Post-wildfire Erosion Research Articles

A Case Study and Numerical Modeling of Post-Wildfire Debris Flows in Montecito, California

Wildfires and their long-term impacts on the environment have become a major concern in the last few decades, in which climate change and enhanced anthropogenic activities have gradually led to increasingly frequent events of such hazards or disasters. Geological materials appear to become more vulnerable to hazards including erosion, floods, landslides and debris flows. In the present study, the well-known 2017 wildfire and subsequent 2018 debris flows in the Montecito area of California are examined. It is found that the post-wildfire debris flows were initiated from erosion and entrainment processes and triggered by intense rainfall. The significant debris deposition in four major creeks in this area is investigated. Numerical modeling of the post-wildfire debris flows is performed by employing a multi-phase mass flow model to simulate the growth in the debris flows and eventual debris deposition. The debris-flow-affected areas estimated from the numerical simulations fairly represent those observed in the field. Overall, the simulated debris deposits are within 7% error of those estimated based on field observations. A similar simulation of the pre-wildfire scenario indicates that the debris would be much less significant. The present study shows that proper numerical simulations can be a promising tool for estimating post-wildfire erosion and the debris-affected areas for hazard assessment and mitigation.

Post-wildfire erosion rates and triggering of debris flows: A case study in Susa Valley (Bussoleno)

Post-wildfires geological hazards are an emerging problem in many places, including areas not typically associated with these events such as the Alpine Region. Hazards connected with post-fire processes such as debris-flows and flood-type events threatens people, infrastructures, services and economical activities. Apart from a few examples, there is a lack of models available to quantify the increase in susceptibility as a result of the modification induced by the wildfires. In this work we test the application of a modified version of the RUSLE, on GIS, to quantify the post-fire erosive phenomena for a case study in the north-western Italian Alps. The results of its application, taking advantage of high-resolution rainfall series and data deriving from field surveys, highlight the marked increase (more than 20 times) in erosion rates, quantified by expressing both the EI (erodibility index), the A (monthly soil loss) and the SL (monthly sediment loss) rise. The months of April, May and June represents the larger share of the total quantities. This is a consequence of the noticeable increase of the Erodibility Index EI, which for the post-fire scenario is more than one order of magnitude higher than the pre-fire one.

Probability and Consequence of Postfire Erosion for Treatability of Water in an Unfiltered Supply System

AbstractForested catchments are critical to water supply in major cities. Many of these catchments face the threat of postwildfire erosion, which can contaminate reservoir water. The aim of this paper is to determine the probability and duration of disruptions to treatability due to runoff‐generated debris flows in the first year after a wildfire, before substantial vegetation recovery takes place. We combine models of reservoir hydrodynamics, postfire erosion, and stochastic rainfall to determine probability and magnitude of sediment concentration at the reservoir water offtake. Central to the paper is our technique for linking model components into a risk framework that gives probabilities to the number of days that the turbidity threshold for treatment is exceeded. The model is applied to the Upper Yarra reservoir, which is the linchpin of the water supply system for Melbourne in SE Australia. However, the framework is applicable to other unfiltered water supply systems where suspended sediment is a risk to treatability. Results show that postwildfire erosion poses a substantial threat, with a relatively high probability (annual exceedance probability = 0.1–0.3) of water being untreatable for >1 year following a high‐severity wildfire. Important factors that influence the risk include postwildfire runoff potential, reservoir temperature, and the amount of clay‐sized grains in eroding headwaters. Assumptions about spatial‐temporal rainfall attributes, reservoir hydrodynamics, and the catchment erosion potential are all important sources of error in our estimate of risk. Our approach to risk quantification will help support planning, risk management, and strategic investment to mitigate impacts.

