Classic Bed Partitioning

The transport capacity for each grain class is the transport potential multiplied by the percentage of that grain class in the bed. Therefore, the total transport capacity is:

Where: T_c is Total transport capacity, n is the number of grain size classes, B_j is the percentage of the active layer composed of material in grain size class "j", and T_j is the Transport potential computed for the material in grain class "j". Partitioning capacity based on the gradation of the active layer is a classic assumption based Einstein's (1950), who proposed sediment discharge of a size class is proportional to the fractional abundance of that size class in the bed (Vanoni, 1975).

Modeling Note – Partition Gradations for Empty Sediment Control Volumes

As long as the sediment control volume has bed sediment, it can partition the transport potential into capacity. However, if the active layer thickness = 0, either because it is a concrete channel with no starting sediment thickness (Initial Max Depth = 0) or because it scoured through the entire erodible depth, the bed partitioning assumption will run into trouble. If the active layer has no sediment, the fraction of each grain class is zero (_j=0 in the previous equation). Therefore, regardless of the computed potential, this approach will compute no transport capacity over a fixed bed. If the model computes no transport capacity, it will deposit all of the sediment in one time step, and erode in the next, causing oscillating errors and decreasing transport in half, because it only transports every other time step.

To offset this numerical artifact, recent versions of HEC-RAS uses the initial bed gradation to partition potential into capacity if the control volume has no bed sediment. This is also why HEC-RAS requires bed gradations for concrete channels. It uses the gradation to compute capacity.