Mixing coefficients for longitudinal and vertical mixing in the near field of a surface pollutant discharge

Abstract

Laboratory experiments conducted in a rectangular open channel on passive mixing of pollutants in longitudinal and vertical directions due to a transverse line source on the flow surface are reported. A finite numerical difference scheme developed by the authors was extended to determine optimum values of longitudinal and vertical mixing coefficients from the observed concentration versus time curves. The vertical mixing coefficient was also computed from the measured velocity distribution and compared with the derived optimum. The velocity distribution-based values of vertical mixing coefficient corresponds well with the optimum derived using the numerical scheme. Through a sensitivity analysis, the errors in the predicted concentration versus time curves are quantified by using the numerical scheme proposed by the authors for two-dimensional dispersion in streams. The sensitivity analysis indicated that predictions of concentration versus time curves can be made with a maximum error of about ±30% if a ±100% error is contained in the values of longitudinal and vertical mixing coefficients.

Keywords:

• Mixing coefficient
• open channel flow
• passive mixing
• slug source
• transverse mixing