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Purpose: To learn about waves, how waves act to modify shorelines, and how humans try to stabilize this dynamic environment for development.
Objectives: In this exercise you will:
1) Learn the differences between deep- versus shallow water waves.
2) Learn about wave refraction and longshore drift.
3) Learn to recognize different coastal geomorphic features.
4) Learn to recognize and understand different means used to stabilize shorelines.
INTRODUCTION
INTRODUCTION
Although shorelines are some of the most dynamic environments on the planet, constantly changing, humans continue to populate the world’s coasts for commerce, food extraction, and recreation. Shorelines are constantly changing shape as coastal sediments are eroded, transported and deposited due to natural processes such as:
(1) wave energy
(2) tides
(3) changes in sea level
These processes are exacerbated by human activities. On the other hand, individuals and communities often go to great lengths – expense and engineering wise – to preserve real estate and structures along shorelines. On the other hand, they also destroy them at an even faster rate by building too close to the shore, exploiting resources including sand and continuing to engage in activities like burning fossil fuels that lead to sea level rise due to climate change.
In this lab, we will specifically focus on how waves form and how they shape the shore, including some landforms that are created due to wave action and engineered structures built to protect shores.
Fig. 5.1. Sandbags are used to stop erosion in Ocean Isle, NC where the coastal erosion rate due to sea level rise is amongst the highest in the country.
WAVES
WAVES
Even if you have never surfed a day in your life, you have probably been to a beach (or a big lake) and noticed the sometimes small ripples, other times massive and powerful waves sweep across the surface of the water. In order to understand wave processes, let's go over some jargon first so you can learn what the different parts of a wave are called and its main characteristics (Fig. 5.2a)
Figure 5.2a Parts of a wave. Notice that waves cause particles to move below the wave that you can actually see.
KEY TERMS:
• Crest ~ highest point of a wave.
• Trough ~ lowest point of a wave.
• Wavelength (λ) ~ distance between two crests or two troughs.
• Wave amplitude ~ distance between a crest or trough and a line of zero displacement (the rest position).
• Wave height ~ distance between crest and trough (Note: height = 2x amplitude)
• Wave base ~ “bottom of wave motion”. Wave base = 0.5 x wavelength. As a wave passes through the water, it induces water movement along a circular orbital whose diameter is equal to wave height. The size of the orbital dissipates with depth to wave base, at which point the water no longer feels the effect of the passing wave.
• Frequency ~ the number of times a wavelength passes a point in a given amount of time.
• Period ~ the time a wave takes to complete a wavelength.
Even though waves can be caused by tides and things like tsunamis, the most common waves in oceans and lakes are generated by the wind. There are two types of wind-generated waves, categorized as deep- or shallow-water waves.
Deep-water waves travel in water that is deeper that their wave base (ie. deeper than half of their wavelength)
Shallow water waves travel in water that is shallower than one twentieth of their wavelength (Fig. 5.2b).
In summary (keep in mind that D= water depth and λ= wavelength)
Deep water waves (D> λ/2)
Shallow water waves (D< λ/20)
Figure 5.2b Deep-water waves transition to shallow water waves when wave base intersects the ocean floor.
If you haven't watched the Intro video for this lab, please do so now, it will really help you understand the properties of waves as you read through the text and answer the questions.
DEEP WATER WAVES & FULLY DEVELOPED SEAS
DEEP WATER WAVES & FULLY DEVELOPED SEAS
Waves form as wind energy is transferred from the atmosphere to the water’s surface. The size of a deep water wave developing in the open sea depends on specific wind conditions including:
wind speed,
fetch – the open water distance across which the wind blows, and
wind duration, or how long the wind blows.
A fully developed sea forms as waves reach the maximum height possible for a certain wind speed and can only form when the wind blows long enough at the particular wind speed and over a large enough of an open area (Fig. 5.3). Table 5.1 summarizes wind conditions and wave characteristics for fully developed seas at different wind speeds.
