Lab10_CoastlinesHurricanes_LLL
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Lab 10–Coastal systems and hurricanes
15 Questions
Lab goals
Coastlines are fascinating and very dynamic places that lie at the juncture of the land (terrestrial lithosphere) and the sea (the hydrosphere), with the atmosphere above. They are also incredibly diverse in nature, and an interesting and fruitful place to apply an earth system science perspective. Finally, they are characterized by high population densities (of humans) and therefore, understanding shorelines is of distinct utility and consequence. This lab starts out by familiarizing you with some of the
features and processes that characterize a chosen suite of shorelines, and then considers what happens during a defining event such as a hurricane. Because of the combination of a dynamic character and high density of human habitation, coastlines are also a place where issues of sustainability have significant implications and consequences. Recognition of coastal features (9 pts)
In this portion of the lab you will first be introduced to a specific type of coastal feature by navigating to a given locality in Google Earth and then working through guiding questions. You will then be asked to find a similar feature of your choosing elsewhere
along the adjacent coastline and answer some
questions about it. Deltas
: A delta is a body of sediment that forms where a
river meets the sea (or other body of water), as the
current weakens and loses the ability to transport
sediment, which then comes to rest. As the river
continues to supply sediment, with time the delta
gets larger, and builds out. However, deltas vary
incredibly because of the different degrees to
which wave and tidal processes reshape the
sediment supplied to the shoreline by the river,
and because of the nature of the shoreline it builds
against.
The image to the right shows how a delta built out
with time in the Puget Sound area of Washington
State. In this particular case the delta filled an
embayment, an indentation into the land. In other
cases deltas build out from the shoreline and
protrude into the ocean. This example also shows
how deltas are connected to other aspects of earth
systems – in this case to sea level rise due to the
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end of the last Ice Age (which formed the embayment), and then to pulses of eruptions, which supplied very large volumes of sediment. Image taken from USGS site: http://walrus.wr.usgs.gov/geotech/pugetposter/model.html
Navigate in Google Earth to
62
57.255 N 164 8.25 W
You can copy and paste these coordinates directly into Google Earth search window to navigate there. Zoom in and out to get your bearings, then frame your window view so that it is about 100 km across (the ‘eye alt’ number which changes as you zoom in and out)
. Orient N as shown in the upper right corner of your screen so that it is “up” (this is the default position and conventional view). What you are looking at is the delta for the Yukon River in Alaska where it enters the Bering Sea. The Yukon River is large and carries a lot of sediment, and this is a large delta. - Use the path tool to trace out the two major channels which presently reach the sea and mark them in bright green and make them wider so that your tracings are clearly visible. This is where
the river is presently providing sediment to the front of the delta. - Note that there are many well developed channel forms on this delta, which are of significant size, but which no longer reach the sea. Find two of these and use the path tool trace them out and assign them a bright yellow color and increase the line thickness as you did above so that they are also clearly visible. Why do these channels no longer reach the shore? Channels get abandoned and plugged as a new channel develops elsewhere. Deltas are dynamic places where rivers are constantly changing their paths. - On the west side of the delta note the light-colored ridges that are parallel to the present-day shoreline but are presently inland of the shore. These are old beach ridges, formed as waves reshape the sediment supplied by the river to the shoreline area. As more sediment is supplied, new beach ridges form seaward to the older one and in this way the delta shoreline can build out. Trace the most prominent beach ridge as far as you can go, and assign that line a bright red coloration.
Question 1:
Copy and paste an image that shows the delta, with your colored line traces of active river channels, abandoned river channels, and beach ridges below. - Using either the line or path tool to make an estimate of how far in kilometers this delta protrudes out into the Bering Sea and has built out with time. Question 2:
Estimate: _______ km
Explore the edges of the delta in Google Earth. Where do you think the prevailing wind direction comes from and why? One thought path that may be able to help you answer this question is to consider that waves are a function of wind direction and strength, and then to consider how wave-related features are developed (or not) along different parts or sides of the delta, with the idea that the beach ridges will be best formed where the waves are the largest. The geomorphology of the delta is connected to climate in several ways. Since we see well-formed beach ridges on the Northwest side of the delta, that
is a good indication that the prevailing wind is from that direction.