Assessment of post-wildfire erosion risk and effects on water quality in south-western Australia

Investigations of wildfire impact on water resources have escalated globally over the last decade owing to an awareness of climate-related vulnerabilities. Within Australia, research into post-wildfire erosion has focused on water supply catchments in the south-eastern region. Here, we examine post-wildfire erosion risk and its potential for water quality impacts in a catchment in south-western Australia. The catchment of the Harvey River, which drains from forested escarpments onto an agricultural coastal plain and into valuable coastal wetlands, was burnt by wildfire in 2016. The aims of this study were to determine erosion risk across contrasting landforms and variable fire severity, using the Revised Universal Soil Loss Equation (RUSLE), and to determine whether post-fire water quality impacts could be detected at permanent river monitoring stations located on the coastal plain. RUSLE outputs showed erosion hot-spots at intersections of steep terrain and high fire severity and that these areas were confined to forested headwaters and coastal dunes. Monthly water quality data showed conspicuous seasonal patterns, but that sampling frequency was temporally too coarse to pick up predicted event-related effects, particularly given that the pre-existing monitoring sites were distal to the predicted zone of contamination.

Post-wildfire Erosion in Mountainous Terrain Leads to Rapid and Major Redistribution of Soil Organic Carbon

Catchments impacted by wildfire typically experience elevated rates of post-fire erosion and formation and deposition of pyrogenic carbon (PyC). To better understand the role of erosion in post-fire soil carbon dynamics, we determined distribution of soil organic carbon in different chemical fractions before and after the Gondola fire in South Lake Tahoe, CA. We analyzed soil samples from eroding and depositional landform positions in control and burned plots pre- and post-wildfire (in 2002, 2003, and 10-years post-fire in 2013). We determined elemental concentrations, stable isotope compositions, and biochemical composition of organic matter (OM) using mid-infrared (MIR) spectroscopy for all of the samples. A subset of samples was analyzed by 13C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy (CPMAS 13C-NMR). We combined the MIR and CPMAS 13C-NMR data in the Soil Carbon Research Program partial least squares regression model to predict distribution of soil carbon into three different fractions: 1) particulate, humic, and resistant organic matter fractions representing relatively fresh larger pieces of OM, 2) fine, decomposed OM, and 3) pyrogenic C, respectively. Samples from the post-fire eroding landform position showed no major difference in soil organic carbon (SOC) fractions one year post-fire. The depositional samples, however, had increased concentrations of all SOC fractions, particularly the fraction that resembles PyC, one year post-fire (2002), which had a mean of 160 g/kg compared with burned hillslope soils, which had 84 g/kg. The increase in all SOC fractions in the post-fire depositional landform position one year post-fire indicates significant lateral mobilization of the eroded PyC. In addition, our NMR analyses revealed a post-fire increase in both the aryl and O-aryl carbon compounds in the soils from the depositional landform position, indicating increases in soil PyC concentrations post-fire. After 10 years, the C concentration from all three fractions declined in the depositional landform position to below pre-fire levels likely due to further erosion or elevated rates of decomposition. Thus, we found, at this site, that both fire and erosion exert significant influence on the distribution of PyC throughout a landscape and its long-term fate in the soil system.

Post-wildfire landscape change and erosional processes from repeat terrestrial lidar in a steep headwater catchment, Chiricahua Mountains, Arizona, USA

Flooding and erosion after wildfires present increasing hazard as climate warms, semi-arid lands become drier, population increases, and the urban interface encroaches farther into wildlands. We quantify post-wildfire erosion in a steep, initially unchannelized, 7.5ha headwater catchment following the 2011 Horseshoe 2 Fire in the Chiricahua Mountains of southeastern Arizona. Using time-lapse cameras, rain gauges, and repeat surveys by terrestrial laser scanner, we quantify the response of a burned landscape to subsequent precipitation events. Repeat surveys provide detailed pre-and post-rainfall measurements of landscape form associated with a range of weather events. The first post-fire precipitation led to sediment delivery equivalent to 0.017m of erosion from hillslopes and 0.12m of erosion from colluvial hollows. Volumetrically, 69% of sediment yield was generated from hillslope erosion and 31% was generated from gully channel establishment in colluvial hollows. Processes on hillslopes included erosion by extensive shallow overland flow, formation of rills and gullies, and generation of sediment-laden flows and possibly debris flows. Subsequent smaller rain events caused ongoing hillslope erosion and local deposition and erosion in gullies. Winter freeze-thaw led to soil expansion, likely related to frost-heaving, causing a net centimeter-scale elevation increase across soil-mantled slopes. By characterizing landscape form, the properties of near-surface materials, and measuring both precipitation and landscape change, we can improve our empirical understanding of landscape response to environmental forcing. This detailed approach to studying landscape response to wildfires may be useful in the improvement of predictive models of flood, debris flow and sedimentation hazards used in post-wildfire response assessments and land management, and may help improve process-based models of landscape evolution.

