Excess Shear Stress Model Research Articles

Relationship between soil erodibility by concentrated flow and shear strength of a Haplic Acrisol with a cationic polyelectrolyte

To clarify the relationship between rill erodibility in an excess shear stress model for concentrated flow erosion and soil particle–particle interactions, we examined the rill erodibility and aggregation-dispersion behavior of Shimajiri Maaji soil, classified as a Haplic Acrisol, with different amounts of cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC). The effect of particle–particle interaction on rill erodibility was determined by performing shear strength measurements and rill erosion experiments as a function of the PDADMAC dose without changing other soil conditions. Moreover, the mechanisms of particle–particle interactions were examined by comparing the electrophoretic mobility (EPM) with the transmission of the supernatant of the soil suspension and soil shear strength. The EPM and transmission showed that the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction governed the aggregation-dispersion behavior. At the same time, the shear strength monotonically increased with increasing PDADMAC dose, even when charge reversal and electric double layer (EDL) repulsion occurred. This result suggests that the shear strength is sensitive not only to the EDL interaction but also to the interactions, the so-called non-DLVO forces, such as bridging interaction. The difference between transmission and shear strength should be due to the difference in the characteristic length because the transmission is attributed to the collision/approaching process between particles, whereas the shear strength is attributed to the broken/retracting process of particle–particle contact. Rill erodibility decreased with increasing shear strength. Furthermore, our quantitative analysis demonstrated that rill erodibility can be represented by a linear function of shear strength. These results clearly show that a high attractive force between soil particles results in low rill erodibility. The strong particle–particle attraction protects soil particles from peeling off by increasing the shear stress of the flowing water. Our study suggests that shear strength measurement can be used to quickly estimate the effect of soil conditioners on rill erodibility in the field.

Study on Parameters of Two Widely Used Cohesive Soils Erosion Models

The erosion rate of cohesive soils was typically modeled with the excess shear stress model and the Wilson model. Several kinds of research have been conducted to determine the erodibility parameters of the two models, but few attempts have been made hitherto to investigate the general trends and range of the erodibility parameter values obtained by the commonly used Erosion Function apparatus. This paper collected a database of 177 erosion function apparatus tests to indicate the variability of all erodibility parameters; the range of erodibility parameters is determined by data statistics and parameter theoretical value derivation. The critical shear stress (τc) and erodibility coefficient (Z0) in the over-shear stress model have a positive proportional relationship when the data samples are sufficient. However, there is no such relationship between the erodibility coefficient (b0) and erodibility coefficient (b1) in the Wilson model. It is necessary to express the soil erosion resistance by considering all erosion parameters in the erosion model. Equations relating erodibility parameters to water content, plasticity index, and median particle size were developed by regression analysis.

Experimental Investigation on the Erosion Threshold and Rate of Gravel and Silty Clay Mixtures

Abstract. The field of cohesive and noncohesive mixture erosion is not fully understood because of the numerous factors that influence soil erodibility. In this study, erosion experiments were conducted on mixtures of gravel and silty clay in proportions varying from 0% to 100% by weight. The critical shear stress of erosion and the erosion rate were quantified using an erosion function apparatus (EFA). Experimental data revealed that the mixture critical shear stress first decreased and then increased with an increasing cohesive fraction for mixtures with silty clay contents up to 50%. The critical shear stress of the mixture showed an increasing trend as the silty clay content varied from 60% to 100%. A transition from noncohesive to cohesive erosion behavior occurred at silty clay contents between 30% and 35%. The appropriateness of a dimensionless nonlinear excess shear stress model and the Wilson model was tested based on the EFA experimental data. The dimensionless excess shear stress model was shown to be appropriate for noncohesive mixtures, while the Wilson model performed better than the dimensionless excess shear stress model for cohesive mixtures. Keywords: Critical shear stress, Erosion rate, Dimensionless nonlinear excess shear stress, Soil mixture, Wilson model.

