Connectivity of post-fire runoff and sediment from nested hillslopes and watersheds

Wildfire increases the potential connectivity of runoff and sediment throughout watersheds due to greater bare soil, runoff and erosion as compared to pre-fire conditions. This research examines the connectivity of post-fire runoff and sediment from hillslopes (< 1.5 ha; n = 31) and catchments (< 1000 ha; n = 10) within two watersheds (< 1500 ha) burned by the 2012 High Park Fire in northcentral Colorado, USA. Our objectives were to: (1) identify sources and quantify magnitudes of post-fire runoff and erosion at nested hillslopes and watersheds for two rain storms with varied duration, intensity and antecedent precipitation; and (2) assess the factors affecting the magnitude and connectivity of runoff and sediment across spatial scales for these two rain storms. The two summer storms that are the focus of this research occurred during the third summer after burning. The first storm had low intensity rainfall over 11 hours (return interval <1–2 years), whereas the second event had high intensity rainfall over 1 hour (return interval <1–10 years). The lower intensity storm was preceded by high antecedent rainfall and led to low hillslope sediment yields and channel incision at most locations, whereas the high intensity storm led to infiltration-excess overland flow, high sediment yields, in-stream sediment deposition and channel substrate fining. For both storms, hillslope-to-stream sediment delivery ratios and area-normalised cross-sectional channel change increased with the percent of catchment that burned at high severity. For the high intensity storm, hillslope-to-stream sediment delivery ratios decreased with unconfined channel length (%). The findings quantify post-fire connectivity and sediment delivery from hillslopes and streams, and highlight how different types of storms can cause varying magnitues and spatial patterns of sediment transport and deposition from hillslopes through stream channel networks.     

Evaluating post-wildfire logging-slash cover treatment to reduce hillslope erosion after salvage logging using ground measurements and remote sensing

Continuing long and extensive wildfire seasons in the Western US emphasize the need for better understanding of wildfire impacts including post-fire management scenarios. Advancements in our understanding of post-fire hillslope erosion and watershed response such as flooding, sediment yield, and debris flows have recently received considerable attention. The potential impacts of removing dead trees, called salvage logging, has been studied, however the use of remotely sensed imagery after salvage logging to evaluate spatial patterns and recovery is novel. The 2015 North Star Fire provided an opportunity to evaluate hillslope erosion reduction using two field experiments and coincidental remotely sensed imagery over 3 years. Simulated rill experiments with four flow rates were used to quantify hillslope erosion on skidder trails with and without added logging slash compared with a burned-only control. Seven replicated hillslope silt fence plots with the same treatments were also evaluated for natural rainfall events. WorldView-2 satellite imagery was used to relate ground cover and erodible bare soil between the two experiments using multi-temporal Normalized Differenced Vegetation Index (NDVI) values. Results indicate that the skid trails produced significantly more sediment (0.70 g s⁻¹) than either the slash treated skid trail (0.34 g s⁻¹) or controls (0.04 g s⁻¹) with the simulated rill experiment. Similarly, under natural rainfall conditions sediment yield from hillslope silt fence plots was significantly greater for the skid trail (3.42 Mg ha⁻¹) than either the slash treated skid trail (0.18 Mg ha⁻¹) or controls (0 Mg ha⁻¹). An NDVI value of 0.32 on all plots over all years corresponded to a ground cover of about 60% which is an established threshold for erosion reduction. Significant relationships between NDVI, ground cover, and sediment values suggest that NDVI may help managers evaluate ground cover and erosion potential remotely after disturbances such as a wildfire or salvage logging.     

西昌市泸山地区典型火后泥石流特征与成因机制研究——以响水沟左岸3#支沟为例

2020年3月30日,四川省凉山州西昌市经久乡发生森林大火,山火燃烧后形成的山火灰和烧焦土体在强降雨作用下极易启动形成泥石流,给西昌市泸山、琼海景区生态环境及当地百姓生命财产安全带来潜在地质灾害风险。以西昌市安哈镇响水沟左岸3^#支沟为例,采用野外调查、无人机航拍、地球物理探测、现场及室内实验等手段,对2020年5月1日该沟发生的火后泥石流流体特征、动力学参数及成因机制等开展研究,系统分析了山火及降雨过程对该次火后泥石流形成的影响机制。该成果可为系统认识西昌泸山“3·30”山火地区火后泥石流的运动特征、物源规模、激发雨量及其发展趋势研判提供科学参考。     

An analytical solution for rapidly predicting post-fire peak streamflow for small watersheds in southern California

