Steps to Develop Early Warning Systems and Future Scenarios of Storm Wave-Driven Flooding Along Coral Reef-Lined Coasts

ABSTRACT: Tropical coral reef-lined coasts are exposed to storm wave-driven flooding. In the future, flood events during storms are expected to occur more frequently and to be more severe due to sea-level rise, changes in wind and weather patterns, and the deterioration of coral reefs. Hence, disaster managers and coastal planners are in urgent need of decision-support tools. In the short-term, these tools can be applied in Early Warning Systems (EWS) that can help to prepare for and respond to impending storm-driven flood events. In the long-term, future scenarios of flooding events enable coastal communities and managers to plan and implement adequate risk-reduction strategies. Modeling tools that are used in currently available coastal flood EWS and future scenarios have been developed for open-coast sandy shorelines, which have only limited applicability for coral reef-lined shorelines. The tools need to be able to predict local sea levels, offshore waves, as well as their nearshore transformation over the reefs, and translate this information to onshore flood levels. In addition, future scenarios require long-term projections of coral reef growth, reef composition, and shoreline change. To address these challenges, we have formed the UFORiC (Understanding Flooding of Reef-lined Coasts) working group that outlines its perspectives on data and model requirements to develop EWS for storms and scenarios specific to coral reef-lined coastlines. It reviews the state-of-the-art methods that can currently be incorporated in such systems and provides an outlook on future improvements as new data sources and enhanced methods become available

An investigation of wave-dominated coral reef hydrodynamics

The coastal zone is of great societal and economic value. Understanding anthropogenic impacts and natural processes is a prerequisite to effective management of coastal resources, and a key part of this understanding is the prediction (both past and future) of the coastal zone's hydrodynamics. Methods of predicting the hydrodynamics of coral reef systems, which tend to be morphologically complex and subject to a variety of oscillatory and non-oscillatory motions over a large range of space and time scales, remain poorly developed.\ud \ud Recent advances in numerical modeling have allowed the practical solution of the two- and three-dimensional shallow water Navier-Stokes equations at spatial scales on the order of tens of meters. This has allowed unprecedented prediction of coastal hydrodynamics, and its use is expanding, particularly in mid- to high latitude continental margins regions. Few researches have yet applied these advances to coral reef systems, however.\ud \ud The goal of this work is to improve the understanding and prediction of relevant hydrodynamic processes in coral reef systems. This is accomplished by the combined analysis of in situ oceanographic instrument data and climate information, as well as the application of a coupled wave-flow numerical model at two different study sites. The study sites, Hanalei Bay and Midway Atoll, both in the Hawaiian Islands (Figure 1.1), constitute two fundamentally different reef morphologies, a fringing reef embayment and an atoll, respectively. Both are subjected to a wide range of wind-wave energy, which is shown to force the most energetic hydraulic motions at both sites.\ud \ud Results include an evaluation of the numerical models used, a statistical analysis of wind-wave climate that identifies major modes of coastal circulation, and the calculation of flushing times and other coastal hydrodynamic metrics under different conditions. Model evaluation shows that if the spatially varying hydraulic roughness and wave dissipation approximations presented here are used, coupled wave/flow numerical model skill for steep and morphologically complex coral reefs may approach that of milder sloped mid-latitude continental margin coasts. The results also highlight important hydrodynamic differences between prevailing (wind and wave) conditions and episodic storm wave events. These events incur water levels, current velocities, flushing rates, and inferred sediment transport several orders of magnitude greater than those of prevalent conditions. For instance, flushing (residence) times at both study sites are on the order of 1-3 days during prevalent conditions, whilst during large storm wave events flushing time may reduce to several hours. The high near-bed flows and associated shear stresses episodically mobilize and transport seafloor sediment and heavily impact the benthic biological community.\ud \ud The number and magnitude of these episodic events are shown to exhibit high interannual variability linked to climate indices for El Niño/Southern Oscillation (ENSO) and the North Pacific Index (NPI). The historically small (but variable) number of these events (between 0 and approximately 20) indicate that annual differences in net sedimentation and water quality are very large at both sites, and most likely sensitive to long-term changes in annual recurrence. Additionally, large changes in sea level anomaly during these large wave events, evident in model predictions and confirmed by tide gauge data at Midway Atoll, introduce an unaccounted for variable in contemporary sea-level trend analyses, possibly at many in situ sea level monitoring sites in the Pacific and Indian Oceans

Wind and Wave Setup Contributions to Extreme Sea Levels at a Tropical High Island: A Stochastic Cyclone Simulation Study for Apia, Samoa

Wind-wave contributions to tropical cyclone (TC)-induced extreme sea levels are known to be significant in areas with narrow littoral zones, particularly at oceanic islands. Despite this, little information exists in many of these locations to assess the likelihood of inundation, the relative contribution of wind and wave setup to this inundation, and how it may change with sea level rise (SLR), particularly at scales relevant to coastal infrastructure. In this study, we explore TC-induced extreme sea levels at spatial scales on the order of tens of meters at Apia, the capitol of Samoa, a nation in the tropical South Pacific with typical high-island fringing reef morphology. Ensembles of stochastically generated TCs (based on historical information) are combined with numerical simulations of wind waves, storm-surge, and wave setup to develop high-resolution statistical information on extreme sea levels and local contributions of wind setup and wave setup. The results indicate that storm track and local morphological details lead to local differences in extreme sea levels on the order of 1 m at spatial scales of less than 1 km. Wave setup is the overall largest contributor at most locations; however, wind setup may exceed wave setup in some sheltered bays. When an arbitrary SLR scenario (+1 m) is introduced, overall extreme sea levels are found to modestly decrease relative to SLR, but wave energy near the shoreline greatly increases, consistent with a number of other recent studies. These differences have implications for coastal adaptation strategies