What is a Watershed Model?
A watershed model is a three-dimensional representation of land that drains to a single river, stream, lake, or ocean. A watershed model is used as a tool to educate people about their watershed and why they should care about them. Watershed models help people make a connection between land use and water quality by demonstrating how we treat our land impacts the body of water to which the land drains.
Why Should I Create a Watershed Model?
A recent survey of Bay residents by the Chesapeake Bay Program found that only half of the respondents could properly define a watershed from a list of four choices. This is especially disturbing because understanding the watershed concept is crucial in helping people make the connection between their actions in preventing pollution on land and the health of their local waterway.
Creating a watershed model is one way of educating children and adults about watersheds. It visually demonstrates the watershed concept and it helps show other difficult to understand concepts such as non-point source pollution, the importance of wetlands, the connection of storm drains to local streams, groundwater recharge, and the reason for having forested riparian areas. In short, watershed models help demonstrate the effects of storm water runoff to school and community groups and reveals their role in preventing pollution of the rivers and streams.
Developing Your Goals and Budget
|Watershed models come in all sorts of sizes, shapes and prices! You can build a portable watershed model for under $50 or you can make a large-scale model for around $1,500! Models can be purchased for as little as $40 for a small model to $1,000 for a portable model with a carrying case and supporting materials. Consider both your budget and your reasons for having a watershed model before determining which to build or buy.
In this write up, we will cover creating the most expensive of the models a large-scale model that will be a permanent fixture. We have chosen to discuss this model because it is the most expensive option and because we think it is really cool! You will use the same techniques in building other models, but the prices will be more reasonable.
The options available for paying for a watershed model installation vary depending on where the model will be installed. If the watershed model will be installed on public land such as in a schoolyard or a park, grants are often available to fund construction. Grantors include the Chesapeake Bay Trust, the Chesapeake Bay Foundation, Pennsylvania’s Growing Greener Program, the Chesapeake Bay Restoration Fund, and the National Fish and Wildlife Foundation’s Small Watershed Grants Program. Your local government also may help pay for the model’s installation. If the watershed model will be on private lands, the options for financing may be much more limited.
You may be installing a watershed model in the schoolyard of a local school or for a local park; you may be creating it to educate the public about the concept of a watershed and how they can reduce their impact on the watershed, or to teach students about the geography of their local watershed. Whatever the case, you will want to write out your specific goals and let them guide your work plan.
Some example goals:
- Educate students and the public about the watershed concept;
- Enhance community awareness of storm water runoff and non-point source pollution and ways to reduce them;
- Teach the importance of specific watershed protection and restoration measures such as riparian forest buffers, wetlands, and low-impact development to students and members of the community.
Creating a Timeline
The amount of time that the project will take depends on the age and motivation of the volunteers. A project coordinator can expect to spend around 120 hours, with volunteers to help with all steps. The high school youth were fast! That entire model was built in 15 half-days of work.
Your timeline will vary depending on how you have set up your project. For example, if you are planning on financing your project through a grant, your first steps are to find an appropriate grant vehicle and apply for the grant. Below is a sample timeline for a watershed model installed in a schoolyard funded through a grant where volunteers will aid in the installation.
- Determine the site where you will install the watershed model;
- Determine the size of the watershed model;
- Calculate the cost of installing the watershed model;
- Solicit volunteers to aid in the design and creation of the model (for example a watershed group, a school group, or boy or girl scout troop);
- Using the information above, apply for a grant to fund the project;
- Assuming that the grant is funded, arrange for a location to store the supplies needed for the watershed model and purchase them;
- Arrange for supplies to be delivered or to pick up them up (NOTE: this may be an additional cost);
- If possible, arrange for public officials and media to be present at the model’s dedication;
- Purchase or rent the necessary tools and equipment for the creation of the model;
- Issue press releases and follow up with media about the watershed model construction to ensure publicity;
- Create and print educational materials to provide to reporters, public officials, and those who aid in the construction;
- Construct the model (can be done in advance of a dedication event that involves the media and public officials);
- Follow up with participantssending public officials and volunteers thank you letters
Locating your Watershed Model
|Finding a good location for a watershed model involves balancing a number of different factors. You want to place the model in a place that is easily visited by your target audience and with access to water for use in demonstrations. The area should be relatively flat and surrounded by grass so that water will flow off of your model and can be absorbed by the grass. Decide whether you want your model to be very visible, or somewhat out of the way there are advantages and disadvantages to both!
