High-efficiency sediment-transport requirements for operation of the Xiaolangdi Reservoir in the Lower Yellow River Open Access

Sediment transport efficiency is a very important topic of riverbed evolution (Ni et al. 2004). In river systems with sediment-laden flow, water use for agriculture and industry increases rapidly with increasing population and economic growth (Vörösmarty et al. 2010), while the water used for sediment transport in rivers decreases gradually. This mismatch between water and sediment leads to the siltation of the river and main channel shrinkage, which lower the bankfull discharge (Cheng et al. 2021). The ‘small floods and big disasters’ phenomenon occurs occasionally, seriously threatening the flood control safety, lives, and property safety of the residential and arable areas along the river (Willner et al. 2018; Li et al. 2020). Therefore, it is crucial to study how to improve sediment-transport efficiency to reduce river siltation, lower the downstream river flood control pressure, and save water resources (Nash 1994; Guo et al. 2020).

The Yellow River (YR) is the second largest river in China and the sixth largest river in the world and is characterized by high sediment yields, an incompatible relationship between flow and sediment, and frequent disastrous floods (Zhao et al. 2019; Hou et al. 2021). The majority of the YR basin is located in a semi-arid climate, and the YR supports the water demands of 12% of China's population and 17% of the arable land with 2% of China's river runoff. As industrial and agricultural production have developed, water resource shortages have become a bottleneck that restricts socioeconomic development in the YR basin (Kong et al. 2022). However, the YR is also known as an ‘Earth Suspended River’ due to rapid sedimentation rates in its lower reaches (Wang & Li 2011). Therefore, improving the sediment-transport efficiency (i.e., transporting more sediment with as little water as possible) is conducive to maintaining the ecological environment and ensuring the long-term stability of the YR (Shen et al. 2019).

The sediment-transport water volume is an important indicator used to characterize the sediment-transport efficiency. The sediment-transport water volume can be expressed as (1) the water volume required to transport sediment per unit mass (Qian et al. 1993; Yue et al. 1996; Zhao et al. 1997), which is the unit sediment-transport water volume; (2) the volume of clear water used to transport sediment per unit mass to the lower reaches or into the sea (Fei 1998; Shi & Wang 2001; Wang & Shi 2002), which is the unit sediment-transport clear water volume (the volume of clear water in the water body containing sediment per unit mass); and (3) the water volume required to transport a specific amount of sediment (Gao & Wang 1999; Yan et al. 2012). The first two concepts are unit sediment-transport water volumes, while the third is the total sediment-transport water volume. Generally, the unit sediment-transport water volume or the unit sediment-transport clear water volume is used. A smaller unit sediment-transport water volume or unit sediment-transport clear water volume indicates higher sediment-transport efficiency, whereas the opposite indicates lower sediment-transport efficiency.

Research has been conducted on the sediment-transport water volume in the lower reaches of the Yellow River (LYR). Qian et al. (1993) and Yue et al. (1996) analyzed the sediment-transport water volume during flood season, non-flood season, and peak flow period in the LYR, and defined the peak flow discharge of high-efficiency sediment transport as the peak flow discharge with a sediment-transport water volume of less than 20 m³/t, while the quantity of silt in the river channel accounted for less than 20% of the incoming sediment load. Shi & Wang (2001) proposed that the unit sediment-transport water volume at an optimal combination of river channel sedimentation conditions and sediment-transport efficiency is the most economical sediment-transport water demand, and that normal functions can be maintained in the LYR if the sediment-transport water volume is maintained at 25 m³/t. Ni et al.(2004) determined that the optimal discharge for controlling sediment transport was 2,390–2,900 m³/s when the sediment concentration was 20 kg/m³ at Huayuankou Station in the LYR. Zhang & Shen (2009) found that the average sediment concentration at Huayuankou Station was 105 kg/m³ and the average water consumption for sediment transport was 9.52 m³/t during controlled discharges of 3,800 m³/s from Xiaolangdi Reservoir (XLDR) during flood season. Yan et al. (2012) analyzed the process of sediment transport in hyperconcentrated rivers based on data collected before 2000, and found that the average sediment concentration at Xiaolangdi Station during flood season was 30.17–49.19 kg/m³ after XLDR operations, for which the unit sediment-transport water volume in the LYR was 17.48–27.82 m³/t. These studies have made initial progress regarding determining the sediment-transport water volume in the YR. However, few studies have performed in-depth systematic investigations of how to reduce the sediment-transport water amount.

