Response and Recovery of a Piedmont Watershed From Tropical Storm Agnes, June 1972

Author:
  • JCJohn E. Costa
    John E. Costa
Publisher website
Google Scholar
Abstract
Tropical storm Agnes generated flooding with a recurrence interval much greater than 100 years in a Piedmont watershed: 633 cfs/mi2 from 60 mi2. Deposits on floodplains were scarce and could be mapped as sand and gravel and as sand. Deposition occurred where tributaries joined the main channel, across meander bends, and below valley constrictions such as roads and bridges. Despite the magnitude of the rainfall and flooding there were no slope failures in the basin. The only modifications to the watershed were observed in the lowlands along the main channels. Channel widening was the most spectacular modification. Within 1 year of the flood, channel cross sections were well along recovery toward pre‐Agnes dimensions. Meandering reaches of the channel recover faster than straight reaches. The Agnes flood is nearly the largest ever reported from a Piedmont watershed of this size. Large floods in the Piedmont apparently play a minor role in shaping the landscape, whereas large floods in the Blue Ridge/Valley and Ridge and Appalachian Plateau provinces, and consequent slope failure, are very active landscaping agents.