Evaluating a Beach-Profile and Longshore-Drift Investigation
A Grade 11 class investigates whether a 60 m rock groyne at a coastal resort is causing erosion downdrift. Working for one day in a 25 km/h south-westerly wind, five groups measured beach profiles at 200 m intervals along 3 km of beach using ranging poles and a 30 m tape, recording the position of the high-water mark and the width of dry sand. They also released 100 painted pebbles at three points and recovered 78 after 48 hours, finding a mean displacement of 41 m and a range from 6 m to 120 m. Results: dry beach width had increased by 2.4 m on the updrift side of the groyne and fallen by 6.8 m immediately downdrift, and by 2.1 m at the furthest survey point 800 m downdrift. Two groups admitted holding the tape slack on the steep upper beach. The pebbles were flat, disc-shaped and all of similar size. Local residents say the beach 'always gets narrower at the end of winter'.
1. Interpret the results. What do they show about longshore drift and the effect of the groyne, and what alternative explanations must be considered?
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Direction of drift: the pebbles moved a mean of 41 m with a range of 6–120 m, and the beach widened 2.4 m updrift while losing 6.8 m immediately downdrift. Material accumulates where drift is interrupted and is starved on the far side; both lines of evidence therefore indicate drift running from the widened side towards the eroded side - i.e. the groyne is trapping sand on one side and starving the other.
Pattern of erosion: the loss is greatest (6.8 m) right at the structure and decreases to 2.1 m at 800 m downdrift. That down-drift decay is the classic signature of a groyne's shadow zone; a general regional cause would erode the whole 3 km more evenly.
Alternatives that must be tested before blaming the groyne:
- Seasonality. Residents say the beach narrows at the end of winter, which fits a seasonal cycle of swell direction and storm waves. A single-day survey cannot separate a seasonal low from a permanent loss.
- Rip currents and local bathymetry can move sand offshore in one embayment without longshore drift.
- Sand mining, river-sediment starvation or a harbour elsewhere could reduce supply along the whole coast.
- Pebble shape and size bias. Flat discs slide further than spheres, so 41 m may overstate drift of real beach sand. The honest conclusion: the data are consistent with groyne-induced erosion but do not prove it, because there is no before-and-after and no seasonal control.
2. Evaluate the reliability and validity of the class's method and describe how you would redesign the investigation.
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Validity (are they measuring the right thing?):
- Dry beach width is a weak measure of erosion; the position of mean high water or the profile shape is better because dry width shifts with wind-blown sand and tide state.
- One day and one tidal cycle cannot separate seasonal change from the groyne effect - this is the biggest design flaw.
- Flat, uniform pebbles overstate drift rate for natural sand; mixing sizes and shapes, or using fluorescent sand, would be more representative.
- 25 km/h wind moves dry sand and affects both the profile and pebble recovery.
Reliability (would it repeat?):
- Two groups held the tape slack, which inflates every distance on the steep upper beach by an unknown amount - that alone could be a metre or more, comparable to the measured changes.
- Only 78% of pebbles recovered; the missing 22% are likely the fastest-moving ones, so 41 m is probably an underestimate - a bias, not random error.
- No repeats, no inter-group check on the same profile.
Redesign: survey the same five profiles monthly for at least 12 months to capture seasonality; standardise the datum (e.g. a permanent benchmark and a levelled staff, tape kept taut and checked); measure the full profile from dune toe to low water rather than dry width only; take three readings per profile and average; use numbered, mixed-size pebbles plus fluorescent sand; add a control beach without a groyne 5 km away; record wind, tide and wave height each visit. Only with a control site and a seasonal run can the class claim the groyne is responsible.