This study proposes a new definition of plane bed regimes. The upper
plane bed is defined as a flat bedform that appears under conditions of
active suspended load, whereas the lower plane bed forms when sediment
particles are moved only by bed-load. Previous studies have recognized
that two types of plane bed could be distinguished by shear stress,
particle size, or flow regime. However, the compilation of a large
amount of existing open-channel flow data indicates that the plane bed
regimes cannot be differentiated by existing definitions. Newly produced
phase diagrams indicate that plane bed data plot in two separate
regions, and the gap between the two regions corresponds to the
threshold condition of suspension. Further investigation is needed to
understand the physical mechanisms generating lower plane beds and
characterizations of parallel laminae from lower and upper plane beds.