MG
M.J.J. Giliam
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Low-Frequency Wave Dynamics over a Fringing Reef
Observations from La Saline Reef, La Réunion
Coral reefs provide natural coastal protection by strongly dissipating incident gravity-wave (GW) energy. However, over shallow fringing reefs, low-frequency (LF) waves can dominate water-level variability and contribute substantially to shoreline run-up. The present study aims to identify the processes and forcing conditions that govern the magnitude and transformation of infragravity (IG) and very-low-frequency (VLF) waves over a fringing reef. For this purpose, an observational dataset was analysed, collected over approximately three months between June and September 2024 at the La Saline fringing reef on the microtidal western coast of La Réunion. It consisted of pressure and velocity measurements at multiple locations extending from the reef slope across the reef flat. In addition, pressure and velocity measurements close to the shoreline and video-derived run-up observations were available for 11 July 2024.
This study showed that the reef acts as a strong low-pass filter, with GW height decreasing on average by 97.6% from the reef slope to the mid-reef, causing IG and VLF waves to dominate the reef-flat wave field. Variability in both IG and VLF wave heights was mainly explained by the offshore GW power and was further modulated by the water level. IG wave heights at the mid-reef remained approximately equal to their offshore magnitude, whereas VLF wave heights were on average amplified by 235% relative to offshore conditions. In addition, observations on 11 July showed that IG wave heights increased again shoreward, while VLF wave heights remained approximately constant between the mid-reef and the nearshore station.
High reflection near the shoreline, together with cross-shore spectral patterns consistent with the expected locations of nodes and antinodes, indicates the development of standing LF waves across the reef flat. The reflection coefficient decreased offshore, suggesting that the wave field becomes more progressive in character towards the reef crest.
Individual events showed indications of a resonant VLF response consistent with an open-ended basin configuration of the reef flat. However, VLF energy was also present when no such resonant signature was observed. This suggests that resonance alone cannot fully explain the VLF energy on the reef flat. Other generation mechanisms, such as non-linear interactions between IG components, may therefore contribute as well, but require further investigation.
The run-up observations showed that wave set-up formed the largest contribution at 65.9%, while the IG band dominated the oscillatory part at 28.6%, against 3.3% for GW and 2.2% for VLF. Moreover, the 2% exceedance run-up level was approximately 28% higher than the corresponding surface-elevation level close to the shoreline. This indicates that nearshore water-level measurements may underestimate the water levels reached at the shoreline.
Overall, the results show that LF wave heights at La Saline are strongly linked to offshore GW forcing and water levels over the reef, while their spatial structure is shaped by (partly) standing-wave patterns. These LF motions dominate the nearshore wave field and contribute substantially to shoreline run-up. Understanding their full dynamics, including the role of resonance, is therefore important for future flood risk assessment. ...
This study showed that the reef acts as a strong low-pass filter, with GW height decreasing on average by 97.6% from the reef slope to the mid-reef, causing IG and VLF waves to dominate the reef-flat wave field. Variability in both IG and VLF wave heights was mainly explained by the offshore GW power and was further modulated by the water level. IG wave heights at the mid-reef remained approximately equal to their offshore magnitude, whereas VLF wave heights were on average amplified by 235% relative to offshore conditions. In addition, observations on 11 July showed that IG wave heights increased again shoreward, while VLF wave heights remained approximately constant between the mid-reef and the nearshore station.
High reflection near the shoreline, together with cross-shore spectral patterns consistent with the expected locations of nodes and antinodes, indicates the development of standing LF waves across the reef flat. The reflection coefficient decreased offshore, suggesting that the wave field becomes more progressive in character towards the reef crest.
Individual events showed indications of a resonant VLF response consistent with an open-ended basin configuration of the reef flat. However, VLF energy was also present when no such resonant signature was observed. This suggests that resonance alone cannot fully explain the VLF energy on the reef flat. Other generation mechanisms, such as non-linear interactions between IG components, may therefore contribute as well, but require further investigation.
The run-up observations showed that wave set-up formed the largest contribution at 65.9%, while the IG band dominated the oscillatory part at 28.6%, against 3.3% for GW and 2.2% for VLF. Moreover, the 2% exceedance run-up level was approximately 28% higher than the corresponding surface-elevation level close to the shoreline. This indicates that nearshore water-level measurements may underestimate the water levels reached at the shoreline.
Overall, the results show that LF wave heights at La Saline are strongly linked to offshore GW forcing and water levels over the reef, while their spatial structure is shaped by (partly) standing-wave patterns. These LF motions dominate the nearshore wave field and contribute substantially to shoreline run-up. Understanding their full dynamics, including the role of resonance, is therefore important for future flood risk assessment. ...