Sediment-transport water volume is directly related to water and sediment conditions (e.g., flow discharge, sediment concentration, and incoming sediment coefficient) (Yao et al. 2006; Li et al. 2010; Yan et al. 2013), river boundaries (Shen et al. 2019), and reservoir operations (Shi & Wang 2003). Interactions between the channel morphology and water-sediment movement, such that the channel's cross-sectional shape affects the flow characteristics, subsequently affect sediment transport (Cheng et al. 2021). Moreover, the goal of reservoir operations is to transfer water and sediment; thus, the XLDR can transport sediment with bed-forming discharge and hyperconcentrated flow (the recommended method for saving water and reducing siltation in the LYR), thereby efficiently transporting sediment to the sea (Bi et al. 2019; Kong et al. 2022; Lu et al. 2022). However, during the early stages of XLDR operations, the main goal of reservoir regulation was flood control and reducing river siltation in the LYR, and the sediment-transport efficiency was not high. During the last 20 years, the underlying surface conditions in the YR basin have undergone tremendous changes, and the amount of sediment has decreased gradually (Wang et al. 2016). The water and sediment that enter the downstream region, as well as the river boundary conditions, have changed substantially; therefore, the XLDR operational requirements have changed to improve the sediment-transport efficiency while also ensuring downstream flood control safety and achieving lower siltation of the LYR.

Under the current water and sediment conditions and riverbed boundaries in the LYR, we used data from 306 non-bankfull floods in the LYR from 1960 to 2016 to examine the requirements for XLDR operation to ensure high-efficiency sediment transport in the LYR. The goals of this study were: (1) to analyze the unit sediment-transport water volume based on different water and sediment conditions in the LYR; (2) to identify scouring and silting responses of the LYR to variations in sediment-transport efficiency along the channel; and (3) to determine an optimized operational scheme for the XLDR to improve sediment-transport efficiency in the LYR. The results are of reference significance for saving sediment-transport water volume in sandy rivers and increasing water availability for socioeconomic development of the river basin.

The XLDR is located in the middle reaches of the YR, 128 km above Huayuankou Station in the LYR. The dam site controls a 694,000 km² basin, which accounts for 92.3% of the total YR basin area. The XLDR was developed primarily for flood control (ice prevention) and reducing sedimentation, and secondarily for water supply, irrigation, and power generation (Chen et al. 2016). The reservoir has a total storage capacity of 12.65 billion m³, including 4.05 billion m³ for flood control, 1.00 billion m³ for water and sediment regulation, and 7.55 billion m³ for sediment trapping (Chen et al. 2016). The river closure for the project began on October 28, 1997, and the gate was closed for impoundment on October 25, 1999. The amount of sediment in the reservoir reached 3.45 billion m³ by April 2019.

The YR mainstream extends 881 km through the North China Plain, from below Xiaolangdi Dam into the Bohai Sea in Kenli County, Shandong Province. The LYR experiences serious sediment accumulation, making it a world-famous overground river. Seven hydrological control stations are located in the LYR: Huayuankou, Jiahetan, Gaocun, Sunkou, Aishan, Luokou, and Lijin stations. A wandering reach is located above Gaocun, which is wide and shallow, with a distance between the dikes of 4.1–20.0 km and a main channel width of approximately 1.5–10.0 km. A transitional reach extends from Gaocun to Taochengpu (28.24 km upstream of Aishan Station), with a distance between the dikes of 1.4–8.5 km and a main channel width of approximately 0.7–3.7 km. A curved reach extends from Taochengpu to Lijin, with a distance between the dikes of 0.4–5.5 km and a main channel width of 0.3–1.5 km. The estuary reach is located below Lijin. According to the river pattern and the distribution of hydrological stations, the LYR is divided into three reaches: above Gaocun, Gaocun–Aishan, and Aishan–Lijin. We analyzed the sediment-transport characteristics of each reach based on water and sediment data collected at Huayuankou, Gaocun, Aishan, and Lijin stations.

