exercise 7 coastal processes and...

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Exercise 7 Coastal Processes and Hazards James S. Reichard

Georgia Southern University Student Name _________________ Section _______ In this lab you will:

examine different types of shorelines and associated coastal hazards. You will also explore how coastal processes are disrupted by various shoreline engineering techniques.

Background Reading and Needed Supplies

Prior to doing this exercise you should read Chapter 7 in the textbook. With respect to supplies, you will need a calculator, ruler, and colored markers. Part I – Coastlines and Human Development

Recall from the textbook that tectonically active shorelines are usually rugged and irregular, with beaches often restricted to coves. In contrast, passive shorelines have little to no tectonic activity, which commonly results in relatively straight coastlines with flat-lying terrain and extensive beaches. The cross-section in Figure 7.1 illustrates the general tectonic setting of active and passive shorelines. For a variety of reasons, people have historically built settlements in coastal areas. Today, the rate of population growth is significantly higher in coastal zones than in the interior of continents. Figure 7.1

1) Describe the general types of coastal hazards one would expect to find along tectonically

active shorelines and passive shorelines.

active shorelines:

passive shorelines:

Ex 7 – Coastal Hazards


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2) The satellite photo in Figure 7.2 shows the area around Hanauma Bay on the Hawaiian island of Oahu. Note the extinct cinder cones, one of which has been breached by wave action, forming a small bay.

a) Using a red-colored marker, outline the boundary between the developed and non-

developed areas on this photo. b) Note how abrupt the boundary is between developed and non-developed areas on the

photo. In terms of the landscape, what topographic feature or landform have the developed areas been built on?

c) Describe the basic reason why people preferentially chose to build on the type of landform

you listed above.

3) What type of geologic hazards do you think might exist in the developed areas? Explain.

4) Based on what you know about plate tectonics and the fact that the Hawaiian Islands are located in the middle of the Pacific Ocean, what type of tectonic-related hazard might the Hawaiian coastline be subjected to? Explain.


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Figure 7.2 - Hanauma Bay on the Hawaiian island of Oahu (Courtesy of NASA)

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5) Figure 7.3 shows a coastal city in Indonesia that was completely obliterated by the 2004 tsunami. This tsunami was generated by a magnitude 9.1 earthquake that formed in the nearby subduction zone. The massive waves killed nearly 250,000 people in various countries around the Indian Ocean.

a) To get a better sense that there was once a vibrant community of people living here, use a red marker and outline the trace of the more obvious roads still visible on the photograph.

b) Notice that rugged terrain of the coastal zone is indicative of a tectonically active area.

Based on the landforms that you see on the photo, explain why people would have originally chosen to build a town at this site.

6) When waves approach shore and begin to drag on the seafloor, the height of the waves naturally increase, a process called run-up. What is it about the shoreline in Figure 7.3 that would have magnified the run-up of the tsunami waves as they approached shore?

7) It is quite clear that the coastal strip in this photo is extremely vulnerable to tsunami waves. Explain then why people would live and work in such a hazardous area?

8) Describe two preventative steps that could be taken to minimize the loss of life should another tsunami strike this region.



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Figure 7.3 – Coastal community near Aceh, Indonesia, that was obliterated by the 2004 tsunami (Courtesy of U.S. Navy, Tyler J. Clements).

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9) The before and after photos from 2008 in Figure 7.4 show the effects of Hurricane Ike on the developed portions of the barrier islands along the Texas Gulf coast. In contrast to the previous photos, the U.S. coastline along the Gulf of Mexico has been tectonically inactive for millions of years.

a) What evidence do you see in the photograph that indicates this area is tectonically

inactive? b) Explain the effect your previous answer would have on the amount of development that

can occur along a coastline.

10) The homes in the photos in Figure 7.4 were destroyed during a major hurricane, named Ike, in 2008. How can you tell from the photos that the homes where destroyed primarily by storm surge, not high winds?

11) Zoom in on the pre-hurricane (Sept. 9) photo and take a closer look at the homes right along the beach. Describe the building design used in the construction of these homes whose purpose was to protect the structures from storm surge.

12) Clearly, the building design in question failed to protect the overwhelming majority of homes in these photos. Provide an explanation as to why the design failed in this case.

13) Note the red arrows that have been added to the photos. Use the zoom tool to take a closer look at the area around the red arrow in which the house was destroyed.

a) In addition to the house being gone, how has the land physically changed in this area? b) The physical changes you described above are indicative of an important shoreline

process. What is the name of this process?


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Figure 7.4 – Bolivar Peninsula near Galveston Bay, Texas (Courtesy USGS).


