Coupled prediction of flood response and debris flow initiation during warm- and cold-season events in the Southern Appalachians, USA

J Tao,A P Barros

doi:10.5194/hess-18-367-2014

Abstract

Abstract. Debris flows associated with rainstorms are a frequent and devastating hazard in the Southern Appalachians in the United States. Whereas warm-season events are clearly associated with heavy rainfall intensity, the same cannot be said for the cold-season events. Instead, there is a relationship between large (cumulative) rainfall events independently of season, and thus hydrometeorological regime, and debris flows. This suggests that the dynamics of subsurface hydrologic processes play an important role as a trigger mechanism, specifically through soil moisture redistribution by interflow. We further hypothesize that the transient mass fluxes associated with the temporal-spatial dynamics of interflow govern the timing of shallow landslide initiation, and subsequent debris flow mobilization. The first objective of this study is to investigate this relationship. The second objective is to assess the physical basis for a regional coupled flood prediction and debris flow warning system. For this purpose, uncalibrated model simulations of well-documented debris flows in headwater catchments of the Southern Appalachians using a 3-D surface–groundwater hydrologic model coupled with slope stability models are examined in detail. Specifically, we focus on two vulnerable headwater catchments that experience frequent debris flows, the Big Creek and the Jonathan Creek in the Upper Pigeon River Basin, North Carolina, and three distinct weather systems: an extremely heavy summertime convective storm in 2011; a persistent winter storm lasting several days; and a severe winter storm in 2009. These events were selected due to the optimal availability of rainfall observations; availability of detailed field surveys of the landslides shortly after they occurred, which can be used to evaluate model predictions; and because they are representative of events that cause major economic losses in the region. The model results substantiate that interflow is a useful prognostic of conditions necessary for the initiation of slope instability, and should therefore be considered explicitly in landslide hazard assessments. Moreover, the relationships between slope stability and interflow are strongly modulated by the topography and catchment-specific geomorphologic features that determine subsurface flow convergence zones. The three case studies demonstrate the value of coupled prediction of flood response and debris flow initiation potential in the context of developing a regional hazard warning system.

Highlights

The Southern Appalachians have been prone historically to devastating landslides, due to the combination of steep terrain, poorly consolidated colluvium soil mantle, and regional climate (Wieczorek et al, 2009; Radbruch-Hall et al, 1982)
A fully distributed hydrologic model coupled with slope stability models was used to investigate the mechanisms triggering initiation of debris flow in two headwater catchments of the Pigeon River Basin in the Southern Appalachian Mountains, USA, for both warm- and cold-season debris flow events
Two slope stability models derived from the infinite slope model using the limit equilibrium method were utilized in this study: one is the modified slope stability index (SSI) calculated from soil wetness and slope neglecting cohesion and suction effects; the other is the factor of safety (FS) accounting for most of the dominant factors controlling slope instability

Summary

Introduction

The Southern Appalachians have been prone historically to devastating landslides, due to the combination of steep terrain, poorly consolidated colluvium soil mantle, and regional climate (Wieczorek et al, 2009; Radbruch-Hall et al, 1982). Landslide hazard risk assessments indicate that up to 50 % of the area of the Pigeon River Basin in the Southern Appalachians is highly unstable (Witt, 2005a, b). Past climatological attribution studies have established that widespread landslides in the Southern Appalachian Mountains are primarily induced by heavy rainfall (Wieczorek et al, 2009) associated with. P. Barros: Coupled prediction of flood response in the Southern Appalachians, USA tropical storms: e.g. the remnants of hurricanes Frances and Ivan in 2004 triggered at least 155 landslides and caused ten fatalities (Wooten et al, 2008). There is an implicit bias in the interpretation of rainfall– debris flow statistics toward the widespread events associated with summertime tropical systems, which remain short of explaining the over 5000 events mapped so far in the Southern Appalachians

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Conclusion