Evaluating the effectiveness of agricultural mulches for reducing post-wildfire wind erosion

Abstract Post-wildfire soil erosion can be caused by water or aeolian processes, yet most erosion research has focused on predominantly water-driven erosion. This study investigates the effectiveness of three agricultural mulches, with and without a tackifier, on aeolian sediment transport processes. A wind tunnel was used to simulate post-wildfire wind erosion at three wind speeds (6, 11 and 18 m s−1). Shallow trays containing soil collected after a wildfire were treated with chopped rice, wheat or chopped wheat mulch; mulch treatments were also compounded with liquid treatments, tackifier to water ratios of 1:6, 1:3 and water. The mulch treatments were generally easily moved at all wind speeds with cover reductions greater than 90% at the highest wind speed. As expected, sediment loss was greatest for the bare soil treatment, ranging from 6.5 g m−2 at the lowest wind speed which increases to 6258 g m−2 at the highest wind speed. Adding wheat or chopped wheat mulch significantly reduced sediment loss by an order or magnitude (698 and 298 g m−2, respectively) at the highest wind speed. Adding chopped rice straw reduced sediment loss by a half to 3573 g m−2 at the highest wind speed, but the effect was not significant due to mobilization of the mulch. The most effective sediment loss mitigation was achieved with liquid tackifier treatments when applied to bare soil and when compounded with various mulch treatments, particularly at the highest wind speed. These results may aid management decisions when mitigating aeolian sediment transport after wildfires.

Effects of aridity in controlling the magnitude of runoff and erosion after wildfire

AbstractThis study represents a uniquely high‐resolution observation of postwildfire runoff and erosion from dry forested uplands of SE Australia. We monitored runoff and sediment load, and temporal changes in soil surface properties from two (0.2–0.3 ha) dry forested catchments burned during the 2009 Black Saturday wildfire. Event‐based surface runoff to rainfall ratios approached 0.45 during the first year postwildfire, compared to reported values <0.01 for less arid hillslopes. Extremely high runoff ratios in these dry forests were attributed to wildfire‐induced soil water repellency and inherently low hydraulic conductivity. Mean ponded hydraulic conductivity ranged from 3 to 29 mm h−1, much lower than values commonly reported for wetter forest. Annual sediment yields peaked at 10 t ha−1 during the first year before declining dramatically to background levels, suggesting high‐magnitude erosion processes may become limited by sediment availability on hillslopes. Small differences in aridity between equatorial and polar‐facing catchments produced substantial differences in surface runoff and erosion, most likely due to higher infiltration and surface roughness on polar‐facing slopes. In summary, the results show that postwildfire erosion processes in Eucalypt forests in south‐east Australia are highly variable and that distinctive response domains within the region exist between different forest types, therefore regional generalizations are problematic. The large differences in erosion processes with relatively small changes in aridity have large implications for predicting hydrologic‐driven geomorphic changes, land degradation, and water contamination through erosion after wildfire across the landscape.

Quantifying the time scale of elevated geomorphic response following wildfires using multi-temporal LiDAR data: An example from the Las Conchas fire, Jemez Mountains, New Mexico

Sediment yields commonly increase following wildfires in mountainous landscapes. Quantifying the magnitude and duration of elevated sediment yields following wildfires is important for quantifying the relative importance of wildfire-affected versus non-wildfire-affected erosion in the long-term evolution of landscapes and for understanding the processes of post-wildfire erosion. In this study we use bi-annual terrestrial laser scanning (TLS) surveys conducted over a 2-yr period together with repeat airborne laser scanning (ALS) surveys conducted over a 1-yr period to estimate the dependence of sediment yield on time following the high-severity Las Conchas fire of 2011 within two small (approximately 1.3km2) watersheds. The dependence of sediment yield on time since the wildfire was quantified using DEMs of Difference (DoDs) of the upland watersheds (to measure erosion) and of the adjacent piedmonts (to measure the deposition of eroded sediment). Over the 2-yr study period, sediment volume decreased linearly with time and sediment yield decreased exponentially with time in one watershed, while the other watershed showed a more complex relationship with time influenced by the specific sequence of rainstorms that impacted that watershed. Both watersheds had a recovery time of approximately 1yr, suggesting that vegetation regrowth and the stabilization of incised valley bottoms were primarily responsible for the decline in sediment yield versus time following the wildfire. Multi-temporal DoDs of the piedmonts show how they responded to the delivery of elevated sediment yields from the upland watersheds, i.e. by (1) incision of the proximal portion of the piedmont, triggering distal deposition, (2) infilling of the incised channel and avulsion, and (3) incision of the distal portion, creating an incised channel connecting the proximal and distal ends of the piedmont. The dynamic nature of incision/aggradation and channel shifting on piedmonts following wildfires has important implications for post-wildfire flood-hazard assessments.