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Barrier islands
: A barrier island is an elongate body of predominantly sandy sediment that forms off shore from the mainland. They typically occur on low-relief and sediment-rich coastlines. Waves help to form the sediment body in the first place, but as it becomes vegetated, the vegetation traps and stabilizes sediment and plays a crucial role in the building and maintenance of the barrier island. People love to live on barrier islands, and have significantly altered their behavior in some cases. Barrier islands are very common along the southern Atlantic coast of the U.S. and along the Gulf Coast. The map to the left shows two barrier islands off mainland Maryland. The oblique air photo above shows the Ocean City inlet portion of the map area with highly populated Fenwick Island to the north (farther away) and Assateague Island to the south with the open ocean to the right. Assateague is where wild horses live, and has been protected from development. The two islands used to be aligned, but are now offset, and the map shows how Assateague has retreated almost a mile since 1849. This retreat is due to coastal engineering that has kept Fenwick Island in place, while Assateague has continued to move. In addition, note in the oblique air photo image, the fan shaped bodies of sediment on the back side (bay side to the left) of the island. How did these get there? Image source: http://pubs.usgs.gov/circ/c1075/conflicts.html
Navigate to 28 4.7 N 96 53 W
Zoom in and out to get your bearings, then frame the horizontal distance in your window view to be about 35-40 km across
. You are looking at part of Matagorda Island in Texas, a barrier island. Note the white strip on the seaward side of the island. This is the shore face, the sandy beach, and if you zoom in you can see the waves in the image. Now note the delta-like feature with channels that fan out into the lagoon between the island and the mainland to the left. Where did this sediment come from? The answer can be found in the area the delta channels converge on, where the beach is wider. This used to be a tidal channel, moving sediment back and forth and forming the delta, but it has been “plugged” 3
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as the waves moved sand into that area. The tidal currents move sediment. When the tide rises it moves the sediment in toward the lagoon, which since that area is more protected (from waves), forms what is known as a flood tide delta on the inside of the tidal channel. When it moves sediment out to sea an ebb tide delta
can also form on the seaward side, but often the waves redistribute this sediment (since it is in the open ocean), and so ebb tide deltas associated with tidal channels are often missing or are well less developed and harder to see.
- Navigate along the length of Matagorda Island and find another flood tide delta (there is one just north). Zoom in a bit and use the path tool to trace 2 or 3 of the relict channels that distributed sediment to form the delta. Question 3:
Insert a Google Earth image of that flood tide delta with traced channels below. Question 4:
For this image, describe whether it is an active flood tide delta or an inactive one where the tidal channel has been plugged with sand. Navigate to 32 34.774 N 80 8.693 W
Frame your window so that the field of view is 2 km wide
. You
should be viewing the very end of Kiawah Island, South
Carolina, with the Atlantic Ocean to the right. A fair-sized tidal
channel marks the southern end of the island here. Note the
white shore face and the distinctive curved or hooked ridges
of white sand that merge with the shore face to the northeast. This end of the island is a distinctive landform feature known
as a spit
. A spit forms due to a process known as longshore
drift
, which in turn is a function of prevailing wind directions
and the resulting waves. Longshore drift describes the
movement of sand along a beach; when the waves come in at
a consistent angle to the beach, sand is washed up the shore
face by the waves at the same angle of the incoming waves,
and then as the waves retreat, the sand is carried back down
the shore face straight out to sea. The net effect is that the
sediment is transported parallel to the shore (as shown in the
diagram to the left). Since this is a dynamic process, it often
helps to see the process animated. Watch this short YouTube
video explaining this process at: https://www.youtube.com/watch?
v=U9EhVa4MmEs
.
It is difficult to understand shorelines without
considering longshore drift which shapes and reshapes
beaches constantly.
Image to right and above is a diagram showing how longshore drift works. The zigzag arrows on the beach indicated the movement of sediment (sand). Image source is USGS site http://pubs.usgs.gov/circ/c1075/change.html
In the case of Kiawah Island, the longshore drift is from the northeast towards the southwest. When sediment comes to a tidal channel where the waves bend around (refract), it enters deeper water and 4
the sand builds out on the one side forming distinctive curved sand bodies called spits
. If you look at the Google image of Kiawah you can see older sand spit ridges where vegetation has had a chance to be established. The other side of the tidal channel is typically characterized by erosion. With some more thought you can see that the above processes suggest the tidal channel here has moved to the southwest with time, and will continue to do so. Image to right is an oblique aerial view of a spit that
formed along the end of a South Carolina barrier island.
