2020 International Conference on Advanced Materials, Electronical and Mechanical Engineering (AMEME 2020) ISBN: 978-1-60595-067-9

Sustainable Improvement of Water Quality in Urban River Network by Rotating Overflow Weir—Case Study of City in

Yang Liu1, Chen Xie1,*, Ziwu fan1, Fan Yang1,2, Chang Yang1, and Guoqing Liu1 1Hydraulic Engineering Department, Hydraulic Research Institute, Nanjing 210029, China 2College of Water Conservancy and Hydropower Engineering, , Nanjing 210098, China

Abstract. Water diversion is a practical and commonly used approach to improve the water quality of rivers in urban plains in China. This paper researches using rotating overflow weirs as the primary engineering solution to facilitate water diversion and improve the water quality of the river network in urban plain area of Changzhou, including hydrodynamic modeling, field tests, and application. The water diversion scheme, including clean water source, water transfer paths, quantity, and rate, was designed based on its current terrain and river systems. The River was selected as a high-quality water source, and its two tributaries were utilized as clean water paths. Four rotating overflow weirs were proposed to create three stages of hydraulic gradient. Clean water can flow naturally and orderly in the researched urban river network without pumps. The required clean headwater level and clean water distributions were modeled hydrodynamically. Four temporary weirs were constructed at design locations to conduct field tests. And the water diversion scheme and its water quality improvement had been verified by field tests. Afterward, four permanent rotating overflow weirs were built. Currently, the monitoring results show the design water transfer scheme reaches expectations. The flow velocity in most rivers has been increased to 7cm/s from standstill in Changzhou's main urban plain area. The composite water quality of 93% river cross-sections monitored has been improved from Class V to Class Ⅲ and all have been improved to Class IV and above.

1 Introduction In China, most urban plains are located downstream of the Yangtze River, , Haihe River, and the Pearl River. These regions are economically developed, highly urbanized and extremely high populated. In recent years, with rapid social and economic development in China, the contamination loads into river systems keep increasing. The water quality in most urban rivers in China is rated Class V( Environmental quality standards for surface water in China, GB3838-2002) or even worse[1-2]. The carrying capacity of the water environment is becoming insufficient in urban rivers, which restricts the sustainable development of the society and economy and has an impact on the livability.

476 Therefore, some sustainable approaches are in urgent demand to improve the water quality in urban river networks. Currently, in China, the existing contamination source control measures can not completely prevent high-intensity contamination load into urban rivers. Water diversion is an efficient and commonly used method to improve water quality in urban rivers [3-10]. The introduction of clean water increases flow quantity, velocity and dissolved oxygen (DO), and dilutes the concentration of contaminants. As a result, the aquatic environmental capacity and self-purification capacity of rivers are improved. Based on current features of the urban river network in Changzhou and the existing sluice gates and pump stations, this paper studied using rotating overflow weirs as the primary engineering solution to facilitate water diversion for the river network in urban plain area of Changzhou. A highly precise hydrodynamic model was built to predict different scenarios and determine an optimal scheme. Four temporary rotating overflow weirs were constructed first at the design locations to validate the model results. Afterward, four permanent rotating overflow weirs were built in the validated locations, and further on- site monitoring tests proved expected water quality improvement. This research outcomes can provide reference and be applied to water quality improvement of river networks in urban plain area.

2 Study area and problem diagnosis for rivers in Changzhou city Changzhou City is located in the south of Yangtze River, west of Taihu Lake, east of Zhenjiang and Nanjing City and north of Anhui Province. It is in the area of Taihu Lake plain in the Yangtze River delta. This study focused on the main urban area of Changzhou City, where gets increasingly more populated and urbanized. The water quality in the studied river network urgently needs to be improved. The study urban area is 179.2 km2, bounded by Xinlong river to the north, Dingtanggang River to the east, the Beijing- Hangzhou to the south and Desheng River to the west, as shown in Fig. 1. There are 113 rivers in this area, and the total river length is 285km, 50km of which are 43 beheaded rivers. Currently, the problems in this river network are: the water quality is fair in big river channels, but bad in small river channels. There are many beheaded rivers, and connectivity is limited. Water is more natural to flow into big rivers; however, water transferring is weak in small and medium rivers. The contaminant source is not entirely cut off along river banks. Visually, the transparency of the water in the rivers is very low, about 30-50 cm.

Figure 1. Changzhou’s urban river network map.

