The main canal of the middle route of the South-to-North Water Diversion Project has the risk of excess head loss in crossing water-conveyance structures, but the assessment of this risk faces difficulties such as the lack of sufficient monitoring points inside the structures, the current water-conveyance flow rate being lower than the design maximum flow rate, and the lack of verification of monitoring data. Monitoring data of the main canal were collected in 2022, prototype observations were carried out, the monitoring data were verified, and a method for calculating the head loss using the combined head loss coefficient was proposed. The assessment of 143 structures showed that 40 structures had excess head losses, including 31 inverted siphons, four aqueducts, four underdrains, and one culvert. The 143 structures had a total residual head of 3.05 m, accounting for 9% of the distributed head. In addition to natural aging, freshwater mussel and algal attachment, sediments, and undesirable flow regimes were all important influencing factors that caused the head loss to increase. It is recommended to take measures such as regular removal of sediments and algae and freshwater mussel attachments, optimization of inlet and outlet shapes, and application of roughness-reducing materials.

The roughness of the aqueduct in the Middle Route of the South-to-North Water Diversion Project has increased due to factors such as the natural aging of concrete and biological attachment. To increase the flow capacity of the project, a roughness reduction test was carried out in April 2021 by installing a nano-rubber coating on the Fangshui River aqueduct, which consists of three aqueduct bodies arranged in parallel. Before and after the test, as well as two years after the test, three field observations were conducted. The analysis revealed that compared to aqueduct body 2, which was only cleaned of surface attachments, after excluding the difference in the background roughness, the nano-rubber coating reduced the roughness of aqueduct body 1 by 0.0013 (10.00%). After two years of operation, the roughness was 0.0010 (7.41%) lower, indicating that the nano-rubber coating had a good and lasting effect on the roughness reduction. The roughness field observation results are sensitive to flow rate and water level measurement errors. For aqueducts with a low head and a high flow rate similar to those of the Fangshui River aqueduct, non-contact measuring instruments should be preferentially utilized, and sufficiently accurate flow rate measurements should be ensured to improve the accuracy of the roughness calculation.