From the extreme to the mean: Acceleration and tipping points of coastal inundation from sea level rise

Abstract

Relative sea level rise (RSLR) has driven large increases in annual water level exceedances (duration and frequency) above minor (nuisance level) coastal flooding elevation thresholds established by the National Weather Service (NWS) at U.S. tide gauges over the last half-century. For threshold levels below 0.5 m above high tide, the rates of annual exceedances are accelerating along the U.S. East and Gulf Coasts, primarily from evolution of tidal water level distributions to higher elevations impinging on the flood threshold. These accelerations are quantified in terms of the local RSLR rate and tidal range through multiple regression analysis. Along the U.S. West Coast, annual exceedance rates are linearly increasing, complicated by sharp punctuations in RSLR anomalies during El Niño Southern Oscillation (ENSO) phases, and we account for annual exceedance variability along the U.S. West and East Coasts from ENSO forcing. Projections of annual exceedances above local NWS nuisance levels at U.S. tide gauges are estimated by shifting probability estimates of daily maximum water levels over a contemporary 5-year period following probabilistic RSLR projections of Kopp et al. (2014) for representative concentration pathways (RCP) 2.6, 4.5, and 8.5. We suggest a tipping point for coastal inundation (30 days/per year with a threshold exceedance) based on the evolution of exceedance probabilities. Under forcing associated with the local-median projections of RSLR, the majority of locations surpass the tipping point over the next several decades regardless of specific RCP.

An updated USACE approach to the evaluation of coastal total water levels for present and future flood risk analysis

“…The seasonal cycle is typically resolved by the Sa and Ssa tidal constituents, but transport variability can cause large anomalies [7]. Figure 6b shows the relative contribution of to high SWL events at 45 long-term tide gauges around the U.S. as shown in [8]. A low ratio of the average to maximum water level observations helps identify regions where the tide range is a much larger contributor than the magnitude.…”

Climate Change, Coastal Vulnerability and the Need for Adaptation Alternatives: Planning and Design Examples from Egypt and the USA

“…Global mean sea level was relatively stable for the past several thousand years under a mild climate until the mid-19th century, but during the 20th century sea level began rising due to global warming resulting from human activities at a global average rate of 1.2 mm/year. The current average global rise rate is 3.2 mm/year, 2.5 times faster [2,3]. Many coastal regions, particularly deltas, however, are experiencing much greater local or relative sea-level rise (LSLR), defined as global mean sea-level rise plus subsidence, plus sediment compaction, and in minor cases land emergence.…”

“…These higher than global rates have significant areal variations and are due to local geophysical (subsidence, faulting), oceanographic, and human factors (oil, gas, and ground water extraction, as well as wetland reclamation). Combined with storms, this rise is resulting in greater surge elevations, more frequent tidal nuisance flooding, and record increases in damage to coastal infrastructure and loss of life [2]. Projected GSLR by 2100 [4][5][6][7] is 0.2 m to 2 m and would be in addition to local rise factors such as subsidence and oceanographic processes (Figure 2).…”

Effective inundation of continental United States communities with 21st century sea level rise

“…Sea level rise is expected to make recurrent tidal flooding both more frequent and more extensive (Sweet & Park 2014;Moftakhari et al 2015;Dahl et al 2017;Kulp & Strauss 2017). While the tidal datums associated with the mean higher high water (MHHW) mark could be revised upward as sea level rises, the water level at which a community begins to flood will not change, thus leading to an increase in flood frequency.…”

“…and frequent enough to make adjusting to this disruption costly-in some cases prohibitively so-or untenable (Spanger-Siegfried et al 2014;Sweet & Park 2014;Moftakhari et al 2015;Moftakhari et al 2017). Investments into protective measures such as bulkheads or pump systems can make a substantial difference to community-level flood severity (Allen 2016).…”