Road segment effect on surface flow pattern and soil loss
Generally, the net effect of non-paved road networks in a forest was
reported to increase basin-wide sediment production (Brooks et al.,
2006; Soulis et al., 2014; Wemple et al., 2001;). Nevertheless, sediment
deposition is also likely where sediment-laden overland runoff reaches
low gradient road surfaces, (Finkner, Nearing, Foster & Gilley, 1989;
Gucinski et al., 2001; MacDonald and Coe, 2008). In post-fire
conditions, Sosa-Pérez and MacDonald (2017) found that sediment deposits
were much more likely on roads with gradients less than or equal to 5
percent. In this study, Road 1, the highway, had a gradient of about 4
percent, and experienced considerable deposition, whereas Road 2 had a
gradient of 7 percent, and little deposition was observed in the road
drains. The results in Table 3 and Figure 4 show that after the fire,
road segments intercepted upstream runoff: and in Figures 5 and 7, were
sites of sediment deposition. In the case of continuous upstream runoff
accumulation along natural hillslopes, discharge is assumed to increase
linearly and thus flow shear stress can be expressed as a function of
downslope distance (Finkner et al., 1989). This explains the increasing
soil loss rate below the ridge (Figures 6a and 7) and before the
deposition on road surface (Figure 7).
Two processes account for the high onsite soil loss rate within the 10 m
buffer belt below road segments (Figures 6 b and c) and the sharp soil
loss increase after leaving the road in Figure 7. First, with the large
amount of post-fire sediment deposition along road surfaces, the
increased flow below roads has a relatively low sediment concentration
and therefore can detach more soil particles (Finkner et al., 1989).
Secondly, the road segments have locally oversteepend fillslopes,
because the excavated material uphill of the road centerline is placed
downhill of the centerline. Further downhill, the slope decreases as
does the predicted erosion rate, until ultimately, deposition begins to
occur (Figure 5).
Figures 3, 4 and 5 all show different ways of seeing how a road can
alter the boundary of sub-catchments and hydrological units on a
post-fire landscape. Table 4 and Figure 3 show that a road segment can
act as a boundary between sub-catchments, and in some cases, forming new
sub catchments downhill from a road (Figure 4). The new sub catchments
below roads tend to have shorter flow paths, and hence, are at less risk
to concentrated flow erosion (Table 5). On the other hand, the flow
length accumulation along roads in Figure 4 shows that road segments can
also function as flow diverters, transporting sediment from one
sub-catchment to a nearby sub-catchment. Road 2 is a good example of
flow diversion as it has been identified as a channel in Figure 2,
increasing the runoff and sediment yield from the adjacent subwatershed.