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Thresholds for post-wildfire flood, erosion, and mass wasting processes
Covert, S. Ashley
Flooding, erosion, and mass movement following wildfires is an example of a threshold for watershed hydrologic function being exceeded, which has significant risk implications for downslope and downstream values, and for management of forest land affected by wildfire. Changes to the soil following fire, such as water repellency and loss of forest floor, can create a threshold in hydrologic response, whereby overland flow generates exceptionally high peak runoff on a watershed scale. Normally, forest soils rarely or never generate extensive overland flow, and hydrologic response is dominated by shallow and deep subsurface flow. Widespread overland flow often causes severe erosion, especially where the forest floor and soil structure have been altered. In steep areas, debris flows and landslides can result. In British Columbia, large-scale flooding and erosion had not been documented following wildfires until after the 2003 fire season. This contrasts with the western US, where severe flooding and erosion are common after wildfires, and where considerable resources are devoted to assessing post-wildfire risks, and applying treatments to affected areas. Climate change may increase the the likelihood of these events in BC. Following the severe 2003 fire season, major erosion and mass movement incidents were observed on 5 fires: the October 2003 flood/erosion event in Kelowna; the August 2004 debris flows on Kuskonook Creek near Creston, which destroyed several houses; the August 2004 debris flood event on Lamb Creek near Cranbrook; the July 2004 debris flows/floods at the Cedar Hills fire near Falkland; and in October 2005, 15 debris flows and landslides occurred in the Mt Ingersoll fire near Burton. Some of these events were documented in Jordan et al (2004 and 2006) [1,2], and several involved substantial property damage and risks to the public. Since then, BC Forest Service staff have worked on developing an interim procedure for post-wildfire risk assessment, and a number of critical knowledge gaps have been identified. (See 'Objectives' below.) The fundamental question addressed by this research project is: What vegetation and soil burn severity and extent is necessary to generate sufficient overland flow in a watershed to cause flooding and erosion, and what are the roles of water repellent soils and forest floor alteration in these processes? The proposed 3-year project seeks to address these knowledge gaps by a research program which emphasizes field data collection and analysis, and which integrates the methods of soil science, hydrology, and geomorphology. Work will be done by MOFR staff scientists, with cooperation from our partners at UBC and Selkirk College. Specific tasks to be undertaken include: - Further field investigation of the events which followed the 2003 fires (and to a much lesser extent, 2004-2005 fires), to better document and analyse the processes which caused the flooding, landslides, and debris flows. - In new fires which occur during the study: establish plots to measure relevant soil parameters, and measure erosion, in burned areas of various severity classes, and reference plots in adjacent unburned areas. - Refine field procedures for measuring burn severity, water repellency, and forest floor properties at burned and unburned sites, in terms of the hydrologic function and antecedent moisture content of the mineral soil, duff, and litter layers. - Rainfall simulation experiments to investigate infiltration and overland flow processes in the study plots. - Monitor rainfall, runoff, erosion, and sediment transport, in small instrumented watersheds. - Data analysis will be done using appropriate statistical, laboratory, and GIS methods, with which the MOFR research team has good in-house resources and experience. This field work program depends on wildfires which occur during the course of the study. In most years, at least several high-risk fires occur in the region which are likely to be suit ...
Report Number
Executive Summary
CMI Poster
Post-Wildfire Soil Erosion, Flood,and Landslide Risks (Presentation)

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