Assessment of Potential Coastal-Change Impacts
06-25-2012, 1100 EDT (National Hurricane Center Forecast Advisory 9)
Elevated water levels and waves during tropical storms can lead to dramatic coastal change through erosion of beaches and dunes. The likelihood of coastal change during storms is predicted using a USGS-developed storm-impact scale that compares modeled elevations of storm-induced water levels to known elevations of coastal topography in order to define three coastal change regimes. These regimes describe the dominant interactions between beach morphology and storm processes and the resulting modes of coastal change along beaches and dunes that serve as the "first line of defense" for many coasts exposed to tropical storms and hurricanes.
The regimes include collision, when waves attack the base of dunes and cause dune-front erosion. Under higher surge or wave runup conditions, waves can overtop dunes leading to overwash, dune erosion, landward dune migration, and overwash deposition on low, narrow islands. The most extreme coastal change regime is associated with inundation, where the elevation of storm surge plus wave setup exceeds the elevation of the primary dune or beach berm. Under these conditions the beach and dune can be severely eroded and low, narrow islands may breach.
The probabilities of collision, overwash, and inundation associated with Tropical Storm Debby were assessed for the sandy beaches along the Florida gulf coast.
Coastal change probabilities were based on estimating the likelihood that the beach system will experience erosion and deposition patterns consistent with collision, overwash, or inundation regimes. The probabilities were estimated by calculating the difference between modeled total water levels (including storm surge and wave runup) and dune or berm elevations. The storm surge elevations along the open coast were obtained from the National Oceanic and Atmospheric Administration's (NOAA) probabilistic surge forecast (psurge), which is based on conditions specific to the landfalling storm. Errors in hurricane forecasts are included in order to identify probable surge levels. The 50% exceedance surge level was used to represent the best-estimate scenario. Maximum wave heights obtained from the NOAA WaveWatch3 model 7-day forecast were used to compute runup elevations. Dune elevations were extracted from lidar surveys collected in May 2006 (FL west coast) and July 2005 (FL panhandle).
In the maps below, red colors indicate high probability while white indicates 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 the present storm forecasts.
Note that the computations assume landfall during mean tide. Landfall of a storm at high tide would lead to higher coastal change probabilities.
Florida Central West Coast
The central west coast of Florida is composed of both marshy wetlands and sandy barrier islands. The sandy beaches are very vulnerable to extreme coastal change during storms because of low dune heights – the mean elevation of the dune crest is 1.8 m (6 ft).
Probabilities of collision, overwash, and inundation for Tropical Storm Debby. Based on NHC forecast advisory 9, beach and dune erosion was likely for 61% of the coast. Approximately 13% of the coast was likely to overwash. Inundation of the beach system was not expected. [larger version]
The dunes and beaches along the Florida panhandle are some of the highest along the Gulf of Mexico coast, making this area less vulnerable to extreme coastal change during storms. The average dune elevation is over 3 m (10 ft); however, high spatial variability makes some areas more vulnerable than others.
Probabilities of collision, overwash, and inundation for Tropical Storm Debby. Based on NHC forecast advisory 9, beach and dune erosion was likely for 25% of the coast. Only 4% of the coast was likely to overwash. Inundation of the beach system was not expected. [larger version]