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Vulnerability of Protected Coastlines in a Changing Environment

Johan Reyns, Ap van Dongeren, Dano Roelvink, Ryan Lowe, James Falter, Bryan Boruff (Eds.)

A report to the Asian Development Bank within the framework of the ADB ‐ UNESCO‐IHE Knowledge Partnership

This project was sponsored by the Asian Development Bank under the ADB‐UNESCO‐IHE Knowledge Partnership

Correspondence to:

Prof. Dr. ir. J.A. Roelvink Department of Science and Engineering UNESCO‐IHE Institute for Water Education Westvest 7 2611 AX Delft The Netherlands d.roelvink@unesco‐ihe.org +31 15 2151838

Copyright front page picture: National Geographic / Nick Caloyianis

White Paper

‐ Vulnerability of Coral Reef Protected Coastlines in a Changing Environment ‐

Editors: Reyns, J. (UNESCO‐IHE) Van Dongeren, A. (Deltares) Roelvink, D. (UNESCO‐IHE) Lowe, R. (University of Western ) Falter, J. (University of ) Boruff, B. (University of Western Australia)

Authors: Boruff, B. (University of Western Australia) Brinkman, R. (Australian Institute of Marine Science) Clifton, J. (University of Western Australia) Falter, J. (University of Western Australia) Huang, Zh. (Nanyang Technical University) Kench, P. (University of Auckland) Kruger, J. (SPC Applied Geoscience and Technology Division ‐ SOPAC) Lowe, R. (University of Western Australia) Monismith, S. (Stanford University) Péquignet, Ch. (University of Western Australia) Poerbandono (ITB Bandung) Pohler, S. (University of the South Pacific) Reyns, J. (UNESCO‐IHE) Roelvink, D. (UNESCO‐IHE) Storlazzi, C. (US Geological Survey) Symonds, G. (CSIRO) Van Dongeren, A. (Deltares)

Table of Contents

Table of Contents ...... I Executive Summary ...... 1 1 Introduction ...... 5 2 Hydrodynamics ...... 7 2.1 Knowledge ...... 7 2.1.1 Introduction ...... 7 2.1.2 Surges ...... 7 2.1.3 Tsunamis ...... 8 2.1.4 Waves ...... 8 2.2 Data availability ...... 13 2.3 Measurement/monitoring techniques ...... 16 2.4 Modeling capability and needs ...... 17 2.5 Summary and Actions ...... 23 3 Reef Dynamics ...... 25 3.1 Knowledge ...... 25 3.2 Data availability ...... 27 3.3 Measurement/Monitoring Techniques ...... 28 3.3.1 Existing tools: Measurement of seabed properties ...... 28 3.3.2 Existing tools: Measurement of Properties ...... 29 3.3.3 Needs ...... 30 3.4 Modeling capabilities and needs ...... 31 3.5 Summary and Actions ...... 33 4 Reef biogenic production processes ...... 37 4.1 Knowledge ...... 37 4.1.1 Production and destruction of biogenic reef carbonate ...... 37 4.1.2 Reef Accretion and Accretion Rates ...... 39 4.1.3 The Detrital Sediment Budget and Reef Associated ...... 41 4.1.4 Guilds concept ...... 44 4.1.5 Reef types ...... 45 I White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

4.2 Data availability ...... 47 4.3 Measurement/monitoring techniques ...... 48 4.4 Modeling capability and needs ...... 48 4.4.1 Biologically driven rates of carbonate production ...... 48 4.4.2 Deposition rates in reefal environments ...... 48 4.5 Summary and Actions ...... 49 5 Coastal response ...... 53 5.1 Knowledge ...... 53 5.1.1 Conceptual model ...... 53 5.1.2 influence ...... 55 5.2 Availability of data ...... 55 5.2.1 Coastline mapping from historic RS imagery ...... 55 5.2.2 Topographic mapping ...... 56 5.3 Measurement/monitoring techniques ...... 56 5.3.1 Topography and bathymetry ...... 56 5.3.2 Edge of vegetation, toe of from remote sensing ...... 56 5.3.3 Long‐term hydrodynamic monitoring at toe of beach ...... 57 5.4 Modeling ...... 57 5.4.1 Shoreline translation modeling ...... 57 5.4.2 Process‐based modeling ...... 58 5.4.3 Conclusions on modeling capabilities ...... 60 5.5 Summary and Actions ...... 61 6 ...... 63 6.1 Knowledge ...... 63 6.1.1 Inundation and overland flow ...... 63 6.1.2 ...... 64 6.1.3 Saline water intrusion ...... 64 6.2 Data availability ...... 67 6.2.1 Inundation and overland flow ...... 67 6.2.2 Erosion ...... 68 6.2.3 Saline water intrusion ...... 68 II White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

6.3 Measurement/monitoring techniques ...... 69 6.3.1 Inundation and overland flow ...... 69 6.3.2 Erosion ...... 69 6.3.3 Saline water intrusion ...... 70 6.4 Modeling capability and needs ...... 70 6.4.1 Inundation and overland flow ...... 70 6.4.2 Erosion ...... 70 6.4.3 Saline Water Intrusion ...... 71 6.5 Summary and Actions ...... 73 7 Vulnerability ...... 77 7.1 Knowledge ...... 77 7.1.1 Defining vulnerability ...... 77 7.1.2 Evolution of Vulnerability ...... 77 7.2 Data availability ...... 78 7.3 Measurement/Monitoring Techniques ...... 80 7.4 Modeling Capability and Needs ...... 81 7.5 Summary and Actions ...... 83 8 References ...... 85

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IV White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

Executive Summary

Coral reefs are abundant features of tropical ocean environments. They are among the most biodiverse and economically important on the planet and are vital to numerous human societies in the Asia‐ Pacific region. In fact, much of the world’s coral reef are located off the of ADB member states in the Indian and . Coral reefs form an integral part of many and coastline ecosystems and their health and function determine to a large extent the resilience of coastal communities.

However, coral reefs are in peril because of local and global causes. Local causes such as from growing population, and economic development in combination with a lack of effective management of these have led to degradation of reef health. The consequence is that coral reefs may no longer be able to fulfill their many services, which makes the coastal zone and its population more vulnerable to hazards. This is a vicious cycle which needs to be halted, also because global causes such as pose important additional pressures on these ecosystems.

In to effectively manage the consequences of these numerous changes to reefs on human populations, we need the capability to reliably predict the impact of such as , inundation and groundwater salinization on the short‐term scale of natural events such as tsunamis, typhoons and other weather events. As this white paper shows, some of the elements to achieve this capability are now present, but essential aspects and especially the integration of knowledge is currently lacking. Likewise, we lack the capability to connect reef health, global climate change and the reefs’ protective function which are essential in order to make long‐term predictions of the development of low‐lying and coastal areas. Both these capabilities are important in order to design effective risk‐informed management plans in terms of prevention, mitigation and preparedness measures, so that the vulnerability of the coastal zone can be reduced.

In most ADB member states, short‐term and long‐term coral reef processes have not been studied well. This is detrimental for the region since it deprives regional centers of accumulating knowledge about local systems, prevents capacity building and the ability to forecast local effects of changing conditions. It is critical that this capability is developed in the Asia‐Pacific region with a focus on addressing regional problems. However, the development of this capability cannot be borne solely by the individual ADB member states with extensive coral reef coastlines, because of financial and knowledge‐infrastructural reasons, and needs to be developed in a supra‐national effort in which the transfer of knowledge from regional and international experts to ADB member states is central.

We have started addressing this need to develop regional capability in the following way. First of all, with funding by the ADB, we have organized a week‐long workshop in Kota Kinabalu, , Malaysia in May 2013, hosted jointly by UNESCO‐IHE and the Borneo Marine Research Institute. This event brought scientists and local experts from the region together to discuss the state‐of‐the‐art and data availability in modeling, observation techniques and vulnerability assessment related to Coastal Hazards in Coral Reef Environments, to identify knowledge gaps and to prioritize actions. This group identified the knowledge, ability,

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gaps and needs in the field of reef hydrodynamics, sediment dynamics, reef biogenic production processes, coastal response, hazards and vulnerability. The detailed results are laid down in the present report.

In synopsis we can state that current knowledge acquired for and model capability developed for sandy coast environments form a solid basis for application on coral reef coasts. However, there are important physical differences which should be stressed and which hinder application of knowledge and tools developed for sedimentary coasts to coral reef environments.

One difference is the effect of the rough topography on the wave and sediment motion and hence the associated coastal change. Another is that limited field measurements to date have shown that longer‐period waves, not incorporated in many current‐state models, are dominant over coral reef platforms and thus important in evaluation. These differences between sandy and coral coasts require theory development and detailed process measurements either from the field or from the lab, for calibration and validation. With these models in place, modeling of coastal hazards (erosion, inundation and ground water salinization) due to short‐term events such as tsunamis, typhoons and events should be initiated on a number coral reef coasts in the region, in conjunction with observations of these hazards. This dedicated field scale verification of model results to data of these hazards must involve a coordinated collection of standardized data sets that can be utilized by coastal researchers and management agencies operating the region.

Another difference is that in contrast to sandy coasts, coral reefs are constructed by communities of live organisms, which react to their environment. However, the knowledge base to connect biogenic processes of reef building and degradation to long term coastal change is lacking. At this point we don’t even know if islands are capable of growing with level rise, and/or what the limit of their growth rate is. The same applies to the connection between hazards and exposed population and assets and their vulnerability, which have both been poorly quantified in coral reef environments. But in order to make risk‐informed decisions, the elements of hazards and impacts need to be much better integrated in order to improve coastal zone management.

The best way to achieve these goals is to actively integrate on the one hand local knowledge of coral reef related processes and events to – on the other hand – ongoing and available model development and measurement techniques, mostly present in a number of ADB donor countries. In this way regional scientists and managers not only have access to models and observational techniques but also have the opportunity to develop and obtain intellectual ownership of model development, and exchange information about observational techniques with varying levels of sophistication. Even in the donor countries, resources such as observational equipment should be pooled and applied in dedicated field cases, so that a synoptic view of coastal processes and underlying data (topography, wave and data) can be obtained. In the end, models, data and knowledge can be combined into Early Warning Systems which can help prepare local populations and government officials of impending hazards.

In order to build capability, improve knowledge transfer and foster cooperation, scientists and managers need to come face‐to‐face. The best way to achieve this is to organize multi‐day workshops in the region with audiences of no more than 30 persons at a time, develop short courses at hosting institutes which can be attended by people from the region, but also to develop joint experiments on reef processes with various

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levels of sophistication in the observational techniques as described above. Models, knowledge and data should be shared on a common open web platform, such as the OpenEarth database (www.openearth.nl).

The knowledge hub is meant to include specialists in the physical sciences and social sciences with a focus on hazards and vulnerability. These specialists will come from member states of the ADB, but also from outside the ADB. These specialists will interact with academics (insofar as they are not specialists, but are stakeholders in a particular coral reef environment) from local institutes as well as government and management officials, including NGOs and supra‐national organizations such as the Initiative.

A knowledge hub requires a focal point with a secretariat. This location should be in the proximity of an excellent research group and in or near the region. For this purpose we propose to host the secretariat at the Oceans Institute of the University of Western Australia in Perth, Australia. While not in an ADB developing nation, Australia is a member of the ADB. Perth is located reasonably close to most of the coral countries in the region and the UWA has developed an extensive research program on physical processes behind coral reefs in the Asia‐Pacific region.

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1 Introduction Coral reefs (broadly categorized as barrier reefs, fringing reefs and ) are abundant features of tropical ocean environments. Apart from the Sea, sections of the East African Coast and the , most of the world’s coral reef coasts are located off the coast of ADB member states in the Indian and Pacific Ocean. Coral reefs are among the most bio diverse and economically important ecosystems on the planet and are vital to numerous human societies in the Asia‐Pacific region. Moreover, coral reef structures protect many of these mainland and island coasts from the impacts of severe waves and storm surges (e.g., associated with tropical cyclones), the impact of tsunamis, and help limit coastal erosion.

Rapid global environmental changes have put many coral reefs in peril for a number of reasons:

• Climate change is leading to ocean warming, increased CO2 absorption in the ocean (), and a gradual rise in . Warming has already led to the mass bleaching (death of reef‐building ) in many reef systems in the Indo‐Pacific region, and there is evidence that ocean acidification is reducing coral reef recruitment and growth and accretion – both of these impacts are only expected to become more severe in the future. Sea level rise will reduce the protective function of coral reef structures, thus enhancing the wave energy transmitted to the coastline, unless coral reef growth keeps apace. Ultimately the decline of coral reefs will combine to increased wave on coasts, thus enhancing shoreline erosion, inundation and salinization of the islands with consequences for marine and terrestrial , agriculture and drinking water supplies.

• Direct human‐induced stresses on the coral reef ecosystems result from coastal development adjacent to reefs, which is impacting coastal water quality, shoreline erosion, loss of fisheries, and contributing to a loss of coral reef . The rapid expansion of shipping activities in the Asia‐ Pacific region has led to the expansion of ports and harbors leading to large‐scale activities, many of which occur in the vicinity of coral reefs. Dredge spills may be advected to reefs, leading to enhanced sedimentation, leading to further coral decline. Coral reefs covered with fine and may die off with similar consequences as bleaching.

For the assessment of the protective nature of coral reef ecosystems and the associated shoreline change of the under these changing environmental conditions, it is important to understand wave dynamics, wave‐ and ‐induced water circulation, run‐up (Péquignet et al., 2009; Pomeroy et al., 2012; Van Dongeren et al., 2013), and sediment fluxes in reef systems. Especially the latter is still poorly understood compared to open sandy coasts (Perry et al., 2011).

Until now, the hydrodynamics, sediment fluxes and shoreline changes behind reef systems in developing countries in the Asia‐Pacific region have not been studied well, especially when compared to sandy coast environments and coral reef systems in developed nations (e.g., U.S. Pacific and Australia). This lack of focus is detrimental for the region since it deprives regional centers of accumulating knowledge about local systems, prevents capacity building and the ability to forecast local effects of changing conditions.

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We have started bridging this knowledge gap in the following way. First of all, with funding by the ADB, we have organized a week‐long workshop in Kota Kinabalu, Sabah, Malaysia, hosted jointly by UNESCO‐IHE and the Borneo Marine Science Institute. This event brought scientists and local experts from the region together to discuss the state‐of‐the‐art and data availability in modeling, observation techniques and vulnerability assessment related to Coastal Hazards in Coral Reef Environments, and to identify knowledge gaps and to prioritize actions. The findings of this workshop are presented in detail in the following chapters, dealing subsequently with hydrodynamics (Chapter 2), reef sediment dynamics (Chapter 3), reef biogenic production processes (Chapter 4), coastal response (Chapter 5), hazards (Chapter 6) and vulnerability (Chapter 7).

