Wave, Tide and Topographical Controls on Headland Sand Bypassing. Issue 8 (22nd August 2021)

• Record Type: Journal Article
• Authors: King, E. V.; Conley, D. C.; Masselink, G.; Leonardi, N.; McCarroll, R. J.; Scott, T.; Valiente, N. G.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-22
• Subjects: headland bypassing -- sediment transport -- Delft3D -- sediment budget -- wave‐current interactions -- embayed beaches -- wave‐tide interactions -- sediment availability; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC017053
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Embayed beaches separated by irregular rocky headlands represent 50% of global shorelines. Quantification of inputs and outflows via headland bypassing is necessary for evaluating long‐term coastal change. Bypassing rates are predictable for idealized headland morphologies; however, it remains to test the predictability for realistic morphologies, and to quantify the influence of variable morphology, sediment availability, tides and waves‐tide interactions. Here we show that headland bypassing rates can be predicted for wave‐dominated conditions, and depend upon headland cross‐shore length normalised by surf zone width, headland toe depth and spatial sediment coverage. Numerically modeled bypassing rates are quantified for 29 headlands under variable wave, tide and sediment conditions along 75 km of macrotidal, embayed coast. Bypassing is predominantly wave‐driven and nearly ubiquitous under energetic waves. Tidal elevations modulate bypassing rates, with greatest impact at lower wave energies. Tidal currents mainly influence bypassing through wave‐current interactions, which can dominate bypassing in median wave conditions. Limited sand availability off the headland apex can reduce bypassing by an order of magnitude. Bypassing rates are minimal when cross‐shore length >5 surf zone widths. Headland toe depth is an important secondary control, moderating wave impacts off the headland apex. Parameterisations were tested against modeled bypassing rates, and new terms are proposed to include headland toe depth and sand coverage. Wave‐forced bypassing rates are predicted with mean absolute error of a factor 4.6. This work demonstrates wave‐dominated headland bypassing is amenable to parameterization and highlights the extent to which headland bypassing occurs with implications for embayed coasts worldwide. Plain Language Summary: It is important to understand the inputs and outputs of sand to beaches to effectively predict long term coastal change. This study focuses on the movement of sand between embayed beaches, around headlands, known as headland bypassing. We use a numerical model of a highly energetic 75 km stretch of coast to predict how much sand moves around the headlands under different wave and tide conditions. We find that bypassing is mostly driven by energetic waves. Changes in water level with the tide has a secondary effect. Tidal currents interact with waves, and this interaction drives bypassing when waves are less energetic. The coverage of sand was also important, with more bypassing if sand is present off the headland toe. The depth of water off the headland apex is an important control on bypassing rate. We test how well a formula to calculate bypassing works, and propose new terms to improve it. This work indicates headland bypassing is potentially ubiquitous along exposed embayed coasts globally. Key Points: Headland bypassing is potentially widespread on energetic embayed coasts Bypassing can be predicted for realistic morphology and sand coverage; key parameters are headland extent, surf zone width and toe depth Tides are a secondary control on bypassing rate under energetic waves. Wave‐current interactions can dominate bypassing for median waves