Nutrient's response to water transfer in an urban river-network

Using hydraulic structures such as pumps, sluices, and dams to transfer and distribute water and enhance the carrying capacity of the regional water environment are important measures to improve the water quality of rivers and lakes (Zhang et al. 2018; Daga et al. 2020). In the flat delta area of large rivers, under the action of estuarine sedimentation, it is easy to form a plain river network with dense rivers and crisscross channels under the action of estuarine sedimentation (Gu et al. 2018). In the region, the drop along the river course is small, the hydraulic driving condition is limited, and the flow is slow (Gan et al. 2012). Under natural conditions, material transport and diffusion is slow, and coupled with the increase of pollutants into the river caused by urbanization, the phenomenon of water eutrophication induced by high nutrient load occurs from time to time (Deng et al. 2018). Excessive nutrient concentrations can weaken the ecological function of water bodies and reduce the diversity of structure, which poses a serious threat to human health (Carpenter & Lathrop 2008; Taylor et al. 2014). Increasing the water diversion of the plain river network can improve the hydrodynamic conditions of the river and increase the content of dissolved oxygen in the water, which is beneficial to the degradation of pollutants by microorganisms, but at the same time, high flow velocity will increase the cutting effect of water flow on the riverbed, aggravating the release of nutrients from sediment into the overlying water (Schindler 2006; Graham et al. 2016; León et al. 2017; Shakibaeinia et al. 2017), and the water quality will deteriorate further. Therefore, it is of great significance to quantitatively analyze the variation of nutrient load in different regions of the river network under different water transfer schemes.

The water quality–hydrodynamics response mechanism is affected by many factors. Decomposing the mechanism and quantifying the weight of various components is helpful in determining the water-dispatching threshold and planning the dispatching scheme as a whole. Tang et al. (2014) studied the migration and diffusion law of pollution components in the middle route of the South-to-North Water Transfer Project and developed a water quality guarantee scheme accordingly. Chen et al. (2013) and Zhou et al. (2014) used a mathematical model to analyze the mechanisms of flow movement and pollutant transport and diffusion under different regulation modes and combined it with the interpolation method to obtain a calculation formula for controlling the transport of pollution clouds. Zhang et al. (2001) studied the behavior of phosphorus release and adsorption at the water–sediment interface of Taihu Lake by using the simulated disturbance environment constructed by a constant temperature oscillator, and the results show that there is a strong release of phosphate under high disturbance. Through the experiment of Wang et al. (2015), the relationship between the TP release rate and disturbance intensity was obtained, and it was found that there was a peak value in the increase of TP concentration. Pauer & Auer (2000) showed that disturbance increased the dissolved oxygen content in water, coupled with the upward movement of high nitrogen concentration during sediment initiation, which promoted nitrification at the sediment–water interface. However, most of these studies focus on the analysis of a single way or conclusions about the integrity of the research region, while few people pay attention to the separation of water diversion effects. For a densely populated urban plain river network, the formulation of the water quality improvement plan needs to be further refined, to maximize the effect of water quality improvement brought by water transfer.

Therefore, this paper takes the eastern lakeside river network of Meiliang Bay of Taihu Lake as an example to study the following three aspects. (1) A hydrodynamic water quality coupling mathematical model considering pollutant degradation and transport is constructed and calibrated and verified. (2) A vertical release experiment of in situ sediment in an indoor flume is carried out. The coupling curve of nutrient factor release rate and bed shear stress is obtained, and the contribution value of sediment release in the actual environment is calculated. (3) The response mechanism weights of the flood, dry season TN, and TP concentrations in the characteristic section of the river network to water diversion are analyzed, and the respective contributions of DI, II, and SI in the water environment under different hydrodynamic conditions are obtained. The research method proposed in this paper evaluates the influence degree and comprehensive effect of water transfer on the positive effect of nutrient dilution and degradation and the negative effect of the endogenous release of pollutants in rivers with different characteristics and quantifies the contribution weight of each part. The results address the question of how many pollutants will be released while promoting the transport and degradation of nutrients in the river, which provides a basis for determining the best hydraulic control threshold, optimizing the dispatching scheme, and taking comprehensive river control measures. If the DI weight of the river is high, the quality of diverted water should be considered and clean water sources should be introduced. When the II weight of the river is high, measures such as riverbed desilting or adding pollutant release inhibitor (Fukushima et al. 2018) should be taken before water diversion to control the endogenous release; when the river SI weight is high, some chemical and biological treatment measures can be taken.

