprone. Since the 1980s, numerous studies have focused on the deltaic sedimentation, riverbed aggradation and river mouth bar formation on the delta-coast of the
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Fig. 1. Location map of the modern Huanghe River delta and historical migration of deltaic channels (modified after Xue, 1994). (1) 1855–1889; (2) 1889–1897; (3) 1897–1904; (4) 1904–1929; (5) 1929–1934; (6) 1934–1938 and 1947–1964; (7) 1964–1976; (8) 1976–present.
Huanghe River (Pang and Si, 1982; Wiseman et al., 1986; Wright et al., 1988, 1990; Qian et al., 1993; Van Gelder et al., 1994; Ye, 1996; Li, 1997; Shi and Ye, 1997; Li et al., 1998b; Yin, 1999; Shi and Zhang,
2003). However, full understanding of the formation mechanisms of the river-mouth bar, in relation to the riverbed aggradation and channel migration, is lacking, and further understanding of hydrological and sedimen-
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tological processes that control delta-coast morphology is vital to coastal science and management, in relation to landuse planning, river channel maintenance, and hazard risk mitigation for sustainable regional development. Therefore, this paper discusses the river mouth bar formation, and its interactions with downstream riverbed aggradation and distributary migration in the Huanghe River delta since 1976.
0.2, 0.6, and 0.8 were measured, from which the depth-averaged velocity and SSC were derived: u¯ ¼
1 ðu0:2 þ u0:6 þ u0:8 Þ 3
ð1Þ
c¯ ¼
1 ðc0:2 u0:2 þ c0:6 u0:6 þ c0:8 u0:8 Þ 3¯u
ð2Þ
2. Data and methods Daily hydrological measurement has been carried out by the Yellow River Conservancy Commission (YRCC) at the four cross-sections of the Lijin station, each of which having 16–24 column measurement points (Li et al., 1998a). The river water level used in the present study is that at a discharge of 3000 m3 s1, and is referred to the Dagu datum, a sea-level datum in China. The consecutive 21-year hydrological data, including discharge, suspended sediment concentration (SSC), and the river water level were collected from YRCC (1976–1996a). Additional hydrological data, including current velocity and direction, SSC, salinity, and water depth in the mouth bar reach were measured during the Coastal Zone Survey Project in mid May (dry season) and late July (flood season) of 1984 (SPSTC, 1991). For each period, seven observation stations were arranged along the mouth bar (Fig. 2). The measurement was made simultaneously at each station for the water column with three to five measuring horizons according to the different water depths. Measurement was taken every one hour both for flooding and ebbing cycles. The depth-averaged flow velocity (u¯ ) and depth-averaged SSC (c¯ ) were calculated using the following conventional methods (Chen, 1996): (i) When water depth is less than 1 m, three water column horizons at a relative water depth of
Fig. 2. Location of survey stations in May and July 1984.
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(ii) When water depth is greater than or equal to 1 m, five measurements at a relative water depth of 0.0, 0.2, 0.6, 0.8, and 1.0 were used to yield the depth-averaged velocity and SSC: u¯ ¼
1 ðu0:0 þ 3u0:2 þ 3u0:6 þ 2u0:8 þ u1:0 Þ 10
c¯ ¼
1 ðc0:0 u0:0 þ 3c0:2 u0:2 þ 3c0:6 u0:6 10¯u þ 2c0:8 u0:8 þ c1:0 u1:0 Þ
ð3Þ
ð4Þ
The topographic data of the river-mouth bar, measured during the Coastal Zone Survey Project and by the YRCC once or twice a year since 1988, were compiled (YRCC, 1988–1995; SPSTC, 1991). These data were imported into a GIS platform for mapping and analyzing mouth bar formation. Morphological changes in some cross-sections along the river channel between Lijin and Qing-3 (Fig. 1) were measured twice a year, namely, before and after the flood season (YRCC, 1976–1996b). By multiplying the neighboring cross-sectional areas and their distance, the volume of sediment accretion and/or erosion of the riverbed can be estimated. Landsat Multi-Spectrum Scanner (MSS) and Thematic Mapper (TM) images were used to study the influence of the shear front on the formation of the river-mouth bar, and the migration of the mouth channel below the Lijin Hydrological Station since 1976. All the images acquired from 1976 to 2003 were corrected for removing atmospheric effects by subtracting the radiance of a ddark pixelT within each band image (Lavery et al., 1993), and were georeferenced and rectified following the procedure by Serra et al. (2003). The false-color composition of the enhanced images was constructed for extracting river channels. The SSC distribution in the Huanghe River mouth was also interpreted via the density slicing of Landsat TM band 3 (Aranuvachapun and Walling, 1988).
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3. Observations and results 3.1. Water and sediment supply from the Huanghe River From 1976 to 1996, a runoff of 252.4 108 m3 year1 and a sediment load of 6.3 108 t year1 discharged into the sea. Of these, 65.2% runoff and 89.6% sediment load occurred during the flood season, when the average SSC in the river channel often reached more than 30 kg m3. The runoff and sediment load tend to decrease with time (Fig. 3a,b), and their changes can be roughly divided into two phases, i.e. before and after 1990. Prior to 1990, the Huanghe River had a mean water discharge of 291.9 108 m3 year1 and a sediment load of 7.1 108 t year1, but since then they have reduced to 173.5 108 m3 year1 and 4.8 108 t year1, respectively, which are equivalent to 40.6% and 33.3% reduction. Hydrological records also display
that, during the period of 1976–1996, the deltaic river channel experienced seasonal dry-up events for 627 days in total, in which 488 days occurred in the 1990s (Fig. 3c). In general, synchronous fluctuations of runoff and sediment load occurred (Fig. 3a,b). However, asynchronous fluctuations took place in 1977, 1978, 1988 and 1994, when small runoff and high sediment load values appeared with a mean SSC of about 40 kg m3. 3.2. River-mouth bar The river mouth bar in the study area consists of three components: bar back, bar crest, and bar front (Fig. 4). Longitudinal length of the bar back during the observed period was 0.3–4.4 km with its slope ranging from 1:5000 to 1:100. Longitudinal length of the bar front was 0.2–1.3 km with its slope ranging from 1:400
Fig. 3. Temporal changes in (a) water discharge, (b) sediment discharge, and (c) duration of channel dry-ups at the Lijin Hydrological Station (after YRCC, 1976–1996a).
