Module 8 Formative Assessment
Student Activities: What Makes Coastal Communities in Louisiana
so Vulnerable to Coastal Flooding?
Students will:
1. Use Earth Explorer or Google Earth to find and explore location and elevation of Isle
de Jean Charles, LA;
2. Measure and plot linear distance and elevation to calculate slope of Louisiana coast;
3. Manipulate a 1-D surge model by entering parameters for shelf slope, wind velocity,
and water depth in order to appreciate how the angle of slope of the bed and the
nearshore coastal land influences the height of storm surge. Source of model: (LSU
Department of Civil and Environmental Engineering)
http://www.coastalhazards.org/1-d-surge-model.html;
4. Estimate storm surge heights for an community located several km from the
shoreline (our example is Isle de Jean Charles in Louisiana);
5. Compare Isle de Jean Charles with a community nearby that is protected by at more
than one other layer of defense (levee, natural ridge, etc.) – example community –
Montegut.
1. Investigating a coastal community in Louisiana using Google
Earth
1. You will need to use Google Earth on a computer (rather than an app on a mobile
device)
2. Dowload Google Earth (if you have not done so already)
3. First set your units of measurement to metric units by going to Tools - > options - >
3-D view and find “Units of Measurement” and select to meters/ kilometers. This
will allow you to measure elevation and distance in meters.
4. Navigate to Isle de Jean Charles, Louisiana, by entering the name in a search. Google
Earth will fly you there.
5. Adjust the altitude to an eye altitude of approximately 20 km.
Isle de jean Charles is in the center, bottom third of the view (see below). Note the
linear pattern of ancient waterways that were distributary channels of a long
abandoned delta lobe built by the Mississippi River. Each of these represents a ridge
of higher elevation ground on which the long, linear communities are establishes.
Locate Louisiana Highway 665 and then find Island Road. You may need to zoom
back in to find the label. Island Road connects Highway 665 to Isle de Jean Charles
(“The Island”). Island Road is the only land connection to the town of Isle de Jean
Charles, which is actually located on the remnant of a natural ridge of higher ground
created by an old, abandoned distributary channel of the Mississippi River. Notice
the broken-up nature of the surrounding marshes and the pattern of old oil and gas
access canals throughout this area. These are the indicators of the rapid loss of land
that has taken place over the past century in this area. As the vegetated marsh turns
to open water, the protective buffer between the community and the Gulf of Mexico
is reduced.
6. Zoom in to Isle de Jean Charles to less than 500 m eye altitude. Observe the
elevation of the ground as you mouse over the community. Note a levee that runs
around the “island” (does it completely surround the island?). Record the greatest
and least elevation of ground (not water). How high is the levee? What is the
average elevation of the land, not counting the levee?
Levee height:
Average land elevation:
7. Zoom out so your eye altitude view is at 85 – 90 km.
8. Select the ruler icon at the top of the view. Select km. as your unit of map length (or
ground length). Draw a line (directly N- S orientation) with your mouse between
Isle de Jean Charles and Timbalier Island, directly south of the community. Record
the distance.
Distance from Timbalier Island to Isle de Jean Charles:
9. Use the mouse to continue south out from Timbalier Island into the Gulf. Find the
first location where the elevation is -10 m (10 m below sea level).
10. Use the ruler tool to measure from this point to Isle de Jean Charles.
Distance from -10 m to Isle de Jean Charles:
Island Road, Isle de jean Charles. Photo Credit: Gary Allen.
2. Visualizing the Louisiana Coast in Cross Section:
Calculating slope (example):
Simple calculation:
If a house is located 1 km (1000 m) from the shoreline and is 1 meter above mean
sea level, then the slope can be expressed as slope = rise/run = 1/1000 = .001 %
If we were to draw this on graph paper and use a scale of:
X axis: 1 square = 10 meters of horizontal distance so 100 squares = 1 km (1000m).
Y axis: 1 square = .2 meters of vertical distance (5 squares = 1 meter)
This slope can be expressed as a percentage or a decimal:
Slope = Rise/run
1/1000 = 0.001
Calculating the slope of the Louisiana Coastline at Isle de Jean Charles:
You will now use your measurements from Google Earth to plot the slope of the
coast line. This will enable you to use a model to find projected storm surge levels
with different storm strengths.
Use the grid on the next page to plot your distance from the -10 m point offshore to
Isle de Jean Charles (your X axis. Choose a scale that fits the grid. 2 squares could
represent 1 km.
On the Y axis you will plot the elevation. The lowest elevation will be -10 m and the
highest is +2 m (the elevation above sea level of the levee around Isle de Jean
Charles).
You will need to draw two “legs” of the slope: 1. from -10 m water depth to the
shoreline (Timbalier Island), and 2. From the shoreline to Isle de Jean Charles. Your
final slope will go from the -10 m depth to Isle de Jean Charles.
Use the guidelines above and below to help you complete the plot.
After completing your plot, calculate the slope as “rise over run”
Slope = ______
Next, you will use this number in the storm surge model.
