Erosion and Deposition at the Shore

Scenario/Summary:

In this lab, we’ll look in depth at how erosion and deposition affects modern shorelines.

Deliverables:

Read through the lab instructions document below before executing the lab steps and creating the report. Refer to Owen, Earth Lab: Exploring the Earth Sciences, ISBN-13: 9780538737005, Lab 14: Shorelines and Oceans, pages 320–327. Follow all procedures in the lab instructions for the items you will need to include in your report. After executing all steps contained in the lab instructions, submit a single Word document containing your report to the Week 5 Dropbox.

Grading Rubric

 Category

Points

Description

Questions 1a and 1c [See Figure 14.8: Development of Spits, Bay-Mouth Bars, and Tombolos, page 323.]

5

The answers are complete and accurate.  Interpretations of the results are included and correct.

Questions 2a–2g [See Figure 14.9: Barrier Islands, page 324]

10

The answers are complete and accurate.  Interpretations of the results are included and correct.

Questions 3a–3j [See Figure 14.10: Beach Protection Strategies, page 326]

15

The answers are complete and accurate.  Interpretations of the results are included and correct.

 Total

30

A laboratory report will meet or exceed all of the above requirements.

iLab Steps

Step 1: Longshore Drift

Waves usually wash up the beach at an angle to the shoreline then run directly down the slope of the beach. Since the slope of the beach is perpendicular to the beach, the water does not wash back exactly where it washed up the beach. This continues repeatedly, causing water to move along the shore, generating the longshore current (see Figure 14.8a: Development of Spits, Bay-Mouth Bars, and Tombolos, page 323.) Each wave also carries some sediment along the shore. The movement of sediment parallel to the shore is the longshore drift or beach drift.

At the mouths of bays or estuaries, longshore drift tends to deposit sediment across the opening, creating spits, which go part way across the bay mouth, or bay-mouth bars. Deposition occurs where an island close to shore blocks the wave action, creating a sandy connection between the island and shore called a tombolo. A breakwater parallel to the shore can act in a similar way and accumulate sand behind it.

Use Figure 14.8: Development of Spits, Bay-Mouth Bars, and Tombolos, page 323 to answer the following questions.

Study Figure 14.8b. Look for features that might tell you the direction of the longshore current and drift.

Figure 14.8b

Depositional features along an imaginary coastline. Tombolos form behind islands where sand settles from the island blocking wave action.

Describe on Figure 14.8b the location and direction of the longshore drift and the probable travel direction of predominant waves.  Explain your reasoning and list various pieces of evidence for the longshore drift and wave directions. Explain why the tombolo has the orientation it has.

Step 2: Barrier Islands

Sandbars above sea level that are completely separate from land but are near and parallel to the mainland are barrier islands. Many of these long islands form when waves breach a long spit or when rising sea level turns wind-blown dunes, normally high up above the shoreline, into an island. Barrier islands typically migrate shoreward with time. One piece of evidence for this is that we see peat (deposited in the lagoon behind the island) at the ocean side of the island (Figure 14.9 a–c.).

Barrier island migration is so rapid that it is historically detectable, as with Assateague Island in Maryland (Figure 14.9d). Despite the natural migration of barrier islands, people commonly build on them. Not surprisingly, the beaches in front of resort hotels and other buildings on these islands are likely to erode, and then the buildings come under wave attack during storm surges (high water levels during storms), hurricanes, and cyclones.

Figure 14-9

Barrier Islands. (a–c) Sequence showing sea-level rise and migration of a barrier island with a typical peat bed (lagoon sediments) beneath it. (a) Low sea level.  (b) Sea level rises. (c) A barrier island showing how a peat bed can eventually underlie the whole island after it has migrated landward over the former location of the lagoon. (d) Migration of Assateague Island, Maryland. The island is retreating more rapidly than Fenwick Island (north) because of the jetties (black) at Ocean City Inlet. (g) 1853 and (h) 1978 coastal subsidence and sea-level rise combine to change Isle Derniere and the various bays surrounding it in the Gulf of Mexico, Louisiana. (a-c) From MONROE/WICANDER/HAZLETT, Physical Geology, 6E. 2007 Brooks/Cole, a part of Cengage Learning, Inc. Reproduced by permission. www.cengage.com/permissions

Use Figure 14.9: Barrier Islands, page 324 to answer the following questions.