Quantifying Soil Erodibility Parameters Due to Wastewater Chemicals

Many factories and hospitals dumped their wastewater into Tigris River without any treatment in Baghdad city. This could influence on soil erodibility on Tigris Riverbanks. Phenol, Nitrate, and Phosphate were considered as huge concentrations in wastewater chemicals. The main objectives of this research were: 1) to investigate the influence of three wastewater chemicals on erodibility parameters of cohesive soils using Blaisdell's solution and scour depth solution techniques from jet erosion tests (JETs); 2) to develop relationship between erodibility parameters and soil hardness. An excess shear stress model was utilized to measure soil erodibility based on two empirical soil parameters: critical shear stress (τc) and erodibility coefficient (kd). The results showed a constantly increased in kd for all three wastewater chemicals at different curing times and concentrations, while τc values were decreased. An inverse relationship was developed between τc and kd for both Blaisdell's solution and scour depth solution techniques.

Watershed Variability in Streambank Erodibility and Implications for Erosion Prediction

Two fluvial erosion models are commonly used to simulate the erosion rate of cohesive soils: the empirical excess shear stress model and the mechanistic Wilson model. Both models include two soil parameters, the critical shear stress (τc) and the erodibility coefficient (kd) for the excess shear stress model and b0 and b1 for the Wilson model. Jet erosion tests (JETs) allow for in-situ determination of these parameters. JETs were completed at numerous sites along two streams in each the Illinois River and Fort Cobb Reservoir watersheds. The objectives were to use JET results from these streambank tests to investigate variability of erodibility parameters on the watershed scale and investigate longitudinal trends in streambank erodibility. The research also determined the impact of this variability on lateral retreat predicted by a process-based model using both the excess shear stress model and the Wilson model. Parameters derived from JETs were incorporated into a one-dimensional process-based model to simulate bank retreat for one stream in each watershed. Erodibility parameters varied by two to five and one to two orders of magnitude in the Illinois River watershed and Fort Cobb Reservoir watershed, respectively. Less variation was observed in predicted retreat by a process-based model compared to the input erodibility parameters. Uncalibrated erodibility parameters and simplified applied shear stress estimates failed to match observed lateral retreats suggesting the need for model calibration and/or advanced flow modeling.

Erodibility Parameters Derived from Jet and Flume Erosion Tests on Root‐Permeated Soils

AbstractFlume tests and in situ jet erosion tests (JETs) allow for the process‐based quantification of erodibility parameters of cohesive soils. In the excess shear stress model used to predict soil detachment, the critical shear stress (τc) corresponds to the stress at which fluvial forces can detach soil particles or aggregates and the erodibility coefficient (kd) governs the rate at which detachment occurs when the imposed shear exceeds τc. The primary objective of this research was to derive and compare erodibility parameters from flume tests and JETs on various root‐permeated soil samples. Based on statistical analysis, erodibility coefficients from the flume tests were typically statistically similar to those derived with JETs. In order to further determine the capability of the JET to indicate differences in erodibility of root‐permeated soils, this research then performed JETs on side‐by‐side in situ soils. The JETs on root‐permeated soils estimated significantly higher τc but insignificant differences in kd compared to JETs on adjacent bare soil. A significant correlation was observed between exposed root surface area (TRSA) and τc, but no correlation between TRSA and kd or the equilibrium scour depth was observed. More flume and JET experiments are needed to clarify the relationship between root‐permeated soils and erodibility parameters, and address a wider range of average root diameters than was considered in this research.

