Predicting the longshore-variable coastal response to hurricanes

The longshore variability of the coastal response to hurricanes may be examined within the framework of a storm-impact scaling model that compares spatially-variable beach morphology and fluid forcing. The relative elevations of dune height and storm-induced water levels are used to define three impact regimes (swash, collision, and overwash), within which the magnitudes and processes of sediment transport are expected to be unique. Maximum total water-levels are modeled as the sum of astronomical tide, storm surge, and wave runup. The 2% exceedence level for runup, the sum of wave setup and swash, is calculated using a parameterization found to be accurate to 38 cm (rms error) based on comparisons to 491 data runs from ten field experiments. Techniques have been developed to extract accurate (15-cm rms) and detailed measures of large-scale coastal morphology and change from high-resolution topographic laser altimetry (lidar) surveys, allowing for quantification of relevant dune heights as well as the magnitudes and patterns of shoreline, dune, beach slope, and beach volume change in response to hurricanes. Based on the relative elevations of modeled hurricane-induced water levels and lidar-derived measures of pre-storm (1997) dune morphology, the potential impact regimes for Hurricanes Bonnie (1998) and Floyd (1999) were defined at 20-m increments along a 70-km stretch of coast in Onslow Bay, North Carolina. Comparisons to the observed impact regime, quantified from calculations of dune erosion and overwash deposition, indicate that the predictive accuracy of the model was 55.4%, an improvement over the 33.3% accuracy associated with random chance. Regime-specific model sensitivity was highest within the overwash regime (86.9%), decreasing to 55.8% and 1.5% in the collision and swash regimes, respectively. Shoreline and beach volume change in response to the storms were spatially-variable: the standard deviation of change was the same order of magnitude as the mean. Magnitudes of coastal change scaled with the observed impact regime. Beach volume change within the overwash and collision regimes was over two times greater than that within the swash regime. Little recovery was observed in overwashed locations where sand was transported inland and removed from the nearshore system. Here, the volume of sand removed from the beach was balanced by that in the overwash deposits

Characterizing storm-induced coastal change hazards along the United States West Coast

Traditional methods to assess the probability of storm-induced erosion and flooding from extreme water levels have limited use along the U.S. West Coast where swell dominates erosion and storm surge is limited. This effort presents methodology to assess the probability of erosion and flooding for the U.S. West Coast from extreme total water levels (TWLs), but the approach is applicable to coastal settings worldwide. TWLs were derived from 61 years of wave and water level data at shore-perpendicular transects every 100-m along open coast shorelines. At each location, wave data from the Global Ocean Waves model were downscaled to the nearshore and used to empirically calculate wave run-up. Tides were simulated using the Oregon State University?s tidal data inversion model and non-tidal residuals were calculated from sea-surface temperature and pressure anomalies. Wave run-up was combined with still water levels to generate hourly TWL estimates and extreme TWLs for multiple return periods. Extremes were compared to onshore morphology to determine erosion hazards and define the probability of collision, overwash, and inundation

Changing the Paradigm of Response to Coastal Storms

Federal, state, and local agencies mounted a massive preparation and response to post–tropical storm Sandy, which made landfall along the northern New Jersey coast on 29 October 2012. The data collected and knowledge gained in response to Sandy are unprecedented and provide critical information to agencies, local emergency responders, and coastal managers and planners

Changing the Paradigm of Response to Coastal Storms

Federal, state, and local agencies mounted a massive preparation and response to post–tropical storm Sandy, which made landfall along the northern New Jersey coast on 29 October 2012. The data collected and knowledge gained in response to Sandy are unprecedented and provide critical information to agencies, local emergency responders, and coastal managers and planners

Predictions of barrier island berm evolution in a time-varying storm climatology

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 119 (2014): 300-316, doi:10.1002/2013JF002871.Low-lying barrier islands are ubiquitous features of the world's coastlines, and the processes responsible for their formation, maintenance, and destruction are related to the evolution of smaller, superimposed features including sand dunes, beach berms, and sandbars. The barrier island and its superimposed features interact with oceanographic forces (e.g., overwash) and exchange sediment with each other and other parts of the barrier island system. These interactions are modulated by changes in storminess. An opportunity to study these interactions resulted from the placement and subsequent evolution of a 2 m high sand berm constructed along the northern Chandeleur Islands, LA. We show that observed berm length evolution is well predicted by a model that was fit to the observations by estimating two parameters describing the rate of berm length change. The model evaluates the probability and duration of berm overwash to predict episodic berm erosion. A constant berm length change rate is also predicted that persists even when there is no overwash. The analysis is extended to a 16 year time series that includes both intraannual and interannual variability of overwash events. This analysis predicts that as many as 10 or as few as 1 day of overwash conditions would be expected each year. And an increase in berm elevation from 2 m to 3.5 m above mean sea level would reduce the expected frequency of overwash events from 4 to just 0.5 event-days per year. This approach can be applied to understanding barrier island and berm evolution at other locations using past and future storm climatologies.2014-08-1

Operational forecasts of wave-driven water levels and coastal hazards for US Gulf and Atlantic coasts

Abstract Predictions of total water levels, the elevation of combined tides, surge, and wave runup at the shoreline, are necessary to provide guidance on potential coastal erosion and flooding. Despite the importance of early warning systems for these hazards, existing real-time meteorological and oceanographic forecast systems at regional and national scales, until now, have lacked estimates of runup necessary to predict wave-driven overwash and erosion. To address this need, we present an approach that includes wave runup in an operational, national-scale modeling system. Using this system, we quantify the contribution of waves to potential dune erosion events along 4,700 km of U.S. Atlantic and Gulf of Mexico sandy coastlines for a one-year period. Dune erosion events were predicted to occur at over 80% of coastal locations, where waves dominated shoreline total water levels, representing 73% of the signal. This shows that models that neglect the wave component underestimate the hazard. This new, national-scale operational modeling system provides communities with timely, local-scale (0.5 km resolution) coastal hazard warnings for all wave conditions, allowing for rapid decision-making related to safety and emergency management. The modeling system also enables continued research into wave-driven processes at a broad range of coastal areas

Improving Understanding of Near-term Barrier Island Evolution through Multi-decadal Assessment of Morphologic Change

Observed morphodynamic changes over multiple decades were coupled with storm-driven run-up characteristics at Fire Island, New York, to explore the influence of wave processes relative to the impacts of other coastal change drivers on the near-term evolution of the barrier island. Historical topography was generated from digital stereo-photogrammetry and compared with more recent lidar surveys to quantify near-term (decadal) morphodynamic changes to the beach and primary dune system between the years 1969, 1999, and 2009. Notably increased profile volumes were observed along the entirety of the island in 1999, and likely provide the eolian source for the steady dune crest progradation observed over the relatively quiescent decade that followed. Persistent patterns of erosion and accretion over 10-, 30-, and 40-year intervals are attributable to variations in island morphology, human activity, and variations in offshore bathymetry and island orientation that influence the wave energy reaching the coast. Areas of documented long-term historical inlet formation and extensive bayside marsh development show substantial landward translation of the dune–beach profile over the near-term period of this study. Correlations among areas predicted to overwash, observed elevation changes of the dune crestline, and observed instances of overwash in undeveloped segments of the barrier island verify that overwash locations can be accurately predicted in undeveloped segments of coast. In fact, an assessment of 2012 aerial imagery collected after Hurricane Sandy confirms that overwash occurred at the majority of near-term locations persistently predicted to overwash. In addition to the storm wave climate, factors related to variations within the geologic framework which in turn influence island orientation, offshore slope, and sediment supply impact island behavior on near-term timescales