Sea Level Rise and Its Impact

Sea Level Trends

Global sea level has risen about 8 inches over the last century, and is rising about 4 inches since 1993, according to NASA’s satellite sea level observations³, as shown in the Fig-1 and 2. Its rising rate has increased over the last twenty years. It is estimated to rise an additional 12 to 48 inches by 2100⁴.

When we consider global warming, we often hear the term, sea level rise. In this case, SLR refers to “Global” SLR, which is a planetary average. In the sense that sea level rise is not globally uniform across the planet but varies regionally, we need to take a close look at the SLR specific to the local scale. Some locations are showing greater rates of SLR than the global average, while others are showing lower. The differences are related to geographical features, ocean currents, wind patterns, etc. Throughout this article, SLR refers to “Local or Relative” SLR specific to St. Augustine, Florida.

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Figure 1. Sea level trends from 1990 to present¹.

SLR is caused primarily by the added water from melting ice sheets and glaciers and the thermal expansion of seawater as it warms. The Earth’s oceans absorb approximately 90% of the heat trapped by excess greenhouse gases in the atmosphere. About 40% of the historically observed sea level rise (SLR) can be attributed to thermal expansion from ocean warming, while 60% can be attributed to glacial and ice sheet melt⁵. Because City of St. Augustine is a low-lying coastal city, it is particularly vulnerable to the impact of SLR. There are two tide stations existed near St. Augustine: Fernandina Beach and Mayport. The Fernandina Beach tide gauge records show SLR of about 8 inches in the last 100 years (0.81 inches per decade)⁶, which is similar to the global trend, while the Mayport rate is about 20% higher (0.98 inches per decade)⁷.

SLR Projections

SLR projections are suggested based on reports from NOAA, FLDEO, a Sea-Grant study by the University of Florida, and Climate Central. Sea level trends are used with models of future SLR scenarios to estimate what sea levels might be at a given point in the future. The trends are overlaid with sets of future sea level projections for evaluation and mapping, such as the US Army Corps of Engineers (USACE) and the National Climate Assessment, commonly referred to as the NOAA projections.

Research has used a bathtub method for mapping SLR on top of high tide (Mean Higher High Water (MHHW) – tidal datum) with a Digital Elevation Model (DEM), generated from laser-based Lidar data, to identify low-lying areas.

Florida Department of Economic Opportunity (FLDEO) released a Coastal Vulnerability Assessment for the City of St. Augustine, Florida in 2016. Based on this report, presented in Table-1, SLR is predicted as 0.4 to 0.9 feet for short-term (2045) and 0.7 to 5.2 feet for long-term (2085). “Low scenario” represents a continuation of historical observations, while “High scenario” considers maximum possible glacier and ice sheet loss by the end of the century. Considering Intermediate-High projection, SLR is one-foot in 2040 and three feet in 2080s. Similarly, the UF Resilient Community Initiative used one-foot, three-foot, and five-foot scenarios on top of MHHW in the study titled Adapting to Rising Tides. A scenario of one-foot of SLR would affect approximately 25% of the City’s area and is projected to occur as early as 2030, or as late as 2070. A three-foot SLR scenario would affect approximately 42% of the City and is projected to occur as early as 2070, or as late as 2100. A five-foot SLR scenario would affect approximately 68% of the City and is projected to occur no earlier than 2085. While the impacts to the City under this scenario are the greatest, the time frame for potential impacts is towards the end of this century.

Table 1. SLR projections at Mayport, FL gage based on NOAA/NCA Projections. Increases are in units of feet relative to local mean sea level and calculated from the mid-point of the existing National Tidal Datum Epoch (1992). (Source: FLDEO⁸)

Time Horizon	Low	Intermediate-Low	Intermediate-High	High
Short term (2045)	0.4 ft	0.7 ft	1.2 ft	1.9 ft
Long-term (2085)	0.7 ft	1.5 ft	3.2 ft	5.2 ft

In general, we consider three types in coastal flood events: MHHW, nuisance flooding, and the 1% annual chance floodplain. The coastal flood hazard events are projected by increasing the present-day base surface elevation through addition of each SLR scenario increment to the base flood conditions, as presented in Table-2. After applying SLR projections to each coastal flood event type, inundation and coastal flooding extents are established for each scenario and flood frequency by intersection the water surface elevation models with the topographic elevation models in a Geographic Information System (GIS)6. Simply, MHHW is the current water elevation, nuisance flooding is the water level possible to reach occasionally due to tides and small coastal storms, and 1% annual chance floodplain is the worst case possible to occur once 100 years.