The study area is the lakeside river network in Lihu New City, Binhu District, Wuxi City (31°31′24″–31°32′33″N, 120°16′17″–120°20′26″E). Wuxi City has a humid monsoon climate in the northern subtropics, with abundant rainfall, with an annual average temperature of 15.6 °C and an average annual rainfall of 1,112.3 mm. It has a resident population of 6.57 million in 2018 and the annual regional gross domestic product (GDP) is 1.1439 trillion yuan. Binhu District is rich in water resources, economically developed, with more than 70 large and small rivers, with a network density of 2.05 km/km², and a water surface rate of 6.91%. The annual average water level is 3.06 m, and the ground elevation is between 3.5 m and 6.2 m (Wusong base). It belongs to a typical plain urban river network (Figure 1). The river network extends from the Liangxi River in the north to Caowang River in the south, Li Lake in the west, and the Beijing–Hangzhou Grand Canal in the east. The upstream reaches of Taihu Lake is the third-largest freshwater lake in China, with a surface area of 2,338.1 km². It covers an area of 33.7 km² (Wang et al. 2016). The contents of N and P in Taihu Lake are high, especially in summer (July–September). The enrichment of cyanobacteria bloom is more serious (Ma et al. 2015). Under the action of the seasonal dominant wind, the bloom cyanobacteria in Taihu Lake have drifted and converged to Meiliang Lake and diverted water from the Liangxi River to the river network through the pumping station, which has caused great pressure on the water environment of the river network.

Under natural conditions, the hydrodynamic conditions in the river network area are weak, reciprocating flow, canal water backflow and other phenomena that occur from time to time, and human activities limit the self-purification function of the water body. To supplement water sources and meet the needs of water exchange, the local government has set up 11 water diversion pumping stations of different sizes in the river network, in which Taihu Lake water introduced by Meiliang pumping station and Daxuan pumping station is the main water source of the river network, with the diversion flow and operation as shown in Figure 1, followed by natural injection of Li Lake to realize river–lake linkage. At the same time, the Lihu New Town area where the river network belongs is densely populated, human activities are frequent, and some sewage is discharged directly, so sluices and dams are set up to intercept water and control pollution to reduce the impact of production and domestic water discharge on the overall water environment of the river network. While these hydraulic structures play a role, they also have a certain impact on the structure and connectivity of the water system: 70% of the rivers have a long-term flow velocity of less than 0.01 m/s, the transport and diffusion of pollutants are slow and they are not easily consumed through degradation, and as a result, they settle and accumulate in the sediments. The comprehensive nutrient value of the Binhu River network in 2017 is 57.3, which is in a state of mild eutrophication (Li & Qin 2012; He et al. 2020).

At present, the local government takes measures such as water diversion to release pollution, in situ treatment, ecological floating islands, flow control and pollution interception, sediment dredging and so on to deal with the problems of river eutrophication and algae proliferation. Among these, water quantity regulation can speed up water exchange, promote the degradation and attenuation of pollution factors, and gradually restore the ability of water self-purification, which is a normal method to solve the problem of water quality in a plain river network. Therefore, the quantitative analysis of the hydrodynamic water quality response mechanism can establish the basis for determining the comprehensive treatment methods adopted in different regions (Wang et al. 2020).

According to the previous study of river distribution and water system structure in the study area, the river network is divided into four regions: Liangxi River, internal tributaries of the river network, Mali River and the southeast of the river network. A total of 103 water quality and 26 sediment sampling sites (Figure 1) were selected to carry out four surveys and monitoring of hydrology and water quality, sediments and algae in March, August, September and November 2018. The velocity of the 1/4, 1/2 and 3/4 quartiles of the cross-section was measured by LB70-1C rotary cup current meter, and the water depth was measured synchronously by portable ultrasonic sounder. The sediment samples on the river bottom and the overlying water at the mud sample points were collected by a stainless steel sampler, and the overlying water was stored in a sealed polyethylene bucket. All the samples were stored away from light at 4 °C and brought back to the laboratory. Some of the deposited samples were centrifuged (3,500 r/min, 30 min) to obtain interstitial water, while others were used for indoor sediment release experiments in an extended annular flume. The contents of water quality factors of in situ water samples, sediment interstitial water and experimental water samples were determined. The content of TN was determined by alkaline potassium persulfate digestion and ultraviolet spectrophotometry (GB/T 11894-1989); TP was determined by alkaline potassium persulfate digestion and molybdenum–antimony anti-color spectrophotometry (wavelength 700 nm) (GB/T 11893-1989).

