Environmental Hazards
Storm Surge Induced Flooding in New Orleans

Chris Below, Chris Dierich, Keith Erickson, and Rachel Kjos

 

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What is a Storm Surge?


What Happened During Hurricane Katrina?

Why New Orleans is Vulnerable?

The Future of New Orleans

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What is Storm Surge?

Storm Surge is the common name for what meteorologists and call Meteorological Residual. We will refer to this phenomenon as 'storm surge' to avoid confusion. Storm surge is the most dangerous phenomenon associated with hurricanes. Figures vary, but it is responsible for between 70% and 90% of the death and damage attributed to hurricanes.

Storm Surge is a large rise in sea level caused by severe weather conditions. The general public often confuses this phenomenon with observed sea level, which is the sea level that one can measure at any given time. The height of a storm surge is officially determined by finding the observed sea level and subtracting the mean sea level (average sea level over a long period of time) and the expected astronomical tide, what remains is the surge amount. What is often reported in the news about surge events is the observed sea level, which results in more dramatic figures.


Source: http://www.nhc.noaa.gov/HAW2/english/surge/surge_big.jpg

This illustration shows a visual breakdown of the factors affecting a surge event.

Storm Surge can be an awesomly devastating force. It is important to remember that water weighs one metric ton per cubic meter so its destructive capability is immense.


Source: http://geology.com/articles/storm-surge.shtml

This house in this photograph is weathering a storm surge, notice that it has been constructed on stilts. If this design feature had not been incorporated, the house would not likely be standing.

There are many factors that affect a storm surge event:

  • Storm Winds: Can blow water directly towards the coast, if the winds are blowing perpendicular to the coastline then a greater surge event will occur because water will not be deflected away from land. As the wind-transported water enters shallow areas it slows due to greater friction with the sea floor and "piles" upon itself, resulting in a rise in the observed sea level.

  • Wave Run-up: Waves transfer water up and onto the shore, the effects of wave run-up can be exacerbated by strong winds and the increase in sea level which allows them to reach much farther inland than they could under normal conditions. Wind-driven waves can destroy structures and infrastructure in a short period of time, leaving an area almost unrecognizable to its former inhabitants.

  • Low Barometric Pressure Within the Storm: Observed sea level within the storm can be raised by ~1cm/millibar inversion of barometric pressure, further adding to the increase in sea level. Think of this effect as being similar to what happens when drinking a beverage with a straw; sucking on the straw creates an inversion of pressure within it drawing fluid up into the the area of low pressure from the area relatively high pressure area surrounding it.

  • Astronomical Tides: Can greatly increase the size of a surge event. If the surge coincides with a spring tide then meters may be added to the surge, greatly increasing the potential for harm and the area effected.

Inland Storm Surge

Storm surge can be devastating to low-lying coastal regions, however, a surge event can occur inland as well. An example of such an incident was the tragedy at Florida's Lake Okeechobee. In 1928 Hurricane San Phillippe caused a storm surge that breached the dyke surrounding the lake and over one thousand people lost their lives in the resulting flood.


Source: http://en.wikipedia.org/wiki/Lake_Okeechobee

Image of lake Okeechobee from satellite.


Source: http://en.wikipedia.org/wiki/Image:1928_Okeechobee_Flood.png

Illustration of area around lake that was inundated in the surge caused by Hurricane San Phillippe.

Large, Shallow bodies of water are particularly susceptible to surge-inducing conditions. Lake Okeechobee, for example covers 1890 square Kilometers (730 square miles) and has an average depth of only 3 meters (~9 feet). This shallow depth allows for wind to pile up surprising amounts of water (remember the forces that influence surge) and drive waves against and over barriers, often easily overwhelming them. A more recent, and no less tragic event was the storm surge on Lake Pontchartrain caused by Hurricane Katrina in in 2005. Lake Pontchartrain, which borders New Orleans possesses many of the characteristics that made Lake Okeechobee such a hazard during a Hurricane San Phillippe; e.g. large and shallow body of water with a low lying surrounding area (Lake Pontchartrain covers ~1630 square kilometers, with an average dept of only 4meters).


Source: http://en.wikipedia.org/wiki/Lake_Ponchartrain

Lake Pontchartrain from satellite, notice the city of New Orleans directly below it.

Other areas of this site will discuss exactly why New Orleans was so vulnerable to storm surge on the lake.