Elucidating the role of vegetation in the initiation of rainfall‐induced shallow landslides: Insights from an extreme rainfall event in the Colorado Front Range

Abstract

More than 1100 debris flows were mobilized from shallow landslides during a rainstorm from 9 to 13 September 2013 in the Colorado Front Range, with the vast majority initiating on sparsely vegetated, south facing terrain. To investigate the physical processes responsible for the observed aspect control, we made measurements of soil properties on a densely forested north facing hillslope and a grassland‐dominated south facing hillslope in the Colorado Front Range and performed numerical modeling of transient changes in soil pore water pressure throughout the rainstorm. Using the numerical model, we quantitatively assessed interactions among vegetation, rainfall interception, subsurface hydrology, and slope stability. Results suggest that apparent cohesion supplied by roots was responsible for the observed connection between debris flow initiation and slope aspect. Results suggest that future climate‐driven modifications to forest structure could substantially influence landslide hazards throughout the Front Range and similar water‐limited environments where vegetation communities may be more susceptible to small variations in climate.

Controls on debris‐flow initiation on burned and unburned hillslopes during an exceptional rainstorm in southern New Mexico, USA

“…The preference for south‐facing slopes in debris‐flow initiation locations is not surprising, as south‐facing slopes in this landscape are sparsely vegetated even when not burned, and are frequently steeper, have thinner soils, and less vegetation (Ebel et al ., ). The lack of vegetation also decreases the apparent cohesion of the soil on steep south‐facing slopes, leading to preferential slope failure in the Colorado Front Range (McGuire et al ., ). A preference for south‐facing slopes in debris‐flow initiation locations was also observed in the Colorado Front Range by Coe et al .…”

“…that control debris‐flow initiation densities, locations, and mechanisms. In addition, we can compare the debris flows documented in this study to debris flows generated by the same storm (Gochis et al ., ) in a different part of the southwestern United States (Coe et al ., ; McGuire et al ., ; Rengers et al ., ).…”

“…First, we hypothesize that debris flows will primarily initiate via overland‐flow runoff in burned areas and from discrete landslides in unburned areas. Second, we hypothesize that in unburned areas, debris flows would initiate preferentially in areas with sparse vegetation due to poor root cohesion (similar to the observations of McGuire et al ., and Rengers et al ., ). In burned areas we proposed a third hypothesis, that the debris‐flow initiations of 2013 would occur preferentially in areas that were designated as high burn severity in 2012.…”

Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks

“…Anderson et al . () estimated that debris flows triggered during this 2013 rainstorm resulted in the removal of hundreds to thousands of years of weathered regolith at their study area in the Colorado Front Range west of Boulder, CO. A pair of recent studies concluded that spatial variations in apparent root cohesion conferred by vegetation were most likely responsible for the aspect control on debris flow initiation (McGuire et al ., ; Rengers et al ., ). In addition, Ebel et al .…”

Tree-root control of shallow landslides

“…Next to the constellation of factors well known to influence the triggering of shallow landslides, vegetation has been recognized to play an important role (Sidle and Ochiai, 2006;Schwarz et al, 2010c;McGuire et al, 2016) and its function is considered an important component of ecosystem services provided in mountain regions. The importance of the effects of vegetation is, in some cases, recognized at a po-454 D. Cohen and M. Schwarz: Tree-root control litical level.…”