This report is meant as an opening of a discussion that will need to be had with a number of other potential stakeholders and funding agencies, and which could possibly lead to a Knowledge Hub in the sense of the existing ones under the flag of the Asia Pacific Water Forum.

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2 Hydrodynamics

2.1 Knowledge

2.1.1 Introduction Coastal hazards such as inundation, coastal erosion and aquifer salinization on reef protected coasts are driven by hydrodynamic processes, of which swell wave events (Pomeroy et al., 2012), surges due to typhoons and hurricanes (Ogg and Koslow, 1978; Péquignet et al., 2009) and tsunamis (Kunkel, Hallberg et al., 2006) are the most important. Therefore, a good understanding of these processes is imperative in order to make accurate predictions of hazards and design prevention, mitigation and preparedness measures for coral reef coasts. While important as drivers for other (biological) processes which govern reef health, hydrodynamic processes such as ‐ and ‐driven flows, internal waves and island‐generated vortices are not considered here, as they are already discussed elsewhere.

Waves, tsunamis and surges are considered at two scales: the basin scale (i.e., the size of ocean basins such as the Pacific or marginal such as the South China Sea) and the island/reef scale, usually with domain dimensions of the order of tens of kilometers.

The body of knowledge of processes on the basin scale has been developed by a large worldwide community of researchers as these scales are important for many aspects and applications such as surge and wave forecasting for coastal protection, ship navigation, offshore explorations, etc. In this chapter we will give a short overview of the state of the art, and of knowledge gaps on this scale, in particular where this knowledge is of importance for the downscaling to the reef/island scale.

Hydrodynamic processes on the reef/island scale are much less understood, even though a large proportion of the world’s coastlines, perhaps as high as 80 % (Emery and Kuhn, 1982), contain a broad of submerged reef structures, including tropical coral reefs, relict temperate limestone platforms and rocky coastal features. Abundant as these structures may be, comparatively little work (as compared to sandy beaches) has addressed the range of nearshore hydrodynamic processes in reef environments. Research has shown that the physical processes on reefs do have some similarities to those on sandy coasts (e.g., having submerged bars), albeit with some important differences: the slope of reefs is generally much steeper than the fore slope of sandy , the reef bottom topography is much rougher and more inhomogeneous, and there is typically a larger distance between the breakpoint of the waves and the coastline, which allows for non–zero depth‐integrated cross‐ flow on reefs (Symonds et al., 1995) and has consequences for the wave transformation.

2.1.2 Surges Tropical (cyclones, hurricanes and typhoons) are agents of major change on coral reefs. Increased wave energy generated by storm coupled with increased sea levels due to pressure forcing (inverse barometer effect) and wind setup at coastlines act to dramatically increase the amount of energy impacting coral reefs and associated shorelines. 7 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

On the basin scale, the magnitude of a is strongly dependent on the size, intensity and propagation speed of the storm while the spatial impact of the surge is determined by the storm trajectory. Contemporary numerical weather analysis and prediction systems are able to simulate and predict the development and trajectory of storms, their accompanying wind and pressure fields, and potential storm surge. The uncertainty in the surge prediction is due to the uncertainties in the track, forward speed and pressure drop and asymmetry of the wind field as well as in the formulations which govern the transfer of wind energy to the water surface elevation. There is a need to improve the formulations for predicting transfer of energy from wind to waves as well as to increase the availability of mid‐ocean meteorological data. Both of these needs are global and thus not particular to reefs. Observations of sea level perturbations are subsequently used to assess the model prediction. Of these observations there are comparably few in the Pacific and . A program of installing and integrating tide and wind stations at small islands would improve the capability to assess surge models in the coral reef environments.

2.1.3 Tsunamis On the basin scale, the dominant processes governing tsunamis are those of generation and propagation. The propagation is governed by refraction, especially over ocean ridges which form so‐call wave guides. Qualitatively it is known that tsunamis can be generated by earthquakes, landslides, volcanic eruption and meteorites. However, the quantitative prediction skill is lacking as the process is quite complex. Regarding earthquake generated tsunamis, the Okada fault model (1985) is used to compute the initial displacement of the water surfaces after the quake. This model requires a number of earthquake parameters (i.e. focal depth, strike length and angle, dip angle and rake angle). This model is adequate; the problem lies in the uncertainty of the parameter values once an earthquake occurs.

The initial displacement of the water surface due to a submarine landslide is not as straightforward. The reason is that the dynamic behaviour of submarine landslides is not well known, since very few direct observations of submarine landslides exist. It is however accepted that the tsunami‐genic capability of a submarine slope failure is a function of the slide type, slide volume, slide dimensions, slide velocity, water depth, and orientation of the slide relative to a point of interest (Auffret et al., 1982; Thio, 2009; Tinti et al., 2006). The formulation of an adequate landslide model is still pending.

2.1.4 Waves

Wave generation and propagation On the basin scale, dominant wave processes are those of wave generation by wind input, wave propagation (refraction, diffraction), interaction between wave components (in particular in ocean , four‐wave interactions) and wave dissipation of steep waves (the so‐called white‐capping). On these scales and depths, dissipation due to depth‐limitations and bottom friction is considered of less importance.

The wave processes are governed by the so‐called wave action equation (Holthuijsen, 2007), which is based on the transport equation, and which can be derived from the mild‐slope equation. As discussed below, this equation forms the basis of the class of spectral wave models. The important physical 8 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

processes are for the most part understood well, at least qualitatively. Some notable and active research topics relevant for tropical reef aspects are:

 even though the physics of four‐wave interactions (quadruplets) are well understood, the computational costs prohibits application of the full equations for practical purposes, and approximations need to be made (Cavaleri et al., 2007).

 As the wave action equation does not include diffraction (the bending of wave rays around surface piercing objects) this process needs to approximated as a diffusive term (Holthuijsen et al., 2003). In archipelagoes of small islands diffraction is a process that still needs to be included.

 The generation of waves due to wind input is a complex process occurring in the boundary layer with a large contribution from small scale dynamics of a two‐phase flow interface. Many parameterizations of this source term have been proposed with different levels of success depending on wind speed and spread, and research is ongoing especially for large and variable wind speeds.

For tropical coast applications the existing body of knowledge suffices as it is now possible to describe the transformation of waves generated in the Southern ocean and Bering sea to tropical regions, including the effect of partial blocking by small islands (Tolman, 2003). However, due to the lack of deep ocean and coastal wave buoys in the south Pacific and Indian Ocean, study of nearshore waves in the Asia Pacific region rely only on incoming conditions provided by model or altimetry wave data which are not always accurate in coastal areas.

Dating back to the pioneering work of Munk and Sargent (1948), it is been long appreciated that surface waves can be a primary mechanism for driving flows on the coral reef scale and for changing mean water levels (Hearn, 1999; Lugo‐Fernandez et al., 1998; Symonds et al., 1995). The fundamental conceptual model is one for which remotely generated waves and break on the fore reef. This wave breaking produces spatial gradients in radiation stresses that act as body forces on the flow (Longuet‐Higgins and Stewart, 1964) producing setup and, depending on reef geometry, flow (Lowe et al., 2008b; Symonds et al., 1995; Vetter et al., 2010).

Available observations (Lowe et al., 2005a; Monismith et al., in press) show that short and dissipation on the fore reef prior to breaking follows predictions of classical wave theory (Svendsen, 2005) developed for linear waves over mild slopes, despite the fact that these waves can be quite steep and asymmetrical and the bottom itself can also be steeply sloping and topographically complex (Storlazzi et al., 2003). Dissipation estimates typically give wave friction factors that are O(0.2) corresponding to physical roughness values of O(0.1 m), although as shown by Lowe et al. (2005a) for spectral waves, wave friction is frequency dependent. As observed in the on beaches (Guza and Thornton, 1980) in shallow water over reefs, wave velocities follow linear shallow water wave theory reasonably closely (Lowe et al., 2005a; Monismith et al., in press), given the water surface

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elevation. In deeper water, the depth‐averaged root mean square wave velocities matched linear theory, but vertical structure represented in sigma coordinates, showed weaker near‐bottom velocities and stronger near‐surface velocities than linear theory would suggest. Improvements to linear theory, notably ones that correctly predict asymmetrical shapes and the generation of higher harmonics can be made through the use of Boussinesq models (Nwogu and Demirbilek, 2010) or through spectral nonlinear wave models (Sheremet et al., 2011).

Wave breaking on reefs can be quite dramatic, typically taking the form of plunging breakers (Battjes, 1974). Nonetheless, simple models of breaking developed for beaches work surprisingly well for prediction of bulk behavior like wave setup. The simplest of these models, ones for which breaker height is set equal to a constant fraction, γ, of the depth, require tuning with observations. For example, Vetter et al. (2010) found that γ was 0.9 and 1.1 for two experiments conducted on . Monismith et al (in press) found that γ ≈1 described a wide range of wave conditions including 6m waves generated by a cyclone. In their numerical modeling which used a variant of the more sophisticated Battjes and Janssen (1978) model, Lowe et al. (2009) found that γ =0.64 best described waves on the Kaneohe reef. These variations of γ are generally consistent with what has been observed on beaches, viz. γ is smaller on flatter beaches (e.g. Kaneohe) than on steeper ones (e.g. Guam) (Raubenheimer et al., 1996). The utility of simple models is surprising given that the fundamental behavior of waves breaking on reef geometries appears different than what is seen on beaches: notably, on steep reef faces, waves can begin breaking in one depth of water and then finish breaking in much shallower water, e.g. on the reef crest (Yao et al., 2013), something that is similar to what can take place on barred beaches where waves can begin to break on the bar and then finish breaking in deeper water onshore (Daly et al., 2012). Unlike the beach case, for good practical reasons, there are few measurements of the actual breaking itself or describing what takes place in highly turbulent reef surf zones. Indeed, this aspect of reef wave dynamics is a major unknown.

Lower­frequency wave motions In contrast to the historical focus on the dynamics of short wave energy (periods of 5‐25 seconds) and mean (i.e. averaged over many wave periods) wave‐driven flows on reefs, a relatively small number of field studies have identified the importance of lower frequency wave motions (periods of 25 seconds to tens of minutes), termed infragravity (IG) waves, to the overall water motion over coral reef flats and (Brander et al., 2004; Hardy and Young, 1996; Lugo‐Fernandez et al., 1998). In particular, recent field studies by Péquignet et al. (2009) and Pomeroy et al. (2012) have shown that mean currents and short waves accounted for only a small part of the total observed flow and surface elevation variance in the region between the reef crest and the shoreline of two fringing reefs with very different morphologies. Instead, the bulk of the water level variability was found to be contained within the IG frequency band, despite the response of the IG waves being somewhat different between systems. Péquignet et al. (2009) observed cross‐reef standing waves over a in Guam while Pomeroy et al. (2012) observed dominantly shoreward propagating IG waves across the over a fringing reef in Western Australia.

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Reef flats and lagoons may be considered as seiching basins with normal mode periods increasing with increasing length and decreasing water depth. For typical fringing reefs, the fundamental period often is larger than 10 minutes, and falls in a non‐energetic frequency band. However any increase in water depth on the reef may bring the fundamental mode frequency into the more energetic IG band. Resonant standing modes have been observed across the fringing reef of Ipan, Guam, during large wave conditions associated with tropical storm Man‐Yi (Péquignet et al., 2009). An increase in water depth due to wave setup promoted the establishment of the normal modes across the reef flat by increasing the fundamental mode frequency.

A recent study of the impact of coral reefs on tsunami waves amplitude demonstrated the dissipative effect of reefs through bottom friction with dissipation increasing with increasing reef length (Gelfenbaum et al., 2011). As for shorter waves the dissipation is highly sensitive to bottom roughness. However the length scale of tsunami waves is often of the order of the wave lengths of reefs’ lowest normal modes and resonance has been linked to amplification of tsunami waves on a reef in Tonga (Roeber et al., 2010). Topographic trapping on reefs of tsunami‐generated waves has also been observed in the form of edge waves (Bricker et al., 2007; González et al., 1995).

Wave­driven currents Munk and Sargent (1948) first quantified a mean wave set up of several decimeters relative to mean sea level over the reef at Bikini Atoll. This set up can be explained using radiation stress theory (Longuet‐ Higgins and Stewart, 1964) in which the decrease in wave energy flux due to wave breaking is (partially) balanced by a gradient in the mean water set up over the reef. Field studies have specifically investigated how the transformation of short waves on reefs generates mean wave‐driven currents across reef systems, primarily due to wave breaking (Hench et al., 2008; Jago et al., 2007; Lowe et al., 2009; Monismith et al., in press; Symonds and Black, 2001; Taebi et al., 2011). Symonds et al. (1995) first formulated an analytical model based on a linearized set of momentum equations in order to demonstrate the relative importance of set‐up and onshore wave‐driven flow across an idealized 1D reef system (subsequent 1D analytical models were also formulated by Gourlay and Colleter (2005), Hearn (1999) and Lowe et al. (2009)).

The fundamental dynamics of the wave‐driven flow in many (but not all) cases appears to involve a balance of the pressure gradient produced by setup and the frictional resistance provided by the reef. The picture of how this works is most clear for the case of a barrier reef backed by a lagoon that has a deeper pass. In this case, it is the frictional resistance of the flow through the lagoon system (e.g. over a shallow reef flat and out the deeper ) that sets the overall flow through the system (Lowe et al., 2010). If resistance in the outflow channel is high the through flow will be weak and the lagoon setup will nearly be the same as the wave setup produced by breaking. If the resistance is low enough, the water level in the lagoon will nearly be the same as that offshore, and the flow over the reef crest will be maximal. Overall, this flow dynamics means that roughness is a major control on circulation. Observations made using sets of ADCPs and wave/tide gauges reported in Hench et al (2008), Lowe et al. (2009), Taebi et al. (2011), Pomeroy et al. (2012) and Monismith et al. (in press) provide an excellent observational database for modeling such flows. The flows shown in Monismith et al. (in press) also 11 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

highlight an important feature of flows on the fore reef: the flow is apportioned between wave transport and mean Eulerian flow. This division is important in that the mean Eulerian flow can increase setup if it is offshore () or decrease setup if it is shoreward (Vetter et al., 2010). The former case can occur if the overall transport through the system is small.

While the barrier reef/lagoon geometry has been the subject of much study, the flow through other reef configurations has received much less attention. For example, for the common atoll configuration (Munk and Sargent, 1948), with and without blocked passes seen (e.g.) at Tarawa, there are only older, more limited sets of observations (Atkinson et al., 1981; Kraines et al., 1999; Tartinville et al., 1997), although a measurement campaign like that of Pomeroy et al. (2012) is planned by UWA (R. Lowe) for late 2013.

