New Orleans Flood Impact Analysis
with GIS and Remote Sensing
Introduction
Study Area
New Orleans is a city in Louisiana state, located near the Gulf of Mexico and shaped by the waterways of the Mississippi River and Lake Pontchartrain. It has a humid subtropical climate with mild winters and hot summers. Its population is about 400,000.
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Methods
Results
Elevation Map (3D)
I also combined the 3D buildings polygon feature with the elevation layer, which may help you grasp how extensively the urban areas could get damaged when a hurricane event occurs. Again, in the case of Hurricane Katrina in August 2005, approximately 80% of the city was flooded.
The Day After Katrina
Data source: OrbView-3 (2005)
This black-and-white satellite image was taken on the day after Hurricane Katrina attacked the city. One thing to note is the flooded areas shown in black here approximately correspond to the areas that are lower than the mean sea level shown in the elevation map. Swipe the imagery to compare them.
Impact and Income Level
This map above shows per capita income by ward simply from low to high. The purple color describes high-income communities.
However, this comparison of the two images reveals that even some high-income communities are exposed to the hurricane flood impacts due to the city’s flat grounds and low elevation.
Leaving the urban area, let us take a look at the wetlands area in the northeastern portion of the city.
Land-Use Classification (2000 vs 2017)
Data sources: (Left) U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center (2016a) / (Right) U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center (2017a)
These are Landsat unsupervised land-use classification maps. The light blue areas describe wetlands in the left 2000 Landsat 7 image and the right 2017 Landsat 8 image. Based on peer-reviewed articles, coastal wetlands in Louisiana are supposed to be dramatically decreasing because of a combination of natural and human activities. However, contrary to my anticipation, the results of this classified imagery did not necessarily show the tendency.
Katrina Track (2005)
Data source: IDM801814833_ohiostate (2021)
Then I started to look at other wetlands areas while considering major hurricane tracks because past studies point out that wetlands work as crucial buffer zones to hurricanes and mitigate their impacts on New Orleans (Barbier et al., 2013).
This map shows Hurricane Katrina track in 2005, and you can see Katrina crossing New Orleans from south to north.
Most historical hurricanes have attacked South Louisiana from south to north after getting stronger above the Gulf of Mexico.
Wetlands in South Louisiana
Thus, I decided to add the vast wetlands area located southeast of New Orleans, considering it as a crucial buffer zone to hurricanes that hit the city.
Natural Color Bands Comparison (2000 vs 2017)
I downloaded satellite imagery of 2000 and 2017 from Landsat 7 and 8, respectively, and first compared them with the Natural-Color band combination. Though the images’ brightness is quite different, making it a bit hard to compare them, it looks like the central part of the Landsat 8 image is more eroded than that of the Landsat 7 image.
Land-Water Interface Bands Comparison (2000 vs 2017)
Next, I applied the Land-Water Interface Bands for both images, which is suitable for identifying land-water boundaries clearly. Now it’s obvious that the wetlands in 2017 on the right are sparse, and you can observe huge water areas around the center of the image, which you cannot see in the left image of 2000.
Land-Use Classification Comparison (2000 vs 2017)
Additionally, I performed the unsupervised land-use classification for the imagery. These classified images may also help you see the difference in wetland spaces between the two periods of time.
Land-Use Change between 2000 and 2017
Finally, here is the image describing land-use change between 2000 and 2017, emphasizing the erosion of the coastal wetlands. I have observed a stable decrease in wetlands in this area on Satellite imagery year-by-year, so I suppose the change shown here is not because of particular natural disasters.
While Louisiana has approximately 40% of the U.S. wetlands, over 90% of the total coastal marsh in the country has been lost in the state. Some studies say Louisiana is losing a football field of coastal wetlands per hour (Boesch et al., 1994; Couvillion et al., 2017).
Oil and Gas Wells Locations
Here I put oil and gas wells locations on the land-use change map. Many studies and articles say the oil and gas industry has significantly damaged Louisiana’s coastal wetlands by drilling wells, canal construction, and waste mismanagement (McGill, 2020).