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Fig. 4. Change in the longitudinal profile of the Huanghe River mouth bar (after YRCC, 1988–1995; SPSTC, 1991).
to 1:100. The bar crest, usually occurring as flatter topography, was irregularly elliptical in shape, 2–6 km long, and with an average water depth of 0.8 to 0.1 m. The bar crest above the 2 m isobath had areas of between 5 and 50 km2. These variables are usually changeable with time. Our survey data demonstrate that the river-mouth bar generally prograded seawards, except for from October 1989 to September 1990, when a remarkable landward retreat was seen (Fig. 4). Before 1989, a rapid progradation of river mouth bar took place, but a slower growth was observed after 1990 (Fig. 4). The bar crest length and elevation also changed with time (Fig. 5). Large values of the bar crest length and elevation occurred during 1984–1987, but bar crest elevation decreased during 1988–1991. Then the bar
crest became shorter but its elevation was moderately high (Fig. 5). Three development stages of Qingshuigou river mouth bar can be recognized from its geometry change (Fig. 6). The early stage (before 1990) was characterised by an elongated bar confined by the subaerial and subaqueous levees. During the following stage (1990–1992), the river mouth bar was more laterally extended. The river mouth bar in the latest stage (1993– 1995) prograded only in a southeastern direction (Fig. 6). A high correlation occurs between the progradation length of the mouth bar from Xihekou and the cumulative sediment load at the Lijin station for the past few decades (Fig. 7). Depth-averaged current velocity in the non-flood season (May 15–16, 1984) at the monitoring stations
Fig. 5. Changes in the longitudinal length and mean elevation of the river mouth bar crest (after YRCC, 1988–1995; SPSTC, 1991).
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ing on July 28–29, 1984 (Fig. 9), also indicates high values at S04 (bar), low values at S03, and high values at S02 (landward distributary). SSC decreased consistently with an increasing seaward distance from S04 (Fig. 9). River-mouth shear fronts can be identified from the Landsat TM images. The interpreted image taken on February 13, 1989 presents a high-turbid southward mass flow surrounding the river mouth, embraced by a low-turbid northward mass flow (Fig. 10a). The image taken on October 4, 1995 indicates a similar separation of mass flows but with the opposite directions (Fig. 10b). 3.3. Variations in riverbed and water levels
Fig. 6. Change in the plan distribution of the Huanghe River mouth bar crest (after YRCC, 1988–1995).
in the Qingshuigou distributary (Fig. 2) was less than 0.8 m s1 (Fig. 8a), and that at S05 was the lowest (0.3– 0.4 m s1). At the stations upstream of S05, the current velocity during the flood tide was larger than that during the ebb tide, while seaward, the situation was reversed (Fig. 8a). Higher current velocity occurred during the flood season. Higher values of current velocity (z2.0 m s1) recorded at S01–S04 both for flooding and ebbing dramatically dropped to 0.7 m s1 at S05, and thereafter the velocity remained quite low (Fig. 8b). Depth-averaged SSC on May 15–16, 1984, portraits higher values (N1 kg m3) at S01–S04, in which the highest value appeared at S03, whereas much lower values (ca. 0.5 kg m3) occurred at S06 and S07 (Fig. 8c). From S01 to S04, SSC during the flood tide was higher than that during the ebb tide, while the opposite applies to the other stations. On July 28–29, 1984, the runoff and sediment load were high and SSC in the distributary system was 10 times more than that on May 15–16 (Fig. 8c,d). At all the stations except for S03, SSC during the flood tide was close to that during the ebb tide. SSC at S06 and S07 during both the flood and ebb tidal cycles was much lower than that at S01–S05 (Fig. 8d). The vertical distribution of SSC during flood-
The sediment budget calculated from the cross-sections points to three phases of riverbed topographic variations. From 1976 to 1979, a major siltation of the riverbed occurred between Xihekou and Qing-3, although the rate of the siltation tended to decrease with time (Fig. 11). At the same time, alternate siltation and erosion appeared along the riverbed between Lijin and Xihekou. From 1980 to 1985, the riverbed between Lijin and Qing-3 experienced slight erosion, whereas after 1985, slight siltation occurred (Fig. 11). Our further calculation indicates that from 1976 to 1996, 0.249 108 t sediment was deposited along the river channel between Lijin and Xihekou, and 3.346 108 t between Xihekou and Qing-3. According to the records at the Lijin Hydrological Station, sediment discharge was 128.3 108 t during the same period (YRCC, 1976–1996a). Thus, about 2.8% of incoming sediment was deposited along the river channel between Lijin and Qing-3.
Fig. 7. Relationship between the cumulative sediment supply from the Huanghe River and distance from the front of the bar crest to Xihekou (after YRCC, 1976–1996a, 1988–1995; SPSTC, 1991).
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Fig. 8. Spatial differences in depth-averaged current velocity (a: May 1516, 1984; b: July 2829, 1984) and depth-averaged SCC (c: May 1516, 1984; d: July 2829, 1984) along the Huanghe River mouth bar (after SPSTC, 1991).
An increasing trend of the water level with time was recorded at Lijin and Xihekou (Fig. 12). The water level fluctuations at the two stations are almost syn-
chronous. From 1985 to 1996, the water level had risen by 1.97 m at Lijin with a rate of 0.164 m year1, and by 2.04 m at Xihekou with a rate of 0.170 m year1.
Fig. 9. Vertical profile of suspended sediment concentration along the Huanghe River mouth bar during flood tide on July 28, 1984 (after SPSTC, 1991).
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Fig. 10. Tidal current and shear front at the Huanghe River mouth inferred from satellite images (a: inner-flood-outer-ebb shear front on February 13, 1989; b: inner-ebb-outer-flood shear front on October 4, 1995).
3.4. Morphological changes of the distributary system There have been two major phases of distributary displacement in the Qingshuigou river mouth. The Landsat images demonstrate that during the mid to late 1970s, most distributaries were oriented in a NE direction, except May 1977, October 1977 and September 1979 (Fig. 13). From the 1980s to the 1990s, distributaries tended to shift from easterly to southeasterly. The lateral shift of the distributary before the 1980s was faster than that after the 1980s. In July 1996, an artificial channel diversion below Qing-8 provided the Chahe distributary at the river mouth tip (Fig. 1) (Chen, 2001; Li et al., 2002). 4. Discussion Heavy-laden sediment along the Huanghe River and associated coastal environmental managements have been discussed in the past century (Pang and Si,
1982; Wiseman et al., 1986; Wright et al., 1988, 1990; Qian et al., 1993; Van Gelder et al., 1994; Li et al., 1998b; Shi and Zhang, 2003). The present study highlights the development of the Qingshuigou river mouth bar during the mid 1970s to the 1990s, when both runoff and sediment supply from the upper reach dropped by about 30–40% (Fig. 3) due mainly to damming, water diversion and soil and water conservation practices in the upstream area (Qian et al., 1993; Ye, 1998; Ren et al., 2002; Xu, 2002, 2003, 2004). This reduction directly affected the progradation rate of the river mouth bar: faster before 1989 but slower after 1990 (Fig. 4). In particular, increased seasonal channel dry-ups after 1990 (Fig. 3c) made the river mouth bar totally exposed to the controls of tidal currents, which significantly limited the river mouth bar progradation, and tidal flow with an average discharge of ca. 300–500 m3 s1 intruded more than 20 km into the distributary mouth channel (Zeng et al., 1997; Li et al., 1998a; Chen, 2001). The intrusion of low SSC tidal currents
Fig. 11. Sedimentation in the mouth channel of the Huanghe River (after YRCC, 1976–1996b).
