Abstract:
The behavior of flow in a compound meandering channel, consisting of a deep main
channel bounded by shallow floodplains, is very complex and influenced by several
kinds of forces and hydraulic parameters. Encroachment of floodplains, i.e. various
human settlements and related activities modifies such overbank flow processes due to
the presence of different types of interventions. Hence, in addition to the classical
turbulent interaction at the interface between the main channel and floodplain, the
presence of an obstacle strongly disturbs the flow dynamics of the encroached compound
channels. The location of encroachment, i.e., whether encroachment occurs in the bends
or straight cross-over portion of a meandering channel, also affects the velocity and shear
stress distribution at and near the encroached area. Moreover, the presence of
encroachment in single overbank or both overbanks is likely to have a considerable
effect on the variation of these hydraulic parameters. To investigate these effects, a
systematic experimental study was carried out in the physical modeling facility of the
Department of Water Resources Engineering, BUET. The experiment was conducted in a
compound meandering channel with five different set-ups of various encroachment
conditions for depth ratio of 0.285 and 0.375. A large volume of data was collected using
Acoustic Doppler Velocity-meter (ADV) from ten experimental runs. Analyses of the
study showed that the distribution of flow in a compound meandering channel is affected
by the encroachment condition. When encroachment occurs in bend, the floodplain flow
reduces significantly compared to encroachment in crossover. The non-uniform velocity
distribution near the encroached area increases the value of both the energy and
momentum coefficients, specially when encroachment occurs on both floodplains of a
bend. For this condition the main channel velocity was found maximum. There has been
no rapid change of shear stress at main channel at any conditions but shear stress
increases at the location when encroachment was placed on both side of floodplain at
bend and crossover. Finally, dye tracer technique was utilized to visualize the complex
flow pattern for various encroachment conditions and the observations were found
consistent with the experimental measurements and analyses.
