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

The Yellow River (YR) is well known in the world due to its high sediment concentration and rapid sedimentation rate in its lower reach (Wang & Li 2011). The riverbed in the lower Yellow River (LYR) has been dramatically aggrading for many years, and is now up to 10 m higher than the overbank plains in some places, forming a so-called ‘suspended river’ or ‘perched river’ due to its high sedimentation rate (Wu et al. 2008a). The main-channel width in the LYR varied from 0.9 to 1.4 km after the 1999 flood season, and there are extensive floodplains utilized by local inhabitants on both sides of the LYR, the area of which accounts for over 80% of the whole river (Xia et al. 2014a; Cheng et al. 2021). Therefore, studying the adjustment characteristics and influencing factors of bankfull discharge (BFD) (herein referring to the flow at which the in-channel water level is about to overtop its banks onto the active floodplain) would provide significant insights into additional information to guide further river management and flood prevention in the LYR.

Through long-term practical exploration of YR management, reservoir sediment retention (RSR) and water and sediment regulation (WSR) have been proposed for disposal and utilization of the sediment. Before the operation of the Xiaolangdi Reservoir (XLDR), the LYR was characterized by obvious shrinkage of the main channel accompanied by a sharp decrease in the flood discharge capacity (Bi et al. 2019), and later the situation was improved due to the reservoir operation. Upon its completion in 1999, the XLDR has been used to retain most of the inflowing sediment to the LYR. To further desilt the lower reaches and recover the medium-sized channels, a water–sediment regulation (WSR) project was initiated with the XLDR as the core in 2002. Remarkable effects of the WSR on the LYR have been reported (Wang et al. 2017), including changes in the river channels (Xu & Si 2009; Chen et al. 2012; Ma et al. 2012), hydrological characteristics (Lu et al. 2022), and sedimentation features (Kong et al. 2022), as well as changes in the evolution of the Yellow River Delta (Kong et al. 2015; Wang et al. 2017).

WSR was accompanied with a large-flow discharge process that was conducive to the sediment transport of the lower reaches and the recovery and maintenance of the medium-sized channels (Zhang et al. 2009; Xia et al. 2014a). The BFD at Huayuankou Station increased from 3,700 m³/s in 2000 to 6,900 m³/s in 2012 (Qi et al. 2013) and, overall, the LYR channel has switched from a sediment-deposition state to an erosive state (Xu et al. 2005). Some studies have been conducted on the operation of the XLDR and the variation of BFD of the medium-sized channels in the LYR. Wu et al. (2008a) pointed out that the variation of BFD could be predicted by using the changes in the incoming discharge and sediment load in the LYR. Li et al. (2015), Xia et al. (2010a, 2010b) and Chen et al. (2018) studied the BFD in the LYR, and analyzed the variation of BFD and its relation with water and sediment. With the operation of the XLDR, the flow and sediment regime entering the LYR has been altered dramatically, and the LYR channel is experiencing continuous degradation, leading to a significant variation in the bankfull channel geometry (Xia et al. 2010a, 2010b, 2014a, 2014b). Hu et al. (2012) analyzed the effect of various water and sediment combinations released from the XLDR on the shaping of the main channel and found that under recent incoming flow and sediment conditions, it is possible to shape and maintain a medium-sized channel with a BFD of approximately 4,000 m³/s. Miao et al. (2016) analyzed the impact of WSR of the XLDR on the scouring and silting of the LYR. Bi et al. (2019) analyzed the scouring and silting characteristics of the LYR before and after the operation of the XLDR. Chen et al. (2021) put forward the operation mode of the XLDR based on the maintenance of the medium-sized channels in the LYR. Cheng et al. (2021) estimated the adjustment characteristics of various bankfull parameters over the period 1986–2015 in a braided reach of the LYR. All of these studies have gone into the operation of the XLDR, the scouring and silting of the middle and lower reaches, and the evolution of the medium-sized channels. However, no research has been done into the quantitative contributions of RSR and WSR to medium-sized channel shaping. Whether WSR or RSR plays a greater role in shaping the medium-sized channels in the LYR remains to be explored. In the existing literature, the scale of the restored medium-sized channels is often attributed to the role of WSR, ignoring the contribution of RSR, resulting in misunderstandings, which are necessary to clarify.

