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.     

Effect of soil properties on erosion by wash and splash

This paper describes laboratory testing of 148 samples collected from Southern Alberta for erosion by wash and splash. Rainfall intensity was held constant during these tests. Soil aggregation was the most significant variable explaining soil loss. The significance of other soil properties, such as organic carbon and clay content is variable, depending on the interrelationships among aggregate stability, organic content, and clay content of particular soils. Variations in erodibility of the major soils examined are explained by the resistance of aggregates to compaction and dispersion. Splash detachment and wash transport are the dominant erosion mechanisms in inter-rill areas.     

河北坝上农田、退耕地和天然草地土壤的可风蚀性

对河北坝上农田、退耕地和天然草地土壤干团聚体稳定性（DASt）、土壤可蚀度（EF）和理化性质进行分析,讨论了这3种土地利用类型土壤风蚀可蚀性的差异及其与土壤理化性质的关系。结果表明：农田和天然草地EF与DASt值差异性显著（P<0.05）,天然草地和退耕地、退耕地和农田差异性不显著,对于EF和DASt,天然草地 > 退耕地> 农田;对于平均粒径,农田≈退耕地>天然草地;对于土壤机械组成黏粒与粉粒含量,天然草地 > 退耕地 > 农田;对于砾石含量,农田 > 退耕地 > 天然草地;对于土壤有机质和全氮含量,天然草地 > 退耕地 > 农田。农田、退耕地、天然草地的DASt、EF系数与砾石含量、平均粒径有着显著相关性,有机质、全氮含量和DASt在天然草地上极显著负相关;EF与有机质、全氮含量在3种地类显著负相关。坝上地区草地和退耕地土壤肥力较高,但表层土壤具有很高的风蚀潜力,因此要限制开垦草地,在必要的地区继续退耕还林还草,提高地表植被的覆盖度,防风固沙,提高土壤肥力,减少土壤风蚀。     

Relationships between physical‐geochemical soil properties and erodibility of streambanks among different physiographic provinces of Tennessee,USA

Erosion of cohesive soils in fluvial environments is dependent on physical, geochemical and biological properties, which govern inter‐particle attraction forces and control detachment rates from stream beds and banks. Most erosion rate models are based on the excess shear stress equation where the soil erodibility coefficient (k_d) is multiplied by the difference between the boundary hydraulic shear stress (τ_b) and the soil critical shear stress (τ_c). Both k_d and τ_c are a function of soil properties and must be obtained through in situ field or laboratory testing. Many studies have generated predictive relationships for k_d and τ_c derived from various soil properties. These studies typically were conducted in watersheds within a single physiographic region with a common surficial geology and/or investigated a limited number of soil properties, particularly geochemical properties. With widely reported differences in relationships between τ_c and soil properties, this study investigated differences in predictive relationships for τ_c among different physiographic provinces in Tennessee, USA. Erodibility parameters were determined in the field using a mini‐jet test device. Among these provinces, statistically four unique clusters were identified from a dataset of 128 observations and these data clusters were used to develop predictive models for τ_c to identify dominant properties governing erosion. In these clusters, 16 significant physical and geochemical soil properties were identified for τ_c prediction. Among these soil properties, water content and passing #200 sieve (percentage soil less than 75 μm) were the dominant controlling parameters to predict τ_c in addition to clay percentage (< 2 μm), bulk density, and soil pore water chemistry. This study suggests that unique relationships exist for physiographic provinces that are likely due to soil physical‐geochemical processes associated with surficial geology that determine minerology of the cohesive soil. Copyright © 2017 John Wiley & Sons, Ltd.     

