Debris slides and flows in the Southern Appalachians - Historical, Process, and Environmental GeomorphologyG. Michael Clark
The University of Tennessee at Knoxville
Continent: North America
State/Province: Tennessee/North Carolina
UTM coordinates and datum: Regional area is centered about 35° 30' N; 83° 30' W
Climate Setting: Humid
Tectonic setting: Passive Margin
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The part of the southern section of the Blue Ridge province lying mainly within southeastern Tennessee and western North Carolina is a region with the highest U.S. elevations (up to 2000 m) and relief between ridge crests and valley floors east of the Rocky Mountains. Mean annual precipitation values are high, up to 2500 mm on high mountain summits, as are precipitation-frequency values associated with thunderstorms, occluded low-pressure fronts, and hurricane remnants. Thus, topography and climate, plus human land use and other factors, combine to foster rapid mass wasting processes in this region. Written accounts since about 1800 AD allow us to compile a record of major occurrences of events of multiple debris slide and debris flow mass movements produced by intense rainfall events and such areas can be investigated in the field.
What are some process-response linkage problems associated with present-day mass movement occurrences? The heads of almost all slide scars occur some distance below the ridge crests, occur in slight pre-existing depressions, and expose bedrock, suggesting that surface shear stress of overland flow and/or rapid pore-water pressure buildup at the base of the soil cover might be triggering factors. How could we test these postulates? Another set of questions relates to why certain hillslope hollows fail in a storm, and others nearby on the same mountainside do not. Are there local factors, such as concentrations of bedrock fracture zones or minor variations in soil texture or hillslope angle?
Over succeeding years, the exposed bedrock in older debris slide scars and flow tracks tend to fill in with loose sediment that creeps and washes into the channelways, the debris fans become incised, and revegetation occurs. In addition to debris fans that can be attributed to mass wasting processes since European settlement times, many hundreds of old, very-deeply weathered sediment fans occur in the region, and pose interesting geomorphic questions. Were these pre-settlement fans also produced by cataclysmic rainfall events? Or, might deposits have been deposited by different processes that operated under other climatic conditions? For example, although there is no evidence of glaciation during the Cenozoic Era in the southern Blue Ridge, it is likely that ice age alpine cold-climatic environments occurred at higher elevations, conditions that would have fostered snow avalanches and alpine debris flows events.
Debris slide and flow events displace great volumes of regolith downslope and the resultant erosional scars bare bedrock and regolith surfaces to surface weathering processes. Some geomorphologists believe that these processes are important agents in the formation of hillslope drainangeways in climatic environments ranging from the subarctic to the humid tropics. And, there are practical reasons for learning more about these mass-wasting processes. Debris flows are high volume rapid mass movements that constitute a major geomorphic hazard. The southern Blue Ridge is conveniently located within driving distance of a large percentage of the US population, has many tourist attractions, and has experienced a rapid increase in visitation especially during summer months when high rainfall events are most common. In addition, many scenic mountain areas are being developed for homesites, second homes, and retirement communities. Many developments are sited in hillslope, footslope, and toeslope locations we know are sites where rapid debris flows occur. And, the effects of many construction projects produce landscape disturbance and subsurface effects that foster increased mass movement activity. Finally, what can be predicted from models of global change? Will mountain hillslopes become more or less stable with changing patterns of precipitation, temperature, and their effects on vegetation and weathering processes?
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