In this paper, based on the analysis of the variation of BFD of the medium-sized channels in the LYR, we established the response relationship between the BFD and the inflow sediment coefficient in the flood season and the large water discharge within the year. On this basis, we further constructed different water and sediment scenarios to analyze and calculate the quantitative contribution of WSR of the reservoir to the medium-sized channel shaping in the LYR. In addition, spatial and temporal analyses of the contribution of current WSR on the LYR were conducted. The results will lay a solid technical foundation for WSR of the XLDR, the river regulation downstream of the reservoir, and the sediment disposal of the YR as well as other large rivers with severe sedimentation throughout the world.

The Xiaolangdi Hydroproject is located at the exit of the last canyon in the middle reaches of the Yellow River, 130 km away from Sanmenxia Hydroproject on the upper reaches and 131.9 km away from Huayuankou Hydrologic Station on the lower reaches. As a backbone project to control the WSR system of the YR (Zhang et al. 2021a, 2021b), the Xiaolangdi Hydroproject has been developed primarily for flood control (including ice prevention) and desilting, and secondarily for water supply, irrigation and power generation, with the aim to create favorable conditions to fend off natural disasters and boost comprehensive utilization of resources. The reservoir has a designed normal storage level of 275 m and a total storage of 12.65 billion m³, including 7.55 billion m³ for sediment retention and 1 billion m³ for WSR. The gate was closed for impoundment in October 1999, and the reservoir is currently in the first phase of the later sediment-retention stage (Chen et al. 2016).

The Sanmenxia Reservoir is the first large-scale hydroproject built on the main stream of the middle Yellow River, developed primarily for flood control, and secondarily for ice prevention, irrigation, power generation and water supply. Since the gating for impoundment in September 1960, the reservoir has been reconstructed twice, and experienced three operation stages of ‘storing water and retaining sediment’, ‘detaining flood and discharging sediment ‘ and ‘storing clean water and discharging muddy flow’. To maintain active storage and control sediment deposition from affecting the elevation of river reaches at Tongguan (Yang et al. 2020), the reservoir is limited to operate at a low water level of 305 m in the flood season, with the corresponding storage capacity of only about 50 million m³, rendering its water and sediment regulating capacity weak. The reservoir area is basically under an equilibrium between scouring and deposition.

We first established expressions to describe the response relationship between the BFD and the water and sediment using the measured data. On this basis, we then calculated the variation of BFD according to the water and sediment process in three different scenarios. Finally, we obtained the quantitative contribution of WSR to the medium-sized channel shaping by comparing the variation of BFD between these three different scenarios.

Upon its completion, the XLDR has successfully reduced the inflowing sediment to the LYR, and played a significant role in scouring and silting of river channels and shaping of medium-sized channels. In order to separately analyze the effect of WSR on shaping the medium-sized channels in the LYR, it is necessary to first analyze the role of WSR in controlling the inflowing water and sediment to the LYR. To this end, three scenarios in this regard are developed according to the locations of reservoirs and hydrologic stations and the WSR of reservoirs:

• Scenario 1: The water and sediment processes at Tongguan, Heishiguan and Wuzhi Hydrologic Stations represent the water and sediment process without RSR or WSR (i.e. without reservoir regulation).

• Scenario 2: The water and sediment processes at Xiaolangdi, Heishiguan and Wuzhi Hydrologic Stations represent the water and sediment process with RSR and WSR.

• Scenario 3: The water process at Tongguan Hydrologic Station, the sediment process at Xiaolangdi Hydrologic Station, and the water and sediment processes at Heishiguan and Wuzhi Hydrologic Stations represent the water and sediment process with only RSR.

The annual average water discharges under scenarios 1, 2 and 3 are 26.175 billion m³, 27.176 billion m³ and 26.175 billion m³, respectively (as shown in Table 1), in which the water discharge in the flood season accounts for 48.5%, 36.9% and 48.5%, respectively, and the annual average water discharge above 2,000 m³/s accounts for 16.7%, 23.0% and 16.7%, respectively. The annual sediment loads are 237 million tonnes, 71 million tonnes and 71 million tonnes, respectively, in which the sediment loads in the flood season account for 81.1%, 94.8% and 94.8%, respectively. It is thus clear that the WSR of the reservoir has increased the large-flow discharge to the LYR. The annual average number of days with discharge above 2,000 m³/s has increased from 16.93 days to 24.64 days, and the corresponding water discharge from 4.379 billion m³ to 6.262 billion m³. Through reservoir sediment retention, the annual average sediment inflow to the LYR has dropped from 237 million tonnes to 71 million tonnes.