3. Modeling storm surge:
The next step is to plug your number (slope) into the storm surge model found
at the link: http://www.coastalhazards.org/1-d-surge-model.html
The three parameters to be entered in the model are:
1. Water depth (the depth of the water at the deepest point before storm surge (10 m
depth in our example)
2. Wind velocity (m/s)
3. Bed slope (as calculated above)
In order to understand the relationships between wind, bed slope and storm surge,
we will first plot some numbers.
a. Slope vs. Surge Height at 10m Water Depth and 10 m/s wind velocity.
h0 (m)
10
Slope
Surge (m)
0.000854
2.0838
0.000427
3.4085
0.000214
5.4705
0.000107
8.613
b. Wind Speed vs. Surge Height at 10m Water Depth
slope
0.00016
h0
(m)
10
u10
(m/s)
Surge
(m)
2
0.0685
5
0.3109
10
0.9215
20
2.5631
40
6.6233
80
15.67
4. Investigating influence of wind speed and slope on storm surge:
Use Excel to plot the two sets of numbers above. Answer the questions below.
1. Describe the relationships you see.
2. Using the storm surge model, manipulate the height of the storm surge by entering
each of the numbers in the data sets above in turn. Watch how the storm surge
changes as you enter different values for wind speed and slope.
3. In what way does the angle of the slope of the bed or shoreline influence the height of
the storm surge?
4. In what way does the wind speed affect the height of the storm surge?
Storm Surge and Isle de Jean Charles
Using your calculated bed slope number for Isle de Jean Charles, experiment to find
out how wind velocity values affect the storm surge level. The water depth will stay
at 10 m. The slope will stay at the calculated value. Just manipulate wind speed and
enter the values in the chart below.
Tip: To “visualize” the wind speeds in more familiar units of miles/hour (mph) –
roughly double the m/s value, so 20 m/s = 44.74 mph; 60 m/s = 134 mph) A
hurricane with wind speed of 130 - 156 mph is a category 4, according to the SaffirSimpson Scale.
(Note: the model may not allow you to put in exact values – so use the closest value
it will allow)
Water depth (m)
10
10
10
10
Bed Slope
0.00028
0.00028
0.00028
0.00028
Wind speed
20 m/sec
30 m/sec
40 m/sec
60 m/sec
Storm surge height
Reduction of Storm Surge by protective landscape features
The effect of “lines of defense” (barrier Islands, marshes, ridges of high ground,
levees, etc.) is of course to reduce (or attenuate) the storm surge by offering friction
and reducing the energy of the water. The actual rate at which this happens varies
according to many variables, including the dimensions of the barrier island, the
health or level of degradation of the marsh, the height of the levee, etc. It is hard to
quantify this so we will use an arbitrary (estimated) number for now. We will say
every 1 km of marsh land reduces the surge by 10 cm.
So if there is a 10 m storm surge at Timbalier Island and the distance from Timbalier
Island to Isle de Jean Charles is 35.5 km, what will the storm surge be at Isle de Jean
Charles?
Visualizing storm surge effects:
A storm surge of just 3 meters can do considerable damage. The energy of the water,
combined with the wind can move large objects and flood any houses that are not
raised above this level. Higher storm surges of course do even more damage. The
residents of Isle de Jean Charles receive frequent flooding from storm surges. Many
surges that reach the community are less than 1 meter so the small levee protects
them.
Reflection question:
What if a Category 4 storm hits the Louisiana coast directly south of Isle de Jean
Charles, delivering a 10 meter storm surge at Timbalier Island. If the barrier islands
and marshes reduce the storm surge by 4 meters and the town receives 6 m of
water, what effects would the residents experience?
5. Compare Isle de Jean Charles to a nearby community to be
protected by the “Morganza to the Gulf” hurricane protection levee
Revisit the fact sheet on the Morganza to the Gulf Levee project. Also, read the Times
Picayune article from May 2013:


http://www.mvn.usace.army.mil/Portals/56/docs/PD/Projects/MTG/117.pdf
http://www.nola.com/environment/index.ssf/2013/05/corps_of_engineers_conclu
des_1.html
Examine the maps in these links and locate the towns of Montegut, Chauvin, and
Dulac. These communities are at a similar distance from the Gulf as is Isle de Jean
Charles, but they will be within the footprint of the Morganza to the Gulf Levee. As
the Times Picayune article states, the elevation of the levee will be between 10.5 and
24 feet (3.2 – 7.3 meters). This is an average of about 5 meters. We will use this
number in a calculation.
Use Google Earth to measure the distance from the Trinity Island to Dulac.
Record the distance here: ____________
Using the hypothetical storm surge of 10 m at landfall and the estimated surge
reduction factor used previously (1 km of land reduces the surge height by 10 cm),
to calculate the height of the storm surge be when it reaches Dulac.
1. Will the new levee protect Dulac in this case?
2. What level of storm surge, as measured at the barrier Islands (landfall) will Dulac be
protected from once the levee is built?
Disclaimer: this is just an estimate for the purposes of understanding the
challenges faced by those living in these coastal communities. We cannot
quantify exactly how the various landscape features protect now and in the
future, sea level rise and further erosion of barrier islands and marshes must be
factored in to any calculations.