A Modular, Non-Newtonian, Model, Library Framework (DebrisLib) for Post-Wildfire Flood Risk Management

Abstract. Wildfires increase flow and sediment load through removal of vegetation, alteration of soils, decreasing infiltration, and production of ash commonly generating a wide variety of geophysical flows (i.e., hyperconcentrated flows, mudflows, debris flows, etc.). Numerical modellers have developed a variety of Non-Newtonian algorithms to simulate each of these processes, and therefore, it can be difficult to understand the assumptions and limitations in any given model or replicate work. This diversity in the processes and approach to non-Newtonian simulations makes a modular computation library approach advantageous. A computational library consolidates the algorithms for each process and discriminates between these processes and algorithms with quantitative non-dimensional thresholds. This work presents a flexible numerical library framework (DebrisLib) to simulate large-scale, post-wildfire, non-Newtonian geophysical flows using both kinematic wave and shallow-water models. DebrisLib is derived from a variety of non-Newtonian closure approaches that predict a range of non-Newtonian flow conditions. It is a modular code designed to operate with any Newtonian, shallow-water parent code architecture. This paper presents the non-Newtonian model framework and demonstrates its effectiveness by calling it from two very different modelling frameworks developed by the U.S. Army Corp of Engineers (USACE), specifically, within the one-dimensional and two-dimensional Hydrologic Engineering Centre River Analysis System (HEC-RAS) and two-dimensional Adaptive Hydraulics (AdH) numerical models. The development and linkage-architecture were verified and validated using two non-Newtonian flume experiments selected to represent a range of non-Newtonian flow conditions (i.e., hyperconcentrated flow, mudflow, debris flow) commonly associated with post-wildfire flooding.