sediment availability for bedload in sand transport module

Hi there,

I'm wondering if anyone can point me in the right direction. I've run some simulations using the sand transport module for various different values of grain diameter, using the van Rijn model. I've used a varying current which was the same for each of runs, and I notice that the peak bedload volumetric transport rate is lower for finer grain sizes, whilst the peak suspended load volumetric transport rate always increases for finer sediments.

I assume that this is due to less availability of sediment for bed load transport, but I cant see what I have chosen in the model set up to include this in the model. I've used the pure current, non-equilibrium form of the model. I've not ticked the "varying layer thickness" option in the "model definition" window. I have however ticked the "include feedback on hydrodynamic, wave and sand transport calculation" option in the morophology module "model definition" window. I guess it is this choice which is limiting the volumetric bedload transport?

I don't see any explanation in the scientific documentation or user guide of how the transport rate might be reduced through the choice of this option. I wonder could anyone point me in the direction of more information?