Аннотация:Abstract
In the river flow the hierarchical system of the macroturbulent vortexlike
structures can have a wide range of size from depth order to meander length order.
These structures are formed due to interaction between flow and the channel bed.
The river channel relief, which is also hierarchical system of dunelike features with
the same range of size, is the "imprint" of the macroturbulence on the movable
channel bed. Bed relief and channel morphological changes generates transformation
of the flow structure.
The quantitative basement of stream flow - channel bed interaction description
is the analysis of the initial instability of the wave-like structures of the flow and
channel bottom relief. 3-D equations of momentum and conservation in curvilinear
coordinates with depth-averaging following N.Kartvelishvily were used for this
analysis. This approach allows to take into account the effects of the dynamic pressure
vertical distribution and of the helical flow; and give possibility to linearize the
equations for disturbance with minimal lost of terms. Standard procedure of
stability analysis of these equations with nontrivial boundary conditions leads to
solutions which predict the continuos ( both in longitudinal and lateral directions)
spectrum of amplitude growth of unstable in time channel bed waves.
The topography of this continuos spectrum is complex, the areas of five types
of channel forms were defined: 1) 2- and 3-D ultramicroforms with the length of
depth order, which increase with depth and Froude number; 2) 3-D isometric in plane
microforms; 3) 3-D elongate mesoforms; 4) 3-D macroforms, whose length increase
with depth and as Froude number and bottom resistance decrease; 5) long (up to 100
channel width) and narrow megaforms.
These unstable waves have their analogy in the relief of the river channel. The
ripples in flumes and megaripples in the river channel correspond with
ultramicroforms. The dunes of 1 and 2 order are correlated with microforms. The area
of mesoforms is transitional between dunes and bars. The macroforms are
correspondent to bars, island, channel meanders. Parallel braids are the natural
analogy for megaforms. The main difference between theoretical and empirical
spectra of river channel forms is related to nonlinearity of channel - flow interaction
in the real river. One of such nonlinear processes is the secondary flow circulation
which leads transformation of the bottom elevations due to channel meanders
formation.