Estimating sediment yield from a paved forest road (Asphalt) (Case study: Watershed Leyle, Ravansar)

Introduction Forest roads are among the largest sediment transport sources to drainage network. So, estimating the sediments of the road network is an important factor in the evaluation of the impact of the road construction. One of the biggest problems caused by the road construction in the forest is the increase in various forms of erosion, such as Gully erosion and masses movements around the roads. The main cause of this process is the destruction of vegetation, soil destruction, increasing runoff rate and unstable slopes. Therefore, in designing forest roads, not only the costs, but also the proper management of the water and soil should be considered. Research results have shown that the forest roads produce a high volume of forest land sediments. An optimal management of forest areas requires having a forest road network with a proper distribution on the forest. Hence, it is one of the most important cost factors for the forest management. Forest roads differ from rural roads and highways. They have low traffic and heavy vehicles. In the areas where these roads are built, there are usually different types of erosion. Therefore, the ability to predict erosion correctly can reduce their negative sedimentation effects. Accordingly, the purpose of this study is to estimate the sediment and erosion of different parts of Shabankar forest road in Zagros region, and to determine the final rate of the road sediment yield, as well as to identify the high potential deposits of the sediments by SEDMODEL.
Methodology In this study, Road Sediment Delivery Model (SEDMODL) was used to estimate the erosion of a paved forest road in 24 km northwest of the Rawansar in Kermanshah province. The road was divided into 5 parts and the factors such as geology, elevation and the cover of cut slope, road traffic, the distance from the streams and rainfall were estimated that were the effective parameters of the model. To estimate the amount of the sediment production, after an evaluation of the factors of the model, the erosion and sediment delivery amount to the streams were estimated in 5 sections of the road. In order to implement the model, the geological maps, topography, slope, road and waterway networks with a scale of 1: 50,000 were produced in the ARC GIS environment.
Results The results showed that the fifth part of the road has more sediments delivery to the waterways among the other sectors. According to results, the total sediment production is 11.92 tons and the sediment delivery to the waterways is 2.06 tons while the area of this road makes about 0.03 percent of the total watershed. In addition, according to the Pearson correlation results, the height of the cut slope and the longitudinal slope of the road were recognized as the main factors affecting the erosion of the road. In this area, the factors such as the length of the road, longitudinal slope, cut slopes and vegetation, and sediment delivery factor were different in various parts of the road leading to differences in erosion and sediment production. The third part of the road, due to having the highest amount of road length and wall height and the minimum coverage, has the highest amount of erosion. After that, the highest erosion belongs to the fourth part, while the length of this sector of the road is less than the first and second parts, due to the high height of the excavation walls in the fourth part, as well as the absence of the wall coverings relevant to the parts one and two. According to Table 10, only the second and fifth parts, due to the intervals of less than 60 meters from the closest waterway, were involved in the delivery of the sediment to the currents of the basin. The high erosion rate in the fifth part is greater than the third and fourth parts due to the minimum level of road surface covered by the fifth part compared to that of the second.
Discussion & Conclusions The construction of roads in the forest areas is one of the factors affecting erosion and sediment production. As the results of this study showed, one of the capabilities of the SEDMODL model is identifying the erosion-sensitive points. According to the results of this research and other similar studies, it is necessary for the erosion control activites to be done in the upstream of the roads having high erosion potential due to the steeper slope to the lower parts, as well as in downstream units due to being close to the waterways and having sediment transfer factor to reduce erosion per unit area of the road.