Category 3 Katrina generated a U.S.-record storm surge which impacted a wide region from Grand Isle, LA, to Mobile Bay, AL, and killed about 1350 people with more still missing. Sensitivity experiments by WorldWinds using the ADvanced CIRCulation (ADCIRC) storm surge model show a large hurricane produces water elevations 20-40% higher than a small hurricane with the same intensity and with considerably more widespread inundation. The previous U.S.-record surge was Hurricane Camille (1969) which impacted the same region, an intense but smaller Category 5 hurricane. Camille's hurricane-force winds extended 60 miles from the storm center, while Katrina's extended 120 miles. Camille's tropical storm-force winds reached 180 miles outward, while Katrina was 230 miles. It is likely Katrina's wide eye of 37 miles also played a role (Camille's eye was approximately 11 miles wide).
The inland penetration of Katrina's storm surge was truly remarkable. The Mississippi River levee system held and confined most of the surge east of the river except for the landfall region of Buras, LA. Regions west of the Mississippi River experienced little surge, suggesting that the river levee system may have augmented Katrina's surge on the east side. Most of Plaquemines, St. Bernard, and eastern Orleans Parishes were inundated with surge which overflowed levees and destroyed them with scouring action. Buildings outside the levee system became cement slabs. Tide gauges also show the surge traveled up the Mississippi River, with elevation spikes reaching 14 feet at the Bonnet Carre Spillway 10 miles west of New Orleans. Levees along some canals south of Lake Pontchartrain were not overtopped but experienced failures that are still under investigation, causing well-publicized flooding of New Orleans. The surge also penetrated through inoperative flood pumps which, when combined with the inability to remove rainwater, caused moderate flooding in the suburban region west of New Orleans. The eastern end of St. Tammany Parish suffered an extreme surge which came from Lake Borgne as well as up the Pearl and Bonfouca river systems, traveling miles inland in Slidell. St. Tammany's surge was associated with the wind shift as Katrina moved inland, sloshing piled-up water in Lake Pontchartrain northeastward.
The entire Mississippi coast experienced the storm surge. The western region from Pearl River to Bay St. Louis suffered the worst, as the surge traveled past Interstate 10. The official peak surge occurred in this region, estimated at 28 feet. However, high water marks indicate even higher elevations, although some may be impacted by wave action (Table 1). Comparisons to Hurricane Camille's surge will be shown at the conference. The surge also traveled far up the Jordan River and Biloxi River, decimating towns such as Kiln, MS. The surge occurred at high tide, adding another foot of water.An ADCIRC simulation of Katrina's surge evolution will be shown at the conference.
Because of the possible role of the Mississippi River levee system trapping the surge, simulations are underway to examine the surge evolution if there was no levee. In addition, Louisiana has experienced extreme wetland erosion in the last 50 years. ADCIRC simulations of Katrina with Louisiana's wetland topography from 1920 and 1970 are in development. These sensitivity experiments will be shown at the conference.
Timing of Wind and Surge
An important insurance issue involves the timing of wind versus surge. All tide gauges failed at the peak of the storm in the severely impacted regions. However, 17 USGS gauges in the impact region functioned during tropical storm-force conditions. All gauges show winds of 50-60 mph with storm surge values of 5-8 feet, typically less than would flood most homes. Examples of these tide gauges will be shown at the conference.
To examine the possible timing of surge in the storm center, time series plots of individual locations were produced from the ADCIRC simulation. They generally show the peak winds preceding the peak surge between 30 minutes and two hours. Eyewitness accounts support this fact, and videos also show the timing of wind and surge. These will be shown at the end of the presentation.
The Impact of the Mississippi River Gulf Outlet and the Hypothesized “Funneling Effect”
The Mississippi River Gulf Outlet (MRGO) is a 70-mile, deep-draft, man-made channel, completed in 1963. 40 miles was dredged through marshland in St. Bernard Parish. Originally 750 feet wide, MRGO has eroded to 2000 feet wide in many places, destroyed more than 36,000 acres of wetlands, and disrupted a brackish environment with high salinity. Its role in hurricane storm surges is also controversial, with speculation MRGO acts as a conduit. Its intersection with the Gulf Intracoastal Waterway has also been hypothesized to provide a “funneling effect” for the storm surge.
These issues were studied with an ADCIRC simulation of Katrina for a “filled-in” MRGO. Little difference was observed, because the surge is a widespread event in which one channel will make little impact. Another simulation used a wider levee system at the intersection, and the surge actually increased, because without restricting the flow, the conveyance increased in this region.