Based on the field monitoring results, the local water quality standard Surface Water Environmental Quality Standard was used to evaluate the water quality of the monitoring section in flood and dry seasons. During the flood season, the content of TN in the river network is generally high and serious, with an average concentration of 4.6 mg/L and a maximum concentration of 43.47 mg/L. Among the monitored sections, only three sections are up to the standard, with an over-standard rate of 95.7%, an average over-standard multiple of 2.67, and a maximum of 42.4 times. The average concentration of TP is 0.5 mg/L, the over-standard rate is 47.15%, the average over-standard rate is 1.15 times, the highest over-standard multiple is 50.7 times, and the over-standard rate of TP in Liangxi River is 40%. During the dry season, the average TN concentration of the river network exceeds the standard rate of 4.1 mg/L by 89.3%, with an average exceeding of the standard of 3.22 times, and the TN content of the most serious section exceeding the standard is 31.62 mg/L. The average concentration of TP in the river network is 0.51 mg/L, higher than the average concentration, accounting for 21.3%, the highest concentration of 8.39 mg, the over-standard rate of TP is 40.7%, the highest is 27 times, and the over-standard rate of TP in the monitoring section of Liangxi River is 80%. On the whole, there is little difference in the overall nutrient content of the river network in different water periods, but there are obvious differences in the nutrient concentration of rivers in different regions. For example, the contents of TN and TP in rivers in the southeast of the river network are 4–5 times higher than those in other areas of the river network.

The adsorption and release of pollutants by sediment is in dynamic equilibrium (Han et al. 2020). Colloids and sediment particles have extensive micro-interfaces in natural water bodies, which form the basis of their coupling with pollutants and as pollutant carriers. The nutrients entering the water body are adsorbed on the surface of the particles and deposited into the sediment, and on the one hand, the driving force caused by the concentration gradient of the sediment–water interface is released from the interstitial water to the overlying water, while on the other hand, due to the re-suspension of the sediment caused by the change of hydrodynamic conditions, the attached pollutants spread to the water body, increasing the pollution load of the overlying water body. The hydrodynamics of the plain river network has a significant response to the regulation of water volume, and the critical velocity of sediment swirling driven by bed shear stress and the pollutant release rate depends on the sediment viscosity and the content of pollutants in the sediment. Therefore, the in situ sediments and overlying water of typical sections were collected, and the vertical sediment release experiments under different hydrodynamic conditions were carried out in the laboratory by using an annular flume made of PVC plates. The migration and transformation law of target factors at the sediment–water interface was obtained.

There are great differences in the physical morphology of the river in the study area, besides the impact of human activities on the river that is strongly regional, resulting in differences in the enrichment degree of pollutants in river sediments and the viscosity of sediment. A runner driven by an external motor is used to realize different levels of disturbance at a rotational speed of 0–500 r/min. The flow velocity change caused by water dispatching is simulated, and the relationship between bed shear stress and TN and TP release rate is obtained. After coupling with the hydrodynamic model, the water quality change under each dispatching scheme is predicted, and the action mechanism of water transfer on nutrient concentration is studied quantitatively. The response mechanism of nutrient load to hydraulic regulation under the complex hydrodynamic conditions of the plain river network is revealed.

The annular flume device (Figure 2) consists of a straight road and a bend, with an overall perimeter of 10 m, a width of 0.3 m and a height of 0.45 m. The radius of the inner ring of the bending part is 0.65 m and the radius of the outer ring is 0.95 m. The total length of the two sections of the straight road is 5 m. A disc runner connected to the motor is set at 0.3 m on one side of the straight road to realize hydraulic drive and is equipped with a speed-regulating meter with a measuring range of 0–1,500 r/min. The mud area is set up on the other side of the straight road, and the water velocity is measured by Acoustic Doppler Velocimetry (ADV) at 0.05 m above the mud sample. The measured position can better characterize the average velocity in the trough under different disturbances (Li et al. 2016). A baffle parallel to the flume is set at the bend in front of the mud area, which weakens the deflection force of the water flow out of the bend and ensures that the shear stress on the bed surface is in the same direction as the ADV velocity measurement. The turbidity of the water body is measured between the baffle and the mud through a portable turbidimeter. When the turbidity tends to be stable, the concentration of TN and TP is sampled at the bend behind the mud area, and the disturbance level is improved synchronously.

The year 2018 is selected as the simulation period, and generalization is carried out based on river network channel connectivity and actual water transport and storage capacity. There are 66 rivers in the model, with a total length of 62.11 km, including 49 boundaries and 11 hydraulic structures. The calculated water level points and discharge points are 546. The calculated water points are located in the cross-section of the river, and the calculated discharge points are between the calculated water points (Ahmed 2010). There are 48 inflow boundaries, which are located in the upper reaches of Liangxi River and along the banks of Lihu Lake, and one outflow boundary, which is located in the Beijing–Hangzhou Grand Canal. The data of river bottom elevation, cross-section shape, topography, water level and gate pump dispatching come from Binhu District Water Conservancy Bureau and Binhu District Environmental Protection Bureau. The boundary flow data are provided by Wuxi Hydrology Bureau, in which the daily average flow is used for the main inflow boundary of the river network, the monthly average discharge is used for the rest of the inflow boundary, and the water level boundary is selected for the outflow boundary. The hydraulic structure consists of two diversion pumping stations, each of which adopts the mode of alternating and complementary operation, diverting water together from Meiliang Bay of Taihu Lake, and merging into the Binhu River network at the sluice gate of Liangxi River. The annual average flow of the two pumping stations is 23 m³/s. Nine auxiliary pumping stations are mainly used for the improvement of the hydrodynamic conditions of tributaries and flood control and drainage. The location and scale of the pumping stations are shown in Figure 1.