Table 2. Sources of flood hazard types selected for the St. Augustine vulnerability assessment⁶.

Flood Type	Description	Frequency	Water Elevation	Source
Mean Higher High Water (MHHW)	The higher daily high tide elevation, defining the limit of what land is essentially “inundated” or has very limited use.	Daily	~ 2 ft NAVD88	NOAA VDatum software
Nuisance Flooding	Areas frequently flooded by tides and/or small coastal storms. Results in shallow flooding, which may disrupt or limit use.	12-17 times a year	3.75 ft NAVD88	Tidal gauge analysis and coordination with community
1% Annual Chance Flood Event	Areas subject to flooding by significant coastal storms. Defines the Special Flood Hazard Area as delineated on Federal Emergency Management Agency Flood Insurance Rate Maps. Also known as the “Base Flood”.	~26% chance in 30 years	Range from 6-10 ft	Preliminary FEMA FIS update for St. Johns County, FL.

Based on the UF study, presented in Table-3 and Fig- 2, 25% of the City can be inundated with 1-foot of SLR above MHHW, which can occur as early as 2030, or as late as 2070. When SLR reaches 3 feet, 42% of the City can be inundated as early as 2070, or as late as 2100. When SLR is 5 feet, it impacts 69% of the City, which can occur no earlier than 2085.

Table 3. Area of City Impacted for Three (Relative) SLR Scenarios Over MHHW⁹.

RSLR Above MHHW	Time Frame	City Area	Area Inundated
(Feet)	(Year)	(Acres)*	(Acres)	(Sq. Miles)	(% of City)
Mean Higher High Water (MHHW)	2030 – 2070	5,362	1,353	2.11	25.22%
3.0	2070 – 2100+	5,362	2,260	3.53	42.15%
5.0	2085 – 2100+	5,362	3,672	5.74	68.48%

*Total City Acres of 5,362 or 8.38 square miles were used for the analyses.

Figure 2. One-foot, three-foot, and five-foot SLR scenarios over MHHW in the City of St. Augustine⁷. Figures A to D, generated using Climate Center’s Surging Sea Risk Zone Map, show the focus areas containing (1) Castillo de San Marcos, (2) Oldest Wooden School House, (3) Old City Gate, (4) Cordova St., and (5) Orange St.

Particularly, Salt March areas have a significant impact even from the one-foot SLR projection. It is projected that 60% of Salt March can be sunk with the one-foot SLR projection. In the sense that these salt marches provide critical ecosystem services for the area, it may diminish the protective storm surge buffering function they currently serve after 2030. Furthermore, approximately 922 and 2,291 historic structures among 3,288 are potentially affected by three-foot and five-foot SLR respectively, while four private historic structures are impacted by one-foot SLR.

Storm Surge Effects

Storm surge can be considered as the case of a 1% annual chance floodplain and is often the greatest threat to life and property along the coast from a hurricane. A problem is that hurricane gets more often and becomes stronger over the past four decades. Storm surge is an abnormal rise of water generated by a storm, over and above the predicted astronomical tides. It is defined as the water level rise due to the combination of storm surge and the tide. The rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 20 feet or more in some cases¹⁰. Follow the link for more information: https://youtu.be/ESchgv-JCGg)

Storms are divided into different categories, representing three feet of water surge for Category 1, and up to 9 feet of water surge for Category 3 in the St. Augustine area (Source: NOAA). The maps below, Fig-3, show the potential water levels caused by Category 1-3 Hurricanes. Storm surge from the Cat-2 hurricane potentially impacts most of the historic structures in the City with greater than 3 feet of water level above ground.

