A slow-moving low pressure weather system caused intense and prolonged rainfall over Louisiana and southwest Mississippi in mid-August 2016. Record-breaking river flows were observed and the severe flooding that followed resulted in damage to tens of thousands of homes.
Extent of flooding in Louisiana and southwest Mississippi in August 2016
The US states of Louisiana and southwest Mississippi experienced heavy and prolonged rainfall from 9 to 15 August 2016. A slow-moving low pressure system, similar to an inland tropical depression, brought high atmospheric moisture content northwards from the Gulf of Mexico by the counter-clockwise flow of air around the central low pressure. High atmospheric moisture levels led to intense rainfall, with many areas of Louisiana receiving over 20 inches (approximately 500 mm) in 7 days. This corresponds to over 600% of the normal rainfall for the time of year. This is the second time this year that record-breaking rainfall has affected Louisiana. At least 27 inches (686 mm) of precipitation was reported in Louisiana during the middle of March 2016 and also caused severe flooding.
7-day precipitation totals over the US on 9 – 16 (NOAA)
7-day rainfall totals on 9 – 16 August over Louisiana as a percentage of the average for this time of year (NOAA)
Record breaking rainfall was experienced across Louisiana: the total amount of rainfall in 72 hours reached 31.39 inches (797.306 mm) in Watson, and 27.47 inches (698 mm) in Brownsfield. At least 20 locations in Louisiana recorded more than 12 inches (approximately 300 mm) of rain in 72 hours. During the most intense period of rainfall on 12 to 13 August, 434 mm of rainfall was recorded in 24 hours in Livingston, 299 mm in Baton Rouge and 283 mm in New Iberia.
The heavy rainfall led to high river levels throughout Louisiana and in parts of south-eastern Mississippi.
Several river gauges recorded record-breaking river levels, sometimes exceeding the previous record by several feet. In Magnolia, Louisiana, the Amite River peaked at 58.56 feet (17.8 m) on 13 August. This exceeded the previous highest level by more than 6 feet (1.8 m) from 23 April 1977. On 14 August in Denham Springs, the Amite River peaked at nearly 5 feet (1.5 m) above the previous record set in 1983, making it the highest recorded water level since 1921 at this location. Record levels were also set on the Comite, Tangipahoa and Tickfaw rivers between 12 and 14 August.
River flooding was reported throughout Louisiana, and in parts of southwestern Mississippi. A federal emergency was declared in Louisiana on Sunday 14 August. Around 40,000 homes were damaged by flood water in southwestern Louisiana, and around 10,000 people have been relocated to emergency shelters. Media reports indicate that more than 20,000 people have been rescued from the flood water by emergency crews and 11 people have died as a direct result of the flooding. 200 roads have been closed and 1,400 bridges await inspection before being reopened to traffic.
JBA's Louisiana and Mississippi flood event footprint
JBA has produced a flood event footprint for the flooding in Louisiana and southwest Mississippi in August 2016. The method for development of the flood footprint is as follows:
- Monitor river levels and flood extent
- Convert river levels to flows using ratings curves
- Convert river flows to return periods per catchment
- For each catchment, extract design hazard map for river flooding
- Cross-reference return period to knowledge of defence breaches, local effects, imagery and claims data