Erosion - Teacher Guide: Lesson Plans
Erosion...can you fight it?
How much energy is involved with waves and erosion?
Can humans stop erosion of the shoreline? Should we?
Is it cost effective?
Students will be able to:
Computers with Internet access
Optional: stream tables, water, sand, piece of rock, masonry,
wood, steel, and concrete
Several engineering solutions are used for coastal protection and beach restoration. In the past, construction of hard structures such as groins and sea walls have been used with varying degrees of success. Some solutions have been successful, some require expensive ongoing maintenance and others have caused even more problems. Another method of coastal protection is to artificially nourish the beaches by transporting sand from another location to restore a beach. Once again, this method has varying levels of success.
Groins, usually made of timber, rock or concrete, are built perpendicular to a beach and into the water to trap sand. On beaches where waves arrive at right angles to the shore, a series of groins can trap sand, creating a series of small beaches. On beaches where waves arrive at an angle to the shore and the beach is affected by longshore drifting, a different situation arises. As the water and sand move by, the first groin will trap sand meanwhile starving the beach of sand (and other groin) further along the shore. Groins can be designed to allow some sand to spill around the structure and minimize downstream erosion.
Groins are not successful in all circumstances contributing to further erosion. A careful analysis of wave approach and currents should precede any decision to install a groin, and the structures should be carefully designed for the specific location.
Seawalls may be constructed of timber, rock, steel or concrete and are placed at the back of a beach. Although seawalls can protect the land directly behind, they can also accelerate erosion at the end of the wall and/or cause erosion of the beach in front of the seawall. When waves hit the wall and retreat, the wave action scours sand from the beach back into the water. Ultimately, the beach becomes lower and flatter, creating a condition where waves become larger, which increases the scouring action and the beach is eventually lost.
Artificial beach nourishment (replenishment) is the depositing of sand from elsewhere to replenish eroded beaches. Sand may be trucked in or dredged and pumped from offshore. But it is not as easy as it sounds. The nourishing sand must be as coarse as the sand that is currently on the beach. If the nourishing sand is of a finer grain, the sand will be easily swept away by normal wave action. On beaches where sand has been lost through longshore drifting it is likely that nourishing sand will also be lost. Sometimes in this scenario, groins are constructed to trap drifting sand.
It is important to remember that coastal erosion is a natural process and does not always have a negative outcome. It is the natural erosion process which gets sand on beaches in the first place, but if interference occurs with natural erosion and deposition patterns, undesired outcomes requiring further action can occur.
The decision to take action is the responsibility of coastal managers. Coastal Managers often face difficult decisions involving roads and buildings that are in danger of damage or destruction. In some cases, the preferred long term solution may be to relocate or abandon structures instead of fighting a losing battle with the sea. However, this is usually impractical due to the investment value of coastal properties and financial benefit of coastal tourism.
If the decision is made to construct coastal protection structures, the Army Corps of Engineers usually becomes involved. During the process, the Corps determines the amount of wave energy is unleashed on the beach, then searches for the most economical, environmentally sound and socially acceptable solutions. In some cases, this will involve hard structures or in many other cases, the preferable approach is beach nourishment.
Corps shore protection projects are usually cost-shared with the State, the local jurisdiction where the project is located, or both.
In this lesson, students will work in cooperative groups as engineering teams charged with creating a coastal protection solution.
Your engineering team has been charged to submit a bid for a design for a 600 meter seawall to protect a major coastal highway. Your team must design the wall right at the edge of the water. The structure must be able to withstand the impact of the ocean waves. You cannot spend any more money on the project than is necessary, so it is crucial that the team know what materials can be used in construction and how much each material will cost. It is also important to know that there will be no funding available for beach nourishment (replenishment) in the future. Your team will have to give a 10 minute presentation on the seawall design and submit the bid to the Project Manager (teacher).
1. To determine the amount of wave energy, use an equation to calculate the amount of energy based on the height of a wave. First, determine the amount of energy for every square meter of wave, the energy (joules) is equal to 1260.6 times the square of the wave height.
Wave Energy = 1260.6 (Wave Height)2
2. To determine the Total Energy in a wave, calculate the total surface area of the wave and multiply that by the wave energy.
Total Energy = Wave Energy (surface area of wave)
For example, calculate the energy for an average open water wave that is 2 meters high, 7 meters wide and 500 meters long:
|Wave Energy = 1260.6 (Wave Height)2
Wave Energy = 1260.6 (2)m2
Wave Energy = 1260.6 (4)m2
Wave Energy = 5042.4 Joules/m2
Total Energy = Wave Energy (surface area of wave)
Total Energy = Wave Energy (7 meters x 500 meters)
Total Energy = 5042.4 Joules/m2 (3,500m2)
Total Energy = 17,648,400 Joules or 1.76484 x 107 Joules
3. For this activity, the waves will be 8 meters wide, and the section of the seawall that the waves will hit is 300 meters long. Determine the highest water height for this month:
4. Calculate the Total Energy of the wave.
5. Using the table of materials below, your team must design a wall to withstand the wave energy calculated above.
|Material||Strength||Cost/cubic meter||Amount needed||Total Cost|
|Natural Rock||30 million joules||$50/cubic meter||900 cubic meters|
|Masonry||40 million joules||$150/cubic meter||300 cubic meters|
|Wood||4 million joules||$25/cubic meter||2000 cubic meters|
|Steel||90 million joules||$225/cubic meter||300 cubic meters|
|Concrete||50 million joules||$180/cubic meter||800 cubic meters|
Note: The Strength represents how much energy the material can absorb PER WAVE before it structurally fails. The Amount Needed column represents how much material needed to supply the stated strength. For example, a wall of 2,000 cubic meters of wood can absorb a maximum of 4 million joules from each wave that hits it.
6. One of the highest waves in recorded history for this site was 5 meters high. This wave occurred during an exceptionally large storm. Would this information change your design? If so, explain.
7. The following links may be of assistance for research:
8. Using all of this information, create a bid for a design for the seawall project described in the Problem Statement.
Your team must create a 10 minute presentation on the seawall design and submit the bid to the Project Manager (teacher).
When preparing your project, your group might also want to consider if the project will be cost effective, possible alternatives, tourism dollars, etc.
Any mix of materials is allowable, but remember that your bid and presentation will be judged according to:
Project presentations given by the groups.
This lesson has been adapted from a Brookdale Community College lesson developed by Robert Macaluso.