Rebecca H. Green

This paper will demonstrate how the parallelism and expressiveness of the Chapel programming language are used to achieve an enormous improvement in computational speed for a problem related to coral reef conservation. Chapel’s concise syntax and versatile data structures enable this problem to be solved in under 300 lines of code, while reducing the time to solution from days down to the order of seconds. This improvement is so substantial that it represents a paradigm shift in the way biodiversity can be measured at scale, providing a wealth of novel information for marine ecosystem managers and opening up brand new avenues for scientific inquiry. This paper will review the solution strategy and data structures in Chapel that allowed these improvements to be realized, and will preview future extensions of this work that have been made possible by this drastic speedup.

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.

An accurate representation of physical and biological processes is crucial to resolve larval dispersal pathways and characterize connectivity of coral reef ecosystems. We investigate how hydrodynamic forcings drive larval retention rates during the bi‐annual mass coral spawning of the coral genus Acropora within a coral reef atoll (Mermaid Reef), located off northwestern Australia. By analyzing hydrodynamic conditions during 41 yr of historical spring and autumn coral spawning events, we identify typical and extreme hydrodynamic forcing conditions. Particle tracking using the output from a fine‐scale coupled wave‐flow hydrodynamic model forced with typical hydrodynamic conditions during coral spawning, revealed a mean transport of larvae eastward across the atoll. Transport was mainly driven by a combination of wave and tidal currents, where the residual tidal flow and unidirectional wave flow increased the net export of particles, and the oscillatory tidal (non‐residual) flow reduced the net export of particles from the reef. Importantly, however, numerical simulations forced with extreme hydrodynamic conditions generated by episodic tropical cyclones (11 out of 41 yr) showed large deviations from the typical eastward flow during autumn spawning, generating different connectivity pathways within the reef. Considering the substantial time larvae can be retained within reef systems, overlooking fine‐scale hydrodynamic processes may greatly overestimate larval transport distances between adjacent coral reef atolls. As a result, we emphasize the need to consider fine‐scale hydrodynamic processes within regional connectivity predictions, which is generally not considered yet critical to understand the capacity of reefs to recover following disturbances.

Waves and tides are often the two primary forcing mechanisms responsible for driving hydrodynamic processes within coral reefs worldwide. Although wave‐ and tide‐driven flows are individually well understood, there remain considerable gaps in our understanding of how their interactions control the reef circulation, and consequently how they shape a range of ecological processes. During 11 months of hydrodynamic measurements across Mermaid Reef, a coral reef atoll off northwestern Australia, the atoll was regularly exposed to a range of wave and tidal conditions. Using a validated wave‐flow numerical model, we showed that wave‐ and tidally driven processes interacted to drive the reef's circulation through several mechanisms including wave‐current interactions and tidal water level modulation of wave‐driven flows. The atoll morphology, particularly the higher elevation of the western reef flat, was found to be a key factor controlling the relative importance of waves and tides. Wave‐driven processes dominated for tidal ranges smaller than required to expose the shallower western reef flat. In contrast, tidal processes dominated for larger tidal ranges, when the western reef flat temporarily acted as a physical barrier to incoming and outgoing flows. The residual (tidally averaged) circulation was consistently directed eastward across the atoll. Over time scales of several months to years, Mermaid Reef can be classified as a tide‐dominated reef. However, due to the incident wave energy and spring‐neap tidal range variability, the relative importance of the dominant hydrodynamic drivers can vary on time scales of hours to days allowing wave processes to episodically dominate the reef circulation.

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.