Seasonality of Flooding in North Carolina

This paper examines spatial patterns in the timing of floods in North Carolina. Regions with distinctly different seasonal flood regimes were identified using cluster analysis of monthly flood frequencies for the annual and partial duration series. The partial duration series produced results similar to those using the annual flood series, which was used to delineate three regions: the Appalachian Region, the Piedmont Region, and the Coastal Plain Region. In the Appalachian Region, about three-fourths of all floods occur during winter and early spring (December-April), while less than one fourth occur from May to November. The Appalachian Mountains are an effective barrier to warm, moist air masses that influence the rest of the state during the summer and fall. The frequency of floods in summer and fall increases to the east in the Piedmont and Coastal Plain regions, where maximum frequencies (~25-30%) occur during August-October. Although flooding in summer and fall is greater than in the Appalachian Region, floods still occur most frequently from January to April in the Piedmont (44%) and Coastal Plain (55%) regions. The smallest watersheds tend to experience a larger proportion of floods in summer and fall.