Numerical computations are conducted of the flow across a turbulent mi
xing layer in an open channel of small depth using three turbulence mo
dels: a single-length-scale model, a modified single-length-scale mode
l, and a two-length-scale model. The performance of these turbulence m
odels is evaluated by comparing the numerical results with the availab
le experimental data. The single-length-scale turbulence model is show
n to be biased toward the small-scale turbulence. The coefficients of
this single-length-scale turbulence model have been modified in an att
empt to improve the performance of the model. The results of the modif
ied single-length-scale model are in agreement with the laboratory dat
a but not with the field data. The two-length-scale turbulence model,
by treating the small-scale and the large-scale turbulence as separate
components, has produced results in better agreement with the experim
ental data than those obtained by the single-length-scale model treati
ng both components together as a whole.