Infragravity wave resonance over coral reef lined coasts

Abstract

Coral reefs are vital to the prosperity of the world and the local communities by providing food and coastal protection. Coral reefs are home to 25% of marine life and have therefore gained the nickname ’rainforest of the sea’. However, the reefs are being damaged by climate change and human intervention, resulting in an alarming rate of degradation. The combination of higher water levels on reef flats and the reduced friction due to reef degradation lead to greater risks of flooding and overwash on low-lying islands.

These aforementioned effects already present a threat to small islands that are naturally fronted by coral reefs. However, there is one more threat of damage to these islands: resonance. The characteristic bathymetry of the coral reefs can induce this type of threat, which is likely to be enhanced through climate change. Fringing reefs can display harbour-like resonance under the right conditions, because of their generally steep fore reefs and shallow horizontal reef flats. This has been shown to occur during various occasions, but the conditions that lead to are still unclear. What drives the occurrence and magnitude of resonance?

Resonance has been observed during high-energy events on various pacific islands, and though it occurs only 3.6% of the time, its damaging potential is larger than any other form of infragravity (IG) wave propagation. Various numerical models have been set up to study the behaviour and drivers of resonance, yet the defining characteristics and drivers of the phenomenon remain enigmatic.

To get a better understanding of the parameters that influence the occurrence and magnitude of resonance in fringing reef environments, a SWASH model is used. The amplification of the incoming IG wave is determined
by comparing the incoming IG wave energy at the beach toe for a beach boundary to an absorbing sponge layer boundary. If the ratio of incoming IG energy is greater than unity, resonance is expected to occur.

The magnitude of the AR is largely dependent on the characteristics of the incoming SS wave field and coral reef bathymetry: a great modulation and a gentle fore reef slope lead to a large amplification. Additionally, a lot of incoming SS wave energy corresponds to a limited amplification of the incoming IG wave, likely due to the interaction of the released bound long wave and the IG wave generated through break point forcing, which are 180° out of phase. Therefore, the largest amplification occurs for a strongly modulated wave field without being too energetic. There is no clear relationship between the incoming SS wave periods and the amplification, the height of the incoming IG wave is similar for various situations. The amplification appears to depend on the combination of the fore reef slope and incoming SS wave periods. Different SS wave periods
lead to different amplifications depending on the slope of the fore reef they pass over. The height of the incoming IG wave is largest for the steepest slopes under consideration. Thus, for these conditions, the risk of flooding is largest. However, the greatest amplification occurs for the gentler slopes
under the considered conditions. It should be noted that this relationship is heavily dependent on the combination of incoming SS periods and the steepness of the fore reef slopes.

The period for which resonance occurs is always higher than the theoretical fundamental eigenperiod, which is found through water depth integration from the location of minimum IG wave height to the beach toe. Integrating the water depth from the most offshore point of break point forcing leads to a smaller deviation of the theoretical fundamental eigenperiod from the observed period, yet this method needs further exploration.

The insights acquired by this research can be used as a base for site-specific modelling, which can provide circumstances under which certain areas are at risk of flooding due to IG wave resonance.