Investigation of forest road surface sediment estimation using two experimental models of SEDMODL and WARSEM

Extended abstract 1- Introduction A bare surface on forest roads is created due to road construction. This surface is the main source of erosion and sediment yield to streams in forest areas. The increase of sediment in streams causes dramatic damage to the quality of water ecosystems and the life of aquatic organisms. Therefore, road engineers should pay attention not only to the cost of road construction but also to its environmental damage. So, it is necessary to reduce the amount of erosion and sediment produced by these roads. With the accurate prediction of erosion and sediment yield of roads, it is possible to mitigate the negative effects of sedimentation and manage the region, sustainability. In this regard, several models such as WARSEM and SEDMODEL have been introduced to estimate the sediment and to identify the sensitive erosion points. These models are often used to estimate road surface erosion in forest regions. Therefore, the purpose of this study was to evaluate different models in estimating forest road sediment and compare them to field measurements. The findings of the study are useful in finding a suitable and realistic model in the estimation of erosion and sediment of the forest road.   2- Methodology Two prediction models, including WARSEM and SEDMODEL, were used to estimate the amount of annual sediment production of forest roads. For this purpose, 2602 m of roads in compartments 202, 212, and 244 in the Rezaian forestry plan of ZarrinGol forests were selected. The total length of roads in the region was divided into homogeneous units, and then factors of road length and width, geological condition, road surface, traffic, longitudinal road slope, rainfall, and sediment transport were calculated using GIS maps (road layer, river, waterway, geology, edaphic and topography), forestry plan booklet and field measurements. A sediment trap and suitable container were installed at the end of each unit to measure the actual amount of road sediment after each rainfall. The volume of water was measured (in L) after the deposition of sediment in the container, and then the deposited sediment was taken out of the container and placed in an oven. After drying, the amount of sediment was calculated (in g/m2). In the laboratory, the sediment concentration was obtained by filtering the suspended load sample and passing the runoff sample through the Whatman 42 filter paper. The sediment samples were then placed in the oven at 105 °C for 24 hours. The samples were weighed in a Desiccator using a digital scale (one-thousandth accuracy), and the sediment concentration was obtained by dividing the sediment mass (g) by the runoff volume (L).   3- Results Results showed that the estimated sediment by SEDMODL and WARSEM for different roads and compartments were 23.87 and 20.07 tons per year, respectively. In addition, the longitudinal slope had a significant effect on the amount of sediment production estimated by the two models WARSEM and SEDMODL, at a probability level of 5%. While this factor has no effect on the amount of sediment measured in real conditions amount of sediment estimated by the models in the slope class of 5-10% was significantly higher than the slope class of 0-5%. There was no significant difference between the amount of sediment estimated by WARSEM and SEDMODL models; however, estimated values were significantly higher than the measured value at the probability level of 5%. Validation of WARSEM and SEDMODL models and their comparison with the field measured value showed a significant difference. Both models estimated the amount of sediment more than the field measured value. The results also showed that the sediment delivery potential estimated by WARSEM and SEDMODL was related to the part of the road located in parcel No. 202.   4- Discussion& Conclusion By calculating the amount of sediment production in the road and various parts using WARSEM and SEDMODL and comparing them with the measured value, it was found that the total amount of sediment estimated by WARSEM and SEDMODL was 20.07 and 23.87 tons per year, respectively. In general, there was no significant difference between the amounts of sediment estimated by WARSEM and SEDMODL, but these estimated values ​​were higher than the field measured value at the probability level of 5%. Moreover, the effect of the longitudinal slope of the road on sediment production was also studied. According to the factors measured by WARSEM and SEDMODL, it is necessary to design roads on geologically resistant formations, improve the pavement quality, decrease the level of sediment delivery and reduce the traffic.