On asymmetric compound open channel flows with two distinct width floodplains: revisiting interfacial mixing layer and flow structures

In high-flow events, inland surface flow cross sections are defined as compound staged channels with irregular and asymmetric nature of floodplains. Understanding the transverse exchange processes of mass and momentum in compound channel sections with different floodplain widths is essential as these effects are linked directly to the riverbank stability, sedimentation and nutrient transport. Three configurations were tested to study time-averaged lateral flow, advection transport of momentum, and their interaction with the shear layer turbulence over different-sized floodplains in compound open channels. Three factors, viz. depth-averaged flow, shear layer turbulence, and dispersive term of transverse velocity, are assessed to investigate this interaction. The 3D flow structures over the different-sized floodplains in the compound channels were studied using the power density spectral and the quadrant analysis of Reynolds shear stresses to reveal the coherent structure effect over the transverse exchange of momentum in these new test cases. The results showed that the transverse exchange and eddy viscosity for the asymmetric compound channels with two floodplains have a higher magnitude than the symmetric compound channel due to higher momentum redistribution over distinct floodplain widths. The shear layer tends to shift towards a stronger transverse current side for the new asymmetric compound channels with different floodplain widths. Irrespective of roughness, more significant mixing layers are commonly viable on the smaller floodplain. The floodplains' size and roughness strongly influence the main channel shear layer width dynamics.