Analyzing debris-flow growth in diverse geomorphic settings with Grfin Tools

The initiation and growth of debris flows is an important activity in many settings, including steep mountains, volcano flanks, and recently burned landscapes. Flow volume from growth exerts a fundamental control on behavior – larger volumes typically lead to faster flows, longer runout, more inundation, and greater hazard. Many processes can promote growth, including hillslope-based processes, such as landsliding or soil rilling, and/or stream channel-based processes such as bed entrainment or stream-bank collapse. Explicitly incorporating and parameterizing these diverse processes in physics-based models is an ongoing challenge to assess hazard and minimize risk.

As an alternative to computationally intensive physics-based models, we developed a USGS software package, called Grfin Tools (an acronym for growth + flow + inundation), that includes tools to define a drainage network, compute volumetric growth from various sources, and then delineate debris-flow inundation throughout a DEM. Grfin Tools uses empirical volume-area relations, derived from observed debris-flow events worldwide, with simple geometric rules to delimit debris-flow inundation. Integrated growth factors, applied over upslope source area and/or upstream channel-length, are used to calculate flow volumes along defined growth zones in the drainage network. Additionally, realistic inundation is created where flows traverse unconfined topography. Grfin Tools requires minimal parameters and places an emphasis on regional geomorphic and topographic controls rather than specific material properties.

Grfin Tools can define-flow inundation with varied modes of growth; we apply these tools to three settings with different growth processes: (1) mountain drainages with distributed landsliding, (2) lahars from volcano flanks that travel from the edifice, and (3) surface-runoff generated debris flows in post-fire landscapes. With upslope distributed landslides as debris-flow sources, nonlinear growth with increasing basin size can reduce potential inundation effects. For lahars from volcanoes, growth from channel sediment entrainment can lead to both wider and longer downstream inundation zones. Finally, with post-fire debris flows, growth from surface-runoff mobilization of available sediment in steep upper watersheds can enlarge flows and inundate fans downstream of mountain fronts. These examples demonstrate the ability of Grfin Tools to delineate debris-flow growth and inundation in diverse geomorphic settings.