Graduation of Rieneke van Noort
Resonance of infragravity waves on coral reef lined coasts
Professor of graduation: Prof. Dr. Ir. AJHM Reniers
Supervisors: Dr. MFS Tissier (TU Delft), Dr. Ir. J. A. Alvarez Antolínez (TU Delft)
Coral reefs are vital to the prosperity of the world and the communities living in the area, by providing food and coastal protection. Coral reefs are home to 25 % of marine life found on the planet and therefore deserve the nickname ’Rainforest of the sea’. However, the health of the coral reefs is damaged by climate change and human intervention as they are degrading at an alarming rate. 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.
In addition to the general increased risk in damage for small island preceded by a coral reef due to the aforementioned effects, the characteristic bathymetry of coral reefs induces another devastating phenomenon that is likely to be enhanced through climate change: resonance. Because of the generally steep fore reefs and shallow horizontal reef flats, fringing reefs can display harbour-like resonance under the right forcing. 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 complete 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. At the base of this model lies the schematized bathymetry of Roi-Namur island, RMI. The incoming Sea Swell (SS) wave field is schematized into a bichromatic wave field Simulations are executed with variations in amplitudes and periods of the SS waves, as well as the slope of the fore reef that precedes the reef flat. The amplification of the incoming IG wave is determined by comparing the incoming IG wave height at the beach toe for a beach boundary to an absorbing boundary. If the ratio of these wave heights is greater than unity, resonance is expected to occur.
The amplification ratio (AR) of incoming IG wave energy is largely dependent on the characteristics of the incoming SS wave field and coral reef bathymetry: a great modulation and a mild fore reef slope lead to a large amplification. Additionally, a lot of incoming SS wave energy reduces the amplification of the incoming IG wave. There is no clear relationship between the incoming SS wave periods and AR, yet a dependence appears to exist of the amplification on the combination of the fore reef slope and the incoming SS wave periods. Different SS wave periods may lead to different amplification depending on the slope of the fore reef they pass over. The absolute value for the magnitude of the incoming IG wave is greatest for the steepest slopes under consideration, despite the lower AR. This is due to the fact that steep slopes generate more IG wave energy through the coral reef-dominant break-point forcing than milder slopes.
The insights acquired by this research can be used as a base for site-specific modelling to get a better understanding under which circumstances a certain location is at risk of damage due to IG wave resonance.