Predicting sediment delivery from debris flows after wildfire
Introduction
Debris flows after fire can be a major process by which sediment erodes from hillslopes and headwaters (Wohl and Pearthree, 1991, Meyer et al., 2001, Cannon et al., 2003, Gabet and Bookter, 2008, Kean et al., 2011, Nyman et al., 2011, García-Ruiz et al., 2013, Jordan, 2015). Their frequency and magnitude are therefore important for landscape evolution and sediment availability in upland rivers and streams (Rutherfurd et al., 1994, Gomi et al., 2002, Benda et al., 2003, Miller et al., 2003). Debris flows can also pose a considerable threat to water supply systems (White et al., 2006, Smith et al., 2011a) and other assets (Cannon and Gartner, 2005, Lyon and O'Connor, 2008). Models for predicting sediment delivery and hazards from post-fire debris flows are therefore needed. Currently the major limitation in the development of such models is the lack of fundamental data on event magnitude and the susceptibility of different landscapes to erosion by debris flows. Lack of data also means that the longer-term implications of these high magnitude events remain poorly understood.
Wildfire-related debris flows can be triggered by slope failure due to reduced root cohesion (Benda and Dunne, 1997, Wondzell and King, 2003) or they can be runoff-generated and triggered by overland flow and sediment bulking (Meyer and Wells, 1997, Cannon et al., 2003). Wildfire contributes to runoff-generated (or progressively bulked) debris flows by reducing infiltration and increasing sediment availability on hillslopes (Wells, 1987, Cannon et al., 2001a, Gabet and Sternberg, 2008, Nyman et al., 2013, Nyman et al., 2014a). The processes causing runoff-generated debris flows include a combination of i) runoff production (Cannon et al., 2001a, Kean et al., 2011), ii) rill erosion (Cannon et al., 2001a), iii) thin hillslope failures (Gabet, 2003) and iv) channel incision in converging zero-order headwaters (Cannon et al., 2001b, Gabet and Bookter, 2008). The persistence of debris flow processes in downstream channels depends on local slope and channel configuration as well as changes in flow rheology due to runoff accumulation, sediment entrainment and deposition within channels (Iverson, 1997, Cannon et al., 2003, Pelletier and Orem, 2014, Kean et al., 2013, Staley et al., 2014, Gartner et al., 2015).
Attributes related to infiltration are an important source of variation in debris flow susceptibility in burned areas (Wells, 1987, Cannon et al., 2001b, Jordan et al., 2004, Nyman et al., 2011). Infiltration is affected by burn severity (Moody et al., 2015) but can also vary with intrinsic catchment attributes related to climate, vegetation and soils in the burned area (Jenny, 1994, Wondzell and King, 2003, Nyman et al., 2014a). Aridity (or the balance between net radiation and precipitation) in particular is emerging as an important source of variation in hillslope processes at the local and regional scales (Wondzell and King, 2003, Ebel, 2013, Sheridan et al., 2015). After a fire the runoff response will also depend on the intensity and duration of rainfall (Cannon et al., 2011, Kean et al., 2012, Staley et al., 2012), which can be spatially variable during the vulnerable post-fire period. The role of burn severity, intrinsic landscape attributes, and rainfall patterns in controlling runoff production and debris flow response from burned areas is further complicated by variation in the scale (or drainage area) within which the debris flow processes operate. In low relief terrain for instance, the debris flows are confined to relatively small drainage areas with terminal deposits located in 1st or 2nd order drainage lines (e.g. 0.01–0.1 km2). In high relief terrain the debris flows persist into 3rd and 4th order drainages and thus operating within a much larger spatial domain (e.g. 0.1–10 km2).