Erodibility study of sediment in a fast-flowing river

Determination of sediment stability in the field is challenging because bed shear stress (BSS), a determining factor of sediment erosion, can’t easily be directly measured. To tackle this challenge and reliably assess sediment erodibility in a fast flowing river, a standalone underwater camera system and a new insitu flume (ISF) were developed and applied in this study. The camera system was used to record sediment movement and the new ISF was used for measuring critical bottom shear stress (CBSS). The camera can be deployed alone in water to record videos or take pictures with light emitting diode (LED) lighting and flexible schedule settings. The ISF is based on the concept that the amount of force needed to erode the same particle under different flow conditions should be similar. Two high resolution Acoustic Doppler Current Profilers (ADCP) also were deployed in the field to collect velocity-depth profiles which are used by conventional methods to calculate BSS with the law of the wall. The sediment erodibility was then assessed based on the comparison between the obtained CBSS and BSS and then further verified with the recorded observations from the deployed camera. The results reveal that the widely used conventional method can produce large uncertainties and is not adequate to provide meaningful conclusion under these conditions.     

Impacts of different vegetation restoration options on gully head soil resistance and soil erosion in loess tablelands

Vegetation restoration is identified as an effective approach to control soil erosion and affects soil detachment and resistance to concentrated flow on the Loess Plateau. However, the effects of vegetation restoration at gully heads in loess-tableland remains unclear. This study was performed to investigate the effects of nine vegetation restoration types at gully heads on soil detachment rate (Dr) and soil resistance to concentrated flow (i.e. soil erodibility, K_r and critical shear stress, τ_c). Undisturbed soil samples were collected from nine vegetation-restored lands and one slope cropland (as the control) and were subjected to a hydraulic flume to obtain Dr values of gully heads under six inflow discharges (0.5–3.5 L s^-1). The results showed that the Dr values of nine revegetated gully heads were 77.11% to 95.81% less than that of slope cropland, and the grassland dominated by Cleistogenes caespitosa and the shrubland dominated by Hippophae rhamnoides had a relatively greater decrease in Dr than those of other seven restoration types. The Dr value of nine revegetated gully heads could be better simulated by stream power than by flow velocity and shear stress and was also significantly affected by soil disintegration rate (positively), soil bulk density, saturated hydraulic conductivity, organic matter content, and water-stable aggregate stability (negatively). Additionally, roots with diameters of 0 to 0.5 mm showed a greater effect on Dr than those with larger diameters. Compared to cropland, the nine restored types reduced K_r by 76.26% to 94.26% and improved τ_c by 1.51 to 4.68 times. The decrease in K_r and the increase in τ_c were significantly affected by organic matter content, water-stable aggregate, mean weight diameter of aggregate and root mass density. The combination of grass species (Cleistogenes caespitosa) and shrub (Hippophae rhamnoides) could be considered the best vegetation restoration types for improving soil resistance of gully heads to concentrated flow. © 2019 John Wiley & Sons, Ltd.     

Laboratory and theoretical evaluation of impact of packing density,particle shape,and uniformity coefficient on erodibility of coarse-grained soil particles

Sedimentation – including erosion, transport, and deposition of coarse-grained particles – is a primary and growing environmental, engineering, and agricultural issue around the world. Soil erosion occurs when the hydrodynamic force induced by flowing water exceeds the geotechnical resistance of soils, as measured by critical shear stress for initiation of soil-particle motion. Even though various quantitative methods have been suggested with respect to different types of soil, the most widely accepted formula to estimate critical shear stress for coarse-grained soil is a direct function of the median grain size of the soil particles; however, the erosion resistance of soils also varies with other geotechnical properties, such as packing density, particle shape, and uniformity coefficient. Thus, in this study, a combined rolling–lift model for particle detachment was derived based on theoretical analysis. A series of experimental flume tests were conducted with specimens prepared with standard soil types, as well as laboratory-prepared mixtures of coarse-grained soil to validate the theoretical model and determine the effect of other geotechnical properties on the erosion characteristics of coarse grains, coupled with the effect of median particle size. The results indicated that the median grain size is the primary variable determining the resistance of coarse grains, but the critical shear stress also varies with the packing density of the soil matrix. In addition, angular particles show more erosion resistance than rounded particles, and the erosion potential of a soil decreased when the grain is well graded (higher value of uniformity coefficient). Additionally, regression analysis was performed to quantify the effect of each parameter on the critical shear stress of coarse grains. © 2020 John Wiley & Sons, Ltd.     

