Hurricane Isaac and the Big Storm Surge
Last week, from approximately August 27-30th, Tropical storm and subsequently Hurricane Isaac entered the Gulf Coast and battered the coasts of Louisiana, Alabama and Mississippi. But Isaac wasn’t your typical Category 1 storm, as described according to the Saffir-Simpson hurricane wind scale. While being classified as a storm capable of “some damage” from hurricane winds, Isaac produced extensive storm surge and flooding damages characteristic of a much larger storm.
According to Hal “Hurricane” Needham, Geography and Anthropology graduate student at LSU, Isaac may provide additional evidence that the Saffir-Simpson hurricane wind scale is a poor indicator of storm surge levels. This wind scale, adopted by the National Hurricane Center, rates hurricane intensity from level 1 (weakest) to level 5 (strongest). Hurricane Isaac was categorized as the weakest level of storm on this scale according to its maximum sustained wind speeds, which fell below the threshold of 95mph. However, Needham points out, “while wind damage from this hurricane may have been modest compared to other hurricanes, the storm surge level was substantial, causing much damage.”
Needham provides the following statistics on the storm:
Hurricane Isaac generated a large storm surge in Southeast Louisiana and along the Mississippi Coast. The highest surge levels were observed east of New Orleans, where the storm generated a 13.6-foot surge at the Lake Borgne Surge Barrier (www.nola.com), and 11.02-foot surge at Shell Beach (NOAA Tides and Currents), and a 10.9-foot storm tide at Little Irish Bayou near I-10 (USGS). On the Mississippi Coast, the highest storm surge observed was approximately eight feet at Bay Waveland Yacht Club (NOAA Tides and Currents).
These water levels were high enough to inundate long stretches of roadways and put several coastal communities under many feet of water. For example, water levels were approximately seven feet above ground in the Shell Beach area. Such impacts lasted a long time, as the storm moved very slowly, prolonging the period of inundation.
The prolonged surge event also likely enhanced flooding from heavy rainfall in communities near Lake Pontchartrain. Heavy rainfall for several days in communities like LaPlace, Mandeville, Madisonville, Covington and Slidell probably did not drain as quickly as normal into Lake Pontchartrain because the lake levels were elevated by storm surge. The NOAA Tides and Currents tide gauge at New Canal Station on Lake Pontchartain [recorded] a maximum storm surge elevation of approximately 6.2 feet, and a surge level of at least four feet for approximately 60 hours.
These elevated lake-water levels likely slowed drainage into the lake, as drainage in this area is mostly gravity-fed. Slower drainage of heavy rainfall likely created a rainfall-surge hybrid flood event in many of these communities, even if the flood waters were mostly comprised of fresh water. This flood pattern resembles flooding near Tampa, Florida in June , [when] Tropical Storm Debby’s surge elevated the water level of Tampa Bay, which, in turn, seemed to slow the runoff of heavy rainfall.
So why did Isaac produce such damaging storm surge levels and aggravated flooding, when it was only recognized as a Category 1 storm according to the accepted wind scale? According to Needham, the storm’s large size and slow moving forward motion likely contributed to the significant storm surge observed near the Mississippi River Delta. “Large hurricanes tend to displace more water than small hurricanes, and the slow forward motion of the storm enabled it to push water onshore for a longer period of time,” Needham said. “As the waters off the Louisiana and Mississippi Coast are quite shallow in this region, the water that was pushed toward shore could not be redistributed by underwater currents, and had nowhere to go but ‘up.’”
Storm surge is the rise of water associated with a low-pressure weather system such as a hurricane, measured as the water height above predicted tide levels. Storm surge during a hurricane is largely responsible for loss of life and damages to coastal communities. According to the National Hurricane Center, many of the deaths of the more than 1,500 persons who lost their lives during Katrina “occurred directly, or indirectly, as a result of storm surge."
Storm surge is recognized as a complex problem, the flooding patterns produced in Louisiana by Hurricane Isaac being a good example. “Surge does not fit neatly into generalizations that may be created by the Saffir-Simpson scale (like calling this storm a category-1 hurricane),” Needham said. “Surge seems to affect the wider drainage of some watersheds, likely exacerbating flooding in communities that may be several miles inland.”
The National Hurricane Center has built storm surge risk maps for various areas according to storm surge simulations that take into account storm forward speeds, landfall locations, storm tracks, storm sizes, storm intensities, and astronomical tides. However, the characteristics of tropical storms and hurricanes that contribute to large storm surges are not yet exhaustively understood. Researchers such as Needham are trying to improve our knowledge of storm surge risks, factors that influence them, and return rates for large surges, i.e. how often large and damaging surges can be expected to return to a particular area.
Climatologists and others studying hurricanes at LSU, including Needham, are encouraging researchers to delve deeper into the storm surge characteristics of these storms. Needham and Louisiana Climatologist Barry Keim have created SURGEDAT, an interactive online map and database of storm surges worldwide. One goal of this project is to provide insight into hurricane climatology and the storm physics that contribute to storm surge and flooding damages in addition to those damages caused by high winds. “Further research on these topics may help us better understand the processes that generate storm surge, as well as specific impacts in future storms,” Needham said.
See more stories and pictures on my own Hurricane Isaac experience here.