Control Effect of a Novel Polyurethane (W-OH) on Colluvial Deposit Slope Erosion in the Benggang Area of Southern China

Abstract

A novel polyurethane (W-OH), namely an eco-friendly hydrophilic polymer, has been widely applied in the field of soil erosion. However, recent research has not revealed the process and mechanisms through which W-OH application influences the soil detachment by concentrated overland flow (hereinafter referred to as soil detachment). In this study, the effects of the W-OH concentration on the physical and mechanical properties and the detachment capacity of colluvial deposit slope soil were investigated, and the impact of the relationship between the flow discharge and the W-OH concentration on the soil detachment capacity was examined under the experimental conditions. The results indicated that W-OH application significantly increased the large-particle content in the soil samples, enhanced the strength properties of the soil samples, reduced their separation capacity, and increased their stability. The structural equation modelling results revealed that W-OH application influences the soil detachment capacity primarily by affecting the shear strength, which exerts a significant negative effect on the detachment capacity (path coefficient = −0.57, p < 0.001). The soil detachment capacity prediction equation, which is based on the flow discharge and W-OH concentration, exhibited satisfactory accuracy (Nash–Sutcliffe efficiency (NSE) = 0.964) and can be used to predict the soil detachment capacity with high precision under similar experimental conditions. In addition, at a W-OH concentration above 1.53%, the impact on the soil detachment capacity is greater than that of the flow discharge. This study focused on investigating the process and mechanisms through which W-OH application reduces soil erosion on colluvial deposit slopes, thereby providing reference data for the management of Benggang erosion.

1. Introduction

Soil erosion has caused significant ecological and environmental harm globally [1,2,3]. In the erosion process, the loss of topsoil and nutrients reduces soil fertility [4,5]. Moreover, severe soil erosion can trigger geological hazards, thereby causing notable damage to ecosystems [6,7]. Many soil and water conservation techniques have been employed to control soil erosion, with numerous researchers comprehensively assessing the effectiveness of these techniques [8,9,10]. However, traditional soil and water conservation methods often exhibit issues such as being time consuming, labor-intensive, and slow to take effect [11].

In recent years, many researchers have applied polymeric materials to explore new techniques for controlling soil erosion. Wu studied the effects of different concentrations of polypropylene acid (PPA), polythene alcoholic (PTA), and urea-formaldehyde poly-condensate (UR) on soil structure and soil loss in the Loess Plateau. The results showed that all three compounds had certain effects in reducing surface runoff, decreasing soil loss, and increasing soil permeability [12]. Kumar investigated the effects of polyacrylamide (PAM) and gypsum on surface runoff and sediment yield on steep slopes and found that the combined application of PAM and gypsum achieved the best sediment reduction effect [13]. Rodrick studied the effects of polyacrylamide and biopolymers on aggregate stability and infiltration in silty loam soil and found that, compared to PAM, biopolymers were less effective as soil stabilizers, but they were more effective in controlling permeability loss [14]. Birhanu Kebede’s experiments showed that combining polyacrylamide with other soil amendments could improve soil structure stability and reduce soil erosion [15]. Moreover, a novel polyurethane (W-OH), namely an environmentally friendly hydrophilic polymer, has been widely applied in the field of soil erosion. W-OH can completely react with water to form a homogeneous W-OH aqueous solution, which can effectively penetrate the soil [16]. It then gradually forms an elastic and adhesive gel, which effectively binds soil particles and significantly reduces soil erosion intensity [11]. Numerous scholars have investigated the application of W-OH in soil erosion. Gao et al. explored the sand fixation mechanism of W-OH using wind tunnel experiments and reported that sand particles treated with a W-OH solution with a concentration greater than 3% exhibited suitable resistance to wind erosion [17]. Liang et al. conducted a 5-month field test on a Pisha sandstone slope and reported that the W-OH-treated area exhibited significant resistance to water erosion, with the total sediment yield reduced by more than 99% [18]. Zhu et al. analyzed the use of W-OH solutions of two concentrations (0% and 5%) under 15° conditions in simulated rainfall experiments [19]. They reported that spraying a 5% W-OH solution reduced the sediment yield on a colluvial deposit slope by 62.1%, indicating effective soil erosion control. Qin et al. used W-OH to stabilize and protect red clay highway slopes in southern China [11]. The results of simulated rainfall experiments revealed that at the early stages of protection, W-OH application accelerated slope drainage, solidified the soil structure, and reduced soil loss. The degree of sand reduction ranged from 37.4% to 65.3%. Li et al. conducted tensile tests to investigate the impact of applying W-OH solutions of different concentrations on the soil tensile strength and reported that W-OH treatment significantly increased the soil tensile strength [20].