Following wildfires, the probability of flooding and debris flows increase, posing risks to human lives, downstream communities, infrastructure, and ecosystems. In southern California (USA), the Rowe, Countryman, and Storey (RCS) 1949 methodology is an empirical method that is used to rapidly estimate post-fire peak streamflow. We re-evaluated the accuracy of RCS for 33 watersheds under current conditions. Pre-fire peak streamflow prediction performance was low, where the average R² was 0.29 and average RMSE was 1.10 cms/km² for the 2- and 10-year recurrence interval events, respectively. Post-fire, RCS performance was also low, with an average R² of 0.26 and RMSE of 15.77 cms/km² for the 2- and 10-year events. We demonstrated that RCS overgeneralizes watershed processes and does not adequately represent the spatial and temporal variability in systems affected by wildfire and extreme weather events and often underpredicted peak streamflow without sediment bulking factors. A novel application of machine learning was used to identify critical watershed characteristics including local physiography, land cover, geology, slope, aspect, rainfall intensity, and soil burn severity, resulting in two random forest models with 45 and five parameters (RF-45 and RF-5, respectively) to predict post-fire peak streamflow. RF-45 and RF-5 performed better than the RCS method; however, they demonstrated the importance and reliance on data availability. The important parameters identified by the machine learning techniques were used to create a three-dimensional polynomial function to calculate post-fire peak streamflow in small catchments in southern California during the first year after fire (R² = 0.82; RMSE = 6.59 cms/km²) which can be used as an interim tool by post-fire risk assessment teams. We conclude that a significant increase in data collection of high temporal and spatial resolution rainfall intensity, streamflow, and sediment loading in channels will help to guide future model development to quantify post-fire flood risk.     

Compaction and cover effects on runoff and erosion in post-fire salvage logged areas in the Valley Fire,California

Runoff and erosion processes can increase after wildfire and post-fire salvage logging, but little is known about the specific effects of soil compaction and surface cover after post-fire salvage logging activities on these processes. We carried out rainfall simulations after a high-severity wildfire and post-fire salvage logging to assess the effect of compaction (uncompacted or compacted by skid traffic during post-fire salvage logging) and surface cover (bare or covered with logging slash). Runoff after 71 mm of rainfall across two 30-min simulations was similar for the bare plots regardless of the compaction status (mean 33 mm). In comparison, runoff in the slash-covered plots averaged only 22 mm. Rainsplash in the downslope direction averaged 30 g for the bare plots across compaction levels and decreased significantly by 70% on the slash-covered plots. Sediment yield totalled 460 and 818 g m⁻² for the uncompacted and compacted bare plots, respectively, and slash significantly reduced these amounts by an average rate of 71%. Our results showed that soil erosion was still high two years after the high severity burning and the effect of soil compaction nearly doubled soil erosion via nonsignificant increases in runoff and sediment concentration. Antecedent soil moisture (dry or wet) was the dominant factor controlling runoff, while surface cover was the dominant factor for rainsplash and sediment yield. Saturated hydraulic conductivity and interrill erodibility calculated from these rainfall simulations confirmed previous laboratory research and will support hydrologic and erosion modelling efforts related to wildfire and post-fire salvage logging. Covering the soil with slash mitigated runoff and significantly reduced soil erosion, demonstrating the potential of this practise to reduce sediment yield and soil degradation from burned and logged areas.     

Hillslope sediment fence catch efficiencies and particle sorting for post-fire rain storms

Sediment fences are often used to monitor hillslope erosion, but these can underestimate sediment yields due to overtopping of runoff and associated sediment. We modified four sediment fences to collect and measure the runoff and sediment that overtopped the fence in addition to the sediment deposited behind the fence. Specific objectives were to: (1) determine the catch efficiency of sediment fences measuring post-fire hillslope erosion; (2) assess particle sorting of sand, silt/clay, and organic matter from each hillslope through the sediment fence and subsequent runoff collection barrels; (3) evaluate how catch efficiency and particle size sorting relate to site and rainfall-runoff event characteristics; and (4) use runoff simulations to estimate sediment fence volumes for future post-fire monitoring. Catch efficiency ranged from 28 to 100% for events and 38 to 94% per site for the entire sampling season, indicating a relatively large underestimation of sediment yields by sediment fences. Most of the eroded sediment had similar proportions of sand and silt/clay as the hillslope soils, but the sediment behind the fence was significantly enriched in sand while the sediment that overtopped the fence was more strongly enriched in silt/clay. The sediment fences had capacities of 3 m³ for hillslopes of 0.19–0.43 ha, but simulations of runoff for 2- to 100-year storms indicate that the sediment fences would need a capacity of up to 240 m³ to store all of the runoff and associated sediment. More accurate measurements of sediment yields with sediment fences require either increasing the storage capacity of the sediment fence(s) to accommodate the expected volume of runoff and sediment, reducing the size of the contributing area, or directly measuring the runoff and sediment that overtop the fence. © 2020 John Wiley & Sons, Ltd.     

