In our study of the avalanche dynamics of granular materials in the slumping regime of a 2D rotating drum, we employed digital image processing to measure the slope angle, and speckle visibility spectroscopy (SVS) to measure the time-resolved dynamics of the generated avalanches. It was found that steady and unsteady states alternately occur in an avalanche. In the steady state, granular temperature is low and the flow rate is constant at a given drum rotating speed; whilst in an unsteady state, both the flow rate and granular temperature are significant because there are intense relative motions among the grains in the active layer. This leads to the removal of solids in upper active layer and the decrease of the surface slope angle. Based on our previous work on granular avalanche dynamics in 3D rotating drums [Yang et al., Chem. Eng. Sci., 146 (2016) 1-9], a model is proposed for the analysis of avalanching characteristics using the duration time, the rest time and the steady flow time. One of the interesting observations of the study was that steady flow durations in granular avalanches exhibit a normal distribution, as well as increasing in value with an increase in the filling degree. (C) 2017 Elsevier B.V. All rights reserved.