June/July 2022 ASCE-NH Newsletter June/July 2022 - Page 10

Reference Stations ( CORS ) Global Positioning System ( GPS ) stations that show land rise of about 0.13 ft / 100 yrs . The corresponding land rise for Durham is 0.06 ft / 100 yrs . These land rise rates will decrease the observed local relative rate of sea level rise .
Future Sea Level Rise Projections :


In general the claimed accelerating sea level rise projections are not supported by the observed sea level rise data . There are recent governmental reports presenting projections for accelerating sea level rise caused by global warming . The latest federal government guide is 2022 NOAA Tech Report 01 , Sweet et . al . NOAA 2022 with tabulated values for relative sea level every 10 years starting in the year 2000 . This report did consider land / earth crust vertical movement at selected US cities , and projected changes in local sea level from gravitational changes associated with anticipated ice cap melting . Comparing the actual observed rates of sea level rise from tide data in Portland for years 2000 to 2022 , the actual rate of sea level rise is significantly less than the NOAA projected rate of rise and it is noted that NOAA assumes Portland land subsidence which is not supported by GPS data .
This NOAA report does present five sea level acceleration curves that have been adjusted near term to better match observed sea level rise observations , however they are set to meet the previously projected emissions based global warming sea level rise by 2100 . The report does note high uncertainty and divergence in the sea level rise projections after 2050 .
New Hampshire Sea Level Rise Projections :
The University of New Hampshire ( UNH ) issued a two part report New Hampshire Coastal Flood Risk Summary UNH SLR in 2019 and 2020 , which has been adopted by the state of New Hampshire and is the recommended policy in regulatory permitting by the NH Department of Environmental Services . Both the 2022 NOAA projections NOAA 2022 and the UNH projections use sea level rise projections starting from sea level in the year 2000 , developed by Kopp et . al . ( 2014 ) Kopp et al . The UNH report does list probabilities for multiple sea level rise curves , using different probabilities for different types of projects based on their tolerance for risk . It is important to understand that these probabilities are Bayesian probabilities , based on future expectations , not traditional probabilities calculated from observational data , such as used to establish FEMA flood levels .
The UNH Part II guidance report uses the 83 % probability curve for design of projects with a high tolerance for sea level rise , such as sidewalks . The 95 % probability curve is recommended for design of projects with a medium tolerance for sea level rise , including residential and commercial buildings . UNH does recommend higher 99 % and 99.5 % probability curves , for design of projects with low and very low tolerance for sea level rise . Currently the UNH 83 % projection curve is about 4 inches higher than Portland observations ( 12 month average to remove seasonal variation ), and the UNH 95 % projection curve is about 5 inches higher than Portland mean sea level observations . As the UNH projections include an acceleration in the rate of sea level rise , and the actual observation do not , the divergence between rise projections and actual sea level observations is expected to increase over time . The UNH projection curves are based on older 2014 sea level rise and global warming model and when UNH adopted these 2014 projection curves , they were not adjusted in consideration of actual sea level rise observations over the last 20 years . Since the UNH modelled sea level rise projections are already significantly in higher than sea level observations with a steeper rise trend , they are not recommended for project design .
CNE has found that the observed rate of sea level rise and fall varies over time and is not correlated to the steadily increasing carbon dioxide trace concentration in the atmosphere . The rates of sea level rise for New England are correlated to natural oscillations including sea surface temperature , variations in Earth ’ s spin rate and planetary orbital forcing .
If you are seeking a state or local permit for work on a seacoast property and the UNH model specified by the regulators says the property is going to be flooded by accelerating sea level rise , you need to be aware these guidelines have significantly exceeded observed sea level rise over the past 22 years . With this UNH report being cited in new regulatory rules and local ordinances , it is important to document the outdated and uncalibrated basis for this UNH report and to recommend that better , observation based “ Best Available Science ” be used for design .