In conclusion, W-OH application can significantly increase the soil erosion resistance. However, previous studies often focused on describing only the effect of W-OH application on soil erosion on the basis of experimental phenomena without thoroughly clarifying the pathways through which W-OH influences soil erosion. Additionally, the influence of the relationship between the W-OH concentration and the flow discharge on the soil erosion strength remains unclear. This limits further application of W-OH in controlling soil erosion and restricts the understanding of the mechanisms by which W-OH reduces soil erosion.

Benggang is a type of landform erosion encountered in the red soil region of South China. Typical Benggang landforms mainly comprise four parts, namely the upper catchment, collapsing wall, colluvial deposit, and alluvial fan (Figure 1A) [21]. The accumulation formed by the collapsing wall is referred to as a colluvial deposit (Figure 1B) [22]. Colluvial deposits have a high coarse particle content (with gravel accounting for up to 70% of the total mass), a loose soil structure, low erosion resistance, and a steep slope (up to 40°), rendering them highly susceptible to erosion [23]. The sediment generated from this process accounts for 80% of the total Benggang erosion, making it one of the most critical erosion processes in the occurrence and development of Benggang landforms [22]. At present, research on colluvial deposit erosion has focused mainly on sediment production, rill development, and hydrodynamic mechanisms [24,25,26,27]. Although many scholars have explored the erosion mechanisms of colluvial deposit, studies on using W-OH to control colluvial deposit erosion are still rare. Moreover, the mechanism by which W-OH reduces colluvial deposit erosion remains unclear.

Therefore, in this study, a colluvial deposit was adopted as the research object, with the primary goals of (1) clarifying the impact of W-OH application on the physical and mechanical properties and the detachment capacity of colluvial deposit slope soil; (2) examining the relationships between the soil detachment capacity and the physical and mechanical properties and hydrodynamic parameters; and (3) establishing a soil detachment capacity prediction equation and quantifying the effects of the W-OH concentration and flow discharge on the soil detachment capacity.

2. Materials and Methods

2.1. Study Area

The soil used in the experiment was collected from the Yangkeng small watershed in Longmen town, Anxi County, Fujian Province, Southeast China (Figure 2), with an average elevation of 196 m. The Yangkeng small watershed is characterized by hilly valley landforms, with the rainy season mainly concentrated between May and September. The average annual rainfall is 1800 mm, and the average annual temperature is 19 °C. The frost-free period lasts approximately 330 days throughout the year [27]. The vegetation types in the study area include Masson pine and Eucalyptus trees; understory shrubs of wild peony, myrtle, and Syzygium buxifolium; and herbaceous plants of Dicranopteris dichotoma, Neyraudia reynaudiana, Odontosoria chinensis, and Eriachne pallescens [28]. The soil in this area mainly develops from colluvial deposit. Under the influence of high temperatures and abundant rainfall, the bedrock weathers to form a thick weathering mantle. This mantle has a high coarse particle content, poor structure, and weak erosion resistance. Under the influence of precipitation and gravity, soil slopes easily erode, collapse, and form Benggang landforms. According to the survey, the number of Benggang in Anxi County was 12,828, approximately 50% of the total in Fujian Province. With a density of 40 per km², the Yangkeng small watershed contains 226 Benggang in total, which is 10 times more than that in Anxi County and 200 times more than that in Fujian Province. With severe rates of erosion that cause increased harm potential, the Yangkeng small watershed has become one of the typical representative areas where Benggang occurs in Fujian Province and even in southern China [27].