Mathematical modeling of water and sediment is a common method for simulating water flow and sedimentation processes, as well as the evolution of scouring and silting, in river channels. The one-dimensional river numerical simulation system used herein is a self-developed reservoir-channel water and sediment dynamics model. The parameters in the model's reservoir calculation module were verified by incoming and outflowing water and sediment, cross-section terrain, and sediment scouring and silting data collected since the XLDR was impounded and began operation in 1999. The parameters used in the river channel calculation module were verified by incoming water and sediment, cross-section terrain, and sediment scouring and silting data collected in the LYR since 1960. We verified that the error between the scour–silt quantities obtained from the model and the measured values are within 10%, which meets the requirements for simulation accuracy. The governing equations used in the mathematical model are as follows:

• ① Water flow continuity equation

  

  (1)
• ② Water flow movement equation

  

  (2)

• ③ Non-equilibrium sediment transport equation

In Equations (1)–(8): x is a coordinate along the flow direction (m); t is time (s); Q is the discharge (m³/s); z is the water level (m); B is the width of the river (m); is the inflow (outflow) per unit time and unit river length (m²/s²); A is the section runoff area (m²); n is the roughness coefficient; g is the gravity acceleration (m/s²); is the recovery saturation coefficient; is the settling velocity of the sediment particles in group k (m/s); is the sediment concentration in group k (kg/m³); is the sediment carrying capacity (kg/m³); is the sediment carrying capacity in group k (kg/m³); is the sediment inflow (outflow) per unit time and unit river length (kg/(m·s)); is the dry bulk density of the sediment (kN/m³); u is the water flow velocity (m/s); h is the water depth (m); and are the densities of the sediment and muddy water, respectively (kg/m³); is the Kármán constant, which is related to the sediment concentration; is the sediment concentration by volume ratio (kg/m³); is the representative settling velocity of the sediment carrying capacity of mixed sediment (m/s); is the median diameter of the bed material (mm); is the bed material gradation; is the recovery saturation coefficient in group k; and m is a coefficient of sediment carrying capacity of water flow.

The factors that affect the sediment-transport efficiency in the river channel include water and sediment conditions, riverbed boundaries, and sediment particle compositions. Among these factors, the incoming water and sediment conditions are the decisive factors. Despite their different research focuses, previous studies of sediment-transport efficiency in the LYR have obtained a consistent understanding: sediment transport in the river channel primarily occurs during the flood season and the sediment-transport efficiency is higher than during the non-flood season, and the sediment-transport efficiency during the flood period of the flood season is higher than those in other periods during the flood season. A total of 306 non-overbank floods (1960–2016) that entered the LYR with peak flow discharges greater than 1,000 m³/s were used to analyze the sediment-transport efficiency of the LYR. The total duration of the floods was 2,468 days. The water and sediment data at the hydrological control stations and water and sediment diversion data were all observational data, which were compiled and published by the Yellow River Conservancy Commission.

The incoming water and sediment of the YR are the basic conditions required to calculate the sediment transport and scour–silt quantities in the river channel. Since the mid-1980s, the water and sediment volume of the YR have changed substantially, with obvious decreases in runoff and sediment load. These changes were not only affected by natural climatic factors, but were also closely related to human activity, including water conservation projects, water and soil conservation and ecological construction projects, and activities related to socioeconomic development. Currently, the sediment load of the YR in the future is generally estimated as 300–800 million tons. In this study, we used a scenario in which 600 million tons of incoming sediment load in the middle YR was applied to calculate the effect of the XLDR's current operational mode and to optimize the reservoir's scheduled operations. The annual incoming water volume corresponding to this sediment load is 26.2 billion m³, 52% of which occurs during flood season. The calculated terrain in the XLDR area and in the LYR were based on data collected before the 2018 flood season.

There were 102 floods with an average sediment concentration of less than 20 kg/m³, with a total duration of 995 days. The unit sediment-transport clear water volumes at Huayuankou Station at all discharge levels were higher than those at Gaocun Station, i.e., the sediment-transport efficiency of Huayuankou Station was lower than that of Gaocun Station, and the sediment-transport efficiency of the reach above Gaocun increased along the river channel.

There were 114 floods with average sediment concentrations of 20–60 kg/m³, with a total duration of 925 days. When the water discharge was less than 3,000 m³/s, the unit sediment-transport clear water volume at Huayuankou Station was less than that at Gaocun Station, and the sediment-transport efficiency was greater than that at Gaocun Station, i.e., the sediment-transport efficiency in the reach above Gaocun decreased along the channel. When the water discharge was greater than 3,000 m³/s, the sediment-transport efficiency in the reach above Gaocun increased along the river channel.

