Water diversion is often used to improve water quality to reach the standard of China in the short term. However, this large amount of water diversion can not only improve the water quality, but also lead to a decline in the water quality (total phosphorus, total nitrogen) of Xuanwu Lake. Through theoretical analysis, the relationship between water quality and water diversion is established. We also found that the multiplication of the pollutant degradation coefficient (K) and the water residence time (T) is a constant (N), $K\cdot T=N$. The water quality changed better at first, with the increase of inflow discharge, and then became worse, and the optimal water quality inflow discharge is 180,000 m³/day. By constructing two-dimensional hydrodynamic and water quality models, the optimal diversion water plan is calculated. Through model calculations, it can be seen that reducing the inflow discharge makes the water residence time longer (15.3 days changed to 23.8 days). Thereby, increasing the degradation of pollutants, and thus improving water quality. Compared with other wind directions, the southwest wind makes the water quality of Xuanwu Lake the most uniform. The concentration of water quality first became smaller and then became larger, as the wind speed increased, and eventually became constant. Implementing these results for water quality improvement in small and medium lakes will significantly reduce the cost of water diversion.

Water pollution in urban rivers is serious in China. Eutrophication and other issues are prominent. Taking the artificial Playground Lake in Zhenjiang as an example, a numerical model combining particle tracing, hydrodynamics, water quality and eutrophication was constructed to simulate the water quality improvement in Playground Lake with or without water diversion by pump and sluice. Simulation results using particle tracking showed that the water residence time depended on wind direction: east wind, 125 h; southeast wind, 115 h; south wind, 95 h. With no water diversion, the lower the flow velocity of Playground Lake under three wind fields, the more serious the eutrophication. Under pump diversion, the water body in Playground Lake can be entirely replaced by water diversion for 30 h. When the temperature is lower than 15 °C, from 15 °C to 25 °C and higher than 25 °C, the water quality can be maintained for 15 d, 10 d and 7 d, respectively. During high tide periods of spring tides in the Yangtze River from June to August, the water can be diverted into the lake through sluices. The greater the Δh (the water head between the Yangtze River and Playground Lake), the more the water quality will improve. Overall, the good-to-bad order of water quality improvements for Playground Lake is as follows: pumping 30 h > sluice diversion > no water diversion. This article is relevant for the environmental management of the artificial Playground Lake, and similar lakes elsewhere.

With the rapid development of China, water pollution is still a serious problem despite implementation of control measures. Reasonable water environment management measures are very important for improving water quality and controlling eutrophication. In this study, the coupled models of hydrodynamics, water quality, and eutrophication were used to predict artificial Playground Lake water quality in the Zhenjiang, China. Recommended “unilateral” and “bilateral” river numerical models were constructed to simulate the water quality in the Playground Lake without or with water diversion by pump, sluice and push pump. Under “unilateral” and “bilateral” river layouts, total nitrogen and total phosphorus meet the landscape water requirement through water diversion. Tourist season in spring and summer and its suitable temperature result in heavier eutrophication, while winter is lighter. Under pumping condition, water quality and eutrophication of “unilateral” river is better than “bilateral” rivers. Under sluice diversion, the central landscape lake of “unilateral river” is not smooth, and water quality and eutrophication is inferior to the “bilateral”. When the water level exceeds the flood control level (4.1 m), priority 1 is launched to discharge water from the Playground Lake. During operation of playground, when water level is less than the minimum level (3.3 m), priority 2 is turned on for pumping diversion or sluice diversion to Playground Lake. After opening the Yangtze river diversion channel sluice, priority 3 is launched for sluice diversion to the Playground Lake. When the temperature is less than 15 °C, from 15 °C to 25 °C and higher than 25 °C, the water quality can be maintained for 15 days, 10 days and 7 days, respectively. Corresponding to the conditions of different priority levels, reasonable choices of scheduling measures under different conditions to improve the water quality and control eutrophication of the Playground Lake. This article is relevant for the environmental management of the artificial Playground Lake, and similar lakes elsewhere.

Building Green Infrastructures (GIs) to reduce stormwater runoff has been recognized as an effective approach to mitigate the negative impact of urban sprawl. Due to the significant differences in urban land use, some Land Parcels (LPs) may have difficulty in building enough GIs to meet stormwater mitigation goals. In this paper, we proposed a Capacity Trading (CT) approach that allows some LPs to trade their extra runoff retention capacities with LPs that have building difficulties, so that they can jointly reach the overall mitigation goal together. The rationale behind CT is that, to avoid potential penalties, it may be more economical for some LPs to ‘buy’ credit rather than to ‘build’ GIs. A case study was used to demonstrate CT operations for two trading scales: (1) CT within neighboring LPs (i.e., CT-1), and (2) CT within 20 m-radius LPs (i.e., CT-2). A GI implementation baseline intensity was set up firstly by treating the whole study area as one entity to reach a specified stormwater runoff control target; individual LPs were then examined for their GI building capacities, which may be deficit or surplus against the target. Results showed that the number and area of deficit LPs were reduced significantly through either CT scales; the number of deficit LPs was reduced from 139 to 97 with CT-1 and 78 with CT-2, and the deficit area was reduced from 649 ha to 558 with CT-1 and 478 ha with CT-2, respectively. The proposed method assumes LPs as the basic planning unit and encourages some stakeholders to maximize their GI building potential to compensate for those with disadvantages. The economic incentives for conducting CT among different LPs in urban area can help achieve stormwater mitigation goals more economically and flexibly. Some coordination among LPs in GI implementation is necessary, which presents both opportunities and challenges for city management.

Low impact development (LID) devices or green infrastructures have been advocated for urban stormwater management worldwide. Currently, the design and evaluation of LID devices adopt the Intensity-Duration-Frequency (IDF) method, which employs the average rainfall intensity. However, due to variations of rainfall intensity during a storm event, using average rainfall intensity may generate certain errors when designing a LID device. This paper presents an analytical study to calculate the magnitude of such errors with respect to LID device design and associated device performance evaluation. The normal distribution rainfall (NDR) with different standard deviations was employed to represent realistic rainfall processes. Compared with NDR method, the error in sizing the LID device was determined using the IDF method. Moreover, the overflow difference calculated using the IDF method was evaluated. We employed a programmed hydrological model to simulate different design scenarios. Using storm data from 31 regions with different climatic conditions in continental China, the results showed that different rainfall distributions (as represented by standard deviations (σ) of 5, 3, and 2) have little influence on the design depth of LID devices in most regions. The relative difference in design depth using IDF method was less than 1.00% in humid areas, −0.61% to 3.97% in semi-humid areas, and the significant error was 46.13% in arid areas. The maximum absolute difference in design depth resulting from the IDF method was 2.8 cm. For a LID device designed for storms with a 2-year recurrence interval, when meeting for the 5-year storm, the relative differences in calculated overflow volume using IDF method ranged from 19.8% to 95.3%, while those for the 20-year storm ranged from 7.4% to 40.5%. The average relative difference of the estimated overflow volume was 29.9% under a 5-year storm, and 12.0% under a 20-year storm. The relative difference in calculated overflow volumes using IDF method showed a decreasing tendency from northwest to southeast. Findings from this study suggest that the existing IDF method is adequate for use in sizing LID devices when the design storm is not usually very intense. However, accurate rainfall process data are required to estimate the overflow volume under large storms.