477 3 Hydrodynamic numerical model The study focuses on the river network of Changzhou main urban area (179.2km2). There are 113 rivers in this area, and the total river length is 285km. River cross sections were surveyed every 100, 200 or 500 meters, and also at inlets, outlets, bends and where narrows down. A total of 1154 river cross-sections were imported to the model, and 335 river reaches, 69 sluice gates, and 71 pump stations were created in the model. The modeled river network is shown in Fig.2. The existing condition model was set up and calculated based on current pump stations and sluice gates scheduling observed during the field study. The calibration results of water levels in two typical sites, Beijing Bridge and Chahualu Bridge, are shown in Fig. 2(c) and (d), respectively. The water level changing trend is the same for the calculated and measured data, and the difference is less than 5cm. Fig. 2(b) shows that the error between calculated and measured data for all the calibrated sites, which are smaller than 5%. This indicates the 1-d hydrodynamic model for river network in Changzhou’s main urban area is precise enough for water level and water quantity prediction. (a) (b)

(c) (d) (b)

Figure 2. River Network Model of Changzhou’s Main Uban Area (a); Comparison of Water Levels Between the On-site Measurement and the Calculation. (a) Beijing Bridge; (b) Chahualu Bridge; (c) Comparision of measured and calculated water levels.

4 Sustainable schemes for rivers by numerical model and field test

4.1 Design the water path: in and out Changzhou's main urban area is bounded by the Yangtze River to the north. The water quality of the Yangtze River is rated as class II~III on average, so it can be chosen as the clean water source. Noted that Total Nitrogen (TN) is generally not considered in the

478 comprehensive evaluation of the water quality of rivers in China. The water quality in two rivers, Desheng River and Zaogang River, are relatively good, and both can be utilized as clean water diversion paths. There are two key pump stations, Weicun and Zaogang hydro- junction, at the upstream of Desheng River and Zaogang River respectively. These two pump stations can transfer clean water from Yangtze River into Desheng River and Zaogang River. Then clean water is then distributed to other middle and small rivers downstream in the urban area. Once the clean water arrives in Desheng River, it is pumped through Xinzha pump station, into the rivers in Xuejia District northwestward and the districts south of Ancient Great Canal. Afterward, it outflows through the Xiaolonggang River to the north and Beijing-Hangzhou Grand Canal to the east. When the clean water arrives in Zaogang River, a portion will then inflow into small and middle rivers in the eastern districts and then outflow through Beitang River and Laotaohuagang River to the north; the rest goes into Ancient Great Canal and feed the rivers in the south, and then flow out to the east through Beijing-Hangzhou Grand Canal. These water paths for water diversion also take the flood control and waterlogging for the urban area into account.

4.2 Design of the sustainable water transferring scheme

Due to Changzhou’s plain terrain, the rivers almost have no hydraulic gradient and do not flow fast naturally. To force clean water flows into the urban river network , the hydraulic gradient needs to be increased. Before this study, the required hydraulic gradient has to be driven by pumps, which was energy consuming. So the challenge is increasing the hydraulic gradient without relying on pump stations. The average normal water level of Beijing-Hangzhou Great Canal is 3.4m to 3.6m in the south of Changzhou urban area. The regional warning flood level is 4.3m. For safety reason, the maximum allowable water level in northern part of the main urban area is controlled at 4.0m. As a result, there is 60cm water level difference from north to south. According to the river network features, it is optimum to form 3 stages of hydraulic gradient. The highest water level is in Zaogang River (First Stage Water Level, and the control node is Xujiatang Bridge, HZG=3.80~4.00m) , a bit lower in Ancient Great Canal (Second Stage Water Level, and the control node is Sanbao Street, HSB=3.60~3.80m) and lowest in Beijing-Hangzhou Grand Canal (Third Stage Water Level, and the control node is Changzhou-Three, HCZS=3.40~3.60m). The rotating overflow weirs are placed in proper locations to help create the hydraulic gradients artificially. The structure of the rotating overflow weirs is shown in Fig. 3(a). During water diversion, it can be rotated to a certain angle to detain upstream flow. And when water overflows, a hydraulic gradient is created and accelerates downstream flow. If flooding occurs, it can lie down to ensure maximum flow capacity. The locations of these rotating overflow weirs, assessment of the suitable water quantity, transfer rate, distributions to the rivers were designed based on the hydrodynamic model and the field study. Zaogang River is one of the two paths for the clean water flowing to the urban river network. When the clean water arrives in Zaogang River from the Yangtze River, a portion of the clean water will flow away through the eastern tributary of Zaogang River, named East Zaogang River. To guarantee the water quantity into the urban river network, a rotating overflow weir was built upstream of East Zaogang River, named Panlongyuan weir. Old Zaogang River, a tributary of East Zaogang River, also flows towards the urban area. After some clean water entering the eastern tributary of Zaogang River, to force more clean water flow into urban rivers through Old Zaogang River, another rotating overflow weir, named Dinosaur Park weir, was built slightly downstream of intersection of East Zaogang River and Old Zaogang River. In the past, in the old town area, flow rates are