Figure 3. Storm surge water level maps projected from Category 1 to 3 hurricanes for St. Augustine⁸

Adaptation Strategies

General adaptation strategies, based on the IPCC 2019 report, are 1) Projection, 2) Accommodation, 3) Advance, 4) Retreat, and 5) Ecosystem-based adaptation (EbA)¹¹, as presented in Table-4 and Fig-4. Protection strategies are applicable to valued assets with significant location-dependence, which are unsuitable for infrastructure alteration or relocation. Hard protection (e.g., dikes, seawalls, breakwaters, barriers and barrages) and advance (building into the sea) are economically efficient in most urban context facing land scarcity, but can lead to increased exposure in the long term. Where sufficient space is available, EbA can both reduce coastal risks and provide multiple other benefits. Accommodation strategies, such as flood proofing buildings are applicable to valued assets which are suitable to alteration to reach sufficient elevation thresholds to avoid scenario-based inundation risks. Advance strategies create new land by building seaward, reducing coastal risks for the hinterland and the newly elevated land. Retreat strategies, such as voluntary setbacks and easements, are applicable to extendable assets with higher vulnerability to SLR and coastal hazards. EbA strategies provide a combination of protect and advance benefits based on the sustainable management, conservation and restoration of ecosystems, such as wetlands and reefs.

Figure 4. Different types of strategies responding to coastal risk and SLR (created based on the Figure 1 of the IPCC 2019 report⁹).

Discussion

Climate-change induced Global SLR is caused by ocean mass gain from in land-ice mass and thermal expansion of ocean water. Responses to SLR refer to legislation, plans and actions undertaken to reduce risk and build resilience. These responses range from protecting the coast, accommodating SLR impacts, retreating from the coast, advancing into the ocean by building seawards and EbA. We need to assess their effectiveness, technical limits, costs, benefits, and the specific governance challenges. A variety of adaptation responses to coastal impacts and risks have been implemented around the world, but mostly as a reaction to current coastal risk or experienced disasters. Economic challenges to the adaptation strategies increase with higher sea levels and make adaptation unaffordable before technical limits are reached. Thus, intergovernmental coordination, cooperation and collaboration are the key for the adaptation strategies: time horizon and uncertainty; cross-scale and cross-domain coordination; equity and social vulnerability; social conflict; and complexity9. Vulnerability assessments, community outreach and education can raise awareness of the SLR impacts and strategic implementation of the adaptation is required with planning, regulatory, financial support, etc. Furthermore, sharing best practices to implement SLR responses and lessons learned from practical efforts synthesize its success.

¹ Wilson, Steven G. and Thomas R. Fischetti. (May 2010). Coastline Population Trends in the United States: 1960 to 2008. Current Population Reports. U.S. Census Bureau. p. 4. https://www.census.gov/prod/2010pubs/p25-1139.pdf
² Florida Oceans and Coastal Council. (December 2010). Climate Change and Sea-Level Rise in Florida: An Update of the Effects of Climate Change on Florida’s Ocean & Coastal Resources. pp. 1-2. http://www.dep.state.fl.us/oceanscouncil/reports/
³ NASA’s Goddard Space Flight Center. https://climate.nasa.gov/vital-signs/sea-level/
⁴ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds. 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp. doi:10.7930/J0Z31WJ2. http://www.globalchange.gov/nca3-downloads-materials
⁵ Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds. 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp. doi:10.7930/J0Z31WJ2. http://www.globalchange.gov/nca3-downloads-materials
⁶ Mean Sea Level Trend. Tide Gauge 8720030: Fernandina Beach, FL. (NOAA). http://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?stnid=8720030
⁷ Mean Sea Level Trend. Tide Gauge 8720218: Mayport, FL. (NOAA). http://tidesandcurrents.noaa.gov/sltrends/sltrends_station.shtml?stnid=8720218
⁸ Florida Department of Economic Opportunity (FLDEO), Coastal Vulnerability Assessment: City of St. Augustine, Florida, Table 2, Table 4, Page 9, 2016
⁹ University of Florida Resilient Communities Initiative, Adapting to Rising Tides (Coastal Resilience in St. Augustine: Baseline of Our Past, Beacon for Our Future), Table-2, Figure-2, 2016
¹⁰ National Hurricane Center and Central Pacific Hurricane Center, Storm Surge Overview and Strom Surge Risk Maps, https://www.nhc.noaa.gov/surge/
¹¹ H.-O. Pörtner,. D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E., IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, Chapter 4, IPCC, 2019. https://www.ipcc.ch/site/assets/uploads/sites/3/2019/12/SROCC_FullReport_FINAL.pdf