Finding Volunteers for Your Watershed Model Project
|There are many sources of volunteers for this project, including students, boy and girl scouts and volunteers from your own organization. Ideally you will want to work with those people that are your target audience. If you are building the model on school grounds as a teaching tool for students, use the students to help build it. This might make the project take a little longer, but it will be more rewarding in the end! If you want the model to look really well done, consider using adults they will have more patience for the work. Building a watershed model is also an excellent scout project.
Constructing a Watershed Model
For the topographic layers:
- USGS 7.5-minute topographic map (The 15- minute series is OK if that’s all you can get, but it is much harder to read the topographic lines)
- Fine tip pens
- Permanent markers
- Acetate sheets
- Clear tape
- Rigid foam boards - About 20 of them. They should have a plastic or foil backing so they stay together. We have used both 2 inch and 1.5 inch. The thinner material is much cheaper, but it may not be appropriate for your scale. If you use 2 inch boards, do not use the tar paper backed ones - they have fiberglass embedded in the paper and are nasty to work with.
- Permanent markers
- Overhead projector
- Saber saw (also known as a jig saw)
- Nails (cheapest kind - for pinning together layers)
- Topo map on acetate
For the foundation:
- 2x6 boards as long as each of the sides of the model
- String line
- Batter stakes (to keep the foundation boards from bulging when the cement is poured)
- Tape measures
- Trenching shovel
- Sand or decomposed granite (DG)
- Skill saw
- Pipe for bending rebar
- Tie wire
- Diagonal pliers
For the cement covering:
- Concrete glue - about 2 gallons for this step (we used "Deck-O-Weld")
- U-cart concrete
- Portland cement powder - you might use 3-6 bags
- Small (1/4 inch) gravel - we used 1/4 cubic yard
- Plaster sand - we used about 1 cubic yard
- Cement mixer - rent one or ask a Landscape Contractor to loan you one
- Square shovels
- 5 gallon buckets
- Chopped fiber - 1 lb (from the concrete makers)
- Rubber coated cotton gloves for each volunteer
- Petroleum jelly
- Paint tray
- Cheap paint brushes - 2 - 3 inches wide
- Hose with nozzle
- Scrub brush for cleanup
- Expanded metal lathe
- Metal shears for cutting lathe
- Nails - to pin lathe to structure
- Cement working tools
- White cement powder - 1 bag
- Cement dyes: Greens, blue, yellows, brown (about 1 lb of each, except blue)
Selecting the Appropriate Watershed
In general it is best to use your local watershed because it is something people can relate to by finding their own homes, schools or other local landmarks. Depending on the size of your watershed, however, this may be unrealistic. If you choose too small an area, there may not be enough topography to be very interesting, However, if you live in a large watershed, you might try picking one of the sub-watersheds the watershed for one local creek, for instance. Try to pick a receiving body of water that everyone can relate to, it could be a local lake, a creek that runs through town, or one that is the focus of restoration efforts.
You may decide not to use your local watershed. Using a generic watershed may be useful if you are trying to demonstrate many watershed concepts, some of which aren’t found in your local area. For example, you may want to show the positive effect that forests have on water quality, but you live in an urban watershed.
You need to decide on an appropriate scale. Take the USGS topography maps for your selected watershed. Enlarge them to a size at which you can easily read the topographic lines - depending on the steepness of the topography, this may be as large as two to three feet in at least one dimension, but keep it as small as possible. Try a copy store, since they can enlarge the entire map onto one large sheet of paper, minimizing distortion at the edges. Make a same size copy of the scale section of the map. You will use this to position the overhead projector properly so your proportions work out.
Decide which topo lines you will use and darken them - sometimes you can get the copy machine to do this by lightening the contrast so the other topo lines disappear. You might trace over them with a fine point pen to emphasize them. We suggest using the 200 foot contour lines, but if you have very high, steep terrain it might be better if you use 400 foot contours or if your area has very shallow hills you might use 100 foot contours. You will then have to copy this onto acetate. The copy store may also have large pieces of acetate or you may need to do portions and tape them together. Use clear cellophane tape, not “magic mending tape” since you can see through the clear tape on the overhead projector.
