RECURRENCE INTERVALS FOR PEAK STAGE ALONG TENNESSEE RIVERS IMPACTED BY HURRICANE HELENE

Hurricane Helene made landfall in Florida on September 26, 2024, as a category 4 hurricane. It quickly traveled north into North Carolina and Tennessee, and as a tropical storm, produced high winds and torrential rains in the NC headwaters of several TN streams, leading to flooding in both western NC and east TN. This flooding scoured channels and floodplains, removed riparian forests, and destroyed homes and sanitary infrastructure. Recurrence intervals for the 3-day rainfall event ranged up to 1000-year (0.001 Annual Exceedance Probability) for areas in western NC from Hendersonville to Asheville and northward to Spruce Pine. The New River watershed in southwest VA experienced similar rainfall exceedances.

USGS stream gauges on affected rivers (Nolichucky, French Broad, Pigeon, and Doe Rivers) recorded record water levels (stream stage), and combining these data with existing annual peak stage records, return periods for the Helene-associated stage were calculated, ranging from 100 years (Doe River) to over 2000 years (Nolichucky River). Of note, the Nolichucky River gauge at Embreeville ceased to report data at a peak stage of 3.88 3ft, when floodwaters washed it away. Here, stream stage was rising steeply when reporting ceased and the highest gauge-recorded stage was likely an underestimation. Through field observation, a more accurate peak stage may be inferred from high water marks on trees, surviving vegetation, and bridge supports.