A model for fire-induced sediment yield by dry ravel in steep landscapes

Abstract

[1] Sediment flux from hillslopes to channels commonly increases following wildfires, with implications for the carbon cycle, river habitats, and debris-flow hazards. Although much of this material is transported via dry ravel, existing ravel models are not applicable to hillslopes with gradients greater than the angle of repose, which can constitute the majority of mountainous terrain. To fill this knowledge gap, we develop a continuity model for sediment storage by vegetation dams on steep hillslopes to predict sediment yields following wildfire. The maximum volume of sediment stored prior to wildfire is set to be a function of vegetation density, the capacity of plants to impound sediment, and the contributing hillslope area. Time is required after fire to establish vegetation and replenish hillslope sediment storage, which introduces vegetation regrowth rate, soil production rate, and fire recurrence interval as important variables that affect ravel yield. Model results for the San Gabriel Mountains, California, predict that sediment yield can increase by several orders of magnitude following fire. These results are consistent with field data of ravel yield (∼30 mm per contributing area of hillslope in 5 months) we collected following the 2009 Station Fire, as well as postfire sediment flux recorded by 93 debris basins. In contrast to previous work, our model shows that heightened postfire sediment yields can be explained by a change in hillslope sediment storage independent of major changes in the soil production rate and landscape form over geomorphic timescales.Citation: Lamb, M. P., J. S. Scheingross, W. H. Amidon, E. Swanson, and A. Limaye (2011), A model for fire-induced sediment yield by dry ravel in steep landscapes,

Short-term post-wildfire dry-ravel processes in a chaparral fluvial system

“…In the model, the volume of sediment stored on hillslopes is a function of chaparral vegetation density, capacity of plants to impound sediment, and the contributing hillslope area. As noted by Lamb et al (2011), in models developed by Roering and Gerber (2005) and Gabet (2003), the dry-ravel flux predicted is infinite when slope is greater than the angle of repose, such that on steep slopes without vegetation, flux is limited by supply. Therefore, it is critical to consider the time needed to establish vegetation between fires, the time needed for bedrock to weather and replenish hillslope stored sediment, and the fire recurrence interval.…”

“…Dry ravel is a dominant sediment transport process in steep chaparral regions (Krammes, 1965;Rice, 1974;1982) where slope is greater than the angle of repose (Lamb et al, 2011). Early research showed high sediment flux rates on steep chaparral slopes (Anderson, et al, 1959;Krammes, 1965).…”

“…Therefore, consideration of wildfire in landscape evolution models to predict landform response to climate and tectonic changes has recently been addressed (Benda and Dune, 1997;Gabet and Dunne, 2003;Lave and Burbank, 2004;Roering and Gerber, 2005;Lamb et al, 2011). For example, Lamb et al (2011) developed a mass balance-continuity model for sediment storage by vegetation barriers on steep hillslopes to predict hillslope sediment yield following wildfire. In the model, the volume of sediment stored on hillslopes is a function of chaparral vegetation density, capacity of plants to impound sediment, and the contributing hillslope area.…”

“…On such steep slopes, weathered sediment may be stored upslope of vegetation whose trunks and branches form barriers to downslope sediment transfer. During and after wildfire, sediment stored behind burned vegetation is destabilized; thus, wildfire that burns chaparral vegetation and the organic litter layer acts as an agent to increase surface erosion rates (Florsheim et al, 1991;Gabet, 2003;Lave and Burbank, 2004;Roering and Gerber, 2005;Shakesby and Doerr, 2006;Jackson and Roering, 2009;Lamb et al, 2011;DiBiase and Lamb, 2013). Sediment transported downslope by dry ravel collects in hillslope depressions or forms irregular or inverted cone-shaped deposits at the base of hillslopes near channel margins (Florsheim et al, 1991).…”

Conversion to drip irrigated agriculture may offset historic anthropogenic and wildfire contributions to sediment production

“…Sediment flux generally rises after wildfire due to increases in the erodibility of hillslope surfaces through the removal of vegetation and litter layers, destabilization of soil aggregates by organic matter combustion, and increases in soil mantle slides or overland flow due to the development of subsurface and surface soil hydrophobicity, respectively (Debano and Krammes, 1966;Keller et al, 1997;DeBano, 2000;Gabet, 2003). In systems experiencing dry seasons, such as much of the Western U.S., this results in down-slope 5 dry-ravel transport through gravity alone (Swanson, 1981;Jackson and Roering, 2009;Lamb et al, 2011;Hubbert et al, 2012). Soil heating can also cause hydrophobicity increases in the soil surface that, along with decreases in interception and evapotranspiration, cause increases in surface runoff during the wet season (Shakesby and Doerr, 2006).…”