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Fig. 12. Temporal change in the water level at a discharge of 3000 m3 s 1 for the Lijin and Xihekou Hydrological Stations (after YRCC, 1976– 1996a).
is conducive to the self-dredging of the river mouth bar reach. Thus, human-induced reduction of runoff and sediment loads significantly influenced the evolution of the river mouth bar. An obvious retrogradation of the river mouth bar between 1989 and 1990 (Fig. 4) probably reflects intensive riverbed dredging during the same period (Ye, 1996; Chen, 2001). Dredging and river regulation were conducted from 1988 to 1992 to mitigate the heavy siltation in the river mouth area, because the braided and unconfined river channels in the river mouth area
may disturb living safety and economic development, particularly in relation to an important oil field at the northern coast off the Qingshuigou distributary (Ye, 1996; Zeng et al., 1997; Chen, 2001; Yu, 2002). Changes in the length and elevation of the bar crest (Fig. 5) also point to the effects of dredging. The spatial distribution of the Qinghuigou river mouth bar, characterized by the three phases of different geometry patterns (Fig. 6), further manifests the combined effect of human activities and water/sediment discharge. Large-scale river regulation and dredging
Fig. 13. Changes in the Qingshuigou distributary channel from 1976 to 2003, interpreted from satellite images. The inset shows topographic effect of the river mouth bar on distributary evolution.
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coupled with more abundant river discharge between 1988 and 1989 (Fig. 3), led to the elongated river mouth bar limited by natural levees. From 1990 to 1992, the upstream movement of salt wedge resulted from the impacts of dredging and smaller river discharge (Fig. 3), caused an extensive lateral migration of the river mouth bar, when the bar moved further seaward beyond the levees’ controls. The river bar suddenly prograded southward in September 1992, which probably reflects the rapid sedimentation in association with flood processes (Chen, 2001). After 1993, the river bar primarily headed southeastward and further beyond the river mouth topographic limitation, which may have resulted from the termination of dredging. River mouth bar progradation was basically dependent on the amount of sediment supply (Fig. 7). The river mouth bar formation has been closely associated with the depth-averaged current velocity and SSC during both the non-flood and flood seasons (Figs. 8 and 9). The segment between S05 and S06 served as the critical location in the river mouth area, where river and sea currents strongly interacted. Much higher values of current velocity and SSC at stations landward of S05–S06 (Figs. 8 and 9) indicate dominant fluvial processes, in which a large amount of sediment has been entrapped in the distributary to form the river mouth bar. Li (1997) and Li et al. (1998a) also proposed that the low current–velocity zone near S05–S06 played a significant role in forming the river mouth bar. The high-turbid mass flow during flooding, usually called hyperpycnal plumes, moved southward which confronted the less turbid mass flow with opposite movement due to ebbing (Fig. 10a). A set of reversally directed mass flows was observed at the same place (Fig. 10b), and the shear front occurred about 3–5 km off the river mouth (Fig. 9). These flows with the shear front hindered further seaward progradation of the river mouth bar (Li et al., 1994, 2001). The formation of the Qingshuigou river mouth bar has altered river channel morphology in the adjacent coastal land. The slow but continuous aggradation of the river channel between Lijin and Qing-3, as well as the continuous water level rise after the mid 1980s (Figs. 11 and 12), were associated with the river mouth bar formation. The alternation of siltation and slight erosion of the river channel before the bar formation may reflect more busualQ river processes. Riverbed aggradation generally occurred when the Huanghe River had small runoff and high sediment loads; whereas riverbed erosion occurred when abundant runoff was combined with low sediment loads (Figs. 3 and 11), as suggested by Qian et al. (1993),
Chen (2001) and Xu (2002). In addition, serious riverbed siltation between Xihekou and Qing-3 in the 1970s has been associated with the formation of the Qingshuigou distributary (Yu, 2002; Shi and Zhang, 2003). The spatial distribution of numerous distributaries in the river mouth area was also affected by the river mouth bar formation after the mid 1980s. The radiated distributaries during the 1970s suggest weaker controls of the Qingshuigou river mouth topography on distributary form (Fig. 14 and its inset A). After the mid 1980s, the formation of the river mouth bar let the distributaries extend southeasterly although their distal portion could migrate beyond the topographic constraint (Fig. 14 and its inset B). 5. Conclusions The Huanghe River mouth bar, represented by the Qingshuigou river mouth area in the present study, is dominated by fluvial processes, and its seaward progradation has been closely associated with runoff and sediment discharging into the sea. The fluctuations of these two fluvial variables over the past few decades (1970s–1990s) has played a decisive role in forming the river mouth bar, affecting its growth rate, elevation, length and geometry patterns. Two major time periods of landform development, before and after 1990, have been identified, based on temporal change in the bar growth rate in response to changed runoff and sediment load. Rapid sedimentation in the river mouth area resulted from the low current velocity due to the counteraction between river outflows and tidal currents, where the turbidity maximum zone formed. The shear front, due to different density values of mass flows in front of the bar, hinders seaward bar progradation. From 1985 to 1996, river mouth bar progradation and associated riverbed silation in the Qingshuigou distributary elevated the water level by ca. 2 m at the Lijin and Xihekou hydrological stations. This enlarged the potential flood risk. The formation of SE-directed distributaries after the mid-1980s was strongly influenced by bar formation. Intensified human activity, such as dredging and dyking along the Qingshuigou distributary channel during the late 1980s and the early 1990s, terminated rapid bar progradation and minimized further bifurcation of distributaries. Overall, the Huanghe River mouth bar evolution in the recent decades highlighted the increasing effects of human activities on the geomorphological evolution of the deltaic coast, and such an inference provides useful information for future studies.
