Impacts of road surface shape on soil erosion of rural unpaved road

Abstract: Unpaved rural roads are thought to be one of the major sources of sediment in small watershed. Measurements of erosion from unpaved roads in rural regions will be taken to improve our ability to predict the delivery of sediment and its effects on water quality. Road surface shape is an important parameter of the road micro-structure. The road surface shape change the erosion features and play a great role in altering the runoff-sediment transportation processes of the road system. In this paper, 4 typical road surface shapes in rural regions of China were chosen to study the impact of surface shape on road erosion. Soil was sampled from rural road surface. After air dried and sieved through a 2 mm screen, soil was filled in a soil bin with the bulk density of 1.65 g/cm3, which was the most common soil bulk density of unpaved road in study area by field survey. The surface of soil was shaped in convex, lateral, flat, and concave to simulate 4 common road surface shapes. The slope of lateral pavement was set to 4°, and the radian of arch pavement (including concave and convex) was set to 0.1. All of the simulated roads were tested in treatments with 3 slopes, which were 5°, 10° and 15°. American SPRACO cone nozzles were used in rainfall simulation with the vertical height of 4.75 m from the ground. The rainfall intensity was 60 mm/h, and the rainfall duration was 45 min. Runoff and sediments were collected in every 3 min. The runoff coefficient, soil loss rate and a set of hydraulic parameters such as Reynolds number, Froude number and Manning's roughness coefficient were calculated after experiment. The results showed that the runoff was generated rapidly on road surface. High bulk density of pavement soil led to less infiltration and larger runoff coefficient. The initial runoff-yielding time of all types of roads was in 30 s, and the runoff coefficient was greater than 70%. The impacts of shapes on sediment production were much higher than that on runoff generation, so the capability of shapes in changing the hydrological characteristics of the road surface flow was much more important to sediment. The concave road received the highest soil loss rate due to the highest flow velocity and erosivity. Totally speaking, the flow pattern on concave was determined as torrent flow while that on convex and lateral road was subcritical flow. The eroded sediment particle sizes on convex and lateral were much smaller than that on concave and flat. The soil erosion on convex road was the lightest for the micro-structure on erosion controlling. The sensitivities of concave and flat road to slope degree were higher, and sediment production and flow erosivity had great enhancement with the increasing of slope degree. Differences of road surface shapes led to different erosion characteristics. Optimized combination of different road shapes and "road-canal" can reduce the impacts of road construction on the surroundings in the future road design to the most extent.