Using the validated numerical model of the river network water environment coupled with sediment release, ten typical sections are selected in the Binhu river network, including two in Liangxi River, one in Mali River, three in the southeast of the river network including Caowang River, and four in internal tributaries of the river network (Figure 1). Based on the actual operation of the river network in 2018, we change the water diversion of pumping stations and the opening and closing of auxiliary pumping stations. Four kinds of water quantity control schemes (Table 1) are designed, including the actual situation, focusing on the effect of the design scheme in the improvement of hydrodynamics and water quality. The flow, flow velocity and nutrient content of the cross-section under the corresponding scheme are simulated, and the contributions of II, DI and SI to the change of nutrient content underwater dispatching are further calculated, and the weight analysis is carried out.

The flow velocity of the water body increases with the increase of the disturbance level, which enhances the shear effect of the sediment-overlying water interface. Under its action, the sediment rotates gradually and is suspended in the water body, so the TN and TP in the interstitial water are released into the water body. The release rates of TN and TP in the sediments of each section also change significantly, from the adsorption of nutrients to the release of nutrients at rest. The release rates of most sections increase at first and then decrease with the increase of shear stress, and the release intensities of S5, S7, and S8 increase continuously. When the disturbance level is maximum, the turbidity increases significantly, and the turbidity at 500 r/min is 3–5 times higher than that at 300 r/min. The adsorption and release of nutrients in most sections tend to be in dynamic equilibrium (Figure 3). During the experiment, the spinning of the sediment is roughly divided into three processes. When the disturbance level is lower than 200 r/min, the bed shear stress is less than 0.0005 N/m² and the maximum turbidity is 4.7 NTU. In this state, the rotation of sediment is not obvious, only a few particles slide on the bed, and the movement is random, and the effect on nutrients changes from adsorption to release in this process. The TN release intensity of S9 is the highest at this stage, which is 6,449 mg/(m²·d). The maximum TP release intensity is 3,145 mg/(m²·d) for S2. When the disturbance exceeds 300 r/min, the average flow rate is 0.4 m/s, the bed shear stress is 0.0008 N/m², the turbidity increases by 1–2 times, and for S2 up to 10.3 NTU. Under this condition, the path formed by the movement of particles can be seen with the naked eye, and the bed surface becomes uneven. There is fog on the inside of the pit, and occasionally a large mud mass roll. At this time, the release rate of nutrients is close to the peak, and the increase of the rate slows down. The maximum release rate of TN for S9 is up to 8,712 mg/(m²·d), and the TP release intensity for S6 is 2,119 mg/(m²·d). When the disturbance level reaches 500 r/min, the water body is turbid, mud masses continue to turn up around the sediment, some fall off in lumps, and the mud samples tend to collapse, with a flow velocity of 0.53 m/s, a bed shear stress of 0.0018 N/m² and maximum turbidity of 48.4 NTU. The adsorption and release of TN and TP from the S1, S2, and S6 sections with low nutrient content in the sediment are close to dynamic equilibrium. The rest of the sections are still in a high release state. The river where S8 is located is affected by human pollution, and the load of nutrient factors is high in the sediment. The release rates of TN and TP, which are 15,104 mg/(m²·d) and 2,166 mg/(m²·d), respectively, are the highest at the maximum disturbance level. The entire experimental process accords with the basic law of sediment movement, and the bed shear stress simulated by the disturbance level can also be well applied to the actual hydrodynamic conditions of the river network.

Combined with the hydrological data of the lakeside river network in 2018 and the field monitoring data, the hydrodynamic water quality model is calibrated, and the trial-and-error method is adopted to debug, select and verify the parameters (Figure 4). In order to calibrate the average monthly velocity, the hydrodynamic module selects the river channel where the study section is located from January to June to calibrate the average monthly velocity, which is verified by the data from July to December. The results show that the river roughness can be divided into two types according to the water surface width. When the maximum water surface width is greater than or equal to 30 m, the roughness is between 0.021 and 0.025, and when less than 30 m, the roughness is between 0.03 and 0.035. At this time, the average relative error between the calculated value and the measured value of each channel velocity of the river network is 3.8%–6.2%.

The module on water selects the measured flood data and dry season data for calibration and verification respectively. Based on the study of river pollutant degradation coefficients by Feng et al. (2016) and Zhang et al. (2015), the diffusion coefficient and degradation coefficient of TN and TP are determined by a trial algorithm. It is found that when the diffusion coefficient takes 10 m²/s, TN and TP degradation coefficients are 0.79 d⁻¹ and 0.2 d⁻¹ respectively, the average relative error is 16.2% and 19.7% respectively, which can meet the requirement that the average relative error between the calculated value of the model and the measured value is less than 20%. The verification results show that the model meets the accuracy of water quality simulation of the Binhu River network.

