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.