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Part II – Shoreline Retreat The map in Figure 7.5 shows the barrier island complex located along the coast of Ocean

City, Maryland. Note the solid line that shows just a single barrier island in 1933. The Ocean City Inlet seen today formed when the island was over-washed during a major hurricane in 1933. As the storm surge flowed back out to sea, it eroded a channel that cut the island in two. The island to the north is now called Fenwick and the one to south is called Assateague. Because the newly formed inlet gave people on the mainland valuable access to the open ocean, the U.S. Army Corps of Engineers constructed a pair of jetties on each side of the inlet. With the jetties in place, a navigation channel could be kept open. In a relatively short period of time sand filled in behind the northern jetty on Fenwick Island. Meanwhile, the southern island, Assateague, began to migrate toward the mainland. Note that the southern jetty, labeled “seawall” on the map, now sits in open water.

14) In what compass direction is the longshore current along this part of the coastline? Explain

how you know (hint: look at the width of the beach in front of the islands). 15) Describe the general process by which barrier islands actually retreat (i.e., migrate) toward

the mainland. 16) Note on the map that the position of Fenwick Island has changed very little since the 1933

hurricane, whereas Assateague Island has retreated landward a significant distance. Describe the basic reason why Assateague Island is experiencing rapid retreat while Fenwick Island appears to be rather stable.



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Figure 7.5 - (USGS Ocean City, MD, Quadrangle)

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17) Because the map in Figure 7.5 was last updated (photo-revised) in 1972, this means that we know the position of the islands in both 1933 and 1972. For points X and Y, determine the number of feet that Assateague Island has retreated landward from 1933 to 1972.

distance between X and X'

distance between Y and Y' 18) First calculate the retreat rate (i.e., velocity) in feet per year at both points X and Y, and then

average your results. Remember that velocity equals distance divided by time.

retreat rate at X:

retreat rate at Y:

average retreat rate: 19) Using the average rate of retreat, predict the number of years it will take for the landward

side of Assateague Island at point Z to reach the shore at point Z'. Again, use the formula of velocity equals distance over time.

distance between Z and Z'

travel time at Z: 20) Based on the number of years you calculated above, determine what year (e.g., 1999)

Assateague Island should have reached the mainland, sealing off Sinepuxent Bay. 21) If you traveled to Ocean City today, you would see that Assateague Island has not yet

sealed off the bay. Explain why your prediction was off.


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Part II – Sea-Level Rise and Global Warming As described more thoroughly in Chapter 16 of the textbook, Earth's climate system is very

complex and has changed continuously over geologic time. As average global temperature changes, so too does the amount of glacial ice being stored on the planet's landmasses. During cool (glacial) periods, sea level falls as additional water is removed from the oceans and stored as glacial ice. Sea level then rises when the climate enters a warm (interglacial) period and the ice begins to melt. As shown in Figure 7.6, around 20,000 years ago during the depths of the last ice age, sea level was approximately 460 feet (140 m) lower than today. Of course, whenever sea level changes, so too does the position of the shoreline. Shorelines naturally advance (shift seaward) when sea level falls, then retreat (shift landward) as sea level rises.

Figure 7.6 - Graph showing dramatic changes in sea level during the previous glacial and

interglacial periods. (Data from K. Lambeck and J. Chappell, 2002, and K. Lambeck, Y. Yokoyama, and A. Purcell, 2002)

140 120 100 80 60 40 20 0Thousands of years before present

-500

-400

-300

-200

-100

0

100

Sea

leve

l com

pare

d to

pre

sent

(feet

)

-150

-100

-50

0

(met

ers)

todaypreviousglacial

previousinterglacial

Because climate change is not unusual in terms of Earth history, there is nothing really

special about the position of today's shoreline which humans have grown accustomed to. Although the climate system has been relatively stable for the past 10,000 years, sea level has continued to slowly rise as the climate moved from an ice age to the current interglacial period. For example, the amount of sea-level rise from 1900 to 2000 was only 0.6 feet (0.2 m). One of the major concerns about global warming today is that sea-level rise is accelerating due to the rapid melting of glacial ice and thermal expansion of the oceans. If all of the remaining ice were to melt, sea level would rise an additional 260 feet (80 m). Currently, the worst-case scenario predicted by climate scientists is a 33-foot (10 m) rise over the course of a few centuries. Were this to occur, major population centers around the world would be inundated, creating a human catastrophe of unprecedented portions.



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In order to get a better sense of the potential problems associated with accelerated sea-level rise, we will examine the projected shoreline changes for southern Florida shown in Figure 7.7. Note that the changes here represent the worst-case scenario of a 33-foot (10 m) rise in sea level over a several hundred year period. 22) With the aid of a Florida road map, plot the location of the following cities on Figure 7.7B.

Miami Fort Myers Tampa Cape Canaveral

23) Supposing that the sea-level rise would be slow enough so that people and businesses

could be moved inland, describe some of the problems that such a move would entail. 24) Under the worst-case scenario cities around the world would be facing the same problems

you described. How would the fact that this would be a world-wide problem affect society's ability to adjust to a major rise in sea level?

Figure 7.7 –Relief map of southern Florida showing the shoreline change associated with a 33-

foot (10 m) sea-level rise. (Courtesy of NASA) A) B)

exercise 7 coastal processes and...

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