Impacts of prescribed fire on soil loss and soil quality: An assessment based on an experimentally-burned catchment in central Portugal

Prescribed (controlled) fire has recently been adopted as an important wildfire-fighting strategy in the Mediterranean. Relatively little research, however, has assessed its impacts on soil erosion and soil quality. This paper investigates hillslope-scale losses of soil, organic matter and selected nutrients before and after a ‘worst-case scenario’ prescribed fire in a steep, shrub-vegetated catchment with thin stony soil in central Portugal. Comparison is made with soil erosion measured: (1) on a nearby hillslope burned by wildfire and monitored at the hillslope scale; and (2) on long-unburned terrain at small-plot, hillslope- and catchment-scales. Hillslope-scale pre- and post-fire soil erosion was recorded over periods of 6weeks to 5months for (1) 9.5months pre-fire and 27months post-fire in the prescribed fire catchment, and (2) c. 3years post-fire at the wildfire site. Organic matter content, pH, total N, K2O, P2O5, Ca2+ and Mg2+ were measured in the eroded sediment and in pre- and post-prescribed fire surface soil. Results indicate that: (1) both the prescribed fire and the wildfire caused expected marked increases in erosion compared with unburned terrain; and (2) the hillslope-scale post-prescribed fire soil losses (up to 2.41tha−1yr−1) exceeded many reported plot-scale post-prescribed fire and post-wildfire erosion rates in the Mediterranean. As a comparison, post-fire erosion for both fire types was less than that caused by some other forms of common soil disturbance (e.g. types of tillage) and even that on undisturbed shrubland in low rainfall areas of the region. Total estimated post-prescribed fire particulate losses of organic matter and nutrients represent only 0.2–2.9% of the content in the upper 2cm of soil, suggesting only a modest fire effect on soil quality, although this may reflect in part a lack of extreme rainfall events following the fire. The longer-term implications for soil conservation of repeated prescribed fire in the Mediterranean are explored and future research priorities identified.

Effectiveness of aerial seeding and straw mulch for reducing post-wildfire erosion, north-western Montana, USA

Various methods are available to reduce post-wildfire erosion, but there is limited quantitative information on the relative effectiveness of these techniques. We used rainfall simulations to compare the erosion and runoff rates from adjacent 0.5-m2 plots treated with aerial grass seeding and straw mulch with untreated control plots following the July 2002 Fox Creek Fire in north-west Montana. In the first summer after the fire, plots seeded at a rate of 9 kg ha–1 had a mean of less than 5% ground cover and the seeding treatment had no effect on the rainsplash erosion rate. In contrast, straw mulch application at a rate of 2.24 Mg ha–1 resulted in ~100% ground cover and an 87% reduction in rainsplash erosion relative to the control (P = 0.001). Measurements on a subset of the plots in the second summer after the fire indicated that ground cover in the treatments and the control averaged 39%, and neither treatment provided a significant increase in ground cover or reduction in erosion relative to the control. These results add to the growing weight of evidence that straw mulch application is highly effective in reducing erosion in the first year after fire, whereas grass seeding is often ineffective because of the limited increase in ground cover that it produces.

