Sea-level rise will radically redefine the coastline of the 21 st century. For many coastal regions, projections of global sea-level rise by the year 2100 (e.g., 0.5-2 meters) are comparable in magnitude to today's extreme but short-lived increases in water level due to storms. Thus, the 21 st century will see significant changes to coastal flooding regimes (where present-day, extreme-but-rare events become common), which poses a major risk to the safety and sustainability of coastal communities worldwide. So far, estimates of future coastal flooding frequency focus on endpoint scenarios, such as the increase in flooding by 2050 or 2100. Here, we investigate the continuous shift in coastal flooding regimes by quantifying continuous rates of increase in the occurrence of extreme water-level events due to sealevel rise. We find that the odds of exceeding critical water-level thresholds increases exponentially with sea-level rise, meaning that fixed amounts of sea-level rise of only ~1-10 cm in areas with a narrow range of present-day extreme water levels can double the odds of flooding. Combining these growth rates with established sea-level rise projections, we find that the odds of extreme flooding double approximately every 5 years into the future. Further, we find that the present-day 50-year extreme water level (i.e., 2% annual chance of exceedance, based on historical records) will be exceeded annually before 2050 for most (i.e., 70%) of the coastal regions in the United States. Looking even farther into the future, the present-day 50-year extreme water level will be exceeded almost every day during peak tide (i.e., daily mean higher high water) before the end of the 21 st century for 90% of the U.S. coast. Our findings underscore the need for immediate planning and adaptation to mitigate the societal impacts of future flooding. Sea-level rise is slow, yet consequential 1 and accelerating 2. Upper-end sea-level rise scenarios could displace hundreds of millions of people by the end of the 21 st century 3. However, even small amounts of sea-level rise can disproportionately increase coastal flood frequency 4,5. A multitude of oceanic processes affect both mean and extreme water levels, such as the tide, tropical and extratropical storms, climatic cycles (e.g., El Nino/Southern Oscillation), oceanic eddies, and circulation patterns 6-11. Hence, the frequency and severity of coastal flooding varies on a multitude of time scales. Yet, the persistent trend and acceleration of sea-level rise have a profound interaction with transient extreme events 12. In theory, sea-level rise progressively increases the frequency and severity of flooding 5. In practice, the monotonic increase in flooding, driven by elevating long-term mean sea level, is often overshadowed by interannual variability in extreme events 13 , which will likely continue through the middle of the 21 st century 14. Many have quantified future increases in potential coastal flood frequency by deriving 'multiplying factors' 15 , 'amplification factors' 16 ...

For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS.For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Executive SummaryBeach erosion is a chronic problem along most open-ocean shores of the United States. As coastal populations expand and community infrastructure comes under increasing threat from erosion, there is a demand for accurate information about trends and rates of shoreline movement, as well as a need for a comprehensive analysis of shoreline movement that is consistent from one coastal region to another. To meet these national needs, the U.S. Geological Survey began an analysis to document historical shoreline change along open-ocean sandy shores of the conterminous United States and parts of Hawaii and Alaska. An additional purpose of this work is to develop systematic methodology for mapping and analyzing shoreline movement so that consistent periodic updates regarding coastal erosion can be made nationally.This report on shoreline change on three of the eight main Hawaii islands (Kauai, Oahu, and Maui) is one in a series of reports on shoreline change in coastal regions of the United States that currently include California, the Gulf of Mexico region, the Southeast Atlantic Coast, and the Northeast Atlantic Coast. The report summarizes the methods of analysis, documents and interprets the results, explains historical trends and rates of change, and describes the response of various communities to coastal erosion. Shoreline change in Hawaii was evaluated by comparing historical shorelines derived from topographic surveys and processed vertical aerial photography over time. The historical shorelines generally represent the past century (early 1900s-2000s). Linear regression was used to calculate rates of change with the single-transect method: long-term rates were calculated from all shorelines (from the early 1900s to the most recent), whereas short-term rates were calculated from post-World War II shorelines only.Beach erosion is the dominant trend of shoreline change in Hawaii. However, shoreline change is highly variable along Hawaii beaches with cells of erosion and accretion typically separated by only a few hundred meters on continuous beaches or by short headlands that divide the coast into many small embayments. The beaches of Kauai, Oahu, and Maui are eroding at an average long-term rate for all transects (shoreline measurement locations) of -0.11 ± 0.01 m/yr (meters per year) and an average s...

Sea-level rise (SLR) induced flooding is often envisioned as solely originating from a direct marine source. This results in alternate sources such as groundwater inundation and storm-drain backflow being overlooked in studies that inform planning. Here a method is developed that identifies flooding extents and infrastructure vulnerabilities that are likely to result from alternate flood sources over coming decades. The method includes simulation of flood scenarios consisting of high-resolution raster datasets featuring flood-water depth generated by three mechanisms: (1) direct marine flooding, (2) storm-drain backflow, and (3) groundwater inundation. We apply the method to Honolulu's primary urban center based on its high density of vulnerable assets and present-day tidal flooding issues. Annual exceedance frequencies of simulated flood thresholds are established using a statistical model that considers predicted tide and projections of SLR. Through assessment of multi-mechanism flooding, we find that approaching decades will likely feature large and increasing percentages of flooded area impacted simultaneously by the three flood mechanisms, in which groundwater inundation and direct marine flooding represent the most and least substantial single-mechanism flood source, respectively. These results illustrate the need to reevaluate main sources of SLR induced flooding to promote the development of effective flood management strategies.

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Planning community resilience to sea level rise (SLR) requires information about where, when, and how SLR hazards will impact the coastal zone. We augment passive flood mapping (the so-called “bathtub” approach) by simulating physical processes posing recurrent threats to coastal infrastructure, communities, and ecosystems in Hawai‘i (including tidally-forced direct marine and groundwater flooding, seasonal wave inundation, and chronic coastal erosion). We find that the “bathtub” approach, alone, ignores 35–54 percent of the total land area exposed to one or more of these hazards, depending on location and SLR scenario. We conclude that modeling dynamic processes, including waves and erosion, is essential to robust SLR vulnerability assessment. Results also indicate that as sea level rises, coastal lands are exposed to higher flood depths and water velocities. The prevalence of low-lying coastal plains leads to a rapid increase in land exposure to hazards when sea level exceeds a critical elevation of ~0.3 or 0.6 m, depending on location. At ~1 m of SLR, land that is roughly seven times the total modern beach area is exposed to one or more hazards. Projected increases in extent, magnitude, and rate of persistent SLR impacts suggest an urgency to engage in long-term planning immediately.