Figure 5.3 A fully developed sea has attained maximum wave heights dependant upon wind speed, fetch, and duration. As a wave becomes more developed, it becomes more rounded, longer, and travels faster; up to speeds almost as high as that of the wind that produces the wave. Image from http://lighthouse.tamucc.edu
At this point, open the Lab 5 Answersheet on a separate window so you can answer the questions as they come up.
Q1. What is a "fully developed sea"?
Q2. List the three factors that determine the size of a deep water wave at sea:
___________________, _________________ and ______________________
For questions 3-5 below, you do not need to upload anything, but copy the Table 5.1 into your notes and do the calculations for wind speed and velocity, so you can complete the missing information. You can copy the table by hand, or print it out in the "Extras" section of this lab if it's easier.
Table 5.1 Conditions necessary for a fully developed sea at given wind speeds and the parameters of the resulting waves.
Q3. Calculate and record the wind speed in miles per hour for Table 5.1. Note: 1 knot = 1 nautical mile per hour = 1.15 mph. What are 10, 20 and 30 knots of wind speed equivalent to in mph?
Q4. Wave velocity.
a) Complete Table 5.1, by calculating the velocity of each wave resulting from the selected wind speeds in a fully developed sea. (Recall: velocity = distance/time or wavelength/wave period. Wave period is the time it takes one wave to pass a stationary object.) -See intro video for tips-
Based on the information in Table 5.1, what are the wave velocities for wavelengths of 28, 111, and 251 ft?
b) Do deep-water waves velocities increase, decrease, or remain constant with increasing wavelength and increasing wave period? ________________________
Q5. Based on information in Table 5.1, how do wave height and length change based on changes in fetch, wind speed and wind duration?
...as fetch, wind speed, and wind duration increase - the wave height and length decrease
...as fetch, wind speed, and wind duration increase - the wave height and length increase
...as fetch, wind speed, and wind duration increase - the wave height and length go to zero
SHALLOW WATER WAVES
SHALLOW WATER WAVES
Shallow-water waves form as deep-water waves transition to shallower water and the wave base begins to “feel” the bottom of the ocean floor (i.e. original wave base is greater than the depth to the ocean floor) (look at Fig. 5.2b which is copied again below). Although the top of a shallow water wave will continue to move forward at a velocity determined by wind conditions, the bottom of the wave will slow due to friction or drag along the ocean floor. This causes the top of the wave to compress resulting in an increase in the wave height and decrease in the wavelength (Fig. 5.2). Shallow water waves eventually form breaking waves when the height of the wave is 0.8 times the depth to the ocean floor.
This short video shows a visualization of the waves breaking as they reach shallower water. This is a result of the water closer to the bottom slowing down due to friction with the sediment, while that closer to the surface moves faster and "overtakes" the bottom layer (which is the breaking that you see from the shore).
Because the wave base is not as deep as the ocean, the speed of deep water waves is not affected by ocean depth, but shallow waves ARE affected by depth. As waves enter shallow water, their interaction with the bottom causes them to slow down.
The velocity of shallow-water waves is purely a function of water depth since the bottom of the wave “drags” on the ocean floor, causing it to slow down. The shallower the water, the faster the top of the wave moves in relation to its bottom part.
Figure 5.2b Deep-water waves transition to shallow water waves when wave base intersects the ocean floor.
** So, in shallow water, wave speed decreases, wavelength shortens and wave height increases. **
The velocity of a shallow-water wave (Vs) is given by the following equation:
Vs = √dg
g = acceleration due to gravity 9.8 m/s2 and
d = water depth.
We will work the equation in terms of centimeters, so can factor out the square root of the gravity term (980 cm/s2) and the equation simplifies to…
Vs = 31√d
Vs = 31√d
Q6. Why does wave velocity decrease with decreasing water depth?
Q7. Velocity of deepwater waves is affected by wind speed, wind duration and fetch. What factor is the most influential on the velocity of shallow water waves? _____________________
Q8. a) Using the equation above for Vs, calculate the velocity of a wave in 100 cm of water. Make sure to include right units! ___________________
b) Complete the "Wave Tank Experiment" by following the procedure and filling in the table of results. Plot the graph as described on the sheet and upload it as "Last Name_WavesLab5".