For floods with average sediment concentrations greater than 60 kg/m³, the unit sediment-transport clear water volumes at Huayuankou Station for all discharge levels were less than those at Gaocun Station and the sediment-transport efficiencies were higher than those at Gaocun Station. Thus, the sediment-transport efficiency of the reach above Gaocun decreased along the river channel.

For floods with an average sediment concentration less than 100 kg/m³, the unit sediment-transport clear water volumes at Gaocun, Aishan, and Lijin stations were similar, all of them decreasing at each discharge level along the channel, i.e., the sediment-transport efficiency increased along the river channel.

For floods with an average sediment concentration greater than 100 kg/m³, the unit sediment-transport clear water volumes at Gaocun, Aishan, and Lijin stations increased along the channel, i.e., the sediment-transport efficiency decreased along the river channel.

After the sediment-laden flow entered the LYR, scouring or silting occurred in each reach with increasing or decreasing sediment-transport efficiency along the channel. Table 1 presents the variations in scouring and silting in the LYR at different sediment concentrations.

For floods with an average sediment concentration less than 20 kg/m³, the sediment-transport efficiency of the reach increased along the channel and the sediment load carried by each cubic metre of water increased. Compared with the upstream section, the higher sediment load in each cubic metre of water must be replenished from the riverbed. Thus, scouring generally occurred in the river channel. At this sediment concentration, the measured amount of scoured sediment in the reach above Gaocun was 1.751 billion tons.

For floods with an average sediment concentration of 20–60 kg/m³, when the water discharge was less than 3,000 m³/s, the sediment-transport efficiency in the reach above Gaocun decreased along the channel, and the sediment load carried by each cubic metre of water decreased, causing silting in the river channel. When the water discharge was greater than 3,000 m³/s, silting generally changed to scouring in the reach above Gaocun, with an increase in sediment-transport efficiency along the river channel.

For floods with an average sediment concentration greater than 60 kg/m³, sedimentation occurred in this reach to different extents as the sediment-transport efficiency decreased along the river channel.

For floods with an average sediment concentration less than 100 kg/m³, scouring generally occurred in the reach as the sediment-transport efficiency increased along the river channel. For floods with an average sediment concentration greater than 100 kg/m³, sedimentation generally occurred in the reach as the sediment-transport efficiency decreased along the river channel. Sediment erosion or siltation mainly occurred in the reach above Gaocun, and the variations in scouring/silting in the reach below Gaocun were far less obvious than those in the reach above Gaocun. For example, for floods entering the lower reaches with average sediment concentrations less than 20 kg/m³, the scouring quantity in the reach below Gaocun accounted for 29.5% of the total in the lower reaches, and for floods with average sediment concentrations greater than 100 kg/m³, the silting quantity of the reach accounted for 7.4%–12.3% of the total in the lower reaches.

The XLDR regulates water and sediment discharges to meet the established river management objectives. Before the XLDR began operation, average annual sediment deposition in the LYR was about 200 million tons, and the riverbed increased 10 cm per year, which seriously affected flood prevention and threatened the lives and property security of the populations residing on both sides of the embankment. Since the XLDR began operation in October 1999, the amount of sediment entering the LYR was reduced considerably through sediment retention and water–sediment regulation by the reservoir (Li & Sheng 2011; Kong et al. 2015). Scouring of the entire downstream river region was also initiated. The cumulative sediment scour was 3.02 billion t, and the minimum pre-flood bankfull discharge increased from 1,800 m³/s (2002) to 4,300 m³/s (2019). In addition, a medium-flood channel with an appropriate discharge capacity (4,000 m³/s; Hu et al. 2012; Liu et al. 2016) has been formed in the lower reaches. The bankfull discharge has increased to greater than 6,500 m³/s in the reach above Gaocun. Thus, the goal of reducing deposition in the LYR has been preliminarily achieved, and the flood control ability has also been improved.

