Partitioned by process: Measuring post‐fire debris‐flow and rill erosion with Structure from Motion photogrammetry

Abstract

After wildfire, hillslope and channel erosion produce large amounts of sediment and can contribute significantly to long‐term erosion rates. However, pre‐erosion high‐resolution topographic data (e.g. lidar) is often not available and determining specific contributions from post‐fire hillslope and channel erosion is challenging. The impact of post‐fire erosion on landscape evolution is demonstrated with Structure from Motion (SfM) Multi‐View Stereo (MVS) photogrammetry in a 1 km2 Idaho Batholith catchment burned in the 2016 Pioneer Fire. We use SfM‐MVS to quantify post‐fire erosion without detailed pre‐erosion topography and hillslope transects to improve estimates of rill erosion at adequate spatial scales. Widespread rilling and channel erosion produced a runoff‐generated debris‐flow following modest precipitation in October 2016. We implemented unmanned aerial vehicle (UAV)‐based SfM‐MVS to derive a 5 cm resolution digital elevation model (DEM) of the channel scoured by debris‐flow. In the absence of cm‐resolution pre‐erosion topography, a synthetic surface was defined by the debris‐flow scour's geomorphic signature and we used a DEM of Difference (DoD) to map and quantify channel erosion. We found 3467 ± 422 m3 was eroded by debris‐flow scour. Rill dimensions along hillslope transects and Monte Carlo simulation show rilling eroded ~1100 m3 of sediment and define a volume uncertainty of 29%. The total eroded volume (4600 ± 740 m3) we measured in our study catchment is partitioned into 75% channel erosion and 25% rill erosion, reinforcing the importance of catchment size on erosion process‐dominance. The deposit volume from the 2016 event was 5700 ± 1140 m3, indicating ~60% contribution from post‐fire channel erosion. Dating of charcoal fragments preserved in stratigraphy at the catchment outlet, and reconstructions of prior deposit volumes provide a record of Holocene fire‐related debris‐flows at this site; results suggest that episodic wildfire‐driven erosion (~6 mm/year) dominate millennial‐scale erosion (~5 mm/Ka) at this site. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

The Evolution of Sediment Sources Over a Sequence of Postfire Sediment‐Laden Flows Revealed Through Repeat High‐Resolution Change Detection

“…These measurements are necessary to expand our knowledge of the mechanistic controls on postfire erosional responses considering the significant natural variability that exists in fire‐disturbed landscapes (Moody et al, 2013). Although inferred from previous studies (e.g., Kean et al, 2011; Wells, 1987), the role of decreasing sediment availability can now be explored more fully with the use of high‐resolution change detection techniques such as terrestrial laser scanning (herein referred to as TLS) (e.g., Schmidt et al, 2011; Staley et al, 2014) and unmanned aerial vehicle Structure‐from‐Motion (herein referred to as SfM) (Ellett et al, 2019). TLS methods are able to resolve surface changes down to less than a centimeter such as those associated with widespread hillslope erosion on burned hillslopes (e.g., DeLong et al, 2018; Staley et al, 2014), and UAV‐based SfM is able to resolve changes on the order of a decimeter (e.g., Barnhart et al, 2019; Ellett et al, 2019) that would be associated with the erosion of large rills, gullies, and channels.…”

Wildfire and earth surface processes

“…They showed how a runoff-generated debris flow can be triggered by low intensity precipitation, resulting in a large distributary fan. Importantly, this work also illustrates the contribution of post-fire erosion to long-term erosion rates Ellett et al (2019). documented a post-fire erosion rate of (6 mm yr À1 ), which is higher than the average erosion rate in the Idaho batholith region since the middle Holocene (5 mm yr À1 ).…”

“…Sediment was first transported into swales through aeolian processes and then subsequently eroded by cold season hydrologic processes (rain-on-snow, rain-on-frozen soils, and snowmelt). Ellett et al (2019) 2) the presence of permafrost. Generally, they found that organic carbon tends to decrease in confined valleys and increase in unconfined valleys.…”

“…Sediment was first transported into swales through aeolian processes and then subsequently eroded by cold season hydrologic processes (rain‐on‐snow, rain‐on‐frozen soils, and snowmelt). Ellett et al (2019) used Structure from Motion Multi‐View Stereo techniques to determine post‐fire rill and debris flow erosion. They showed how a runoff‐generated debris flow can be triggered by low intensity precipitation, resulting in a large distributary fan.…”

Partitioned by process: Measuring post‐fire debris‐flow and rill erosion with Structure from Motion photogrammetry

“…In Ellett et al (2019) page 3132, in the original published version, Figure 4 was incorrect and appears to be a duplicate of Figure 2. The figure caption is correctly presented but the corresponding figure was wrongly inserted.…”