An Investigation of the Piping Problems Causing Water-Level Drops in Mountain Lake, Giles County, Virginia, USA

Abstract

Mountain Lake, located in Giles County, Southwestern Virginia, is one of only two natural lakes in the state of Virginia. The lake exhibits rare self-draining (self-dumping) behavior, with recent episodes in 2008, 2011, and 2012 that drained the lake almost completely and revealed the presence of four sinkhole-like depressions, with piping holes at their bottoms and sides, near the northeastern and northwestern margins of the lake. The purpose of this study was to investigate the piping potential of lake-bottom sediment and its role in seepage and lake-level fluctuations. Sixteen grab samples of lake-bottom sediment were collected from the vicinity of the depressions. Additionally, five Shelby tube samples were gathered to determine the sediment’s field density. Grain size analysis, Atterberg limits, and a compaction-mold permeameter test were used to evaluate lake sediment’s susceptibility to piping. Grain size analysis and Atterberg limits tests reveal that the lake-bottom sediment consists predominantly of fine sand and silt of low and high plasticity, which are well known for their susceptibility to piping. Results of the compaction-mold permeameter test show that the hydraulic gradient at which the sediment starts to pipe, the critical hydraulic gradient (i_c), ranges between 1 and 10, depending on the density, grain size distribution, and cohesion values of the sediment. These results demonstrate that piping of lake sediment is the primary mechanism responsible the formation of lake-bed depressions and lake-level fluctuations.