The changing rules of the composite roughness in drainage pipelines with sediments

Abstract

The deposition of sediments with different roughness increases uncertainty in the prediction of effective Manning's n (also known as composite roughness). In this paper, the changing rules of composite roughness and the influence on the discharge capacity of drainage pipes under different flows and slopes were studied by experimental measurement and numerical simulation. When the Froude number is less than 1, the composite roughness decreases gradually with the increase of flow. When the Froude number is more than 1, the composite roughness increases at first and then decreases with the increase of flow. It also showed that the different bed roughness has a great influence on composite roughness, which affects the hydraulic characteristics of flow. The composite roughness of the drainage pipe increases as the bed roughness increases. The water surface rises and the average velocity decreases under a certain slope and flow. The formulas for calculating the composite roughness are obtained by analogy analysis, which can provide theoretical support for the design of the drainage pipeline. HIGHLIGHTS The roughness coefficient is not constant when hydraulic conditions change.; When a sediment deposit is present in a pipe, the overall resistance consists of two components.; The changing rules of composite roughness are related to the flow regime.; The particle size of the sediment in the pipeline will affect the composite roughness.; Bed roughness can be separated from the composite roughness.;

Keywords

• composite roughness
• discharge carrying capacity
• hydraulic characteristics
• pipeline with sediments