Solute mixing in a permeable non-rectangular channel

Abstract

Understanding diffusion and solute transport, is regarded as a main activity in environmental management, sustainable development and sedimentation processes because of their role in diffusion of contaminants. The current paper presents an experimental study which was done in a parabolic channel with permeable and impermeable bed conditions in order to estimate the transverse mixing coefficient (TMC) and the maximum solute mixing length (SML) for different channel bed roughness coefficients and discharges. The cross section of the channel was 0.5 m wide and 0.3 m deep with a parabolic shape (i.e. similar to furrow irrigation). Three levels of bed roughness coefficient of 0.02, 0.04, and 0.06, and three discharges of 5, 10, and 15 L/s were considered. Bed infiltration channel was applied at a low level and medium level as well as a case without infiltration. In the current experiments, sodium chloride was used as a soluble tracer and was injected into the water at mid depth in the upstream cross section. The tracer concentration profile in the water and velocity profile were measured at eight cross sections in the channel. The results showed that the values of SML ranged between 20 and 46 m in impermeable and permeable beds channel beds respectively. The assessment of usual discharge values, infiltration, and roughness coefficient of a furrow (agricultural field) indicated that the SML of the furrows were less than 20 m. In the current research, the average dimensionless mixing coefficient was found to be 0.17 in the nonrectangular channel. Eventually, equations were developed to explain the TMC and SML in a permeable parabolic channel that could be beneficial in prediction of fertilizer transport in furrow irrigation.