Post-Fire Treatment Effectiveness for Hillslope Stabilization

This synthesis of post-fire treatment effectiveness reviews the past decade of research, monitoring, and product development related to post-fire hillslope emergency stabilization treatments, including erosion barriers, mulching, chemical soil treatments, and combinations of these treatments. In the past ten years, erosion barrier treatments (contour-felled logs and straw wattles) have declined in use and are now rarely applied as a post-fire hillslope treatment. In contrast, dry mulch treatments (agricultural straw, wood strands, wood shreds, etc.) have quickly gained acceptance as effective, though somewhat expensive, post-fire hillslope stabilization treatments and are frequently recommended when values-at-risk warrant protection. This change has been motivated by research that shows the proportion of exposed mineral soil (or conversely, the proportion of ground cover) to be the primary treatment factor controlling post-fire hillslope erosion. Erosion barrier treatments provide little ground cover and have been shown to be less effective than mulch, especially during short-duration, high intensity rainfall events. In addition, innovative options for producing and applying mulch materials have adapted these materials for use on large burned areas that are inaccessible by road. Although longer-term studies on mulch treatment effectiveness are on-going, early results and short-term studies have shown that dry mulches can be highly effective in reducing post-fire runoff and erosion. Hydromulches have been used after some fires, but they have been less effective than dry mulches in stabilizing burned hillslopes and generally decompose or degrade within a year

Continued evaluation of post-fire recovery and treatment effectiveness for validation of the ERMiT erosion model

The use and cost of post-fire emergency stabilization treatments continues to grow. To help maximize the impact of these treatments, many assessment teams use the Erosion Risk Management Tool (ERMiT) erosion model to predict postfire erosion and mitigation effects. However, despite several completed JFSP projects, the long-term effects of these treatments remain unknown, and the ERMiT model has not been validated. Long-term post-fire erosion and runoff data on a variety of mulches and erosion barriers were collected using 12 existing sites throughout the Western U.S. The agricultural straw and wood strand mulch treatments were very effective at reducing erosion and runoff. The contour-felled log treatment was effective at reducing runoff and erosion for small storms, but was not effective for larger events. The hydromulch formulations tested in this study were not effective at reducing runoff or sediment yields. Numerous presentations, field trips, and Burned Area Emergency Response (BAER) trainings were conducted. These activities provided much-needed information about the effectiveness of stabilization treatments

Evaluating Post-fire Salvage Logging Effects on Erosion

Legal challenges have delayed numerous post-fire salvage logging operations, which often results in lost economic value of the burned timber and unrecovered legal expenses. The scientific literature has shed little light on the additive effect of salvage logging operations on post-fire runoff, erosion, flooding, and sedimentation. Hence, there is an urgent need to better understand the impacts of post-fire salvage operations so that land managers can evaluate the relative and cumulative effects of different salvage logging practices. Intensive, multi-scale studies are needed because the effects of post-fire logging are superimposed on the effect of wildfires; rates and processes change according to the spatial and temporal scales of the investigation; and the studies to date indicate tremendous variability in the effects of post-fire salvage logging with the type and extent of the logging, site characteristics, and climatic conditions. To address this need, the current project was established in the Northern Rockies to integrate experiments at the hillslope and small watershed scale that focus on erosion processes. Replicated plots were used to measure sediment production rates from burned and unlogged plots, logged areas, tracked areas due to ground-based logging, and tracked areas with added slash as an erosion control treatment. Measured erosion rates were related to detailed measurements of site characteristics including ground cover, rilling, water repellency, amount of area disturbed due to salvage logging operations, number of passes of logging equipment, soil compaction, and the number and type of erosion mitigation practices (e.g., application of logging slash, mulch, and water bars). Runoff and sediment yield data were collected from two pairs of small watersheds to determine how salvage logging affects runoff, peak flow, and erosion rates, and whether the erosion estimates from the hillslope plots can be extrapolated to the small watershed scale. Rill simulation studies were conducted on three sites affected by ground-based salvage logging to evaluate the various types of equipment and identify site factors that affect runoff and erosion rates