479 small in four rivers, Dongshi River, Xishi River, Nanshi River and Beishi River. In order to divert more water into the rivers in the old town area, two rotating overflow weirs, named Xinshi Bridge weir and Yang Bridge weir, were designed to raise the water level in Guan River. The schematic diagram of the diverted clean water flow path in the main urban area of Changzhou is demonstrated in Fig.4, and the locations of these four weirs are shown in Fig. 3(b). The water level and water mobility of the rivers in the urban area are the most sensitive to the First Stage Water Level in Zaogang River. When determining a proper First Stage Water Level, it should both consider the need for water quality improvement and flood control. Five scenarios were simulated, i.e. the water level at Xujiatang Bridge(HZG) is 3.80 m~4.00 m (Wusong elevation). The modeled schemes are shown in Table 1.

(a)

(b)

Figure 3. Clean water flow paths (a) and locations of overflow weirs and the three-stage water levels control points (b). Table 1. Model schemes.

Model Scheme Number HZG/m Number of weirs 1# 3.80 4 2# 3.85 4 3# 3.90 4 4# 3.95 4 5# 4.00 4

4.3 Computational boundary conditions The scenarios are modeled hydrodynamically. During simulation, upstream boundary condition is controlled by a flow-time series, which is constant 40m3/s and 30m3/s, pumped by Zaogang and Weicun hydro-junction, respectively. Downstream boundary condition is controlled by a water level-time series, which is 3.41m, the mean annual water level of urban rivers.

4.4 Simulate the water transfer effects The clean water comes from Yangtze River through Zaogang and Weicun hydro-junction. After the clean water arrives at the upstream of the main urban area, based on the paths demonstrated above, the water quantity and flow rates into urban rivers are listed in Table

480 2. And the percentage of rivers of different velocities in these five scenarios are shown in Table 3. The results show, the higher HZG is, the more water flows into urban rivers, and the higher the flow rates are in the urban rivers, and a higher percentage of the river flowing faster than 0.1m/s. However, in terms of flooding control, it is suggested the water level of Zaogang River (HZG) is controlled below 4.0m, and the flow rate into urban river network can be more than 30m3/s. However, the velocity in some rivers is still hard to be improved. A type of the rivers is connected by culverts, and the flow rates are restricted by the limited capacity of culverts, e.g., Dingjiatang River, Hengtangbang River and etc. Another type of rivers is beheaded river, they are unconnected and the velocity are hardly improved, i.e. Baijiabang River, Tongjiabang River, Chuanxinbang River and etc. Table 2. Flow rates into urban rivers and distributin of flow rates in the old town area for different scenarios.

3 Sce- Flow in different river(m /s) nari urban Xi- Bei- Nan- Dong- Hengt- Long- Xiaolong- Chaiz- Sanji- o area shi shi shi shi ang you gang hibang ng 1 24.0 5.8 8.1 3.5 4.7 3.0 13.2 1.0 1.3 4.3 2 27.0 6.4 8.9 3.9 5.0 3.5 15.5 1.1 1.3 4.5 3 28.5 6.9 9.5 4.2 5.2 3.7 16.0 1.3 1.4 4.6 4 29.5 7.0 9.5 4.3 5.3 3.9 19.0 1.5 1.5 4.7 5 34.0 7.5 10.1 4.6 5.5 4.1 19.4 1.6 1.6 4.7 Table 3. Percentage of the rivers of different velocity by different scenarios (%). Scenario <7 cm/s 7-10 cm/s 10-15 cm/s >15 cm/s 1 48.66 29.60 11.54 10.20 2 45.83 31.07 9.41 13.69 3 45.25 31.37 9.13 14.25 4 45.03 11.53 28.25 15.19 5 43.62 12.28 27.50 16.60

