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.