Site‐scale variability of streambank fluvial erodibility parameters as measured with a jet erosion test

The erosion rate of cohesive streambanks is typically modelled using the excess shear stress equation, dependent on two erodibility parameters: critical shear stress and erodibility coefficient. The jet erosion test (JET) has become the most common method for estimating these erodibility parameters in situ. Typically, results from a few JETs are averaged to acquire a single set of parameters for characterizing a streambank layer; however, this may be inadequate for accurately characterizing erodibility. The research objectives were to investigate the variability of JET results from assumed homogeneous streambank layers and to estimate the number of JETs required to accurately characterize erodibility for use in predictive models. On three unique streambanks in Oklahoma and across a range of erodibility, 20 to 30 JETs were conducted over a span of three days at each site. Unique to this research, each JET was analysed using the Blaisdell, scour depth and iterative solutions. The required sample size to accurately estimate the erodibility parameters depended on the JET solution technique, the parameter being estimated, and the degrees of precision and confidence. Conducting three to five JETs per soil layer on a streambank typically provided an order of magnitude estimate of the erodibility parameters. Because the parameters were log‐normally distributed, using empirical equations to predict erosion properties based on soil characteristics will likely contain high uncertainty and thus should be used with caution. This study exemplifies the need to conduct in situ measurements using the JET to accurately characterize streambank resistance to fluvial erosion. Copyright © 2015 John Wiley & Sons, Ltd.     

Erodibility of fine soil from the composite river bank of Brahmaputra in India

The erosion of a composite river bank critically depends on the erodibility of its fine soils, as the fine soil has higher resistance against erosion. Therefore, for the estimation of the bank erosion in the case of a composite river bank, it is important to determine the critical shear stress and erodibility coefficients of the bank soil and their spatial distribution. In the present study, erodibility parameters of the river bank of Brahmaputra in India have been estimated through 58 in situ submerged jet tests. The significance of spatial and layer‐wise distribution of the erodibility parameters was tested through analysis of variance (ANOVA). Results indicate that the spatial variation of erodibility parameters is highly significant, but layer‐wise variations of the erodibility parameters are not significant. Therefore, the erodibility of the riverbank depends on the particular location, whereas layer‐wise average erodibility parameters can be lumped for the estimation of the bank erosion for the specific site. Using the measured erodibility parameters, yearly river bank erosions at the study locations were computed and found to fall within the reported range of the bank erosion in the Brahmaputra River. Copyright © 2010 John Wiley & Sons, Ltd.     

Erosion prediction on unpaved mountain roads in northern Thailand: validation of dynamic erodibility modelling using KINEROS2

The event‐ and physics‐based KINEROS2 runoff/erosion model for predicting overland flow generation and sediment production was applied to unpaved mountain roads. Field rainfall simulations conducted in northern Thailand provided independent data for model calibration and validation. Validation shows that KINEROS2 can be parameterized to simulate total discharge, sediment transport and sediment concentration on small‐scale road plots, for a range of slopes, during simulated rainfall events. The KINEROS2 model, however, did not accurately predict time‐dependent changes in sediment output and concentration. In particular, early flush peaks and the temporal decay in sediment output were not predicted, owing to the inability of KINEROS2 to model removal of a surface sediment layer of finite depth. After 15–20 min, sediment transport declines as the supply of loose superficial material becomes depleted. Modelled erosion response was improved by allowing road erodibility to vary during an event. Changing the model values of erosion detachment parameters in response to changes in surface sediment availability improved model accuracy of predicted sediment transport by 30–40%. A predictive relationship between road erodibility ‘states’ and road surface sediment depth is presented. This relationship allows implementation of the dynamic erodibility (DE) method to events where pre‐storm sediment depth can be estimated (e.g., from traffic usage variables). Copyright © 2001 John Wiley & Sons, Ltd.