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Acknowledgements Financial support from National Natural Science Foundation of China (Grant No. 50339050) and 973 National Key Project for Basic Research (Grant No. 2002CB412404) is gratefully acknowledged. We are indebted to Prof. Chen Zhongyuan of East China Normal University, Dr. Peter Cowell, University of Sydney Institute of Marine Science, and Mr. John Hudson, from Sydney Olympic Park Authority, for their improvement of an early manuscript. Thanks are also given to Dr. Andrew J. Plater, University of Liverpool for his constructive reviews. References Aranuvachapun, S., Walling, D.E., 1988. Landsat-MSS radiance as a measure of suspended sediment in the lower Yellow River (Hwang Ho). Remote Sensing of Environment 25, 145 – 165. Cauwet, G., Mackenzie, F.T., 1993. Carbon inputs and distribution in estuaries of turbid rivers: the Yangtze and Yellow rivers (China). Marine Chemistry 43, 235 – 246. Chen, Z., 1988. Tidal limit of the modern Yellow River mouth. Transactions of Oceanology and Limnology 4, 83 – 86 (in Chinese). Chen, X., 1996. Hydrology of the Yellow River. Publishing House for the Huanghe River Water Conservancy, Zhengzhou (in Chinese). Chen, Y., 2001. Study on the Yellow River Mouth and Its Regulation Experiences. Publishing House for the Huanghe River Water Conservancy, Zhengzhou (in Chinese). Chen, Z., Yu, L., Gupta, A., 2001a. The Yangtze River: an introduction. Geomorphology 41, 73 – 75. Chen, Z., Li, J., Shen, H., Wang, Z., 2001b. Yangtze River of China: historical analysis of discharge variability and sediment flux. Geomorphology 41, 77 – 91. Hayes, M.O., 1979. Barrier island morphology as a function of tidal and wave regime. In: Leatherman, S.P. (Ed.), Barrier Islands from the Gulf of St. Lawrence to the Gulf of Mexico. Academic Press, New York, pp. 1 – 29. Lavery, P., Pattiaratchi, C., Wyllie, A., Hick, P., 1993. Water quality monitoring in estuarine waters using the Landsat Thematic Mapper. Remote Sensing of Environment 46, 268 – 280. Li, Z., 1997. Formation and evolution of the bar on the Yellow River estuary. Acta Geographica Sinica 52, 54 – 62 (in Chinese with English abstract). Li, S., Brian, F., 1993. Flood management on the lower Yellow River: hydrological and geomorphological perspectives. Sedimentary Geology 85, 285 – 296. Li, G., Cheng, G., Wei, H., Pan, W., Ren, Y., Ding, D., Zhou, Y., Zhao, J., 1994. Shearing zone of the current field off the modern Yellow River mouth. Chinese Science Bulletin 39, 928 – 932. Li, G., Wei, H., Han, Y., Chen, Y., 1998a. Sedimentation in the Yellow River delta, part I: flow and suspended sediment structure in the upper distributary and the estuary. Marine Geology 149, 93 – 111. Li, G., Wei, H., Yue, S., Cheng, Y., Han, Y., 1998b. Sedimentation in the Yellow River delta: Part II. Suspended sediment dispersal and deposition on the subaqueous delta. Marine Geology 149, 103 – 121.
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Wang, Y., Aubrey, D.G., 1987. The characteristics of the China coastline. Continental Shelf Research 7, 329 – 349. Wang, Y., Ke, X.K., 1989. Cheniers on the east coastal plain of China. Marine Geology 90, 321 – 335. Wang, Z., Liang, Z., 2000. Dynamic characteristics of the Yellow River mouth. Earth Surface Processes and Landforms 25, 765 – 782. Wang, F., Su, Z., 1989. The distribution characteristics of tidal currents off coast of the Yellow River estuary gate. Transactions of Oceanology and Limnology 2, 8 – 11. (in Chinese with English abstract). Wang, Q., Cao, L., Yang, Z., 1992. Sedimento-dynamic characteristics of the Huanghe (Yellow) River subaqueous delta. Science China Series B 35, 631 – 640. Wiseman, W.J., Yang, Z.S., Bornhold, B.D., Keller, G.H., Prior, D.B., Wright, L.D., 1986. Suspended sediment advection by tidal currents off the Huanghe River (Yellow River) delta. Geo-Marine Letters 6, 107 – 113. Wright, L.D., Wiseman, W.J., Bornhold, B.D., Prior, D.B., Suhayda, J.N., Keller, G.H., Yang, Z.Sh., Fan, Y.B., 1988. Marine dispersal and deposition of Yellow River silts by gravity-driven underflows. Nature 332, 629 – 632. Wright, L.D., Wiseman, W.J., Yang, Z.S., Bornhold, B.D., Keller, G.H., Prior, D.B., Suhayda, J.N., 1990. Processes of marine dispersal and deposition of suspended silts off the modern mouth of the Huanghe River (Yellow River). Continental Shelf Research 10, 1 – 40. Xu, J., 2002. River sedimentation and channel adjustment of the lower Yellow River as influenced by low discharges and seasonal channel dry-ups. Geomorphology 43, 151 – 164. Xu, J., 2003. Sediment flux to the sea as influenced by changing human activities and precipitation: Example of the Yellow River, China. Environmental Management 31, 328 – 341. Xu, J., 2004. Effect of human activities on overall trend of sedimentation in the lower Yellow River, China. Environmental Management 33, 637 – 653. Xu, J., Cheng, D., 2002. Relation between the erosion and sedimentation zones in the Yellow River, China. Geomorphology 48, 365 – 382.
Xu, J., Li, S., Zhi, J., 1990. Variation of runoff and sediment delivery in the upper and middle Yellow River in recent 70 years. Yellow River 5, 44 – 49 (in Chinese with English abstract). Xue, C., 1993. Historical changes in the Yellow River delta, China. Marine Geology 113, 321 – 330. Xue, C., 1994. Division and recognition of modern Yellow River delta lobes. Geographical Research 13, 59 – 66 (in Chinese with English abstract). Yang, X., Damen, M.C.J., Van Zuidam, R.A., 1999. Use of Thematic Mapper imagery with a geographic information system for geomorphologic mapping in a large deltaic lowland environment. International Journal of Remote Sensing 20, 659 – 681. Ye, Q., 1996. Dynamics state of the bar at the Yellow River mouth. Geographical Research 15, 58 – 65 (in Chinese with English abstract). Ye, Q., 1998. Flow interruptions and their environmental impacts on the Yellow River delta. Acta Geographica Sinica 53, 385 – 392 (in Chinese with English abstract). Yin, G., 1999. Characteristics of self-adjusting in longitudinal stream profile of the lower Yellow River. Journal of Sediment Research 2, 28 – 33 (in Chinese with English abstract). YRCC (Yellow River Conservancy Commission), 1976–1996a. Hydrological Records of Water and Sediment (interior report, unpublished, in Chinese). YRCC (Yellow River Conservancy Commission), 1976–1996b. Data on Fixed Cross-sections in the Huanghe River Mouth Channel (interior report, unpublished, in Chinese). YRCC (Yellow River Conservancy Commission), 1988–1995. Topography Database in the Huanghe River Mouth Bar (interior report, unpublished, in Chinese). Yu, L., 2002. The Huanghe River (Yellow) River: a review of its development, characteristics, and future management issues. Continental Shelf Research 22, 389 – 403. Zeng, Q., Zhang, Sh., Hu, Ch., Yin, X., Li, Z., Jiao, Y., 1997. The evolution and reconstruction of the Huanghe River (Yellow) River mouth. Publishing House for the Huanghe River (Yellow) River Water Conservancy, Zhengzhou (in Chinese).
