Compound section is a general shape for cross section of rivers and natural channels. Flow pattern in compound channels can be very complicated because of the significant difference between depth and velocity in main channel and those in floodplains. Although a two-dimensional (2-D) flow pattern is dominant in many flow situations, one-dimensional (1-D) analysis of flow through compound channels is a common practice for engineers. Many classical concepts and definitions such as critical depth, Froude number and slope classification result from a 1-D analysis of flow. In this regard, an interesting 1-D concept is the possibility of occurrence of multiple critical depths in compound channels which consequently results in the possible formation of five different types of water surface profiles, rather than three typical types in single channels. This research is a study on the agreement between visual observations of flow pattern in compound channels with flow behavior predicted by 1-D analysis of flow. To achieve this, experiments were conducted in a straight fixed-bed laboratory compound channel with rectangular main channel and floodplains having homogeneous roughness, under two different slopes representing mild and steep conditions. Different flow patterns of gradually-varied and rapidly-varied flow including hydraulic jump were developed. This was done by adjusting discharge and boundary condition in the channel. The details of experimental observations are presented. In general it can be concluded that flow pattern predicted by 1-D analysis shows a reasonable agreement with the experimental observations in the case of a backwater formation when depth increases in the direction of flow. Conversely, less agreement is observed in the case of drawdown curves, which is an indication of 2-D dominated flow in these situations.

Keywords