Tidal elevation Tidal elevation in nearshore environment results from an astronomical component that is well understood and local topographic effects (Cordier et al., 2013) which are not always well understood. On atolls and reefs with large tidal regime, the interaction between and waves needs to be investigated to fully understand the 3D circulation. For example, the strong dependence of both breaking and friction on water depth means that tidal modulations of wave energy at all frequencies are expected and observed on reefs (Hearn, 1999; Lowe et al., 2009; Lugo‐Fernandez et al., 1998; Péquignet et al., 2011; Pomeroy et al., 2012). To date, hydrodynamic studies have never been conducted on macrotidal reefs.

Wave runup Wave runup is composed of a mean elevation, wave setup and an oscillatory component, which has often been observed at low frequency (Raubenheimer and Guza, 1996; Ruggiero et al., 2004). On reefs setup compares well with theoretical and increases nearly linearly with increasing offshore wave height (Gerritsen, 1981; Gourlay, 1996a, b; Monismith et al., in press; Tait, 1972; Vetter et al., 2010). To our knowledge, swash oscillations have not been measured directly on reef environments. However, sea surface elevation close to reef‐fringed shore has been observed to be dominated by low frequency oscillations (Nakaza and Hino, 1991; Péquignet et al., 2009). This is similar to dissipative sandy shore swash zone (Raubenheimer and Guza, 1996; Ruggiero et al., 2004).

Extrapolating these results to consider the effects of rising sea levels, we should expect to see less setup when water depth over the reef increases (Péquignet et al., 2011). However, any decrease in runup due to decreased setup may be more than offset by an increase in the amplitude of swash oscillations that would be expected given decreases in friction and a shift of reef normal mode frequencies into the more energetic band of forcing (Péquignet et al., 2009).

Frictional dissipation The discussion above highlights the fact that the dependence on water depth of most hydrodynamics processes on a reef makes knowledge of reef bathymetry necessary to be able to predict wave energy on reef‐fringed shores. In particular high resolution bathymetry is required to represent the rough structures of most reefs. The roughness of corals and reef builders in general is much larger than 12 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

unconsolidated sediments (Jaramillo and Pawlak, 2011; Nelson, 1996; Nunes and Pawlak, 2008) and results in a large range of values of friction factors on reefs (Gerritsen, 1981; Hearn, 1999; Lowe et al., 2005a; Nelson, 1996; Péquignet et al., 2011; Rosman and Hench, 2011), up to 100 times the values observed for sandy shores. Friction has mostly been measured for wind waves and the dissipation of long waves cannot be ignored (Pomeroy et al., 2012). Typically drag coefficients should decrease with decreasing frequency (Lowe et al., 2005a), however a single roughness scale is not representative of the roughness range and complexity observed (Jaramillo and Pawlak, 2011; Nunes and Pawlak, 2008; Zawada et al., 2010). It is expected that larger spatial scale roughness elements will contribute to the dissipation of waves with larger orbital excursions, i.e., larger amplitude, lower frequency waves. Reefs are also porous and because of the difficulty of measuring porosity in particular around the reef crest, the effect of porosity on dissipation of waves on reefs has not been accounted for. Moreover, because of the porosity of reefs, the effect of waves dynamics does not end at the shore line and the influence of wave setup on ground water levels has been observed as far as 5km inland on (Rotzoll and El‐Kadi, 2008). Porosity may be particularly important for atolls and small islands with limited groundwater reservoirs.

2.2 Data availability

On the basin scale in the Pacific and Indian Oceans, there is only limited data from tide gauges and wave buoys. Around the Asia‐Pacific region, wave buoy data are only available in the North Pacific, and around Australia. For the Indian Ocean and the South Pacific, wave data are available from altimetry data and operational global wave models (Young, 1999). However, both numerical models (Bidlot et al., 2002; Wingeart et al., 2001) and altimeter wave heights and periods (Dobson et al., 1987; Queffeulou, 2004; Webb, 1981) are mostly calibrated using existing wave buoys.

Reef scale studies are often limited by the availability of reef topography data. The Scanning Hydrographic Operational Airborne LIDAR Survey () data provide good coverage for the US territories (Guenther et al., 2000). Around Australia, high resolution bathymetry is provided by the Laser Airborne Depth Sounder (LADS) data. In regions where no LIDAR data are available, vessel mounted (single or multibeam) coupled with RTK‐GPS positioning is an accepted practical means to provide local‐scale description of the topography.

Recent field deployments are presented in the Table 1.

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Table 1: Overview of recent hydrodynamic field campaigns in coral reef settings

Authors/ Location Reef type Deployment Reference

USACE/Merrifield Ipan, Guam Fringing reefs Cross‐shore arrays (Péquignet et al., of PUV sensors 2011; Péquignet Mokuleia, reef length 500 to et al., 2009; 900 m and spring Vetter et al., tide~1m 2010)

Lowe Ningaloo Reef‐lagoon Cross‐shore arrays (Pomeroy et al., of PUV sensors 2012; Taebi et al., 2011)

Lowe Kaneohe , Reef‐lagoon Cross‐shore arrays (Lowe et al., 2009) Hawaii of PUV sensors

USGS/Storlazzi Molokai, Hawaii Reef‐lagoon Cross‐shore array (Storlazzi et al., of PUV sensors 2004b); (Storlazzi West Maui, and Jaffe, 2008) Hawaii

Monismith Moorea (‐lagoon Cross‐shore arrays (Monismith et al., Polynesia) of PUV sensors in press)

Kench Lady Elliot Spring GBR Stilling wells (Jago et al., 2007; tide=4m and pressure Kench et al., Lady Elliot GBR sensors 2006)

Bonneton /IRD New Caledonia Reef and lagoon Cross‐shore array (Bonneton et al., of PUV sensors 2007) spring tide=1.4m

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Authors/ Location Reef type Deployment Reference

Cordier Reunion Fringing reef Sediment and Cordier et al. hydrodynamics (2012) Spring tide=0.9m measurements

Andrefouet Tuamotu (French Reef and lagoon (Andréfouët et al. Polynesia) circulation (2012); Dumas et al. (2012))

Data about waves, flows, and setup recorded in carefully controlled lab experiments have provided important test cases for the wave models described below. In this regard, excellent, detailed experiments reported by Demirbilek et al. (2007) (and analyzed in Sheremet et al. (2011)) show the behavior of waves shoaling on a steep, complex reef face installed in a large wave flume. In a similar vein, Yao et al. (2013) and Yao (2012) report measurements of waves and setup for steep slopes with and without a shallow reef ridge (a common geometry). In all of these experiments the advantage gained by going to the lab is the ease of measurement, particularly for processes taking place where the waves are breaking. The lab also enables studies of the effects of various parameters, e.g. wave period or setup on system response.

Lab experiments, together with post‐tsunami surveys, can provide information crucial for validating numerical tools used to study tsunami inundation, run‐up and morphological changes. In contrast to data collected from post‐tsunami surveys, which are very often incomplete and record only cumulative effects, lab experiments allow detailed study of tsunami‐induced morphological changes, runup and inundation over various coastal profiles as well as interaction between tsunami flows and coastal structures. Due to the great length and period of tsunami waves, solitary waves are often used in large‐ scale wave flume tests or wave‐basin tests to study near‐shore tsunami waves. Large‐scale lab tests on solitary wave runup over simple topography have been carried out by Yeh et al. (1989) for a plane beach, Baldock et al. (1998) for a combined slope, and Liu et al. (1995) for a circular island. Recent experimental work of Rueben et al. (2011) studied the effects of buildings in an inundation zone on inundation distances. These experimental results enable us to verify numerical codes developed for simulation of tsunami inundation. While tsunami waves are capable of causing significant morphological changes, pre‐tsunami bathymetry and topography are very often not available in tsunami affected coastal areas. Thus, lab experiments are needed to provide data for model verification. For example, the

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tests by Kobayashi and Lawrence (2004) for a sandy plane beach and Chen et al. (2012) for a sandy combined slope could provide information on the changes of both beach profile and grain size.

The experimental studies of the tsunami forces exerted on light‐frame wood buildings (van de Lindt et al., 2009) and tsunami‐induced scours at a bridge pier (Tonkin et al., 2003) helped understand destruction of coastal buildings by tsunami waves.

2.3 Measurement/monitoring techniques

At broad ocean scales, satellite remote sensing provides basin scale observation of ocean swell climate and storm wave events and delivers improved spatial resolution than that provided by the limited distribution of wave observing buoys in the tropical oceans. Comprehensive in‐situ observation of offshore ocean waves at locations and spatial scales relevant to many coral reefs and islands does not currently occur, despite the existence of appropriate wave buoy technology, as implementation is prohibitively expensive.

At local reef scales, waves observations typically involve the use of fixed Acoustic Doppler Current Profilers or velocity meters and pressure sensor wave gauges sampling at high frequency to provide a Eulerian specification of the temporally evolving across‐ and along‐reef distribution and transformation of wave energy and circulation (see for example, Lowe et al., 2009; Pomeroy et al., 2012). Lagrangian techniques include the use of drogue drifters to map the combined tidal and wave induced circulation (Herdman, 2012; Mantovanelli et al., 2012; Spydell et al., 2007). A particular challenge of contemporary techniques lies in the difficulty of placing instrumentation through the surf zone, with sufficient spatial coverage to adequately resolve the processes taking place during wave breaking. Advances in miniaturization of electronic componentry have the potential to drive innovation in nearshore wave observation. Current novel technologies under development include: pressure sensor strings, that offer the potential to be laid down and secured to the reef and lagoon surface, from the shoreline out the fore reef; miniaturized GPS and accelerometer packages fitted to Lagrangian drifters could, when deployed en masse, provide cost effective means to map the sea surface height variability and circulation; Simple, low cost current/wave gauges (Figurski et al., 2011) provide the means to gather highly spatially resolved descriptions of both wave energy and circulation. The requirement for simultaneous deployment of numerous and varied instruments will benefit from the systems that integrate the control and data management from the observational array. Reef and Island scale sensor networks are an emerging technology that can be used to provide spatially dense bio‐physical measurements in real‐time, over spatial scales suitable for island and reef environments. The term 'sensor network' refers to an array of small, wirelessly interconnected sensors that collectively stream data to a central data aggregation point. Communications with the sensors is bi‐directional and they can be controlled remotely by central land‐based control systems as part of a 'smart' response to real time events as they occur.

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2.4 Modeling capability and needs

The modeling capability of hydrodynamic processes at the basin scale has existed for some time. Models are now routinely operational forecasting of tsunamis, waves and surges. With regard to wave models, the sheer size of basins constrains the level to which wave processes can be directly simulated. Historically, wave action (spectral) models have been used (most notably WAM and WaveWatch III) which are based on the transport equation of wave action and contain all relevant propagation and source and sink terms, as described above. These types of models predict the transformation of the energy (variance) of the sea surface as a function of frequency (inverse of period) and direction, from which integral properties such as wave heights, mean wave directions and wave period measures can be derived. These types of models are essentially linear models and do not predict the wave shape (and be extension wave asymmetry and skewness) and/or extreme waves. We refer to Table 2 for an overview.

On the basin scale, surge prediction models have been based on the 2DH nonlinear shallow water equations with bottom friction and surface wind forcing. For instance, the National Hurricane Center of NOAA uses the SLOSH model which is a 2D hydrostatic model without the effect of waves on inundation, tides, river flow or direct rain flooding. Models such as ADCIRC are 2D hydrostatic, cover entire basins with a flexible mesh grid (Luettich and Westerink, 1991).

Similarly, tsunami propagation models have been based on 2DH nonlinear shallow water equations (with bottom friction). These models are initiated with the surface displacement caused by the earthquake or landside. In the case of the former, these initial water levels are computed using the Okada fault model (Okada, 1985) which requires a number of earthquake parameters (i.e. focal depth, strike length and angle, dip angle and rake angle). After the computation is started, the initial tsunami waves will propagate in the model area and produce tsunami wave heights and inundation levels along the area of interest. The tsunami‐genic capability of a submarine slope failure is a function of the slide type, slide volume, slide dimensions, slide velocity, water depth, and orientation of the slide relative to a point of interest (Auffret et al., 1982; Thio, 2009; Tinti et al., 2006).

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Table 2: Overview of current state‐of‐the‐art hydrodynamic modeling approaches

Model class Background Application Knowledge/ability Need/drawback Examples

(basin, reef)

Spectral models balance equation for B Well established and Four‐wave SWAN, WAM, rd (3 generation) wave action as commonly used interactions, wind WaveWatch function of direction input and frequency (Booij et al., 1999; Tolman, 2003)

Wave action balance equation for R Simple linear wave Only 1D stationary (Gerritsen, 1981; st models (1 bulk wave action (at model bulk parameters, Lowe et al., 2005a). generation) peak frequency, surpassed by 3rd unidirectional generation and time‐dependent models

Spectral wave Expansion of the mild R 1D only Expensive (Freilich et al., evolution models slope equation for 1990; Massel and steep slopes Gourlay, 2000; Sheremet et al., 2011)

Shallow-water shallow-water B/R Stationary and Delft3D (Lesser et equation models equations + stationary slowly‐varying al., 2004; Lowe et forced with wave energy/action flows only al., 2009; Stelling, stationary waves transport model 1984)

Shallow-water shallow-water R Computationally Artificial separation XBeach models forced on equations + wave efficient of long and short wave-group scale energy/action waves (Roelvink et al., 2009; Van

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Model class Background Application Knowledge/ability Need/drawback Examples

(basin, reef)

transport model Dongeren et al., 2013)

Mild-slope models 3D velocity potential + R Wave breaking Essentially linear REFDIF, CGWAVE assumed vertical parametrized processes profile 2DH (Kirby and Dalrymple, 1984; Panchang et al., 1999)

Boussinesq-type 3D free-surface flow R Well developed theory, Computationally FUNWAVE models equations + series wave shape included, expensive expansion in vert.dir.  parametrized breaking, (Nwogu and 2DH no sediment transport Demirbilek, 2010)

Multi-layer 3D free-surface flow R Relatively novel Computationally SWASH (Zijlema, models equations + multiple technique, automatic expensive 2012) horizontal layers  3D breaking

Full free-surface 3D free-surface R (or smaller) Full description of Very expensive COMFLOW visc.flow models Navier-Stokes physics, no sediment equations full 3D transport Not applied in coral reef environments yet

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Model class Background Application Knowledge/ability Need/drawback Examples

(basin, reef)

Smooth 3D free-surface R (or smaller) Full description of Very expensive SPH (Dalrymple and hydrodynamics Navier-Stokes physics, no sediment Rogers, 2006) equations transport Not applied in coral reef environments yet

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On the island or reef scale, despite physical differences between sandy coast and reef environments, simple 1D (cross‐shore) wave transformation models developed for mildly‐sloping beaches have been successfully used to investigate short wave transformation over some reefs. Gerritsen (1981) and Lowe et al. (2005a) used a 1D wave‐energy conservation model (where wave energy is equivalent to wave action for constant wave period) with the Battjes and Janssen (1978) breaker model. Massel and Gourlay (2000) and Sheremet et al. (2011) extended a mild‐slope equation model with a correction for the steeper slopes of reefs.