Explain in your own words how the peat bed (labeled lagoon sediments in Figure 14.9 a–c) could be present under the entire barrier island in (c) when the peat only forms in the lagoon behind the island.  Toward what direction has Assateague Island (Figure 14.9d) migrated?  Which part of Assateague Island (Figure 14.9d) has migrated the most?

If any buildings had been built on the north end of Assateague in 1849, could those buildings still be standing today? If so, where; if not, why not?

What is the fastest rate of retreat of Assateague Island in meters per year? Show how to do the calculations.  At this rate of retreat, how long would it take for the shoreline to erode back to a house built 200 ft. from the shore (1m = 3.28 ft.)? Show how to do the calculations.   How much time passed between the two maps in (g) and (h)? What happened to Isle Derniere in this time?  How have the bays changed?

Step 3: Beach Erosion Control

Attempts to control beach erosion takes advantage of longshore drift. A groin (Figure 14.10b) built perpendicular to the shore acts like a dam to the longshore drift so the sand builds up on the up-current side. However, this construction also tends to cause erosion on the down-current side because the groin has cut off the supply of sand. A breakwater (Figure 14.10c) built parallel to the shore helps sand deposition between it and the shore in a similar manner to a tombolo. For any structure added to a beach, an area of deposition will also cause an area of erosion where the structure interrupts the flow of sand. Another way to keep a shoreline from losing its beach is to import sand, called beach nourishment (Figure 14.10e and f).

Figure 14.10

Beach Protection Strategies. Groins, perpendicular to the shore, slow beach erosion. (a) A beach with oblique waves. (b) The same beach with an older groin at the right showing resulting deposition and erosion adjacent to the groin, and a newly built groin at the left. (c) A breakwater built parallel to the shore. Sand collects behind the breakwater somewhat like a tombolo. (d) Erosion control structures at shoreline at Norfolk, Virginia. (e, f) Built-up barrier islands of Miami Beach, Florida before beach replenishment. (f) The same location after beach replenishment.

Use Figure 14.10: Beach Protection Strategies, page 326 to answer the following questions.

Figure 14.10

Describe the direction of the longshore current in Figure 14.10a.  Describe on Figure 14.10b how the distribution of sand around the new groin on the left will change after a period of longshore drift, deposition, and erosion. Where is deposition and where is erosion?  Describe where deposition and erosion have occurred in Figure 14.10c. Explain the distribution of sand around the breakwater. Why has sand built up where it has, and why has it eroded elsewhere?  What structures attempt to control beach erosion in Figure 14.10d? [Laura: d & e could be deleted if this is getting too long]  Based on the pattern of erosion and deposition, describe the direction of longshore drift in Figure 14.10d.  Use evidence from these figures and describe some of the problems with erosion control structures.   Which are the before and after pictures in Figures 14.10e and f (read the caption)? Could the photo sequence be reversed in time? Explain.

Now look back at the map of Assateague Island in Figure 14.9d. The two jetties (two thick, black lines) at Ocean City Inlet were built for ship navigation. They act somewhat like groins at this location.

Describe the direction of the longshore current direction here. What evidence supports your drawing in part a? What are the consequences to local beaches and barrier islands of building structures like this?

Step 4: Write and Submit the Lab Report

Follow the directions above to write your lab report. Submit your lab report using the Week 5 Lab Report template as Word document to the Dropbox. Name this week’s lab report “iLab Week 5 Report Your Name.

 

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