Validation

The landslide susceptibility model was evaluated using the receiver operating characteristic curve (ROC curve), a non-dependent threshold approach. The ROC curve shows the validity of the diagnostic ability of a binary classifier system as its discrimination threshold [100]. The ROC curve is created by plotting the values of the true positive rate (TPR) against the values of the false-positive rate (FPR) at different threshold settings [101]. This curve validates the model's accuracy regardless of the prediction model since it compares random landslide points and a separated dataset of landslides [102,103]. A synthetic index was calculated for the ROC, utilizing the area under the curve (AUC), which has generally been applied in past studies to evaluate the accuracy of the landslide susceptibility map. A higher AUC value indicates a higher accuracy of the susceptibility map [98]. The AUC value obtained from the AHP model revealed a 0.865 accuracy (Figure 6). Therefore, the results of the model indicate an accurate susceptibility map. A precise LSM model highly depends on conditional factors. It thus assists decision-makers, landscapers, and urban planners in identifying hazard-prone areas for early mitigation. After studying the nature and environmental conditions of the study area, a hydroseeding experiment was introduced and carried out on a high-risk slope within the study location to control and prevent potential landslides.

**Figure 6.** ROC curve.

#### *4.2. Experimental Observations, Monitoring, and the Result of Hydroseeding*

In this study, we evaluated and recorded the germination performance of the four hydroseeded seed species. The main goal was to determine which of these seeds, currently used to hydroseed slopes in different parts of the world, is more effective in the study area. Moreover, the vegetation ground cover and the vegetation root were studied to test their effectiveness in controlling landslides (Figure 7).

**Figure 7.** Hydroseeded vegetation.

#### 4.2.1. Germination Rate

Before the experiment, we studied environmental factors such as rainfall and temperature. The surveys were conducted at the end of each month from August to December 2020. In each subplot, the species were identified and recorded. Couch (G4) seed germinated between the 2nd and 3rd day and showed an approximate 92% germination rate, followed by signal grass (G3) which grew on the 9th day with an 88% germination rate. The survey also recorded a delay in germination on both rye corn (G2) and ryegrass (G1), which germinated between day 15 and day 17, with 84% and 60% germination rates, respectively.

#### 4.2.2. Vegetation Root Length

The vegetation root is the most crucial aspect of the plant for slope stabilization. Adequate subsurface drainage is essential to reduce the pore-water pressure of the subsoil. Over the past decades, several types of research have revealed significant roles played by plant roots to minimize the detachment rates of the soil as a result of concentrated flows and are therefore very effective in controlling landslides [104,105]. Vegetation roots support the slope drainage system and act as a scale preferential flow direction on the hillslope and drain the subsoil's water content from unstable terrain. When vegetation root systems converge, or the subsurface flow ends abruptly in the slope, it may lead to a concentration of water pressure in a critical region of the hill, thereby leading to instability. Flow direction may occur, resulting in both positive and negative outcomes on slope stability. Practical and precise knowledge of the disposition of vegetation roots in the slope is necessary to ge<sup>t</sup> it right. In this study, the vegetation root architecture was determined using the pull-out tests introduced by Yen [91]. Root samples from each of the four species were uprooted from the field, washed, and the root length was measured using a measuring tape, and the values were recorded and plotted in Figure 8.

**Figure 8.** Graph plot of the monthly vegetation root length.

The root architecture of the four seed species in Figure 9a–d shows the typical distribution of the root system. It offers a general idea of how the roots developed and indicates the localization of deep-rooted and fibrous roots within the root system. Ryegrass has a fibrous root system with thick primary roots and thinner lateral branches and showed a poor result. The roots of ryegrass are usually arbuscular mycorrhizal. Flawed due to the environmental and climatic conditions, ryegrass, if under suitable conditions, may germinate faster than some other grass seeds. However, the roots spread slowly and grow naturally into clumps that spread their shoots vertically, called tillers. The shallow roots of ryegrass limit its tolerance for heat and drought. It adapts well to a wide variety of improved soil conditions, such as acidic and alkaline soils. It also thrives better under soil pH. Rye corn showed an extensive, fibrous root system that may expand.

**Figure 9.** The different vegetation root samples: (**a**) Ryegrass (G1); (**b**) rye corn (G2); (**c**) signal grass (G3); (**d**) couch (G4).