More complex two‐dimensional horizontal (2DH) and three‐dimensional (3D) coupled wave‐circulation numerical models have also been developed to predict the spatial distribution of mean wave‐driven currents and water levels within reef‐lagoon systems (Lowe et al., 2009; Ranasinghe et al., 2006; Symonds and Black, 2001). These models are in essence based on the equation of mass conservation and the equations of 2D or 3D horizontal momentum conservation driven by radiation stress gradients, which are computed from the 2D quasi‐steady conservation of short wave energy equation with dissipation terms for wave breaking and bottom friction dissipation.

Nwogu and Demirbilek (2010) and Sheremet et al. (2011) each applied a 1D phase‐resolving wave model to simulate both short wave and IG waves from laboratory flume experiments, with smooth bottom beds. Roeber and Cheung (2012) described an application of a 2D Boussinesq‐type model to fringing reefs. They showed good model comparison to laboratory data of solitary waves incident on a 1D and 2D reef.

Van Dongeren et al. (2013) applied the 2DH XBeach model which is composed of a nonlinear shallow water solver with time varying wave driver (on the scale of the wave groups) to simulate the transformation of infragravity waves on a fringing reef. The directly modeled processes, the parameterized and non‐modeled processes are described in Table 3. It reveals that the model is quite applicable for coral reef enviroments.

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Table 3: Model physics of the hydrostatic XBEACH model (Roelvink et al., 2009)

Process/Modeled Simulated Parameterized Not modeled Applicability on reefs

Dispersion Y As long as waves are near shallow water regime

Diffraction Long waves Short waves As long as there are no surface piercing objects

Refraction Y

Shoaling Y

Reflection Long waves Short waves As long as incident short waves on coastline are small or coast is dissipative

Wave‐wave Y Shallow water: interaction triads

Wave‐current Y Will be important in interaction channels

Wind input Y As long as fetches are short

Wave breaking Y only bulk process important

Bottom friction Y Corals are rough topographies turbulence Y

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2.5 Summary and Actions

Table 4: Summary and actions for hydrodynamic modeling

Issue Score Action Current Need State KNOWLEDGE Waves breaking Details of breaking process 1 5 Laboratory and field largely unknown investigations Circulation 3D description lacking, wave 2 5 current interaction lacking Surge Knowledge about inundation 2 5 and morphological change lacking

DATA AVAILABILITY Waves Limited number of available 1 5 Install more buoys buoys, satellite data needed Breaking waves No detailed field data, limited 2 5 Acquire lab data lab data Wave induced No available data on atolls 1 5 Field experiment circulation Wave run up Lacking 1 5 Field experiment Tsunami Only local in situ data after 1 5 Expand field campaigns recent events, very site specific MEASUREMENT AND MONITORING TECHNIQUES Roughness Side scan sonar analysis 1 5 Develop remote sense techniques for roughness not roughness instruments well developed

Runup Camera or laser 1 5 Develop instruments instrumentation

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Issue Score Action Current Need State MODELING CAPABILITY Tsunami Unknowns about effect of 2 5 Develop submodels roughness and porosity

Surge Need sediment transport 2 5 Develop submodels models for supercritical flow based on first principles and lab data Wave “sandy beach” models applied 2 5 Application to reef cases transformation only to limited number of reef cases; need confirmation

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3 Reef Sediment Dynamics

3.1 Knowledge

Until now, studies of the dynamics of sediment movement in coastal systems have predominately focused on open‐coast sandy beaches and where the sediment is also mostly comprised of siliclastic material (i.e., typically quartz‐based ). From this work, a large body of experimental data (both laboratory and field based) has led to the development of numerous semi‐empirical formulations that are designed to predict sediment (sand) transport rates under a wide range of different current and wave conditions (e.g., Soulsby, 1997). These formulations have thus become the foundation for a number of predictive sediment transport approaches, ranging from simple empirical models to those implemented in complex process‐based coupled hydrodynamic‐sediment transport numerical models.

A key challenge in predicting sediment transport, even the most simple sandy beach environments, is initially how to relate the hydrodynamic (mean and turbulent) flow structure, as derived from field observations or a hydrodynamic model, to the entrainment (mobilization) of sediment that was initially at rest on the seafloor. Ultimately these relationships (e.g., Shields, 1936) rely on predicting the hydrodynamic forces (i.e., the critical bed stress stresses) that are required to overcome sediment resistive forces (i.e., their , interactions between grains, etc.) and hence leading to the initiation of sediment transport in the bottom boundary layer. For uniform sandy beds, empirical formulations to predict these critical shear stresses are traditionally a function of basic properties of the sediment (generally just a representative “spherical” grain size and a representative sediment density), as well as in some cases simple geometrical properties of bedforms if they are present (e.g., ripple heights, lengths, etc.). Once sediment transport is initiated, the horizontal transport is then (somewhat arbitrarily) decomposed into two components: (1) suspended load ‐ sediment that is transported in suspension in the water column; and (2) bed load ‐ sediment transport in which grains remain in regular contact with the seabed (Figure 1). Numerous transport formulations have been proposed to predict these separate rates of suspended and bed load, and these have been implemented in a variety of different predictive models.

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Figure 1: Conceptual model of sediment transport (Soulsby, 1997)

Nevertheless, despite the many significant advances that have been made in predicting sediment transport on siliclastic sandy beaches, many (and probably most) of the underlying assumptions within existing transport models are technically violated when applied to tropical coral reef environments. This is largely due to 1) Differences in grain size, shape and properties between siliclastic and carbonate sands 2) Differences in how biogenic carbonate sands are transported relative to inert sand 3) The influence of the complex topography of reefs ‐ from small to whole‐reef scales ‐ on the actual transport process.

The composition of biogenic carbonate material is known to vary widely among different tropical coral reef systems, and usually also varies strongly across different reef zones in the same reef (see “Reef Processes” Section). In particular, the carbonate material found in different parts of a reef can be dominantly sourced from different key constituents such as coral, coralline , foraminifera, or mollusks (Perry et al., 2011; Yamano et al., 2005). As carbonate sediments originate from skeletal material with diameters on the order of O(10‐1000) mm, the diameters are much larger in size than silicate sand diameters.

This relatively large material (e.g., coral rubble) may be episodically transported across reef environments by energetic forcing events such as large storm waves or tsunamis (e.g., Goto et al., 2007; Nott, 1997). Also, its transport rates may be ill‐predicted using sediment transport formulations based on silicate material.

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There have only been a limited number of dynamical models to predict, for example, the hydrodynamic forces required to explain historical observations of the transport of deposits induced by extreme storm waves and tsunamis (e.g., Buckley et al., 2012; Lorang, 2011; Nott, 2003). However, these approaches still only predict the transport of isolated with an idealized shape (i.e., spherical or rectangular cubes) that are not appropriate for predicting the transport of coral rubble material, where substantial interactions of individual “grains” will occur. There is a clear need for the development of new transport formulations (ultimately derived from empirical data sets that are also lacking), to provide a means to estimate the transport of large biogenic material within reef systems.

Nevertheless, even for carbonate sediment in the sand‐size classes, there still can be much uncertainty in the ability of existing siliclastic sediment transport formulations to correctly predict rates of carbonate sand transport. For example, a limited number of studies have revealed that different carbonate material with the same effective sediment diameter can have grossly different critical shear stress thresholds and sediment fall velocities depending on its source. This can be due to the substantial geometrical differences in carbonate sediments, which depend on both the type of organisms that produced the sediment, how this material was broken down, as well as the porosity of the biogenic material that may substantially reduce its effective density. Ultimately to improve carbonate sediment transport predictions in coral reef environments will require much more empirical data on how different grain types interact with the hydrodynamics, which must capture a wide range of different source types.

Sediment transport in coral reef environments, including both siliclastic and carbonate sediment, is also substantially more complicated by the need to account for how sediment interacts with the large immobile roughness (canopies) formed by coral reef communities. Importantly, sediment transport in the vicinity of reef roughness may be significantly attenuated, which reduce the shear stresses that directly interact with the sediment. Many studies have observed and developed models to predict how the unidirectional flow within canopies (critical to predicting the sediment at the base of the canopy) depends on different morphological properties of the canopies and the overlying flow. These studies have generally focused on canopies comprised of idealized roughness (e.g., Ghisalberti, 2009; Lowe et al., 2005b), vegetation (e.g., Ghisalberti and Nepf, 2002; Nepf and Vivoni, 2000), with a few studies using realistic coral roughness (e.g., Lowe et al., 2008a; Reidenbach et al., 2007). A much more limited number of studies have also shown how the presence of an overlying oscillatory surface‐wave driven flow can substantially enhance flow inside canopies (e.g., Lowe et al., 2007; Luhar et al., 2010). Despite the obvious importance of the reduction of bed shear stresses by reef roughness, presently there is virtually no information about how the corresponding sediment dynamics are modified by these interactions. This presently poses another major gap in our ability to accurately predict sediment transport within coral reef environments.

3.2 Data availability

There are numerous data sets on the grain size distributions and composition of reef sediments, which include a wide variety of systems in the Indo‐Pacific and Caribbean, as well as those incorporating a 27 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

number of different reef types ranging from fringing reefs to atolls. However, most of these studies provide only a descriptive assessment of the distributions of sediment properties across reefs, thus lacking a process‐level understanding of what controls these distributions, whether it be physical or ecological.

Field observations of suspended sediment have been obtained on a wide range of reefs, but accurate measures of sediment fluxes on reefs remain scarce. These tend to be based on point based estimates of suspended sediment transport rates based on co‐located hydrodynamic measurements and logging sensors that provide a proxy for sediment concentrations (see below). Direct measurements of bed load transport on reefs are largely lacking, which may be particularly important within low energy areas of reefs such as within sandy lagoons.

There is limited data presently available that can provide confidence in existing sediment transport models to accurately predict sediment transport in reef environments. Fundamental data on the transport of carbonate sediments is lacking, making it difficult to 1) understand how well existing semi‐ empirical sediment transport formulation originally derived with spherical siliclastic sands perform in reef environments and 2) to support the development of new formulations to improve these predictions. While there is some data on how vegetated canopies formed by e.g. and influence sediment transport, there is currently no data on how canopies of coral reef communities modify these dynamics.

3.3 Measurement/Monitoring Techniques

3.3.1 Existing tools: Measurement of seabed properties

Sampling Systems There are a number of different tools to collect samples of sediment on the seabed for physical and/or chemical analyses (e.g., Draut et al., 2009). These range from simple manual hand sampling to the use of Van Veen‐type grab samplers, up to more complicated box, gravity, piston, and pneumatic coring systems that provide a long record for historical inventories but that generally require larger vessels with a davits or A‐frames with winches.

Sonar Surveys A number of geophysical tools are available that provide spatially‐extensive information on seabed characteristics. Single‐beam have been used for decades and can provide insight on the distribution of hard substrate (e.g., coral and/or coralline algal pavements) and sediment by mapping out differences in reflected by the seabed (i.e., more sound is reflected by hard bottom, coarse, or rippled sediment than by finer or smoother sediment). Multi‐frequency sonars (e.g., Greenstreet et al., 1997) provide better discrimination of the seabed properties by quantifying the differential response to the different incident acoustic frequencies. These systems provide good positional accuracy but with low spatial coverage. Side‐scan swath sonar systems (Chavez et al., 1996) provide greater spatial coverage are now readily available and relatively inexpensive, but often suffer from reduced positional 28 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

accuracy. Multi‐beam sonars provide the positional accuracy of single‐beam sonars with the spatial coverage of side‐scan sonars (e.g., Dartnell and Gardner, 2004), but have less spatial resolution (usually an order of magnitude) than side‐scan sonar systems and are very expensive. New interferermetric side‐ scan sonars provide the spatial coverage and resolution of side‐scan sonar systems with almost the positional accuracy of multi‐beam systems (Storlazzi et al., 2011b); but these, too, are relatively costly.

Fixed Time­series Sonars As described above, sonars can provide insight into seabed sediment character (composition and morphology). Not only can these systems be mounted on vessels to provide spatially‐extensive but temporally‐limited coverage, but they can also be mounted on a fixed structure (e.g., a tripod) using both pencil‐beam and sector‐scanning sonar systems (Lacy et al., 2012) to make time‐series measurements of seabed character in response to changes in oceanographic forcing (waves, currents, etc.) or geologic events (river floods, landslides, etc.).

Optical Systems Visible and multispectral airborne, space‐borne, and underwater optical systems have been used to map the distribution of sediment across reef environments (Gibbs and Cochran, 2009). Such techniques have the ability to provide high spatial coverage of seabed character down to depths for 30‐40 m in the clearest waters. More recently, remote sensing techniques have been applied to high‐resolution microscopic imagery of seabed sediment to provide inexpensive and fast surveys or time series of seabed sediment physical parameters (Rubin et al., 2007).

3.3.2 Existing tools: Measurement of Water Column Properties

Optical The primary sensors used to make measurements of the of sediment in the water column over reefs use the backscatter or transmission of certain wavelengths of light. Transmissometers quantify the amount of light blocked across the sensor by suspended sediment particles (e.g., Wolanski et al., 2003). These sensors are very sensitive and are useful in optically clear waters but become saturated at relatively low particle concentrations and thus not useful in highly turbid environments. Optical backscatter sensors measure the amount of light scattered by sediment back to the sensor and, although not as sensitive as transmissometers, can effectively operate at much higher suspended sediment concentrations (e.g., Storlazzi et al., 2009). Although both transmissometers and optical backscatter sensors are extremely reliable and the industry standards for assessing levels of suspended sediment, they only provide information on concentration of sediment in the water column and react differently to different sized particles. When information on sediment character, such as source (reef versus terrigenous) is desired, some studies have employed underwater visual camera systems to recover optical imagery of the material in suspension to provide insight into the origin of the sediment (e.g., Storlazzi et al., 2009). All of these optical sensors are negatively affected by fouling and typically can only provide clean data unbiased by biofouling for a few days to weeks in coral reef environments unless the sensors’ optics are cleaned regularly. 29 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

Sonars (single and multiple frequencies) Sonars can provide information on suspended sediment concentrations similar to optical backscatter sensors by recording the amount of acoustic energy reflected back to the sensor, and are generally unaffected by fouling. Acoustic sensors, unlike optical sensors, are much more dependent on particle size and thus not as effective in locations with varying grain sizes. Given that sonar frequency is inversely related to the optimal size of particles measured, these approaches tend to target finer‐grained sediment particles based on the higher frequencies (i.e., order megahertz) of typical oceanographic instruments. In some cases the acoustic backscatter recorded by acoustic Doppler velocimeters and acoustic Doppler current profilers developed to measure currents can be used to provide information on suspended sediment concentrations similar to dedicated acoustic backscatter systems (e.g., Storlazzi et al., 2004a) . Multi‐frequency systems have also been developed to increase the number of grain sizes that can be measured by a single unit.

