Search

The wave fields of coastal bays are comprised of waves generated by far-off storms that enter the bay to combine with waves generated locally by winds inside the bay and regionally outside the bay. In any given location, the resultant wave field varies spatially and temporally, and affects coastal features, such as beaches in estuaries and bays (BEBs). However, wave fields in enclosed bays with tidal shoals are poorly studied, limiting the efficacy of coastal protection and restoration projects in these systems, a critical focus in light of ongoing sea level rise. Here we present observations of the wave field in Tomales Bay, a 20-km-long, narrow, semi-enclosed embayment on the wave-dominated coast of Northern California (USA) with a spring-tide range of 2.5 m. We deployed pressure sensors near several beaches along the linear axis of the bay. Low-frequency waves (4 ∗ 10 - 2- 2.5 ∗ 10 - 1 Hz or 4–25-s period) were not observed further than 4 km of the mouth, delineating the “outer bay” region, where remotely generated swell and regionally generated wind waves could dominate. The wave spectrum of the landward “inner bay” was dominated by fetch-limited waves generated within the bay with frequency ≥ 2.5 ∗ 10 - 1 Hz. The energy of both ocean waves and locally generated wind waves across all sites were controlled by the tide, but the former by changes in attenuation and the latter likely by modulation of wave generation. Wave energies were low at low tide and high at high tide, but high-frequency wind wave energy was increased during ebb tides while lower-frequency swell energy was reduced during ebb tides, suggesting different mechanisms of tidal influences. Thus, in addition to fluctuations in winds and the presence of ocean waves, tides exert a strong control on the wave energy spectra at coastal features in mesotidal regions. In general, events that may be impactful for BEB morphology are expected to occur when waves due to high winds or high-swell event arrive during high-tide periods. However, no such events were observed during our study and questions remain as to how rarely such events occur across the bay.

Coral reefs protect coastlines from inundation and flooding and serve over 200 million people globally. Wave transformation has previously been studied on coral reef flats with limited focus on forereef zones where wave transformation is greatest during high‐energy conditions. This study investigates the role of forereef spur and groove (SaG) morphology in wave energy dissipation and transmission at the reef crest. Using XBeach on LiDAR‐derived bathymetry from One Tree Island in the southern Great Barrier Reef, we reproduced dissipation rates comparable to SaG field studies. We examined how wave energy dissipation differs between realistic bathymetry and those with SaG features removed, demonstrating an up to 40% decrease in dissipation when SaG features are absent. We then investigated changes to wave energy dissipation and wave transmission at the reef crest based on IPCC AR5 emission scenarios (RCP2.6 and RCP8.5) and a total disaster scenario (TD) for the year 2100. For RCP2.6, an increase in wave heights of 0.8 m and an increase in water level of 0.3 m resulted in a two‐fold increase in dissipation rates. For RCP8.5 and TD, with no increase in incident wave height, dissipation rates were 29% and 395% lower than RCP2.6. This resulted in increased wave transmission at the reef crest by 1.8 and 2.7 m for the RCP8.5‐ and TD based models, respectively, when compared to the RCP2.6‐based model. The results from our novel modeling approach of using long‐shore varying accurate bathymetry on forereefs show increased wave energy dissipation rates with implications for reducing coastal flooding and island inundation on reef‐lined coasts. Plain Language Summary: Coral reefs protect coastlines from floods and waves, benefiting over 200 million people globally. We studied how waves change over coral reefs, focusing on the forereef zone where wave transformation is most significant during high‐energy conditions. The shape of the forereef, specifically the long comb‐like grooves that cut through coral reefs, known as spur and grooves (SaG), modify wave energy transformation. Utilizing digital representation of waves over accurate reef shapes (known at bathymetry), we simulated wave dissipation rates comparable to real‐world SaG studies. By comparing high resolution, accurate bathymetry to smoothed bathymetry with SaG removed, we demonstrate a 40% decrease in wave energy dissipation. Next, we investigated how wave energy dissipation changes in different forereef environments. We considered low and high emission scenarios (RCP2.6 and RCP8.5) from the IPCC AR5 report and a total disaster scenario (TD) for the year 2100, considering changes to wave power, and water‐level. We found that models based on high‐emission scenarios (RCP 8.5) had decreased dissipation rates, resulting in more water passing the reef crest. Our study highlights the benefits of using accurate reef shapes in simulating wave energy dissipation on coral reefs. Accurate bathymetry can incorporate features such as SaG of different shapes, which increase wave dissipation. Key Points: Accurate bathymetry (<1 m) in forereef wave energy dissipation models can include features such as spurs and grooves (SaG)SaG increases forereef wave energy dissipation when compared to similar bathymetry without SaGForereef dissipation shifts from bed friction to wave breaking in modeled environmental conditions based on future climate projections [ABSTRACT FROM AUTHOR]