Silt City
Silt City explores the concept of how changes in land use in a catchment will create changes in the local and regional waterway environment.
By creating a Silt City model, students will see for themselves that;
- Comparatively more water runs off a city catchment
- Water runs off city catchments much more quickly - this can cause erosion and flooding,
- Less water soaks into a city catchment - leading to changes in groundwater levels and soil in the catchment, and
- How models can be used to investigate and solve problems.
Materials
- 1 metre square piece of black plastic stapled onto a piece of cardboard (to represent the city),
- 1 metre square piece of carpet (to represent the country),
- Four 500ml beakers,
- Blocks of wood or bricks (to prop up the carpet and plastic up and create a slope).
- A stop watch,
- Old stockings, and
- A small quantity of vegetable dye
Why don’t you create a hypothesis about the differences between an urban and country catchment.
Set up this model and see if you were right.
Making the model
Set up your Silt City models in a classroom, a lab or outside in a space where the model catchments can be left to drip overnight.
Step1
Prop up the carpet square and the plastic square at an angle using bricks or blocks. Ensure that there is a dip along the length of the squares, so water will run into two measuring beakers, collecting at the bottom.
Step2
Sprinkle vegetable dye on the ‘uphill’ areas of the plastic and carpet squares. The dye will represent soil, or even pollutants present in the catchment. The dye will help you determine how quickly such a substance can make it through the catchment and into your waterway.
Running Silt City
It is now time to set up two model catchments to explore the changes which take place in the water cycle when a city is built. The carpet catchment is more like a country catchment with plenty of plants (carpet) to slow the water - while the plastic catchment is impermeable and thus more like the city, with its’ impermeable roads and rooftops.
Sprinkle both models with exactly the same amount of water (from two 500ml beakers), and record how long it takes for the water to move through each catchment and into the water collected at the downhill end of the model catchment.
Predict which model will generate the most run-off
Predict which catchment will generate run-off most quickly?
Step 1
Appoint a time keeper, a measurer, and a recorder.
Step 2
Place two 500ml beakers ‘downhill’ so they can collect the run-off from each catchment.
Step 3
Use the other two 500ml beakers to gently sprinkle the ‘uphill’ end of each model (carpet and plastic) with 500ml of water.
Step 4
Measure the water collected in the downhill beakers after 30 seconds, 5 minutes and 1 hour.
Step 5
Allow your model catchments to drip overnight, and measure the level of water in the downhill beakers again in the morning.
Your Silt City Results
Water reaching the containers (in mL)
Run-off times plastic carpet comments
30 seconds
5 minutes
1 hour
Overnight
Did you find that the fastest run-off response occurred in the permeable (country/carpet) catchment or the impermeable (city/plastic) catchment?
Did you find that in a 24 hour period, most run-off was produced by the permeable (country/carpet) catchment or the impermeable (city/plastic) catchment?
Discuss your results in your group. Were there any differences between catchments?
What were they?