Note the multiple curved whitish sand bodies. At one
time each one of these was the active shore face and
new sand ridges have extended the spit towards the
viewer with time. The oldest part of the spit is farthest
away. In this particular case with a careful look one can
also see how the waves are oblique in a manner
consistent with a transport direction toward the viewer,
and consistent with the explanation given above. Image
source is the USGS website: http://pubs.usgs.gov/circ/c1075/barrier.html
Images above showing a bit of what Kiawah Island is like on the ground. To the left
one can see how the sea grass (tolerant of salt) is vegetating and stabilizing the wind-blown sand that has formed small dunes. Dunes are a common component of barrier islands. Wind is also an important sediment transport mechanism in these shore line settings with the beach shore face providing the source of the sand
. To the right
is the sandy expanse of wave ripples exposed at low tide on the spit of Kiawah. Navigate to 32 17.964 N 80 31.843 W
Frame your window so that it covers 3-4 km of image
, which is of Pritchard’s Island, South Carolina. As you progress from right to left across your view (with N in the standard position) you should see: a) the open Atlantic Ocean, b) the thin white sandy shore face, c) the vegetated backbone of a barrier island, and d) an array of meandering and branching tidal channels with tidal flats in between the 5
channels. Further west (left) is the mainland. One can note in the vegetated area (trees here) of the island, linear features that are approximately parallel to the modern shoreline. These are old beach ridges that become stabilized by the vegetation with a new beach being formed on the ocean side. In this way the island can grow with time, building out in a seaward direction. At present the island is being
eroded. Note the nice spit that has formed at the south end of the island. Tidal flats and tidal channels found on the landward side of the island result from the interplay between tides, sediment and vegetation. The tidal channels distribute sediment and the vegetation traps it. Tidal flats and channels are areas of very high biologic productivity. Navigate to 32 17.614 N 80 34.454 W
Frame the window so that it covers about 10 km
from east to west. This is Capers and St. Philips Island
area of the South Carolina coast. Note the additional beach ridges visible in this view. - Use the path tool and what you have learned above to trace examples of the following features using the colors indicated. Zoom in and out as needed to find and trace the features but return to a 10 km eye alt view for your image. Trace: a) tidal channel in light blue, b) shore face in yellow, c) vegetated beach ridge in brown, d) spit in red. Question 5:
Insert a copy of the image with your tracings below.
Question 6:
What is the direction of longshore drift in this case and on what do you base your answer? Emergent shorelines
: Emergent shorelines are ones dominated by high relief and erosion. Features that you will learn about as you go through this section are sea cliffs
, sea stacks
, sea arches
, wave cut benches
, and wave cut terraces
. Perhaps the best introduction here is with some photos from coastal California.
To the left above
one can see sea cliffs that characterize much of the coastline of northern California. Note the flat, now vegetated top of the sea cliffs, which is a wave cut terrace (how they form is 6
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explained below). Also note the many rocks poking up in the surf zone out to sea. In a protected bay to the very left a sandy beach has formed at the foot of the sea cliffs. To the right
above
is a feature known as a sea stack (note it also has a flat top in this particular case, not all sea stacks do). Also note the surf zone which indicates the water is shallow surrounding the sea stack. As the waves erode the base of the sea stack, causing the overlying rocks to fall and get ground up and carried away, the sea stack is gradually getting smaller. To the left
is an offshore island with a rather large sea arch eroded into it. Note also the surf zone around it, indicating somewhat shallower water. There is a submerged area of flat rock in the shallow waters which formed due to wave erosion and is known as a wave cut bench. To the right
is an example of a typical wave on a windy day. Armed with boulders and sediment, the waves hammer against the shoreline and are a significant erosion agent.
