The Role of Altered Landscapes in the Disaster Lifecycle

Abstract:

Most landscapes have some degree of human-induced alteration, including roadways, water diversions, and impacts from historical land use (e.g., grazing, mining). These changes in a landscape can magnify impacts of cascading disasters such as fires and subsequent storm events. The population of the Colorado Front Range is growing rapidly, putting increased pressure on the wildland-urban interface. Many historical (mid 19^th to mid 20^th century) mining districts have since developed into substantial exurban communities. Historical mining activity generated numerous waste rock and tailings piles of various magnitudes, from small areas of trace-metal bearing deposits to the underpinnings of entire towns. Recent natural disasters have exposed this material, enhancing mobilization by wind and water. The Fourmile Canyon Fire burned nearly 2600 hectares and over 160 homes in September 2010. Overland flow during storms in the first year post-fire mobilized mine tailings along denuded slopes, resulting in an increased flux of sediment bearing Hg, As, Cu, and other trace metals to the stream. Record-breaking amounts of rainfall across Boulder and Larimer Counties in September 2013 resulted in flooding that triggered landslides at road cuts, undercut roads at bridges, and displaced structures built directly on mine tailings, notably in Jamestown, north of Fourmile Canyon. Additional tailings mobilized by overland flow and high peak flows contributed to elevated trace metal concentrations in stream sediments downstream from legacy mining activity. Understanding the collective impact of historical and modern changes to a landscape will lead to a more holistic approach to disaster preparedness and post-disaster recovery, particularly community planning. An awareness of the geomorphic, hydrologic, ecologic and geochemical changes resulting from historical land use will enable better assessments of the resiliency of ecosystems and communities during and following disasters.