Detachment characteristics of root-permeated soils from laboratory jet erosion tests

The influence of vegetation on flow and sediment dynamics at various spatial and temporal scales has been well documented. Vegetation may be one of the most effective measures in streambank stabilization. Traditionally, research on the influence of vegetation roots on streambank stabilization has focused on mechanical reinforcement and reduced applied shear stress due to above ground biomass. Few studies have investigated the effect of roots on fluvial detachment of sediment. This study conducted 36 mini-jet erosion tests (mini-JETs) on bare soil samples and 29 mini-JETs on root-permeated soil samples (average root diameters of 0.5 to 1.7mm) to determine the role of roots in erosion resistance. The research also estimated parameters of the linear excess shear stress model (erodibility coefficient, kd, and critical shear stress, τc) and a nonlinear detachment model called the Wilson Model (b0 and b1) and investigated the correlations between parameters of the two models and root characteristics. Root-permeated soil samples were more erosion resistant at higher shear stress as the τc and b1 parameters were on average higher for the vegetated samples than the bare soil samples. As root diameter increased in the soil samples, erosion rates at high shear stress decreased. The erodibility coefficient parameters (b0 and kd) of both the linear and nonlinear detachment models were negatively and significantly correlated to root diameter through power functions. No significant correlation was detected between critical shear stress or b1 and root parameters which supports conclusions of previous studies. Significant correlations were observed among the parameters of the excess shear stress model and the nonlinear detachment model; especially high correlation was observed between τc and b1 for the vegetated samples. In conclusion, root-permeated soils exhibited lower erosion rates primarily through increasing the required shear stress before detachment.

Influence of Lead Pollution on Cohesive Soil Erodibility using Jet Erosion Tests

<p class="1Body">Recent researches were investigated the high concentrations of Lead in Baghdad soils due to the emissions from Leaded fuel of cars, generators, and the industrials. These high concentrations in addition to their impact on human health may impact on the landscape and streambanks and may cause significant issues on soil erodibility. The erosion rate of cohesive soil was usually estimated using two alternative models, excess shear stress model which is depended on two major soil parameters: the critical shear stress, <em>τ<sub>c</sub></em>, and the erodibility coefficient, <em>k<sub>d</sub></em>, and Wilson model which is depended on two mechanistic soil parameters: <em>b<sub>0</sub></em> and <em>b<sub>1</sub></em>. A new miniature version of Jet Erosion Test (“mini” JET) was performed to derive both model parameters. The objective of this study was to investigate the influence of Lead pollution on cohesive soil erodibility using “mini” JET under controlled laboratory setups to predict soil erodibility. In order to observe the Lead contamination on soil erodibility, soil samples were mixed with different quantities of Lead concertation and the samples were packed at ASTM standard mold on two different bulk densities. Results show that the Lead pollution increased soil erodibility when the concentration of Lead increased. An inverse relationship between excess shear stress parameters <em>k<sub>d</sub></em> and <em>τ<sub>c</sub></em> was observed as well as between Wilson model parameters <em>b<sub>0</sub></em> and <em>b<sub>1</sub></em>. The Wilson model parameters were closely resembled the empirical excess shear stress parameters with benefit that Wilson model parameters are mechanistic parameters.</p>

Variability of fluvial erodibility parameters for streambanks on a watershed scale

Typically the erosion rate of cohesive soils is modeled using the excess shear stress equation, which includes two soil parameters: the erodibility coefficient (kd) and the critical shear stress (τc). Alternatively, a mechanistic detachment rate model, the Wilson model, was recently developed to predict the erosion rate of cohesive soils, also based on two soil parameters: b0 and b1. The Wilson model is proposed as advantageous in terms of being a more mechanistic, fundamentally based erosion equation. The objective of this research was to derive the excess shear stress model parameters (kd and τc) from field jet erosion tests (JETs) on numerous streambanks across the Illinois River watershed in Oklahoma to investigate (i) erodibility parameter variability or uniformity at a river basin scale, (ii) correlations between the derived parameters and soil texture, (iii) influence of solution technique on the estimated erodibility parameters, and (iv) the applicability of predictive relationships between kd and τc. The second objective was to demonstrate the applicability of the mechanistic Wilson model using field JET data and to investigate correlations between the excess shear stress model parameters, kd and τc, and the Wilson model parameters, b0 and b1. Erodibility parameters for streambanks of varying soil texture were measured using a miniature version of the JET device (mini JET). Data from the JETs were used to derive the erodibility parameters using a Blaisdell and scour depth approach. Soil samples were acquired at locations of the JETs to quantify particle size distribution, average particle size (d50) and bulk density. No significant relationships existed between kd or τc and bulk density, d50, percent clay, silt, or sand, or percent clay-silt. Existing empirical relationships should be used with caution considering the variability between the results observed in this research and previous relationships proposed in the literature. Strong correlations were observed between b0 and kd (R2=0.62 to 0.89) and between b1 and τc (R2=0.31 to 0.96). Therefore, the Wilson model parameters closely resemble the empirical excess shear stress parameters.