The effect of water transfer and pollution release is related to the amount of water diversion and the concentration of source water and background water quality. Through the upper boundary water transfer pumping station and internal auxiliary pumping station of the river network, the flow of the river and cross-section can be changed, and the increased water transfer can be diverted and lost continuously in the river network. Finally, there are differences in the increase of water quantity in different rivers. Figure 5 is the change and proportion of flow and concentrations of TN and TP of each section under the three dispatching schemes (scheme B, C, and D) based on the actual scheme A during the flood and dry seasons. When the water transfer of the upper reaches of the river network is increased by 20%, the flow of Liangxi River tributaries S3 and S4 increases by more than 20%. The average flow of S4 in the flood season increases from 0.23 m³/s to 0.313 m³/s, an increase of 36.29%. S1 and S2, the mainstream of Liangxi River, are located in the main channel of water transfer, and their discharge increments are increased by 3.93 m³/s and 4.19 m³/s, respectively; in the southeast of the river network, S8, S9 and S10 are further from the diversion pumping station, and there are multiple open boundaries in the area flowing through Lihu Lake, so the flow response is not significant, increasing by 0.01%, 0.07%, and 0.11% respectively in the flood season, while the average discharge of S8 and S9 decreases slightly in the dry season. The flows of S5, S6 and S7 in the middle tributaries of the river network and Mali River increase by 6.72% to 15.42%. Based on the comparison of the simulation results of schemes C, D, and A, the auxiliary pumping station of the river network mainly controls the water volume of the tributaries in the middle of the river network. Under the direct action of the auxiliary pumping station, the S3–S7 flow decreases, the decrease of S7 is small (1.04%), while that of S3–S6 is larger and the flow direction of S3, S4, and S6 changes, in which the flow of S3 decreases from 1.17 m³/s before the pumping station to 0.36 m³/s, and the flow reduction is the largest. S6 traffic decreases the most, by 97.47% (from 0.82 m³/s to 0.02 m³/s). Under the indirect action of the auxiliary pumping station, the flow of S1 and S2 in the mainstream of Liangxi River increases by 7.32% and 1.12% respectively, and the cross-sectional flow in the southeastern region fluctuates only slightly. In the case of the operation of the auxiliary pumping station, when the water transfer of the river network increases by 20%, the flow of S3–S7 remains unchanged due to the control of the pumping station, and the flow of S1 and S2 increases by 27.36% and 16.99% respectively under the comprehensive action, and the effect of the diversion pumping station and the auxiliary pumping station on the rest of the section is limited. In the dry season, the flow rate of S4 decreases under the action of the auxiliary pumping station, which is because the actual flow in the dry season is lower than that in the flood season, and the flow direction changes under the action of the pumping station, but the flow rate changes little. Under each scheme, the flow of S8 increases in the flood season and decreases in the dry season, but the range of change is less than 0.1%, which is caused by the fluctuation of its normal water quantity.

Based on the calculation results, when scheme B increases the water diversion by 20%, the water quality of S2 is improved, TN decreases by 0.038 mg/L in the dry season (1.72%), and TP decreases by 0.002 mg/L (1.69%). Secondly, the TN of S7 decreases by 0.022 mg/L (0.6%) and TP decreases by 0.001 mg/L (0.5%) in the flood season, and the improvement of the cross-section factor in the main river is greater than that in the tributary sections. In the flood season, the TN and TP content in S6 decreases by 0.276% and 0.28% respectively, and by 1.09% and 1.03% in the dry season. The improvement is quite different because the background values of factor concentrations are different in different water periods. In the section where the increase of water diversion leads to an increase in water quality concentration, the factor concentration of S3, S4, and S5 in the tributaries increases greatly, especially in the dry season, and the TN concentration increases by 1.34%, 1.47%, and 2.89%, respectively. The content factors of S8, S9, and S10 are all decreased, but the decrease is not more than 0.01%, and there is no substantial impact of water dilution. After the auxiliary pumping stations are added in schemes C and D, the factor concentration of the river directly regulated by it decreases significantly, and the water quality of the diversion pumping station is no longer affected by the change of water quantity. In scheme C, when the auxiliary pumping station is opened and the water diversion is constant, the concentration of TN and TP in S3–S6 decreases in different water periods, and the decrease in the dry season is greater than that in the flood season. For example, in S4, the concentration of TN decreases by 19.35% in the flood season, and TP by 20.01%, and by 25.33% and 23.34% in the dry season, respectively. The indirect effect of the auxiliary pumping station slightly increases the concentration of nutrient factors in other sections. The TN concentration in S1 and S2 during the flood season increases by 0.015% and 0.359% respectively compared with that before the opening of the auxiliary pumping station. The TP concentration of S7 increases by 0.304% in the flood season, and the concentration of the S8, S9, and S10 factors changes little after the opening of the auxiliary pumping station. Scheme D opens the auxiliary pumping station while increasing the water diversion, and more than 70% of the cross-section factor concentration is improved, in which, the tributary S3–S6 is not affected by the diversion water under the control of the auxiliary pumping station, S2, S9, and S10 can dilute some pollutants under the comprehensive action, and the water quality of S8 increases slightly under each scheme, and the increase of TN concentration in the dry season is the largest (0.051%).