On the other hand, signal grass showed a better root development in the soil with a deep taproot system that can effectively grow deeper on a wide range of soils and adapt to different environmental conditions. The experiment also reveals that the couch has the most extended root system, with a shallow taproot system that can spread, anchoring the shallower soil depth and around the subsoil. The experiment so far compared the effectiveness of the four seed plant roots in stabilizing the soil against possible landslides. However susceptible to incisive landslide occurrence, no research has been done to compare the effectiveness of these four plants in controlling landslides in this study location. Hence, this study introduced the landslide-reducing potential of these species with both fibrous and tap roots systems on the slope of the study area. The experimental results showed that plant roots with taproot systems were more adaptive and efficient in the soil type than the seed with a fibrous root system. According to [106], tap-rooted plant species penetrate more into thick soils than fibrous-rooted plant species. Therefore it is well adapted for use in landslide mitigation and control.

The different vegetation roots showed a significant variation regarding the site characteristics; with an increase in rainfall, the roots considerably showed their best tolerance following an increasing monthly pattern, with the highest length observed from G4 in December, followed by G3. Both G1 and G2, with a fibrous root system, also showed a significant monthly root development. The vegetation root was plotted using an origin software.

#### *4.3. Result of the Hydroseeded Vegetation Ground Cover*

Hydroseeded vegetation ground cover is the percentage of the ground surface covered by vegetation. It controls landslides by anchoring the soil against rainfall and other landslide causative factors. The plants were monitored, and the vegetation ground cover was recorded from August to December.

The species with the highest values was ryegrass (G1). It showed leaf spreading and creeping characteristics. From August to September 2020, the experimental result recorded 40% vegetation cover due to its prolonged germination rate. A significant 180% of coverage was recorded between November and December. Rye corn (G2) was second to ryegrass and reached 160% coverage between November and December. However, the values sharply increased from 60% to 120% between August and October 2020. Signal grass (G3) showed the lowest result with 40% to 80% between August and October and 95% between November and December. Moreover, couch (G4) gave values between 80% and 120% from August to October and remained unchanged until December with 140%.

Vegetation ground cover can help to prevent landslides by protecting the soil surface against the impact of rainfall and surface runoff. It also reduces runoff volume, increases surface roughness, and reduces sediment traps and transportation [105,107]. The result was plotted using the Origin software, as shown (Figure 10) below.

**Figure 10.** A plot of the vegetation ground cover.

At the start of the monitoring period, August 2020, G1 and G3 showed the lowest vegetation cover while the vegetation cover reached 80% in G4. Between September and December, the vegetation covers of G1 significantly increased, topping other species and showing the best choice for control of surface landslide. Test G3 showed the least acceptable result with a 95% vegetation cover.

#### *4.4. Vegetation Surface Runoff*

The experiment focused on analyzing the overall influence of the vegetation root and vegetation ground cover to reduce surface runoff, reducing the effects of landslides on the study area. The four different vegetation species were subjected to rainfall intensities of 35 mm/h in September, 48 mm/h in October, 33 mm/h in November, and 34 mm/h in December on a Malacca and Serdang soil series sloping at 65◦, as shown in Table 5 below. The area's soil type is a mix of sandy, clay, and loam, collectively identified under the

Serdang and Malacca series. In line with past literature, the runoff rate increased due to the percentage of vegetation ground cover and vegetation root characteristics [108].


**Table 5.** Showing selected daily runoff under different rainfall intensities.

The results show that G2 has the highest surface runoff rate followed by the G1 plot, then G3, with G4 having the least runoff amount under different rainfall intensities. The surface vegetation runoff of the vegetation is shown in Figure 11 below.

It shows that the runoff rate changes due to their vegetation characteristics under the same rainfall intensity. One can observe from Figure 11 that the amount of surface runoff in the G1 and G2 plots was significantly higher than the G3 and G4 plots under the same rainfall intensity.

The average surface runoff of G4 is 4.85 mm, while G3 comes behind with an average surface runoff value of 5.89 m3. G2 and G1 also have surface runoff values of 7.01 mm and 7.43 mm, respectively. Couch vegetation offers the most acceptable landslide control benefits to signal grass, rye corn, and ryegrass vegetation. The findings sugges<sup>t</sup> that the selection of couch species over other species is justified based on landslide control benefits.
