Andrew W. M. Pomeroy | Search USGS

USGS2023

Wave-scale observations of sediment resuspension and subsequent transport across a fringing reef flat

During a 3-month deployment on a broad, fringing reef flat in Moloka’i, Hawai’i, we observed over 28,000 wave-driven resuspension (WDR) events of coarse-grained sediment in order to identify major factors. These events were short-lived (2-11 s) and distinct from the longer-duration patterns of water-column backscatter. The wave-driven transport of WDR events was onshore, but the net cross-shore transport was ultimately controlled by water levels. Higher water levels produced larger reef-flat waves, which were requisite for these events to occur. But rising water levels also drove stronger offshore flows. Consequently, onshore net transport of WDR events only occurred within a narrow water-level range, when waves were sufficiently large, but the offshore flow was still weak. Our observations demonstrate how cross-shore transport of coarse-grained material over reef flats is sensitive to changing water levels. Rising sea levels will likely alter transport patterns, which will in turn affect cross-shore delivery of carbonate sand to adjacent shorelines.

Hawaii↗

USGS2023

Combining field observations and high-resolution numerical modeling to demonstrate the effect of coral reef roughness on turbulence and its implications for reef restoration design

Coral reefs are effective natural barriers that protect adjacent coastal communities from hazards such as erosion and storm-induced flooding. However, the degradation of coral reefs compromises their ability to protect against these hazards, making degraded reefs a target for restoration. There have been limited field and numerical modeling studies conducted to understand how an increase in coral reef roughness, as would occur due to restoration, can affect wave energy dissipation for a range of real-world wave and water level conditions. To address this knowledge gap, field measurements were collected over adjacent low-roughness and high-roughness reefs off Molokaʻi, Hawaiʻi, USA, subjected to the same oceanographic forcing. Those field data were then used to calibrate and validate OpenFOAM computational fluid dynamics models of the reef. These calibrated models were then used to explore energy dissipation for a range of wave conditions based on measurements from a suite of existing datasets and values from the literature. In general, wave dissipation scales with incident wave conditions, where greater dissipation occurred for shallow depths and shorter-period waves. This tendency for short-period waves to be more readily attenuated is supported by wave energy dissipation factors in the range of 0.1–5, which decline with increasing wave period. Near-bed turbulent kinetic energy dissipation also scales with incident wave conditions, where the greatest difference in dissipation between low and high relief cases occurs for short wave periods. Turbulence becomes less affected by bottom roughness as the wave period increases. Based on this study, wave attenuation and turbulent energy dissipation could be enhanced by 0.5–1 order of magnitude (45% per across-shore meter) if the seabed roughness at the field site were increased by 13%, an achievable goal in coral reef restoration.

Coastal Engineering↗