The release intensity of TN and TP in sediment mainly depends on the shear stress on the bed surface and the concentration difference between sediment interstitial water and overlying water. The physical and chemical properties of sediments in different rivers are quite different after long-term accumulation. Based on the indoor release tests carried out on the study section, the relationship between release rate and shear stress of different sections is determined, and thus the release amount of nutrients under different hydrodynamic conditions is determined. The variation of the flow velocity of schemes B, C, and D relative to scheme A in the flood and the dry season is shown in Figure 6. During the flood season, the flow velocity of S1–S7 under scheme B increase; S1, which has the most increase in flow rate, is closest to the diversion pumping station, while the flow rate of S4 increases from 0.019 m/s to 0.026 m/s, with the largest increase (35.7%). The flow rate of S8–S10 decreases slightly, with an average decrease of 0.41%. In scheme C, only the flow velocity of S1 and S2 increases, the velocity of other sections decreases, and the S3–S6 velocity decreases significantly, with an average decrease of 112.5%, in which the flow rate of S5 decreases to 0.001 m/s, the flow is almost static, the decrease of S3 is the largest (156.6%), and the change of S8–S10 velocity is not significant (less than 0.07%). In scheme D, the flow velocity of S1 and S2 is enhanced by the increase of flow and the auxiliary pumping station. Due to the control of the auxiliary pumping station, the flow velocity of S3–S6 is no longer affected by the change of water diversion, and the flow rate of S7–S10 decreases by 0.2% to 0.8%. In the dry season, the change of flow velocity under different schemes except S7 is similar to that in the flood season. In the dry season, schemes B, C, and D for S7 can improve the hydrodynamic conditions by 11.3%, 5.6%, and 5.2%, respectively. Therefore, the effect of hydraulic regulation on the adsorption and release of nutrients in river sediments in different regions of the river network is that only increasing the amount of water diversion can improve the hydrodynamic conditions of the river where S1–S7 is located, thus increasing the release of nutrients. Without increasing the flow, and only the auxiliary pumping station being opened, the TN and TP released from the sediment of S1 and S2 increases, the flow velocity of S3–S6 decreases significantly, the concentration of sediment adsorbed TN and TP decreases, and S3–S6 after the opening of the pumping station is no longer affected by the change of water diversion from the river network, so the opening of the auxiliary pumping stations is helpful in controlling the endogenous release of the responding river. When the auxiliary pumping stations are opened, the water diversion capacity is increased, the flow velocity of S1 and S2 is increased, and the nutrient release of sediment is increased. The endogenous release of S7 decreases slightly under schemes C and D in the flood season but increases in other cases. There is little change in S8–S10.

The improvement of hydrodynamic conditions caused by hydraulic regulation can increase the content of dissolved oxygen in water, promote the degradation and transformation of nutrients by microorganisms, drive the rotation of particles, and provide more activation sites (Richey et al. 1985) and so on. The effect of hydraulic control in flood and dry seasons on the degradation of TN and TP in each section is shown in Figure 6. Based on the flood season simulation results, the degradation effects of scheme B on TN and TP are quite different in the sections. The nutrient concentrations of S1, S3, S4, S5 and S8 are increased, while the degradation decreases. The TN content of S3 increases from 1.18 mg/L to 1.22 mg/L, TP from 0.108 mg/L to 0.11 mg/L, and S8 increases slightly, and TN and TP increase by 0.04% and 0.08%, respectively. The nutrient concentration in the other sections decreases, and the degradation is enhanced. The TN and TP degradation of S2 is the most, which is 2.94% and 2.14%, respectively. In scheme C, the TN and TP degradation of S3–S5 increases, and the maximum increase of TN and TP degradation is 52.3% and 54.2%, respectively. The TN degradation of S7 increases slightly (0.28%), TP degradation decreases (0.61%), and S8–S10 degradation changes to zero. It is difficult for auxiliary pumping stations to play a role in the degradation and dilution of pollutants in the southeast of the river network. In scheme D, the improvement of TN and TP degradation in most sections is improved, only the TN concentration of S1 and S8 increases, and the TP concentration of S1, S7, S8 and S10 increases. The degradation of TN and TP of S3–S6 under the action of the pumping station is not affected by the change of water diversion. During the operation of the auxiliary pumping station in the dry season, the TN and TP degradation of S3–S6 increases by 10% compared with the flood season, while the TN degradation of S8–S10 decreases by 0.2% to 0.5%, indicating that the promoting effect of the auxiliary pumping station on the degradation of tributaries in the river network in the dry season is stronger than that in the flood season, and inhibits biochemical degradation in the southeast of the river network.

