Tropical Cyclone Report - National Hurricane Center - NOAA

Plaquemines Parish 10 – 17 ft
St. Bernard Parish 8 – 12 ft
Orleans and St. Tammany Parishes 4 – 8 ft
Jefferson and Tangipahoa Parishes 3 – 6 ft
St. John the Baptist and St. Charles Parishes 1 – 3 ft
Storm tide is estimated to have inundated parts of Plaquemines Parish with as much as 10
to 17 ft of water above ground level, based on data from United States Geological Survey
(USGS) pressure sensors and surveyed high-water marks (Fig. 9a). Isaac piled water up from
Breton Sound against a non-federal back levee that was meant to protect the east bank of
Plaquemines Parish, and several USGS pressure sensors indicated that water depths just outside
the levee in northern Plaquemines Parish reached 9 to 14 ft above ground level. The non-federal
levee heights vary from 8.5 to 12 ft, and as a result, water overtopped the levee and began
spilling into the area between it and the Mississippi River levee, extending from Braithwaite
southward to Bel Air. The deepest water likely occurred in uninhabited areas within the levee
system, which tend to be at lower elevations than major roads and nearby residential and
commercial areas. A USGS pressure sensor secured to the base of a tree in an uninhabited area
near Scarsdale measured a storm tide of 14.0 ft above NAVD88. Since the sensor was surveyed
at an elevation of 2.8 ft below NAVD88, the inundation at that location was almost 17 ft above
ground level. Lower but still significant inundation occurred in more inhabited areas. Data from
a storm tide sensor near the Braithwaite Golf Course indicated that the water was as much as 13
to 14 ft deep there. The highest high-water mark surveyed by the USGS was 14.25 ft above
NAVD88 in a residence in Bertrandville, suggesting an inundation of 6 to 7 ft above ground
level. Farther south in Plaquemines Parish, water depths were 10 to 12 ft above ground level
outside the federal levee system on the east bank and as deep as 5 to 7 ft on the west bank.
Major flooding also occurred in unprotected areas of St. Bernard, Orleans, and St.
Tammany Parishes. In St. Bernard Parish, water depths outside the federal levee system were as
much as 8 to 12 ft based on measurements from several USGS storm tide sensors in Wood Lake,
Delacroix, and Alluvial City. Eastern portions of Orleans Parish near the Rigolets and Lake St.
Catherine had water heights of 4 to 8 ft above ground level based on storm tide sensor data and
several high-water marks. In St. Tammany Parish, on the northeastern shore of Lake
Pontchartrain, maximum water depths were on the order of 4 to 7 ft, and the surge from the lake
penetrated far enough inland to inundate parts of Slidell, Lacombe, Mandeville, and
Madisonville. Widespread storm surge flooding on the western side of Lake Pontchartrain
occurred in St. John the Baptist Parish. USGS high-water marks in the city of Laplace indicate
that maximum water depths were 1 to 3 ft.
The USGS reported that strong winds and storm surge from Isaac forced the Mississippi
River to flow backwards for almost 24 h. At Belle Chasse, Louisiana, a USGS streamgauge
showed that the river rose 10 ft and flowed upstream at 182,000 cubic ft per second. The normal
flow for the river at that location is downstream at about 125,000 cubic ft per second. The storm
surge traveled more than 300 miles upriver, as far as Red River Landing in northern Pointe
Coupee Parish, and it caused the river to rise 8 ft in Baton Rouge.
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