Rill erosion in natural and disturbed forests: 1. Measurements

Abstract

[1] Rill erosion can be a large portion of the total erosion in disturbed forests, but measurements of the runoff and erosion at the rill scale are uncommon. Simulated rill erosion experiments were conducted in two forested areas in the northwestern United States on slopes ranging from 18 to 79%. We compared runoff rates, runoff velocities, and sediment flux rates from natural (undisturbed) forests and in forests either burned at low soil burn severity (10 months or 2 weeks post-fire), high soil burn severity, or subject to skidding of felled logs. The runoff rates and velocities in the natural sites (2.7 L min −1 and 0.016 m s −1 , respectively) were lower than those in all the disturbed sites (12 to 21 L min −1 and 0.19 to 0.31 m s −1 , respectively), except for the 10-month old low soil burn severity site where the velocity (0.073 m s −1 ) was indistinguishable from the natural sites. The mean sediment flux rate in the natural sites also was very small (1.3 × 10 −5 kg s −1 ) as compared to the rates in the disturbed areas (2.5 × 10 −4 to 0.011 kg s −1 ). The hillslope gradient did not affect the runoff or sediment responses. The sediment flux rates generally were greater in the initial stage of each inflow period than in the steady state condition, but there was no corresponding transient effect in runoff rates. Rill erosion modeling implications based on these data are presented in part 2 of this study.

Measurements of the initiation of post‐wildfire runoff during rainstorms using in situ overland flow detectors

“…Runoff did not start simultaneously on the hillslope and in the microdrainages. In general, T r in the micro-drainages were inversely correlated (power law, R 2 = 0.54) with rainfall acceleration, a, reflecting the Abrahams et al, 1986 Desert soils 0·092-0·687 1·1-7·7 6·6-37 Bunte and Poesen, 1994 Bare soils in flume 0·014 1·3-11 5-18 Emmett, 1970 Semi-arid soils 0·029-0·33 0·2-46 0·18-12 Nyman et al, 2013 Burned soils 0·36-0·40 6-14 62-114 Robichaud et al, 2010 Low burn severity 0·24-0·64 6·3 ± 2·2 7·3 ± 5·8 Robichaud et al, 2010 High burn severity 0·23-0·75 6·5 ± 2·0 31 ± 12 This paper, 2005 Harvard fire High burn severity, hillslope facets 0·38-0·79 -8-21 This paper, 2010 Fourmile fire High burn severity, hillslope micro-drainages 0·18-0·65 -7-14 dependence indicated by Equation (2), but they were not correlated with either θ i or maximum I 1 . Observed T r in the micro-drainages ranged over two orders of magnitude from 0·3 to 79·4 minutes (Table II).…”

“…These have been made in laboratory flumes (e.g. Abrahams et al, 1986;Abrahams and Parsons, 1991;Parson et al, 1996;Sheridan et al, 2007;Robichaud et al, 2010;Nyman et al, 2013), and a few have used both types of simulations (e.g. Emmett, 1970;Dunne and Dietrich, 1980;Abrahams and Parsons, 1991;Gilley et al, 1992;Kinner and Moody, 2010) or overland flow simulation (e.g.…”

Effects of gradient, distance, curvature and aspect on steep burned and unburned hillslope soil erosion and deposition

“…In turn, because aspect (the facing direction of a hillslope) impacts soil and vegetation characteristics, it can modulate or amplify erosion. Likewise, a loss of surface litter and vegetative cover following fires can promote increased runoff rates and velocity, increasing sediment flux rates (Robichaud et al, 2010). For the Northern Hemisphere, south-facing slopes are generally warmer, drier, have shallower soils and are more sparsely vegetated than north-facing slopes (e.g.…”

Constraining the relative importance of raindrop‐ and flow‐driven sediment transport mechanisms in postwildfire environments and implications for recovery time scales

“…Raindrop-driven transport refers to sediment being transported by rainsplash as well as sediment that is detached by raindrops and then subsequently transported by overland flow [Kinnell, 2005]. Rates of rill erosion, however, have also been shown to increase substantially in burned relative to unburned soils [Pierson et al, 2008;Robichaud et al, 2010;Wagenbrenner et al, 2010]. In contrast to raindrop-driven transport, the direct detachment and transport of sediment by overland flow (referred to here as flow-driven sediment transport) may be less sensitive to vegetation cover and more sensitive to changes in infiltration capacity, which could increase runoff and lead to the exceedance of critical entrainment thresholds by overland flow.…”

“…Combined with an increase in soil erodibility [Moody et al, 2005], increases in runoff make hillslopes in postwildfire environments prone to high rates of erosion [Meyer and Wells, 1997;Inbar et al, 1998;Prosser and Williams, 1998;Robichaud et al, 2010;Lamb et al, 2011]. Rates of sediment transport remain elevated over a time period of several years, often referred to as the window of disturbance, before gradually returning to the background rate [Shakesby and Doerr, 2006;Nyman et al, 2013].…”