Publication Date

5-2019

Date of Final Oral Examination (Defense)

3-8-2019

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geoscience

Department

Geosciences

Supervisory Committee Chair

Jennifer Pierce, Ph.D.

Supervisory Committee Member

Nancy Glenn, Ph.D.

Supervisory Committee Member

Jaime R. Goode, Ph.D.

Abstract

In mountainous regions burned by wildfires, profound changes in soil characteristics and combustion of vegetation increase hillslope and channel erosion during storm events. Reduced infiltration and abundant loose sediment produce large post-fire erosional events which endanger human lives and infrastructure and contribute significantly to long-term erosion rates. While the influence of fire in increasing erosion has long been recognized, quantifying volumes and sources of eroded material from burned landscapes is difficult. Pre-erosion high-resolution topographic data (e.g. lidar) are often not available in burned areas and determining specific contributions from post-fire hillslope and channel erosion is challenging. Multiple erosional processes mobilize sediment from hillslopes, but the connectivity of hillslopes to channels controls the basin-wide erosional response.

We quantify an important spatial threshold separating hillslope and channel erosion processes in a catchment burned in the 2016 Pioneer Fire. Further, we confirm the impact of post-fire erosion on landscape evolution, demonstrate the applicability of Structure from Motion photogrammetry (SfM) to quantify post-fire erosion without detailed pre-erosion topography, and improve estimates of rill erosion at adequate spatial scales. In this rugged 0.95 km2 watershed in the weathered Idaho Batholith, 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 to derive 5 cm resolution topography of the channel scoured by debris flow. Lacking cm-resolution pre-erosion topography, we created a synthetic surface defined by the debris flow scour’s geomorphic signature and used a DEM of difference (DoD) to map and quantify channel erosion, finding 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%. Next, we delineated sub-basins within the larger study catchment to investigate the evolution of hillslope and channel erosion with varying contributing areas. We document that a drainage area of 20 ha (0.2 km2) represents the threshold from dominantly hillslope to dominantly channel erosion in this setting. Hillslopes contribute less to total erosion as drainage area increases, reflecting increased connectivity and efficiency of channel networks. Our experimental sub-basin results show a positive relationship between sediment yield (mass/area/time) and drainage area; contrary to most literature. The modern deposit volume was 5700 ± 1140 m3, indicating ~60% contribution from post-fire channel erosion. Our measured total eroded volume (4600 ± 740 m3) aligns closely with the preliminary assessment from the US Geological Survey (USGS) post-fire hazard model for similar, modest precipitation intensities.

Holocene alluvial stratigraphic sequences exposed by the 2016 debris flows show fire-related deposition dominates the stratigraphic record. 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. Comparisons of fire-related sediment yields from episodic events with Holocene sediment yields reconstructed from other studies in the region suggest episodic wildfire-driven erosion dominates millennial-scale erosion. Further investigations into spatial thresholds of post-fire erosion, hillslope-channel connectivity, and long-term landscape changes, especially when coupled with high resolution topography, will help to quantify the impacts of wildfire in other settings.

DOI

10.18122/td/1517/boisestate

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