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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 Changzhou City in China Yang Liu1, Chen Xie1,*, Ziwu fan1, Fan Yang1,2, Chang Yang1, and Guoqing Liu1 1Hydraulic Engineering Department, Nanjing Hydraulic Research Institute, Nanjing 210029, China 2College of Water Conservancy and Hydropower Engineering, Hohai University, 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 Yangtze 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, Huai 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 Grand Canal 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).

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