Turbulent velocity fluctuations in natural rivers

The longitudinal and transverse components of flow velocity, as
well as the normal and tangential stresses, must be expected to
fluctuate with time and space when flow at a high Reynolds number
moves between fixed boundaries.  Although these nonperiodic
fluctuations are generally secondary in magnitude compared to the
mean motion, they have profound effects on properties of the
primary mean motion.  Scientists from the Illinois State Water
Survey are involved in collecting and analyzing detailed velocity
data from the Illinois and Mississippi Rivers using 2-D
electromagnetic current meters.  The goal of the present research
is to understand and evaluate the turbulent structure in natural
river systems, especially near the channel border areas.  The
fluctuation characteristics of flow velocity were analyzed
systematically.  Flow velocities were measured with time at six
different lateral locations and at three different vertical
elevations.  Analysis of the velocity data included the cross-
sectional and vertical distributions of longitudinal and
transverse velocity components (u, v), the fluctuating velocity
components (u', v') and their frequency-distribution  curves,
turbulent intensities (�x, �y), turbulent shear stress (- u'v'),
and turbulent kinetic energy.  It appears that velocity, velocity
fluctuation, turbulent intensity, turbulent shear stress, and
turbulent kinetic energy are all relatively high within the main
channel.  Moreover, all of these flow parameters decrease in the
channel border areas.  Thus the main channel area above the
riverbed appears to be the most active zone in the river.  The
study shows that the values of �x, �y, �xy, and average KE are high
at elevations of 10 to 20 percent above the bed.