Samplers A number of tools have been developed to take physical samples of suspended sediment for physical or chemical analyses. Simple traps, deployed both vertically and horizontally, have been used to collect sediment, but these tools often affect flow at the site and can provide spurious data on true sedimentation rates (Storlazzi et al., 2011a). Designed surfaces and collection devices have more recently been developed to try to accurate measure sedimentation rates and provide sediment samples for analyses (Field et al., 2013). A number of pump sediment systems developed for river systems have been modified for use in shallow coastal systems (e.g., van Rijn, 2007a); these systems are generally very cumbersome and not well suited for long‐term autonomous measurements.

3.3.3 Needs

The needs can be summarized as follows: 1) Expand the number of observations of sediment transport (particularly bed load transport) and sediment distribution on coral reef‐lined coasts, with a longer duration per experiment than now usual. 2) Better understanding of the heterogeneity of coral reef sediment properties and their effects and implications for measurement techniques and calibration. 3) Better understanding of the effect of sediment deposition and re‐suspension on reef health.

Although there have been a number of measurements made of sediment dynamics in coral reef environments over the past decade, these are orders of magnitudes less than those made along sandy, siliclastic coasts that have gentler bathymetric variations and are less hydrodynamically rough. The much greater bathymetric complexity of coral reefs and their greater hydrodynamic roughness means that there are extremely limited samples for a given reef configuration. Therefore the primary need is for increased numbers of studies and data from varying reef types and shoreline configurations. Furthermore, most of the previous field studies have been of limited duration (weeks to a few months)

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and thus are insufficient to significantly contribute to the understanding reef and reef‐shoreline annual and longer term sediment budgets.

In addition, there is also great heterogeneity of grain sizes, shapes, and densities in coral reef sediment (Kench and McLean, 1996). Because optical and acoustic sensors are dependent on grain size and calibration procedures that only relate to a particular (single) sediment type, it is apparent that much more information is needed on this heterogeneity on suspended sediment measurements. Furthermore, time‐series sampling of sediment in suspension is needed for time‐varying calibrations of acoustic and optical sensors. At this time, most physics‐based numerical models’ sediment transport formula assume solid spherical sediment particles of a given size and uniform density, but many carbonate sediment particles are vascular and of varying density due to dissolution and re‐precipitation of material (Kench and McLean, 1996). This means that new studies are required to understand how the various shapes, densities, and structures of carbonate sediment particles can be represented by uniform sizes and densities.

Most of the tools used to make time‐series measurements of sediment fluxes in coastal environments specifically target the measurement of suspended sediment loads; there are very few direct measurements of bedload in any coastal environment, let alone in coral reef environments. Because the greatest volume of sediment is transported as bedload, the lack of data on this aspect of sediment dynamics on coral reefs currently limits our ability to accurately constrain net fluxes and sediment budgets on coral reefs and adjacent shorelines.

Lastly, the processes of fine‐scale deposition and resuspension on hydrodynamically‐rough corals and carbonate surfaces that affect biologic and chemical processes relevant to reef health (reef morphology, roughness, and sediment production) are poorly understood. Understanding of such fine‐scale processes are relevant to not only to better constraining the impact to natural events such as river flood plumes, but also anthropogenic events such as the impact of dredge disposal in the vicinity of coral reef environments.

3.4 Modeling capabilities and needs Given the complex and highly variable morphologies of coral reefs that are that are subject to a wide range of different hydrodynamic forcing regimes and sediment types, there is a need to develop process‐based (non‐site specific) models to predict sediment transport in these environments. Essential features for modeling sediment transport in coral reef environments, in general, include:

 Facility to accommodate one or more sources of sediment including those produced internally (e.g., on the fore reef, reef crest, and lagoon) as well as from terrestrial inputs;  Facility to accommodate multiple grain size distributions and settling velocities that may also include particle‐particle interactions including the effects of aggregation (flocculation) and disaggregation; 31 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

 Representation of suspended and bedload transport modes;  Hydrodynamic forcing provided by wind, waves, gradients, surface heat and water fluxes, and open‐boundary conditions including tides and boundary currents;  Resolving three‐dimensional effects to accommodate and the vertical distribution of sediment concentration;  Modification of sediment transport due to canopy or hard reef structures;  Wave‐current interaction processes;  Swash processes on porous beaches;  Representation of all relevant bed processes (e.g. bed level changes due to deposition and resuspension, consolidation, scouring, armoring, and representing hard reef structures).

Numerous numerical modeling studies have focused on predicting sediment transport over relatively smooth sandy bottoms, but as noted above should be used with extreme caution when applied to coral reef environments. In general, the transport of sediment grains by a flow can be modeled as either bedload or suspended load depending on the sediment and flow characteristics. For uniform sandy beds, a number of semi‐empirical bedload transport formulations have been developed (e.g. Van Rijn, 1984) that are applicable to a wide range of flow conditions, including due to the combined effects of waves and currents.

However, the ability of existing bedload transport models to predict the transport of carbonate sands remains unknown, in large part due to the lack of empirical data to develop and validate these formulations for these different sediment types (see section 1.2.3). Furthermore, given the hard (immobile) structures that coral reef communities form on reefs, hydrodynamic shear stresses that interact with adjacent sediment beds may be substantially reduced, thereby reducing bedload transport rates, and furthermore acting to directly obstruct bedload transport (see section 1.1). The complex interactions between these reef structures and surrounding sediment beds remains entirely unknown and currently precludes the use of existing sediment transport formulations over e.g. coral reef flats with even moderate coral coverage.

For suspended load, the vertical distribution of suspended sediment concentration is governed by the relative magnitudes of downward settling due to gravity and, assuming the sediment source is at the bottom, upward due to turbulence. For uniform sandy beds, many numerical simulations of this advection‐diffusion balance are described in the literature (e.g. Van Rijn, 2007b) using different parameterizations of turbulent diffusion (i.e., different turbulent closure schemes). When combined with a sediment entrainment (critical shear stress) threshold and a hydrodynamic transport model, this allows rates of suspended load to be numerically predicted. However, similar to the challenges with predicting bedload, the critical shear stresses responsible for entraining sediment may be strongly modified by the presence of hard reef structures, precluding the use of existing suspended load transport formulations. Moreover, the presence of the large and complex roughness of reefs can

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generate very different turbulent flow fields (Hench and Rosman, 2013), which may require the use of fundamentally different turbulence models to accurately predict how suspended sediment is transported in coral reef environments.

At the shoreline, sediment transport processes in the swash zone are likely very different again. In general, sediment concentrations are much higher in a swash zone than one would experience further out in a surf zone, or in the case of a reef, within the lagoon (e.g. Puleo et al., 2000).During swash events, water filtrates into and out of the beach face and alternates between a decelerating uprush phase and an accelerating backwash phase, both of which modify the sediment transport processes. The hydrodynamics of swash processes on even simple sandy beaches are not fully understood, and even less is known about the associated sediment transport processes (e.g., Butt and Russell, 2000) . Carbonate beaches on coral reef islands are typically even more porous than many sandy beaches and understanding and modeling cross‐shore and alongshore sediment transport in the swash zone on these reef‐protected beaches will be essential to accurately predict shoreline change in reef environments.

Finally, modeling both bedload and suspended sediment transport require knowledge of the settling velocity distribution. Unlike relatively well sorted beach sand carbonate sediments in coral reef environments may be poorly sorted and cover a wide range of shapes and sizes with quite different settling velocity characteristics. It is also known fine‐grained sediments derived from terrigenous sources and dredging operations may flocculate in sea water and alter their size‐distribution towards a more coarse‐grained one (e.g., Kranck, 1975; Pejrup, 1988; Soulsby et al., 2013) and the density of suspended aggregates due to flocculation decreases as their size increases (e.g., Dyer and Manning, 1999; Gibbs, 1985). As a result, the settling velocity of the flocs is different from individual particles. However, the extent and importance of flocculation in non‐cohesive marine sediments in coral reef environments is not well understood and may be important to include in some numerical model applications where flocculation processes may occur.

3.5 Summary and Actions

1) Summarizing the above, the following priority actions can be defined: expand the number of observations of sediment transport (particularly bed load transport) and sediment distribution on coral reef‐lined coasts. 2) Better understanding of the heterogeneity of coral reef sediment properties and their effects and implications for measurement techniques and calibration. 3) Better understanding of the effect of sediment deposition and re‐suspension on reef health. Development of new transport formulations to estimate the transport of heterogenic material with varying properties (grain size, shape, porosity). 4) Development of predictive models which account for sediment production, sediment transport forcing by different agents, and modification of sediment transport by existing (hard) canopies and reef structures.

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Table 5: Summary and actions for sediment transport

Issue Score Action Current Need State KNOWLEDGE Sediment Some data on 3 5 Data from a wider characteristics and sediment grain size range of reefs, both distributions distributions and geographically and composition with different morphologies Carbonate sediment Very limited 1 5 Laboratory flume transport understanding of studies on how how different different sediment carbonate grain grain types respond to types are hydrodynamics transported

Sediment transport Existing knowledge 1 4 Direct observations among reef canopies limited to only a few from field and studies of laboratory studies and canopies DATA AVAILABILITY Sediment property Several data sets for 3 5 Data from a greater distributions reefs in the Indo‐ diversity of reef types Pacific and the and global regions Caribbean Sediment transport Some limited data 2 5 New field and on suspended laboratory data on sediment fluxes exist bed load transport on a very small and sediment number of reefs (i.e. transport influenced Hawaii) by reef canopies

MEASUREMENT AND MONITORING TECHNIQUES Seabed properties Robust analytical 4 5 Improved techniques approaches to infer to automate sediment sediment characterization, characteristics especially in situ

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Issue Score Action Current Need State Suspended load Hydrodynamic 2 5 Improve our measurements are understanding of the robust but accurate response of different measurements of indirect sediment sediment sensors (optical and concentrations are acoustic) to carbonate challenging sediment material

Bed load Accurate bed load 1 5 Develop approaches measurements are that can be used in largely confined to the field controlled laboratory studies MODELING CAPABILITY Carbonate sediment It is unclear how far 2 5 Data is first needed to transport existing formulations assess how existing formulations derived from sediment transport siliclastic sediment formulations perform transport studies can be extended

Canopy sediment Presently no models 1 4 Develop semi‐ transport exist empirical transport formulations formulations that account for reduced bed shear stresses by overlying or adjacent canopies

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36 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

4 Reef biogenic production processes Coral reefs and reef associated landforms (beaches, coastal plains and islands) are unique in several respects. Their sediments, predominantly composed of (CaCO3), tend to be the direct result of ecological processes being that are derived from the skeletons of calcifying reef organisms that induce CaCO3 deposition. Understanding of coastal processes that influence coastal change and drive hazards along reef coastlines, therefore, requires a robust understanding of the carbonate budgets that influence net reef accretion (structure) and the generation of detrital sediment. Understanding the physical dynamics of reef associated coastlines and coral reefs to changing environmental conditions thus also requires consideration of the ecological processes that influence the reef framework and carbonate sediment production and accumulation.

4.1 Knowledge

4.1.1 Production and destruction of biogenic reef carbonate There are many different kinds of calcifying organisms on coral reefs. These range from photosynthesizers such as reef‐building coral, calcareous macro algae, crustose corraline algae (CCA) and benthic forams; to particle‐ and detrital‐feeding organisms such as snails, clams, mussels, and oysters. In the case of coral and benthic forams, these organisms mitigate through closely associated symbiotic microalgae. Arthropods such as , prawns, and lobster also form mixed organic‐carbonate shells, but tend to not contribute substantially to either the formation/accretion of the reef framework, nor to the formation of carbonate sediments. In most cases, coral and algae are by far the largest contributors to the formation of calcium carbonate (CaCO3) on coral reefs by mass since photosynthesis can support much higher rates of the energy and carbon supply to create CaCO3 than can feeding on particles or detritus. Nonetheless, the sediment composition around reefs will often not reflect the production or distribution of the dominant calcifiers living in the adjacent reef communities (see further on). The aggregate rate of net CaCO3 formation by mixed, shallow reef communities occupying coral‐, algae‐, and sand‐dominated substrate has been very well constrained over the past 40+ . These rates typically average around 4 kg m‐2 yr‐1; a rate which (Kinsey, 1985) referred to as reflecting a ‘standard reef ’; however, they can range between a factor of two lower or higher than this value (2 to 8 kg m‐2 yr‐1; (Falter et al., 2013)). Surprisingly, aggregate rates of community net calcification (Gnet) across the Indo‐Pacific and Caribbean do not appear to vary substantially with either latitude or longitude despite known gradients in light, temperature, and saturation state on these planetary scales. However, there does appear to be greater seasonality in Gnet with increasing latitude (Kinsey, 1985). Because Gnet for these mixed communities contain larger sand patches; Kinsey (1985) Kinsey (1985) further estimated that areas of 100% coral and/or algae would calcify at rates in excess of 10 kg m‐2 yr‐1.

