Dear Michael,
For suspended sediment, the concentration at your boundaries domain can be specified in the steering file through keyword CONCENTRATION PER CLASS AT BOUNDARIES. To avoid unwanted erosion or sedimentation at the entrance of the domain, it may be also convenient to use keyword EQUILIBRIUM INFLOW CONCENTRATION = YES to specify the value of the concentration at inflow, according to the choice of the REFERENCE CONCENTRATION FORMULA.
I hope it helps,
all the best,
Pablo