Natural and artificial influences on coastal erosion at Sidmouth, Devon, UK

Like many small coastal towns in the UK, Sidmouth in Devon was founded on the valley sides adjacent to a river outfall that provided a natural harbour. Subsequent expansion of the town in the late 18th and early 19th centuries, when living by or visiting the sea became popular for health reasons, involved the entrainment of the river and building on land that was subject to marine flooding. Engineering works in the 19th and 20th centuries that were designed to protect the low-lying parts of the town included the construction of sea walls, and groynes and offshore bunds to protect a ridge of storm-beach gravels that acts as a natural sea defence. These works have collectively had an effect on erosion rates in the cliffs of Triassic sandstone and mudstone on the east side of the town. Natural landslide mechanisms in the cliffs adjacent to Sidmouth include rock-block and toppling failures induced by marine undercutting, and hydraulic stoping along faults and major joints at the foot of the cliffs. The principal landslide mechanism is the collapse of unconsolidated Head deposits and deeply weathered mudstones in the highest (mostly 3 to 5 m) part of the cliff. The falling material commonly destabilises the underlying well-jointed sandstones and mudstones. Artificial factors that have influenced erosion rates in the cliffs east of the River Sid outfall in the last 100 years have included the refraction of waves adjacent to the end of the river wall, and interference with the easterly longshore drift of the beach gravels. A secondary factor has been the destabilising influence of a Victorian railway tunnel that was dug parallel to and up to 25 m from the cliff face on the east side of the town. In the absence of quantitative monitoring data, published estimates of the rates of cliff erosion are significantly higher than those obtained in the present study from a comparison of maps made between 1802 and 2006.