HR: 1340h
AN: H43F-1687    [Abstracts]
TI: Soil Erosion in a Burned Mountainous Watershed, Kootenay National Park, British Columbia
AU: * Martin, Y E
EM: ymartin@ucalgary.ca
AF: Department of Geography, University of Calgary, 2500 University Dr. NW, Calgary, AB T2N 1N4, Canada
AU: Gallaway, J M
EM: jmgallaw@ucalgary.ca
AF: Department of Geography, University of Calgary, 2500 University Dr. NW, Calgary, AB T2N 1N4, Canada
AU: Johnson, E A
EM: johnsone@ucalgary.ca
AF: Department of Biological Sciences, University of Calgary, 2500 University Dr. NW, Calgary, AB T2N 1N4, Canada
AB: In late July 2003, lightning ignited wildfires in Kootenay National Park, southeastern British Columbia. Following the wildfires, a study was initiated to investigate post-fire soil erosion in Hawk Creek watershed, located with the National Park. Hillslope gradients are moderate (generally less than 30 degrees) in the lower third of the basin and become steeper with increasing elevation. Summer rainfall is delivered by convectional thunderstorms, and average annual rainfall is about 340 mm. The mean fire return interval in this region is about 165 years and fire frequency has changed over time in response to climate. Silt fences were installed to measure soil erosion in the summers of 2004 and 2005. Results indicate a threshold rainfall intensity of 1.6 mm in 15 minutes is required to mobilize sediment and based on this value, a rain day herein refers to days when the maximum 15-minute rainfall intensity exceeds 1.5 mm. Sediment transport data show many rain days producing no sediment, while relatively small amounts of soil erosion occurred during several rain events in 2004 and 2005. An important finding is that post-fire soil erosion rates are very small, and considerably lower than many other results reported in the literature. The sandy soils exhibited no signs of soil hydrophobicity, but rather displayed high infiltration capacities that precluded the development of overland flow. No evidence of hydrophobic soils or rilling was evident in the hillslope plots during sediment collection after the rain events and it is probable that the majority of soil erosion occurred by rainsplash. The increased soil erosion measured on steeper plots would be in accordance with the increased rates in downslope rainsplash erosion expected on higher gradient slopes. Grain sizes trapped by the silt fence relative to the soil profile are based on soil samples collected from silt fences and nearby hillslopes. Most of the sediment collected in the silt fence traps fell in the size range less than 2 mm, indicating stresses associated with rainsplash were generally not sufficient to entrain larger particles.The less than 2 mm fractions for silt fence and hillslope samples were extracted for comparison, with the hillslope samples containing 45-58 percent of particles less than 354 microns, and silt fence percentages ranging from 72-90 percent. The higher percentage of fines in the silt fence samples suggests that fines less than 354 microns are being selectively entrained, transported and deposited in the silt fence. Selective transport of particles less than 0.5 mm occurred, with minimal mobilization of coarse sand. Soil erosion data collected herein provides the necessary data to derive gradient-driven transport equations, which can be combined with a stochastic algorithm for wildfire return interval and precipitation events (such as storm duration, interstorm duration, and average storm rainfall intensity) to explore implications of rainsplash erosion on burned hillslopes in this region over longer time scales. A brief window of opportunity exists after a fire for effective rainsplash or overland flow transport, and high energy storms are required in these same post-fire years to enable such an occurrence.
DE: 1815 Erosion
DE: 1826 Geomorphology: hillslope (1625)
DE: 1862 Sediment transport (4558)
DE: 1865 Soils (0486)
SC: Hydrology [H]
MN: 2007 Fall Meeting