We have all seen lots and lots of maps showing surge flood heights from past hurricanes or forecasts of approaching hurricanes. We all know that the level of damages and the risks to safety are strongly dependent on how high flood waters will be. However, these flood maps are almost always a map of the maximum flood levels during the entire duration of the hurricane. For example, the flood map may show high flood water in Slidell and in Madisonville, but the high water did not actually happen at the same time, because it shows the maximum height regardless of when it happens. So, this is a weakness of the maximum flood maps, because it does not show how or when the water actually got there. Therefore, PC conducts “surge dynamics analysis” which is simply looking at the time-lapse sequence of surge movement over the few days that a hurricane is affecting our coast. The time-lapse analysis reveals aspects of surge that is generally not known, but also suggests answers to the age-old question “what’s the worst hurricane track?”
We basically examined time-lapse for Hurricanes Katrina (tracked east of New Orleans) and Isaac (tracked west of New Orleans) in two different regions: Lakes Pontchartrain/Maurepas and Breton Basin (St. Bernard and east Plaquemines Parish). In both regions, we saw that generally as a storm approaches the coast surge builds in the same areas regardless of the storm track. For the Lake Pontchartrain region, storm surge tends to build in the Laplace/Frenier area, and for the Breton Basin, surge builds along the Mississippi River near Pointe a la Hache or further south. Another aspect of surge movement is that, both regions can have severe flooding when surge is moving back to the gulf. Generally, it is implied that the surge moving inland is of greatest concern. However, these “outflow surge” events have been catastrophic in both areas and continue to present a major threat to both regions.
So, in the Lake Pontchartrain region, surge movement can be described as rotating around the perimeter of the lake. The surge always starts in the Laplace/Frenier area, but if the storm is tracking east (Katrina) of the lake, surge moves counter-clockwise from Frenier and toward New Orleans and then as strong outflow surge toward Slidell. If the storm is tracking west of the lake (Isaac), the surge at Frenier moves clockwise toward Madisonville and then as a weak outflow surge toward Slidell.
In the Breton basin, the surge starts along the river, however what happens afterward is again dependent on the storm track. If the storm passes to the west (Isaac), the initial surge along the river is pushed northward along the Mississippi River levee towards Caernarvon and Braithwaite. If the storm passes to the east (Katrina), the surge along the river is pushed southward creating a very severe outflow surge event. This outflow surge event is especially noteworthy because of compounding factors. For major storms like Katrina and Camille, surge overtopped the Mississippi River levees on both the east and west bank. At the point when the storm is just east of the river, there are three re-enforcing influences making surge much worse, including high surge within the river, a north wind blowing surge south, which is the same direction of river flow. This outflow surge event has happened at least twice with complete devastation to Lower Plaquemines parish’s east and west bank. The interaction of the Mississippi River with surge is now getting more attention, but generally the interaction of river flow, levees and surge have not been well modeled or understood.
We encourage folks to review PC’s two reports on surge dynamics. We believe it reveals critical information to understand risks due to hurricane surge. However, we must also remind everyone that every storm is different in detail, and that we should all act on the side of caution and respect emergency directives during an actual hurricane event.
These reports by PC can be viewed at: