Quantifying sources of fine sediment supplied to post-fire debris flows using fallout radionuclide tracers
Highlights
► We quantify sources of fine sediment supplied to post-fire debris flow deposits. ► Fallout radionuclides (137Cs, 210Pbex, 239,240Pu) were used to trace sediment sources. ► Hillslope source inputs to deposits were 22–69% and 32–74% for two catchments. ► Variability in source contributions was related to sequences of debris flow surges.
Introduction
Debris flows can occur following wildfire in steep forest environments and result in severe erosion and downstream sediment redistribution (Wohl and Pearthree, 1991, Meyer and Wells, 1997, Cannon, 2001). Post-fire debris flows may be characterised according to their primary initiation mechanism. Elevated surface runoff from recently burned hillslopes may progressively entrain material until debris flow conditions are reached either on hillslopes or within channels (Cannon et al., 2001a, Cannon et al., 2001b, Gabet and Bookter, 2008). In contrast, soil saturation and loss of tree root cohesive strength after fire may result in shallow landslides that deliver large quantities of material downstream as a debris flow (Swanson, 1981, Benda and Dunne, 1997, Meyer et al., 2001, Wondzell and King, 2003). In some instances post-fire debris flows may be generated by a combination of these processes (Cannon and Gartner, 2005). Debris flows can scour channels to bedrock and form extensive coarse material deposits behind log-jams and along channel sections with reduced gradients (Meyer and Wells, 1997, Santi et al., 2008). Coarse terminal fan deposits may form at stream junctions causing flow redirection and channel adjustments in trunk streams (May and Gresswell, 2004). Substantial quantities of fine sediment and ash may also be delivered to streams and impact on water quality (Smith et al., 2011a).
Previous research on post-fire debris flows has been largely concentrated in the western United States and Canada, while the occurrence of debris flows following fire has also been reported in the Swiss Alps (Conedera et al., 2003) and in the forested uplands of southeastern Australia (Nyman et al., 2011). In North America, investigations of post-fire debris flow occurrence initially focused on documenting events, with particular attention on debris flow generation processes as well as analysis and interpretation of past debris flow deposits (Wells, 1987, Wohl and Pearthree, 1991, Spittler, 1995, Meyer and Wells, 1997, Cannon et al., 1998, Cannon and Reneau, 2000, Cannon, 2001, Cannon et al., 2001a, Meyer et al., 2001, VanDine et al., 2005). Subsequently, researchers have sought to quantify storm rainfall conditions and thresholds responsible for generating post-fire debris flows (Cannon et al., 2008) and develop empirical models relating the volume of material eroded to various landscape attributes as well as rainfall characteristics (Gabet and Bookter, 2008, Gartner et al., 2008, Cannon et al., 2010). The frequency of post-fire debris flows and their contribution to long-term erosion have also been the subject of both field and modelling based investigations (Meyer et al., 1992, Benda and Dunne, 1997, Istanbulluoglu et al., 2004, Pierce et al., 2004).
Progressive sediment entrainment by surface runoff has been identified as the dominant initiation process of most post-fire debris flows studied in the western United States (Cannon and Gartner, 2005). Likewise, in southeastern Australia, Nyman et al. (2011) reported the occurrence of multiple runoff-generated debris flows after fire in steep headwater catchments under dry eucalypt forest burned at high severity. Sources of material transported by runoff-generated debris flows have received some attention. Santi et al. (2008) presented an analysis of sources based on a survey of 46 debris flows in the western United States and found that channel erosion was the dominant source of material with rills making only a minor contribution (0.1–10.5%). Interrill erosion was not measured in this study. Nyman et al. (2011) measured both hillslope and channel erosion across three catchments affected by post-fire debris flows in southeastern Australia and found that hillslopes contributed 25–65% of the total estimated mass of eroded material.
The previous studies reporting sources of post-fire debris flow material are based on volumetric surveys that include the complete range of particle size fractions. However, quantifying hillslope and channel source contributions of fine sediment (clay and silt) to debris flows may also provide valuable insights. The supply of fine sediment and ash has been identified as an important factor contributing to the process of progressive sediment entrainment that is associated with the initiation of runoff-generated debris flows after fire (Meyer and Wells, 1997, Cannon et al., 2001b, Gabet and Sternberg, 2008, Gabet and Bookter, 2011). More generally, fine sediment is important for producing debris flow conditions through its contribution to shear strength via cohesion in clays and inter-particle friction (Costa, 1988). Therefore, identification of the sources of fine sediment transported by debris flows can offer a means to better understand debris flow generation after fire. Furthermore, reconstructing temporal sequences of source contributions to debris flows may be possible by combining knowledge of source inputs to individual debris flow deposits with analysis of the spatial arrangement of deposits within channels.