It can be seen from the table that the BFD in the three scenarios shows different variations with time. In scenarios 1 and 3, the calculated BFD fluctuates, showing a trend of increasing first and then decreasing, except for the Huayuankou Hydrologic Station, where the BFD at the beginning of the period is greater than that at the end of the period. In scenario 2, the calculated BFD at all stations shows an overall increasing trend, with the BFD at the beginning less than that at the end. The variation of calculated BFD at the beginning and end of the period in the three scenarios is compared as follows.

In scenario 1, the calculated BFD during the period from 2002 to 2015 increased from 3,075 m³/s to 4,152 m³/s at Huayuankou Station, but decreased from 1,717 m³/s to 1,470 m³/s at Gaocun Station, from 1,679 m³/s to 1,069 m³/s at Sunkou Station, and from 2,540 m³/s to 2,123 m³/s at Lijin Station. The minimum BFD during this period also decreased from 1,630 m³/s to 1,162 m³/s. It is thus clear that in this scenario, the BFD decreased at all other stations except at Huayuankou Station, so this scenario represents the variation trend of BFD without WSR of the reservoir.

In scenario 2, the calculated BFD during the period from 2002 to 2015 increased from 3,890 m³/s to 7,247 m³/s at Huayuankou Station, from 2,244 m³/s to 6,027 m³/s at Gaocun Station, from 1,985 m³/s to 4,407 m³/s at Sunkou Station, and from 2,902 m³/s to 5,036 m³/s at Lijin Station. The minimum BFD during this period also increased from 1,953 m³/s to 4,265 m³/s. It is thus clear that in this scenario, the BFD at all stations increased, and the variation of calculated BFD was consistent with the measured one in the lower Yellow River, so this scenario represents the variation of BFD under the combined effect of RSR and WSR under natural water and sediment conditions.

In scenario 3, the calculated BFD during the period from 2002 to 2015 increased from 3,717 m³/s to 4,382 m³/s at Huayuankou Station, but decreased from 1,984 m³/s to 1,531 m³/s at Gaocun Station, from 1,747 m³/s to 1,081 m³/s at Sunkou Station, and from 2,690 m³/s to 2,518 m³/s at Lijin Station. The minimum BFD during this period also decreased from 1,738 m³/s to 1,183 m³/s. It is thus clear that in this scenario, the BFD decreased at all other stations except at Huayuankou Station close to the dam, indicating that it is impossible to recover the medium-sized channels in a large scale from a long distance only by RSR.

Through comparison of the calculation results in these three scenarios, we can obtain the quantitative contributions made by RSR and WSR to the recovery of medium-sized channels, To be specific, the difference in the calculation results between scenarios 1 and 2 shows that the quantitative contributions made by RSR and WSR to the BFD at Huayuankou, Gaocun, Sunkou and Lijin, and to the minimum BFD are 2,016 m³/s, 2,540 m³/s, 1,829 m³/s, 1,571 m³/s and 1,712 m³/s, respectively (as shown in Table 3). The difference in the calculation results between scenarios 2 and 3 shows that the quantitative contributions of WSR to the BFD at Huayuankou, Gaocun, Sunkou and Lijin, and to the minimum BFD are 1,361 m³/s, 2,263 m³/s, 1,766 m³/s, 1,437 m³/s and 1,612 m³/s, respectively. The difference in the calculation results between scenarios 1 and 3 shows that the quantitative contributions of RSR to the BFD at Huayuankou, Gaocun, Sunkou and Lijin, and to the minimum BFD are 655 m³/s, 277 m³/s, 63 m³/s, 134 m³/s and 100 m³/s, respectively. It is thus clear that RSR and WSR have increased the average BFD at each station. To be specific, the average BFD at Huayuankou is increased by 2,016 m³/s, including 1,361 m³/s (68%) contributed by WSR and 655 m³/s (32%) by RSR. That at Gaocun is increased by 2,540 m³/s, including 2,263 m³/s (89%) contributed by WSR and 277 m³/s (11%) by RSR. That at Sunkou is increased by 1,829 m³/s, including 1,766 m³/s (97%) contributed by WSR and 63 m³/s (3%) by RSR. That at Lijin is increased by 1,571 m³/s, including 1,437 m³/s (92%) contributed by WSR and 134 m³/s (8%) by RSR. The average minimum BFD is increased by 1,712 m³/s, including 1,612 m³/s (94%) contributed by WSR and 100 m³/s (6%) by RSR.

