PORTRAYING THE REGIONAL EFFECTS OF WIDESPREAD DEBRIS FLOWS WITH INTEGRATED GROWTH

Debris flows can modify channels and sculpt hillslopes in many environments with flow volumes exerting a primary influence on mobility, inundation area, and erosion potential. Debris flows can grow in volume through a variety of processes such as channel-bed entrainment, bank failure, aggregation of multiple landslides, coalescence of flows, and/or hillslope rilling. As an alternative to explicitly embedding these diverse processes into flow routing models, our approach combines these effects into a growth factor. Growth factors can be integrated over contributing area (m³/m²), similar to basin erosion rates, or over channel length (m³/m), analogous to yield rates. Zones of growth typically occur in upper parts of a basin where channels are steep and confined. We use growth factors and growth zones to estimate volumes throughout a network, and then use empirical volume-area relations to delineate potential debris-flow inundation. This method is implemented in the USGS software package Grfin Tools (growth + flow + inundation), currently under development.

Two examples illustrate the use of integrated growth factors over large regions. Each example involves widespread debris-flow growth and uses one growth factor to represent the dominant growth processes. Both area and channel-length growth factors were obtained by analyzing photographic or lidar images before and after major debris-flow events. The first example focuses on Puerto Rico, where in 2017 Hurricane Maria triggered thousands of debris flows that grew mostly from the aggregation of shallow landslides, indicating area-controlled growth. Using growth zones defined by channel gradients ≥ 6º and an area-growth factor of 0.01 m³/m², our results reveal inundation extents similar to those observed after the hurricane. The second example is in the Oregon Coast Range, where numerous debris flows were generated by a large storm in 1996. Here, many debris flows grew by entrainment of channel-bed sediment, making this channel-growth dominant. With growth zones having channel gradients ≥ 10º and a channel-length growth factor of 11 m³/m, inundation extent is also similar to that observed from the event. Our simple, empirical approach works well to rapidly evaluate the effects of diverse debris-flow growth processes in different landscapes.