River mouth bar formation, riverbed aggradation and channel migration in the modern Huanghe (Yellow) River delta, China Hui Fan a,b,c,*, Haijun Huang a, Thomas Q. Zeng d, Kairong Wang e a
b
Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Institute for Development Strategy of Science and Technology, Shandong Academy of Sciences, Jinan 250014, China c Graduate School of the Chinese Academy of Sciences, Beijing 100039, China d Division of Geography, School of Geosciences, University of Sydney, City Road, Sydney 2006, Australia e Institute of Hydraulic Research, Yellow River Conservancy Commission, Zhengzhou 450003, China Received 19 July 2004; received in revised form 8 August 2005; accepted 8 September 2005 Available online 25 October 2005
Abstract This paper addresses the recent (1970s–1990s) processes of river mouth bar formation, riverbed aggradation and distributary migration in the Huanghe River mouth area, in the light of station-based monitoring, field measurements and remote sensing interpretation. The results show that the morphological changes of the river mouth bar have been closely associated with the largely reduced fluvial discharge and sediment load. Landform development such as bar progradation occurred in two phases, i.e. before and after 1989, which correspond to faster and lower bar growth rates, respectively. Fast riverbed aggradation in the mouth channel was strongly related to river mouth bar progradation. During 1976–1996, about 2.8% of the total sediment loads were deposited in the river channel on the upper to middle delta. Therefore, the river water level rose by a few meters from 1984 to 1996. The frequent distributary channel migration, which switched the radial channel pattern into the SE-directed pattern in the mid-1980s, was linked with mouth bar formation. Marine conditions also constrain seaward bar progradation. Furthermore, the history of river mouth bar formation reflects human impacts, such as dredging and dyking in order to stabilize the coastal area. D 2005 Elsevier B.V. All rights reserved. Keywords: Huanghe (Yellow) River delta; River mouth bar; Sedimentation; Channel migration; Remote sensing; Human activity
1. Introduction Sediment transport from the source to the sink of the world large rivers, and its impact on the delta-coast morphology have been the major research topics in the past few decades (Milliman and Meade, 1983; Milliman and Syvitski, 1992; Chen et al., 2001a). The discharge of river water and sediment into the sea is vital for delta-coast geomorphological changes (Qian et * Corresponding author. Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China. E-mail address: [email protected] (H. Fan). 0169-555X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.08.015
al., 1993; Chen et al., 2001b). Recent changes in water and sediment of the Huanghe (Yellow) River, closely associated with intensifying human activity, have been widely noticed (Qin and Li, 1986; Aranuvachapun and Walling, 1988; Wright et al., 1988, 1990; Xue, 1993; Wang and Liang, 2000; Xu, 2003). The Huanghe River originates in Qinghai Province in western China, and it has a length of 5464 km and a catchment area of 752 000 km2 (Qian et al., 1993). Flowing through the Loess Plateau, it carries a huge amount of sediment, i.e. 1.4 109 t year1 on average (1919–1989) at the Sanmenxia Hydrological Station, about 1024 km from the river mouth (Chen, 1996).
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About 0.3 109 t year1 of the sediment have been deposited in the river channel below Sanmenxia (Xu et al., 1990; Qian et al., 1993). This has resulted in a rise of the riverbed by more than 10 m above the surrounding floodplains (Li and Brian, 1993; Qian et al., 1993; Yu, 2002). Therefore, the lower Huanghe river channel is called dHanging RiverT (Xu and Cheng, 2002; Yu, 2002). Furthermore, about 23% of supplied sediment is entrapped in the delta coast, about 40–55% is sequestrated in the nearshore, and only 21–37% is transported to the outer ocean (SPSTC, 1991). The Huanghe River serves as world’s largest contributor of fluvial sediment load to the ocean (Ren and Shi, 1986; Wang and Aubrey, 1987), accounting for ca. 5% of the global river sediment budget (Milliman and Meade, 1983; Milliman and Syvitski, 1992). During the Holocene, about 8.0 1012 t sediment was delivered into the Yellow Sea and the Bohai Sea (Shi et al., 2002) to alter the coastal morphology (Qin and Li, 1986; Wang and Aubrey, 1987; Ren, 1989; Wang and Ke, 1989; Cauwet and Mackenzie, 1993). Dramatic sediment accumulation on the delta-coast has caused the large-scale avulsion of the river channel in the last few thousand years (Pang and Si, 1979; Xue, 1993; Ren and Walker, 1998). The latest one occurred in 1855 when the river swung back from the southern Yellow Sea Coast (Jiangsu Coast) to the Bohai Sea of northern China, leading to the formation of the modern Huanghe River delta (Fig. 1). Since 1855, the Huanghe River has changed its course more than 50 times, and 8–12 of them were treated as the major historical events (Pang and Si, 1979; Xue, 1993, 1994). On average, the major shifts of the river course occurred about every 10 years (Fig. 1) (Pang and Si, 1979; Ren and Shi, 1986; Qian et al., 1993; Xue, 1994; Wang and Liang, 2000). The Qingshuigou distributary, which resulted from artificial geoengineering in 1976, is the current prime outlet of the Huanghe River to the sea. A mean discharge of 1330 m3 s1 and a mean suspended sediment concentration (SSC) of 25.5 kg m3, recorded at the Lijin hydrological station about 100 km from the coast (Fig. 1), are also applicable to the delta-coast (SPSTC, 1991). Although the river discharge has decreased significantly since the Qingshuigou distributary formed, high SSC (N100 kg m3) has often been measured in the channel during the flood period from July to October (Li et al., 1998a; Xu, 2002). The Huanghe River mouth can be classified as a low to mesotidal estuary based on Hayes (1979). The tidal range is about 1.2–1.8 m (SPSTC, 1991; Li et al., 1998b; Chen, 2001). Tidal currents around the river
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mouth are irregular, semi-diurnal and bidirectional. The flood current generally flows SSE and the ebb current directs NNW (Chen, 1988; Wang and Su, 1989; SPSTC, 1991). The depth-averaged velocity of the flood current is usually larger than that of the ebb current, and the maximum flood velocity in the delta front reaches 2.2 m s1 (SPSTC, 1991; Zeng et al., 1997). The maximum tidal current intrusion into the distributary system is about 20 km inland when the recent river discharge is extremely low, whereas it is only 2–3 km when the discharge is 1000 m3 s1, and no intrusion takes place when the discharge is more than 2000 m3 s1 (Zeng et al., 1997; Li et al., 1998a; Chen, 2001). The residual current in the Huanghe River mouth is mainly driven by wind, with a mean velocity of 0.10–0.25 m s1. The surface residual current and local waves are subject to the monsoon, and generally direct southward in the winter and northward in the summer (SPSTC, 1991). The wave height in the river mouth is about 0.8 m under the normal weather conditions (Chen, 2001). Weak tidal and wave sediment dynamics, and a large riverine sediment supply has resulted in deposition of more than two-thirds of the annual sediment load in the Qingshuigou distributary delta lobe (Fig. 1; Wright et al., 1990; Qian et al., 1993; Shi and Zhang, 2003). The average sediment accumulation rate for 1976–1992 is about 1.2 m year1 and the river mouth area has been an estuarine depocenter (Wang et al., 1992; Li et al., 1998b). As a consequence, the active river-mouth bar prograded with a rate of 1–4 km year1 and the delta lobe extended with a rate of 20–25 km2 year1 (SPSTC, 1991; Van Gelder et al., 1994; Li et al., 1998b; Yang et al., 1999; Wang and Liang, 2000; Li et al., 2002; Yu, 2002). Although the active rivermouth bar is not large in size, it has a profound effect on the rapid growth of the Huanghe River delta (Li et al., 1998b). The rapid deposition on the river-mouth bar causes its seaward progradation that has been an important control on the upstream siltation in the lower channel of the Huanghe River, serving as a stimulus to the river channel migration (Qian et al., 1993; Shi and Zhang, 2003; Shi, 2005). Heavy sedimentation in the lower reaches of the river channel causes the riverbed to aggrade at a rate of several centimeters per year (Qian et al., 1993; Van Gelder et al., 1994; Xu and Cheng, 2002). This aggradation increases the flood risk on the floodplain, making the river channel avulsionprone. Since the 1980s, numerous studies have focused on the deltaic sedimentation, riverbed aggradation and river mouth bar formation on the delta-coast of the
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Fig. 1. Location map of the modern Huanghe River delta and historical migration of deltaic channels (modified after Xue, 1994). (1) 1855–1889; (2) 1889–1897; (3) 1897–1904; (4) 1904–1929; (5) 1929–1934; (6) 1934–1938 and 1947–1964; (7) 1964–1976; (8) 1976–present.