Concerns over the impact that rising ocean and pCO2 levels will have on the growth of coral, algae, and other calcifying organisms over the past decade has driven much of the recent research on how the growth of calcifying organisms are affected by different environmental variables (e.g.; light, temperature, nutrients, pH, pCO2, food availability etc.). Most of this work has been done in controlled 37 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

experimental aquaria which can, at best, only approximate the full suite of environmental conditions representative of real reef environments. Furthermore, scaling‐up the growth rates of individual organisms (particularly sessile ones) to entire reefs can be challenging. For this reason, other studies have focused more or measuring the growth rates of calcifying reef organisms in situ at the scale of an individual organisms or communities. While community scale measurements are generally reported in areal‐specific rates of CaCO3 production by mass, organism‐scale growth rates have been reported as either areal rates of calcification (grams of CaCO3 per area per time) or rates of vertical extension (cm per ). Numerous rates of in situ coral calcification on the order of 1 to 2 g cm‐2 yr‐1 have been reported for the massive coral spp. (see Cooper et al., 2008); however, few such in situ areal measurements for branching coral are available due to their geometric complexity. Rates of coral extension can range from on the order of 1 cm yr‐1 for massive coral such as those belonging to the family Poritidae to rates in the general range of 5 to 10 cm yr‐1 for fast‐growing branching coral such as those belonging to the family Acroporidae (Bak et al., 2009; García et al., 1996; Gladfelter et al., 1978; Harriott, 1998, 1999; Shinn, 1966). Because of their generally simple morphology, rates of CCA growth have been reported as either extension rates of 0.5 to 3 cm yr‐1 (Adey and Vassar, 1975; Matsuda, 1989; ‐2 ‐1 Steneck et al., 1998), or 1 to 10 kg CaCO3 m yr (Chisholm, 2000; Mallela et al., 2004; Pari et al., 1998). While there are many different taxa of calcareous algae, most studies interested in the production of reef CaCO33 have focused on , which grows through the accumulation of discrete segments making measuring its growth tractable. Reported growth rates depend on the and biomass ‐2 ‐1 density of the Halimeda stands, but are on the order of several kg CaCO3 m yr (Drew, 1983; Hart and Kench, 2007).

There are four main processes governing the breakdown of biogenic CaCO3 produced on reefs: dissolution, , boring, and mechanical erosion. Rates of net community and system calcification derived from changes in total alkalinity (Smith, 1973) by definition represent total rates of CaCO3 formation minus dissolution; therefore, rates of dissolution need not be specifically addressed in carbonate budgets at these scales as long as the primary rates of CaCO3 production are based on these estimates derived from net changes in water chemistry. Tribollet and Golubic (2011) have provided an excellent review of rates of various bioeroders belonging to each of these general ecological guilds. They further divide the more traditional borer guild into microborers (micro‐algae, bacteria, and fungus) and macroborers (e.g.; , bivalves, sipunculids, , and forams). Rates of microborer erosion are generally on the order of 1 kg m‐2 yr‐1, but can reach several kg m‐2 yr‐1. While many of these boring agents act through dissolution of the carbonate material, rates of macro‐bio erosion tend to be similar to rates of micro‐bio erosion (several kg m‐2 yr‐1); however, a more profound effect of macroborers on the biogenic carbonate structure may be through reducing its structural integrity making it more susceptible to breakage by storms or other strong hydrodynamic forces. It is not yet clear how much the action of microborers undermine the structural integrity of biogenic carbonates given their size (tens of microns), but these may also play an important role. Grazers of reef algae (particularly and sea urchins) are by far the dominant bioeroders on coral reefs. The destruction of carbonate material caused by grazers typically ranges between 5 and 20 kg m‐2 yr‐1. Given that much of this material passes through the of the grazing organism, it is not certain how much of the material 38 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

is dissolved in the low‐pH environment of the gut versus being excreted as intact carbonate sands and muds (Perry et al., 2008). However, it is generally believed that most of the material eroded by the action of herbivorous grazers is not dissolved making these organisms a potentially important source of carbonate sediment in some reef systems. The sum total rates of bio erosion from each of these classes is estimated to thus be between 5 and 15 kg m‐2 yr‐1; rates that are similar to those for production by the dominant calcifiers in areas of 100% coral and algal cover. Thus, it would appear that rates of carbonate production and destruction in many reef communities are generally well‐balanced.

Figure 2: The flow and partitioning of calcium carbonate (CaCO3) and sediment in a coral reef system.

4.1.2 Reef Accretion and Accretion Rates

The development of a reef’s geomorphic structure is largely dependent upon coral growth and the biologically‐induced precipitation of calcium carbonate (Figure 2). There is a significant difference between the ability of individual coral colonies to calcify and extend their skeletons and the ability of an entire reef platform to vertically accrete. From a geomorphological perspective the growth of a coral reef reflects the net processes of biological construction, bio erosion, mechanical and chemical deposition (e.g. detrital sediment and cementation) and mechanical and chemical erosion (transport and ). These processes can be highly variable at short timescales and between reefs (Perry and Hepburn, 2008; Scoffin, 1992).

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Of particular relevance to assessing future coastal hazards in coral reef environments is the rate of accretion of the reef structure, which modulates hydrodynamic processes and in turn governs sediment fluxes, shoreline change and flooding. Rates of reef accretion can be considered at different timescales.

Historical rates of reef accretion Past rates of reef accretion inferred over geological timescales have reconstructed reef accretion over the past 10000 years in response to the last major period of sea level change (18000 – 3000 years ago). Such studies use radiometric dating of coral cores to derive net long‐term rates of coral reef accretion. Data show significant variability in reef growth between different regions and between different framework types. Rates of vertical accretion of reef framework range from 1 to 30 mm yr‐1 (modal rate of 6‐7 mm yr‐1, (Hopley et al., 2007; Montaggioni, 2005). Highest rates of growth have been observed in highly porous frameworks of tabular and branching corals, while the lowest rates occur in foliose and encrusting corals and coralline algal reefs. Detrital‐dominated reefs exhibit vertical accretion rates that range from 0.2 – 40 mm yr‐1 and can be divided into three groups that reflect different hydrodynamic energy regimes. Lower modal rates of accretion of 1‐3 mm yr‐1 occur in low energy lagoonal settings with mud dominated materials. Rates of detrital reef accumulation on the order of 4‐8 mm yr‐1 characterize sand and rubble facies on moderate energy reef flat and backreef settings. Highest detrital rates of framework accumulation (> 10 mm yr‐1) occur in storm settings in which sand and gravel sheets are able to be deposited episodically (Montaggioni, 2005).

However, the examination of net rates of reef development tends to mask spatial and temporal variations in reef growth behaviour. Spatial variations in reef accretion exist within individual reef systems according to ecological zone and energy exposure. For instance in the barrier reefs of (Montaggioni, 2005) and (Kayanne et al., 2002) windward outer reef margins were characterized by rates of approximately 6 mm yr‐1 whereas leeward reef growth followed a catch‐up mode at rates of 3‐4 mm yr‐1. Temporal variations in reef growth rate have also been identified as a result of shifts between reefs catching up and keeping up with sea level change. Globally reefs have exhibited a reduction in growth rate as they approach their vertical growth limit near the sea surface (Hopley et al., 2007; Montaggioni, 2005).

There have been comparatively few studies of rates of lateral reef growth that lead to widening of reef structures. For Indo‐Pacific reefs, rates of lateral reef accretion range from 15 to 84 mm yr‐1 (modal rate of 50 mm yr‐1) for reefs in semi‐exposed to sheltered settings. Highest rates of lateral progradation occur in high energy reef margins and range from 24 to 300 mm yr‐1 (modal rate of 90 mm yr‐1, Montaggioni, 2005; Yamano et al., 2003).

Contemporary rates of reef accretion Because net reef accretion is essentially a time integrated process that occurs over decades to millennia there have been comparatively few attempts to determine contemporary rates of reef accretion. 40 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

Measurements of changing alkalinity have been used to establish consistent ‘standards of metabolic performance’ ‐ in terms of photosynthesis, production/ ratios and calcification – for a range of coral substrates at a wide range of reef locations (Kinsey, 1983). Differing rates of vertical accretion can be clearly associated with different reef production zones. Using this approach estimates of reef accretion have ranged between ~0.9 and 9.0 mm yr‐1 (Kench et al., 2009b; Spencer, 1995), which are consistent with geological analogues.

Future Reef Accretion There has been considerable scientific discussion concerning future rates of reef accretion. Central to such discussions are consideration of anthropogenic and environmental change stressors (e.g., increasing ocean temperature and ocean acidification) that may compromise the ability of reefs to accrete as sea level rises over the coming century. Collectively, studies of future reef response have been largely speculative and based on conceptual models of environmental change and reef adjustment.

Biological assessments of future reef accretion have been driven by considerations of changes in reef and health and tend to provide gloomy projections of reef physical deterioration (structural loss) and lack of accretion. Some environmental change scenarios, associated with high rates of sea level rise and deleterious impacts on reef ecosystems envisage coral reef ecosystems being ‘drowned out’ over centennial timescales. It is clear from the presence of extensive drowned banks that some reef systems did fail in the (Vecsei, 2003).

In contrast, geological assessments of future reef performance provide more optimistic scenarios of reef accretion potential based on evidence from the geological record. For example, the past modal rate of reef accretion ~6‐7 mm yr‐1 (range of 1 – 30 mm yr‐1) are of a similar magnitude to future projections of sea level of ~0.5 mm yr‐1. All other things being equal, this suggests reefs are unlikely to be ‘drowned out’ by near future sea level rise (summarized in Kench et al., 2009b).

Whether reefs will accrete to track near‐future sea level rise (the ‘keep up’ mode) or not accrete, resulting in increased water depths over reefs, has implications for shoreline processes, reef ecology and groundwater hydrology. Historical records of long‐term net accretion over the past 10,000 years show that framework‐dominated reefs have recorded rates of vertical growth of 1 – 30 mm yr‐1, with a modal rate of 6 –7 mm yr‐1. Such accretion rates are of a similar magnitude to future projections of sea level change and suggest that, all other things being equal, reefs are unlikely to drown and that reefs have the potential to more than keep pace with expected rates of near‐future sea level change (Pachauri and Reisinger, 2007; Spencer, 1995).

It is clear that development of more robust models of future reef accretion potential is a high priority.

4.1.3 The Detrital Sediment Budget and Reef Associated Landforms The generation of detrital sediment on reef platforms and its transfer by physical wave and current processes is critical for the formation, maintenance and ongoing development of a suite of geomorphic features on reef systems (Figure 2). The primary sources of detrital material in reef systems are the skeletons of corals and other organisms or that contribute to CaCO3 deposition. In addition to 41 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

ecological processes, chemical precipitation can be important in the production of carbonate material (e.g. ooids) on some reefs, while terrestrial inputs can also impact locally in the sediment systems of fringing and barrier reefs.

The production of bioclastic sediment is reliant on a complex set of processes that include calcification by calcareous organisms and its conversion to the sediment reservoir. Calcification is mediated by organism specific metabolic processes and varies significantly between species and between primary framework builders (coral, ) and secondary producers (foraminifera, molluscs). The contribution of secondary benthic organisms to the sediment reservoir is largely a function of organism growth rate, fecundity and turnover as skeletons contribute directly to the sediment reservoir upon death (Figure 2). However, conversion of primary framework to detrital sediment is reliant on a complex set of additional processes that include mechanical and biological breakdown of substrates (Madin, 2005; Scoffin, 1992). Thus, the relationship between calcification and sediment generation is not linear, with time lags for the decay of reef framework that may span 101‐103 years (Perry et al., 2011).

The dominance of ecological processes in generating sediment on coral reefs imparts a number of unique characteristics on reef sediment systems. First, the source of sediment production is local with the transport and deposition occurring on or near reef platforms over distances hundreds of meters to several kilometers. Second, sediment may or may not be produced on a continual basis; however, current understanding environmentally and taxonomically driven variation in contemporary sediment production rates remains poorly understood.

Figure 2 shows that carbonate and terrigenous sediments within the reef sediment reservoir are cycled within reef systems and are retained in a number of distinct zones. Some detrital sediment may be exported off the reef platform, down the fore‐reef, along sand chutes or through the flushing of fine material from reefs in suspension (Hubbard et al., 1990; Hughes, 1999). These off‐reef sediment fluxes are net losses to the reef calcium carbonate budget and make no contribution to geomorphic development of the reef platform surface. Sediment retained on reef platforms can be reincorporated and cemented into reef framework, contributing to the long‐term development of reef morphology (discussed earlier, see Hubbard et al., 1990; Perry and Hepburn, 2008). Alternatively, material can be transferred to central‐ or back‐reef zones where depositional landforms such as rubble ridges, gravel sheets, reef islands or coastal plains can accumulate (Maragos et al., 1973; Stoddart and Steers, 1977; Woodroffe et al., 1999). These landforms form in the intertidal to subaerial zones and act as medium‐ term stores of sediment within the reef platform system. Transport of sediment to backreef lagoons promotes subtidal deposition of material and contributes to lagoon infill. Deep lagoons act as long‐term sinks of sediment (Kench, 1998; Macintyre et al., 1987; Purdy and Gischler, 2005).

At the intra‐reef platform scale, subtle variations in processes produce a suite of reef sedimentary units that can be differentiated based on the location of sediments relative to the reef surface, reef type, presence of non‐carbonate substrate and elevation of landforms with respect to sea level. In fringing reef settings, detrital carbonate deposition typically occurs toward the leeward edge of reefs at

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the interface between the reef and terrestrial environment. Typically, subaerial coastal plains, beaches, spits and barriers form through progradation of sediment across the backreef zones (Figure 3A, B). In these settings, terrigenous sediments delivered to the coast mix with biogenic sediments and further contribute to landform development. In contrast, in barrier reefs, lagoons separate the reef structure and non‐carbonate shoreline. Sediment generated from reefs can actively contribute to lagoon infill and the formation of subaerial deposits is reliant on transport of sediment to backreef or central reef locations. Land building in barrier reef environments may also occur through transport of sediment from reefs into lagoons and its subsequent reworking and deposition at the shoreline where mixing with terrigenous sediments may also contribute to landform accumulation (Figure 3). In atoll environments or isolated platform reefs, sediments can accumulate directly on the reef surface or over lagoon sediments, forming reef islands (Figure 4).

From a geomorphic perspective the development of reef sedimentary landforms is controlled by a number of key boundary conditions. First, sea level and its modulation of coral platform development, act as a critical control on the stability of sedimentary landforms. Second, accommodation space defines the available volume for sediment deposition as controlled by substrate gradient, elevation and sea level (Cowell and Thom, 1994). For reef islands the lower boundary defining accommodation space is governed by reef margin, the reef flat elevation and lagoon depth. Third, energy exposure and reef top hydrodynamics play an important role, which in coral reef settings are modulated by the relationship between reef elevation, sea level and incident ocean swell. Fourth, sediment supply is also controlled by reef and sediment generation processes.

In contrast to the geomorphic development of reef platforms, knowledge of the evolution and morphodynamics of reef sedimentary landforms is not well developed.

