AIR Worldwide: Louisiana’s Record Rainfall Caused by Low-Pressure System

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Catastrophe modeling firm AIR Worldwide reports that the extensive flooding in southern Louisiana, from west of Baton Rouge to Mississippi, was caused by a slow-moving tropical depression–like low-pressure system that crawled across the region over the past week.

“Rainfall amount ranged from 12 to more than 24 inches, with 31.39 inches recorded near Watson and 27.47 inches in Brownfields; 8.49 inches fell at the Baton Rouge Metropolitan Airport in one day,” said Dr. Hemant Chowdhary, principal scientist at AIR Worldwide. “Parts of the region received two to four times the average total monthly rainfall for August in just three days.”

The low pressure system “intensified as it slowly drifted across the state. Although the storm featured a closed warm core low, because it formed and was maintained inland it was technically not considered a tropical depression. Nevertheless, the counterclockwise (southerly) flow off the Gulf of Mexico — where sea surface temperatures are almost at record warmth levels — brought tremendous amounts of moisture inland across southern Louisiana. Also, precipitable water amounts in the atmosphere were at near record amounts of 2.50-2.75 inches. The combination of these factors, with a lift from the low-pressure system, resulted in slow-moving torrential rainfall and thunderstorms from Lafayette to Baton Rouge,” Chowdhary said.

As of Monday, Aug. 15, at least 10 rivers had surpassed moderate-to-major flood stages at several locations along their course: Amite, Bogue Chitto, Calcasieu, Comite, Mermentau, Pearl, Tangipahoa, Tchefuncte, Tickfaw and Vermilion.

“As many as eight rivers have reached record flood levels, including the Amite River, which crested at Magnolia at about 6.5 feet above the 65-year historical record of 51.91 feet (set in 1977), and the Comite River, which crested near Comite Joor Road about 4 feet above the 70-year historical record of 30.99 feet (set in 2001),” Chowdhary said.

States of emergency have been declared for Louisiana and impacted counties in Mississippi, and the president has designated parts of Louisiana as federal disaster areas.

The Louisiana Department of Transportation (DOT) reported that approximately 200 roadways became impassable due to flooding, including parts of both interstates 10 and 12. The DOT added that as many as 1,400 bridges need to be inspected before they can be reopened. In some areas, stranded cars remain on highways.

More than 40 percent of buildings in the U.S. Gulf Coast region meet Flood Insurance Rate Map (FIRM) standards set in 1980, according to AIR. While the specifics of the FIRM standards vary somewhat by building construction and occupancy class, these standards dictate that buildings must be elevated above base flood elevation, as defined by FEMA, and that any parts of a building that remain below base flood elevation must be constructed of flood-resistant materials.

According to AIR, in addition, only 0 percent – 10 percent of buildings constructed since 1980 in the Gulf States have basements (which increase vulnerability to flood damage). Buildings in the region are more typically built on slab foundations, which are designed to withstand flotation, collapse, or lateral movement that can be inflicted by floodwaters. Crawl space foundations in residential buildings in the region are required to have flood openings no more than 1 foot above grade.

AIR added that in Louisiana, more than 80 percent of the residential construction is wood, with an estimated 5 percent having basements. Over half of the commercial buildings are steel and concrete. Unlike residential structures, commercial buildings often are engineered and built to stricter standards, and are thus less vulnerable than single-family homes. Still, mechanical, electrical, and plumbing (MEP) systems can experience severe damage, which results in high losses.

Source: AIR Worldwide

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