Coastal Change Probability
The coastal change probability is based on estimating the likelihood that the beach system will experience erosion and deposition patterns consistent with collision, overwash, or inundation regimes. The probabilities are estimated by calculating the difference between the model-derived total water levels (including storm surge and wave runup) and lidar-derived dune or berm elevations. Beach slopes (needed to estimate runup) are obtained from the lidar elevation data. The probability of COLLISION is computed as the likelihood that the total water level exceeds the elevation of the dune base. The probability of OVERWASH is computed as the likelihood that the total water level exceeds the elevation of the dune/berm crest. The probability of INUNDATION is computed as the likelihood that surge levels exceed the dune/berm crest elevation. Uncertainties considered arise from topographic elevation errors (e.g., lidar error and interpolation error) and errors associated with predicting wave runup elevations.
Generalized coastal change probability for a category 3 hurricane based on observed dune elevations and potential interactions with storm waves, runup, and surge. The probabilities incorporate what is known and what is uncertain about the data, including uncertainties in dune elevations, beach slopes, waves, and storm surge. [larger version]
Coastal change probabilities can be estimated prior to hurricane landfall using storm scenario information, and then updated as forecasts of a specific event become available. Prior to a storm event, predictions from the National Oceanic and Atmospheric Administration (NOAA) storm surge model (SLOSH) were used to simulate the surge resulting from numerous hypothetical hurricane landfall scenarios. The envelope representing the maximum simulated surge elevations was used to define a worst-case surge value at each location where potential coastal change is being evaluated. Predicted maximum wave heights at the 20-m depth contour along the area of interest are obtained from the NOAA WaveWatch3 model 7-day forecast and used to compute predicted runup elevations. Probabilities are updated using event-specific storm surge when NOAA operational surge forecasts become available.
A generalized estimate of each coastal change regime (collision, overwash, inundation) is determined for Category 1-5 hurricanes, based on predicted maximum surge and 2% exceedance of the runup elevations for each hurricane intensity category. These predictions are presented as five separate maps, each with three parallel color strips that are aligned with the coastline. Red colors indicate high probability and white indicate low probability. The probabilities can be interpreted as indicating that the specified coastal change regime is very likely (probability >90%), likely (>66%), about as likely as not (33% to 66%), unlikely (<33%), and very unlikely (<10%) given each hypothetical storm scenario (i.e., we consider the errors associated with the topography and its interaction with wave runup and do not factor in the probability of storm occurrence.)
This analysis assumes a hypothetical hurricane landfall scenario such that each location on the map would experience the most severe impacts that would be associated with the right front quadrant of the hurricane. In reality, the coastal change probabilities will be localized and will not occur along the entire stretch of coast for a single storm landfall. Furthermore, the computations assume landfall at mean astronomical tide. Landfall at high tide would lead to higher coastal change probabilities.
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