Friday, October 4, 2013

Karen's Surge May Be a 5 or 6 Year Coastal Flood Event for Portions of SE Louisiana and Mississippi

The National Hurricane Center is predicting that the highest storm surge levels from Tropical Storm Karen may occur from the Mouth of the Mississippi River east to Mobile Bay. The 7AM CDT advisory this morning says that water levels could reach 3 to 5 feet above ground level in this area if the peak surge occurs at high tide. Considering the tidal range and land elevations above sea level in this region, this prediction converts over to approximately a 6 to 8 foot storm surge.

The National Hurricane Center's Storm Surge Exceedance Product predicts the highest storm tide levels could occur off the coast of Southeast Louisiana and Mississippi.

Interestingly, this coastal region also took the brunt of Hurricane Isaac's surge last August, as well as the highest water levels in Hurricane Gustav (2008) and Katrina (2005). So how often does this region experience surge events, and, more specifically, how frequently would a 6-foot surge inundate this area?

SCIPP is developing a storm surge frequency tool that can answer these questions. The tool is still experimental and will not be launched until 2014. However, Tropical Storm Karen provides a great opportunity to give this web tool a test run.

SCIPP is developing a storm surge frequency web tool that will enable users to better understand the frequency of storm surge inundations in a specific area. This image provides customized surge data for a region within 25 miles of Gulfport, Mississippi.

In the image above, I clicked on Gulfport, Mississippi and chose a circle radius of 25 miles. The tool instantly gave me a list of all the historical storm surge inundations that have occurred in this region since 1900. In this region, SURGEDAT has historic water levels from 37 unique tropical cyclones (hurricanes and tropical storms).

Check out the graphs in the upper right corner of the screen. The bar graphs tell me that a 100-year storm surge in this region is a surge height of 24.75 feet. Meanwhile, the logarithmic scale below the bar graph provides return periods for specific water levels. As the latest NHC advisory mentions the possibility of a 6-8 foot surge in this region (3-5 feet above ground level if at high tide), I select 6 feet on this scale. The tool tells me that this water level is reached or exceeded in a tropical surge event every 6 years on average in this area, which would make Karen a 6-year event if this water level is reached.

This image provides surge history for an area within 25 miles of Shell Beach, Louisiana. SURGEDAT provides historical surge data for 48 unique tropical cyclones (hurricanes and tropical storms) that have generated high-water marks in this region.

I also used this tool to generate storm surge statistics for an area within 25 miles of Shell Beach, Louisiana. SURGEDAT has identified high water marks for 48 unique tropical cyclones (hurricanes and tropical storms) within this area. All of these high water marks occurred in Louisiana. The tool tells me that the 100-year storm surge in this area is 19.15 feet and a level of 6 feet should be reached or exceeded approximately every 5 years. The tool also tells me that a 6-foot storm surge within 25 miles of the mouth of the Mississippi River would occur every 9 years on average.

If you paid close attention to these numbers, you've realized that the Mississippi Coast has a higher 100-year storm surge level than SE Louisiana, while SE Louisiana has a higher 5-year storm surge return period. This water level was reached at least 18 times along coastal Mississippi since 1900, and 24 times in Southeast Louisiana (within 25 miles of Shell Beach).

Although data quality may slightly affect these numbers, these values are likely accurate. The profile of the Mississippi Coast drives some of the world's largest storm surges into this area. While SE Louisiana also has observed many high storm surges throughout history, this region also captures many small-magnitude surge events, as the coastal profile sticks out into the Gulf of Mexico and tends to capture high water from tropical cyclones that are positioned almost anywhere in the Gulf of Mexico.

By contrast, other portions of the Gulf Coast, like the West Coast of Florida, are generally less vulnerable to storm surge. SCIPP's Storm Surge Frequency tool told me that a 6-foot storm surge in the Tampa Bay area only occurs about every 20 years, based on historical data going back to 1900. Such analysis reveals that storm surge inundations have considerable geographic variation, making areas like coastal Mississippi and Southeast Louisiana more vulnerable to surge inundations than other locations along the Gulf Coast.

1 comment:

  1. The NHC exceedance graphic show less than a 20% chance of going over 2 - <3 feet...

    I'm curious why the public bulletins say 3-5 feet? I'm not sure how that's all coordinated though. Any insight? I asked NHC Surge on Twitter but they've not replied.