Evaluation method for restoration effect of different sections of small and medium-sized rivers: A case study of the Qingyi River, China.

Due to severe disturbances caused by rapid urbanization and industrialization, river ecosystems have evolved into complex natural-social composite ecosystems. At present, there is no unified evaluation method for the effect of river restoration in China. The existing research fails to fully integrate the "natural-social" characteristics of rivers, and pays less attention to the river section scale, which is not conducive to the development of accurate pollution control work. In this paper, we first put forward the division method of river section type which conforms to the "nature-society" dual attribute characteristics of the river, and divides the river section into "ecological/service/dual" functional "urban/rural" river section. Then a method for evaluating the restoration effect of different river sections based on index weight optimization was proposed. Based on the type of river section, the weights of 17 indexes were optimized, and the common evaluation method of restoration effect-multi-index evaluation method was improved. In addition, the application and verification of the established method were carried out on the main stream of Qingyi River and its typical tributaries. The evaluation results show that only the restoration effect of Baling River decreased from average in 2019 to acceptable in 2021, and ΔH (the change of the mean score of the target factor after repair) decreased from 15.6% to 6.2%. It was found that for the river sections where the repair effect cannot be maintained stably (such as Baling River), the previous methods paid more attention to the overall effect and easily ignored the specific problems. The improved method can identify problems more easily and facilitate timely post-maintenance. To further optimize the repair effect of Qingyi River, it is suggested that the restoration effect should be strengthened from the aspects of pollution source control, water quality improvement, aquatic restoration, continuous monitoring and evaluation. The evaluation results can provide a reference for the design, operation, and maintenance of restoration projects in small and medium-sized rivers.