Huanghe River (Pang and Si, 1982; Wiseman et al., 1986; Wright et al., 1988, 1990; Qian et al., 1993; Van Gelder et al., 1994; Ye, 1996; Li, 1997; Shi and Ye, 1997; Li et al., 1998b; Yin, 1999; Shi and Zhang,
2003). However, full understanding of the formation mechanisms of the river-mouth bar, in relation to the riverbed aggradation and channel migration, is lacking, and further understanding of hydrological and sedimen-
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tological processes that control delta-coast morphology is vital to coastal science and management, in relation to landuse planning, river channel maintenance, and hazard risk mitigation for sustainable regional development. Therefore, this paper discusses the river mouth bar formation, and its interactions with downstream riverbed aggradation and distributary migration in the Huanghe River delta since 1976.
0.2, 0.6, and 0.8 were measured, from which the depth-averaged velocity and SSC were derived: u¯ ¼
1 ðu0:2 þ u0:6 þ u0:8 Þ 3
ð1Þ
c¯ ¼
1 ðc0:2 u0:2 þ c0:6 u0:6 þ c0:8 u0:8 Þ 3¯u
ð2Þ
2. Data and methods Daily hydrological measurement has been carried out by the Yellow River Conservancy Commission (YRCC) at the four cross-sections of the Lijin station, each of which having 16–24 column measurement points (Li et al., 1998a). The river water level used in the present study is that at a discharge of 3000 m3 s1, and is referred to the Dagu datum, a sea-level datum in China. The consecutive 21-year hydrological data, including discharge, suspended sediment concentration (SSC), and the river water level were collected from YRCC (1976–1996a). Additional hydrological data, including current velocity and direction, SSC, salinity, and water depth in the mouth bar reach were measured during the Coastal Zone Survey Project in mid May (dry season) and late July (flood season) of 1984 (SPSTC, 1991). For each period, seven observation stations were arranged along the mouth bar (Fig. 2). The measurement was made simultaneously at each station for the water column with three to five measuring horizons according to the different water depths. Measurement was taken every one hour both for flooding and ebbing cycles. The depth-averaged flow velocity (u¯ ) and depth-averaged SSC (c¯ ) were calculated using the following conventional methods (Chen, 1996): (i) When water depth is less than 1 m, three water column horizons at a relative water depth of
Fig. 2. Location of survey stations in May and July 1984.
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(ii) When water depth is greater than or equal to 1 m, five measurements at a relative water depth of 0.0, 0.2, 0.6, 0.8, and 1.0 were used to yield the depth-averaged velocity and SSC: u¯ ¼
1 ðu0:0 þ 3u0:2 þ 3u0:6 þ 2u0:8 þ u1:0 Þ 10
c¯ ¼
1 ðc0:0 u0:0 þ 3c0:2 u0:2 þ 3c0:6 u0:6 10¯u þ 2c0:8 u0:8 þ c1:0 u1:0 Þ
ð3Þ
ð4Þ
The topographic data of the river-mouth bar, measured during the Coastal Zone Survey Project and by the YRCC once or twice a year since 1988, were compiled (YRCC, 1988–1995; SPSTC, 1991). These data were imported into a GIS platform for mapping and analyzing mouth bar formation. Morphological changes in some cross-sections along the river channel between Lijin and Qing-3 (Fig. 1) were measured twice a year, namely, before and after the flood season (YRCC, 1976–1996b). By multiplying the neighboring cross-sectional areas and their distance, the volume of sediment accretion and/or erosion of the riverbed can be estimated. Landsat Multi-Spectrum Scanner (MSS) and Thematic Mapper (TM) images were used to study the influence of the shear front on the formation of the river-mouth bar, and the migration of the mouth channel below the Lijin Hydrological Station since 1976. All the images acquired from 1976 to 2003 were corrected for removing atmospheric effects by subtracting the radiance of a ddark pixelT within each band image (Lavery et al., 1993), and were georeferenced and rectified following the procedure by Serra et al. (2003). The false-color composition of the enhanced images was constructed for extracting river channels. The SSC distribution in the Huanghe River mouth was also interpreted via the density slicing of Landsat TM band 3 (Aranuvachapun and Walling, 1988).
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3. Observations and results 3.1. Water and sediment supply from the Huanghe River From 1976 to 1996, a runoff of 252.4 108 m3 year1 and a sediment load of 6.3 108 t year1 discharged into the sea. Of these, 65.2% runoff and 89.6% sediment load occurred during the flood season, when the average SSC in the river channel often reached more than 30 kg m3. The runoff and sediment load tend to decrease with time (Fig. 3a,b), and their changes can be roughly divided into two phases, i.e. before and after 1990. Prior to 1990, the Huanghe River had a mean water discharge of 291.9 108 m3 year1 and a sediment load of 7.1 108 t year1, but since then they have reduced to 173.5 108 m3 year1 and 4.8 108 t year1, respectively, which are equivalent to 40.6% and 33.3% reduction. Hydrological records also display
that, during the period of 1976–1996, the deltaic river channel experienced seasonal dry-up events for 627 days in total, in which 488 days occurred in the 1990s (Fig. 3c). In general, synchronous fluctuations of runoff and sediment load occurred (Fig. 3a,b). However, asynchronous fluctuations took place in 1977, 1978, 1988 and 1994, when small runoff and high sediment load values appeared with a mean SSC of about 40 kg m3. 3.2. River-mouth bar The river mouth bar in the study area consists of three components: bar back, bar crest, and bar front (Fig. 4). Longitudinal length of the bar back during the observed period was 0.3–4.4 km with its slope ranging from 1:5000 to 1:100. Longitudinal length of the bar front was 0.2–1.3 km with its slope ranging from 1:400
Fig. 3. Temporal changes in (a) water discharge, (b) sediment discharge, and (c) duration of channel dry-ups at the Lijin Hydrological Station (after YRCC, 1976–1996a).
