Wave, Tide and Morphological Controls on Embayment Circulation and Headland Sand Bypassing

Embayed beaches separated by irregular rocky headlands represent around 50% of the world’s shoreline and are important zones ecologically and commercially. Accurate determination of sediment budgets is necessary for prediction of coastal change over long timescales in these zones. Some headlands have been shown to permit sediment bypassing under particular forcing conditions, therefore knowledge of sediment inputs and outflows via headland bypassing are important for sediment budget closure. Recent modelling work demonstrates bypassing rates are predictable for an isolated headland, however, it remains to test this predictability using a range of real headland morphologies, and to examine the influence of embayment morphology, sediment availability and tidal effects.

We show that bypassing rates are strongly influenced by the relative proximity between adjacent headlands, and the degree of embaymentisation. Tidal currents are secondary to wave forcing, mildly moderating bypass rates, whereas tidal elevation strongly influences bypassing rates largely through variations in apparent headland and embayment morphology.

A fully coupled (3D hydrodynamics and waves) numerical model was used to simulate sand transport along a 75 km long macrotidal, embayed coast in the north of Cornwall, UK. Twenty-five embayments were included in the analysis. Nine wave conditions were simulated and bypass rates were analysed for three tidal elevations. Simulations were performed with both uniform sediment availability and a realistic spatial distribution of sediment, and both including and excluding tidal currents. It is shown that many of the embayments along this stretch of coast exhibit headland bypassing under energetic wave forcing, highlighting the need for accurate bypass rate predictions for sediment budget determination on embayed coasts.

Headland extent relative to surf-zone width was a critical control on sand bypass rates in line with previous work. Predictive expressions were accurate to within a factor of 4 for beaches exhibiting a ‘normal’ circulation pattern (embayment length long relative to surf zone width), however, they did not predict well cases where embayment cellular circulation was dominant (embayment length short relative to surf zone width).  Tidal currents exhibited a secondary control relative to wave forcing, moderating bypass rates by up to 20% in this macrotidal environment. Large differences in the apparent morphology of the embayments between high and low tide strongly impact bypassing rates, with greatest bypassing occurring at low-tide when headland cross-shore length is smallest. Bypass rates were reduced for realistic sediment distributions versus uniform sediment availability, due to larger transport magnitudes when sediment is available off the headland toe.

This work highlights the extent to which headland bypassing occurs along this embayed coast with implications for similar coasts worldwide. It also emphasises the need for accurate predictions of headland bypassing in these regions and suggests areas for further efforts to focus to refine future predictive parameterisations.