Retreat rates of soft-sediment cliffs: the contribution from dated fishweirs and traps on Holocene coastal outcrops

The Holocene lithological sequence in the Gwent Levels of southeast Wales dictates that most of the eroding coast is composed geomorphologically of a pair of platforms and surmounting cliffs. Situated at high-tide level and protected by engineering works, the upper cliff has been in a stable position for several centuries. The lower cliff occurs at about mid-tide level and exposes estuarine silts and thin peats capped by the main peat bed (mid-Holocene), resistant to wave attack and other erosive forces. The disposition of a variety of wooden fishweirs and traps dating from the last 1000 years over the lower foreshore seaward of the mid-tide cliff in the Magor Pill area suggests that the latter is retreating at a long-term rate with a maximum value of the order of 0.87 m a−1 and at short-medium term rates from the order of a few decimetres to a few metres annually. The retreat of the mid-tide cliff is a threat to the substantial and mainly hard-engineered sea-defence works erected on the platform (upper foreshore) created by the main peat.