2.2. Collection and Treatment of Soil Samples

The collected soil was air dried. Since the particle size of gravel in colluvial deposits mainly ranges from 2 to 10 mm, debris with a diameter greater than 10 mm and impurities such as dead branches and leaves were removed. The soil samples were passed through 10 and 2 mm sieves to obtain particles with a diameter of 10–2 mm and soil samples with a particle diameter smaller than 2 mm, respectively. In total, 58.40%, 37.41%, and 4.18% of the obtained soil samples contained 0.05–2 mm (sand), 0.002–0.05 mm (silt) and <0.002 mm (clay) particles, respectively, which can be classified as sandy loam soil (USDA Soil Texture Classification). The soil was acidic, with a pH of 5.15, and presented a very low organic matter content of 1.68 g kg⁻¹. Previous studies have shown that the average gravel mass content of the colluvial deposit in this area is 30% [29], and an average bulk density is 1.4 g cm⁻³ [30]. Soil samples were collected from three different locations (upper, middle, and lower parts of the colluvial deposit), with three samples from each location, to measure the soil moisture content. The average soil moisture content of these nine soil samples was used as the mean soil moisture content for this experiment (15%). As a result, the ratio of soil particles between approximately 2 and 10 mm in size and soil particles smaller than 2 mm was set to 3:7. The two components were thoroughly mixed, and water was added to adjust the field moisture content before storage.

2.3. Experimental Design and Setup

In the experiment, W-OH solutions of five concentrations (1.0%, 2.0%, 3.0%, 4.0%, and 5.0%) and a blank control (a W-OH concentration of 0.0%) were employed. The W-OH solution was sprayed uniformly across the soil surface following the manufacturer’s recommended dosage of 3.0 L m⁻².

(1) Measurement of the soil physical and mechanical properties

The soil aggregate size and content significantly influence the soil resistance to erosion. In the experiment, the prepared soil samples were evenly packed into 60-cm⁻³ ring molds (with a diameter of 61.8 mm and a height of 20 mm) with a bulk density of 1.4 g cm⁻³, followed by spraying the corresponding W-OH solution. The samples were left to stand and naturally dry. After drying, 50 g of each sample was weighed and subjected to wet sieving (using a DIK-2012 soil aggregate analyzer) to classify six particle size ranges (2–10 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm, 0.035–0.25 mm, and 0–0.035 mm). The mass ratio of water-stable aggregates larger than 0.25 mm and the mean weight diameter (MWD) were then calculated. Each treatment was measured three times, and the average of the three measurements was taken as the final measurement value for that treatment.

To investigate the changes in the mechanical properties of the soil after spraying with the W-OH solution, we performed shear and compressive tests. The prepared soil samples were uniformly placed into 60 cm⁻³ ring molds (diameter, 61.8 mm; height, 20 mm) with a bulk density of 1.4 g cm⁻³. Afterwards, the W-OH solution of the required concentration was sprayed, and the samples were allowed to stand and naturally dry. After the samples had dried, distilled water was sprayed across their surface to achieve saturation, and the samples were then placed in an incubator at 25 ± 1 °C with 50 ± 5% humidity until the moisture content reached the target ±1% [20]. The shear strength of the processed samples was analyzed via a direct shear apparatus (LH-DDS-4). Each treatment was measured three times, and the average of the three measurements was taken as the final measurement value for that treatment. The compressive test was conducted by filling the prepared soil samples uniformly into 100 cm⁻³ ring molds (diameter, 50.46 mm; height, 50 mm) with a bulk density of 1.4 g cm⁻³, which were then treated in the same way as the samples employed for the shear test. After the samples reached the target moisture content, the unconfined compressive strength was obtained using an unconfined pressure apparatus (TKA-WXY-1F). Each treatment was measured three times, and the average of the three measurements was taken as the final measurement value for that treatment.

(2) Measurement of the soil detachment capacity

The prepared soil samples were uniformly placed into 500 cm⁻³ ring molds (diameter, 100 mm; height, 63.7 mm) with a bulk density of 1.4 g cm⁻³, followed by spraying with the required W-OH solution. After spraying, the samples were left to stand and dry for 72 h. The indoor runoff scouring test is used to simulate soil detachment under controlled conditions, replicating the actual soil erosion process. By precisely measuring parameters, such as soil detachment capacity, flow velocity, and shear stress, this method allows for the scientific quantification of various factors affecting soil detachment capacity and helps explore its variation mechanisms. Following similar methods reported in previous studies [31], a 5 m-long, 0.2 m-wide flume (Figure 3) was used in the flume scouring experiment, with the slope of the flume set to the average slope of the colluvial deposit (30°). On the basis of local rainfall records [32], six flow discharge levels of 4, 8, 12, 16, 24, and 32 L/min (converted into unit flow discharge levels of 0.33 × 10⁻³, 0.67 × 10⁻³, 1.00 × 10⁻³, 1.33 × 10⁻³, 2.00 × 10⁻³, 2.67 × 10⁻³ m² s⁻¹) were set, with the flow discharge controlled by a peristaltic pump (WT600-4F). Soil samples treated with W-OH solutions of the same concentration were subjected to three scouring tests under each flow discharge condition, resulting in a total of 108 tests. The average of the three measurements was taken as the measurement value for this treatment.

