Augmentation of WRF-Hydro to simulate overland-flow- and streamflow-generated debris flow susceptibility in burn scars

In steep wildfire-burned terrains, intense rainfall can produce large runoff that can trigger highly destructive debris flows. However, the ability to accurately characterize and forecast debris flow susceptibility in burned terrains using physics-based tools remains limited. Here, we augment the Weather Research and Forecasting Hydrological modeling system (WRF-Hydro) to simulate both overland and channelized flows and assess postfire debris flow susceptibility over a regional domain. We perform hindcast simulations using high-resolution weather-radar-derived precipitation and reanalysis data to drive non-burned baseline and burn scar sensitivity experiments. Our simulations focus on January 2021 when an atmospheric river triggered numerous debris flows within a wildfire burn scar in Big Sur – one of which destroyed California's famous Highway 1. Compared to the baseline, our burn scar simulation yields dramatic increases in total and peak discharge and shorter lags between rainfall onset and peak discharge, consistent with streamflow observations at nearby US Geological Survey (USGS) streamflow gage sites. For the 404 catchments located in the simulated burn scar area, median catchment-area-normalized peak discharge increases by ∼ 450 % compared to the baseline. Catchments with anomalously high catchment-area-normalized peak discharge correspond well with post-event field-based and remotely sensed debris flow observations. We suggest that our regional postfire debris flow susceptibility analysis demonstrates WRF-Hydro as a compelling new physics-based tool whose utility could be further extended via coupling to sediment erosion and transport models and/or ensemble-based operational weather forecasts. Given the high-fidelity performance of our augmented version of WRF-Hydro, as well as its potential usage in probabilistic hazard forecasts, we argue for its continued development and application in postfire hydrologic and natural hazard assessments.</p

Parameter calibration for high-porosity sandstones deformed in the compaction banding regime

This paper discusses the parameter calibration procedure for an elastoplastic constitutive model for high-porosity rocks. The model selected for the study is formulated in the frame of the critical state theory, which is here used in a form able to accommodate non-associated plastic flow and softening effects due to volumetric and deviatoric plastic strains. The goal of this study is to generate a set of model constants able to capture both the stress-strain response and the compaction localization characteristics (e.g., stress and inclination at the onset of the deformation bands). For this purpose, data about the compaction localization properties of four extensively characterized sandstones have been considered. In particular, the strain localization theory has been used as a calibration tool, using explicitly information about the pressure-dependence of the localization mechanisms observed in experiments. The model constants have been defined by matching the constitutive response upon hydrostatic compression, as well as the stresses at the transition from high-angle shear bands to pure compaction bands, and from compaction bands to homogeneous cataclastic flow. It is shown that such procedure generates a set of model constants able to capture satisfactorily both the rheological response upon triaxial compression and the salient features of the compaction localization process

Instability criteria for quasi-saturated viscous soils

This paper presents a theoretical framework to interpret the inception of unstable undrained creep in quasi-saturated soils. For this purpose, the effect of gas bubbles occluded in the fluid phase is embedded into an augmented compressibility of the fluid mixture, while the mechanical characteristics of the solid skeleton have been simulated through a viscoplastic strain-hardening model. This constitutive framework has been been used to formulate a theoretical platform able to detect runaway failures resulting from extended stages of undrained creep. It is shown that the conditions identifying the onset of spontaneous accelerations are governed by the same stability index associated with the initiation of static liquefaction. At variance with soils saturated by incompressible fluids, the conditions for undrained instability are altered by the appearance of the Skempton coefficient B, thus reflecting the beneficial effect of the fluid compressibility and its ability to decrease the liquefaction potential. The capabilities of the theory are verified through a sequence of undrained creep simulations showing the transition from stable to unstable behavior resulting from an increase of the degree of saturation. The proposed findings provide a conceptual framework to interpret the effects of gas bubbles in loose soils, as well as to assess effectiveness and longevity of liquefaction mitigation strategies based on desaturation technologies

A Generalized Backward Euler algorithm for the numerical integration of a viscous breakage model

This paper discusses the formulation and the numerical performance of a fully implicit algorithm used to integrate a rate-dependent model defined within a breakage mechanics framework. For this purpose, a Generalized Backward Euler (GBE) algorithm has been implemented according to two different linearization strategies: The former is derived by a direct linearization of the constitutive equations, while the latter introduces rate effects through a consistency parameter. The accuracy and efficiency of the GBE algorithm have been investigated by (1) performing material point analyses and (2) solving initial boundary value problems. In both cases, the overall performance of the underlying algorithm is inspected for a range of loading rates, thus simulating comminution from slow to fast dynamic problems. As the viscous response of the breakage model can be recast through a viscous nucleus function, the presented algorithm can be considered as a general framework to integrate constitutive equations relying on the overstress approach typical of Perzyna-like viscoplastic models

Constitutive modeling approaches for cross-anisotropic porous rocks

Simulating the anisotropy of rock properties is a major challenge for geomechanical modeling. Despite numerous techniques that have been proposed to account for the orientation of the material reference system with respect to the loading directions, such methods often involve a major increase in the number of model parameters, especially if the anisotropy influences both elastic and plastic properties. In this paper, two approaches to model the mechanical behavior of cross-anisotropic porous rocks are examined. The first approach relies on a tensorial projector used in conjunction with standard strain-hardening plasticity. It is illustrated how such projector is able to map the stress conditions into a modified stress space, thus distorting the yield surface and inducing the dependence of yielding on the direction of sample coring. Afterwards, a novel energy-based approach to replicate such mapping effects is discussed. The methodology relies on the Breakage Mechanics theory, i.e. a constitutive framework expressing the yielding of granular rocks in terms of a strain energy threshold associated with the release of the elastic energy stored in the brittle grains constituting its skeleton. It is shown that energy-based yielding produces the same benefits of a tensorial projection without the need of introducing additional model parameters, but solely relying on the directional properties of the elastic stiffness tensor. The capabilities and performance of both approaches with respect to data available for a porous rock are outlined, discussing their relative merits and providing guidelines for the formulation of constitutive laws able to reduce the number of model parameters by relying on more detailed insights on the microscopic causes of inelasticity

Stability analysis of unsaturated deposits: effect of soil properties and hydromechanical coupling

This paper investigates the mechanical conditions governing the activation of shallow landslides in unsaturated deposits. Particular attention is given to the effect of saturation processes applied under a constant total state of stress. For the sake of generality, multiple failure mechanisms are considered and are interpreted in the light of a unified theoretical framework. The analyses rely on the scheme of infinite slope and on the use of a coupled hydro-mechanical constitutive model. Simple shear test simulations are used to investigate the role of stress anisotropy, initial suction and soil properties. The simulations show that the predicted mechanism of activation depends on the material constants that reproduce the coupling between retention properties and stress-strain behavior. These constants affect the magnitude of the triggering perturbations and can play a relevant role in collapsible/liquefiable deposits. In particular, it is shown that the range of slope inclinations prone to originate a runaway instability can be a function of such hydro-mechanical coupling terms, thus depending on material constants that are not directly associated with the shearing resistanc