title: THE IMPACT OF EXTREME STORM WAVES AT THE COAST; THE ROLE OF INFRAGRAVITY WAVES
Creators: name: Billson, Oliver
Publisher: name: University of Liverpool Repository
Year: 2021

description: Infragravity (IG) waves are long period waves with frequencies lower than wind-waves and swell, usually in the frequency band 0.003 to 0.05 Hz. IG waves are known to dominate hydrodynamic and sediment transport processes close to the shoreline on low sloping sandy beaches, especially when the incoming swell and wind-driven waves (incident waves) are large. However, in extreme wave conditions, how their importance varies on coarser grain sized and steeper beaches, and with different mixes of incoming swell and wind-waves, is largely unknown. Here, a new dataset comprising in-situ and remote observations from five contrasting sites (one low-sloping sandy beach, two steep gravel beaches and two compound/mixed sand and gravel beaches), under extreme wave conditions is used to assess infragravity response across a wider range of wave heights &amp; periods, beach slopes and grain sizes than has been previously explored. The beaches studied ranged in slope from tanβ = 0.02 – 0.35 with median grain sizes (D50) of between 0.25 – 60 mm. During the experiments significant wave heights (Hs) of up to 7 m and peak periods (Tp) up to 20 s were observed. During the five storms recorded, waves in excess of the 95th percentile of the long-term record of Hs were observed at all sites, with waves representative of a 1-in-1, 1-in-5, 1-in-10, 1-in-20 and 1-in-40 year event at the five sites respectively. Video observations of a 1-in-40 year storm, ‘Emma’, impacting a steep gravel beach revealed that significant infragravity swash height (Sig) dominated over significant gravity swash height (Sg) at the shoreline when offshore wave height (H0) exceeded 1.5 m, where ‘dominance’ was defined by the ratio of Sig/Sg exceeding 1. Sig increased linearly with offshore wave height (H0), as has been reported in previously published field work on sandy beaches. However, for a given wave height, Sig was between one third and three times larger on the steep gravel beach than values quoted in the literature for sandy beaches. Observations collected on the steep gravel beach during storm ‘Emma’ were compared to data collected at an additional four sites (a low-sloping sandy beach, a second steeper gravel beach and two compound/mixed sand and gravel beaches). Sig at the shoreline in excess of 0.5 m was consistently observed at all five contrasting beaches. The largest infragravity swash heights were observed at the steeper gravel beach (Sig up to 11.4 m), followed by the low-sloping sandy beach (Sig up to 3.2 m), and the less steep gravel beach (Sig up to 2.6 m) and were lowest at the compound/mixed sites. Due to contrasting incident wave breaking and dissipation processes, infragravity frequencies were observed to be most dominant over gravity frequencies on the low-sloping sandy beach (Sig/Sg up to 4.4), occasionally dominant on the gravel beaches (Sig/Sg up to 2.5), and rarely dominant on the compound/mixed beaches (Sig/Sg up to 1.1). An existing equation commonly used to parameterize Sig on sandy beaches was tested on the new dataset, performing well on data from the sandy beach but less well on data from the gravel beach. An existing equation commonly used to parametrize runup on gravel beaches was modified to produce a new gravel specific parametrization of Sig, which performed well on the gravel sites and less well on the sandy site. Both equations performed poorly when applied to the dataset combining sand and gravel beaches. H02T, proportional to deep water wave power, was found to accurately predict Sig on both the sand and gravel beaches, demonstrating that, under extreme storm wave conditions, combined wave height and period are the main drivers of infragravity oscillations at the shoreline, with the beach morphology playing a secondary role. In-situ observations were collected seaward of the incident wave breakpoint by bed-mounted acoustic Doppler current profilers and through the surf zone by intertidal arrays of pressure transducers at two of the five sites (the low-sloping sandy beach and the less steep of the two gravel beaches). Analysis revealed that energy transferred to the IG band seaward of the surf zone at the sandy beach and landward of breakpoint at the gravel beach. The surf beat similarity parameter (ξSurfbeat) indicated that bound long wave release was the dominant IG wave generating mechanism on the low sloping sandy beach (ξSurfbeat &lt; 0.05) whilst breakpoint forcing was the dominant mechanism on the steep gravel beach (ξSurfbeat &gt; 0.1). The findings presented in this thesis highlight the importance of collecting field data over a wide range of conditions. When deep water significant wave height (H0) exceeds 2 m, IG energy dominates the inner surf zone and swash on both sand and gravel beaches. Therefore, in addition to their well-known importance on sandy sites, infragravity waves are also implicated in the inundation and erosion of gravel beaches during storms.