After calculation, the contribution degree of DI, II, and SI in the process of nutrient concentration change caused by water diversion is obtained (Figure 7), to refine the best water quality improvement measures that should be taken in different regions. On the whole, the contribution of DI to increase the diversion volume of the river network is less than 36.87%. Both the weights of score of II and SI are more than 90%. This is because in the water diversion schemes B and D, the amount of water is increased based on the original water diversion without changing the quality of water diversion, and the reduction of pollutants in the process of the water body from the diversion pumping station to the target section is the basic condition for DI to play a role, such as at S1, which is closest to the diversion pumping station. The time from the pumping station to the cross-section is short, and the role of II and SI is limited. The nutrient factor content of the newly added water in the cross-section is not significantly different from the background value of the receiving water body. Under the dilution effect, the contribution of DI to TN and TP is 8.6% and 4.5%, respectively. For a section far away from the diversion pumping station, such as S7, II and SI play a full role before reaching the fault, so that the diversion water quality has a more significant change, and the highest contribution of DI to TN and TP is 32.63% and 33.05%, respectively. At the same time, water diversion is continuously diverted in the river network, and the limited amount of water reaching the S9 and S10 sections at the distal end of the river network is the reason for its low contribution to DI. The area where the flood of the river network is most affected by DI is the central and eastern region, such as the length of the water flow to S8 is large, and there are more main rivers and fewer tributaries along with the river network, which is conducive to dilution.

Specifically, the concentration of TN and TP of S1 in the upper reaches of Liangxi River increases under different schemes, and the contribution degree of the three pathways is II > SI > DI. The weight of II is 75.77% to 99.12%, under the same scheme, the dry season is larger than the flood season, and in the same water period C + D > B, which means that water diversion can have an effect on S1 within a short time after entering the river network, with less water loss, significant improvement of hydrodynamics, and little difference between the water quality and the actual scheme, so the sediment release has become the main factor for the increase of S1 nutrient content. Compared with S1, the II weight of S2 in the lower reaches of Liangxi River decreases, and SI and DI increase. In flood and dry seasons, the SI and DI of TN and TP in scheme B and D are beneficial to the reduction of its content, and the nutrients of the sediment are released in the design scheme; this is because the diversion is partially diverted before reaching S2, and the increase of bed shear stress is less than at S1. At the same time, the TN in diversion goes through biochemical degradation and diffusion in the middle and upper reaches of Liangxi River. The concentration decreases, so the weight of DI increases, and the maximum decrease of TN content is scheme B in the dry season, when the weights of II, SI, and DI are 30.05%, 45.51%, and 24.42%, respectively. Under the comprehensive action, the TP content of each scheme of S2 increases, and it is difficult for the contribution value of dilution and degradation to make up for the release of sediments. For example, the sediment release, degradation, and dilution of scheme D increase during the flood season, and the values of II, SI, and DI are 91.85%, 5.3%, and 2.85% respectively, and the release amount is much higher than that of dilution and degradation.

In S3–S6 of the tributaries of the river network, when the auxiliary pumping station is opened by schemes C and D, the input water and flow velocity are controlled, and the decrease of flow velocity is beneficial for reducing the release of sediment and the consumption of nutrients by stable biochemical reactions of microorganisms. The pumping station also controls the input of nutrients and reduces the exogenous input, so the effects of the three pathways are beneficial for the reduction of TN and TP concentration in flood and dry seasons, and the weight of SI is the largest. In scheme C, the weights of DI, II, and SI of TN concentration of S6 in the flood season are 2.34%, 2.78%, and 94.88%, respectively. When scheme B does not open the auxiliary pumping station, only increasing the amount of water diversion is not conducive to the reduction of nutrients in the tributary section. For example, the three effects of S3–S5 in scheme B will increase the concentration of TN and TP, in which the TN weights of II, SI, and DI of S5 in the flood season are 2.15%, 63.7%, and 31.16% respectively, and the weights in the dry season are 0.81%, 64.58%, and 36.6%, respectively.

The concentration of TN and TP of S7 located in the middle of Mali River decreases under scheme B, while the concentration of TN and TP increases in the flood and dry season in scheme C and D. Compared with the other sections, S7 is indirectly affected by the pumping station and is at a longer distance from the diversion pumping station, so the hydraulic regulation is more complex. The weight of TN and TP is SI > DI > II, and the weight of SI is between 58.8% and 66.46%. The DI is between 30.9% and 33.05%, and the weight of II is small (0.49%–10.3%).