Measure your map size. Decide how big you want the footprint of the relief map to be in final form. For example if your map is 2 feet across by 3 feet long and you want the final model to be about 10 feet wide, you will have to blow the whole thing up five times the size (2x5=10).
Now you need to figure out the vertical elevation. Most relief maps stretch the vertical elevations. The earth is really quite flat, although it doesn’t appear that way to us little humans. By exaggerating the vertical, the map will look more “real” to most people. Mathematically this means that you might have a horizontal scale where one inch on the map might equal 300 feet in real life but for your vertical scale one inch on the map might equal 100 feet in real life.
We suggest 3:1 or 4:1 vertical exaggerations, but don’t go any higher than this or things will start to look too pointy. A good idea is to figure out how tall a local landmark will be in each of the scales and decide what looks appropriate. If one of your hills or mountain is only been 6 inches high on a 1:1 scale, stretching it to a 3:1 scale will make it 18 inches tall.
Tracing the topographic lines
Note: This step is going to drive you crazy because none of the lines will ever match up perfectly. It’s the fault of the distortion of both the copy lens and the lens on the overhead projector. It’s OK!!! Do the best that you can, and realize that all of the little details will get covered up with cement anyway and it will all come out in the end looking gorgeous!
- Push a table against a long wall. Put the foam board on top of the table, as vertically as you can get it. You can tape the top corners to the wall so the board doesn’t fall over.
- Make sure the overhead projector is facing square to the board so that its light “box” is as square as possible in order to minimize distortion of the map. Raise or lower the entire overhead projector with books rather than tilt the lenses to minimize distortion.
- Take the scale that you blew up to the same size as your acetate map and put it on the overhead. Move the overhead forward or backward until your scale shows you have blown things up to the proper size. For example, if your acetate map is 3 feet across and you want the final relief map to be 12 feet across (3x4=12). If the scale you have has a line that is 2 inches long, you need to have that scale line projected so it becomes 8 inches long (2x4=8) on the foam board.
- Place masking tape in front of the overhead projector so if it gets bumped, you know how to position it again.
- Start in the upper corner of the board. Trace your lowest contour (perhaps for you it will be the 200 foot contour). You’ll have to move the board and map to use the entire 8x4 foot board since the projection won’t be large enough to cover the whole board. Move the board and map, not the overhead projector!
- Use fine-point dry-erase markers to mark on the acetate map the boundaries of what part of the map you used for a single set of foam boards. This way you know where to start the next set of boards so they fit together on the ground.
- Label the part of the board that is the “keeper” - the part you’ll use on the model - with an appropriate name, the elevation, and add an arrow facing north. If you don’t label it, you’ll throw out the wrong piece or try to put it in upside down!
- Set that board aside to be cut, place another foam board against the wall and trace the next selected topo line (for example, the 400 ft. line.)
- Label the keeper part of that board too with name, elevation and north arrow.
- Keep going until you have traced all of the topo lines for that section of the map.
- Move on to another section of the map. Remember that fitting the pieces together along those seams will be frustrating because of the distortion from the overhead projector. Keep in mind that when you cover things over with an inch or two of cement, the finer details will be covered up and it will look great!
- As pieces are cut, stack them on top of each other and pin with nails to keep them from sliding all over the place or blowing away.
- You may eventually get to “floater pieces” - little peaks. For example say you have an 800 ft. piece that extends to the back of the model and the next piece - the 1,000 ft. piece is just a little sausage shaped piece on top. Before you remove the 800 ft. board from the wall, trace the position of where the 1,000 ft. piece will sit. Label that with the elevation and a “Do Not Cut”. This will help you place that little 1000 ft. floater piece.
- If you are building in a public place, you will have to build the sections inside some secure room and take them outside to the foundation on the day you plan to cover it with the first cement layer.
