Road segment effect on hydro-geomorphology characteristics
Road-related flow paths were extracted with different CSA values and a detailed analysis was carried out on the effects of roads on predicted erosion rates. ArcMap flow paths that intersected with the upslope side road edges were recognized as inflow, and outflow paths were those that intersected with road edge on the downslope side (Table 3). As the CSA was reduced, the number of flow paths increased and the average distance between flow paths decreased. Table 3 shows that the number of outflow paths is always less than that of the inflow paths. This effect is especially noticeable for the smaller CSAs with more dense flow paths, where a larger portion of inflow paths are intercepted by road segments and result in relatively fewer outflow paths than that of the 1000-m2 CSA. Road 2, which falls below road 1 showed a lower density of outflow paths with a similar inflow density when CSA was 1000 m2. In the case of the 200 m2 CSA, the average distance between outflow paths for road 2 is more than twice the distance than that of the road 1.
Further analyses were conducted to demonstrate road segment effect on surface hydrogeomorphic characteristics. Road segments that fall inside the burn boundary were chosen and the surrounding sub-catchments were delineated with the critical source area (CSA) of 0.5 ha (Figure 3). The hydrological units were then overlaid with the surface topography and road segments to determine the effects of roads on sub-catchment delineation. All sub-catchments in Figure 3 are connected to roads and can be classified as five types according to the spatial position with road segments (Table 4). There are 17 sub-catchments which pass through road segments and finally reach the fire boundary. These hydrologic units account for 47% of the total area. Small channel diversions were predicted when the sub-catchment channels intersected with road segments. On the other hand, 43% of the sub-catchments which started from ridge were intercepted and ended by the two road segments. In addition, 9 new hydrological units were formed downslope of road segments. In spite of being shorter, road 2 with its numerous drainage features, is more efficient than road 1 in intercepting upstream sub-catchments and generating new ones.
The effect of road segments on surface hydrological process was further studied in the context of flow length. Upstream flow length was overlaid with road segments (Figure 4). The highly detailed 2-m LiDAR DEM shows a large number of flow initial points (flow length = 0, blue color) along the hillslope. This illustrates that road segments may stop the upstream flow length accumulation and start new flow paths below roads, depending on the road drainage features. Especially for road 2, which has ditches, most of the upstream flow paths were intercepted. For road 1, which has no ditches or culverts for most of its length, many upstream flow paths are not intercepted and pass over road 1 after a slight direction change. The flow then continues to accumulate length in the downslope direction.