Modeling wave processes over fringing reefs with an excavation pit

Abstract

The excavation of reef-flat sand and aggregate for engineering projects is a common practice on atoll islands, with attendant coastal management and hazard mitigation issues. To assess the impact of reef-flat excavation pits on wave transformation, a numerical study is carried out based on one-dimensional weakly dispersive, highly nonlinear Boussinesq equations. Model simulations are compared to field observations made at Majuro Atoll, Republic of the Marshall Islands (Ford et al., 2013a) at a 75-m wide fringing reef flat with a 17-m wide, 4-m deep excavation pit and at an adjacent unmodified reef flat. With a calibrated empirical breaking model and uniform friction coefficient, the numerical model is shown to satisfactorily reproduce the observed sea and swell (SS) and infragravity (IG) frequency band wave heights for a moderate amplitude wave event. By removing the lateral morphological differences of the field experiment, the model shows that an excavation pit reduces/increases shoreline IG /SS wave heights compared to numerical tests without the pit, in qualitativeagreement with Ford et al. (2013a). An EOF analysis demonstrates that the reduction in IG wave heights is associated with modifications of the ¼ and ¾ wavelength IG standing modes across the reef caused by the pit. Model tests are used to evaluate the effects of cross-reef pit location and width on wave transformation. Shoreline wave heights are least affected by a pit located near the reef crest, and both SS and IG wave heights increase as the pit location moves shoreward.Shoreline wave heights are an increasing function of pit width as the increase in SS wave height exceeds the decrease in IG wave height produced by the 3 pit. The effects of incident wave height and spectral bandwidth are also examined for the reef with and without pit. It is found that both shoreline SS and IG wave heights increase with the increasing incident wave height for both profiles. The numerical model results also demonstrate that the shoreline response is sensitive to the bandwidth of the incident wave spectrum, and larger IG shoreline wave heights are excited for narrower spectral bandwidths until wave reflection limits the forcing.

Impact of Coral Reef Mining Pits on Nearshore Hydrodynamics and Wave Runup During Extreme Wave Events

“…Additionally, due to less energy transfer from the SS peak to SS tail through triad nonlinear wave–wave interactions, there is increased wave energy in the SS peak band and decreased wave energy in the SS tail band. For increasing pit width, relative to the reef flat width, SS wave height also increases compared to narrow pits (Figures a and b), which generalizes findings from Yao et al (). Wider pits result in larger wave heights in both the SS peak and SS tail bands compared to narrow pits, mainly due to less wave breaking and to a lesser extent due to lower transfer of wave energy from the SS to the IG band.…”

“…However, the findings by Ford et al () and Yao et al () are not applicable in a more generic sense, and are likely to change for different reef morphologies, pit designs, and wave conditions. Additionally, Yao et al () emphasized that further research is needed to understand the broad implications that mining pits can have on the coastal environment.…”

“…A numerical wave modeling study of the same reef showed that pit width and cross‐shore location can influence the nearshore wave heights (Yao et al, ). Yao et al () also suggested that the decrease in IG wave energy due to the presence of the pit was likely caused by a modification of the two first two natural frequencies of the reef, which could have resulted from a change of the reef open basin structure (Wilson, ).…”

“…A numerical wave modeling study of the same reef showed that pit width and cross‐shore location can influence the nearshore wave heights (Yao et al, ). Yao et al () also suggested that the decrease in IG wave energy due to the presence of the pit was likely caused by a modification of the two first two natural frequencies of the reef, which could have resulted from a change of the reef open basin structure (Wilson, ). The observed increase in SS wave energy at the shoreline was attributed to a decrease in hydrodynamic roughness by removal of the rough reef surface, as well as an increase in water depth that lead to a decrease of breaking induced dissipation (Yao et al, ).…”

“…Yao et al () also suggested that the decrease in IG wave energy due to the presence of the pit was likely caused by a modification of the two first two natural frequencies of the reef, which could have resulted from a change of the reef open basin structure (Wilson, ). The observed increase in SS wave energy at the shoreline was attributed to a decrease in hydrodynamic roughness by removal of the rough reef surface, as well as an increase in water depth that lead to a decrease of breaking induced dissipation (Yao et al, ). Payo and Muñoz‐Perez () argued that the observed changes in SS and IG energy could be coupled, as a decrease in wave breaking due to the presence of the excavation causes a lower transfer of wave energy from the SS to the IG wave band.…”