The Role of Basal Roughness and Assemblage Grain‐Size Distribution in Shaping Granular Rheology and Basal‐Force Signals

We present idealized 3‐D discrete element simulations exploring how varying boundary roughness and grain‐size distribution affect bulk granular‐rheology and basal‐force distributions. Boundary roughness has a direct control on the internal static friction coefficient. Further, when flows enter a highly energetic collisional regime, relatively smooth substrates promote the development of a dense plug‐like flow, that is, supported by a basal granular temperature an order of magnitude greater than what is measured within the bulk. We show that in the low velocity limit, basal‐force distributions are not only sensitive to averaged roughness but also sensitive to how the substrate is constructed. Additionally, varying the grain‐size distribution by increasing the proportion of fine‐particles increases internal energy fluctuations. We use this observation to define a dimensionless parameter comparing the changes in basal‐force fluctuating energy and internal energy fluctuations as measured by the granular temperature. This scaling shows that a redistribution of fluctuating energy ultimately decreases the total power of basal‐force spectra. We conclude this study by performing 1‐D steady‐state surface‐wave propagation calculations, using basal‐force data from our simulations as a source. Using the method of spectral moments shows that the dispersion of power in observed signals can be used to track changes in flow regime. The Green's functions used in the present study yield data that underscores the importance of receiver placement and suggests there is an optimal distance at which these changes can be observed. We suggest that a similar exercise can be used with empirical Green's functions to inform deployment strategies.