Journal of Waterway, Port, Coastal, and Ocean Engineering

Forecasts of sea-level rise by 2100 have such a wide range that objective planning is difficult. In 2007, the Intergovernmental Panel on Climate Change (IPCC) forecast a sea-level rise from 1990 to 2100 between 0.18 and 0.59 m, but they did not completely consider contributions from Greenland and Antarctica in determining the upper limit. Recent projections by others have been larger, typically 1–2 m, with Greenland and Antarctica being the greatest contributors. However, these projections are usually maximum possible rises without probabilities of occurrence, whereas the maximum IPCC projection is at the 95% confidence level with a 2.5% probability of being equaled or exceeded. Project planning and design generally require projections with associated probabilities to determine risk. This paper shows what worldwide tide gauge data tell us about sea-level rise trends and accelerations in the twentieth century, and satellite altimeter recordings tell us about the rise since the early 1990s. Using IPCC methodology, sea level projections with associated probabilities are made from 1990 to 2100. Projections of Greenland and Antarctica contributions are estimated based on satellite measurements of current ice-mass-loss trends and accelerations, and then they are added to contributions from thermal expansion and melting of glaciers and ice caps based on the latest information. Sea-level rise projections from 1990 to 2100 are 0.18, 0.48, and 0.82 m at 5, 50, and 95% confidence levels, respectively.

The relative contributions of sea-level rise (SLR) and increasing extratropical storminess to the frequency with which waves attack coastal features is assessed with a simple total water level (TWL) model. For the coast of the U.S. Pacific Northwest over the period of wave-buoy observations (approximately 30 years), wave height (and period) increases have had a more significant role in the increased frequency of coastal flooding and erosion than has the rise in sea level. Where tectonic-induced vertical land motions are significant and coastlines are presently emergent relative to the mean sea level, increasing wave heights result in these stretches of coast being possibly submergent relative to the TWL. Although it is uncertain whether wave height increases will continue into the future, it is clear that this process could remain more important than, or at least as important as, SLR for the coming decades, and needs to be taken into account in terms of the increasing exposure of coastal communities and ecosystems to flooding and erosion.

Sea-level rise (SLR) estimates vary broadly, but most estimates predict significant increases in sea levels within the next century. Through the use of validated, coupled wave and hydrodynamic models, this study investigates SLR effects on storm surge and nearshore waves, including variation with coastal landscape type and storm characteristics. Both the wave and hydrodynamic models account for bottom friction based on land-use type, so the modeling procedure accounted for SLR-related changes to land cover, in addition to higher mean sea levels. Simulation of storms with similar meteorological characteristics and track but with landfall in the northeastern and southwestern coastal areas allowed examination of how the coastal features influenced the storm surge and wave response to increased sea levels. The simulation results analyzed here demonstrate that the relationship between storm surge and relative SLR (RSLR) varies between geographic region and storm scenario. The increase in an inundated area is linear in the north, and in the south, the inundated area approaches the northern values for similar storms asymptotically. Nearshore wave results indicate, as expected, that larger water depths created by positive RSLR and amplified surge allow larger waves to propagate into inland areas. For the Texas simulations, it was found that an increase in hurricane wind speeds of 25% is approximately equivalent to a RSLR of 0.5 m in terms of increased area of inundation impact. Because of the complexities of storm-surge dependency on storm strength, track, and local topography, there is no one-size-fits-all response to RSLR descriptive of all locations. Site-specific computer modeling should be used to evaluate the risk facing coastal communities.

The impact of sea-level rise on salt marsh sustainability is examined for the lower St. Johns River and associated salt marsh (Spartina alterniflora) system. A two-dimensional hydrodynamic model, forced by tides and sea-level rise, is coupled with a zero-dimensional marsh model to estimate the level of biomass productivity of S. alterniflora across the salt marsh landscape for present day and anticipated future conditions (i.e., when subjected to sea-level rise). The hydrodynamic model results show mean low water (MLW) to be highly spatially variable with a SD of ± 0.18 m and mean high water (MHW) to be less spatially variable with a SD ± 0.03 m. The spatial variability of MLW and MHW is particularly evident within the tidal creeks of the salt marsh. MLW and MHW are sensitive to sea-level rise and respond in a nonlinear fashion (i.e., MLW and MHW elevate by an amount that is not proportional to the level of sea-level rise). The coupled hydrodynamic-marsh model results illustrate the spatial heterogeneity of biomass productivity and indicate marsh vulnerability to sea-level rise. The model is then used to demonstrate an application of engineered accretion that can help sustain a marsh that is exposed to sea-level rise.

Reliable hurricane flood probability estimates are essential for effective management and engineering in the coastal environment. However, uncertainty in future climate conditions presents a challenge for assessing future flood probabilities. Studies suggest that in the future, sea-level rise may accelerate, and hurricanes may intensify and occur less or more often. Here, methods are presented for incorporating sea-level rise and future hurricane conditions into extreme-value flood statistics analysis. By considering an idealized coast, surge response functions are used with joint probability statistics to define time-varying continuous probability mass functions for hurricane flood elevation. Uncertainty in the flood estimates introduced by uncertainty in future climate is quantified by considering variance in future climate and sea level projections. It will be shown that future global warming can increase the flood elevation at a given return period by 1–3% per decade, but that climate-related uncertainty only marginally contributes to the overall uncertainty associated with hurricane flood statistics. Finally, it will be demonstrated that adaptive management practices are the most effective means of optimizing future coastal engineering activities in the face of climate change.

Fundamental uncertainties governing sea level rise projections are associated with the rate of acceleration in global sea level rise and the local factors affecting relative sea level. Recent models and observations of global ice sheets and their contributions to sea level, as well as climate models, suggest that significant sea level acceleration is imminent, yet observational data from tide gauges and satellites have not yet found rates consistent with some of the projections. To allow coastal planners a way to incorporate the uncertainties of acceleration, a synthesis of observed sea level data with selected distributions of acceleration and the current rate of rise is proposed to provide probabilistic estimates of future sea levels. The resulting distributions can be adopted in a risk-based framework to assess project vulnerability. As geophysical understanding of the climate-forced sea level dynamics improves, the selection and veracity of these distributions and their sea level projections will improve. As a case study, the methodology is demonstrated by application in Key West in southern Florida.

A method is presented to consistently tie future mean sea level rise (MSLR) scenario projections to local geodetic and tidal datums. This extends the U.S. Army Corps of Engineer (USACE) guidance for incorporating the effects of future MSLR into coastal projects. While USACE relies on the National Oceanic and Atmospheric Administration (NOAA) 19-year National Tidal Datum Epoch (NTDE) for its datum relationships, the approach proposed herein generalizes this guidance by choosing the appropriate 19-year epoch centered on the start year of the MSLR scenario under consideration. The procedure takes into account the local annual sea level variability, which confounds the matching to any given single year while generalizing and preserving the 19-year averaging long used by NOAA to calculate the NTDE. Examples of the MSLR scenario matching procedure are given using actual data and projections for La Jolla, California, and Sewells Point (Hampton Roads), Virginia.