Sea Level Rise in Virginia – Causes, Effects and Response

Abstract

Sea level rise (SLR) along Virginia’s coasts and around the Chesapeake Bay as measured by tide gauges is analyzed and discussed. It is shown that the SLR rates vary between one location to another and in most locations the rates increase over time (i.e., SLR is accelerating). The latest science of SLR is reviewed and the causes of the high SLR rates in Virginia are discussed. The impacts of land subsidence and ocean currents (changes in the Gulf Stream in particular) on sea level are especially notable and important for predicting future SLR in Virginia. The consequences of SLR on increased duration and severity of floods are demonstrated and potential responses are discussed. INTRODUCTION One of the environmental consequences of climate change that have been the most visible in Virginia is sea level rise (SLR). While sea level along the coasts of Virginia is slowly rising, the impacts of waves and storm surges increase as waters are pushed farther into previously unaffected coastal areas and low-lying streets. Both natural features such as marshes and barrier islands and also the built features such as docks, shipyards, tunnels, homes and hotels constructed along the shoreline are all affected. People living on the coast do not always recognize sea level rise itself, but they clearly see that there is more frequent flooding and that areas that were not flooded in the past are now becoming new flood-prone areas (Atkinson et al. 2013, Mitchell et al. 2013, Ezer and Atkinson 2014, Sweet and Park 2014). The relative SLR rate (i.e., local water level relative to land) on Virginia’s coasts is one of the highest of all U.S. coasts and the rate appears to be accelerating (Boon 2012, Ezer and Corlett 2012, Ezer 2013, Sallenger et al. 2012, Kopp 2013). SLR rates from tide gauges in Virginia over the past 10-30 years are ~4-6 mm/year, which are higher than the global mean SLR rate of ~1.7 mm/year over the past century as seen from tide gauges and even higher than the ~3.2 mm/year over the past 20 years as seen from satellite altimeter data (Church and White 2011, Ezer 2013). Note that SLR of 3 mm/yr is equivalent to about 1 foot/century. Relative SLR is primarily the result of 1 Corresponding author: tezer@odu.edu, latkinso@odu.edu 356 VIRGINIA JOURNAL OF SCIENCE three processes: 1. global SLR due to warming ocean temperatures and melting land ice, 2. local land subsidence (sinking) and 3. ocean dynamics. The impact of land subsidence and ocean dynamics is especially evident in Virginia. The Virginia coast is experiencing subsidence due to human activities such as groundwater extraction and historic geological processes (Boon et al. 2010, Eggleston and Pope 2013). Changes in the flow of offshore currents and the Gulf Stream in particular can result in water level anomalies and flooding (Sweet et al. 2009, Ezer and Atkinson 2014). Since much of Virginia’s coastal areas are flat, small amounts of SLR can have dramatic impactsincreased flooding and coastal erosion, and altering marshes. Dealing with these issues requires knowledge on future SLR to design and plan accordingly. CURRENT TRENDS IN SEA LEVEL RISE Water level measurements from 13 locations around the Chesapeake Bay and the Virginia coast were analyzed (Figure 1)8 stations with long records (~40-110 years) and 5 stations with shorter records (10-20 years). Water levels along the U.S. coast are measured by tide gauges maintained by the National Oceanic and Atmospheric Administration (NOAA) (Zervas 2009). Hourly data are obtained from the NOAA website (www.tidesandcurrents.noaa.gov); these data are used for calculations of potential flooding and storm surge impacts (Atkinson et al. 2013, Ezer and Atkinson 2014, Sweet and Park 2014). Monthly mean data for stations around the globe are archived by the Permanent Service for Mean Sea Level (PSMSL, www.psmsl.org, Woodworth and Player 2003). The PSMSL monthly data were used for the stations with long records, while the NOAA data were used for the stations with short records (Figure 1); monthly means were calculated from hourly data before calculating SLR rates. Note that the statistical accuracy of calculating SLR rates from linear regression (fitting the data with a straight line, the slope of which represents the mean rate) depends on record length. For example, a record of 60 years would yield an error in SLR of less than ±0.5 mm/yr (at 95% confidence level), while a record of 30 years would have an error of less than ±1.5 mm/yr (Zervas 2009, Boon et al. 2010). However, there are only 2 tide gauge stations in Virginia with observations of over 60 years (86 years at Sewells Point in Norfolk and 62 years at Kiptopeake on the eastern shore). Therefore, long records from Maryland and short records from Virginia are analyzed as well. The analysis of the long records is shown in Figure 2 and that for the shorter records is shown in Figure 3. Also shown (smooth black line in Figure 2) are inter-annual variations after removing high-frequency variations using Empirical Mode Decomposition (EMD, Huang et al. 1998, Ezer and Corlett 2012). SLR rates are calculated for the past 30 years, and the 30 years before that, to see if the rates are constant or changing. Our results reveal that everywhere within the region sea level is rising faster than the global rates. However, SLR rates are not constantthey vary in time (due to climatic changes in the ocean) and in place (due to local and regional land subsidence, see discussion later). SLR is largest in the lower Chesapeake Bay (Chesapeake Bay Bridge Tunnel (CBBT) and Norfolk), and a little lower in the northern REVIEW OF SEA LEVEL RISE IN VIRGINIA 357 FIGURE 1. Map of the Chesapeake Bay region and location of tide gauge stations. Long and short records are indicated and analyzed separately in figures 2 and 3, respectively. 358 VIRGINIA JOURNAL OF SCIENCE FIGURE 2. Monthly sea level in the Chesapeake Bay for stations with long records (from 40 years in Chesapeake Bay Bridge Tunnel, CBBT, to 110 years in Baltimore). Inter-annual variations are shown by black heavy lines and linear trends by dash lines. SLR rates in mm/yr are shown for two 30-year periods. REVIEW OF SEA LEVEL RISE IN VIRGINIA 359 FIGURE 3. Monthly sea level and trends as in Figure 2, but for tide gauge stations in Virginia with relatively short records. The SLR rates in mm/y are listed under the