5 Field tests by temporary weirs to verify the water quality improvements To verify the water environment improvements of the design scheme, four temporary overflow weirs were built in the design locations shown in Fig. 4. Field tests were mainly to check the hydraulic gradients created by these weirs. The tests were conducted from 1st to 13th May 2017. It took the first eight days to build four tempory weirs. At that time, the construction of Xinzha Pump Station had not been finished. As a result, during the test, only the First and Second Stage Water Level could be formed; therefore, the field test was restricted within the northeastern part of the urban area. From 9st to 13th May 2017, 40m3/s of clean water was transferred from Yangtze River through Zaogang River. By operating the pump stations and sluice gates in Changzhou city, the water level at the intersection of Zaogang River and East Zaogang River was controlled at 4.0m. The weir crest levels were set based on the numerical model. At the same time, synchronous prototype observation was done. Data of key locations in urban river network was collected, i.e. water quality index, flow rate, water level, water directions and etc. The water level difference between Sanbao Street (HSB) and Zaogang River(HZG) is 20cm, which reaches the expectation. The flow distribution is shown in Fig. 5(a). It shows that the clean water flow from the Yangtze River into Zaogang River is 38.3m3/s,of which 30.0 m3/s flows to Zaogang River and 7.2 m3/s flows to East Zaogang River. In the old town area, the flow rate is 6.8 m3/s in Xishi River, 5.5 m3/s in Beishi River, 2.1 m3/s in

481 Nanshi River and 3.5 m3/s in Dongshi River. The flow rates were increased significantly compared to the past. There were nine water quality sampling sites as shown in Fig.5(b). The testing results of water samples. Including NH3-N, DO are shown in Fig.6. After water diversion started on 9th May, all sampling sites’ DO concentration increased from Class IV or V, and some even th increased above Class III . The concentration of NH3-N dropped rapidly from 9 May, from class V or worse to above class IV. The concentration of CODMn decreased slightly. In general, almost all the key water quality indexes can meet the Class IV standard.The suggested scheme achieved the expected water quality improvement for urban river network.

(a) (b) (c) (d)

Figure 4. Temporary overflow weirs. (a) Xinshi bridge; (b) Yang bridge; (c)Panlongyuan; (d) Dinosaur park.

(a) (b)

Figure 5. Distribution of flow rates in rivers in Changzhou’s main urban area (a) Observation locations of water quality(b).

(a)

(b)

Figure 6. Variation of water quality indexes at various observation locations. (a) NH3-N concentration curve; (b) DO concentration curve.

482 6 Field operation of water diversion after construction of rotating overflow weirs completed

6.1 Arrangement of field operations On November 2018, the key part of the project, four permanent rotating overflow weirs had been built and passed the inspection. To further verify the scheme and to adjust an optimum water diversion routine, a field operation was conducted in the northeastern urban area from 19th to 30th, November, 2018, with the assistance of Changzhou Water Resource Bureau. The photos of these four weirs are shown in Fig.7. (a) (b) (c) (d)

Figure 7. Photos of rotating overflow weirs. (a) Panlongyuan; (b) Dinosaur Park; (c) Xinshi Bridge; (d) Yang Bridge. By the operation of four weirs integrated with other existing sluice gates and pump stations, the water level difference for these rivers were raised, and the hydraulic gradient were reorganized. The flow distributions were optimized, and the water quality were improved. The clean water was pumped from Yangtze River through Zaogang hydro- junction. Most of the existing sluice gates were open, and the rest were partly open. All the pump stations were not running. By adjusting the rotation angle of the overflow weirs, the water level difference between upstream and downstream of the weirs was raised 10cm to 30cm. As a result, the water can flow naturally without using pump stations. Most of the rivers in the field test area are improved by this economical and sustainable scheme. The field operation involved in 33 rivers, comprising of 9 main stems and 24 tributaries. During 12 days, hydrodynamic index and water quality indicators were monitored, including 26 flow rate monitoring sites (1 site for each river on average) and 76 water quality monitoring sites (2 sites for each river on average).

6.2 Results analysis

After several days’ water diversion, when the water levels become steady. The water difference between HZG and HSB is about 20cm, which reached the expectation. And the flow rates of Chaizhibang River, Xishi River, Beishi River and Nanshi River increased 11 times, 31 times, 61 times and 8 times, respectively. In general, it shows that the discharge increased most significantly after the rotating overflow weirs constructed. The scheme is cost saving and very low-power consumption. Only 1 pump station was used (Zaogang hydro-junction). Other projects, including 4 wires and sluice gates, are controlled to certain opening size, and the rivers can flows. On 29th November 2018, the water samples were collected and analyzed for 76 monitoring sites. The comparison between the results before and after the construction of rotating overflow weirs are shown in Fig.8. 42%, 51% and 7% of the monitoring sites were improved to Class II, III and IV, respectively, excluding some unconnected rivers. None of

483 these monitoring sites were Class V or worse than Class V any more. Overall, the water quality of 95% of the monitoring sites was significantly improved. Because Hengtang River was being dredged and the maintenance of Beitang hydro- junction during the test, the clean water cannot be transferred into the rivers along Hengtang River and east of Hengtang River. The flow rates and water quality were not improved in this area during this test.

