NOAA tide gauges are measuring rapid changes across the entire severity-spectrum of coastal flood risk along U.S. coastlines due to RSL rise. The most noticeable impact of RSL rise is the increasing frequency of HTF (sometimes referred to as ‘nuisance’ or ‘sunny day’ flooding), which typically causes minor and disruptive impacts. However, within many rural and urban U.S. coastal communities, the cumulative effects of more HTF upon public-works systems, roads, first floors of businesses, and residences (among others) is becoming a serious problem. Because of this, communities need projections for ‘next year’ and for the coming decades for preparedness and planning purposes to respond to the growing RSL-related HTF threat.
In 2018, the national annual HTF frequency reached 5 days (median value) and tied the historical record set in 2015 as measured by 98 NOAA tide gauges along U.S. coastlines. In all, 12 individual locations broke or tied their HTF records. There are now over 40 locations whose HTF trends reveal significant acceleration (nonlinear increase) and 25 locations whose HTF trends are linearly increasing, implying that impacts soon will become chronic without adaptation. HTF was most prevalent during 2018 along the Northeast Atlantic Coast and broke 15 records along the Chesapeake Bay and the Eastern Gulf of Mexico Coast, where major flooding also occurred from hurricane-induced storm surge and heavy rainfall/river flows.
HTF for 2019 is projected to be more likely than normal at about 40 locations along the U.S. West and East Coasts in part due to a minor El Niño that is predicted to persist until early next year. The national median HTF frequency is projected in 2019 to be more than 100% greater than it typically was in 2000 and regionally HTF is projected to occur:
• 8 days along the Northeast Atlantic (140% increase since 2000)
• 5 days along the Southeast (190% increase over 2000)
• 3 days along the Eastern Gulf (100% increase since 2000)
• 6 days along the Western Gulf (130% increase since 2000)
• 2 days along the Southwest Pacific (80% increase since 2000)
• 6 days along the Northwest Pacific (20% increase since 2000)
Annual flood records are expected to be broken again next year and for years and decades to come. Projecting out to 2030 and 2050 provides vital information for communities who are already taking adaptation steps to address coastal flooding impacts and those who are beginning to assess future flood risk in their communities. Bounded by a range of RSL rise under a lower and continued-high emission rate, today’s national HTF frequency of 5 days (national median) is likely to increase to about 7–15 days by 2030 and 25–75 days by 2050 (HTF range: low emission – high emission values), with much higher rates in many locations