June/July 2022 ASCE-NH Newsletter June/July 2022 - Page 7


Date : May 10 , 2022 Duncan Mellor , PE Principal Coastal Engineer Civilworks New England
If you have applied for a state and local environmental permits recently in the New Hampshire seacoast , you will have noticed the rules require design for more extreme rainfall and accelerating sea level rise . As these rules may restrict what you can do on your property and may adversely impact your property value , we should be asking if these extreme forecasts of future conditions are accurate . This review looks at how the accelerating sea levels projected by the state , don ’ t match up with actual observed data . The acceleration projections are already significantly higher than observed sea level data , and the divergence is increasing .
In 2013 the New Hampshire Senate passed Bill SB 163 , Chapter 188 which established a New Hampshire Coastal Risk & Hazards Commission to prepare for projected sea level rise and other coastal hazards . This was followed in 2016 with Senate Bill SB 452 requiring state agencies to make changes to statues , rules and policies to prepare for coastal flood risks using the best available projected coastal flooding risks , such as the 2014 Coastal Risks & Hazards report .
The first 2014 Coastal Risks & Hazards ( CRH ) report adopted projection curves developed by National Oceanic and Atmospheric Administration ( NOAA ), with acceleration in sea level rise starting in 1992 . This report indicated that projects with high tolerance for flood risk to commit to 3.9 feet of sea level rise and prepare for 6.6 feet of sea level rise by 2100 . Projects with low tolerance for flooding should commit to 6.6 feet of sea level rise by 2100 . It is significant to note that by the time the 2014 CRH report was issued , the outdated sea level rise acceleration projections were above the observed sea level rise . It is apparent that the rise acceleration curves were not recalibrated to better fit the 22 years of sea level rise observations after 1992 that existed prior to the CRH report release .
In 2020 the Coastal Risks & Hazards report was updated and issued by the University of New Hampshire ( UNH ) and the New Hampshire Department of Environmental Services ( NHDES ) in two volumes in late 2019 and early 2020 UNH SLR . This report adopted sea level rise acceleration curves starting in 2000 , and assigned Bayesian probabilities to the different sea level rise projection curves . For residential and commercial buildings they assign a “ 95 % probability ” category with 1.6 feet of sea level rise by 2050 and 3.8 feet of sea level rise by 2100 . Note that Bayesian probabilities include the use of opinion and extrapolation modelling , and these probabilities are different from observational data based probabilities , such as used to establish FEMA 100 year ( 1 %) flood elevations . Other governmental climate change forecasting reports also use Bayesian probabilities , but are usually careful not to assign specific percentages , preferring to use more vague term , such as “ likely ”.
The 2020 CRH report , like the prior report , presents sea level rise acceleration curves , but with the rise acceleration beginning in 2000 . As this report was not issued until 2020 , we have the opportunity to see how 21 years of actual sea level rise compares to the forecasted acceleration curves . Figure 1 plots the 2020 CRH acceleration curves (“ UNH ”) and projected acceleration 2022 curves by NOAA , in comparison with actual sea level rise observed by global tide stations , global satellite sea level rise data and locally observed sea level rise in Portland , Maine ( the reference tide gauge for most NH tide prediction locations ).