Prior to the experiment, the treated soil samples were saturated via immersion for 24 h and then removed and left to stand for 0.5 h to remove excess water from the samples gravitationally. The soil samples, after gravitational water removal, were placed in the soil sample box at the bottom of the flume (Figure 3), thereby ensuring that the surface height of the soil was level with the flume surface. Then, a sliding cover was placed. After adjusting the flow discharge and allowing the flow to stabilize, the sliding cover was removed. The timing process began, and sediment was collected. Scouring was stopped when the erosion depth of the soil sample reached 2 cm [33]. For difficult-to-erode soils, the scouring time was limited to 5 min. The flow velocity was measured via the potassium permanganate staining method (0.8% (w/w) KMnO₄). Ten measurements were obtained in each scouring test, and the maximum and minimum values were removed. The average of the remaining eight values was adopted as the surface flow velocity, and corrections were made using the appropriate correction factors for the laminar, transitional, and turbulent flow regimes (0.67, 0.70, and 0.80, respectively) [34]. The water depth (h) was measured by using a water depth probe (0.1 mm precision). Our previous laboratory study revealed that in fixed-bed sand transport tests, runoff remained the most stable within 1 m from the lower end of the outlet. Therefore, the left, middle and right positions, at 0.5, 0.7, and 0.9 m away from the sampling port, respectively, were selected to determine h, with a total of 9 h values. The average value was adopted as the final h value.

2.4. Indicator Calculation

(1)

Mean weight diameter

$$MWD={\sum }_{i=1}^{n}xiwi/{\sum }_{i=1}^{n}wi$$

(1)

The data in the equation come from the aggregation test. MWD represents the mean weight diameter (mm), xi represents the average particle size of two adjacent particle classes (mm), wi represents the mass percentage composition of sediment particles under each particle level (%), and n represents the number of particle sizes classified.

(2)

Shear strength

$${\tau }_s=\sigma tan\phi +c$$

(2)

The data in the equation come from the soil shear test. ${\tau }_s$ represents the soil shear strength (kPa), σ is the normal stress acting on the soil (kPa), φ is the internal friction angle of the soil (°), and c is the soil cohesion (kPa).

(3)

Soil detachment capacity

$$D_c={\displaystyle \frac{W}{tA}}$$

(3)

The data in the equation come from the scouring test. $D_c$ is the soil detachment capacity by rill flow (kg m⁻² s⁻¹), W is the weight of the detached soil (kg), t is detachment time (s), and A is the projected area of the soil sample (m²).

(4)

Hydrodynamic parameters

$$v=k{v}_s$$

(4)

$$\tau =\rho ghS$$

(5)

$$\omega =\tau v$$

(6)

The data in the equation come from the scouring test. $v$ is the flow velocity (m s⁻¹), k is the flow velocity correction factor, $v_s$ is the surface flow velocity (m s⁻¹), τ is shear stress (Pa), ρ is the water mass density (kg m⁻³), g is the gravity constant (m s⁻²), h is the flow depth (m), S is the slope gradient (m m⁻¹), and ω is stream power (W m⁻²).

2.5. Data Analysis

To quantify the accuracy and validity of the soil detachment capacity model, the relative root mean square error (RRMSE) and Nash–Sutcliffe efficiency coefficient (NSE) were used. These metrics can be calculated as follows:

$$RRMSE={\displaystyle \frac{\sqrt{{\frac{1}{n}{\sum }_{i=1}^n(O_i-P_i)}^2}}{O_m}}$$

(7)

$$NSE=1-{\displaystyle \frac{\sum _{i=1}^n(O_i-P_i{)}^2}{\sum _{i=1}^n(O_i-O_m{)}^2}}$$

(8)

where $O_i$ is the measured value of the ith sample, $P_i$ is the predicted value of the ith sample, $O_m$ is the measured mean value, and n is the number of samples. Notably, the model performs well for NSE ≥ 0.70, fairly well for 0.40 ≤ NSE < 0.7, and poorly for NSE < 0.40 [35].

