Mark L. Buckley

Submerged coastal structures (e.g., submerged breakwaters and natural and artificial reefs) are common in nearshore waters globally. By locally reducing water depths from the surrounding bathymetry, they modify the incident wave field, mean currents, runup and sediment transport around them. Although submerged structures are generally assumed to promote coastal protection by dissipating waves offshore and creating sheltered conditions in their lee, their interaction with waves can result in wave-driven circulation patterns that may either promote shoreline accretion or erosion (Ranasinghe et al., 2006). A meta-analysis of the coastal changes resulting from the construction of submerged breakwaters found that, contrary to expectation, in the majority of the cases erosion occurred in their lee (Ranasinghe and Turner, 2006). Therefore, a detailed understanding of the wave structure interactions and resulting hydrodynamics is paramount to predict coastal changes in their lee. Here we explore the detailed wave-driven hydrodynamics in the lee of submerged structures using phase-resolved modelling.

Approximately 20 to 30 percent of the world’s coastlines are fronted by shallowly buried or outcropping shore platforms overlain by perched beaches (Kirk, 1977; Marshall and Stephenson, 2011; Trenhaile, 2002). Seasonal erosion of perched sediment can shift the beach state from ‘accreted’ to ‘exposed’ (Gallop et al., 2011), and the effect this has on wave-induced flood risk (known as wave runup) is unknown. As sea levels rise and storm severity increases, understanding how beach state influences wave runup is crucial for minimising coastal hazard risk and managing perched beach coastlines. In this work, idealised numerical modelling and field observations along a perched beach in southwestern Australia were used to quantify the influence of beach state on wave runup, setup, and swash processes.

Storms are one of the main causes of coastal erosion, causing substantial property and infrastructure losses in coastal communities. Coastal erosion is particularly damaging in underrepresented communities that are unable to meet building and zoning regulations due to limited resources [Lam, 2014]. The United States Geological Survey (USGS) has developed a probabilistic assessment of hurricane-induced coastal erosion for sandy coastlines along the eastern United States and the Gulf of Mexico. However, the current USGS product incorporates an empirical equation with inputs of beach slope and deep-water wave height and period [Stockdon, 2006] to predict the wave runup. Though this is appropriate for open coast sandy beaches, it is not valid for the rocky and reef lined coasts of Puerto Rico due to their complex bathymetries, steep slopes, and large bottom roughness. To expand this tool to Puerto Rico, the USGS has created 1-dimensional XBeach [Roelvink, 2009] models around the island.

Nearshore rocky reefs with scales of order 10–100 m are common along the world's coastline and often shape wave‐driven hydrodynamics and shoreline morphology in their lee. The interaction of waves with these reefs generally results in either two or four‐cell mean circulation systems (2CC and 4CC, respectively), with diverging flows behind the reefs and at the shoreline in the 2CC case and flows that diverge in the lee and converge at the shoreline in the 4CC case. By applying a phase‐resolving wave‐flow model to conduct a detailed analysis of mean momentum balances for waves interacting with nearshore reefs, we develop an understanding of the drivers of 2CC and 4CC flow dynamics and how they vary for different reef geometries and wave and water level conditions. The 2CC or 4CC patterns were primarily driven by alongshore pressure gradients toward the exposed (nonreef fronted) or reef‐fronted beach. These alongshore pressure gradients were dependent on the cross‐shore setup dynamics governed by the balance between pressure (i.e., related to the setup) and radiation stress gradients, and mean bottom stresses exerted on the water column. If shoreline wave setup in the lee of the reef was less than the exposed beach, a 4CC pattern developed with convergent flow at the shoreline in the lee of the reef; otherwise, a 2CC emerged with divergent flow at the shoreline. Across the parameter space investigated, reef roughness, distance to the shoreline, and beach slope were the three parameters most likely to change the flow patterns between 2CC and 4CC.

