Braided rivers easily form wide and shallow floodplains when there is no
constraints on both sides of the river. During floods, rising water
level submerges the floodplain of the bifurcated channel, resulting in
the Compound-Braided River. The generalized model was established based
on statistical data from the braided river reach of Heilongjiang. In
this paper, the flow field of the straight compound-braided river was
measured in flume experiments, and then the effect of the interaction of
floodplain and main channel on the flow pattern, water level, flow
structure and resistance force were studied under overbank flow
conditions. The split ratio variation trend is further discussed. The
results show that hydraulic factors in diverge segment were mainly
related to braided reach, with high longitudinal velocity observed in
inner floodplain. The exchange flow between floodplain and main channel
accelerates transverse flow and promotes sediment transport intensity
laterally. Secondary flow of compound section within the influence range
of bifurcated flow was obviously inhibited. Boundary shear stress
analysis showed that the diversion ratio of the main tributary under
overbank condition decreased slightly and would maintain constant values
as surface rise.