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Fig. 4. Change in the longitudinal profile of the Huanghe River mouth bar (after YRCC, 1988–1995; SPSTC, 1991).
to 1:100. The bar crest, usually occurring as flatter topography, was irregularly elliptical in shape, 2–6 km long, and with an average water depth of 0.8 to 0.1 m. The bar crest above the 2 m isobath had areas of between 5 and 50 km2. These variables are usually changeable with time. Our survey data demonstrate that the river-mouth bar generally prograded seawards, except for from October 1989 to September 1990, when a remarkable landward retreat was seen (Fig. 4). Before 1989, a rapid progradation of river mouth bar took place, but a slower growth was observed after 1990 (Fig. 4). The bar crest length and elevation also changed with time (Fig. 5). Large values of the bar crest length and elevation occurred during 1984–1987, but bar crest elevation decreased during 1988–1991. Then the bar
crest became shorter but its elevation was moderately high (Fig. 5). Three development stages of Qingshuigou river mouth bar can be recognized from its geometry change (Fig. 6). The early stage (before 1990) was characterised by an elongated bar confined by the subaerial and subaqueous levees. During the following stage (1990–1992), the river mouth bar was more laterally extended. The river mouth bar in the latest stage (1993– 1995) prograded only in a southeastern direction (Fig. 6). A high correlation occurs between the progradation length of the mouth bar from Xihekou and the cumulative sediment load at the Lijin station for the past few decades (Fig. 7). Depth-averaged current velocity in the non-flood season (May 15–16, 1984) at the monitoring stations
Fig. 5. Changes in the longitudinal length and mean elevation of the river mouth bar crest (after YRCC, 1988–1995; SPSTC, 1991).
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ing on July 28–29, 1984 (Fig. 9), also indicates high values at S04 (bar), low values at S03, and high values at S02 (landward distributary). SSC decreased consistently with an increasing seaward distance from S04 (Fig. 9). River-mouth shear fronts can be identified from the Landsat TM images. The interpreted image taken on February 13, 1989 presents a high-turbid southward mass flow surrounding the river mouth, embraced by a low-turbid northward mass flow (Fig. 10a). The image taken on October 4, 1995 indicates a similar separation of mass flows but with the opposite directions (Fig. 10b). 3.3. Variations in riverbed and water levels
Fig. 6. Change in the plan distribution of the Huanghe River mouth bar crest (after YRCC, 1988–1995).
in the Qingshuigou distributary (Fig. 2) was less than 0.8 m s1 (Fig. 8a), and that at S05 was the lowest (0.3– 0.4 m s1). At the stations upstream of S05, the current velocity during the flood tide was larger than that during the ebb tide, while seaward, the situation was reversed (Fig. 8a). Higher current velocity occurred during the flood season. Higher values of current velocity (z2.0 m s1) recorded at S01–S04 both for flooding and ebbing dramatically dropped to 0.7 m s1 at S05, and thereafter the velocity remained quite low (Fig. 8b). Depth-averaged SSC on May 15–16, 1984, portraits higher values (N1 kg m3) at S01–S04, in which the highest value appeared at S03, whereas much lower values (ca. 0.5 kg m3) occurred at S06 and S07 (Fig. 8c). From S01 to S04, SSC during the flood tide was higher than that during the ebb tide, while the opposite applies to the other stations. On July 28–29, 1984, the runoff and sediment load were high and SSC in the distributary system was 10 times more than that on May 15–16 (Fig. 8c,d). At all the stations except for S03, SSC during the flood tide was close to that during the ebb tide. SSC at S06 and S07 during both the flood and ebb tidal cycles was much lower than that at S01–S05 (Fig. 8d). The vertical distribution of SSC during flood-
The sediment budget calculated from the cross-sections points to three phases of riverbed topographic variations. From 1976 to 1979, a major siltation of the riverbed occurred between Xihekou and Qing-3, although the rate of the siltation tended to decrease with time (Fig. 11). At the same time, alternate siltation and erosion appeared along the riverbed between Lijin and Xihekou. From 1980 to 1985, the riverbed between Lijin and Qing-3 experienced slight erosion, whereas after 1985, slight siltation occurred (Fig. 11). Our further calculation indicates that from 1976 to 1996, 0.249 108 t sediment was deposited along the river channel between Lijin and Xihekou, and 3.346 108 t between Xihekou and Qing-3. According to the records at the Lijin Hydrological Station, sediment discharge was 128.3 108 t during the same period (YRCC, 1976–1996a). Thus, about 2.8% of incoming sediment was deposited along the river channel between Lijin and Qing-3.
Fig. 7. Relationship between the cumulative sediment supply from the Huanghe River and distance from the front of the bar crest to Xihekou (after YRCC, 1976–1996a, 1988–1995; SPSTC, 1991).
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Fig. 8. Spatial differences in depth-averaged current velocity (a: May 1516, 1984; b: July 2829, 1984) and depth-averaged SCC (c: May 1516, 1984; d: July 2829, 1984) along the Huanghe River mouth bar (after SPSTC, 1991).
An increasing trend of the water level with time was recorded at Lijin and Xihekou (Fig. 12). The water level fluctuations at the two stations are almost syn-
chronous. From 1985 to 1996, the water level had risen by 1.97 m at Lijin with a rate of 0.164 m year1, and by 2.04 m at Xihekou with a rate of 0.170 m year1.
Fig. 9. Vertical profile of suspended sediment concentration along the Huanghe River mouth bar during flood tide on July 28, 1984 (after SPSTC, 1991).
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Fig. 10. Tidal current and shear front at the Huanghe River mouth inferred from satellite images (a: inner-flood-outer-ebb shear front on February 13, 1989; b: inner-ebb-outer-flood shear front on October 4, 1995).
