The steep slopes and large bottom roughness of coral reefs contrast with the milder slopes and smoother bottoms of sandy beaches where most historical research on surf zone processes has been targeted. These physical characteristics of reefs may thus explicitly violate the underlying assumptions of existing surf zone models when applied to reef systems. Given the importance of surf zone dynamics to a range of physical dynamics (e.g., coastal inundation and sediment transport) and biological processes (e.g., larval and nutrient transport and dispersal), there is a need to rigorously assess existing theory and, where necessary, develop new theory appropriate for reef environments. This thesis was directed at addressing these open questions through three chapters. In chapter two, the suitability of commonly used numerical models in steepslope reef environments was assessed using existing laboratory observations on a smooth reef profile. In chapter three, the dynamics of wave setup were evaluated with higher spatial-resolution data collected in a 1:36 scale wave flume with a smooth bottom and characteristic fringing reef profile. Finally in chapter four, the effect of large bottom roughness on wave setup was determined by comparing the results from the smooth laboratory flume experiments with the same experiments conducted over scaled roughness elements.
In chapter two, we find that existing nearshore numerical models are capable of reproducing observations of waves and wave setup across a laboratory reef profile with reasonable accuracy, despite the steep (1:11) reef slope explicitly violating the mildslope and other fundamental assumptions that underpin the numerical models. However, a partial explanation for the good agreement with observations reported in chapter two, and elsewhere, may be owing to the tuning of empirical coefficients and hence not be physically meaningful. Indeed, in many cases tuning model parameters to best reproduce the observed wave height decay resulted in a reduction in the accuracy of wave setup predictions, which indicates a fundamental breakdown in the dynamical relationship between the predicted radiation stress gradients and the observed wave setup. However, as detailed surf zone measurements were not available, it was not possible to further evaluate the details of wave and setup dynamics.
The general lack of detailed surf zone measurements on reefs was rectified by conducting the high-resolution laboratory experiments detailed in chapters three and four. In these chapters, we utilized new datasets to evaluate the time-averaged crossshore momentum equation from observations of water levels and velocities, rather than the traditional approach of using empirical models to predict wave transformation through the surf zone. For runs without roughness (chapter three), the momentum equation collapsed to a balance between pressure and radiation stress gradients, and revealed a significant underestimation of wave setup when radiation stress gradients were approximated from observations using linear wave theory alone. Including the radiation stress contribution from wave rollers was found to reduce the under prediction of setup from 21% to only 3% on average.
Using an array of roughness elements affixed to the same reef profile as in chapter three, the influence of the large bottom roughness typical of coral reefs was evaluated in chapter four. Despite the presence of roughness modifying the setup profiles near the reef crest and changing the partitioning between wave and bottom stress forces, roughness did not affect the net setup on the reef flat. These results are in contrast to the often reported large increases in setup predicted by the presence of roughness in numerical models. The consistency between the setup on the smooth and rough laboratory profiles is accounted for by an 18% (on average) reduction in setup generated by radiation stress gradients in the rough versus smooth cases coupled with a 16% (on average) increase in setup generation by bottom roughness resistance forces in the rough cases. These results highlight the need for precise predictions of the wave field and the time-averaged bottom stress to accurately predict the setup response on steep-slope reefs. This thesis contributes to the understanding and prediction of the complex wave and setup dynamics on steep slopes and in areas of large bottom roughness.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2016|