S8–S10 are located at the lower boundary of the river network, which is far away from the diversion pumping stations. After the increased water is continuously diverted, the improvement effect on the hydrodynamic water quality in this area is not significant. And secondly, the regulation and control effect of the auxiliary pumping station on this area is also limited. From the small changes of TN and TP concentration in this area under different schemes, it was found that the nutrient concentration of S8 increases in the flood and dry season, while that of S9 and S10 decreases under all schemes in the flood season. In the dry season, except for the increase of TN concentration in scheme C, the others also decrease slightly. SI plays a leading role in the change of nutrient concentration in S8, with a weight of 60%–70%. II plays a leading role in the change of nutrient concentration in S10, and the weight is kept above 90%. The weight of the three pathways of S9 in scheme B and D is II > SI > DI, while in scheme C it is SI > II > DI, and this is because increasing 20% water diversion will have a certain disturbance effect on the S9 sediment. Based on the indoor experiment on the nutrient release rate of S9 sediment (Figure 3), the shear stress produced by the flow velocity under the corresponding scheme exceeds the shear stress corresponding to the peak nutrient release rate in this section. Therefore, the flow velocity of scheme B and D is higher than that of scheme A, but the nutrient release intensity is lower than that of the initial scheme.

The change of hydrodynamic conditions caused by the water transfer of an urban plain river-network has an important influence on the content of nutrients in the rivers, but little attention has been paid to the influence weight of the various ways. This paper compares and analyzes the three action ways of II, DI, and SI caused by the change of hydrodynamics in different regions of the river network, and quantifies the contribution of each way to the concentration of nutrient factors, which can provide a theoretical basis for the formulation of a dispatching scheme and hydrodynamic regulation threshold based on the improvement of water quality in the river network area. On the whole, in the Binhu River network, as a channel for the discharge of lake water with high nutrient content into the Beijing–Hangzhou Grand Canal under the target of reducing the total amount of pollutants in Meiliang Bay, the DI contribution caused by hydraulic regulation is not more than 37%, while II and SI are more than 90% sometimes. Therefore, it is difficult to improve the water quality of the river network by diluting pollutants. This mainly depends on increasing water diversion to speed up the flow of water and promote the diffusion and degradation of nutrients. But at the same time, it will also lead to the release of nutrients in the sediment, so we need to make a quantitative analysis of the contribution of different pathways in different regions. Specifically, for the rivers in the southeast of the river network, the interaction between the water quantity and hydrodynamic conditions and the river network is not enough, and the response of the nutrient load to the designed regulation schemes is weak. For the rivers to the west of the Mali River, simply increasing the amount of water diversion can promote the biochemical degradation of TN and TP, but it is not enough to make up for the sediment release and the carrying capacity in the diversion water caused by the acceleration of water flow, so it is disadvantageous to the nutrient content of the river near the water diversion pumping stations of the river network. With the movement of water flow, after a period of transport and degradation, it can improve the water quality of medium- and long-distance rivers. By simply opening the auxiliary pumping station, the velocity and flow rate of the tributaries are reduced, the external input and endogenous release of pollutants are controlled, and the water quality is improved, but this indirectly increases the pollution load of the mainstream. For example, the pollutant input and sediment release of Liangxi River have increased under the action of auxiliary pumping stations. Through increasing the amount of water diversion and the joint regulation of auxiliary pumping stations, the nutrient concentration in other sections of the river network has been improved except that the upper reaches of Liangxi River is impacted by water flow, which leads to the release of a large number of nutrients in the sediment and the increase of nutrient content.

Therefore, we suggest that floodgates should be set up in the upper reaches of Liangxi River to control the flow rate or carry out dredging to reduce the release of pollutants caused by flow disturbance, and pump stations should be set up in the southeast to realize the linkage with the river network, the elevation of regional water level and that the flow rate is conducive to the degradation of pollutants (Gao et al. 2018). At that time, the joint regulation and control mode of increasing the water diversion of Meiliang and Daxuan pumping stations and opening the auxiliary pumps of the river network will be adopted, and the nutrient content of the regional water body will be generally improved. In the next step, we plan to set up more water diversion schemes and auxiliary pumping station operation schemes, and couple the eutrophication mathematical model to study the hydraulic regulation threshold under the target of more water quality improvement, so that the water environment of the lakeside river network can be significantly improved. This provides a reference for the formulation of the hydraulic regulation and control scheme of an urban river network.

This work was supported by the Major Science and Technology Program for Water Pollution Control and Treatment of China (2017ZX07203002-01), Shanghai Water Bureau Research Project (Assessment of the Impact of Salinity Fluctuation in the Yangtze Estuary on Water Quality of Drinking Water Sources, 2019-09), National Natural Science Foundation of China (No. 51779075), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (No. 51479064), and Qing Lan Project of Jiangsu Province (2018-12). The authors express thanks to Dr Yuan Weihao and Dr Yan Yu for their help with data collection.

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