(a) (b)

Figure 8. Water Quality Rating. (a) Before; (b) After Water Diversion Test.

7 Conclusions In recent years, rapid urbanization in China intrigues severe surface water pollution problems in rivers of urban plain cities. It restricts the social and economic development and impacts people’s life. This paper studied using rotating overflow weirs as the primary engineering solution to facilitate the water quality improvement in the urban river network of Changzhou, a typical city in plain area. A 1-d hydrodynamic model for urban river network of Changzhou was established and calibrated to less than 5 cm of water level error. Five scenarios were simulated to predict the flow velocity distributions. A water diversion scheme has been proposed to improve the flow velocity and water quality in urban river network. The clean water from the Yangtze River was diverted to the urban river network through two main rivers. Four rotating overflow weirs were designed and built to form three stages of hydraulic gradient. The field test results after the weirs were constructed show the solution is useful for improving urban river network in Changzhou City. By operating the weirs and other sluice gates, the flow velocities of most urban rivers are significantly improved, and the flow rate has been increased by 8-60 times, compared with the past. Comprehensive water quality of 93% of the monitored river cross sections are improved from worse than Class V or Class V to Class IV or above. The water environment is improved in Changzhou’s urban area. The research approach and engineering solutions in the present study can be applied to the water improvement of other urban river networks in plain areas.

This research was funded by National Key R&D Program of China (2018YFC0407205), (2019YFB2102000) and Water Conservancy Science & Technology Program of (2016005), (2016009), (2017001ZB).

484 References 1. Xu, Z.X.; Zhang, C. ; Li, H.Z. Classification and systematic treatment of black odor water bodies in urban rivers in China. Water & Wastewater Engineering, 10, 2- 6+40(2018). 2. Ministry of Water Resources of the People’s Republic of China. China Water Resources Bulletin 2015~2018. Beijing: China Water & Power Press(2018). 3. Xu, G.Q.; Zhu, J.D. Water Environment Improvement by Clean Water Diversion in Shanghai City. Water Resources Protection, 3, 26-30(2001). 4. Xiong, W.Y. Practice and understanding of Fuzhou inland water diversion project. China Water &Wastewater, 7, 26-28(2000). 5. Lu, Q. Current situation of water quality in Suzhou creek and the experiment of using water division from conservancy works on it. Journal of Shanghai Agricultural College, 1, 62-67(1999). 6. Cai, J; Wang, C.S.; Wang, W. Discussion on improving the water environment in the river course by diverting clean water and conducting better management. China Water Resources, 7, 39-41.(2011). 7. Hao, W.B.; Tang, C.Y.; Hua, L. Acharya, K. Effects of water diversion from Yangtze River to Taihu Lake on hydrodynamic regulation of Taihu Lake. Journal of Hohai University (Natural Sciences), 2, 129-133(2012). 8. Sun, J.; Ruan, X.H. Study on Water Diversion and Drainage to Improve the Environmental Effect of River Water in Nanjing City. China Rural Water and Hydropower, 3, 29-31(2008). 9. Shang, Z. Y.; Zhang, Y.Z.; Dai, J.J.; Li, Y.T.; Wei, T.T. Study on water environment improvement scheme by living water flow in Kunshan main city and its surrounding areas. Water Resouces Protection, 06, 129-136(2017). 10. Lu, X.Y.; Wu, S.Q.; Zhang, Y.; Dai, J.Y.; Li, T. Progress of ecology and environment effect of water diversion project. Journal of Water Resources & Water Engineering, 4, 38-45(2014). 11. Wang, C.; Wei, Z.; Zhang, L.; Zuo, J.Y. Experimental study on improvement of water environment by water diversion in plain river networks. Journal of Hohai University (Natural Sciences), 2, 136-138(2005). 12. Cui, G.B.; Chen, X.; Xiang, L.; Zhang, Q.C.; Xu, Q. Evaluation of water environment improvement by interconnected river network in plain area. Shuili Xuebao, 12, 1429- 1437(2017) 13. Li, Y.; Acharya, K.; Yu, Z. Modeling impacts of Yangtze River water transfer on water ages in Lake Taihu, China. Ecological Engineering, 2, 325-334(2011). 14. Xu ,Y.X.; Wang, W.C.; Zeng, W.F. Li, Y.P.; Lai, Q.Y. ;Yin, X.H.; Zhang, S.S. Simulation on improvement of water environment in plain river network by water diversion. Water Resources Protection, 1, 70-82(2018).

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