SEICHE WAVE MODEL FACTILIATION GUIDE

Transcription

1 SEICHE WAVE MODEL FACTILIATION GUIDE This project was brought to you by the LakeViz project and funded by the National Science Foundation

2 Table of Contents Overview and Background Learning Goals Materials Preparation and Procedure Credits

3 OVERVIEW This facilitation guide is intended for educators who plan to give demonstrations to public visitors and school-aged children who visit a museum or science center. Overview to Facilitation Guide and Background This guide includes instructions on how to introduce the activity, set up the demonstration, and facilitate visitor questions. In addition, this guide provides some background content about lake science, possible extensions to activities, and links to additional resources. BACKGROUND INFORMATION If you ve ever gone swimming in different lakes, you may have noticed some are warm and others are cold. Why is this? A lake s temperature is affected by its size and shape, the change of seasons, and the climate where the lake is found. We ll explore what happens under a lake s surface, including how changes in temperature and density lead to an important process called lake mixing. Density measures how heavy something is compared to how big it is. Denser water has more water molecules squeezed into a given amount of space. Denser water always sinks while less dense water rises. 2

4 Lake Stratification As the seasons change from winter to spring and then summer, lake water separates into different temperature layers from top to bottom. This is known as stratification. In summer, the Sun heats the lake s top layers, while the lake s bottom layers stay cold (warmest layer on top). Stratification happens because the warmer water at the surface has a lower density than the cool, deeper water and literally floats on top of it. The lake s top water layer, or epilimnion, is always less dense than the bottom water layer, or hypolimnion. The water layer in between them, the thermocline, is where water temperature changes a lot even though lake depth changes very little. Ice Formation When the winter air temperature gets very cold ice can form at the lake s surface, while warmer water stays below the ice. Why does that happen? Why doesn t the lake freeze all the way through, or from the bottom up? Water is an unusual substance when its temperature falls below 4 degrees Celsius (or 39.2 degrees Fahrenheit). As water cools below 4 ºC it starts to become less dense (lighter). And so as the surface of the lakes continues to lose heat and cool, the cold surface water now floats on top of the warmer 4 ºC water below it. When that surface water freezes (which occurs at 0 ºC), it stays floating on top of the water it is less dense than liquid water. That s why ice floats in a glass of water and on a lake s surface. This layer of ice acts as insulation on top of the 4 ºC water below it. 3 Lake Mixing Lake mixing is a progressive process. As we move from summer to fall, and then winter, the water at the very top cools, becomes denser and then mixes with the layer immediately underneath it. This will often continue until this mixing has reached the very bottom and lake overturn has occurred. Now water temperature and density are the same throughout the lake. Photo courtesy of Lake George Association

5 How Mixing and Stratification Affect Lake Life When a lake is stratified, algae in the surface layer can consume all the life giving nutrients, and then sink to the bottom of the lake. This leaves the surface layer starved of nutrients, and creates the conditions for harmful phytoplankton that can produce their own nutrients. When lake turnover occurs, it brings nutrients back to the surface, which increases growth of algae, a food source for many aquatic animals. Turnover also moves oxygen from the surface to deeper waters, supporting aquatic life throughout the lake. If lake mixing didn t bring oxygen to the organisms living in the bottom of the lake and nutrients to the organisms in the top of the lake, they might not be able to survive! Stratification affects where fish and other organisms live within the lake. In summer most living things, including fish, hang out in top layers of the lake where it s warmer and there is more sunlight and available food. In winter, fish tend to stay near the lake bottom where the temperature is warmer. Some even burrow into the lake s muddy bottom. Water s interesting physical properties make it possible for animals to survive the winter in a frozen lake. Because ice is less dense than liquid water, the ice stays at the top of the lake. The ice insulates the water below from the cold winter air this keeps the whole lake from freezing like a Popsicle, and the fish and other organisms from freezing in it! Density/Stratification Model This model can be used to explain lake physics or lake hydraulics, and can include topics like lake stratification, lake mixing, and seiche (internal) waves. This graph is helpful in explaining why the cold water sinks to the bottom of the model while the warm water goes to the top. It is also helpful in explaining why lakes like Lake Tahoe don t freeze during the winter. 4

6 Learning Goals This activity models density stratification, lake mixing, and seiche waves by showing how water moves in model deep lake cross-section, using containers with different shaped bottoms and plastic and rubber partitions to model waves in lakes. Visitors can also compare what happens in the various models, to see how lake shape can impact how water moves and flows. The key ideas that visitors should take away is that lakes vary in size, depth, shape and that physics control water movement within the lake and affect water quality by the distribution of dissolved substances, nutrients, microorganisms and plankton. Large-scale water motions include changing water levels, seiche waves, currents, waves and turbulence. Wind, solar radiation, stratification, and earth s rotation are important forces causing water movements of lakes. Photo courtesy of ECHO Lake Aquarium and Science Center 5

7 Detailed List: Model Tanks (plastic model with divider) - Flat Bottom (1) - Sloped Bottom (1) - Curved Bottom (1) Tank Partitions - Acrylic - for Flat Bottom Tank 9.5 x 3.25 (3) - Rubber (3) Fluorescent Light - 18 (1) Wooden Supports for Fluorescent Light (2) Thermometers (3) Suction cups for securing Thermometers (6) Rubber stoppers (3 in sizes: 00, 2, and 2) Drain hose assemblage (1) Stir sticks (3) Pitchers or thermos with at least 2 liter capacity (3) Various colors of Liquid Watercolor Paints or Food Coloring (1 set, preferred colors blue and yellow) Clear Plastic Tubing with at least 30 cm length (1) or Blow dryer Funnel (1) Coffee Urn with at least 30 cup capacity (1) Water Cooler with at least 2 gallon capacity (1) Cooler (1) Ice Straws to simulate wind Waste Bins - if a drain is not easily accessible Materials Simple List: Plastic model with plastic divider Hot water Ice cold water Yellow food coloring Blue food coloring Blow dryer or straws Waste water bucket if planning to do the demonstration multiple times For Extensions: Optional Materials: - Dry Erase Marker (1) - Container of Salt (1) Photo courtesy of Lindsay Chan 6

