Free Infragravity Waves on the Inner Shelf: Observations and Parameterizations at Two Southern California Beaches

Numerical predictions of nearshore waves and shoreline runup are usually initialized on the inner shelf, seaward of the surfzone, with sea‐swell (SS) waves from local wave buoys or regional wave models. Lower frequency infragravity (IG) waves are not reliably measured by buoys or included in regional models. Here, co‐located pressure and velocity observations are used to characterize IG waves in 10–15 m depth in southern California. Shoreward propagating IG waves are often dominated by free waves, with the boundwave energy fraction <30% for moderate and low energy incident SS waves. Only 5% of records, with energetic long swell, show primarily bound waves. The shoreline slope of concave beaches increases by ∼3 between spring high and low tides, and free seaward and shoreward IG energy in 10–15 m vary tidally. The observed linear dependency of free IG energy on SS energy and period is consistent with Ardhuin et al. (2014, https://doi.org/10.1016/j.ocemod.2014.02.006)'s parameterization (R2= 0.71). Including the tide level as a proxy for beach slope and modifying the SS frequency dependency increases R2to 0.91. The ratio of free seaward to shoreward propagating IG energy suggests between 50 and 100% of the energy radiated seaward in depths of 10–15 m is trapped offshore and redirected shoreward. Free (random phase) and bound (phase‐coupled) IG waves are combined to initialize the SWASH numerical model. SWASH predicted runup is only weakly influenced by waves at the offshore boundary. Nonlinear IG generation and dissipation in the shoaling and surfzone overwhelm the effects of shoreward propagating waves observed at the offshore boundary. Infragravity (IG) waves are long‐period (25 s–2.5 min) waves that contribute to coastal flooding and beach erosion. IG waves, generated near the shoreline by short‐period sea‐swell (SS) wave groups (known by surfers as “sets”), have long wavelengths (100s of m) and do not typically curl and break like ordinary sea and swell waves. Instead, they can be reflected off the beach face and propagate seaward. Our study concerns IG waves on the inner shelf (10–15 m depth, ∼500–700 m offshore), seaward of the main region of IG generation. Similar to previous observations in Hawai'i and North Carolina, we find most of the reflected, seaward‐going IG energy cannot reach deep water and is trapped on the continental shelf. We develop an observation‐based estimate of IG wave energy on the inner shelf as a function of SS wave energy and tide level. Finally, we show with a numerical model that IG wave runup at the shoreline is influenced only weakly by IG waves on the inner shelf. Infragravity (IG) waves on the inner shelf (10–15 m depth) in San Diego, USA are often dominated by refractively trapped free wavesFree IG energies are parameterized as a function of local sea‐swell conditions and tide levelNumerically modeled wave runup is only weakly influenced by the shoreward propagating IG waves observed at the offshore boundary Infragravity (IG) waves on the inner shelf (10–15 m depth) in San Diego, USA are often dominated by refractively trapped free waves Free IG energies are parameterized as a function of local sea‐swell conditions and tide level Numerically modeled wave runup is only weakly influenced by the shoreward propagating IG waves observed at the offshore boundary