Abstract:

[Objective] In the early stage operation of long-distance water conveyance pipelines, the actual roughness coefficient is generally lower than the design value. After years of operation, factors such as erosion, sedimentation, and the growth of aquatic organisms cause the roughness coefficient to gradually increase, which directly affects the water conveyance capacity and operational safety of the project. This study systematically investigates the influence of roughness coefficient variations on the hydraulic characteristics of complex water conveyance pipelines under both stable operation and transient process. [Methods] The 206 km-long pressurized PCCP pipeline section of Chuo’er River to Xiliao River Diversion Project was taken as a case study (with surge protection measures of impedance-type surge tanks and flow and pressure regulating valves). The one-dimensional method of characteristics was used to analyze the influence of roughness coefficient variations on hydraulic characteristics during stable operation and transient process. Operational risks were evaluated, and strategies to address roughness uncertainty were proposed from the perspective of operational scheduling. [Results] Under stable operating conditions, when pipeline roughness coefficient was relatively high, the reference flow rate decreased, posing a risk of failing to meet the design flow rate. Additionally, the head loss along the pipeline increased, while the flow rate decreased. When the opening degree of the flow and pressure regulating valve at the mid-section of the main pipeline remained unchanged, the head drop at the valve location diminished accordingly. This resulted in decreased pressure on the main pipeline upstream of the flow and pressure regulating valve and increased pressure downstream, necessitating special attention to the influence of pipeline pressure variations. During the transient process, the water hammer waves generated by pipeline pressure fluctuations superimposed with the mass waves caused by water level fluctuations in the surge tank. After the valve was closed, the fluctuations propagated independently in the upstream and downstream sections of pipelines and gradually attenuated. With increasing roughness coefficients, the frictional damping of the water hammer waves increased, leading to reduced amplitude of head fluctuations before and after the valve, decreased magnitude of surge tank water level fluctuations, and lower stabilized water levels. Therefore, an adequate pressure margin should be considered in engineering design. It was possible to address the uncertainty caused by roughness coefficient variations by adjusting the opening degree of the flow and pressure regulating valve on the mid-section of main pipeline and terminal valves on branch pipelines, thereby ensuring the design flow rate. However, after the valve operation mode was adjusted, the pipeline pressure became dependent on the roughness coefficients and valve opening degree, making it necessary to verify in advance whether the pressure along the entire pipeline met the water hammer protection requirements. [Conclusions] In the design of long-distance water conveyance pipelines, the influence of pipeline roughness coefficient on both flow rate and hydraulic characteristics must be considered, and the upper and lower limits of roughness coefficient variations should be reasonably predicted. The upper limit is used to ensure the water conveyance capacity during long-term operation, while the lower limit is used to cope with the relatively large residual head in the early stage of operation. The findings of this study serve as a references for addressing roughness coefficient uncertainty in the design of long-distance water conveyance pipelines.

Key words: long-distance water conveyance pipelines, roughness coefficient, water hammer protection, transient process, flow capacity, hydraulic characteristics