Flow of lava, avalanches, mudslides, and many geophysical and planetary flow systems are examples of free-surface flows of Yield Stress Fluids (YSFs). This category of fluids is known for its dual behavior below- and above a yielding threshold for the applied shear stress on each fluid element. The material behaves as an amorphous elastic solid below the yielding threshold and fluidizes above it. This will lead to the presence of unyielded plug regions translating and rotating as solid-like segments within the yielded surrounding fluids. The existence of macroscopic particles in the fluid adds to the complexity of the flow setting. Transport of debris in the riverbeds and avalanches, dispersion of the cooled-down agglomerates of lava in the molten medium, and migration of solid material such as icy rocks in high pressure YSF-like, sub-terranean oceans of Europa (Jupiter’s moon) are among numerous natural examples of particle-laden flows of YSFs. To replicate the conditions experimentally, aqueous solutions of Carbopol with yield stress  are used in combination with hydrogel particles. The elastic hydrogel particles have been used in volume fractions φ = 0, 10, 20, and 30 % as mono- and duodispersed suspensions. The excellent refractive index matching of these elastic particles with Carbopol permits accurate recording of the illuminated flow field seeded with tracer particles for PIV measurements, without optical blockage of the macro particles in the optical path. Measurements are performed with channel inclinations ranging from zero to 18°, with controlled deployment of gate opening ranging from 3 cm (i.e. 50 % of the channel width) to a full open dam-break situation. Stream-wise PIV recordings of the transient and semi-steady field are complemented with span-wise recordings targeting statistical results on the particle migration and sedimentation. The results are put in context with the experiments on Newtonian and YSFs in free surface flumes containing rigid particles [1,2], as well as duct flow experiments on Carbopol with the same elastic particles [3].      References [1] Christophe Ancey, Nicolas Andreini, Gaël Epely-Chauvin, The dam-break problem for concentrated suspensions of neutrally buoyant particles, J. Fluid Mech. (2013), vol. 724, pp. 95–122.[2] G Rousseau, C Ancey, An experimental investigation of turbulent free-surface flows over a steep permeable bed, J. Fluid Mech. (2022), vol. 941, A51.[3] Sagar Zade, Tafadzwa John Shamu, Fredrik Lundell, Luca Brandt, Finite-size spherical particles in a square duct flow of an elastoviscoplastic fluid: an experimental study, J. Fluid Mech. (2020), vol. 883, A