The physical processes contributing to the initiation and propagation of runoff generated debris flows are complex and difficult to model. Using a mechanistic modelling approach Kean et al. (2013) showed that the initiation, frequency and magnitude of debris flow surges are regulated by channel slope with low gradient sections acting as ‘sediment capacitors’ temporarily storing eroded sediment before its released as mass failure due to the downstream forces exceeding the resisting forces. This process is sensitive to runoff production, sediment availability, grain-size and topographic attributes. The parameters needed to model these processes are rarely available for large areas, and for practical purposes the debris flow magnitude and susceptibility are often modelled though empirical analysis of inventories (Van Den Eeckhaut et al., 2006, Blahut et al., 2010, Cannon et al., 2010). Statistical models of debris flow initiation and magnitude have been developed specifically for burned areas to help mitigate post-wildfire risks (Gartner et al., 2008, Gartner et al., 2014, Pak and Lee, 2008, Cannon et al., 2010). These models use information on catchment attributes and rainfall to produce estimates of debris flow probability and magnitude at the catchment outlet where infrastructure, homes and people may be at risk of direct impact from the debris flow.
In southeast Australia, debris flows in burned areas were only recently recognised as an important erosion process (Nyman et al., 2011, Smith et al., 2012). The lack of data on post-fire debris flows in this region mean that their magnitude, sediment sources and the factors contributing to variability in susceptibility remain poorly understood. In this study the overall goal is therefore to collect the data needed for developing models of sediment delivery from debris flows. The study takes a landscape-scale approach towards predicting debris flow probability while seeking to explicitly represent the distribution of erosion and deposition within debris flow producing catchments. The study was designed with three main objectives:
- 1.
Use field surveys of post-fire debris flows to quantify magnitude of erosion, the sediment sources and the triggering storm conditions
- 2.
Analyse erosion and deposition rates with respect to slope and drainage area
- 3.
Develop a model of debris flow susceptibility for large burned areas using data on burn severity, landscape attributes (e.g. slope and aridity) and rainfall intensity that are available at regional scales
Section snippets
Study area
The study is carried out in the eastern uplands of Victoria, southeast Australia (Fig. 1). The region forms part of the Great Dividing Range, and is described by Jenkins (1991) as a belt of “ridges, plateaus and corridors”. Elevation ranges from 200 m above sea level (a.s.l.) in foothills to 2000 m a.s.l. in the alpine regions along ridges and on upland plateaus. A large majority of the bedrock is of Palaeozoic origin and consists of marine sedimentary rocks (mudstone, shales and sandstone). The
Post-fire debris flows: rainfall intensity and sediment yield
Erosion surveys of debris flows were carried out in 10 catchments at seven sites (Fig. 1, Table 1). These sites were selected because i) information on the date of the debris flow response was available from catchment management authorities and/or local landowners, ii) the debris flows could be readily accessed, and iii) rainfall data (usually rainfall totals) were available from nearby rain gauges. Hillslope and channel erosion measurements at Rose River 1, Yarrarabula 2 and Germantown 1 (data
Post-fire debris flows: rainfall intensity and sediment yield
Rainfall totals for debris flow triggering storms ranged from 16 to 34 mm (Fig 4). The average 30-min (I30) and 15-min (I15) rainfall intensity of debris flow triggering rainfall events ranged from 17 to 60 mm h− 1 and 35–85 mm h− 1, respectively, corresponding to average return intervals (ARI) between 2 and 10 years. The range of 15-min peak intensities were higher than the minimum intensities reported by Kean et al. (2011) and Staley et al. (2012) for post-fire debris flows in the San Gabriel
Conclusions
The study set out to quantify sediment yields from post-fire debris flows in southeast Australian highlands and model the effects of landscape attributes on debris flow susceptibility. We found that post-fire debris flows produce sediment yields of 113–294 t ha− 1 which are 2–3 orders of magnitude higher than background erosion rates in undisturbed forests. Sediment yields could be modelled along the drainage network by expressing erosion as a function of local slope and contributing area and
Acknowledgments
The research was funded by the Bushfire Cooperative Research Centre for the Victorian Department of Environment, Land, Water and Planning and the Melbourne Water Corporation. We are also thankful to Philip Noske, Matt Richardson and Rene van der Sant for support in the field. Comments from two anonymous reviewers and the editor helped improve the manuscript.
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