Sources of fine sediment transported by post-fire debris flows may be quantified using fallout radionuclides as sediment tracers. Caesium-137 (137Cs) and excess lead-210 (210Pbex) have been widely applied as tracers to determine the relative hillslope surface and channel bank source contributions to sediment stored and transported in river networks (e.g. Wallbrink et al., 1998, Wallbrink et al., 2003, Walling et al., 1999, Walling et al., 2008, Russell et al., 2001, Collins and Walling, 2002, Whiting et al., 2005, Smith and Dragovich, 2008). Recently, plutonium-239 and 240 (239,240Pu) have been investigated as potential sediment tracers, with 239Pu successfully employed to trace sediment sources in a river basin in Queensland, Australia (Everett et al., 2008, Tims et al., 2010). We propose to use 137Cs, 210Pbex and 239,240Pu as tracers to quantify the proportional contributions of fine sediment from hillslope surface and channel bank sources to material stored in channel deposits formed by debris flows. The study focuses on two small forest catchments that experienced runoff-generated debris flows following wildfires in southeastern Australia. While there has been limited application of fallout radionuclides to trace sediment sources in catchments after wildfire (Blake et al., 2009, Wilkinson et al., 2009, Smith et al., 2011b), the present study represents the first application of these tracers to investigate fine sediment sources of post-fire debris flows. In addition to the sediment tracing analysis, we also date past debris flow deposits found in one of the catchments. This leads to consideration of the possible contribution made by such high magnitude erosion events after fire to long-term erosion and soil profile change based on available data from the region.
Section snippets
Study area
The study sites are located in the forested uplands of central and northeastern Victoria, Australia (Fig. 1a). Sampling focused on two small catchments situated within Myrtle and Sunday Creeks, which are located in the Goulburn and Ovens River basins, respectively. The study sites were included in a catalogue of high magnitude erosion events that occurred after wildfire in the eastern uplands of Victoria and were classified as runoff-generated debris flows on the basis of field evidence of
Fallout radionuclides as sediment tracers
Most sediment tracing studies using fallout radionuclides are situated in agricultural catchments with comparatively few focusing on forest environments or forest areas affected by wildfire (Wallbrink et al., 2002, Motha et al., 2003, Blake et al., 2009, Wilkinson et al., 2009, Smith et al., 2011b). Despite this, fallout radionuclides presently provide the best available tool for quantifying proportional source contributions to sediment transported through burned forest catchments. Other
137Cs and 210Pbex concentrations in sources and channel deposits
Concentrations of 137Cs and 210Pbex in hillslope surface deposits substantially exceed those in channel banks in both the Myrtle and Sunday Creek study catchments (Table 1). Hillslope deposits in Myrtle Creek had mean concentrations of 137Cs and 210Pbex that were 17 and 149 times those in the channel banks, respectively, while in Sunday Creek hillslope deposit concentrations were 11 (137Cs) and 46 (210Pbex) times the levels measured in bank samples. Concentrations of both radionuclides in
Conclusion
This study employed the fallout radionuclides 137Cs, 210Pbex and 239,240Pu as tracers to quantify proportional contributions of fine sediment from hillslope surface and channel bank sources to channel deposits formed by post-fire debris flows. It represents the first application of fallout radionuclide tracers to examine sediment source contributions to post-fire runoff-generated debris flows. The estimated hillslope source inputs to individual deposits in the Myrtle Creek study catchment
Acknowledgements
This project was supported by a University of Melbourne Early Career Researcher Grant (2010) awarded to the first author and Melbourne Water. An Australian Institute of Nuclear Science and Engineering (AINSE) Award (ALNGRA10094P), also held by the first author, provided funding for AMS analysis. The authors would like to acknowledge the contribution of Gabrielle Josling for field support, Gabi Szegedy for sample preparation, and Chris Leslie from the CSIRO Black Mountain Laboratory for gamma
References (87)
- et al.
Deriving hillslope sediment budgets in wildfire-affected forests using fallout radionuclide tracers
Geomorphology
(2009) - et al.
Wildfire-related debris-flow initiation processes, Storm King Mountain, Colorado
Geomorphology
(2001) - et al.
Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern California
Geomorphology
(2008) - et al.
Selecting fingerprint properties for discriminating potential suspended sediment sources in river basins
Journal of Hydrology
(2002) - et al.
Source type ascription for fluvial suspended sediment based on a quantitative composite fingerprinting technique
Catena
(1997) - et al.
Concentration and characterization of plutonium in soils of Hubei in central China
Journal of Environmental Radioactivity
(2010) - et al.