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Figure 3: Typical sedimentary landform settings in coral reef environments (Kench et al., 2009a, Figure 7.10)

4.1.4 Guilds concept The geological view of reefs emphasizes the functions and interactions of reef organisms with regard to the physical structure of a reef. In this view the reef organisms are assigned to different reef guilds (Fagerstrom, 1991). The five main reef guilds are:

1. Builders; massive scleractinian corals are the principal builders of reef frameworks with contributions from other coral groups such as the Octocorals Sinularia and Heliopora in certain settings. 2. Binders; calcareous encrusting coralline algae such as Lithoporon (“reef cement”) and other genera bind reef rubble together. Interestingly many soft or weakly lithified organisms also play an important role in binding, notably sponges, Palytoa, fleshy algae, and certain worms. Sea grasses and algae also constitute important binders in the reef system. 3. Bafflers; many branching corals such as cervicornis () act as bafflers, trapping sediment at their bases by reducing wave and current flow through their structures. Some of the reef binders (algae, seagrasses) also act as bafflers. 4. Destroyers; bio eroding organisms which break down the skeletons of calcareous organisms are instrumental in the “sand cycle” that is the grain diminuation and fragmentation of large calcareous reef components. Most important bioeroders are certain (e.g., parrot fishes), sea urchins (e.g.,Crown‐of‐Thorns, Echinometra); gastropods (e.g., Drupella); boring sponges (Cliona and Pione). 5. Dwellers; all the reef organisms which live on the reef and hidden in the reef framework are reef dwellers (sponges, fishes, arthropods, etc,). They can have multiple functions within the reef system and some are important sand generators such as the large benthic foraminifera (Marginopora, Calcarina, Baculogypsina) 44 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

It should be noted that in this framework an organism may be part of several guilds at the same time, e.g., the parrot fish is a dweller within as well as a destroyer of the reef framework.

4.1.5 Reef types Most tropical coastlines are surrounded by reefs which form a shallow platform of variable width seaward of the shoreline. Three different reef types are commonly distinguished (Figure 4):

 Fringing reefs

 Barrier reefs

 Atoll reefs

Fringing reefs Definition: Fringing reefs are located in front of a landmass of variable composition; they are always connected to the shore or land.

Morphology: Fringing reefs vary in their width but are typically between 100 m and 1000 m wide. Exceptions to that rule are not uncommon particularly where recent landslides have reduced the width of the reef platform and a new narrow fringing reef is forming. Wider fringing reefs often become transitional to barrier reefs. Fringing reefs consist of a fore reef with spur and groove (S.a.G.) system, a reef crest, back reef and beach. Most fringing reefs are dissected by channels which allow for drainage of land‐derived water. The distance of the channels to each other is highly variable and channels are more frequent in humid climates than in arid regions and more abundant on windward sides of islands than on lee‐ward sides.

Characteristics: Because fringing reefs are connected to the shoreline they tend to receive more terrestrial sediments than do barrier reefs. Where large rivers drain into the sea from high islands fringing reefs often cannot establish themselves. Islands with high terrestrial sediment influx often have soft shorelines covered with mangroves instead of fringing reefs.

Regional distribution: Fringing reefs are common all throughout the tropical Pacific and Indian Oceans where freshwater and sediment drainage is limited and where Island shelves adjacent to landmasses are established.

Barrier reefs Definition: Barrier reefs are separated from a land mass by a lagoon of variable width and depth.

Morphology: The morphology of barrier reefs is principally similar to fringing reefs but the backreef grades into a lagoon with significantly reduced wave energy. Large patch reefs, pinnacle reefs and other reefal structures can grow in the lagoons. Barrier reefs also have channels leading from the lagoon into the open ocean except in a few cases where the reef rim is completely cut‐off from the open ocean and tidal flow is only through the reef framework.

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Characteristics: Barrier reefs are detached from land and consequently less exposed to freshwater run‐ off from land and terrestrial sedimentation. Barriers located on windward sides of islands often grow more prolific than those on the lee‐ward sides.

Regional distribution: Barrier reefs are common around the larger landmasses of tropical and subtropical island arcs, continents (SE Asia). They also found around infra‐oceanic islands that are transitional to atolls (, , Hawaii). Some islands have barrier reefs in addition to fringing reefs. Double barriers can also occur (e.g., , New Caledonia).

Atoll reefs Definition: Atoll reefs are encircling a lagoon and a central landmass is missing.

Morphology: The morphology of atoll reefs is principally the same as for barrier reefs with a fore reef, reef crest, back reef and lagoon. The morphology of the reef varies with regard to the main wind direction and history. Lagoons can measure 70 km in diameter or more but most are smaller. Channels dissecting the annular rim are called "hoa" and are common on some atolls but missing on others. They are important for the water and sediment fluxes in and out of the central lagoon.

Characteristics: Atoll reefs are typically far removed from any terrestrial sedimentation. on the reef rim are small and low‐lying. As such they are particularly vulnerable to over‐topping. Islets in high‐ energy settings are often composed of coarse sediments ("motu") whereas those in low energy settings have sandy beaches (sand cays).

Regional distribution: Atolls are most common in the central and south Pacific where intraplate volcanism is fueled by migration of the large Pacific Plate over numerous thermal plumes. Atolls are the final stage of subsidence of a in a tropical environment and testimony to the reef’s ability to keep up with sea level rise. Atoll formation is intricately linked to geological history and wave and wind climate. Atolls in the Indian Ocean (e.g. Maldives) are often more complex and contain “rings within ring” called “faroes” (Agassiz, 1902).

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Figure 4: Diagram showing the three principal reef types discussed in the text. (Image credit: http://geology.uprm.edu/Morelock/7_image/rfmorp.gif)

4.2 Data availability There have been abundant measurements of total reef community calcification rates, dissolution rates, and long‐term accretion rates reported in the literature (e.g. Andersson and Gledhill, 2013; Falter et al., 2013; Kinsey, 1985). There have also been an abundance of bio erosion measurements reported in the literature (e.g. Tribollet and Golubic, 2011); however, the accuracy and repeatability of these measurements are not quite as good as for measurements of total reef accretion and net calcification owing to the difficulty of measuring and integrating the various forms of bio erosion and properly partitioning total erosion amongst the relevant taxa responsible. Nonetheless, the various agents of sediment production and breakdown (or ‘guilds’) have been well‐described decades ago. Similarly, basic morphological descriptions of the various reef types inhabiting the Indo‐Pacific and Caribbean regions have been well‐known for a long time (e.g. Stoddart, 1969; Wiens, 1962) Rates of actual sediment production, retention, residence time and transport still remain poorly constrained and are much more limited in number. Indeed, sediment production remains one of the least studied aspects of coral reef sedimentology (Kench, 2013; Perry et al., 2011). We suspect that spatial maps and vertical cores of reef sediment facies could be available from government agencies charged with mapping and monitoring of coastal reef systems; however, accessing and systematically making these available in digital format would require a very large effort.

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4.3 Measurement/monitoring techniques Total rates of system‐ and community‐scale net calcification are usually made by measuring changes in total alkalinity across a system and calculating net benthic fluxes using either a combination of the depth and residence time or a combination of the distance traversed and the depth‐integrated transport (Falter et al., 2008; Smith, 1973; Teneva et al., 2013). Fredsoe and Deigaard (1992) indicate that in many instances a more fully explicit treatment of the non‐conservative transport equations involving both ‐2 time‐dependent and advective terms is necessary. Experimental rates of coral calcification (g CaCO3 cm hr‐1) in controlled aquaria are generally made by also measuring changes in total alkalinity; however, there are alternative methods for measuring calcification from changes in the buoyant weight of the skeleton (Davies, 1989). Field measurements of coral calcification have generally focused on simple geometric extension rates since in situ rates of organism‐scale calcification are difficult to make. Extension rates are generally estimated by measuring changes in the distance between a reference point (usually some sort of band) to the tip of a coral finger. Similar measurements of calcification and extension rates of CCA communities have been made in both aquaria and in the field; however, there is far less data for these calcifiers than for corals given its lower attention. There are even less measurements of the growth rates of Halimeda spp. These are usually made by staining the actively growing plates or banding the at specific linkage point and measuring the number and mass of plates formed over a given period of time. Rates of bio erosion from boring organisms (macro and micro) are usually estimated by measuring the rate of invasion in specially cut blocks of bare coral skeleton and the volume of material removed over a defined period of time. Rates of bio erosion from grazing organisms are usually made by comparing changes in the mass of carbonate blocks cut from coral skeletons exposed to the open reef and those protected by cages aimed at excluding the dominant grazers.

4.4 Modeling capability and needs

4.4.1 Biologically driven rates of carbonate production Given the uncertainty in making field measurements of biologically driven rates of carbonate production and destruction, we recommend that suitable ranges in rates of reef accretion and detrital sediment formation be assigned based on spatial maps of benthic reef community type and/or derived from an evaluation of the literature prior to model implementation. However, it is clear that the prevailing physical processes such as wave transformation, generation, dissipation, circulation, sediment suspension and transport will be highly dependent on the morphology of the reef surface (e.g., roughness) and composition of the sediment being produced or imported (e.g.; size, density, and shape). These attributes should be explicitly included in any numerical model developed.

4.4.2 Deposition rates in reefal environments

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As more than 40% of the world's oil and gas reservoirs come from carbonate rocks, reefs have historically been assessed by geologists as carbonate depositional systems in order to understand reservoir capacities. Resulting products are typically three‐dimensional facies models which utilize geophysical methods and coring to assess the geometry of potential reservoirs. An outcome of this activity is the study of extant reef systems to understand the processes which lead to the formation of reservoirs.

To identify and quantify the attributes of facies tracts satellite images can be integrated and processed in GIS, followed by statistical analysis. For example Vlaswinkel et al. (2008) mapped 9 different facies classes from Landsat images for 19 modern carbonate platforms from Caribbean and Indo‐Pacific regions which were digitized using ER Mapper. The reservoir facies that were recognized included fully aggraded reefs, partially aggraded reefs, reef aprons, shoals and shallow platform interiors. Quantitative characterization of the reef complexes were derived by measuring for every polygon of each facies distinguished various attributes such as area, perimeter, width, length, orientation, and the variability within those metrics. Statistical analyses confirmed the existence of certain predictive “rules” between configuration and composition of facies tracts on and among carbonate platforms (e.g. size of platform versus number and/or abundance of facies or size of platform versus shape complexity.) These kinds of “rules” have provided some general concepts and raw data that could be used as input for enhanced carbonate models. In other words, if facies tracts could become better linked to carbonate production rates, then these could facilitate more accurate predictions about sediment distributions on reef flats and carbonate budgets.

Currently an extremely limited number of reefs globally have been assessed in terms of their facies distribution and the link to carbonate production rates remains to be established.

4.5 Summary and Actions

Table 6: Summary and actions for reef biogenic functioning

Issue Score Action Current Need State KNOWLEDGE Carbonate Isolating sediment 2 5 Measure abundance production production from and growth rates of total community and dominant sediment‐ system rates of net producing organisms calcification

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Issue Score Action Current Need State Reef accretion rates Measuring 4 3 Robust model climatically driven development changes in already slow growth rates characteristic of the Holocene

Reef carbonate Bringing together 2 5 More holistic studies budgets robust of coastal systems measurements of dominated by local sediment production of biogenic production, carbonates (eg, reefs). transport, and retention within the same

Coastal reef Accurate spatial 3 5 Determine availability maps of reef habitat, of regional databases dominant calcifiers, from relevant and system government agencies. morphology. Also Fill data gaps with needed are vertical remotely sensed cores of sediment techniques or line facies. transect surveys.

DATA AVAILABILITY Net Calcification Only a handful of 5 2 Plenty of system‐scale measurements in the measurements literature. Measurements are 4 3 Plenty of prone to larger measurements in the errors and span wide literature, but few range of taxa and bio relate directly to areal erosion mechanisms. rates of detrital Integration of sediment production. organism‐scale measurements to community and ecosystem scales difficult.

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Issue Score Action Current Need State Sediment budgets Very limited number 2 5 Conduct more holistic of sediment budgets system‐scale studies. MEASUREMENT AND MONITORING TECHNIQUES Sediment production Identify those 4 3 Target only those processes which processes which generate detrital generate detrital sediments sediments MODELING CAPABILITY Sediment production Still limited 1 5 Start developing semi‐ quantitative empirical relationships understanding of the describing physical and dependency of biological sediment production mechanisms driving on relevant sediment production environmental and (see above) ecological factors

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5 Coastal response

5.1 Knowledge

5.1.1 Conceptual model A useful way to describe the changes to the coastal system starts by dividing it up into a number of larger units as depicted in Figure 5 below, where we distinguish between reef flat, shallow lagoon, deep lagoon, beach and regions. Changes in these units as a whole are characterized by their sediment volume; together they share the so‐called sediment reservoir. Similar approaches have been taken in e.g. de Vriend et al. (1993) for tidal and lagoons. The units are linked through the transports between them, denoted by arrows in the figure. We distinguish the following transport pathways:

 Reef flat to lagoon: dominated by wave motions and mean currents. Swell waves, infragravity waves, tidal and wave‐drive circulation all may contribute to the bottom shear stresses that are responsible for sediment mobilization and transport (Van Dongeren et al., 2013).

 Lagoon to beach: these transports are governed by what remains of the waves after passing the reef; usually this is dominated by infragravity waves and mean currents (Beetham and Kench, 2011; Pomeroy et al., 2012).

 Beach to coastal plain: sediment can be transported onto the plain by overwash processes, either by infragravity waves during swell or storm events, or more infrequently by tsunamis. Another mode of transport is wind‐driven, leading to formation.

Inputs to the sediment reservoir can be both in the form of carbonate sediment production on the reef (see Chapter 4) and from rivers discharging terrigenous sediment at or nearby the reef (e.g. Draut et al., 2009). Losses to the active sediment system are either by export from the reef‐lagoon system, driven by waves and currents (often dominated by the strong seaward currents through channels) (e.g. Morgan and Kench, 2012), or by cementing to the reef platform.

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Figure 5: Conceptual model of the detrital sediment reservoir in coral reef systems showing functional relationships (transfers) between major depositional depositional zones (geomorphic units). Note, arrows denote whether sediment fluxes are unidirectional or bidirectional

To a first approximation the sediment balance is linked to changes in the height of the coastal plain and in its planform area (e.g. Reading, 1996). Quantification of the transport processes within reef systems (see Chapter 3), both on short time‐scales and integrated over longer periods, is extremely complex and has hardly been attempted, though basic model concepts are available (e.g. Douillet et al., 2001; Fernandez et al., 2006). Alternatively, from observed volume changes in the different units, net sediment transport magnitudes can be inferred (e.g. Browne et al., 2013); these long‐term averages may be used to validate transport models.

At this level of aggregation we cannot describe net movement of units or changes in coastal planform or profile shape. This requires more detailed description of transports across the profile and along the coastline. Longshore transport can lead to significant seasonal and long‐term changes, particularly on relatively small islands (Perry et al., 2011). While general conceptual models exist, it is unknown whether standard process‐driven modeling approaches applied in are applicable to these more complex environments, where a mix of infragravity and swell waves resulting from complex refraction and wave‐current interaction processes create a swash‐dominated longshore transport along the beaches.