TSUNAMIS ARE SHALLOW WATER WAVES
TSUNAMIS ARE SHALLOW WATER WAVES
Shallow-water waves do not ONLY occur at the shore, tsunamis (usually caused by submarine earthquakes) are examples of shallow water waves even if they occur out in the deep ocean. This is because even the deep ocean is shallow compared to a wave with a huge wavelength like a tsunami (sometimes a wavelength of up to 310 miles!).
WAVE REFRACTION
WAVE REFRACTION
You might have noticed that waves often "bend" as they arrive at the shore, they come in sideways rather than perpendicular to the shore. This is called wave refraction.
Wave refraction occurs when waves bend as the wave part in shallow water slows relative to the wave part in deeper water. The effects of wave refraction are clearly illustrated if you examine the spacing between wave orthogonals (lines perpendicular to the wave crest), which show wave energy concentrated at headlands and dispersed in embayments (Fig. 5.5).
Erosion is expected on high-energy beaches where the orthogonals get closer together and energy is concentrated.
Deposition is expected on low-energy beaches where orthogonals spread apart and energy is more dispersed.
Overtime, the concentration of energy will erode headlands and work to “straighten out” irregular shorelines.
Figure 5.5 Wave refraction causes wave energy to be concentrated at headlands and dispersed in embayments. The wave energy coefficient can be calculated by comparing the distance between two orthogonals in deep water relative to the distance between the same two orthogonals in shallow water.
WHAT IS AN ORTHOGONAL?
It is just a line drawn perpendicular to another line. In this case, wave orthogonals are lines perpendicular to wave crests.
WHAT IS AN ORTHOGONAL?
It is just a line drawn perpendicular to another line. In this case, wave orthogonals are lines perpendicular to wave crests.
Figure 5.6 Image of Isla Vista, CA.
Q9. Waves on the image of Isla Vista in figure 5.6 are bending as they approach the coastline. This process is called _________________________.
Q10. Consider the velocity of a shallow water wave. Use the equation: Velocity = distance/time or wavelength/wave period. (Wave period is the time it takes one wave to pass a stationary object.)
Calculate the velocity (ft/sec) of the waves in this image given an average wavelength of 22 feet and a wave period of 7 seconds.
Shallow water wave velocity = __________
Q11. Assume the shallow water waves in the Isla Vista image form as deep-water waves wash in from fully developed seas. Do you expect the velocity of the shallow water waves to be greater than or less than their deep-water counterparts? What is the reason for your answer? _______________________
Q12. Given an average wavelength of 22 feet, at what depth will the waves begin to ‘feel’ the seafloor? EQUATION: wave base = wavelength/2.
Wave base = __________
Q13. Stronger wind conditions sometimes occur at Isla Vista and result in waves much larger than the average wavelength in the proceeding problems. The Office of Beach Management and Tourism would like to construct a pier off the shoreline that is safe from the highest breaking wave that could form off the Isla Vista coastline.
a) Given that a shallow water wave will break when its height is [~ 0.8 x (depth to the ocean floor)], what is the maximum height to which a wave could grow on a beach face at 20-foot depth?
b) Add the height you just determined to 20 feet (the depth of the seafloor) to determine how high you would need to construct a pier to be safe from the largest wave.
LONGSHORE DRIFT
LONGSHORE DRIFT
One natural process that modifies shorelines is longshore transport. Longshore transport refers to the component of flow (longshore current) and sediment (longshore drift) parallel to the shoreline and occurs as wind-generated waves approach the shoreline at an angle, then wash back out to sea perpendicular to the shoreline (Fig. 5.7). This results in overall net migration of sediment parallel to the shoreline in the direction of the approaching waves. Whether or not longshore drift causes a beach to grow or shrink in size depends on the location of the beachface (up- or down-drift) and the balance between erosion and deposition rates caused by longshore drift. If the erosion and deposition rates are balanced, the beach remain unchanged.
Figure 5.7 Longshore drift occurs as waves wash in at an angle to the beach face and wash back to sea perpendicular to the beach face. This results in net migration of sediment and current parallel to the shore face.
