Using 2D LiDAR to Investigate Swash-Level Gravel Barrier Dynamics

Gravel barriers are efficient absorbers of wave energy due to the high permeability of gravel sediments. They are thus highly valued as a coastal defence and have numerous other benefits including biodiversity and recreation. Given the probable increase in storm intensity under climate change, this importance will only increase in the future. However, these systems can themselves be damaged during storm events. To effectively manage and monitor them, it is necessary to understand how they respond to energetic forcing. Thus far, morphological and hydrological measurements of sufficient resolution to resolve individual swashes are rare on gravel environments, especially over a protracted period time. Hence, our current knowledge is lacking.

As part of the #gravelbeach project, this study addresses this knowledge gap by investigating swash-level change at different gravel barrier typologies. Here, we focus on Borth, a drift-aligned macrotidal composite barrier in West Wales. A 6 m LiDAR tower was deployed between 28 November 2024 – 25 March 2025. Three-second scans were taken every low tide, while one-to-two hour scans were taken every high tide. The shoreline is extracted every 1 s, from which the total water level and bed profile are calculated. To extract berm location and characteristics, a novel semi-automated approach is developed.

During the transition from neap to spring tide, a series of berm construction and destruction events is observed, which is in contrast to prior observations of a gradual translation. At the swash level, four different types of berm response to energetic events were identified. They could flatten, overtop, aggradate vertically, or possess an upper zone of accretion (making the local profile gentler) and a lower zone of erosion (making the local profile steeper). It is also demonstrated that a gravel barrier can change between these types rapidly; in one example, three types are displayed in a single two-hour period. This diversity in barrier slope, both temporally and spatially, is often not taken into account by modelling approaches and highlights the need for continued research into swash-level dynamics at gravel barriers.