The results revealed that the WSR is more efficient for shaping and maintaining a medium-sized channel. After the implementation of the WSR, the downstream main channel obtained comprehensive erosion, which reduced the deposition of the LYR, increased the BFD of the main channel, and improved the flow capacity (Xu & Si 2009; Hu et al. 2012; Miao et al. 2016). However, it is noteworthy that the scale of the restored medium-sized channels is not only attributed to the role of WSR; RSR also contributed to approximately 6%–32% of the medium-sized channel shaping in the LYR.

In addition, in 2002, the first year of WSR, the contributions of WSR to the BFD at Huayuankou, Gaocun, Sunkou and Lijin and to the minimum BFD were 21%, 49%, 78%, 59% and 67%, respectively (as shown in Figure 8). In subsequent years, the contribution showed an upward trend. The contribution to the BFD of the reaches downstream of Huayuankou was gradually stabilized after 2006, keeping at over 90% in most years. The Huayuankou Station was closer to the dam, so it was greatly affected by the sediment retention and clear water discharge of the reservoir. The riverbed was easily scoured in the early stage, and the discharge of clear water during the sediment-retention period would definitely be conducive to recovering the medium-sized channel. Nevertheless, with the gradual coarsening of riverbed sediment, the effect of scouring by clear water, especially at a small discharge, would inevitably decrease (Shen et al. 2020). Miao et al. (2016) have suggested that the current WSR is no longer as effective as it was initially for the LYR, exhibiting a tendency towards functional degradation. Hence, the contribution of WSR to the BFD at Huayuankou generally showed a slow increase, accounting for more than 90% in 2014.

Through analysis of the temporal and spatial variations of BFD in the LYR and its influencing factors (i.e. water and sediment), it is clear that there is a good response relationship between the BFD at Huayuankou, Gaocun, Sunkou, Lijin and the minimum flow section and the inflow sediment coefficient in the flood season and the water discharge above 2,000 m³/s during a year. Considering the delayed response of BFD to water and sediment, expressions of response relationships are established, to fully describe the impact of the equilibrium between water and sediment and the large-flow discharge process on the variation of BFD in the lower Yellow River.

Three water and sediment scenarios are constructed based on the WSR of the XLDR. On this basis, the quantitative contributions of WSR of the reservoir to the medium-sized channel shaping in the LYR are calculated as per the expressions of response relationships. From 2002 to 2015, the contributions of the reservoir operation to the discharge capacity recovery of the medium-sized channels at Huayuankou, Gaocun, Sunkou, Lijin and the minimum flow section in the LYR were 2,016 m³/s, 2,540 m³/s, 1,829 m³/s, 1,571 m³/s and 1,712 m³/s, respectively. The contributions of WSR to the recovery of the medium-sized channels gradually increased with time and became stabilized eventually, i.e., for the Sunkou section, which increased from 63.5% in 2003 to 98.3% in 2007, and remained at around 97.8% during 2008–2015. With the increase of the distance from the dam, such contribution also increased and eventually became stabilized in narrow reaches, reaching over 90%. Along the LYR, such contribution also increased from 68% to 97% and eventually became stabilized in narrow reaches, reaching over 90%. The research results indicate that the large discharge process during WSR of the XLDR plays a significant role in shaping the medium-sized channels in the LYR.

With the changes in the natural environment of the YR Basin and the increasingly stringent requirements on basin protection and management, and given the fact that the YR has a small amount of water resources and there is a tension between supply and demand, it is recommended to focus research on water discharge for WSR in the future, in order to optimize the discharge process through reservoir operation, thus improving the efficiency of sediment transport and medium-sized channel shaping during WSR.

The current investigation was financially supported by Young Elite Scientists Sponsorship Program by Henan Association for Science and Technology (grant number 2022HYTP022), the National Key R&D Program of China (grant number 2016YFC0402503 and 2018YFC1508404) and Science-Technology Program for Excellent Talents in Yellow River Conservancy Commission of the Ministry of Water Resources (HQK-202319).

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We declare that we are not affiliated with or involved with any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this paper.