Friday, July 23, 2010

Peak U.S. Surge Estimate for TS Bonnie





Peak U.S. Surge Estimate for TS Bonnie, based on historical comparisons

Surge Height: 3-5 feet
Peak Surge Location: Southeast Louisiana

Discussion:
Storm surge levels should build in Southeast LA late Saturday, and peak Saturday night or Sunday morning, depending on the time of landfall. The estimate of peak surge level, based on historical comparisons, is 3-5 feet.

Look for water to pile up along the eastern border of the MS River Delta, areas east of New Orleans, and even western Lake Pontchartrain. Particular areas that may be affected include towns along the MS River Delta, such as Venice and Buras, communities east of New Orleans, such as Shell Beach, Yscloskey and Delacroix, and western portions of Lake Pontchartrain, around Frenier. Expect surge levels to increase as easterly winds pickup on Saturday.

Water tends to pile up quickly along this stretch of the U.S. Gulf Coast, because the water is very shallow, and easterly winds push water onshore along this stretch of coastline. Given the current NHC track, the strongest winds in this region should be from the east or southeast, enhancing surge levels. Fortunately, Bonnie is forecast to move quickly, so tropical storm force winds from the east should occur for less than six hours in most areas, limiting surge heights.

The SCIPP Program identified three historical surge events that may provide insight into Bonnie's peak surge levels. Information on these events is listed below, and tracking maps from the Unisys Corporation are included in this post. Bonnie's peak surge level should be slightly lower than these other historical events because the storm is moving so quickly, not enabling enough time for surge to build up very high.

Unnamed Storm, 1940
Peak Surge Level: 6.4 feet
Peak Surge Location: Frenier, LA (western end of Lake Pontchartrain)

An unnamed storm in 1940 tracked from east to west in the northern Gulf of Mexico. It passed south of Venice, LA as a tropical storm, then became a hurricane south of Grand Isle, eventually making landfall near the TX/ LA border. This system produced a peak surge level of 6.4 feet at Frenier, on the west end of Lake Pontchartrain, even though the center of this storm was never closer than 200 miles to Frenier! This is a testament to how efficiently water piles up along the western shores of Lake Pontchartrain, given a strong easterly wind. Even though this system became a hurricane, it was a tropical storm south of Venice, LA, so it is included in this historical comparison. This storm moved much slower than Bonnie, enabling east winds to persist for several days, which enhanced the surge.

TS Brenda, 1955
Peak Surge Level: 6 feet
Peak Surge Location: Shell Beach, LA

TS Brenda formed off the SE coast of LA and curved towards NW, making landfall near Shell Beach, LA. This storm formed much closer to coast than Bonnie, but still was a Tropical storm that made landfall in SE LA, so it is a helpful comparison. Bonnie is tracking over more water, enabling higher surge levels, but moving faster, which should reduce surge levels.

TS Bertha, 1957
Peak Surge Level: 4.7 feet
Peak Surge Location: West end of Vermillion Bay, LA

Even though TS Bertha made landfall farther west than where Bonnie will likely make landfall, historical data from this storm is helpful because it was a TS for at least one full day and tracked from SE to NW, similar to Bonnie. The peak surge location was at Vermillion Bay, LA, probably farther west than Bonnie's peak surge.

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