Debris Flows and Alluvial Fan Development in the Appalachians

Wilson, Gregory

Alluvial fans formed by debris flows were investigated at sites in Tennessee, Virginia, West Virginia, New York, and New Hampshire, Fans have developed unique characteristics due to differences in local geologic, topographic, and climatic conditions. However some characteristics are shared by all the Appalachian debris fans, because similar processes were involved in fan development. Appalachian debris fans are typically small (<0.5 2 km), and irregular in shape. Fan shapes and sizes are often determined by local topography. Fan growth is limited by steep hillsides and truncation of tributary fans by trunk streams. Correlation between fan and basin areas was found to be very low. Topography also plays a major role in determining fan locations and numbers within an area. Fans were found to be most prominent in areas where there was a sudden change in flow confinement and slope, and less common where change was more gradual. Sedimentological characteristics common to most of the fans included extremely poor sorting, angular clasts, and an absence of sedimentary structures. Relative amounts of clasts and matrix varied significantly between areas, probably due to differences in source material, and the nature of the flows depositing the fan sediment. Most of the sediment appears to have been transported downslope by debris flow processes. The abundance of clast-supported sediments at some sites suggests that the sediment-rich debris flows may have been diluted upon entering tributary channels, and sediment was transported to the fans by hyperconcentrated flow. All historical Appalachian debris flows have resulted from intense rainfalls, associated with either local thunderstorms, regional thunderstorms, or tropical cyclones. These intense rainfalls appear to be closely related to the interaction between tropical air masses and mountainous terrain. Debris flows appear to be more frequent in areas north of the glacial border. Analysis of storm tracks of tropical cyclones suggests that the frequencies of tropical cyclones are approximately the same for the northern Appalachians as the southern Appalachian regions. This suggests that the higher debris flow frequencies in the north are related to a lower rainfall threshold for slope failure rather than a higher storm frequency• Dating climatic change within the Appalachians during the Late Quaternary met with limited success due primarily to the lack of datable material. In Nelson County, Virginia a date of 13,170 + 190 yr B.P. was obtained from a solifluction deposit which had been buried by debris flow material. The oldest dated debris flow material in the area is 10,840 + 200 yr B.P., therefore debris flow activity in this area must have begun sometime between 13,170 + 190 yr B.P. and 10,840 + 200 yr B.P.