Evaluating the Effectiveness of Wood Shreds on Post-fire Erosion

Agricultural straw mulching is a commonly used post-fire hillslope erosion control treatment that is aerially applied by helicopter. While widely used and reasonably effective at reducing erosion, agricultural straw is not native to the forest environment. There is a growing consensus among Burned Area Emergency Response (BAER) teams that mulch made from native forest material would be preferable to agricultural straw. Wood shred mulch made from post-fire road hazard trees is an alternative to agricultural straw. An optimized blend of sizes of wood shreds was effective in reducing sediment yields in both indoor rainfall simulation and outdoor field experiments. Several post-wildfire field experiments showed that wood shreds and agricultural straw were effective in reducing sediment yields as compared to the controls but neither treatment had an effect on runoff. Erosion reductions from wood shred treatments ranged from 50-96% in these experiments, and the presence and effectiveness of wood shreds appears to outlast both agricultural straw and hydromulch. Wood shreds are denser than agricultural straw and, as a consequence, about 4 times more wood shreds (by weight) than straw are needed to provide the same ground cover in a designated area. As a result, a helicopter with cargo nets required about four to five times as many round trips to treat an acre with wood shreds as with agricultural straw. This made wood shred application take longer and cost more than agricultural straw ($1,500 to$2,000 per acre [$3,750 to$5,000 per ha] and $500 per acre [$1,250 per ha], respectively). Field tests using a Heli-Claw, an alternative to a cargo net for heli-mulching, suggest that the Heli-Claw is a viable option for the aerial application of wood shreds. Results from these studies were disseminated through publications and a wide range of presentations, such as webinars, national meetings, and regional specialists meetings; thus, research findings have been directly conveyed to BAER teams and land managers. [Note: Throughout this report customary (English) units are stated first and metric equivalents are parenthetical where appropriate. The use of the symbol “t” is for ton (2000 lbs) in the customary system and the symbol “t” is for tonne (1000 kg [~2200 lbs]) in the metric system.

Evaluating Post-fire Salvage Logging Effects on Erosion

Legal challenges have delayed numerous post-fire salvage logging operations, which often results in lost economic value of the burned timber and unrecovered legal expenses. The scientific literature has shed little light on the additive effect of salvage logging operations on post-fire runoff, erosion, flooding, and sedimentation. Hence, there is an urgent need to better understand the impacts of post-fire salvage operations so that land managers can evaluate the relative and cumulative effects of different salvage logging practices. Intensive, multi-scale studies are needed because the effects of post-fire logging are superimposed on the effect of wildfires; rates and processes change according to the spatial and temporal scales of the investigation; and the studies to date indicate tremendous variability in the effects of post-fire salvage logging with the type and extent of the logging, site characteristics, and climatic conditions

Infiltration and interrill erosion rates after a wildfire in western Montana, USA

© 2015. The 2000 Valley Complex wildfire burned in steep montane forests with ash cap soils in western Montana, USA. The effects of high soil burn severity on forest soil hydrologic function were examined using rainfall simulations (100 mm h-1 for 1 h) on 0.5-m2 plots. Infiltration rates, sediment yields and sediment concentrations were compared among three treatments: control (unburned and undisturbed); bare (unburned with all surface vegetation, litter, and duff removed prior to each simulation); and burned. Rainfall simulations were done immediately after the fire and repeated in 2001, 2002, and 2005. Soil moisture, water repellency, and understory canopy and ground cover were measured and related to infiltration rates and sediment yields. The unburned forest soil was water repellent at the mineral surface. This surface repellency was no longer detected after it was burned at high severity, but a post-fire water repellent soil layer was observed at 1-2 cm below the surface. The control plots had high ground cover (90% overall), infiltration of 44-48 mm, and very low sediment concentrations (median values of 0.1-0.6 g L-1) and sediment yields (6-54 g m-2) for all years despite changes in soil moisture and strong water repellency. The bare and control plots had similar water repellency values, but the interrill erosion in the bare plots was high throughout the study (624-1277 g m-2). In the year of the fire, the burned sites had high rates of soil water repellency (88%) and little ground cover (10%). This resulted in low infiltration rates (30 mm), high sediment concentrations (median value 21 g L-1), and high sediment yields (1157 g m-2). By 2005, the fire-altered water repellency decreased in occurrence (48%) and severity, and the ground cover increased (42%). This resulted in much greater infiltration (84 mm), lower sediment concentration (median value 0.5 g L-1), and lower sediment yields (15 g m-2) on the burned plots. The importance of ground cover for preventing interrill erosion was demonstrated by the very low sediment yields on the control plots as compared to the bare and burned plots. The strength and occurrence of water repellency in both the unburned and burned sites decreased as soil moisture increased; however, strong soil water repellency was detected at the soil surface whenever unburned soils were dry. Fire-altered soil water repellency influenced the infiltration capacity and increased runoff rates immediately after the fire; however, the loss of protective ground cover was a more significant factor for the increased sediment concentrations and sediment yields