Hurricane Isaac - Forecast and Documentation of Coastal Change

Science Center Objects

Hurricane Isaac coastal change forecast, pre- and post-storm photos, and lidar elevation maps document coastal change.

Aerial photos of Dauphin Island taken in 2004, 2005, and 2008 show erosion

Repeated attacks by hurricane waves and surge have taken a toll on Dauphin Island, Alabama, carving a breach in the narrow island, toppling beachfront homes, and eroding large stretches of coast. (Public domain.)

As Hurricane Isaac churned toward the northern gulf coast in August 2012, USGS scientists worked to determine where and how the storm's waves and surge might reshape the beaches and dunes that stand between the storm and coastal developments. Through a complex modeling process that uses coastal elevations, wave forecasts, and potential storm surge, they predicted coastal change hazards, such as shoreline and dune erosion, that might be expected during an Isaac landfall.

Response activities included:

  • Forecast of potential coastal change (see next section)
  • Pre-and post-storm photos documenting coastal change 
  • Comparison of pre- and post-storm lidar topography and coastal change

Forecast of Potential Coastal Change

Elevated water levels and waves during tropical storms can lead to dramatic coastal change through erosion of beaches and dunes. USGS has developed a storm-impact scale that predicts the likelihood of coastal change by comparing modeled elevations of storm-induced water levels to known elevations of coastal topography in order to define three coastal change regimes. These regimes describe how beach morphology (physical form) and storm processes tend to interact, and the resulting modes of coastal change along beaches and dunes, which often serve as the "first line of defense" for many coasts exposed to tropical storms and hurricanes.

The three regimes are collision, overwash, and inundation. Collision occurs 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, which can include dune erosion, landward dune migration, and overwash deposition on low, narrow islands. In extreme cases, such deposition can bury roads and parts of buildings. 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 Hurricane Isaac were assessed for the sandy beaches in the northern Gulf of Mexico.

Coastal change probabilities give 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 tide, storm surge and wave runup) and dune or berm elevations. The calculations assumed landfall at high tide, as represented by the 20-year mean high water level. 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% exceedence 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 2008.

In the gallery of images above, 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.

Pre-and Post-Storm Photo Comparisons and Lidar Elevation Maps

Hurricane Isaac made landfall in southeastern Louisiana near the mouth of the Mississippi River on August 29, 2012. It was a relatively weak Saffir-Simpson Scale Category 1 hurricane with peak-sustained winds of 70 Knots (or 80 MPH). However, the storm's large size and slow speed as it approached the coast allowed it to impact the coast over many hours, much like a winter storm. In the storm's right-front quadrant, where the winds were onshore and strongest, storm surge and superimposed waves inundated and overwashed the barrier islands that lie to the east of the River, e.g. the Chandeleur Islands, LA and Dauphin Island, AL, both severely impacted during 2005's Hurricane Katrina. During Isaac, these islands suffered considerable changes, including the apparent destruction of remnants of the oil protection berm built on the Chandeleurs after the Deepwater Horizon oil spill. Changes are visualized in comparisons of pre-Isaac (August 8, 2012) and post-Isaac (September 2, 2012) aerial photos.

The USGS acquired an airborne lidar survey of post-storm topography between the 8th and 18th of September, 2012, along the barrier islands of Louisiana, Mississippi, and Alabama to measure coastal change resulting from Hurricane Isaac. Comparisons of the post-storm elevation data to lidar data collected prior to Isaac's landfall are used to characterize the nature, magnitude, and spatial variability of hurricane-induced coastal changes, such as beach erosion, overwash deposition, and island breaching. We have focused our preliminary lidar analysis on Dauphin Island, AL, and on Chandeleur Island, LA.

The lidar elevation maps show pre-storm elevation, post-storm elevation, and elevation difference at each highlighted location. In the pre- and post-storm elevation maps, orange and red colors indicate higher elevations while yellow and green colors indicate lower elevations. In the elevation difference maps, red colors indicate erosion such as shoreline retreat, and blue-green colors indicate accretion, such as overwash deposits where waves and surge have moved sand landward. Location numbers are consistent with those from the photo pair comparison and maps are oriented with the Gulf of Mexico on the bottom.