Quantifying the Erodibility of Streambanks and Hillslopes Due to Surface and Subsurface Forces

<abstract> <bold><sc>Abstract.</sc></bold> The erosion rate of cohesive soils due to fluvial forces is usually computed using an excess shear stress model. However, no mechanistic approaches are available for incorporating additional forces, such as seepage, into the excess shear stress model parameters. Recent research incorporated subsurface (seepage) forces into a mechanistic detachment rate model for streambeds. The new detachment model, the Modified Wilson model, was based on two modified dimensional parameters (b₀ and b₁) that included seepage forces. The objective of this study was to modify the parameters (b₀ and b₁) to quantify the influence of seepage on erodibility of cohesive streambanks and to compare the results to those obtained from tests on horizontal beds. A new miniature version of a submerged jet erosion test device (“mini” JET) and a seepage column were utilized to derive the parameters of the Modified Wilson model for a silty sand soil and a clayey sand soil across a range of uniform seepage gradients. The experimental setup was intended to mimic a streambed and a streambank when the “mini” JET and seepage column were placed in vertical and horizontal orientations, respectively. The soils were packed in a standard mold at a uniform bulk density (1.5 to 1.6 Mg m^-3) near the optimum water content. Seepage forces influenced the observed erosion with a non-uniform influence on b₀ and b₁ as functions of the hydraulic gradient and density. Expected theoretical differences between Wilson model parameters for streambanks and streambeds were not consistently observed for these erodible soils, most likely due to variability in streambed and streambank samples in terms of soil preparation, packing, and seepage gradient establishment. The influence of seepage forces can be predicted by the Modified Wilson model parameters in both vertical and horizontal experimental setups using JETs on soils without seepage. Additional research is needed on the behavior of several mechanistic soil parameters under the influence of seepage.

Mechanistic Detachment Rate Model to Predict Soil Erodibility Due to Fluvial and Seepage Forces

AbstractThe erosion rate of cohesive soils is typically computed using an excess shear stress model based on the applied fluvial shear stress. However, no mechanistic approaches are available for incorporating additional forces, such as groundwater seepage into the excess shear stress model parameters. Seepage forces are known to be significant contributors to streambank erosion and failure. The objective of this research was to incorporate seepage forces into a mechanistic fundamental detachment rate model to improve the predictions of the erosion rate of cohesive soils. The new detachment model, which is referred to as the modified Wilson model, was based on two modified dimensional soil parameters (b0 and b1) that included seepage forces due to localized groundwater flow. The proposed model provided a general framework for studying the impact of soil properties, fluid characteristics, and seepage forces on cohesive soil erodibility. Equations were presented for deriving the material parameters from bot...

