Earth and Environmental Science 11–12 · Year 12
Runoff from a model catchment: how land cover changes the flood peak
Module 6: Hazards
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The idea
Paving and clearing raise the fraction of rain that runs off at once, so the same storm produces a higher, earlier flood peak from a built-up catchment.
What you need
- two identical sloped trays (30 × 40 cm) with a single outlet: one lined with soil and turf, one with a plastic sheet (paved)
- watering can with a rose delivering 1 L in 60 s, measuring cylinders, stopwatch
- a third tray with soil and a ponding hollow as a detention basin
How to do it
- Apply the same 1 L over 60 s to each tray and collect the outflow in 15 s intervals for 3 minutes.
- Plot outflow rate against time for each tray (a hydrograph) and read the peak and its timing.
- Compute the runoff coefficient, total outflow / water applied, for each tray.
- Repeat with the detention-basin tray and compare peaks.
- Repeat the turf tray with the same litre spread over 10 minutes, 50 mm/h on the 0.12 m² tray, collecting until the outflow stops, and compute its runoff coefficient at that intensity.
- Use the rational method to estimate the peak flow from a 1 ha paddock and a 1 ha car park in a 50 mm/h storm, taking the turf coefficient from the 50 mm/h run; the 1 L in 60 s storm is 500 mm/h on the 0.12 m² tray, ten times the design intensity, so coefficients measured in it overstate runoff from turf in the 50 mm/h storm.
What you should see
The paved tray peaks early and high and returns most of the litre; the turf tray peaks later and lower and holds part of the water; the basin tray has the lowest peak. The 60 s storm is 8.3 mm of rain in one minute, 500 mm/h, so the turf tray returns a larger share of the litre than it does when the same litre falls over 10 minutes at 50 mm/h. Rational method Q = C i A / 360 (Q in m³/s, i in mm/h, A in ha): with example coefficients C = 0.9 (paved) and C = 0.2 (grass) and i = 50 mm/h on 1 ha, Q = 0.125 m³/s and 0.0278 m³/s, 22 % of the paved value.
What changes
- What you change
- surface cover (turf, paved, basin)
- What you measure
- outflow hydrograph: peak rate, time to peak, total volume
- What you keep the same
- rain volume and rate
- slope
- tray size
- starting moisture
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Floods only happen near big rivers.
- Drains remove flood risk (they move water faster to the next point).
Safety card
Hazards
- water on floors
Controls
- work outdoors or over a large tray
Note
No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-13EES11/12-1EES11/12-2EES11/12-3EES11/12-4
- Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-02
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.