Geography K–10 · Years 7–8
Creek discharge from a cross-section and float timing
Water in the world (ACARA Year 7 sub-strand); NSW Stage 4 focus area Water in the World (2015 syllabus) and Water in the world (2024 syllabus)
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.
The idea
Discharge is cross-sectional area multiplied by velocity, so a creek's flow in litres per second can be measured with a tape, a ruler and a floating orange.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- drowning and slips at the water edge
- contaminated water
- sharp objects in urban channels
Controls
- reach no deeper than 0.3 m and slow; no entry after heavy rain or when the water is above the depth limit; adult ratio per the school excursion policy
- no entry to stormwater channels or drains at any time
- gloves, cuts covered, hand-washing after the fieldwork
- throw rope and first aid on site
Note
Fieldwork near water is run under the school excursion risk assessment; RiskAssess (riskassess.com.au) holds a fieldwork template. No chemicals are used.
What you need
- A shallow (no deeper than 0.3 m, below gumboot height), slow, straight reach of a local creek, 10 m long, approved by the land manager and the school
- 30 m tape, 1 m ruler or metre stick, 6 marker pegs; a 10 m string line with a line level (or a clinometer) to measure the fall of the water surface along the reach
- 3 oranges (they float just below the surface and are visible), stopwatch, a net or stick to retrieve them
- Data sheet: widths, depths at 0.5 m intervals, three float times
- Gumboots, throw rope, first-aid kit
How to do it
- Peg a 10 m reach along the bank. At the upstream and downstream ends stretch the tape across the water and read the wetted width.
- At each end measure the depth every 0.5 m across; area of each end = Σ (0.5 m × depth). Mean area A = average of the two ends.
- Measure the bed slope rather than assuming one: stretch the string line along the reach just touching the water surface at the upstream peg, level it with the line level, and measure the gap down to the water surface at the downstream peg. Slope S = fall ÷ 10 m, so a fall of 10 mm over 10 m is S = 0.001; read the fall to the nearest 5 mm and state the precision that gives. Enter the measured slope in the calculator in place of its default.
- Release an orange 1 m upstream of the first peg in mid-channel; start the watch as it passes the first peg and stop at the last. Repeat three times; mean time t.
- Surface velocity v_s = 10 m ÷ t. Mean velocity is lower than the surface velocity because of bed friction; record v_s and, for the discharge, use the mean of the three floats and state that the result is an upper estimate.
- Discharge Q = A × v_s in m³/s; multiply by 1000 for L/s. Report it as an upper estimate, because v_s is the velocity at the surface and the mean velocity over the whole cross-section is lower.
- Repeat on a later day after rain, once the water is back below the depth limit and flowing slowly, and compare.
What you should see
A cross-section drawing at each end, three float times that agree closely on a steady reach, and Q in L/s. For a channel 2.0 m wide averaging 0.15 m deep (A = 0.30 m²) with a float taking 25 s over 10 m (v_s = 0.40 m/s), Q = 0.12 m³/s = 120 L/s. The calculator's Manning estimate for the same channel, with its default n = 0.048 and S = 0.001, is 50.8 L/s, less than half the float figure. The two are not the same quantity: the float gives a surface velocity and the model returns a mean velocity from an assumed roughness and an assumed slope, so the gap is the class's to account for. With v = 0.40 m/s the roughness that would fit is n = 0.020, or, holding n = 0.048, the slope would be 0.0056 rather than the assumed 0.001; this is why the slope is measured with the line level instead of assumed, and the class states which assumption its own measurement contradicts. After rain the same reach is expected to give a larger area and a shorter float time, so Q rises through both factors. It worked if the three times agree closely and the two end areas are similar; a large difference means the reach is not uniform and a straighter one should be chosen.
What changes
- What you change
- day (before and after rain)
- What you measure
- discharge Q (L/s)
- What you keep the same
- same reach and pegs
- float released at mid-channel
- three float trials
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A wide river always carries more water than a narrow one.
- Water at the surface moves at the same speed as water on the bed.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Geography K–10 Syllabus (2015), NESA; current and taught in 2026, replaced from 2027. The outcome codes are not printed on the NESA landing page for this syllabus, which only links the document, so the url given is the syllabus document itself (geography-k-10-syllabus-2015.docx); the code and its outcome text were read in that document on 23 September 2026.GE4-2GE4-7
- Geography 7–10 Syllabus (2024), NESA; implementation from 2027, not yet taught; code read on this page on 22 September 2026GE4-PRI-01GE4-TAP-01
- Australian Curriculum v9AC9HG7K01AC9HG7S02AC9HG7S04
Sources
The pages the author read to write this activity.