Dear Bastien,
By default, Sisyphe has zero solids inflow imposed at inlets.
I advise you to take a deeper look at Sisyphe user manual (v7p3 the last one), how to impose inflow boundary conditions is indicated on chapter two (solid discharges inflow) and chapter three (suspended sediment inflow).
Nevertheless, find attached a boundary conditions file I used when I started evaluating a similar case (it was a synthetic channel my case with only one inlet and one outlet).
I hope this helps.
Best Regards,
José Andrés.