The cliffs characteristic of emergent shorelines form as the waves cut at and undermine the base, and then the overlying rocks fall into the sea, where they are ground up and carried away by the continued wave action. With time the waves cut a notch into the coast and the cliff retreats (moves inland) as material is lost. An example is given below. Image to left
is of coastline at Pacifica, California before 1997. During larger storms the waves cut at
the base of this cliff made up of soft and relatively loose material and trigger mass wasting. The large
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rocks and earth-moving equipment were from an attempt to protect the cliff from the waves. The 30-
meter-high cliff failed during the 1997-1998 El Nino storms, the cliff retreated some 10 meters, and the
front row of houses you see here were a loss. Most sea cliff retreat is episodic and associated with
larger storms. Image source is USGS website http://pubs.usgs.gov/pp/pp1693/
. Image
to the right
is
for comparison purposes this is the same stretch of coastline along Pacifica, California, but in 1998.
Image source is http://coastal.er.usgs.gov/hurricanes/elnino/
. A YouTube video of the Pacifica case can
be found at http://www.youtube.com/watch?v=Hw6sDulj7GA
A rather amazing YouTube video of mass wasting triggered by sea cliff erosion can be viewed at http://www.youtube.com/watch?v=ZVjr4mii3cE
Navigate to 39 18.455 N 123 48.405 W
Frame your view so that the image captures a 2-3 km
wide window. This is the Mendocino, California coastline area. Once again, note all the sea cliffs
here. These are one indication that this is an emergent coastline, created in this case by uplift of the crust relative to sea level. To have emergent shorelines in California makes perfect sense, because of the active tectonics and earthquakes that characterize this part of the world, and which are related to the juncture between the Pacific plate and North American plate (which the well-known San Andreas fault is part of), each moving their own separate ways. Note the extensive areas of white, foaming, crashing waves that extend from the cliff out to sea in your Google Earth scene of Mendocino. This area is a wave cut bench
, where the waves wear away at the rocks, flattening them. The rocks found along coastlines are often different types and are fractured to different degrees, so that some are easier to erode than others, which accounts for much of the irregularity you see in such coastlines, including all the small bays. The bays are the result of areas where the softer and/or more fractured rocks allowed the waves to erode further back into the land. Sometimes, the waves erode right around a more resistant section of rocks and leave a small island or rock spire behind as the cliff retreats. These resistant rocks that become separated are known as sea stacks
.
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Image above
is an example from the Oregon coast of a sea stack that has become separated from the mainland. Especially striking is the fairly flat surface that the town of Mendocino sits on. It contrasts with the rather
hilly topography further inland. This used to be a wave cut bench, but was uplifted above sea level by the tectonics of the area, to form a feature known as a wave cut terrace
. Old beach deposits can often be found covering the very top of such wave cut terraces. To really see the nature of this terrace, you will want to use the navigation tools so that you have an oblique view. Another example of both a wave cut bench and uplifted wave cut terrace can be seen in the image below from the Oregon coast. Image above
shows a wave cut bench (foreground) and a wave cut terrace (background with trees on top). The terrace is an older wave cut bench that has been uplifted by tectonic forces. Location is Sunset Bay, Oregon.
-
On average about how high is the Mendocino terrace above sea level? You should be able to just move your cursor around and see the elevation change in the status bar. If it isn’t working, make sure you have ‘terrain’ checked under Layers.
This is the amount of tectonic uplift relative
to sea level that has occurred since the wave cut bench formed. Since sea level globally has been increasing in the last 10,000 years, it is clear this terrace is due to crustal uplift rather than sea level drop.
Question 7:
Average height of wave cut terrace: ________m - Navigate along the coast (south is best direction to go) until you find another example of a wave cut terrace. Use the path or polygon tool to trace and identify examples of the following features using the colors indicated: a) sea cliff – blue, b) wave cut terrace – yellow (polygon tool can work well here), c) sea stack – red. Again, make the lines wide enough they are easily seen. Question 8:
Insert your resulting map here.
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Reefs
: These are very different coastal systems, where biologic activity dominates. A great variety of organisms in the reef environment, including corals and calcareous algae, make homes and skeletal elements out of the ions in the seawater. This hard mineral material gets broken up and reworked after the organisms die resulting in abundant carbonate sediment. Most of southern Florida is made of such carbonate sediment. The reef itself where the interlocking corals create a rigid framework acts as a wave barrier creating a sheltered
lagoonal environment behind
which sediment collects and
houses a host of organisms
adapted to these conditions. If
you ever get the chance,
snorkeling or diving on these
reefs is an incredible experience.
