A forecast of nuisance flooding of Charleston peninsula is presented, based on an analysis of tide records from Charleston Harbor, SC. The forecast was based on past trends in local sea level and tidal harmonics, including the 18.6-yr lunar nodal and annual cycles. The data document an exponential rise in mean sea level. Extrapolating to year 2060 shows that the sea-level trend already is equivalent to the RCP4.5 scenario and on track to exceed NOAA's intermediate low sea-level rise scenario of 0.5 m this century. If the trend continues, MSL will have risen by 0.22 m in 50 yr at an annual rate of 0.5 cm/yr in 2069. Simulations to 2064-2068, based on an empirical relationship between the annual number of flood events, defined as a water level exceeding 1.17 m NAVD (North American Vertical Datum of 1988), and the annual sum of monthly mean high water (r 2 = 0.84), predict annual flood events will rise to the 60 to 75 range. Application of the hourly tidal harmonics to the long-term sea-level trend provided estimates of total land area flooded and duration of flooding. Flood duration is expected to rise to 6.5% by 2046-2050 and 8.2% of time by 2064-2068. The area exposed to flooding will be 4.23 km 2 in 2046-2050 and 4.46 km 2 in 2064-2068, corresponding to about 20-21% of peninsular area on what was formerly marshland and creeks, filled in earlier centuries. Finally, the estimated cost of defending the city and a proposal for a climate tax are discussed.
The responses of marsh elevation in four National Parks affected by Hurricane Sandy were examined using empirical data from surface elevation tables (SET) and modeling. The parks examined were Fire Island National Seashore and Gateway National Recreational Area in New York; Cape Cod National Seashore, Massachusetts; and Assateague Island National Seashore, Maryland. Observed vertical accretion rates were compared with calculations made with the Marsh Equilibrium Model (MEM). MEM predicts vertical accretion resulting from the accumulation of organic material in soil and the capture of suspended inorganic material at the marsh surface. MEM simulations of a decade or more of marsh elevation change at 52 SET stations were generally consistent with observations. Park-specific averages of observed vertical accretion ranged from 0.16 ± 0.33 (± 1 SD) to 0.51 ± 0.21 cm/year, while the range of calculated rates was 0.15 ± 0.03 to 0.22 ± 0.05 cm/year, depending on the park. Grand means of observed and calculated rates were 0.36 ± 0.34 and 0.19 ± 0.06 cm/year, respectively. We defined a novel metric termed normalized elevation capital (NEC) that incorporates information about tide range and elevation capital. All but 2.3% of biomass collections from all the parks fell within 0 < NEC < 1. Consistent with marsh equilibrium theory, long-term vertical accretion rate tended to be greatest, 0.4 ± 0.2 cm/year, in the range 0.4 < NEC < 0.6 where vertical accretion is dominated by organic production. Average episodic accretion during the storm from mineral deposition also was greatest and positive, 0.6 ± 0.9 cm in the range 0.4 < NEC < 0.6. Finally, one marsh in Gateway NRA, restored by an application of sediment to NEC = 0.55-0.68, had post-treatment vertical accretion rates of 0.36 ± 0.31 cm/year, not statistically different from SET stations elsewhere in Gateway, 0.57 ± 0.54 cm/year. The sediment amendment placed restored sites in the range of NEC where theory predicts that biogenic accretion should dominate vertical accretion. Model simulations suggest that current rates of vertical accretion in the parks are close to their theoretical limits, and in the absence of new sediment, extant marsh communities in these parks are unlikely to survive continued acceleration of sea-level rise in the absence of periodic sediment renourishment.
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