3. Results

3.1. The Effects of W-OH on the Physical and Mechanical Properties of Colluvial Deposits

Soil aggregates are the fundamental units of the soil structure, and an increase in aggregates is beneficial for enhancing the soil quality and fertility [36]. After each W-OH solution was sprayed, the content of water-stable aggregates in the experimental soil increased with increasing spray concentration (Figure 4). Compared with that in the control group, after the samples were sprayed with 1%, 2%, 3%, 4%, or 5% W-OH solutions, the content of water-stable aggregates in the soil samples increased by 6.58%, 8.23%, 9.69%, 11.93%, and 12.59%, respectively, suggesting that the applied W-OH solution facilitated the effective aggregation of the loose colluvial deposit soil. The MWD is commonly adopted by researchers to characterize the size and distribution of soil particles [37]. Figure 5 shows the relationship between the MWD of the experimental soil samples and the W-OH concentration. The MWD first slowly increased and then rapidly increased with increasing W-OH concentration. After each W-OH solution was sprayed, the average MWD increased by 30.07% compared with that in the control group. The results indicate that applying the W-OH solution imposes a favorable aggregation effect on the experimental soil, thereby significantly increasing the content of large particles, which benefits nutrient retention in colluvial deposit soil and helps reduce erosion.

Soil mechanical properties are important factors influencing soil stability and the resistance to erosion. The W-OH solution exhibits satisfactory permeability [17], allowing it to penetrate through the soil pores, providing a solid basis for exploring the impact of W-OH application on the characteristics of the soil strength. Figure 6 shows the relationship between the shear strength of the experimental soil and the W-OH solution concentration. With increasing W-OH solution concentration, the shear strength of the soil samples significantly increased, indicating an exponential relationship. Compared with those of the untreated samples, the shear strengths of the soil samples treated with the 1%, 2%, 3%, 4%, and 5% W-OH solutions increased by 10.95%, 20.62%, 25.70%, 41.41%, and 63.42%, respectively. Figure 7 shows that the unconfined compressive strength of the experimental samples increased rapidly at first with increasing W-OH solution concentration and then slowly increased and eventually stabilized. The relationship can also be modelled by an exponential function. Compared with that of the untreated samples, the average unconfined compressive strength of the soil samples treated with a W-OH solution increased by 322.37%. The above analysis revealed that spraying with W-OH solution effectively enhanced the ability of the soil to resist external forces, and after solidification, the applied W-OH significantly increased the bonding between the soil particles and the soil hardness, thereby reducing erodibility.

3.2. The Effects of W-OH on Soil Detachment Capacity

Soil detachment capacity is a key indicator for quantifying soil erosion intensity [38]. Figure 8 shows the relationship between the soil detachment capacity and the W-OH solution concentration under various unit flow discharge levels. Figure 8 shows that, overall, under a constant unit flow discharge, the soil detachment capacity decreased with increasing W-OH solution concentration. At a unit flow discharge of 0.33 × 10⁻³ m² s⁻¹, there was no significant change in the soil detachment capacity with spray concentration variation. As the unit flow discharge was increased to 0.67 × 10⁻³–1.33 × 10⁻³ m² s⁻¹, the soil detachment capacity initially decreased rapidly with increasing concentration and eventually stabilized at a concentration of 2%. When the unit flow discharge was increased to 2.00 × 10⁻³–2.67 × 10⁻³ m² s⁻¹, the soil detachment capacity also decreased rapidly with increasing spray concentration, but stabilization occurred when the concentration reached 3%. As indicated in

Table 1. Fitted equations of W-OH concentration and soil detachment capacity under different experimental conditions.

