The representative grain size is only used in the dispersive term of the O'Brien equation, and only becomes active if that method is selected. However, the value is squared in one of the quadratic terms of the equation, so results can be sensitive to this user decision. Various sediment transport analyses that collapse the particle size effects of a particle-size distribution into a representative grain size use the median grain size (d50) or geometric mean of the distribution. However, it can be difficult to determine the median particle size of an event with substantial clay components that also moves car-size boulders. It is also an open question whether the median particle size is appropriate for the dispersive process Bagnold was describing (in a single particle-size theoretical model) under transport conditions that include substantial grain class fractions between 0.004 mm and 4,000 mm. Evaluate the uncertainty and sensitivity of this value in the simulation and determine if it should be a fixed parameter or a calibration parameter.
