Background and Objective: Most forest roads in Golestan Province are unpaved or gravel-surfaced and generate considerable amounts of dust during the dry season, particularly in summer, due to vehicle traffic and wind action. Besides reducing driver visibility and traffic safety, dust emissions contribute to environmental pollution, diminish the recreational value of forest areas, and accelerate surface deterioration of forest roads. Therefore, identifying sustainable dust control measures is essential. One practical approach in arid and semi-arid regions is the application of gravel mulch, which increases surface roughness, reduces near-surface wind velocity, and traps fine particles. This study aimed to evaluate the effects of gravel type and surface cover percentage on dust suppression along the Arab-Dagh forest roads in eastern Golestan Province.
Material and Methods: A portable wind tunnel was used to simulate wind-induced soil erosion and dust emission under field conditions. The tunnel had a contact area of 60 × 38 cm, and road sections with similar geometric characteristics (slope, curvature, and width) were randomly selected. Three gravel types, including small cobbles (>3 cm), almond-shaped gravel (1.5–3 cm), and fine gravel (<1.5 cm), were applied at four surface cover levels (0, 25, 50, and 75%). After application, the treated surfaces were compacted using a manual cylindrical roller to ensure initial stability. Experiments were conducted at wind velocities of 2 and 4 m s⁻¹, representing typical airflow generated by moving vehicles. Each treatment was replicated three times, resulting in a total of 72 experimental units. Dust collected in the silicone chamber at the end of the wind tunnel was weighed using a digital balance with a precision of 0.1 mg. Data were analyzed as a factorial experiment using the General Linear Model (GLM) in SPSS at the 99% confidence level.
Results: Gravel type, surface cover percentage, and wind velocity each had significant effects on dust production (P < 0.01). The interaction between gravel type and cover percentage was also significant, whereas the three-way interaction with wind velocity was not (P > 0.05). All gravel treatments significantly reduced dust emissions compared with the control, with small cobbles showing the greatest effectiveness. Mean dust production decreased from 15.17 g m⁻³ in the untreated control to 9.72, 11.09, and 11.87 g m⁻³ for small cobbles, almond-shaped gravel, and fine gravel, respectively. Increasing surface cover significantly enhanced dust suppression. At 75% coverage, small cobbles reduced dust emissions by 81% and 71% under wind speeds of 2 and 4 m s⁻¹, respectively. Under the same coverage, almond-shaped gravel reduced dust by 44% and 50%, while fine gravel achieved approximately 44% reduction at both wind speeds. The lowest dust suppression occurred at 25% surface cover. Increasing wind velocity from 2 to 4 m s⁻¹ significantly increased dust emissions. The results further demonstrated that gravel size and particle shape play a key role in improving soil surface stability. Small cobbles increased surface roughness, reduced near-bed airflow velocity, filled surface voids, and effectively trapped suspended particles, thereby providing the highest dust control efficiency.
Conclusion: Depending on particle size and surface coverage, gravel mulch can substantially reduce wind erosion and dust emissions from forest roads. Given the availability of suitable rock borrow sources in eastern Golestan Province, applying small cobbles at approximately 75% surface coverage is recommended as an economical, durable, and environmentally friendly method for controlling dust along the Arab-Dagh forest road network. Further studies are recommended to evaluate the long-term durability and seasonal performance of gravel mulch under natural rainfall and field conditions.