Laboratory experiments were conducted to understand the dynamics of gravity-driven free surface flow of multi-phase viscoplastic fluids. Foam and sand-foam mixtures were employed to represent multi-phase yield stress fluids. Two different foam-water ratios of 0.1 and 0.2 were used and a wide range of sand concentrations from 0 to 0.82 were selected. A series of flume tests were conducted to model the two-dimensional flow of sand-foam mixtures for two slopes of 10 degrees and 15 degrees. Effects of sand concentration c(0) on the dynamics of free surface flow were investigated. Three-dimensional spreading tests were carried out using an inclined plane with four bed slopes of alpha = 10 degrees, 12 degrees, 15 degrees and 18. Single and multi-discontinuity were observed in the spreading of foam mixtures. It was found that the number of discontinuities can be correlated with the bed slope and multi-discontinuity occurred for alpha >= 12 degrees. Flow heights and frontal velocities were measured for different experimental conditions (slope angle, rheological parameter). The results of spreading tests were properly scaled using a non-dimensional time scale and Froude number. Experimental results were used to predict the rheological characteristics of sand-foam mixtures based on the Herschel-Bulkley constitutive law. Predictions were compared with independent measurements of the rheological parameter. The inclined plane test results showed that the shear stress predictions were independent of the bed slope and the uncertainty of the predictions was slightly higher than the rheometery results from the literature. (C) 2015 Elsevier Ltd. All rights reserved.