- This entire process may take several days. It is somewhat slow and tedious so if kids are involved, it is best to have older youth working on this part - 5th grade and up. Refer back to the paper topographic map that you used to make the acetate one if you have any questions as to how things fit together. You may be reduced to hand cutting some pieces to make things fit together properly at all of the boundaries. Remember - it will all work out in the end!
Making the foundation
For this part of the work, find a volunteer who has had some experience with concrete. Having someone familiar with the material will make this section go very smoothly - plus they will probably have the necessary tools!
- Lay the boards on the ground to form the frame and make sure it is square.
- You will want to build a mild slope into the foundation of the model in order to ensure that it will drain properly. We recommend a 6 inch gain over 12 feet. To make a 6 inch gain, frame the entire shape of the model with the 2x6 lumber and add another layer of 2x6 to the back of the model so that the back wall is 12 inches high. Cut side boards on the diagonal so the sides slope from 12 inches in the back to 6 inches in the front.
- Nail the batter boards to the frame to keep everything in place. Leave enough of the nail sticking out that you can take the nails out easily and remove the framing once the concrete foundation is cured overnight.
- Back fill the foundation box with sand and stomp down on it to compact it. Wet the fill down and stomp again. Use a trenching shovel to cut a 6-8 inch wide trench the whole way around for the foundation. Building the foundation in this way makes digging easier since you are making a trench into the sand rather than the soil.
- Lay out the rebar around the foundation frame. Bend it where necessary to fit into the trench by putting the pipe over the rebar and moving the pipe along the rebar until one end of the pipe is where the bend should be. At this point, stand on the rebar and use the pipe as a lever to bend the rebar.
- Tie the pieces of rebar together with tie wire and lay the rebar into the trench. Use tie wire to suspend the rebar in the trench so it does not touch the boards. To do this, tie the wire off to nails that are hammered lightly into the outside of the boards. Do not hammer nails inside the trench if you do they will be covered with cement and it will be very difficult to get the foundation boards off.
- Carefully measure your trench and calculate how much concrete you will need. It’s better to have too much than too little, since you can always make stepping stones from the extra.
- Pour concrete into the foundation trench. Poke the cement with a stick or jiggle it with cement tools to try to get it to settle into the trench evenly. We recommend renting a cement trailer for this rather than try to mix our own because it takes a fair amount of concrete.
- Allow it to cure at least overnight before removing the form boards. Cut the tie wires where they go into the cement.
Placing the first cement coat
Again, if you have a volunteer who has ever mixed cement, it would be a great help but realize that cement is pretty forgiving stuff and mixing it does not take a rocket scientist. The mix for this layer is 3 parts sand, 3 parts gravel, 1 part Portland cement powder. We recommend a 5-gallon bucket and marked with the 3 part and 1 part levels.
- Add the sand, gravel, cement powder and a handful of chopped fiber together in the mixer and set the mixer rolling so they are well mixed before you add the water. Add the water slowly since it is very easy to go from being too dry to too wet very quickly. When the mixture seems close to ready let it mix for a few minutes and then add about 2 cups of concrete glue. Again mix thoroughly. If you need to adjust the mixture, add either more water if it's too dry or more powder/sand if it's too runny. The mixture should be creamy but not too runny. Remember it has to stick in place on the model!
- Make relatively small batches of cement until you get a feel for how fast your volunteers are using it.
- While the first batch is mixing, mix about 1 part glue and to 2 parts water and paint the entire structure to be covered - this helps the cement stick to the foam.
- Have everyone grease their hands with petroleum jelly up to their elbows. Use generous amounts, since this is what prevents alkali burns from the cement. Have everyone put on the rubber coated gloves.
- Pour the cement into the wheelbarrow and wheel it to the model. You can use buckets to get the cement to the center of the model.
- Cover the foam model with the cement. Your task on this layer is to cover the foam and get rid of the “stair-step” effect. It is okay if some of the edges of the foam are still exposed by the end of this layer.
- Pay particular attention to make sure people are not filling in all of the creek channels with concrete. It is best to have a “Quality Control Inspector” whose sole job is to run around and check to make sure valleys are not filled with cement! Remember - if it hardens in the wrong place there is nothing to be done but to start again!
- Pay particular attention to the streams. Try to get as even a slope as you can on them, but realize that the next coat is where you will fine-tune the flowing of the creeks.
