Slack-Water Deposits and the Magnitude and Frequency of Flash Floods, Eastern Kentucky

The potential for predicting flood magnitude and frequency using sediments deposited in backwater areas during flash floods was investigated on the Cumberland Plateau of eastern Kentucky, a region n9torious for flash flooding. Slack-water deposits are abundant in the area at tributary mouths and bedrock channel expansions. They are identifiable on geologic quandrangle maps, and are locally good potential indicators of maximum flood-crest elevations. However, in this humid region, flash floods could not be distinguished from non-flash floods using slack-water sedimentology. The results from the slack-water deposits studied indicate that they offer limited potential for predicting flash floods because 1) intense weathering, erosion, and bioturbation of slack-water deposits has rapidly altered and generally destroyed the flood stratigraphy to the extent that flood-frequency data could not be obtained; 2) mineralogic differences of bedrock units in tributaries of drainages prone to flash flooding are small and inadequate for distinguishing tributary and main channel flood deposits, and 3) flash floods in tributaries commonly but not always cause backwaters and slack-water deposits in main channels, preventing use of the conventional slope-area method of estimating discharge. The results suggest that the slack-water method may be successful if first a quantitatively comprehensive method of describing flash floods using meteorologic, hydrologic, geomorphic, and sedimentologic parameters could be developed and used to distinguish them from non-flash floods