Coral reefs provide natural coastal protection by strongly dissipating incident gravity-wave (GW) energy. However, over shallow fringing reefs, low-frequency (LF) waves can dominate water-level variability and contribute substantially to shoreline run-up. The present study aims to identify the processes and forcing conditions that govern the magnitude and transformation of infragravity (IG) and very-low-frequency (VLF) waves over a fringing reef. For this purpose, an observational dataset was analysed, collected over approximately three months between June and September 2024 at the La Saline fringing reef on the microtidal western coast of La Réunion. It consisted of pressure and velocity measurements at multiple locations extending from the reef slope across the reef flat. In addition, pressure and velocity measurements close to the shoreline and video-derived run-up observations were available for 11 July 2024.
This study showed that the reef acts as a strong low-pass filter, with GW height decreasing on average by 97.6% from the reef slope to the mid-reef, causing IG and VLF waves to dominate the reef-flat wave field. Variability in both IG and VLF wave heights was mainly explained by the offshore GW power and was further modulated by the water level. IG wave heights at the mid-reef remained approximately equal to their offshore magnitude, whereas VLF wave heights were on average amplified by 235% relative to offshore conditions. In addition, observations on 11 July showed that IG wave heights increased again shoreward, while VLF wave heights remained approximately constant between the mid-reef and the nearshore station.
High reflection near the shoreline, together with cross-shore spectral patterns consistent with the expected locations of nodes and antinodes, indicates the development of standing LF waves across the reef flat. The reflection coefficient decreased offshore, suggesting that the wave field becomes more progressive in character towards the reef crest.
Individual events showed indications of a resonant VLF response consistent with an open-ended basin configuration of the reef flat. However, VLF energy was also present when no such resonant signature was observed. This suggests that resonance alone cannot fully explain the VLF energy on the reef flat. Other generation mechanisms, such as non-linear interactions between IG components, may therefore contribute as well, but require further investigation.
The run-up observations showed that wave set-up formed the largest contribution at 65.9%, while the IG band dominated the oscillatory part at 28.6%, against 3.3% for GW and 2.2% for VLF. Moreover, the 2% exceedance run-up level was approximately 28% higher than the corresponding surface-elevation level close to the shoreline. This indicates that nearshore water-level measurements may underestimate the water levels reached at the shoreline.
Overall, the results show that LF wave heights at La Saline are strongly linked to offshore GW forcing and water levels over the reef, while their spatial structure is shaped by (partly) standing-wave patterns. These LF motions dominate the nearshore wave field and contribute substantially to shoreline run-up. Understanding their full dynamics, including the role of resonance, is therefore important for future flood risk assessment.
This study showed that the reef acts as a strong low-pass filter, with GW height decreasing on average by 97.6% from the reef slope to the mid-reef, causing IG and VLF waves to dominate the reef-flat wave field. Variability in both IG and VLF wave heights was mainly explained by the offshore GW power and was further modulated by the water level. IG wave heights at the mid-reef remained approximately equal to their offshore magnitude, whereas VLF wave heights were on average amplified by 235% relative to offshore conditions. In addition, observations on 11 July showed that IG wave heights increased again shoreward, while VLF wave heights remained approximately constant between the mid-reef and the nearshore station.
High reflection near the shoreline, together with cross-shore spectral patterns consistent with the expected locations of nodes and antinodes, indicates the development of standing LF waves across the reef flat. The reflection coefficient decreased offshore, suggesting that the wave field becomes more progressive in character towards the reef crest.
Individual events showed indications of a resonant VLF response consistent with an open-ended basin configuration of the reef flat. However, VLF energy was also present when no such resonant signature was observed. This suggests that resonance alone cannot fully explain the VLF energy on the reef flat. Other generation mechanisms, such as non-linear interactions between IG components, may therefore contribute as well, but require further investigation.
The run-up observations showed that wave set-up formed the largest contribution at 65.9%, while the IG band dominated the oscillatory part at 28.6%, against 3.3% for GW and 2.2% for VLF. Moreover, the 2% exceedance run-up level was approximately 28% higher than the corresponding surface-elevation level close to the shoreline. This indicates that nearshore water-level measurements may underestimate the water levels reached at the shoreline.
Overall, the results show that LF wave heights at La Saline are strongly linked to offshore GW forcing and water levels over the reef, while their spatial structure is shaped by (partly) standing-wave patterns. These LF motions dominate the nearshore wave field and contribute substantially to shoreline run-up. Understanding their full dynamics, including the role of resonance, is therefore important for future flood risk assessment.