FLOODS AND PALEOFLOODS IN THE GREENBRIER RIVER BASIN, WEST VIRGINIA

Meteorological floods are generated by the interplay of atmospheric and landscape conditions, and can occur over various spatial and temporal scales. Floods are recorded within the instrumental record and also within geological archives. In eastern North America, evidence of paleofloods is not well represented in many geomorphic settings, but is sometimes preserved within cave sediments. Here we present a flood record for the Greenbrier River in southeastern West Virginia based upon modern hydroclimatology and clastic sediments preserved within Greenbrier River Cave (GRC) and Colonial Acres Cave (CAC). Evidence of frequent, low magnitude floods appear to be preserved in CAC, which is closer to the Greenbrier River, whereas evidence of large floods is better preserved in GRC, which sits higher above the river.

Four USGS gaging stations exist on the Greenbrier River; two upstream and two downstream of GRC and CAC. Evidence of the three largest floods at the two downstream gages (1985, 1996 & 2016) are preserved within GRC and represent three different flood generating mechanisms. Using paleostage indicators in GRC and the gage discharges it has been possible to back-calculate Manning roughness coefficients (n) for the channel outside GRC during multiple floods using hydraulic modeling (HEC-RAS). Minimum n-values are calculated to be 0.027 to 0.035.

To date, numerous Holocene paleofloods have been successfully documented in the two caves. Three prehistoric large floods have been recognized in GRC and a sequence of lower magnitude paleofloods are present in CAC slackwater sediments. The discharges of the prehistoric floods recorded in GRC are similar to those in the historical record. However, the largest prehistoric flood had a minimum stage that was close to the peak (maximum) stage of the largest historic flood, so may have been substantially larger.