Post-wildfire erosion response in two geologic terrains in the western USA

Volumes of eroded sediment after wildfires vary substantially throughout different geologic terrains across the western United States. These volumes are difficult to compare because they represent the response to rainstorms and runoff with different characteristics. However, by measuring the erosion response as the erodibility efficiency of water to detach and transport sediment on hillslopes and in channels, the erosion response from different geologic terrains can be compared. Specifically, the erodibility efficiency is the percentage of the total available stream power expended to detach, remobilize, or transport a mass of sediment. Erodibility efficiencies were calculated for the (i) initial detachment, and for the (ii) remobilization and transport of sediment on the hillslopes and in the channels after wildfire in two different geological terrains. The initial detachment efficiencies for the main channel and tributary channel in the granitic terrain were 10 ± 9% and 5 ± 4% and were similar to those for the volcanic terrain, which were 5 ± 5% and 1 ± 1%. No initial detachment efficiency could be measured for the hillslopes in the granitic terrain because hillslope measurements were started after the first major rainstorm. The initial detachment efficiency in the volcanic terrain was 1.3 ± 0.41%. The average remobilization and transport efficiencies associated with flash floods in the channels also were similar in the granitic (0.18 ± 0.57%) and volcanic (0.11 ± 0.41%) terrains. On the hillslope the remobilization and transport efficiency was greater in the volcanic terrain (2.4%) than in the granitic terrain (0.65%). However, this may reflect the reduced sediment availability after the first major rainstorm (30-min maximum rainfall intensity ∼ 90 mm h − 1 ) in the granitic terrain, while easily erodible fine colluvium remained on the hillslope after the first rainstorm (30-min maximum rainfall intensity = 7.2 mm h − 1 ) in the volcanic terrain. The erosion response in channels and on hillslopes of the granitic and volcanic terrains was similar when compared using erodibility efficiencies.

Effects of post-wildfire erosion on channel environments, Boise River, Idaho

What is the geological or ecological context that earth scientists, biologists, and resource managers use to understand large-scale watershed disturbances, such as fires, mass wasting, and floods? We address this question using a field study of post-fire channel changes in the Boise River basin in central Idaho based on surveys of over 27 km of channels. Intense rill and gully erosion from the Rabbit Creek fire (1995) greatly increased sediment supply to numerous third- through sixth-order valley floors. We concentrated our field study where recently aggraded and enlarged alluvial fans impinged on channels in drainage areas of 100–350 km 2. Alluvial fans that had enlarged because of post-fire sedimentation triggered a number of morphological changes in channels and valley floors. Alluvial fans created nick points in receiving channels that caused an increase in channel gradient immediately downstream of fans and a decrease in channel gradients upstream of fans for distances up to 4 km. Wide floodplains, side channels, and the beginning of terrace construction were associated with increased sediment storage in proximity to aggraded fans. Fan-related changes in channel gradients also affected the spatial distribution of channel substrates. Studies across western North America indicate that periodic, large influxes of sediment to channels are a fundamental part of stream ecosystems. In addition, new perspectives in riverine ecology focus on the patchy distribution of aquatic habitats. Our study integrates those two perspectives by illustrating how fire-related sediment production coupled with irregularly spaced tributary junctions contributed to the formation of certain types of riverine habitats.

Soil Heating, Nitrogen, Cheatgrass, and Seedbed Microsites

Heat-induced changes in the soil-solution and post-wildfire erosion can create chemically and texturally diverse seedbed microsites. We quantified organic carbon, extractable NH4+ after incubation (aerobic and anaerobic), and emergence of cheatgrass (Bromus tectorum L.), by particle size fractions, in unburned and simulated burned sagebrush (Artemisia tridentata spp. tridentata Nutt.) subcanopy soil. For all particle size fractions, significantly (P less than or equal to -O.05) more extractable NH4+ and significantly less extractable N03- were measured in heated material as compared to unheated material. Heated treatments had significantly more NH4+ and significantly less N03- mineralized after 11 days aerobic incubation than after unheated treatments; net N mineralized tended to be higher for all particle fractions in heated treatments than in unheated treatments. Emergence of cheatgrass under aerobic conditions was significantly retarded in all heated treatments. Elevated NH4+ to N03- ratios in the soil-solution following heating does not explain suppression of cheatgrass emergence. Nitrogen mineralization, before and after simulated burning, is adequate in all particle size fractions to support the needs of germinating seeds. Nitrogen mineralization was not enhanced by the presence of growing cheatgrass plants.