3.4. Morphological changes of the distributary system There have been two major phases of distributary displacement in the Qingshuigou river mouth. The Landsat images demonstrate that during the mid to late 1970s, most distributaries were oriented in a NE direction, except May 1977, October 1977 and September 1979 (Fig. 13). From the 1980s to the 1990s, distributaries tended to shift from easterly to southeasterly. The lateral shift of the distributary before the 1980s was faster than that after the 1980s. In July 1996, an artificial channel diversion below Qing-8 provided the Chahe distributary at the river mouth tip (Fig. 1) (Chen, 2001; Li et al., 2002). 4. Discussion Heavy-laden sediment along the Huanghe River and associated coastal environmental managements have been discussed in the past century (Pang and Si,
1982; Wiseman et al., 1986; Wright et al., 1988, 1990; Qian et al., 1993; Van Gelder et al., 1994; Li et al., 1998b; Shi and Zhang, 2003). The present study highlights the development of the Qingshuigou river mouth bar during the mid 1970s to the 1990s, when both runoff and sediment supply from the upper reach dropped by about 30–40% (Fig. 3) due mainly to damming, water diversion and soil and water conservation practices in the upstream area (Qian et al., 1993; Ye, 1998; Ren et al., 2002; Xu, 2002, 2003, 2004). This reduction directly affected the progradation rate of the river mouth bar: faster before 1989 but slower after 1990 (Fig. 4). In particular, increased seasonal channel dry-ups after 1990 (Fig. 3c) made the river mouth bar totally exposed to the controls of tidal currents, which significantly limited the river mouth bar progradation, and tidal flow with an average discharge of ca. 300–500 m3 s1 intruded more than 20 km into the distributary mouth channel (Zeng et al., 1997; Li et al., 1998a; Chen, 2001). The intrusion of low SSC tidal currents
Fig. 11. Sedimentation in the mouth channel of the Huanghe River (after YRCC, 1976–1996b).
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Fig. 12. Temporal change in the water level at a discharge of 3000 m3 s 1 for the Lijin and Xihekou Hydrological Stations (after YRCC, 1976– 1996a).
is conducive to the self-dredging of the river mouth bar reach. Thus, human-induced reduction of runoff and sediment loads significantly influenced the evolution of the river mouth bar. An obvious retrogradation of the river mouth bar between 1989 and 1990 (Fig. 4) probably reflects intensive riverbed dredging during the same period (Ye, 1996; Chen, 2001). Dredging and river regulation were conducted from 1988 to 1992 to mitigate the heavy siltation in the river mouth area, because the braided and unconfined river channels in the river mouth area
may disturb living safety and economic development, particularly in relation to an important oil field at the northern coast off the Qingshuigou distributary (Ye, 1996; Zeng et al., 1997; Chen, 2001; Yu, 2002). Changes in the length and elevation of the bar crest (Fig. 5) also point to the effects of dredging. The spatial distribution of the Qinghuigou river mouth bar, characterized by the three phases of different geometry patterns (Fig. 6), further manifests the combined effect of human activities and water/sediment discharge. Large-scale river regulation and dredging
Fig. 13. Changes in the Qingshuigou distributary channel from 1976 to 2003, interpreted from satellite images. The inset shows topographic effect of the river mouth bar on distributary evolution.
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coupled with more abundant river discharge between 1988 and 1989 (Fig. 3), led to the elongated river mouth bar limited by natural levees. From 1990 to 1992, the upstream movement of salt wedge resulted from the impacts of dredging and smaller river discharge (Fig. 3), caused an extensive lateral migration of the river mouth bar, when the bar moved further seaward beyond the levees’ controls. The river bar suddenly prograded southward in September 1992, which probably reflects the rapid sedimentation in association with flood processes (Chen, 2001). After 1993, the river bar primarily headed southeastward and further beyond the river mouth topographic limitation, which may have resulted from the termination of dredging. River mouth bar progradation was basically dependent on the amount of sediment supply (Fig. 7). The river mouth bar formation has been closely associated with the depth-averaged current velocity and SSC during both the non-flood and flood seasons (Figs. 8 and 9). The segment between S05 and S06 served as the critical location in the river mouth area, where river and sea currents strongly interacted. Much higher values of current velocity and SSC at stations landward of S05–S06 (Figs. 8 and 9) indicate dominant fluvial processes, in which a large amount of sediment has been entrapped in the distributary to form the river mouth bar. Li (1997) and Li et al. (1998a) also proposed that the low current–velocity zone near S05–S06 played a significant role in forming the river mouth bar. The high-turbid mass flow during flooding, usually called hyperpycnal plumes, moved southward which confronted the less turbid mass flow with opposite movement due to ebbing (Fig. 10a). A set of reversally directed mass flows was observed at the same place (Fig. 10b), and the shear front occurred about 3–5 km off the river mouth (Fig. 9). These flows with the shear front hindered further seaward progradation of the river mouth bar (Li et al., 1994, 2001). The formation of the Qingshuigou river mouth bar has altered river channel morphology in the adjacent coastal land. The slow but continuous aggradation of the river channel between Lijin and Qing-3, as well as the continuous water level rise after the mid 1980s (Figs. 11 and 12), were associated with the river mouth bar formation. The alternation of siltation and slight erosion of the river channel before the bar formation may reflect more busualQ river processes. Riverbed aggradation generally occurred when the Huanghe River had small runoff and high sediment loads; whereas riverbed erosion occurred when abundant runoff was combined with low sediment loads (Figs. 3 and 11), as suggested by Qian et al. (1993),
Chen (2001) and Xu (2002). In addition, serious riverbed siltation between Xihekou and Qing-3 in the 1970s has been associated with the formation of the Qingshuigou distributary (Yu, 2002; Shi and Zhang, 2003). The spatial distribution of numerous distributaries in the river mouth area was also affected by the river mouth bar formation after the mid 1980s. The radiated distributaries during the 1970s suggest weaker controls of the Qingshuigou river mouth topography on distributary form (Fig. 14 and its inset A). After the mid 1980s, the formation of the river mouth bar let the distributaries extend southeasterly although their distal portion could migrate beyond the topographic constraint (Fig. 14 and its inset B). 5. Conclusions The Huanghe River mouth bar, represented by the Qingshuigou river mouth area in the present study, is dominated by fluvial processes, and its seaward progradation has been closely associated with runoff and sediment discharging into the sea. The fluctuations of these two fluvial variables over the past few decades (1970s–1990s) has played a decisive role in forming the river mouth bar, affecting its growth rate, elevation, length and geometry patterns. Two major time periods of landform development, before and after 1990, have been identified, based on temporal change in the bar growth rate in response to changed runoff and sediment load. Rapid sedimentation in the river mouth area resulted from the low current velocity due to the counteraction between river outflows and tidal currents, where the turbidity maximum zone formed. The shear front, due to different density values of mass flows in front of the bar, hinders seaward bar progradation. From 1985 to 1996, river mouth bar progradation and associated riverbed silation in the Qingshuigou distributary elevated the water level by ca. 2 m at the Lijin and Xihekou hydrological stations. This enlarged the potential flood risk. The formation of SE-directed distributaries after the mid-1980s was strongly influenced by bar formation. Intensified human activity, such as dredging and dyking along the Qingshuigou distributary channel during the late 1980s and the early 1990s, terminated rapid bar progradation and minimized further bifurcation of distributaries. Overall, the Huanghe River mouth bar evolution in the recent decades highlighted the increasing effects of human activities on the geomorphological evolution of the deltaic coast, and such an inference provides useful information for future studies.
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