| Construction and development of road network is the foundation for regional economic development, and plays an important role in energy flow, information flow and logistics exchange between different regions. However, acted as a pure artificial landscape, the road network significantly effects the surrounding environment. The soil erosion formed by the road construction activity has become a kind of loss of soil types which should not be ignored in the Three Gorges reservoir area, effects the watershed hydrological processes, influences the receiving water quality, and accelerates the siltation of rivers and reservoir. Carry out the researchs of road erosion mechanisms and erosion protection (especially on the low-volume unpaved road network), can partly explain the problem of basic research lagged practice in Three Gorges Reservoir area.In this paper, an unpaved road was selected as the study objective, the road network was departted into road cutslope, road surface and road fillslope. Seven vegetation restoration model were selected for road sideslope vegetation recolonization, such as Natrual restoration (NR); Grass and shrub (GS); Sodded strips (SS); Terrace with grass and shrub (TGS); Grass (GR); Farmland (FL); Farmland with grass (GF). Rainfall simulation, scouring simulation and in-situ soul strength test were employed in this study to study the impacts of vegetation on topsoil structure and soil shear strength improvement, runoff and sediment reduction, and then selected the optimal model for different road sideslope erosion control. Four road shape surfaces were tested under rainfall simulation to identify the influence of shapes on road surface erosion processes, runoff hydrological characteristics, and runoff and sediment transportation. Five native road segments were selected to study the impacts of traffic load and nature rainfall on soil loss and contaminants first flush effects. Some results are list as:1. To study effects of roadside slope vegetation restoration on shear strength (τ) of and surface soil loss from the slopes, seven vegetation restoration models were adopted in in-situ shearing tests. It was found that vegetation root systems decreased in density with increasing soil depth, the root length density (RLD) in the 0-10 cm soil layer accounted for 34～78% of the total RLD And the vegetation root magnitude was positively related to soil water content (SWC) showing a power function correlation. Shear strength of the surface soil layer (0-10cm) was mainly affected by vegetation root systems and vegetation coverage and increased as a power function of RLD, root weight density (RWD) and vegetation cover (VC). The controlling factors on τ varied with soil depth, and regression analysis showed RLD, RWD and VC had statistically significant effects on τ in the 0-10 cm soil layer. However, none of the predominant factors onτwere founded in the 10-30 cm soil layer, which was so defined as a transition layer and was the lowest shear strength τ as compared with the 0-10 cm and 30-50 cm soil layers. The influence of soil bulk density (p) and SWC got bigger onτ with soil depth, showing a positive linear relationship and a negative power function relationship, respectively in 30-50 cm soil layer.2. Vegetation recolonization has often been used to control roadside slopes erosion, and in this part, four restoration models-Natural Restoration, Grass, Grass & Shrub, Sodded Strip-were chosen to recolonize the plants on a newly built unpaved roadside slope in the Three Gorges Reservoir Area. After eight months growth, eight rainfall simulations (intensity of 90 mm h-1 for 60 min) and in-situ soil shear strength test were then carried out to identify the impacts of vegetation on roadside slope erosion and soil shear strength. The erosion on cutslopes was higher than that on fillslopes. The runoff coefficient and soil detachment rate were significantly lower on the Grass & Shrub model (4.3% and 1.99 g m-2 min-1, respectively) compared with the other three, which had the highest surface cover (91.4%), aboveground biomass (1.44 kg m-2) and root weight density (3.94 kg m-3). Although revegetated with the same grass species, the soil erosion on the Sodded Strip model was much higher (38.5 g m-2 min-1) and had a lower root weight density (1.50 kg m-3) and root length density (2.11 km m-3) compared with Grass (11.5 g m-2 min-1 of soil erosion,3.01 kg m-3 of root weight density and 4.73 km m-3 of root length density, respectively) for the thin and not fully developed roots. The runoff coefficient and soil detachment rate on roadside slopes showed a logarithmic decrease with the root weight density, root length density and aboveground biomass. The soil shear strength measured before and after the rainfall was higher on Grass & Shrub (59.29 and 53.73 kPa) and decreased on Grass (46.93 and 40.48 kPa), Sodded Strip (31.20 and 18.87 kPa) and Natural Restoration (25.31 and 9.36 kPa), whereas the soil shear strength reduction during rainfall was smaller on Grass & Shrub (5.56 kPa). Negative linear correlations were found between the soil shear strength reduction and aboveground biomass, root weight density and root length density. The variation of soil shear strength reduction was closely related to the roadside slope erosion, a positive linear correlation was found between runoff coefficient and soil shear strength reduction, and a power function was shown between soil detachment rate and soil