ANALYSIS OF LAKE SEDIMENTS RESULTING FROM A PARTIAL DAM FAILURE LOCATED IN WESTERN NORTH CAROLINA

A partial dam failure provided a unique opportunity to study downstream sedimentation effects. In fall of 1999, a large amount of sediment was released from the dam of Randall Lake (RL), near Highlands, North Carolina, during maintenance operations. Residents along Lake Sequoyah (LS), 1.7 km downstream, noticed a significant loss of water depth. The purpose of this study was to analyze the LS sediments to characterize pre-event and post-event sediment sequences. In this study, 20 cores were collected from LS, using a 1m-piston corer, and 8 streambed samples from between the two lakes. All cores were described for texture, color, mineral composition, etc. Selected cores and streambed samples were analyzed for texture using hydrometer and sieve methods, and for organic carbon % using loss on ignition. In addition to analysis of physical data, the sediment was interpreted in context of weather data, local water authority reports, and accounts by local residents.

Analysis of lake cores shows a common stratigraphic sequence: a lowermost sandy loam overlain by up to 1+ m of medium to coarse sand. The sand unit, in most places, consists of an upper and lower sand separated by a fine-grained, organic-rich layer up to 14 cm thick. The lowermost sandy loam had an average loss-on-ignition of 10% with 64% sand; the overlying sand had an average 3% loss-on–ignition and 81% sand. The sand unit extends at least 250 m down the axis of LS and thins abruptly towards the sides.

The sand unit in LS is interpreted to have resulted directly and indirectly from a massive influx of sand released from the partial dam failure of RL. It is believed that two sedimentation events occurred. The major event was the failure of the dam resulting in a large pulse of water and sediment; this resulted in a rapidly deposited, fining upward sequence of sand. After this initial pulse, low-energy, fine-grained sedimentation resumed. The second phase of sand sedimentation was likely caused by the flushing of sand stored in the stream due to a high discharge resulting from a high-magnitude rain. Alternatively, it is possible that the upper sand sediments were remobilized as the water level in Randall Lake was lowered, although there is no record of this taking place. Additional sand remains stored in the stream connecting LR with LS, which will result in continued sand inputs into LS.