Evaluating the Impact of Road Longitudinal Slope Angle in Forest Roads (Main Access and Main Roads) on Runoff and Soil Loss. (Case Study: Gardeshi District, Mazandaran)

Forest is a sustainable ecosystem with a high balance, and is characterized by non-eroded soils and sediment-free water. Forest roads serve as the largest construction activities and a component added to the forests which are required for the accessibility to forests and forest resources, logging, transportation, conservation, tourism, etc. However, these changes in the dynamic environment of forest structure cause many disruptions in the natural behaviors of environmental factors affecting the forest structure. Roads create surfaces without vegetation which can serve as a starting point for runoff, disrupting and diverting the natural flow and sudden accumulation of water, diverting the natural down-slope drainage route, accumulation of water and transmitting it to other sub-basins, changing the runoff pattern and carrying deposits due to the increased water velocity and ultimately the destruction of the forest. Studies have shown that forest roads are among the main sources of sediment transport into rivers, and approximately 90% of the sediment coming from a forested area into rivers originates from forest roads. Increased sediment transport into rivers can cause irreparable damages to water quality, ecosystems and aquatic organisms. Therefore, forest road experts should consider not only the road construction costs but also the environmental damages caused by the road operation.
Among the challenges for forest managers is the accurate design and construction of forest roads with required standards and minimum possible costs. Ignoring these defined measures and standards will result in the financial and environmental damages. One of the most important standards for road construction is road longitudinal slope angle. Determining the relationship between road longitudinal slope angle with runoff amount and the volume of produced sediment helps to manage the road network for constructing new roads in accordance with the standards, to define new standards, and to predict the maintenance of roads.
This research aims at evaluating the impact of road longitudinal slope angle in the forest roads (Main Access and Main Roads) of Gardeshi district (Sari, Mazandaran Province) on runoff and soil loss through applying simulation techniques and artificial rainfall simulator in a completely randomized design on 1 m2 plots with 4 replicates in 3 slope classes in each road. Then, sediment and runoff during rainfall simulation performed with a constant intensity of 60 mm h-1 was measured for 30 minutes in each slope class. The district was selected so that the other parameters such as road surface area, traffic, and rainfall were fixed, and only the effect of road longitudinal slope angle on runoff and sediment was addressed.
Results showed the average produced runoff, runoff coefficient and sediment concentration in slope classes of 1 (3 – 5%), 2 (5 – 8%) and 3 (more than 8%) on grade 1 road. The runoff volumes were 15.9, 17, and 18.78 L m2, with the corresponding runoff coefficients of 53%, 56.66%, and 62.58% and soil loss values of 6.12, 9.05, and 12.38 g L-1, respectively. Results also showed the average produced runoff, runoff coefficient and sediment concentration in slope classes of 1 (0-3%), 2 (3-6%) and 3 (6-9%) on grade 2 road. The runoff volumes were 19.92, 21.25, and 22.8 L m2, with the corresponding runoff coefficients of 66.41%, 70.83%, and 76% and soil loss values of 8.51, 10.17, and 16.82 g L-1, respectively. Results of ANOVA showed a significant difference between the amount of runoff and sediment in road slope classes. The results of Tukey test indicated that runoff and sediment significantly increased with an increase in the slope. Results of Independent t-test showed that the mean values of runoff volume, runoff coefficient and sediment concentration were 23.33 L, 71% and 11.84 g per liter in the grade 2 road compared to 17.23 L, 57% and 9.18 g per liter in the grade 1 road. According to the F (0.85) and P (Discussion & It can be concluded that road longitudinal slope angle is one of the most important factors determining the amount of runoff and sediment on the road, so that the amount of runoff and sediment will increase with an increase in the slope, which should be considered according to the location of road.