Deriving Parameters of a Fundamental Detachment Model for Cohesive Soils from Flume and Jet Erosion Tests

Abstract. The erosion rate of cohesive soils is commonly quantified using the excess shear stress model, which is dependent on two major soil parameters: the critical shear stress (I„ c ) and the erodibility coefficient (k d ). A submerged jet test (jet erosion test, or JET) is one method that has been developed for measuring these parameters. The disadvantage of using the excess shear stress model is that parameters I„ c and k d change according to erosion conditions, such as soil structure, soil orientation, type of clay, presence of roots, and seepage forces. A more mechanistically based detachment model, called the Wilson model, is proposed in this article for modeling the erosion rate of soils using hydraulic analysis of a JET. The general framework of the Wilson model is based on two soil parameters (b 0 and b 1 ). The objectives of this study were to: (1) develop methods of analysis of the JET to determine parameters b 0 and b 1 for the Wilson model in a similar fashion to the previous methodology developed for open-channel flow, and (2) compare the excess stress model parameter (k d ) and the Wilson model parameters (b 0 and b 1 ) determined from the flume tests and JETs for two cohesive soils. Flume tests, treated as the standard test method, and original and “mini” JETs were conducted on two soils to independently measure the excess shear stress model parameter (k d ) and the Wilson model parameters (b 0 and b 1 ). Soil samples of two cohesive soils (silty sand and clayey sand soils) were packed in a soil box for the flume tests and the JETs at water contents ranging from 8.7% to 18.1%. No statistically significant differences were observed for the excess shear stress model parameter (k d ) and for the Wilson model parameters (b 0 and b 1 ) when determined from the flume tests and JET devices, except for b 1 with the original JET. The Wilson model is advantageous in being a more mechanistic, fundamentally based erosion equation as compared to the excess shear stress model. The Wilson model can be used in place of the excess shear stress model with parameters that can be estimated using existing JET techniques.

Modeling Soil Detachment on Unpaved Road Surfaces on the Loess Plateau

Soil loss from unpaved rural roads contributes largely to watershed sediment yield. A road erosion model is therefore urgently needed to evaluate road erosion production. Field flow simulation experiments were conducted on unpaved loess roads under five flow rates and three slope gradients in order to quantify road surface hydraulics and develop soil detachment prediction models. The results showed that flow velocity was highly related to flow rate but generally independent of slope gradient. The hydraulic roughness of the road surface was much less than that of cropland and was influenced significantly by relatively intensive erosion on steep slopes. Easily obtainable parameters such as flow rate and slope gradient could be used to estimate soil detachment accurately. Furthermore, power functions of chosen hydraulic parameters, (i.e., shear stress, stream power, and unit stream power) could be used to predict detachment rates. Among these parameters, flow shear stress was the best predictor. A detachment prediction model similar to that used in the WEPP model was proposed. Corresponding to the excess shear stress model in WEPP, loess road surface soil erodibility is 0.0066 s m-1 and critical shear stress is 3.25 Pa.

A physically based model to predict hydraulic erosion of fine‐grained riverbanks: The role of form roughness in limiting erosion

Hydraulic erosion of bank toe materials is the dominant factor controlling the long‐term rate of riverbank retreat. In principle, hydraulic bank erosion can be quantified using an excess shear stress model, but difficulties in estimating input parameters seriously inhibit the predictive accuracy of this approach. Herein a combination of analytical modeling and novel field measurement techniques is employed to improve the parameterization of an excess shear stress model as applied to the Lower Mekong River. Boundary shear stress is estimated using a model (Kean and Smith, 2006a, 2006b) for flow over the irregular bank topography that is characteristic of fine‐grained riverbanks. Bank erodibility parameters were obtained using a cohesive strength meter (Tolhurst et al., 1999). The new model was used to estimate annual bank erosion rates via integration across the Mekong's annual flow regime. Importantly, the simulations represent the first predictions of hydraulic bank erosion that do not require recourse to calibration, thereby providing a stronger physical basis for the simulation of bank erosion. Model predictions, as evaluated by comparing simulated annual rates of bank toe retreat with estimates of bank retreat derived from analysis of aerial photographs and satellite imagery, indicate a tendency to overpredict erosion (root‐mean‐square error equals ±0.53 m/yr). Form roughness induced by bank topographic features is shown to be a major component (61%–85%) of the spatially averaged total shear stress, and as such it can be viewed as an important factor that self‐limits bank erosion.