Unit Flow Discharge (×10−3 m2 s−1)	Equation	R2	p
0.33	Dc = −0.007 + 0.053e−0.497C	0.886	0.001
0.67	Dc = −0.003 + 0.524e−1.084C	0.994	0.001
1.00	Dc = −0.061 + 0.909e−0.710C	0.978	0.001
1.33	Dc = −0.097 + 1.501e−0.723C	0.971	0.001
2.00	Dc = −0.219 + 2.434e−0.622C	0.940	0.001
2.67	Dc = −0.176 + 3.200e−0.724C	0.987	0.001

Note: $D_c$ is the soil detachment capacity by rill flow (kg m⁻² s⁻¹), e is a natural constant, C is W-OH concentration.

, the relationship between the soil detachment capacity and the W-OH solution concentration under the various unit flow discharge conditions can be described by an exponential function, and the fit of the equation is excellent, with R² values greater than 0.8. To analyze the relationship between the soil detachment capacity and the rate of reduction with the W-OH solution concentration under the experimental conditions, one-way analysis of variance (ANOVA) (

Table 2. Changes in the average soil detachment capacity and the reduction rate of detachment capacity with W-OH concentration.

	W-OH Concentration (%)
	0	1	2	3	4	5
Average soil detachment capacity (kg s−1 m−2)	1.31 a	0.7394 ab	0.158 b	0.013 b	0.012 b	0.007 b
Reduction rate (%)	-	42.068 b	88.134 a	99.022 a	99.291 a	99.555 a

Note: Lowercase letters represent significant differences between treatments (p < 0.05).

) was conducted. The results revealed that spraying with a 1% W-OH solution does not significantly reduce the soil separation capacity compared with that of the untreated soil samples. Compared with that in the control group, the soil separation capacity decreased by 42.07–99.56% under spray concentrations ranging from 1–5%. Furthermore, when the spray concentration reached 2%, no significant changes in the reduction rate of the soil detachment capacity were observed.

3.3. Relationship Among the Soil Detachment Capacity, Physical and Mechanical Properties, and Hydrodynamic Parameters

The soil detachment capacity is jointly influenced by hydraulic characteristics and soil properties [39,40]. To investigate the relationships between the soil physical and mechanical properties and the hydrodynamic parameters with the soil detachment capacity, we performed a correlation analysis between the soil detachment capacity and various variables (Figure 9). As shown in Figure 9, the soil detachment capacity was highly significantly negatively correlated with the W-OH concentration, shear strength, unconfined compressive strength, and water-stable aggregate content; significantly negatively correlated with the MWD; and significantly positively correlated with the unit flow discharge, mean flow velocity, shear stress, and stream power. Additionally, we applied the structural equation modelling path analysis method to investigate the mechanism underlying the changes in the soil separation capacity under W-OH spraying (Figure 10). We selected various parameters such as the unit flow discharge, mean flow velocity, W-OH concentration, MWD, shear strength, and soil detachment capacity for model fitting. The reason for choosing these parameters is that, compared with the shear stress and stream power, the mean flow velocity is easier to measure. Furthermore, the MWD is more representative of the overall soil particle size than water-stable aggregates are, and the shear strength is a better indicator of the resistance of soil to lateral water flow than the unconfined compressive strength is. Notably, Figure 10 shows that the Chi/df, P, GFI, and NFI results of the model conform with the structural equation modelling evaluation criteria [41], which indicates that this model is well suited for investigating the logical relationships between the various factors in this study. Moreover, Figure 10 shows that the flow discharge mainly influenced the change in the detachment capacity via the mean flow velocity, which exerts a significant positive effect on the detachment capacity (path coefficient = 0.42, p < 0.001). W-OH application influences the change in the detachment capacity primarily by affecting the shear strength, either directly or indirectly, with the shear strength significantly negatively affecting the detachment capacity (path coefficient = −0.57, p < 0.001).

3.4. Magnitude of the Influences of the Flow Discharge and W-OH Concentration on the Soil Detachment Capacity

In this experiment, the W-OH concentration and flow discharge were the primary factors influencing the detachment capacity of colluvial deposit soil. To better understand the influence of the relationship between the W-OH concentration and the flow discharge on the detachment capacity, we employed nonlinear regression of the concentration and the unit flow discharge to establish the following detachment capacity prediction equation:

Dc = 5640.932q^1.266e^−0.825C       R² = 0.964 p < 0.001

(9)

where Dc is the soil detachment capacity by rill flow (kg m⁻² s⁻¹), q is the unit flow discharge (m² s⁻¹), and C is the W-OH concentration (%).

