Vegetation stabilisation and slope failure of eroding sand dunes

Published source details Carter R.W.G. (1980) Vegetation stabilisation and slope failure of eroding sand dunes. Biological Conservation, 18, 117-122.
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Summary

In the UK and elsewhere, coastal sand dune stabilisation by planting vegetation is widely used and often effective. However, at many sites foredunes need to retain some instability to remain effective buffers against erosion, too much stabilisation may be detrimental to the beach-dune system. This study compared stability of vegetated (previously planted with marram grass Ammophila arenaria) and non-vegetated dune slopes at Portrush, County Antrim (Northern Ireland). The site is characterized by little available fresh sand, foredune growth is thus largely absent, 18-20 m high dunes abut the beach. This study investigated response of the dunes following seawater undercutting in 1975.

At the eastern end of the beach a length of about 250 m of mobile seaward-facing dune slope was planted and thatched during the 1960s to counter erosion. Marram was planted, along with a few buckthorn Hippophae rhamnoides bushes, gorse Ulex brushwood was laid. By 1975, grasses, herbs and bryophytes had colonized, leaving little exposed sand. Adjacent dunes were not planted (apart from occasional marram clumps) and comprised mostly loose sand.

In February 1975, high tides and waves combined to erode the seaward dune bases. Slope profiles were surveyed immediately after the event and again 3 months later. A comparison of the vegetated and non-vegetated seaward-facing dune slopes was made.

Along the non-vegetated section erosion scars (up to 0.3 m in height) were quickly filled by sand collapses over the 18 m high slope. A near constant angle of 33.5° was maintained. An estimated15 m³ sand/m dune length was supplied to replenish the depleted beach levels.

The vegetated section responded very differently. Erosion scars of up to 2 m appeared alond the dune base and much less sand (about 2 m³/m) was supplied to replenish beach. Numerous slides and slumps occurred. Over the three months, 47 slips occurred (lateral dimensions ranging 1.5 to 9.2 m, with downslope displacement of 1.0 to 2.5 m).

In conclusion, at Portrush the vegetated dune slopes are prone to slips following seawater undercutting and sand supply to the eroding beach is greatly reduced in comparison with the mostly unvegetated dune sections.

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