Geology Lab

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GEOL110L_Lab8_CoastalProcesses2.pdf

Coastal Processes

LABORATORY 8

Name: Section: Date:

Part 1 - Wave Characteristics

1. The coast is a unique boundary where three spheres of the earth meet. What are these three spheres?

2. Wind blowing across a body of water pushes the water surface into waves because of _________________ between the moving air (atmosphere) and water (hydrosphere) causes wa- ter to move.

3. The height, length, and period of a wave depends on the combination of three factors. List the three factors:

4. Review: Draw and label the characteristics of an idealized wave. Include: crest, trough, wave height, wavelength, and wave base.

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5. When waves approach shore, they are said to “feel the bottom”. The depth at which circular motion becomes negligible, or flattens out, is called the wave base depth. This depth is calcu- lated using the equation below:

Calculate the wave base depth for a wave with a wavelength of 60 meters and a wave height of 6 meters.

wave base depth = 1/2 X wave length

Figure 1. Evolution of a coastal wave.

When deep-water waves approach shore, the water depth decreases and the wave base starts to intersect the sea floor. At this point, the wave starts to change. Figure 1 below depicts these changes to the waves.

6. Match the descriptions below to a letter on the diagram above.

____ Wave base

____ Deep-water waves with constant wavelength

____ Velocity decreases, wave height increases, wavelength decreases (wave eventually crash- es) ____ Approaching shore waves begin to touch bottom (wavelength decreases)

____ Surf zone

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Exercise 7. Types of breakers.

Match the descriptions below to one of the three main types of breakers: surging breaker, spilling breaker, and plunging breaker. Then, indicate the figure (a, b, or c) which corresponds with each of these types of breakers.

Has a curling crest that moves over an air pocket and breaks with a sudden loss of energy. They form on moderately steep beach slopes and are the best waves for surfing:

Forms on the steepest shorelines, causing wave energy to compress very suddenly right at the shoreline and break onto the beach:

Form on gently sloping beaches, where the energy of the wave is dissipated gradually. The wave slowly increases in height, then slowly collapses on itself:

a b

c

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Figure 2. Wave refraction along a straight shoreline.

The slowing and bending of waves in shallow water is called wave refraction.

As deep-ocean waves approach the shore, the portion of the wave in the shallowest water will “feel the bottom” and its wave velocity will slow down. The deeper-water portion of the wave crest keeps moving at a relatively rapid speed. The higher speed of the deep water wave caus- es the entire wave crest to progressively rotate toward the shoreline and eventually the wave crest becomes parallel to the shoreline (Figure 2).

8. On Figure 3 below, draw a “D” at area of deposition indicating the areas of low energy and will bedominated by depositio and an “E” at area of erosion indicating the areas of high ener- gy and will be dominated by erosion.

9. The red arrows represent the paths of incoming waves. As the waves reach shallow water they bend and tend to line up parallel to the shore. What is this called?

10. Wave refraction affects the distribution of ______________ along the shore, influencing where and to what degree _________________ , sediment transport, and ______________ will occur.

Figure 3

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Exercise 11. Rip currents.

At times, incoming waves can pile large volumes of water on the beach--much larger than nor- mal. The only way for the water to return back into the deeper waters is to ebb back in a self- made channel through the surf zone, creating a rip current. This flow is often incorrectly called a “rip tide”. The rip current flows back out perpendicular to the shore.

On Figure 4 below, please label the surf zone. Draw arrows and label incoming deep-water waves. Circle crashing waves. Draw and label arrows indicating direction of water flow in the rip current.

Figure 4

If you are caught in a rip current, what should you do?

Part 2 - Currents

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Longshore current is the net motion of water parallel to the beach as a result of the zigzag movement of water due to run-up and backwash within the swash zone along a shore. Swim- mers can be inadvertently carried by longshore current and eventually find themselves car- ried far from where they initially entered the water.

Longshore drift (beach drift) is the movement of sediment in a zigzag fashion caused by the longshore current. The sediments are carried up the beach by swash and back down by backwash, moving large amounts of beach sand downshore in the direction of the longshore current. By various processes, rivers, longshore currents and longshore drift, water and sed- iments are carried from an upshore region to a downshore region. As a result the beach is considered a “river of sand”. Rivers transport sediments from their source (mountains) to the coast via channelized water flowing down hill under the force of gravity. Once reaching the river’s mouth the sediments are then transported along the beach via the longshore drift in a zigzag motion.

12. Figure 5 shows incoming waves and the impact waves have on the erosional process and movement of sediments. The result of this interaction is longshore drift and longshore cur- rent. Match each letter on the figure with the correct step of the process listed below.

Deep water ____ Incoming deep ocean waves ____ Longshore drift ____

The beach ____ Area of wave refraction ____ Longshore current ____

Shallow water ____ Beach drift ____

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Part 3: Coastal Landforms

The Pacific Coast is made up of tectonically active margins, with landforms such as sea cliffs and marine terraces. These landforms provide much of the material found along the beach- es. Sea cliffs are undercut by waves creating sea caves and arches at the cliff’s base, until eventually the overhanging part of the cliff collapses. Wave action, combined with the fallen material, shapes and smoothes a wave-cut platform. People choose to build houses on top of these cliff-tops for sea views, not realizing the potential hazards of collapse these landforms present.

13. Figure 6 below is an overview of a coast. Next to each feature listed below, please write the correct letter that matches the image:

Wave-cut cliff ____ Wave-cut platform _____ Marine terraces ____

Sea arch ____ Sea stack ____

Part 4: Human Mitigation

Exercise 14. Hard stabilization.

Hard stabilization involves processes of preventing coastal erosion to provide protection for property along the coast. Structures built to protect coastal communities from erosion commonly contribute to further erosion by altering wave energy and sediment movement, making communities vulnerable to hazards. These structures can trap sand, reducing the sand migration along the beach causing heavy erosion on the down-current side of the barrier. Instead of eliminating erosion, the structures relocate the area of erosion. The building of groins and jetties prevents the continual supply of sand by rapping sand on the up-current side of the structure and thus eroding away of the beach down-current.

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Figure 7. Hard stabilization cont.

Figure 7 below is an aerial view of Santa Barbara Harbor, CA. Match the correct letter (A-F) to each of the following descriptions. Note that one letter (D or E) will not be used.

Breakwater ____ Direction of longshore drift (D or E?) ____

Area of wave refraction ____ Area threatened by erosion after construction of the breakwater ____ Area of sand accumulation ____

More human mitigation techniques. Soft stabilization.

Soft stabilization (beach nourishment) is primarily accomplished by pumping sediment onto the shoreline/beach from some other area, temporarily replenishing the sediment supply. Common sources of sand used for beach replenishment includes onshore beach areas of heavy sand accumulation or dredging sand and silt that has accumulated along the ocean floor in a region that is deeper than the wave base. Sometimes sand is collected from the seafloor and transported to the depleted beach areas on large barges. Sand is suction-pumped from the source and pumped through huge pipes as a slurry of sand and water. As sand is being dumped onto desired area, it is spread across the beach using heavy earth moving equipment. One dis- advantage of this technique is that it can be very expensive.

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15. Can you think of how soft stabilization may affect the natural cycles of the coastal envi- ronment?

16. Figure 8 below is a familiar view to coastal California. These houses are sensitive to coastal processes which put them at risk. Propose three ways risk to these houses could be reduced:

Figure 8