Testing a Retreat Rate on the Kendrapara Coast
A student team surveyed a 3 km stretch of the Kendrapara coast near Satabhaya (Rajnagar block) for its project record. Data collected: (i) the high-water line printed on Survey of India toposheet 74 L/1, surveyed in 1974-75, lies 1,150 m seaward of the high-water line the team fixed by GPS in December 2023; (ii) two erosion stakes set in 2015 show retreat of 6 m/yr at one and 9 m/yr at the other; (iii) a hand-auger core behind the beach ridge shows 0.4 m of coarse sand with shell hash resting on 1.2 m of grey estuarine silt, then a second sand layer, then peat at 2.6 m depth; (iv) the ridge crest is 2.1 m above mean sea level, the mean spring tide range is 2.4 m, and the back-barrier surface stands at 0.9 m; (v) the nearest tide gauge shows mean sea-level rise of about 1.2 mm/yr over the same period. The draft report states: 'The coast retreated 24 m every year because of sea-level rise.' Your task is to test that sentence.
1. Recompute the retreat rate from the map-to-GPS comparison and explain, with at least three reasons, why it must not be quoted as an annual erosion rate. Which single check would you make first?
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Arithmetic. 2023 − 1975 = 48 years. 1,150 m ÷ 48 yr ≈ 24 m/yr. The division is correct; the error lies in what was divided.
Why the figure is not an erosion rate
- Different datums. The sheet is referenced to the datum and mean sea level in use at the time of survey; the GPS line is on WGS-84. Datum shift alone can displace a plotted coastline by tens of metres, and it must be removed before any subtraction.
- Different tide states. A 'high-water line' on a survey sheet is a defined feature, usually high water at ordinary spring tides. The GPS line was fixed on one calm post-monsoon December day, probably near neap tide. On a flat beach, comparing a spring mark with a neap waterline inflates retreat.
- Generalisation. At 1:50,000, 1 mm on the sheet equals 50 m on the ground. Half a millimetre of drawing error is 25 m of ground uncertainty.
- Different windows. The stakes cover 2015-2023, a short and possibly storm-dominated period; the map comparison covers 1975-2023. Neither is a constant process.
Defensible statement. The only sustainable number in the data is 6-9 m/yr from the stakes, and even that is an eight-year record. The honest sentence is: 'the shoreline has moved roughly 1.15 km inland over 48 years, but current measured retreat is of the order of 6-9 m/yr.'
First check. Re-measure the same feature at the same tide stage on a dated satellite series (Landsat 1990, 2000, 2010, 2023) all acquired at similar tidal height, then compare. If those rates cluster near 6-9 m/yr, the 24 m/yr figure is an artefact of mismatched lines.
2. Using the core and the elevation figures, decide which process dominates - gradual sea-level rise, storm overwash, or compaction of the back-barrier peat. Show the arithmetic.
Read the worked answer
Test 1: can sea-level rise alone do it? 1.2 mm/yr × 48 yr = 57.6 mm, about 6 cm. Sea-level rise of 6 cm cannot produce 1,150 m of horizontal retreat by itself; something else supplies the energy.
Test 2: is the ridge overtopped by ordinary tides? Spring high water is half of the 2.4 m range, i.e. 1.2 m above mean sea level. The crest is 2.1 m, giving 0.9 m of freeboard. Normal tides therefore do not overtop it - but a surge of 0.9 m or more plus wave run-up does. Cyclonic surges on this coast routinely exceed that.
Test 3: what does the core record? Coarse sand with shell hash is a high-energy deposit - exactly what overwash leaves behind the ridge. The grey estuarine silt is low-energy tidal-flat deposition; the second sand layer is a second event; peat at 2.6 m is a former marsh surface now buried 1.7 m below the present back-barrier surface.
Verdict. The dominant process is episodic storm overwash and landward rollover, superimposed on a slow background, with burial/compaction of the peat as a secondary contributor. Gradual sea-level rise is not the driver; it only sets the slow background. That also explains why the stakes 6 m/yr and 9 m/yr differ on the same shore - one stake has been through more events.
Confirmation. Date the two sand layers (optically stimulated luminescence on quartz, or radiocarbon on shell) and match them to known cyclone years. If they line up with major storms, the argument is closed.