Image to right
is of New
Caledonia to the east of Australia
in the Pacific Ocean. The fringing
white line here is a framing coral
reef complex that creates
sheltered (and lighter blue)
waters, with much deeper waters
further offshore. Image is from
NASA Visible Earth http://soundwaves.usgs.gov/2006/07/awards.html
Navigate to 18 26.120 N 87 45.505 W Zoom in and out to get your bearings and then scale your view to about 2 km across
. You should be seeing part of the southeastern coastline of the Yucatan Peninsula, and the imagery here shows shallow features. There is a lot that can be seen here, but it may not be obvious at first. Start by finding the line of white about 700 meters off shore. These are breaking waves due to shallowing of the sea bed, and they mark the location of the main barrier reef
underneath which is a linear feature roughly parallel to the shore with distinctly deeper water offshore. Note the slightly different coloration of the shallow coral reef with a brownish tinge that can be seen. This reflects the color of living coral. The coral carbonate material is bleached white once the coral that used to live in it dies. Note how as you trace the edge of the reef, there are breaks and/or interruptions. These are tidal channels
that cut through the reef. 10
Between the reef and the beach sand is a white expanse with brownish dots. The white area represents
carbonate sand material made from the waves breaking down the coral and other skeletal elements. The brownish dots are small patch reefs
that grow in the protected lagoonal area. Different types of corals favor the patch reefs over the main reef. Along the mainland shoreline one can see the thin sandy beach with the jungle on the landward side. If you navigate along the beach at points you will see small islands of vegetation along the shore. This vegetation must be able to tolerate saltwater, and much of it is populated by mangrove, which are small
trees that can grow in salt water. Mangroves
are also an important biologic component of this shoreline
system (in addition to the corals). Sediment gets trapped in amongst the roots of mangroves, and thus these small islands are there because of the mangroves. Finally, note on the seaward and deeper side of the main reef (the fore reef area) a streaked character where the streaks are almost perpendicular to the reef. These are storm related features, where, during
large storms (and this is an area where hurricanes occur on a regular basis) strong currents move coral
debris and carbonate sand down the ocean-side slope and deeper offshore. - Use the path tool to trace out a) the main reef with a red line, b) circle one or two patch reef areas with a purple line, c) circle any mangrove patches you can find with a bright green line, d) trace a tidal channel that cuts through the reef with a light blue line. Question 9: Make a copy of your image and paste it below.
The effects of hurricanes (2 pts)
You have already been exposed to the idea that the rate of coastal processes is not constant. Sea cliff retreat is accelerated during storms. One can contemplate the relative contributions of day-to-day events versus the rare but more powerful events, such as a hurricane. More sea cliff retreat often occurs during big storms than during all the time between big storms. More change occurs on a barrier island during a hurricane than all the time between hurricanes. Therefore it is appropriate to focus on hurricanes in particular. Hurricanes are tropical cyclonic storm systems that originate in equatorial Atlantic waters and have winds in excess of 74 mph. On the west side of the Pacific they are known as typhoons. Sandy and Katrina are hurricanes in recent memory for many in the U.S. One of the critical processes during a hurricane is the storm surge. What is storm surge
and what does it do?? Storm surge causes most of the risk and devastation associated with hurricanes, and can also move a large volume of sediment. As the hurricane comes on shore, on one side of the eye (to the north or east for the Atlantic and Gulf coasts of the U.S. respectively) the winds are blowing onshore, and this pushes the water on shore. Remember that these
are winds in excess of 70 mph. It is a bit like blowing on a bowl of soup strong enough that you push it up and over the bowl edge. What can be surprising is the magnitude of storm surge – up to 30 feet in extreme cases. For Sandy at the Battery in New York City in 2012 it was a bit over 13 feet and associated waves reached heights of 32 feet above mean sea level. For more details visit NOAA’s (National Oceanic and Atmospheric Agency, federal) website on storm surges: http://www.nhc.noaa.gov/surge/
.