shear strength reduction. This study demonstrated that Grass and Grass & shrub were more suitable and higher cost-effective in controlling initial period slope erosion of newly built low-volume unpaved road.3. Roads play a significant role in altering hydrological processes. Roads cause more erosion to be generated from the fillslope for both road-concentrated flow and rainfall. In this study, six treatments (Natural Restoration, Grass, Grass & Shrub, Sodded Strip, Grass & Farmland, and Farmland) were used to recolonize fillslope plants on a newly built unpaved road. Rainfall simulation (rainfall intensity of 90 mm h-1 and 120 mm h-1) and scouring simulation (scouring flow rate 15 L min-1 and 20 L min-1) tests were conducted to identify the effects of plants on soil erosion. In the rainfall simulation test, Grass & Shrub was more effective at reducing fillslope erosion than the other treatments, and Grass & Shrub also had a lower runoff coefficient, soil detachment rate, and higher efficiency in trapping runoff and sediment. The hydrological responses of all of the tested plots in the scouring test were much faster than in the rainfall simulation, as indicated by the lower lag time to runoff generation. The fillslope erosion in the scouring test was significantly higher than in the rainfall simulation. The water-stable aggregate, saturated hydraulic conductivity, vegetation cover, root length density, and root weight density were important factors that conditioned the runoff generation and sediment yield from the fillslope in both experimental tests. In the scouring test, in infiltration improvement and flow erosivity reduction, Grass was more effective at trapping runoff and sediment because of its dense well-developed system of fine roots. Therefore, except for an immediate surface cover, in the areas where the fillslope has a risk of road-concentrated flow scouring, the enhancement of topsoil roughness was also very important in weakening the impact of road-concentrated flow on the fillslope.4. The impact of road construction and road networks on soil erosion has been examined in recent years. However, minimal attention has been given to studying and comparing shorter road sections. In this study, laboratory rainfall simulation was performed to evaluate the soil erosion and flow hydrological characteristics of simulated roads with four road surface shapes, such as convex (CV), inslope&outslope (I/O), flat (FL) and concave (CC) shapes. Simulated ditches, which were composed of stainless steel, were established on both sides of the roads. The results indicated that the road surface shape significantly affected the erosion process. The FL and CC shapes generated higher runoff rates and runoff volumes than the CV and I/O shapes. The CC shape exhibited the highest soil detachment rate, which was 11-14 times larger than the soil detachment rate from the CV shape. The flow hydrological characteristics were also influenced by the road surface shape. The mean flow velocity, Reynolds number and Froude number increased significantly in the order CV<I/O<FL<CC. The water depth, Darcy-Weisbach resistance coefficient and Manning’s roughness coefficient decreased in the order CV>I/O>FL>CC. The road surface shape caused a re-routing of the surface flow paths and subsequently affected the role of roadside ditches in runoff and sediment delivering. For the CV shape, 67-82% of the runoff and 85-94% of the sediment produced on the road surface were delivered by the roadside ditch. For the CC shape, however, the rates decreased to 3-5% for runoff and 4-7% for sediment.5. The existence and development of the road is a great threat to the soil erosion and the safety of water quality. In this paper, two unpaved roads with different traffic load, which located in the Jieya watershed Three Gorge Reservoir area, were selected to discuss the soil and No-point source pollutants loss processes under the nature rainfall events. The results show that both the runoff rate and erosion rate were higher on main road (high traffic load) than that on branch road (low traffic load). The rainfall precipitation was the key factor in altering the runoff generation. Except for the rainfall precipitation, rainfall intensity also played great role in affecting the sediment yield. The rainfall process had a significant effect on the No-point source pollutants loss, under different rainfall events, the time lag effects of pollutant loss process and rainfall differed. The nutrient of nitrogen was lost in the form of TN, which contained of 80% of the total. The nutrient of phosphorus was lost in the form of particulate phosphorus (PP), which contained of 80%, and the dissolved phosphorus (DP) contained of 20%. The rainfall process altered the first flush effect of the No-point source pollutant, according to the investigation, all the pollutants had the first flush effect (FFE) phenomenon, the FFE on suspended soil was slight, but the FFE on TN, NH4-N, PP and DP were much heavy.The results in this paper provided practical approaches for controlling roadside slope erosion, and can be used in an adaptive design to guide future road construction projects and roadside slope protection. Although the results obtained here were specific to the relevant condition of the Three Gorges Reservoir Area in Hubei Province, the approaches can be adapted for many other regions of the world. |
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