Equation (9) indicates that under the experimental conditions, the W-OH concentration and the unit flow discharge could explain 96.4% of the total variation in the detachment capacity. The Nash–Sutcliffe efficiency (NSE) of Equation (9) is 0.964, and the relative root mean square error (RRMSE) is 0.349, indicating a favorable fitting accuracy. Figure 11 shows the 1:1 line of the predicted detachment capacity versus the measured detachment capacity using Equation (9). The data points occur close to the 1:1 line, indicating a satisfactory accuracy of the prediction equation, and it can be used to predict the detachment capacity with a suitable precision under the experimental conditions. However, it is uncertain how accurate the prediction equation is outside the experimental conditions.

In summary, under the experimental conditions, the soil detachment capacity increased with increasing flow discharge and decreased with increasing W-OH concentration. Both factors significantly affect the detachment capacity (Figure 9). To compare the magnitudes of the effects of both factors on the soil detachment capacity and determine the appropriate application amount of W-OH, we calculated the ratio of the relative changes in the soil detachment capacity to the relative changes in these two factors [27,42]. In regard to the flow discharge, the relative changes in Dc and q (Rq) can be calculated as follows:

$$Rq=\left|{\displaystyle \frac{\left({Dc}_2-{Dc}_1\right)/{Dc}_1}{\left(q_2-q_1\right)/q_1}}\right|=\left|{\displaystyle \frac{∆Dc{q}_1}{∆q{Dc}_1}}\right|=\left|{\displaystyle \frac{\partial Dc}{\partial q}}{\displaystyle \frac{q_1}{{Dc}_1}}\right|=1.266$$

(10)

where ${Dc}_1$ and ${\displaystyle \frac{\partial Dc}{\partial q}}$ are calculated via Equation (9). Similarly, the ratio (Rc) of the relative changes in $Dc$ to the relative changes in $C\left(C_2-C_1\right)$ can be obtained with Equation (11):

$$R_C=\left|{\displaystyle \frac{\left({Dc}_2-{Dc}_1\right)/{Dc}_1}{\left(C_2-C_1\right)/C_1}}\right|=\left|{\displaystyle \frac{∆Dc{C}_1}{∆C{Dc}_1}}\right|=\left|{\displaystyle \frac{\partial Dc}{\partial C}}{\displaystyle \frac{C_1}{{Dc}_1}}\right|=0.825C$$

(11)

In conclusion, unlike the flow discharge, the relative change in the soil detachment capacity with the W-OH concentration is not constant but follows a linear relationship (Equation (11)). The higher the concentration is, the greater the effect on the soil detachment capacity. When the W-OH concentration exceeds 1.53%, the effect of the W-OH concentration on the detachment capacity exceeds that of the flow discharge.

4. Discussion

Larger soil particles generally exhibit a greater ability to resist physical forces and are more stable than smaller soil particles are [43]. Figure 4 and Figure 5 show that spraying the W-OH solution significantly increased the proportion of large particles in the soil. This occurs primarily because the polar and active groups in the W-OH solution adhere to the surface of soil particles through physicochemical interactions, creating strong adhesive forces [17], allowing soil particles to bond and form larger aggregates (Figure 12). Zhu et al. found that the particle size of red soil and colluvial deposit soil treated with W-OH increased with the treatment concentration, which is consistent with our research [19]. During soil erosion, fine particles are usually the first to be eroded [44]. The application of W-OH increases the proportion of larger particles in the soil, which contributes to reducing its erodibility.

The experiment demonstrated that W-OH significantly enhanced the strength properties of the colluvial deposit soil (Figure 6 and Figure 7). This is primarily because the adhesive gel formed by W-OH after solidification often wraps envelops and generates bonds with the soil particles [20], thereby generating more compact connections between the particles and enhancing soil stability. Figure 13 shows scanning electron microscopy (SEM) images of the soil samples treated with the 0% and 5% W-OH solutions for comparison. After W-OH treatment, we clearly observed thread-like and sheet-like bridges between the soil particles (Figure 13), which significantly enhanced the bonding between the particles, leading to a change in the soil mechanical properties. Liang et al., Ma et al., and Lin et al. also reported similar bridging phenomena in their experiments [18,20,45]. Gao et al. also indicated that the hardness and compressive stress of their soil samples increased with increasing W-OH concentration [17]. Furthermore, Lin et al. observed that the tensile strength of their soil samples increased with increasing W-OH concentration [20], which is similar to our results.