Comparison of Pu and 137Cs as tracers of soil and sediment transport in a terrestrial environment
Journal of Environmental Radioactivity
(2008) - et al.
The longevity of hillslope soil in SE and NW Australia
Catena
(2010) - et al.
The ANTARES AMS facility at ANSTO
Nuclear Instruments and Methods in Physics Research B
(2004) - et al.
A morphometric analysis of gullies scoured by post-fire progressively bulked debris flows in southwest Montana, USA
Geomorphology
(2008)
The effects of vegetative ash on infiltration capacity, sediment transport, and the generation of progressively bulked debris flows
Geomorphology
Empirical models to predict the volumes of debris flows generated by recently burned basins in the western U.S
Geomorphology
Interpreting particle size effects in the adsorption of 137Cs and unsupported 210Pb by mineral soils and sediments
Journal of Environmental Radioactivity
Late Quaternary erosion in southeastern Australia: a field example using cosmogenic nuclides
Quaternary International
Radiocesium and plutonium: still together in “background” soils after more than thirty years
Chemosphere
Using fallout plutonium as a probe for erosion assessment
Journal of Environmental Radioactivity
Actinides AMS for nuclear safeguards and related applications
Nuclear Instruments and Methods in Physics Research B
Radiocarbon: a chronological tool for the recent past
Quaternary Geochronology
Distribution and characteristics of 239,240Pu and 137Cs in the soil of Korea
Journal of Environmental Radioactivity
Chemical associations of artificial radionuclides in Cumbrian soils
Journal of Environmental Radioactivity
Spatial and temporal patterns of debris-flow deposition in the Oregon Coast Range, USA
Geomorphology
Contrasting rainfall generated debris flows from adjacent watersheds at Forest Falls, southern California, USA
Geomorphology
Evidence of debris flow occurrence after wildfire in upland catchments of south-east Australia
Geomorphology
Fire as an agent in redistributing fallout Cs-137 in the Canadian boreal forest
The Science of the Total Environment
Suspended sediment sources in two small lowland agricultural catchments in the UK
Journal of Hydrology
Sources of debris flow material in burned areas
Geomorphology
Improving precision in sediment source and erosion process distinction in an upland catchment, south-eastern Australia
Catena
Wildfire effects on water quality in forest catchments: a review with implications for water supply
Journal of Hydrology
Measurements of Pu and Ra isotopes in soils and sediments by AMS
Nuclear Instruments and Methods in Physics Research B
Plutonium as a tracer of soil and sediment movement in the Herbert River, Australia
Nuclear Instruments and Methods in Physics Research B
Soil erosion rates on forested mountain hillslopes estimated using 137Cs and 210Pbex
Geoderma
A tracer budget quantifying soil redistribution on hillslopes after forest harvesting
Catena
Quantifying the contributions of sediment, sediment-P and fertiliser-P from forested, cultivated and pasture areas at the landuse and catchment scale using fallout radionuclides and geochemistry
Soil and Tillage Research
Tracing suspended sediment and particulate phosphorous sources in catchments
Journal of Hydrology
Fallout radionuclide tracers identify a switch in sediment sources and transport-limited sediment yield following wildfire in a eucalypt forest
Geomorphology
Debris flows as geomorphic agents in the Huachuca Mountains of southeastern Arizona
Geomorphology
Postfire erosional processes in the Pacific Northwest and Rocky Mountain regions
Forest Ecology and Management
A fast switching electrostatic deflector system for actinide isotopic ratio measurements
Nuclear Instruments and Methods in Physics Research B
Stochastic forcing of sediment supply to channel networks from landsliding and debris flow
Water Resources Research
Magnetic enhancement in wildfire-affected soil and its potential for sediment-source ascription
Earth Surface Processes and Landforms
Bayesian analysis of radiocarbon dates
Radiocarbon
Debris-flow generation from recently burned watersheds
Environmental and Engineering Geoscience
Wildfire-related debris flow from a hazards perspective
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2019, GeomorphologyCitation Excerpt :One only used fine particles to circumvent the selection effects to focus on the source contributions of finer and the other used a sediment size correction factor (Collins et al., 2012). The fine size fractions often utilized were <0.01 mm (Smith et al., 2012; Wilkinson et al., 2013; Olley et al., 2013; Laceby and Olley, 2015), <0.053 mm (Fox and Papanicolaou, 2007; Zhang et al., 2016b), and <0.063 mm (Collins et al., 2010a; Zhou et al., 2016; Vale et al., 2016; Zhao et al., 2017; Koiter et al., 2018); and sometimes group particle sizes were used (Haddadchi et al., 2015). It was noted that which particle size to use should be based on sampled sediments (Laceby et al., 2017).