Study on the characteristics and formation mechanism of the typical post-fire debris flow in Lushan area of Xichang City: A case study of 3# branch on the left bank of Xiangshuigou

On March 30, 2020, a forest fire broke out in Liangshan area of Xichang City in Sichuan Province. After the fire, the ash and burnt soil easily developed into debris flow in case of heavy rainfall, which brought potential geological hazard risks to the ecological environment of Lushan and safety of local people's lives and properties in Qionghai and Lushan areas. Taking 3^# branch on the left bank of Xiangshuigou in Anha Town as an example, the authors conducted field investigation, UAV aerial photography, geophysical exploration, field and indoor experiments to study the fluid characteristics, dynamic parameters and genetic mechanism of the post-fire debris flow on May 1, 2020. The features of movement, provenance scale and deduced precipitation were also systematically analyzed. This achievement could provide some reference for the study of the development trend of the post-fire debris flow and the mitigation of disasters in this area.     

高强度螺栓摩擦型连接火灾后抗剪试验研究

建筑火灾发生频繁且对钢结构危害严重,但总体而言绝大多数火灾并未造成钢结构的根本性破坏,尽快鉴定其火灾后安全性并进行加固修复,对于减小灾后间接经济损失意义重大。高强度螺栓摩擦型连接是钢结构最常用的连接方式之一,其火灾后受力性能对整个结构灾后承载的安全至关重要。但目前国内外对高强度螺栓连接高温过火冷却后的受力性能研究极少。本文对高强度螺栓摩擦型连接进行了火灾后抗剪试验研究,得到了连接的抗滑移承载力、极限承载力以及荷载—变形曲线,研究了过火温度、冷却方式对连接受力性能的影响。为模拟火灾中的可能情况,试验考虑了自然冷却、泼水冷却两种冷却方式。试验表明,当高强度螺栓连接的过火温度不超过400°C时,连接的抗滑移承载力、极限承载力下降小,可判定连接仍能承受外部设计荷载,不需要对连接进行加固或替换螺栓;当过火温度超过400°C时,连接的抗滑移承载力、极限承载力开始显著下降,需连接更进一步的检测并进行仔细的结构分析,以确定连接能否继续承受外部设计荷载,以及是否需要采取必要的加固与修复措施。     

Tree Planting:How Fast Can It Accelerate Post-fire Forest Restoration?——A Case Study in Northern Da Hinggan Mountains,China

In 1987,a catastrophic fire burned over 1330000 ha in the densely forested area of the Da Hinggan Mountains in the northeastern China.After the fire,intensive management including burned trunk harvesting and coniferous tree planting had been conducted to accelerate forest restoration.To study the long term effect of these activities on forest recovery,we used a simulation modeling approach to study long-term(300 years) forest dynamics under current planting and natural regeneration scenarios.Results indicat...     

Post-fire management effects on sediment (dis)connectivity in Mediterranean forest ecosystems: Channel and catchment response

Forest fires and post-fire practices influence sediment connectivity (SC). In this study, we use the ‘aggregated index of connectivity’ (AIC) to assess SC in five Mediterranean catchments (198–1090 ha) affected by a wildfire in 2012 in south-eastern Spain. Two temporal scenarios were considered, immediately after the fire and before post-fire management, and 2 years after the fire including all practices (hillslope barriers, check-dams, afforestation, salvage logging and skid trails). One LiDAR (light detection and ranging)-derived digital elevation model (DEM, 2 m × 2 m resolution) was generated, per scenario. The five catchment outlets were established as the computation target (AIC_OUT), and structural and functional SC were calculated. Index outputs were normalized to make the results of the non-nested catchments comparable (AIC_N-OUT). The output analysis includes the SC distribution along the catchments and at local scale (929 sub-catchments, 677 in the burned area), the hillslope and channel measures' effect on SC, and a sedimentological analysis using observed area-specific sediment yield (SSY) at 10 new (built after post-fire practices) concrete check-dams located in the catchments (SSY = 1.94 Mg ha⁻¹ yr⁻¹; σ = 1.22). The catchments with more circular shapes and steeper slopes were those with higher AIC_N-OUT. The structural SC maps – removing the rainfall erosivity influence – allowed evaluating the actual role played by the post-fire practices that reduced SC ( $\overline{x}$= − 1.19%; σ = 0.41); while functional SC was linked to the actual change of SC ( $\overline{x}$= + 5.32%; σ = 0.62). Hillslope treatments resulted in significant changes on AIC_N-OUT at sub-catchment scale with certain disconnectivity. A good and positive correlation was found between the SSY and the changes of AIC_N-OUT. However, the coarse DEM resolution explained the lack of effect of the rock check-dams – located on the secondary channels – on AIC_N-OUT. AIC_N-OUT proved to be a useful tool for decision making in post-fire restoration, but an optimal input data is still necessary to refine calculations.