For the past 20 years, the XLDR has mainly been used for sediment trapping to reduce sedimentation in the LYR. Hyperconcentrated flows that are likely to cause sedimentation in the LYR are blocked and stored, while clear water or large discharge flows with low sediment concentrations are discharged; thus, the sediment-transport efficiency of the LYR is relatively low. Statistically, 45 of the 49 large discharge flows discharged from the XLDR had average sediment concentrations less than 20 kg/m³, for which the unit sediment-transport clear water volumes were greater than 40 m³/t. To improve the sediment-transport efficiency in the LYR, it is necessary to discharge high sediment concentration flows from the XLDR. After this high sediment concentration flow enters the downstream channel, most of the sediment will be deposited in the reach above Gaocun. Before the flood season in the following year (when clear water from the XLDR is discharged), the sediment deposited in this part of the river can be transported downstream to the sea. Therefore, it is feasible to optimize sediment discharges from the XLDR by accounting for silt reduction and high-efficiency sediment transport in the LYR.

The current sediment discharge mode of the Xiaolangdi Reservoir (current scheme) focuses on water storage and silt retention. Water and sediment regulation are carried out according to the incoming flow and water storage of the reservoir before or during the flood season, while clean water or large flows with low sediment concentrations are occasionally discharged to scour the downstream channel (Table 2). According to our results, when the discharge of water that enters the LYR is greater than 3,000 m³/s with a sediment concentration greater than 60 kg/m³, the sediment-transport efficiency of the LYR is relatively high. A flow with this sediment concentration entering the downstream river channel will cause sedimentation in the reach above Gaocun, which has a relatively high channel discharge capacity, but will not have a major impact on the reach below Gaocun, which has an appropriate channel discharge capacity. The XLDR can discharge water and sediment at these levels. Therefore, to improve the sediment-transport efficiency in the LYR, the XLDR can lower the water level for sediment discharging and increase the non-overbank flow with an outflow discharge greater than 3,000 m³/s when an incoming water flow has a sediment concentration greater than 60 kg/m³. This optimized scheme is shown in Table 2. In the future, the operation mode of the XLDR should be adjusted over time according to changes along the downstream channel.

In the current scheme, the XLDR stores water and retains sediment during the sediment retention period. Before or during the flood season, the reservoir discharges large flows to scour the LYR. The average sediment concentration at Huayuankou Station during large flows is 56.4 kg/m³ and the average water consumption for sediment transport is 17.35 m³/t.

In the optimized scheme, the reservoir would increase the chances of sediment discharge in large flows with high sediment concentrations. The average sediment concentration at Huayuankou Station would be 77.1 kg/m³ for large flows and the average water consumption for sediment transport would be 12.59 m³/t, indicating that the same amount of sediment transport requires 27% less water consumption than the current scheme. During the period when water levels are lower and sediment is discharged in high sediment concentration flows, the average sediment concentration at Huayuankou Station would be 122.9 kg/m³ and the average water consumption for sediment transport would be only 7.8 m³/t.

In general, after optimizing the sediment discharge operation mode of the XLDR, the sediment-transport efficiency of the LYR improved, the sedimentation rates in the XLDR and the LYR slowed, and the time for which the channel size maintained a flow of 4,000 m³/s increased.

The sediment-transport water volume is directly related to the sediment concentration. As the sediment concentration increased, each station's unit sediment-transport clear water volume decreased. However, the regulation of sediment-transport water volume via water discharge is constrained by the sediment concentration. According to Figure 3, the relationships between flow discharge and the sediment-transport water volume in the different reaches varied, or no obvious correlation was observed between them. This was attributed to two reasons: (1) adjustments to the river channel terrain occur due to sediment scouring and silting under different water and sediment conditions; (2) owing to the uncertainty of river channel terrain in response to water and sediment from a single water flow, for a single water flow, the conditions during the early stage of the riverbed, the incoming water and sediment, and the riverbed material gradation will have an impact on the evolution of water and sediment. Thus, the relationship between flow discharge and sediment-transport water volume becomes more random. Therefore, the sediment-transport efficiency of a river channel cannot be improved simply by regulating the discharge of water that enters the LYR. The operation of the XLDR over the last 20 years also indicates that it is impossible to improve the sediment-transport efficiency in the LYR simply by discharging large water flows with low sediment concentrations.

