HR: 08:45h
AN: H41A-04    [PDF]
TI: Hillslope morphology and sediment production modulated by fire in the Oregon Coast Range
AU: * Roering, J J
EM: jroering@uoregon.edu
AF: UO, Department of Geological Sciences, Univ. of Oregon, Eugene, OR 97403-1272 United States
AU: Gerber, M
EM: mgerber@darkwing.uoregon.edu
AF: UO, Department of Geological Sciences, Univ. of Oregon, Eugene, OR 97403-1272 United States
AB: Since the 1970s, the Oregon Coast Range (OCR) has proven to be a fertile study area for testing, calibrating, and validating process-based models for sediment transport and production, but few of these studies have considered the influence of episodic fires, which were more prevalent during the Holocene and Late Pleistocene than today. Although recent conceptual models have considered the role of fire in regulating root reinforcement and the frequency of debris flows, geomorphic response at recent burn sites (1999, 2001, and 2003) suggests that the influence of fire may be far-reaching, affecting transport processes in areas not prone to debris flows. Although we observed sparse evidence for accelerated erosion by overland flow, unchanneled valleys in each of our study sites exhibited significant deposition due to dry ravel. At the 1999 burn site (Austa Fire), the pulse of accelerated transport has been sufficient to strip a significant portion of the soil mantle, leaving a series of ridges and valleys with a patchy soil cover. In addition, bedrock has been progressively emerging on steep sideslopes in the 2002 burn site (Siuslaw Fire). Similar to previous studies, the ravel response appears to result from the incineration and removal of understory vegetation (particularly ferns) that act as natural sediment traps. Using a series of artificial sediment traps, Bennett (M.S. thesis, OSU, 1982) quantified the effect of hillslope angle on post-fire, ravel-driven sediment flux. Our re-analysis of her data indicates that sediment flux increases nonlinearly with slope angle and is well represented by a physically-based, nonlinear transport model previously tested and calibrated in the OCR (Roering et al., WRR, 1999). Importantly, model parameters calibrated for the post-fire dataset suggest that transport rates may increase by nearly an order of magnitude in the years following an intense burn. In addition, the critical gradient parameter, which governs the angle at which flux rates increase rapidly, is lower than the previously calibrated value, suggesting that post-fire ravel transport may preferentially strip the soil mantle along steep sideslopes. We quantified the morphologic implications of fire by modeling the evolution of hillslopes subject to episodic pulses of accelerated transport using the field-calibrated parameters from the post-fire data. Our results suggest that fire may not only regulate the magnitude and frequency of sediment production in the Oregon Coast Range, but also impart a signature in hillslope morphology.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1869 Stochastic processes
SC: Hydrology [H]
MN: 2003 Fall Meeting