ALLUVIAL FAN HAZARD MAPPING, RISK ASSESSMENT, AND HAZARD MANAGEMENT STRATEGIES: CANYON AND JONES CREEKS CASE STUDIES, WHATCOM COUNTY, WASHINGTON

Western Whatcom County is home to well over 100 alluvial fans formed by a combination of clear-water floods, sediment-laden floods, and debris flows. While human habitation is absent on many of the fans, others host extensive development due to their relative elevation above adjacent floodplains, typically well-drained soils suitable for on-site septic systems, and the aesthetic attraction of living adjacent to an apparently benign stream. Year-round residences have replaced seasonal recreation-based occupation of the fans in recent years. Significant storm events over the last 25 years have resulted in property damage on several alluvial fans. Alluvial fan assessments following events in 1983, 1989, and 1990 raised local government awareness of the need to regulate land-use on alluvial fans and resulted in some increased regulatory scrutiny. These assessments were also valuable for public education regarding alluvial fan hazards, but their qualitative nature limited use as a regulatory tool. Recent hazard mapping efforts on the Canyon and Jones Creek alluvial fans (KWL 2003, 2004) produced more quantitative risk assessments that provided discrete hazard management strategies. A significant scientific outcome was that the events of the last 25 years are relatively minor within the geologic record. Armed with this information, Whatcom County government chose to work with local, state and federal partners to purchase empty lots, homes, and a family-owned resort in high hazard areas of the Canyon and Jones Creek fans. The County also recently revised the geological hazards section of the critical areas ordinance to further discourage development in high hazard areas and to set a more consistent standard for geotechnical reports and designs should a landowner propose building on a fan. While these changes denote more informed decision-making, homes continue to be constructed in moderate to low risk areas of the fan with building in high hazard areas still allowed. The end result is a shift of liability onto the designing engineer or geologist and the County's technical administrator in the absence of the political will to negotiate the mine field that is the “reasonable use”of property ownership.