Process Domains in Synthetic Landscapes: Slope-Area Relationships in the Mountaintop Mining Region of Central Appalachia.

Abstract:

Landscapes and the governing geomorphic processes that shape them have been described in a conceptual framework of process domains. At a coarse scale, process domains are segregated between hillslope, colluvial, and alluvial processes, which can be distinguished by governing erosional processes and partitioned by local slope-drainage area relationships. In landscapes that have experienced dramatic topographic alteration such as the mountaintop coal-mining (MTM) region of central Appalachia, the resulting modified environment may be considered a synthetic landscape. Such a landscape has process domains that are decoupled from prior landscape evolution trajectories. In particular, landslide and debris flow processes, which are a predominant geomorphic agent in these steep mountain systems and a primary sediment delivery mechanism to the downstream fluvial network, may be eliminated from this landscape and detectable through changes in slope-area relationships. We evaluate differences in slope-area relationships using 10-m DEMs between two time periods, pre-mined and post-mined. At five study site located within the MTM region in the central Appalachian Mountains, US, we compare slope-area changes to adjacent unmined landscapes over the same time periods. Distinct differences exist in the character of slope-area relationships between unmined and MTM sites and local slopes are systematically and considerably reduced in all process zones of mined sites. In particular, there is an expansion of the unchanneled valley zone through either an individual or simultaneous upslope shift into the hillslope region and downslope shift into the debris flow region. In addition, local slopes are markedly reduced (33% to 44%) in the post-mined period relative to the pre-mined period at all sites and are generally below the threshold required to trigger landslides and debris flows. The consequence of altered erosion processes in this upper portion of the catchment, particularly the potential elimination of landslide and debris flow processes, is a downstream river network starved of its primary sediment source. Decreases in sediment supply will translate to increases in hydraulic driving forces with the fluvial network and increased bank erosion and bed incision processes.