Mudflow rheology: A review and analysis for Earth and planetary science disciplines

Modelling the rheology of mud is intrinsic to studying mudflows in several Earth and planetary science disciplines. Rheological models enable the flow behaviour of mud particle-water suspensions to be understood and predicted with the use of equations relating the shear-stress to the shear-rate, providing insight into subsurface conditions, improving hazard management, and understanding planetary surface conditions on extra-terrestrial bodies like Mars. This review firstly provides an introduction to rheology, then examines and evaluates the importance of mud rheology and its underpinning variables (e.g., particle volume fraction, particle size and shape distribution). We explore how mud rheology is used in different disciplines of Earth and planetary science to understand the dynamics of various natural processes. Rheology models are examined, and we collate data from multiple published studies on mud suspensions. Four of the most commonly used rheology models are presented (Bingham, Herschel-Bulkley, Power Law, and Casson) and we find the Herschel-Bulkley model to provide the best fit to experimental data. The importance of model choice is explored, and we give a direct comparison of mudflow velocity predictions when using the Bingham and Herschel-Bulkley models. Disparities in the calculated velocity are significant, emphasising the challenges of comparing two different model outputs. Furthermore, as these mud suspensions are non-Newtonian (and thus their viscosity depends on the shear-rate applied), we define the shear-rate ranges typical in different environmental settings (e.g., mud volcanoes, drilling, debris flows, and coastal muds), and give subsequent model recommendations depending on the corresponding shear-rate range.