Compared with the findings of previous research, the water consumption for sediment transport was lower in this study. For example, Zhang & Shen (2009) established the relationship between the ratios of sediment erosion and deposition, average flow, and average sediment concentration during the flood season through data analysis. For a sediment amount of 600 million tons, they proposed that the average sediment concentration at Huayuankou Station was 105 kg/m³, and the average sediment-transport water consumption was 9.52 m³/t during the flood season (XLDR controlled discharge is 3,800 m³/s). We propose that the optimization of sediment discharge operations from the XLDR will result in the average sediment concentration at Huayuankou Station of 122.9 kg/m³, and the average water consumption for sediment transport of only 7.80 m³/t, which is 18% smaller than the value obtained by Zhang & Shen (2009). Zhang & Shen (2009) selected a representative constant flow of 3,800 m³/s, while in this paper the flow was a variable process greater than 3,000 m³/s discharged by XLDR . We also optimized reservoir sediment discharge operations, which increased the sediment discharge opportunities for flows with high sediment concentrations.

Shi & Wang (2003) used Equation (9) to propose that the minimum sediment-transport water volume at Huayuankou Station was 16.29 m³/t, and that the corresponding sediment concentration of the water flowing into the LYR was 60 kg/m³ under the premise of constraining the sedimentation ratio in the channel to below 20%. However, based on the current boundary conditions of the LYR, we believe that the XLDR has preliminarily achieved reductions in siltation in the LYR, and the channel scale of the reach above Gaocun Station has far exceeded the target value. Therefore, the future goal of reservoir regulation should not be blindly to pursue the scouring of the LYR, but should also be to improve the sediment-transport efficiency of the LYR. The XLDR can discharge flows with sediment concentrations greater than 60 kg/m³, for which most of the siltation caused by the flow will only occur above Gaocun Station. In addition, this siltation can be transported to the sea when the XLDR discharges clear water prior to the following flood season.

The following conclusions were drawn based on the findings:

• (1)

  The unit sediment-transport clear water volume of the river channel decreased with increasing sediment concentration. When the sediment concentration of the water flow was less than 20 kg/m³, the unit sediment-transport clear water volume of the river channel was greater than 50 m³/t. When the sediment concentration increased to 60 kg/m³, the unit sediment-transport clear water volume decreased to ∼16 m³/t. When the sediment concentration increased to greater than 100 kg/m³, the unit sediment-transport clear water volume decreased to less than 10 m³/t and gradually leveled off. Flows with high sediment concentrations not only improved the sediment-transport efficiency in the LYR, but also caused sediment deposition.

• (2)

  It is necessary to optimize the current sediment discharge from the XLDR to improve the sediment-transport efficiency and conserve water resources by accounting for silt reduction in the LYR. In the future, given current scheduling, when an incoming water flow has a sediment concentration greater than 60 kg/m³, the water level can be lowered and non-overbank large water flows with an outflow discharge greater than 3,000 m³/s can be increased. In a scenario in which the incoming sediment load in the middle YR is 600 million tons, the average water consumption for sediment transport at the downstream Huayuankou Station would be reduced from 17.35 m³/t (current scheme) to 12.59 m³/t (optimized scheme), which could save 27% of the sediment-transport water volume. This consumption would be reduced further to 7.8 m³/t for water flows with high sediment concentrations. The proposed optimized scheduling scheme for the XLDR could reduce the cumulative silt quantity in the downstream channel by 1.1 billion tons more than the amount under the optimized scheme. In addition, the optimized scheme could increase the time of maintaining the bankfull discharge at ∼4,000 m³/s for 11 additional years, extend the sediment-trapping period of the XLDR by ten years, and increase the annual amount of sediment transported into the sea by 109 million tons.

• (3)

  In the future, the operation mode of the XLDR should be optimized according to changes in downstream channel erosion/silting and sediment transport. For example, when the river channel fills with silt and the minimum bankfull discharge is less than 4,000 m³/s, the reservoir should continue to act mainly as a sediment trap and discharge clear water or large water flows with low sediment concentrations to scour and reshape the downstream channel, thereby ensuring safe flood control in the LYR.

This study was supported by the Young Elite Scientists Sponsorship Program of the Henan Association for Science and Technology (No. 2022HYTP022), and the National Key Research and Development Programme of China (No. 2016YFC0402503, No. 2017YFC0404402). We are grateful to Dr Yao Yue of Wuhan University for her valuable comments.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.