- Do not try to smooth the concrete to a polish as they do for sidewalks - the rough surface will stick to the next layer much better than a smooth surface. Remember it's going to look pretty rough and ugly at this point covered with handprints, lumps and bumps, and pebbles. Don't worry - the stucco and color coats will fix all of that.
- Do not run water over it until it has set you will wash the cement powder out of the concrete!
- Wash your tools when you are finished.
- Let the cement harden overnight. It is probably best to let it cure for a few days before applying the next coat.
- If possible, wet the concrete periodically as it cures. The slower the cure, the harder it becomes and the less cracking it will do.
Placing the stucco coat
The next layer is stucco. This is the last structural coat you will add, so make sure you have it as thick as you need it for strength. As long as you have at least 0.5 - 1 inch of stucco over everything, it should be fine. Remember that you already have several inches of concrete that evened out the layers.
The mixture for stucco is 6 parts sand to 1 part cement powder. Add a generous handful of chopped fiber to the cement as it mixes. Again, add water slowly. Add at least 2 cups of concrete glue per 12 shovels of sand. The concrete glue adds extra hardness to the concrete.
- Before you start on this layer, run water down the creeks and note how the water flows. This will guide you in making sure that the creeks flow how you want them to. Try to get even flows and avoid pockets that collect standing water, unless you want them for lakes. This washing will also get rid of any loose particles.
- Use a scrub brush to scrub any pockets where stuff settles. If you add stucco over loose powder or crumbs, it will tend to peel off.
- Mix 2 parts water/1 part concrete glue and paint the entire model to help the layers of concrete stick to each other.
- Grease and glove all participants.
- Place the stucco layer on the model. The stucco layer should be about .5 - 1 inch thick over the entire structure in order to ensure a strong model.
- Use the "Quality Control Inspector" again to make sure the creek valleys are not filled in!
- For the vertical sides of the model you will need to add expanded metal lathe. First lay the lathe against the vertical side and cut it to fit the shape of the side. Then pin the cut piece of lathe to the model with nails to hold it in place. Finally, use cement tools to work enough cement onto the side of the model, fill in all the cracks, and cover the lathe. This will take some effort but it will be worth it the metal lathe adds extra strength to the walls. If necessary, apply two layers of cement to fully cover the lathe and make it disappear.
- Wash your tools when you are finished
- Again, keeping the model damp will help it cure. This is not essential, but it will help it set better. Remember do not run water over it until it has set or you will wash the cement powder out of the concrete.
- Allow this layer to cure for at least a day, preferably two or three days.
Placing the color cement coat
This layer adds color, but not much in the line of strength to the model. You should use concrete dyes because they will become part of the cement and will not peel off like paint will do. Theses dyes are available from stores that sell stucco, tile, or cement professionals. If you are having problems locating such a store, contact a local stucco contractor and ask for advice they may even have some samples that they can give you.
To paint your watershed you will need several colors of green and perhaps some browns or yellows and blue. Ask to see samples of the dye mixed in white concrete if possible. You will need no more than 1 pound of each color with the exception of blue. The blue is very expensive and you may not be able to afford to buy it. We suggest asking the store to order a free sample and in return you will make sure to publicize their store.
Consider using white cement for this layer. Although white cement is twice as expensive as the Portland variety the colors will be much more vibrant. It may take several short days to place the color cement layers since you need to let one color harden before you try to put another one over it. The blue line creeks should go on last and the colors under it should be well hardened and set or you will end up with muddy-looking creeks.
- Run water through the creeks again, noting any final adjustments you need to make to make the creeks flow properly. Scrub and flush off all loose particles.
- Decide where you want which color keeping in mind that you can apply one color over top of another once the first color is hardened.
- Mix small batches of white cement in the mixer - about 5 parts sand to 1 part cement powder. Again, add concrete glue and fiber. Go heavy on the concrete glue for this layer - it will go on more smoothly and you won't need to paint the entire structure with glue.
- Grease and glove all hands.
- Mix colors in individual buckets. Add small amounts of dye at a time remember that they are very potent! Realize, however, that the cured color will be lighter than the color you see in your bucket. You may wish to experiment with small amounts of the color ahead of time.
