The Oktober Flut: An anomalous extreme high-water event in Annapolis, MD, USA, October 2021

Abstract

Between 28 and 30 October 2021, Annapolis, Maryland, USA, experienced the third highest flood event on record, referred to as the “Oktober Flut.” Unlike many of the extreme coastal flood events that have impacted Annapolis in the past, the Oktober Flut was not associated with a tropical or post-tropical cyclone. Instead, sustained wind forcing from a series of passing extratropical storms drove anomalously high water levels. The maximum observed hourly water level during the event was 4.88 ft above mean lower low water (MLLW). This exceeded the Moderate Flood Stage, defined by the National Weather Service (NWS) as 3.3 ft (MLLW), and had an annual exceedance probability of 5.2%. In total, flood waters exceeded the NWS Minor Flood Stage (2.6 ft MLLW) for 78 non-consecutive hours over multiple tidal periods. Relative sea level rise and increasing instances of coastal nuisance flooding events, like the Oktober Flut, have the potential to impact the local economy in Annapolis, MD, and operations at the geographically adjacent U.S. Naval Academy. Coastal nuisance flooding events also challenge existing community resilience efforts and initiatives, particularly those related to preparing for natural hazards. This paper provides an overview of the meteorological factors that drove the anomalously high-water levels and coastal flooding at the U.S. Naval Academy. In addition to data analysis from fixed, long-term sensors maintained by federal partners, observations from spotdeployments in advance of the storm are also presented. The results are discussed with respect to the storm impacts on infrastructure and facility preparedness. Lessons learned are documented with the aim of informing other coastal communities on how to better assess local flood-specific action plans, based on the accuracy and timing of forecast data.