Wave breaking on the steep fore‐reef slopes of shallow fringing reefs can be effective at dissipating incident sea‐swell waves prior to reaching reef shorelines. However, wave setup and free infragravity waves generated during the sea‐swell breaking process are often the largest contributors to wave‐driven water levels (wave runup) at the shoreline. Laboratory flume experiments and a two‐dimensional vertical phase‐resolving nonhydrostatic wave‐flow model, which includes a canopy model to predict drag forces generated by roughness elements, were used to investigate wave‐driven water levels for along‐shore uniform fringing reefs. In contrast to many previous studies, both the laboratory experiment and the numerical model account for the effects of large bottom roughness. The numerical model reproduced the observations of the wave transformation and runup over both smooth and rough reef profiles. The numerical model was then extended to quantify the influence of reef geometry and compared to simulations of plane beaches lacking a reef. For a fixed offshore forcing condition, the fore‐reef slope controlled wave runup on reef‐fronted beaches, whereas the beach slope controlled wave runup on plane beaches. As a result, the coastal protection utility of reefs is dependent on these slopes. For our examples, with a fore‐reef slope of 1/5 and a 500 m prototype reef flat length, a beach slope of ∼1/30 marked the transition between the reef providing runup reduction for steeper beach slopes and enhancing wave runup for milder slopes. Roughness coverage, spacing, dimensions, and drag coefficient were investigated, with results indicating the greatest runup reductions were due to tall roughness elements on the reef flat.

Two‐dimensional mean wave‐driven flow and setup dynamics were investigated at a reef‐lagoon system at Ningaloo Reef, Western Australia, using the numerical wave‐flow model, SWASH. Phase‐resolved numerical simulations of the wave and flow fields, validated with highly detailed field observations (including >10 sensors through the energetic surf zone), were used to quantify the main mechanisms that govern the mean momentum balances and resulting mean current and setup patterns, with particular attention to the role of nonlinear wave shapes. Momentum balances from the phase‐resolved model indicated that onshore flows near the reef crest were primarily driven by the wave force (dominated by radiation stress gradients) due to intense breaking, whereas the flow over the reef flat and inside the lagoon and channels was primarily driven by a pressure gradient. Wave setup inside the lagoon was primarily controlled by the wave force and bottom stress. The bottom stress reduced the setup on the reef flat and inside the lagoon. Excluding the bottom stress contribution in the setup balance resulted in an over prediction of the wave‐setup inside the lagoon by up to 200–370%. The bottom stress was found to be caused by the combined presence of onshore directed wave‐driven currents and (nonlinear) waves. Exclusion of the bottom stress contribution from nonlinear wave shapes led to an over prediction of the setup inside the lagoon by approximately 20–40%. The inclusion of the nonlinear wave shape contribution to the bottom stress term was found to be particularly relevant in reef regions that experience a net onshore mass flux over the reef crest.

Coral reefs generate substantial volumes of carbonate sediment, which is redistributed throughout the reef‐lagoon system. However, there is little understanding of the specific processes that transport this sediment produced on the outer portions of coral reefs throughout a reef‐lagoon system. Furthermore, the separate contributions of currents, sea‐swell waves, and infragravity waves to transport, which are all strongly influenced by the presence of a reef, is not fully understood. Here, we show that in reef‐lagoon systems most suspended sediment is transported close to the seabed and can, at times, be suspended higher in the water column during oscillatory flow transitions (i.e., near slack flow) at sea‐swell wave frequencies, and during the peak onshore oscillatory velocity phase at infragravity wave frequencies. While these wave frequencies contribute to the transport of suspended sediment offshore and onshore, respectively, the net flux is small. Mean currents are the primary transport mechanism and responsible for almost 2 orders of magnitude more suspended‐sediment flux than sea‐swell and infragravity waves. Whilst waves may not be the primary mechanism for the transport of sediment, our results suggest they are an important driver of sediment suspension from the seabed, as well as contributing to the partitioning of sediment grain sizes from the reef to the shoreline. As the ocean wave climate changes, sea level rises, and the composition of reef benthic communities change, the relative importance of mean currents, sea‐swell waves, and infragravity waves is likely to change, and this will affect how sediment is redistributed throughout reef‐lagoon systems.