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In this simplified image from NOAA
note how the storm surge can be
added on top of the normal tide to
cause a higher storm tide. Also note
that the storm waves are above (on
top of) the storm tide level. The
highest land elevation on some barrier
islands can be less than the height of
the storm surge and thus the water
can cover the entire island during the
storm. Image source: http://www.nhc.noaa.gov/surge/
This image from the same source
shows how the pushing of the
hurricane winds causes most of the
surge, but a small percentage is also
due to the atmospheric low pressure,
which causes an increase in the local
sea level. Naturally, the larger the
hurricane the larger the potential
surge, but other factors such as the
profile of the sea bed can also make a
significant difference. One way to get an appreciation of storm surges is to see examples and the following YouTube video, which you are encouraged to watch, is of Hurricane Sandy storm surge: http://www.youtube.com/watch?v=hBsvNfrUN0I
. There are plenty of other videos of storm surge for your further education. You can also explore the storm surge risk map from NOAA - http://www.nhc.noaa.gov/surge/risk/
. There are also quite a few YouTube video clips of the storm surge associated with Typhoon Haiyan that hit the Philippines in 2013, killed 6000 plus people, and may have been one of the largest typhoons in history. Here is one: https://www.youtube.com/watch?v=rS0gv4Xbw7w
Navigate to Mantaloking, NJ
Zoom in to about 200-500 m
. This is one area where damage from Hurricane Sandy was concentrated. Under the View option on the top menu bar, select the Historical Imagery option. Note the new slider bar in the upper left corner of the Google Earth view window. View imagery from before Hurricane Sandy (2011 and earlier) and after (2012 and later). Click through the Historical Imagery to get an idea of how the coastline was impacted. You can navigate north and south to see more damage.
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-
Find areas of obvious sand transport and damage. Make sure you compare the pre and post images to get a true idea of the extent of change and damage. Choose two
example locations that document the changes and the damage from Hurricane Sandy and copy and paste the before and after images below. Then write captions that include which shoreline elements you learned about in this lab that can be seen in your image, and what types of changes and damage are evident in your image.
Question 10: Location 1: before and after images and caption:
Question 11: Location 2: before and after images and caption:
An important piece of information for understanding the contribution of hurricanes to shoreline behavior and to planning for hurricanes if one lives along such a shoreline is how often they occur. It is also important to consider the size of the hurricane, as smaller hurricanes occur more often than larger hurricanes. The predicted frequency of hurricanes is often cast as the recurrence interval for a given size hurricane
for a certain section of coastline. For example, one estimate based on the age of distinctive storm layers these larger hurricanes leave in the geologic record is that the larger 4 to 5 category hurricanes have hit the Alabama coastline with a recurrence interval of roughly 600 years (
http://nsstc.uah.edu/aosc/al_hurricanes.html
). Remember that this does not mean that hurricane arrival
in Alabama is cyclic, but that in the long term the average time between such hurricanes is 600 years. One way to think of it is that your chances of having a hurricane that size along the coast of Alabama each year is 1 out of 600. The odds may seem low, but the cost is high.
Summary (4 pts)
Our claim in this lab is that coastlines are especially active areas with a high degree of linkages among the various spheres that make up the earth. You should explore this by answering the 4 questions below, given what you know and have learned. Since these are open ended and the answers could get quite long, please use 100 words or less for each question (but use those 100 words effectively). This will help you decide which are the most crucial processes to include. Question 12:
What role do atmospheric processes play in shaping a coastline?
Question 13:
What role do biosphere processes play in shaping a coastline?
Question 14:
What role do hydrosphere processes play in shaping a coastline?
Question 15:
What role do lithosphere processes play in shaping a coastline? 13
Image to right is one example depicting
the various factors that influence
shoreline behavior, especially that of
barrier islands. Note that as
comprehensive as this diagram is, from
our perspective there is at least one
element that has been left out. You can
use it to help you answer the questions
above. Image source is USGS website http://pubs.usgs.gov/circ/c1075/web.html
. Additional reading if interested or if you plan to live along a coast:
Coastal Change Hazards: Hurricanes and Extreme Storms, USGS http://coastal.er.usgs.gov/hurricanes/
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