Quantifying beach variability along the Teignmouth-Dawlish seawall

Abstract

An investigation into the variation of beach sediment levels fronting a seawall was conducted for both Teignmouth and Dawlish, on the south coast of Devon, UK. The aim was to quantify the levels of beach variability along the length of the seawall and discuss this in line with plans to re-develop sections of the existing structure. It was predicted that Sprey Point, a section of the seawall that protrudes into the surf-zone, would interrupt the littoral transport of sediment at Teignmouth. At Dawlish, it was expected that the presence of groynes positioned along the beach would also influence the transport of sediment in the longshore direction. Plymouth Coastal Observatory offers bi-annual topographic beach profile surveys using RTK GPS, as well as a catalogue of wave data measured using a Directional Waverider MK III Buoy. This data was used to create historic profiles that show how the cross-shore beach profile has varied over the 7-year survey period (2013-2019). Historic profiles were analysed alongside local wave data plots showing the significant wave height, Hs, peak wave period, Tp, and wave direction, Dirp, in order to understand how the wave conditions have influenced the sediment levels on the beach over the years. The vertical variability in front of Teignmouth town was found to range between 0.85 – 1.35 m for the survey lines in this section of the beach. In Addition, Sprey Point appears to function as a barrier to the longshore drift of sediment along the coastline. To the south-west of this protrusion the beach displayed its largest vertical variability of 3.52 m, positioned approximately 310 m upcoast of Sprey Point. To the north-west of this obstruction Teignmouth beach experienced a depletion of sediment levels and a reduction in vertical variability. Here, the beach displayed its lowest vertical variability of 0.62 m, located 80 m downcoast of Sprey Point. In front of Dawlish town, the vertical variability ranged between 1.03 – 1.32 m in a 400 m section of the beach lying between the two groynes known locally as the Colonnade breakwater and the Coastguard breakwater. To the north-east of these groynes, the beach displayed a depletion in sediment levels. Central to the location of the seawall failure (335 m from the Coastguard breakwater) the beach exhibited a minimum sediment level of -1.2 m, and the lowest vertical variation of 0.9 m. The historic profiles showed a general movement in the longshore direction towards the north-eastern end of the beach near Langstone Rock, where the vertical variability was measured to be 1.78 m. The results confirmed that the presence of coastal structures has interrupted the longshore transport of sediment and consequently influence the acceleration of erosional processes.