Patterns and Projections of High Tide Flooding along the U.S. Coastline Using a Common Impact Threshold.

Abstract

For forecasting purposes to ensure public safety, NOAA has established three coastal flood severity thresholds. The thresholds are based upon water level heights empirically calibrated to NOAA tide gauge measurement s from years of impact monitoring by its Weather Forecast Offices (WFO) and emergency managers. When minor (more disruptive than damaging), moderate (damaging) or major (destructive) coastal flooding is anticipated (not associated with tropical storms), NOAA issues either a flood advisory (for minor) or warning (for moderate or major). L ess than half of NOAA tide gauges located along the U.S. coastline have such ‘official’ NOAA flood thresholds, and where they exist, the heights can vary substantially (e.g., 0.3 –0.6 m eter within minor category). They differ due to the extent of infrastructure vulnerabilities, which vary by topography and relief , land -cover types or existing flood defenses. We find that all official NOAA coastal flood thresholds share a comm on pattern based upon the local tide range (possibly in response to systematic development ordinances). Minor, moderate and major coastal flooding typically begin about 0.5 m , 0.8 m and 1.2 m above a height slightly higher than the multi -year average of the daily highest water levels measured by NOAA tide gauges. Based upon this statistical (regression -based) relationship, a ‘derived’ set of flood threshold proxies for minor, moderate or major impacts are permissible for almost any location along the U.S. c oastline. The intent of this report is not to supplant knowledge about local flood risk. Rather, the intention is to provide an objective and nationally consistent set of impact thresholds for minor/moderate/major coastal flooding. Suc h definitions are currently lacking, which limits the ability to deliver new products as well as the effectiveness of existing coastal flood products. Coastal communities along all U.S. coastlines need consistent guidance about flooding, which is 1) forecasted in the near fut ure (e.g., severity/depth of 4 -day predictions of storm surge heights ‘above ground level’), 2) likely in the coming season or year (e.g., probabilistic outlooks) or 3) possible over the longer term (e.g., decadal to end-of -century scenarios). Our primary emphasis is to use the derived threshold for minor flooding, which we refer to as ‘high tide’ flooding (also known as ‘nuisance’, ‘sunny day’ and ‘recurrent tidal’ flooding), to assess nationally how exposure —and potential vulnerability —to high tide floodi ng has and will continue to change with changing sea levels