Choosing the right data for a coastal erosion claim
A Year 12 student is investigating whether a 2.4 km stretch of sandy beach has experienced net erosion over the past 40 years. Available data: (1) aerial photographs from 1983, 2001 and 2023 at 1:5000 scale, with the 2001 set partly cloud-affected; (2) a local council survey of 12 cross-shore transects measured annually since 2010, showing a mean shoreline retreat of 1.8 m per year with a standard deviation of 0.9 m; (3) anecdotal reports from long-term residents describing 'much less sand than when I was a child'; (4) a single drone survey from 2023. The student's draft conclusion states: 'The beach is eroding at 1.8 m per year and will disappear in 60 years.' The teacher asks the student to justify the data selection and evaluate the conclusion.
1. Evaluate the student's selection and use of the four data sources. Identify which sources provide the strongest evidence for the erosion claim and explain why.
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Strongest evidence: the council transect survey (source 2). It is systematic, repeated annually, spatially distributed across 12 transects, and quantified. The mean retreat of 1.8 m/yr is a measured rate, not an impression. However, the standard deviation of 0.9 m signals substantial variability between transects and years, so the mean conceals local differences (some transects may be accreting).
Aerial photographs (source 1) are valuable for extending the record back before 2010 and for spatial coverage, but the 1:5000 scale limits positional precision to roughly a few metres, and the cloud-affected 2001 set weakens the mid-period comparison. They are useful corroboration, not primary proof.
Drone survey (source 4) gives high-resolution contemporary detail but a single snapshot cannot establish a trend.
Resident reports (source 3) are anecdotal, subject to recall bias and shifting baselines, and cannot be quantified. They may corroborate a perception of change but carry little evidentiary weight alone.
Judgement: the claim is best supported by combining source 2 (rate and trend) with source 1 (longer temporal baseline and spatial pattern). Sources 3 and 4 are supplementary only.
2. Critique the draft conclusion: 'The beach is eroding at 1.8 m per year and will disappear in 60 years.' What is wrong with it, and how should it be rewritten?
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Three distinct problems:
- Overgeneralisation from a mean. 1.8 m/yr is an average across 12 transects since 2010. With SD 0.9 m, some transects likely show far less retreat or accretion. Stating a single rate for the whole 2.4 km misrepresents spatial variability.
- Unjustified extrapolation. A linear projection over 60 years assumes constant processes. Coastal systems are non-linear: storms, sediment supply changes, sea-level rise acceleration and management interventions all alter rates. The data cover only 13 years, so a 60-year forecast extends far beyond the evidence.
- A category error about 'disappearing'. A beach does not simply vanish at a fixed rate; it may migrate landward, steepen, or stabilise depending on backshore geology and sediment budgets. 'Disappear' is undefined.
Rewritten conclusion (example): 'Between 2010 and 2023, surveyed transects recorded a mean shoreline retreat of 1.8 m per year (SD 0.9 m), indicating net erosion across the monitored section, though with considerable local variation. Aerial photography from 1983–2023 is consistent with this trend. Projecting this rate forward is uncertain because it assumes unchanged processes; longer records and process data would be needed to forecast future shoreline position.'