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Activity: Current Observation Methods

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

Materials for Part A

  • Graph paper
  • Pencil

Materials for Part B

  • Plaster of Paris
  • Mixing bowl
  • Safety goggles
  • Dust mask
  • Dishwashing or exam gloves
  • Water
  • Spoon
  • Ice cube trays
  • Marine epoxy
  • Plastic tray (approx. 10 cm x 10 cm., size will depend on size of weights)
  • Nail and hammer
  • Dive weights (or other dense, flat, waterproof objects, like bricks)
  • Zip ties or line to tie weights to plastic backing
  • Scale
  • Table 3.3
  • Paper towels
  • Watch to keep time

Materials for Part C

  • Stopwatch
  • Data sheet
  • Current meter
  • Plant material
  • Transect tape (or other method of measuring long distances)


Safety Note: Be mindful of the risks of drowning when working near bodies of water. Follow all posted safety signs and check local ocean and weather conditions.


Safety Note: Plaster of Paris will become hot when mixed with water. Supervise mixing and handling of plaster to prevent burns. No bare skin should contact the newly mixed plaster. Wear eye protector, gloves, and a dust mask while handling plaster powder and marine epoxy. Work in well-ventilated areas when using epoxy.


Safety Note: Epoxy should only be used in a well-ventilated area.

A. Design your study protocol.

  1. If possible, visit the location of your water current research. Make observations about local wind patterns, tides, waves, weather trends, and coastal features. If you cannot visit your study site in advance, obtain this information from your instructor.
  2. Draw a map of your study site using graph paper and label key geographical features.
  3. Hypothesize where you will find the lowest water flow. Mark this location on your map.
  4. Where do you hypothesize that you will find the highest water flow? Mark this location on your map.

Clod cards are a simple way of measuring relative water flow by using blocks of plaster of Paris. As water flows over the blocks, they dissolve. The faster the flow, the more they dissolve, a process called dissolution. To compare the rates of water flow among different locations, scientists measure the amount of dissolution over time. If clod cards from one area weigh less than the cards from other areas after a certain amount of time, then that area has the higher rate of water flow.

  1. Design an experiment with the goal of determining where the water current is highest and lowest at your study site. Determine
    1. the number of clod cards needed (consider the importance of replication),
    2. where to place clod cards (e.g., 3 in slow area, 3 in fast area), and
    3. other environmental factors to take into account (e.g., tides and waves).
  2. Determine the amount of time clod cards will be deployed. In slow moving water, it may take days to start noticing dissolution. In fast moving water, the cards may dissolve in hours. It is generally best to leave clod cards in place at least two hours.

B. Make and deploy clod cards.

  1. Mix Plaster of Paris and water in a bowl until smooth. Make enough plaster to make the number of clod cards you will need according to your study protocol.
  2. Pour the plaster slurry into ice cube trays. Do not overfill the trays. Gently shake the trays side-to-side to release any air bubbles from the slurry.
  3. Allow the plaster to dry for at least two days before removing the clod cards from the tray (Fig. 3.17 A).

<p><strong>Fig. 3.17.</strong> (<strong>A</strong>) Clod cards removed from their ice cube tray mold</p><br />
<p><strong>Fig. 3.17.</strong>&nbsp;(<strong>B</strong>) Clod card attached to a plastic tray using epoxy adhesive. Note the four holes in the tray for securing a weight.</p><br />

  1. Punch holes in your plastic tray using a nail (Fig. 3.17 B). These holes will be used to attach the plastic tray to the weight. They should be large enough for a zip-tie to go through. The location of the holes will depend on the size of your weight.
  2. Epoxy the cards to the plastic tray (Fig. 3.17 B). Work outside or in a well-ventilated area. Closely follow the direction for the brand of epoxy you are using. Allow the epoxy to fully dry.
  3. Number and weigh each clod card with its epoxy and plastic tray. Record on your data sheet (see sample in Table 3.3).
  4. Secure the weights to the clod cards using zip-ties through the plastic tray holes (Fig. 3.18).

<p><strong>Fig. 3.18.</strong> (<strong>A</strong>) Clod card attachment using zip-ties</p><br />
<p><strong>Fig. 3.18.</strong>&nbsp;(<strong>B</strong>) Clod card, labeled #7, secured to a weight, and deployed in the intertidal</p><br />

  1. Deploy your clod cards in the field according to your study protocol.
    1. Record the number of the clod card and the location of the card in the field.
    2. Record the time of deployment.
  2. Retrieve clod cards from their field location after the amount of time you have specified in your protocol.
    1. Record the time of retrieval.
    2. Determine the amount of time the clod cards were in the field.
  3. Remove the clod cards from the weights. Rinse and clean all clod cards to remove sand and debris. Dry clod cards for at least two days.
  4. Weigh the cards and their attached epoxy and plastic trays. Calculate the difference between the start and end weights of the clod cards.
  5. Use graph paper or computer graphing software to explore the relationship between clod card dissolution (determined by weight loss) and their location of deployment in your study site. Make a map of the rate of dissolution of your clod cards in comparison to their location.

C. Collect additional data on currents

  1. Current Meter (Optional): If you have a current meter, you may deploy it at the same locations as your clod cards, and additional locations of interest, to further your physical description of the location. Record your findings.
  2. Floating Object: You may use local plant material, like palm fronds or branches, that will float to estimate long shore currents.
    1. Tie a collection of fronds or branches together to simulate a raft.
    2. Throw the fronds into the water, as far offshore as you can. Observe how the fronds move.
    3. Tie a new collection of fronds together and, based on your observations in part b, estimate current speed. For example, if your fronds moved quickly toward the west,
      1. Position your partner 50 to 100 meters from you toward the west and measure the distance.
      2. Start your stopwatch when the fronds hit the water and stop it when they pass your partner.
      3. The speed of the long shore current can be calculated by dividing the distance between you and your partner by the time it took the fronds to travel.
      4. Record your findings.
    4. If your fronds do not travel alongshore, record what you do observe.


Activity Questions: 
  1. Compare data collection methods.
    1. How do clod cards and floating objects differ in the way they measure currents?
    2. What problems did you experience when using clod cards to measure currents?
    3. What problems did you experience when using current meters and/or floating objects to measure currents?
    4. Which method would you use in future research projects? Explain your reasoning.
  2. Explain your results.
    1. Where was the current fastest?
    2. Where was the current slowest?
    3. How did your predictions compare to your actual observations?
    4. Suggest some explanations for these differences in local currents. Explain your reasoning.
  3. Explain your experimental replicates.
    1. What is meant by the term “replication” in scientific experiments? Define the term using your own words.
    2. What is the importance of replication?
    3. How was replication used in your experiment?
  4. What is the importance of water currents? Provide three real-world examples where knowledge of water currents would be useful.
  5. If you could repeat this activity, how would you change your experimental design?
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.