planebed

Planebed, also called planar bed, is a term used in sedimentology and hydraulic engineering to describe a sedimentary bed that is essentially flat with little long-wavelength relief. In this regime, bedforms such as ripples and dunes are absent or greatly diminished, and the surface appears planar over large areas.

Formation and conditions: Planebeds form under conditions where the flow has sufficient energy to mobilize a substantial portion of the bed material, preventing the growth of larger bedforms. This can occur at high flow velocities relative to grain size, high transport capacity, or limited sediment supply that inhibits dune and ripple development. The result is a smooth or gently undulating surface produced by continuous bed-load movement, rolling, or partial suspension of grains rather than organized bedforms.

Characteristics: A planebed is characterized by low morphological relief and a relatively uniform grain surface. Microtextures and small-scale roughness may be present, but broad-scale bedforms such as dunes are suppressed. The hydraulic roughness of a planebed tends to be lower than that of bedform-dominated regimes, influencing flow resistance and sediment transport modeling.

Significance and occurrences: Planebeds are observed in certain rivers during high-flow periods, in braided channels, and in controlled laboratory flumes. They are also noted in turbidity currents and other high-energy sedimentary environments. Understanding planebeds helps in modeling hydraulic resistance, predicting sediment transport rates, and interpreting channel evolution during flood conditions.

See also: bedforms, ripple, dune, channel flow, sediment transport.