Wave runup on fringing reefs with paleo-stream channels

Abstract

Many tropical coastlines are fronted by coral reefs and are increasingly exposed to wave attack and wave-driven marine flooding. This problem demands immediate attention as safe habitability of and social and economic activity in reef-lined coastal regions are under serious threat, while these regions are known to have some of the world’s highest population densities.
High runup events and flooding on coral reef-lined coasts have been subject of recent studies, and some valuable insights are gained. It was found that the recent increases in wave attack, high runup events, and coastal flooding are primarily due to high offshore water levels coinciding with high energy swell events, circumstances which will become more frequent with sea-level rise. Increasing water depth over the reef changes the hydrodynamics across the reef in such a way that larger incident-band and infra gravity-band waves reach the shoreline, causing high runup levels and flooding of the land behind the shoreline.
However, little is known about the influence of longshore variations on runup and flooding of reef-lined coasts, while most show significant longshore variations, of which shore-normal paleo-stream channels are a prevalent one. This study aims to fill in that knowledge gap by examining the influence of paleo-stream channels on runup along reef-fronted shorelines, specifically during extreme wave conditions.
With a system analysis we determine the range of naturally occurring topographies of reef-channel systems and determine a representative reef. Results of this analysis are a useful starting point for future studies on this subject.
With a parametric study using the numeric model XBeach, we show that the presence of a channel results in a strong circulation on the reef flat and significant longshore variation of runup. Depending on the geometry and forcing, runup levels are increased next to the channel or inside the channel. This impact of the channel increases for higher incident waves, lower incident wave steepness, wider channels, a narrower reef and shorter channel spacing. Longshore variation of infragravity wave height is responsible for large scale variations in runup, while setup, short waves and very low frequency wave heights cause a local increase of runup inside the channel.
Results of the parametric study are valuable as they provide insight in which locations on a coast are most vulnerable to high runup events, using only widely available data such as reef geometry and offshore wave conditions. This is relevant for prediction of coastal hazards and to guide coastal management policies. Furthermore, this study provides insights for future studies on flood risk of reef-lined coasts, as it illustrates the importance of accounting for longshore variations while schematizing a coastline to predict high runup and coastal flooding, for instance to assess when a 1D schematization is sufficient and when a 2D model is required.