- Apply the colored cement remembering that this color coat is thin. We suggest smearing this layer on with your hands and using damp sponges to pat the cement down. You want this layer to be relatively smooth, but not too polished. Let the cement stand for perhaps an hour before using the sponges in order to allows the cement to begin to set. A light rubbing with a dampened sponge will give the model a nice finish. Remember, if there is anything you don't like, you can always go over it again with another layer! Cement is cheap.
- We suggest a very deep green for the vertical exterior walls. This is a nice neutral color that sets off the rest of the model and, if you like it will allow you to add underground geology to your model.
- Wash your tools when you are finished.
- Do not run water over it until it has set, however, or you will wash both the cement powder and the color out of the concrete.
Using Your Watershed Model
You can use your watershed model to teach many different lessons. Some of the different ideas you can demonstrate include:
The Concept of a Watershed
To demonstrate this concept, use a bucket of water or hose to show how water flows off of the land and gathers into creek or lake draining the land. If your watershed shows several streams, you can also show how a large watershed is made up of smaller sub watersheds (the tributaries to your larger stream).
The problem of soil erosion
Use fine sand or sifted soil to demonstrate how exposed soil flows off of the land into streams. You can also show how erosion rates are greater on steeper than flatter ground. To demonstrate this idea, spread the sifted soil or sand evenly on the model and then gently add water to the different areas in a fashion that mimics different types of rainfall.
You can also teach how erosion rates differ on contour plowed land versus fields plowed vertically. Place package string on the model in different plowing directions and add the sifted soil or sand. Again gently add water to the different areas in a fashion that mimics different types of rainfall.
The positive effect of forest buffers
To demonstrate how a buffer of trees or unmowed vegetation can help prevent soil from entering streams, place green fleece fabric along the edge of your stream. Leave part of the stream unvegetated and repeat the demonstration with soil and rain water.
The impacts of pesticide and fertilizer run-off
This is demonstrated much the same way as showing erosion. If you can identify areas of your watershed where there are concentrations of homes, a golf course, or agricultural fields, use there spots for your demonstration. This time, instead of the sand or soil, use a bright colored drink powder it really gets people’s attention!
The problem of detergents in streams
Urban and suburban streams are often impacted by detergents from washing cars or illegal hookups that dump washing machine water into curbs and gutters. For this demonstration, use toy cars and pour soapy water around them to demonstrate how the detergents flow into the stream. Suggest that adults take their cars to the car wash where the water that is used is treated at a treatment plant rather than going into the stream as is.
Pollution from oil and grease
Again use toy cars for this lesson. You can also use plastic tracks for toy cars to demonstrate roads. Cut sections out of the sides of the tracks to represent storm drains. This time, use blue or green food coloring to represent the oil and model how oil and grease run-off from roads and streets into the stream system. Say that adults should take their car to a professional to have the oil changed, or if they are doing it themselves, they should bring the used oil to a place that recycles it. Let them know to keep cars in good working condition and to drive as little as possible.
Bacterial contamination in streams
Bacteria from uncollected pet and animal waste can wash off and contaminate waterways. You can use plastic animal figures and small balls of brown paper to demonstrate both the problem of pet waste in urban environments and waste from farm animals such as from cows or hog farms.
Infiltration, storage and slow release of water in pervious areas
Watersheds that have very little impervious surfaces (roads, buildings, sidewalks, and driveways) tend to absorb water, hold it, and release it slowly over time to streams through ground water. This absorption is important because if the water just runs off quickly, streams go dry or their flow is greatly diminished and they tend to become warm. Warm streams or ones that have gone dry are not hospitable to fish and other aquatic life. Furthermore, we get our drinking water from either ground water or streams and rivers. If pervious land areas are not absorbing rain water, we are losing our drinking water!
To demonstrate this concept, use a large piece of green fleece fabric folded over several time or large fat sponges to mimic a pervious watershed. Pour water over this area and show on lookers how water continues to flow out of the sponge or fleece after you have stopped pouring. Then show them how the pervious surface is still full of water. Compare this with an impervious area by pouring water directly onto the cement watershed and watching it runoff quickly.
|Source: The University of California Cooperative Extension 4-H Project