8 Photo courtesy of 3dh2o.org The plexiglass container, divided into two sections by a partition (rubber or plastic), is filled with colored water (red and blue or yellow and green). After the water has been added to each compartment, the partition between the containers is removed to allow mixing. This experiment can be done in many ways: where both sections have water at the same temperature; where the sections have different initial temperatures; etc. A tube can be used to add water of a different temperature to the lake to mimic stream insertion (yellow and green water, stream in red). Visitors can see how different temperatures and types of water mix. Visitors can also add ice and observe the depth the ice sinks to, and how the water from the melting ice mixes. Furthermore, dye representing point source pollution or invasive species can be introduced, observed, and compared between models. The flow of the brightly colored fluids, due to differences in temperature or salt content, helps visitors visualize important concepts including lake currents, wind effects on mixing, and thermoclines. Water can drain through a tube at the bottom of the tank to start over. 7 Photo courtesy of UC Davis Tahoe Environmental Research Center

9 PREPARATION Before the demonstration: 1. Obtain hot water. Dye the hot water yellow. Make sure it stays hot until you do the demonstration or else it will not work. 2. Obtain ice cold water. Dye the ice cold water dark blue. Remove the ice from the water just before the demonstration. 3. Put empty plastic model tank in the location you wish to do the demonstration and make sure the center divider is in place. PROCEDURE 1 2 Introduce the topic of lake physics which describes how water moves within a large, deep lake. The main topics within lake physics we re going to look at are lake stratification, lake mixing, and seiche (or internal) waves. With younger audiences, you might have to explain what stratification is. Introduce the demonstration. Tell the students you re going to add warm yellow water to one side of the model and cold blue water to one side of the model and remove the divider. We re going to observe how the two different temperatures of water interact. Ask the students to make a hypothesis of what they think is going to happen when you remove the divider. Have a few students share their ideas with the group. Preparation and Procedure Make sure you press down firmly 3on the center divider as you add the cold water to one side of the model and warm water to the other side of the model. (Note: It works best to add equal amounts of water on each side of the divider at the same time.) Remind the students to observe what happens when you remove the divider. Remove the divider. 4 Give the model a minute to settle after you remove the divider and ask the students to share their observations. What happened? Why did this happen? There is one key factor that causes the two different temperature waters to separate, does anyone have an idea what that factor is? Explain how the warm water has a much lower density than the cold water. You can reference a laminated copy of the graph given above to show students that cold water really is denser than warm water. 8

10 5 Explain how stratification like this actually does occur in large, deep lakes like Lake Tahoe. Ask the students how they think the top layer of the lake might warm. Explain how the sun heats the top layer of the lake and that the intensity of the sun is different in each season (most intense in the summer, least intense in the winter) which leads to different stratification levels in the lake throughout the year. (Test tubes filled with blue water and olive oildarker yellow/canola oil-lighter yellow are useful to illustrate the cold and warm layers in the different seasons as shown to the right.) In the spring the water on the surface of the lake starts to warm, during the summer the warm layer gets deeper due to the more intense heat from the sun, in the fall the warm layer begins to lose heat to the air above the lake as the air begins to cool for the season, and in the winter the air above the lake is cool enough and the intensity of the sun is low enough that the lake becomes a uniform temperature at about 39 degrees F (refer to the first graph for the reasoning behind this). 6 Lake Mixing: When the lake is at a uniform temperature and density from top to bottom (during the winter), it can achieve full lake mixing, bringing oxygenated water from the surface to the bottom of the lake and cycling nutrient rich water from the bottom to the surface of the lake. This natural cycle is essential for the health of the lake and the organisms living in it. 9

11 7 Seiche Waves: Now you can talk about seiche waves. Seiche waves are internal waves that occur beneath the surface of the lake due to forces like wind and tectonic activity. Photo courtesy of UC Davis Tahoe Environmental Research Center These waves usually do not show on the surface of the lake. While the water in the model is still stratified you can demonstrate the effect of wind on seiche waves in a lake. Using either a straw or a blow dryer you can apply a wind force laterally across the surface of the lake. Before you do the demonstration, ask the students to make a hypothesis of what they think is going to happen when you apply the wind to the model. As you do the demonstration, the students should see the cold blue water from the bottom of the model begin to move upward toward the surface as the yellow warm water is pushed by the wind away from the source in the direction the wind is going. During this demo you don t want to cause too much turbulence on the surface of the water. You can explain that while seiche waves cause movement to happen within a lake, they don t cause the lake to mix fully from top to bottom for that to happen you need the lake to have a uniform density. 8covered, End with a recap of Lake Physics and the topics that you including density stratification, seasonal changes, lake mixing, and seiche waves. Photo courtesy of UC Davis Tahoe Environmental Research Center 10

12 Credits This project was supported by the National Science Foundation under grant number DRL Any opinions, findings, conclusions, or recommendations expressed in this program are those of the author and do not reflect the views of NSF. This project was created as part of the LakeViz project funded by the above-named grant. Content provided by ECHO Lake Aquarium and Science Center. Find out more at for an archive of learning resources related to 3D visualizations and freshwater ecosystems. The collaborating institutions include the ECHO Lake Aquarium and Science Center, the University of California at Davis Tahoe Environmental Research Center, and the Lawrence Hall of Science at the University of California, Berkeley. 11