Under the test conditions, the soil detachment capacity decreased exponentially with increasing W-OH concentration (Figure 8). This occurs primarily because as the W-OH solution concentration is increased, its consolidating effect on the soil particles increases, thereby forming a thicker consolidation layer on the soil surface [46]. This enhances the strength properties of the soil and increases its resistance to scouring. Additionally, after W-OH solidification, an elastic gel was produced around the soil particles, which bonded the loose particles together. When the soil is disturbed, the gel between the particles can provide a notable pulling effect (Figure 14), helping to counteract external forces. Table 2 reveals that when the spraying concentration reached 3%, the soil detachment capacity decreased by more than 99% compared with that in the control group. These findings demonstrate that a 3% W-OH solution can effectively prevent colluvial deposit erosion. In simulated rainfall tests, Qin et al. determined that under light and moderate rainfall conditions, the sediment yields at 3%, 5%, and 7% W-OH concentrations were nearly equal, while under heavy rainfall, the sediment yields at 5% and 7% W-OH concentrations were similar [11]. This finding is similar to our results.

The soil detachment capacity is jointly influenced by the soil properties and hydraulic characteristics [39,40]. Under the test conditions, soil properties affect the detachment capacity mainly by influencing soil aggregation and mechanical properties. An increase in soil aggregation and changes in the soil mechanical properties increase soil stability and enhance its strength characteristics [17,19,20], thereby increasing its ability to resist erosion. This is the main reason for the reduction in the soil detachment capacity. The structural equation modelling results (Figure 10) also indicated that W-OH application reduces the soil detachment capacity by directly or indirectly affecting the shear strength of the soil. Hydraulic characteristics affect the soil detachment capacity mainly via increased runoff flow, which results in a higher flow velocity, thus increasing the runoff energy [3,41] and ultimately enhancing the scouring power of runoff and the erosive capacity of water, which increases the soil detachment capacity.

Traditional soil and water conservation measures primarily reduce soil erosion by enhancing soil stability, improving water infiltration capacity, and reducing surface runoff. While these measures are effective in preventing soil and water loss, they generally suffer from high costs, complex implementation, short-term ineffectiveness, and high maintenance requirements. Studies have shown that spraying a certain amount of polymeric materials such as polyacrylamide and polythene alcoholic can also effectively prevent soil erosion [12,13,14,15]. However, their solutions typically have high viscosity, which hinders infiltration into the soil, and when applied alone, their effectiveness in reducing soil erosion still needs improvement. In contrast, W-OH can completely react with water to form a homogeneous W-OH aqueous solution, which can effectively penetrate the soil [17]. Additionally, the gel formed by W-OH can effectively encapsulate and bind soil particles [Figure 13], thereby enhancing soil erosion resistance.

5. Conclusions

This experiment focused on analyzing the impact of W-OH application on the soil detachment capacity of colluvial deposit soil in the Benggang erosion area of southern China, and the main conclusions are as follows: W-OH application significantly increased the content of large soil particles, with the average MWD of the experimental soil increasing by 30.07% compared with that in the control. Both the shear strength and unconfined compressive strength of the treated soil increased with increasing W-OH solution concentration, and both exhibited an exponential relationship with the W-OH concentration. In contrast, the detachment capacity decreased with the increase in W-OH concentration. Compared to the control, spraying 1–5% W-OH solution leads to a 42.07–99.56% decrease in soil detachment capacity. The structural equation model indicates that mean flow velocity has a significant positive effect on detachment capacity (path coefficient = 0.42, p < 0.001). W-OH mainly affects detachment capacity by directly or indirectly influencing shear strength, and shear strength has a significant negative effect on detachment capacity (path coefficient = −0.57, p < 0.001). Detachment capacity can be predicted using a function based on flow discharge and W-OH concentration, with the equation showing an NSE of 0.964, indicating good accuracy. Furthermore, when the W-OH concentration exceeds 1.53%, its effect on the detachment capacity exceeds that of the flow discharge. These results increase the understanding of the mechanism by which W-OH application reduces the soil detachment capacity and provide new insights into controlling colluvial deposit erosion. The empirical equation can also provide guidance for obtaining the appropriate W-OH dosage under similar conditions.