Watch in longshore drift in action in this videoclip. Shows the difference between swash and backwash really well.
COASTAL GEOMORPHOLOGY & ENGINEERED STRUCTURES
COASTAL GEOMORPHOLOGY & ENGINEERED STRUCTURES
Coastlines are ever-changing as they are subjected to wave energy (refraction and longshore drift), tidal energy and even changes in sea level. Geologists study the coastal geomorphology, or landforms and the processes that shape them, to better understand our coastal systems. This information is critical for understanding how man-made, engineered structures modify our shorelines, for better or worse (Figure 5.8).
Figure 5.8 Shorelines experiencing long shore transport of sediment are often modified by engineered structures. In this case, sand is moving to the right; groins halt the movement of sand and trap it on the updrift side of the structure. As a result, there is erosion on the downdrift section of the beach. From usgs.gov.
Q14. Which direction is the long shore drift in the image with 4 groins? Looking at the image...
left to right
right to left
top to bottom
Engineered Structures to Stabilize Coastlines
Engineered Structures to Stabilize Coastlines
Man-made structures help slow down the rate of shorelines erosion, this is called shoreline stabilizing. Some engineered structures that are used to stabilize shorelines to prevent further erosion are:
Breakwater- hard structure or rock pile in the water and parallel to the shoreline. Its purpose is to intercept wave energy. Often protects boat moorings and harbors. May connect to land.
Groin- hard structure or rock pile perpendicular to the shoreline.
Jetty- structure, usually a pair, to stabilize migrating inlets and/or river mouth and mitigate sediment in-filling.
Sea wall- hard structure or rock pile on land and parallel to the shoreline.
Geomorphic Coastal Features
Geomorphic Coastal Features
Many engineered structures for shoreline stabilizing are based on principles exhibited by natural, geomorphic coastal features (landforms). These include: barrier islands, cuspate spits, deltas, estuaries, lagoons, tombolos and tidal inlets.
Q16. Use the descriptions of each type of geomorphic coastal feature below, to identify the structures shown in pictures 1-7. Each definition matches one picture. Enter your answers in the answersheet form.
Figure 5.10. Examples of geomorphic features found on various coastlines.
Barrier Island- Massive sand ridges parallel to, but separated from the mainland by an intervening body of water (i.e. tidal marsh, sound, estuary).
Cuspate Spit- long, narrow sand ridge that is attached at one end but free on the other. Becomes “curved” or “cuspate” as sediment is deposited in front of a water body as a results of longshore drift.
Delta- a deposit of sediment that forms where a stream enters a standing body of water such as a lake or ocean. The name is derived from the Greek letter delta (D) because these deposits often have a triangular shape in map-view.
Estuary- a partially enclosed coastal body of water with one or more rivers or streams flowing into it and with a free connection to the open sea.
Lagoon- shallow body of brackish water with no or very restricted outlet to the ocean.
Tidal Inlet- channel or break in a barrier island or spit that connects an estuary to the open ocean.
Tombolo- rock island connected to land by a sandbar that forms as incoming, refracted waves sweep sand together behind both sides of the rock island.
Beach Nourishment in Miami, FL
Beach Nourishment in Miami, FL
Miami and the surrounding coastal communities have highly developed shorelines boosting multi-million dollar engineering projects and artificially nourished beaches. Figure 5.11 shows a before-and-after aerial view of the beach following a 1970’s nourishment project. Notice that erosion dominates the before photo with groins and pocket beaches lining the shoreline. Beach erosion at this location is caused by a lack of a sediment supply. The 18 million m3 of sand used to replenish the beach was dredged from offshore and pumped onto the beach. The after photo shows a wide recreational beach.
Q17. Tourism plays a major role in the economy of Miami Beach. With no beach – there would be no tourists. What other role do beaches play for communities like Miami? (Hint: think about late summer-fall major storms.)
Figure 5.11 Miami beach before and after beach nourishment.
YOU MADE IT to the END!!
YOU MADE IT to the END!!
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