Abstract
Abstract. Post-wildfire landscapes are highly susceptible to rapid geomorphic changes, and the resulting downstream effects, at both the hillslope and watershed scales due to increases in hillslope runoff and erosion. Numerous studies have documented these changes at the hillslope scale, but relatively few studies have documented larger-scale post-fire geomorphic changes over time. In this study we used five airborne laser scanning (ALS) datasets collected over 4 years to quantify erosion and deposition throughout the channel network in two ∼15 km2 watersheds, Skin Gulch and Hill Gulch, in northern Colorado after a wildfire followed by a large, long-duration flood 15 months later. The objectives were to (1) quantify the volumes, spatial patterns, and temporal changes over time of erosion and deposition over a nearly 4-year period, and (2) evaluate the extent to which these spatially and temporally explicit changes are correlated to precipitation metrics, burn severity, and morphologic variables. The volumetric changes were calculated from a differencing of DEMs for 50 m long segments of the channel network and associated valley bottoms. The results showed net sediment accumulation after the wildfire in the valley bottoms of both watersheds, with greater accumulations in the wider and flatter valley bottoms in the first 2 years after burning. In contrast, the mesoscale flood caused large amounts of erosion, with higher erosion in those areas with more post-fire deposition. Only minor changes occurred over the 2 years following the mesoscale flood. Volume changes for the different time periods were weakly but significantly correlated to, in order of decreasing correlation, contributing area, channel width, percent burned at high and/or moderate severity, channel slope, confinement ratio, maximum 30 min precipitation, and total precipitation. These results suggest that morphometric characteristics, when combined with burn severity and a specified storm, can indicate the relative likelihood and locations for post-fire erosion and deposition. This information can help assess downstream risks and prioritize areas for post-fire hillslope rehabilitation treatments.
Highlights
Wildfires alter hydrologic response by creating conditions that can lead to greatly increased runoff and erosion rates
The ash and sediment transported into the Cache la Poudre River after the High Park Fire greatly increased turbidities and suspended sediment concentrations (Writer et al, 2014), but our observations indicated that these sediment inputs generally did not alter the channel morphology of the main stem other than at a few tributary confluences, immediately behind a diversion dam, and much further downstream where the river suddenly emerges from the foothills into an unconfined valley bottom
We should believe that the difference in the amount of deposition and net change between T1 and T3 and T4 is much larger than what we have shown here (e.g., Fig. 5) primarily because the first airborne laser scanning (ALS) data were collected after the first summer when there were very large amounts of post-fire sediment deposited in the channels and valley bottoms of both watersheds (Brogan et al, 2019), and because of the poorer accuracy of the first ALS dataset
Summary
Wildfires alter hydrologic response by creating conditions that can lead to greatly increased runoff and erosion rates. At the hillslope scale these fireinduced changes increase a variety of erosional processes, including rainsplash, sheet flow, rilling, gullying, landslides, and debris flows (e.g., Benda and Dunne, 1997; Inbar et al, 1998; Cannon et al, 2001; Gabet and Dunne, 2003; Roering and Gerber, 2005; Wagenbrenner and Robichaud, 2014; Rengers et al, 2016b). As spatial scale increases channel erosion can become important (e.g., Meyer et al, 1992; Legleiter et al, 2003; Wagenbrenner and Robichaud, 2014), Published by Copernicus Publications on behalf of the European Geosciences Union. Brogan et al.: Sediment storage and erosion after fire and flood but at larger scales the predominant post-fire response is deposition, including alluvial fans, channel infilling, floodplain accretion, reservoir filling, and a sediment super slug (e.g., Moody and Martin, 2001; Reneau et al, 2007; Santi et al, 2008; Orem and Pelletier, 2015; Moody, 2017)
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