Debris Slides and Flows on Anakeesta Ridge within the Great Smoky Mountains National Park, Tennessee, U.S.A
Date of Award
Master of Science
G. Michael Clark
Eric C. Drumm, Don W. Byerly
Debris slides and flows along Anakeesta Ridge in the GSMNP have been investigated utilizing dendrochronology, aerial photography, erosion stations, precipitation data and rockslope engineering techniques.
Based on the slides for which the specific dates of occurrence are known, the corresponding precipitation that was at least in part responsible for triggering the mass-wasting events varies from 1 .5 inches of rain per day to 4 inches of rain for a 6 hour period. Unfortunately, intensity records are not available to provide at what rate the precipitation was delivered. Based on TV A precipitation records it was determined that 1273 storms (1 or more inches of rain per 24 hour period) occurred during the period of 1951-1987 in the general area of the Great Smoky Mountain National Park. An abundance of moisture is available to effect erosional processes.
Transportational processes operating on Anakeesta Ridge include creep, overland flow, and debris sliding. Additional slope modifiers include: needle ice, slaking, and bank slumping. Slope retreat is primarily accomplished through sheet wash, mass movement and tree throw. Appreciable amounts of fine sediment are moved downslope by slope wash. Tree-throw continues to operate proximally to the Anakeesta Ridge slide scars. Additionally, tree-throw is present at every breach of a ridge crest in the study area and is common along the unfailed slopes of Anakeesta Ridge. In terms of biogenic transport, tree-throw importance is clearly expressed by the microtopography created by decaying tree-throw mounds. Recent slide scar retreat has been nonexistent in some areas, primarily side-slopes, whereas retreat has been as high as 37.2 em over a 7 month period in scar head areas.
Log jams act as an effective debris dam in slide-track constrictions until that time when the logs are structurally weakened by decay and can be overcome by a new debris torrent. The logs then become part of the ensuing debris slide and contribute to vegetal debris in the fan.
Periodic aerial reconnaissance of areas of interest may be sufficient to detect incipient slide development. The Anakeesta Ridge slides have developed through head ward erosion; this was easily tracked through sequential aerial photographs. Incipient slides are followed by additional, ongoing sliding.
The compound slide scars on Anakeesta Ridge have increased in area and volume: 4,300 m2 and 1790 m3 in 1953 to 128,000 m2 and 84,100 m3 in 1987. The scar head and upper slide track areas are the primary debris volume contributors. Anakeesta Ridge is in a stage of accumulation in the mid-slope regions of the slide scars. The scar heads continue to erode headwardly, supplying material to the lower regions of the scar. Aerial imagery for Anakeesta Ridge indicates that slope failure is initiated in the mid-slope region. The upper slope segments average 43.4°,the mid-slope segments average 33.6° and the lower slope segments average 25.6°. Anakeesta phyllite has an abundance of release surfaces in the form of cleavage planes, joints, faults and bedding planes. The chute morphology is characterized by wedge failure planes as formed by the intersection of cleavage/bedding and joints.
Direct shear testing of Anakeesta phyllite yielded an internal friction angle (phi) of 58.2° and a cohesion (c) of 6134 pounds/foot2. Utilizing these values, and the slope and failure plane orientations, a factor of safety range from 1.19 to 2.53 was generated.
A high percentage of the draws about Anakeesta Ridge are associated with debris sliding activity. Debris fans along the ridge, vary in approximate minimum dates of origin from 1749 to 1971 as determined from dendrochronological data. Tree coring data yields dates which range from 6 to 87 years between events and average 13.8 years. This length of time may represent the average amount of time required for slope ripening (weathering, accumulation) to occur, the time between major precipitation events or a combination of the two.
Anakeesta Ridge slope instability is due to the coincidence of weathering, regolith accumulation, tree levering against shallow root networks, and high precipitation events. Slides will continue to cause problems along U.S. 441, therefore a need exists for locational and temporal predictors and a precise accounting of slide localization factors. Debris sliding cannot be prevented, however, hazardous areas can be delineated, and risks be assessed. In this way, landslides will not be studied simply for forensic purposes. A debris slide in a particular area is not a one-time event. Portions of Anakeesta Ridge have failed in the past, and under the present climatic regime, will continue to do so. Landslide potential is limited only by the availability of excess water, steep slopes and material.
Ryan, Patrick T., "Debris Slides and Flows on Anakeesta Ridge within the Great Smoky Mountains National Park, Tennessee, U.S.A. " Master's Thesis, University of Tennessee, 1989.