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5.1.2 Grain size influence The beach and island response to hydrodynamic forcing is strongly determined by the distribution of sediment properties. Typically, sediment diameters have some relationship to the local wave energy; however, this may not always be the case due to the biogenic nature of sediment production in coral reef environments (Cordier et al., 2012). The hydrodynamic processes responsible for mobilizing sediments are often strongly influenced by the width and depth of the reef flat. For narrow reefs, the remaining wave energy that reaches the toe of the beach is stronger and may cause these beaches to be predominantly made up of coarse and coral rubble. Conversely, behind wide reefs the swell energy is mostly filtered out and finer sandy beaches can be maintained (Kench and Brander, 2006b). The selective transport processes leading to often distinct spatial distribution of sediment types and sizes are still poorly understood; even though existing sediment transport formulations take grain size into account, different formulations vary widely in their sensitivity and none consider the effect of the very different grain shapes and densities of carbonate sediments.

5.2 Availability of data

5.2.1 Coastline mapping from historic RS imagery Mapping shoreline change using historical datasets provides a powerful approach to document background rates and patterns of planform coastal change in reef associated coastlines. Such data provides valuable information to determine long‐term rates of coastal change for hazard planning, and to verify geomorphic models.

A number of recent studies have begun to quantify the planform changes in reef associated coastlines from historical data (Biribo and Woodroffe, 2013; Ford, 2013; Ford et al., 2012; Rankey, 2011; Webb and Kench, 2010). However, these studies have primarily focused on mid‐ocean atoll settings and collectively have analysed only a small number of islands in the Pacific Ocean (~200). Similar analyses for high and continental coastlines are few and there is a need to undertake additional studies that sample across a range of reef types.

While there are still few systematic studies of historical shoreline change on reef associated coastlines, datasets do exist for many reef systems in the Asia‐Pacific region that may allow coastal changes to be quantified. Specific types of data include:

 Earliest geological maps and charts (some dating back a century);

 Aerial photography. Many locations have coverage starting in the 1940’s;

 Satellite imagery (e.g., Ikonos, Quickbird); There is no single repository of these datasets. Furthermore, the different types of data can be difficult to source and can be costly.

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5.2.2 Topographic mapping There is a pressing need to establish systematic monitoring records to document and monitor reef and coastal topography. Such monitoring is necessary to establish changes in sediment volumes along the coast, and determine vertical changes in coastal elevation.

There are currently few medium‐term monitoring sites in reef settings designed to determine topographic changes in the coast. Key sites include 13 islands in the Maldives ((Kench and Brander, 2006a)) and a network of locations in Hawaii (Rooney and Fletcher III, 2005). Coastal monitoring has also been undertaken by SPC at a number of anthropogenically impacted sites in Pacific islands. Most recently monitoring has been established at Fatato Island, atoll and Maui Bay Fiji (Waves and Coasts in the Pacific Project). Currently, these monitoring datasets reside with individual research groups and there is no common platform for public access.

5.3 Measurement/monitoring techniques

5.3.1 Topography and bathymetry The techniques used for measuring coastal topography depend on the variables that are to be recorded, the required accuracy, the required surface area to be covered during the survey, the temporal interval, and the available budget (Mason et al., 2000).

Methods to undertake topographic monitoring include: LIDAR; RTK Global Positioning System Surveys; Total station surveys, and dumpy level surveying. It is necessary to ensure any surveys are reduced to a common datum. A number of low‐cost coastal monitoring approaches, suitable for community‐scale involvement, can also be used to document coastal change. In particular, measurements of the distance to the edge of vegetation (from fixed benchmarks in the inland) and beach width are able to be recorded using simple tape measure techniques. Within this framework, beach profiling can be performed using the Emery method (Emery, 1961). If a larger area is to be surveyed, possible methods are RTK GPS, possibly on a quad bike, terrestrial or airborne LIDAR, or multibeam mapping for inshore areas. A fourth possibility is aerial photogrammetry using stereo‐pairs of photos. A comparison of different topographic methods is given by Mason et al. (2000) and Moore (2000). Recently, methods were developed that allow for beach and nearshore bathymetry mapping using video systems. An overview of the state‐of‐the‐art in this field is given by Holman and Stanley (2007).

5.3.2 Edge of vegetation, toe of beach from remote sensing Planform changes of islands may be derived from the position of the vegetation line or the momentary position of the water line on satellite or video imagery. This latter momentary shoreline may be related 56 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

to an absolute reference through a nearby tide gauge. These sharp boundaries are relatively easily recovered using structure extraction techniques from the appropriate combination of imagery bands from remote sensing platforms, if the image resolution is sufficient.

5.3.3 Long­term hydrodynamic monitoring at toe of beach A number of hydrodynamic measurement instruments may be used in standalone mode, or cabled to a shore station. Water levels are measured by using visual or acoustic techniques, or through pressure variations in the water column. Current velocities can be measured using electromagnetic (classic ECM) or acoustic devices (ADCP, ADV). Waves may be measured using pressure sensors, wave staffs or correlation between the beams of acoustic devices, like ADCP's. Points of attention are site accessibility, power supply, measurement continuity, biofouling and high investment costs.

5.4 Modeling

5.4.1 Shoreline translation modeling The Shoreline Translation Model (STM) was originally developed by (Cowell et al., 1992) for sandy barrier coasts. It is based on the assumption that the response of coastal sand deposits to sea level rise can be modeled based only on principles of sand‐mass conservation and geometric rules for shoreface and barrier morphology. The model attempts to predict the horizontal and vertical displacement of coastal sand bodies over the existing coastal substrate that undergoes reworking as a consequence. This produces changes in position of the coastline as well as reconfiguration of the backshore and shelf morphology and stratigraphy. This results in either barrier transgression or the classical behaviour described by the Bruun rule ((Bruun, 1962) depending on external sediment supply and time lags in profile response (Cowell et al., 1992).

Kench and Cowell (2001) and Cowell and Kench (2001) extended the STM by relating the closure depth to the reef elevation, thus truncating sediment exchange potential. They used their model to study the three possible responses of atoll islands to sea level rise according to (Woodroffe and McLean, 1992): (1) Bruun rule response, (2) equilibrium response through washover processes, analogous to barrier rollover, and (3) continued island growth through excess sediment supply from the reef flats. Using data from Tarawa Atoll, (Kench and Cowell, 2001) show that atoll islands do not show one response mode to sea level rise, although they all recede. This recession does not necessarily cause narrowing. (Cowell and Kench, 2001) show the extreme sensitivity of reef island shorelines to barrier rollover effects in absence of external sediment supply, and the importance of reworking the local geological framework in providing sediment to the retreating islands. Because of this extreme sensitivity, small changes in coastal sediment transport patterns by secondary global warming effects such as a change in wave climate may have severe consequences in terms of shoreline change (Cowell and Kench, 2001). STM results indicate that atoll islands have the potential to keep up with sea level rise, as washover is a

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predominant process causing vertical aggradation, and recession rates are generally small compared to island width (Kench and Cowell, 2001).

5.4.2 Process­based modeling In process‐based models we start from a detailed description of processes, by incorporating the transport of sediment and how this in turn influences the evolution of bathymetry or topography. It is a bottom‐up approach, where we do not specify any equilibrium situation beforehand but let the model evolve freely to wherever it wants to go. Obviously, this raises the question of whether such an approach can ever lead to sensible solutions in the longer term. For other types of systems such as tidal , this has been shown to be possible (e.g. van der Wegen et al., 2010); whether it is true for coral reef environments is as yet unresolved.

Principles of process­based morphodynamic modeling The term 'morphodynamic model' is short for 'dynamic morphological model', where 'morphological' means the study of shapes, in this case of the sea bed, and 'dynamic' indicates that we consider how sea bed changes as a result of processes acting on it. The basic flow chart given in Figure 6 is valid for most types of morphodynamic models. We call it process‐based modeling since we take a detailed description of the physical processes leading to sediment transport and bottom changes as a starting point.

bottom depth

waves current

sediment transport

bottom

change loop back

Figure 6: Flowchart of a morphodynamic model

We start from a bathymetry, given on a detailed two‐dimensional grid (in case of area models) or one dimension (in case of coastline or coastal profile models). Given boundary conditions for waves and 58 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

currents, we predict the wave and current fields, which usually interact; together, these processes determine the sediment transport. The sediment transport gradients lead to bottom changes, which then feed back into the bathymetry, the currents and waves and the sediment transports, and so on. Different models may differ greatly in the way they compute currents, waves and transports and in the way the bottom changes are computed and fed back into the bathymetry.

In the case of modeling coastal response in coral reef environments we can consider three approaches distinguished by the wave processes that are resolved.

Wave resolving area models In such an approach the individual swell waves, infragravity waves and the wave‐induced currents are all resolved. This allows us to resolve complex diffraction patterns and the effects of velocity skewness and asymmetries on sediment transport, as well as the swash processes on beaches. However, the numerical grids used for this have to be very fine (multiple grid points per short wave length), so for large areas and longer timescales this becomes prohibitively expensive. Still, as a reference model or to investigate particular short‐term events this approach can be useful. So far, very little experience has been built up using this approach, though some first applications are given in Roelvink et al. (2013)

Wave group resolving area models In this approach, individual waves are not resolved but their effect is parameterized in terms of wave energy, wave action, radiation stress, skewness and asymmetry. However, their variation on the wave group scale is resolved and so are the resulting infragravity waves that are often dominant behind reefs. Such models can used much coarser grids given that only the wave group lengths have to be resolved, as well as the relevant morphological features. The model XBeach (Roelvink et al., 2009) that uses this approach has already been widely used to model dune erosion, overwashing and breaching in sandy environments and its hydrodynamics has been tested in reef environments (Van Dongeren et al., 2013); validation of the model for coral reef coastal morphology change has not been performed to our knowledge.

Wave averaged area models Wave‐averaged modeling such as in Delft3D, ROMS or MIKE21 has been widely applied for assessing flow patterns, residence times and water quality on reefs (e.g. Grady et al., 2013; Villanoy et al., 2012), but much less for sediment transport and morphology. Some experience has been gained in consultancy in modeling transports over unerodible layers but beach change on coral reef‐protected beaches has been difficult to model because of the lack of swash zone processes. Heuristic approaches linking the beach and dune changes to accretion and erosion nearshore have been applied with some success in other environments but have yet to be developed in reef environments.

Profile models For approximately longshore uniform coasts, profile models focusing on cross‐shore behavior can be effective tools, e.g. Walstra et al. (2012). They can be used in wave resolving, wave group resolving or wave averaged mode and since they are much less computationally intensive can bridge the gap in time scales for cases where the coast is not very curved or otherwise three‐dimensional. 59 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

Coastline models In coastline models the change of planform shape due to gradients in the total longshore transport and due to cross‐shore net gains and losses can be modeled, under the assumption that the shape of the cross‐shore profile does not change (e.g. Ashton and Murray, 2005; Pelnard‐Considere, 1956). While this technique can be highly successful in sandy coastal environments, its success is mainly due to the simplified response of the longshore transport as a function of the relative angle of the coastline with respect to the incident waves. This simple connection is lost behind coral reefs, and some other, more complicated modeling of the wave transformation and resulting longshore transport is needed. Still, it could be useful to use the coastline model concept to integrate detailed process model output to longer‐term planform behaviour.

Upscaling methods In order to apply the detailed process‐based models described here to predict coastal evolution on longer timescales, a range of upscaling techniques developed in literature can be considered; for a review see e.g. Roelvink and Reniers (2011). The most important ones likely to be used are:

 Input reduction: instead of simulating through detailed time series of boundary conditions for tide, wind, surge and wave conditions, these are categorized into classes or bins of conditions with associated probability of occurrence; the transports and morphological changes for each bin are computed and the resulting change is obtained by integrating over all bins. In the case of strong seasonality different schematizations per season can be considered (e.g. Walstra et al., 2013).

 Morphological factor approach (Roelvink, 2006): the morphological changes dynamically computed during a simulation are multiplied by a factor, thus speeding up the time evolution; instead of simulating changes over 10 tides one simulates changes over one tidal cycle while multiplying bottom changes by a factor 10. This technique is succesfully and routinely used in sandy coastal environments and can likely be used in some form in reef environments. This is an active field of research.

Deriving macroscopic relationships from process­based modeling Apart from directly using the process‐based modeling approach to simulate longer‐term coastal response, these models can also be used to systematically investigate sediment transport responses to events and their sensitivity to the morphology itself; from such investigations macroscopic relationships could be derived, which can be used in more aggregated model approaches such as the STM.

5.4.3 Conclusions on modeling capabilities So far, only the more aggregated STM model approach has been applied to modeling morphodynamics of actual reef environments. While this approach elucidates a number of interesting aspects, it has fundamental limitations, such as the assumption of longshore uniformity. Process‐based approaches seem to be promising tools and their hydrodynamic capabilities have been established to some extent; their suitability for morphodynamic simulations, especially over longer periods, is yet to be verified and likely needs much further development.

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5.5 Summary and Actions

Table 7: Summary and actions for coastal evolution

Issue Score Action Current Need State KNOWLEDGE Conceptual model Coastal response 2 5 Adapt process‐based modeling only on model formulations to reef aggregated level environments

Influence of grain size Mechanisms 2 4 Develop transport affecting spatial formulations suitable for grain size biogenic sediment variations poorly understood DATA AVAILABILITY Coastline mapping from Few systematic 2 5 Develop central repository remote sensing studies on for imagery. Analyze variety of available data coastline types systematically Topographic mapping Few sites with 1 5 Establish monitoring medium‐ or long‐ network covering key sites term coverage in relevant settings. Centralize data. MEASUREMENT AND MONITORING TECHNIQUES Topography and bathymetry Collect data on a 5 5 Organize community‐level large scale across monitoring programs Asia and the Pacific Vegetation edge, beach toe Collect data on a 3 5 Develop central repository large scale across for imagery. Analyze Asia and the available data Pacific systematically Long‐term hydrodynamic Long‐term 5 5 Establish permanent monitoring hydrodynamic monitoring centre where sampling of reef measurement continuity is system garantueed

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Issue Score Action Current Need State MODELING CAPABILITY Aggregated profile modeling Only suited for 5 3 very long time scales, highly simplified Process‐based modeling Hydrodynamics 2 5 Adapt/develop specific possible to a physics for morphological high degree; modeling of reef morphodynamics environments unproven

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6 Hazards

Coastal hazards such as storm surges, tsunamis, high tides, land subsidence, and sea‐level rise are the cause of a number of impacts, such as inundation, coastal erosion and saltwater intrusion. In terms of coastal flood/inundation extensive literature exist documenting major events, notably the 2004 Indian Ocean tsunami (Okal, 2006), 2011 Tohoku earthquake and tsunami (Mori, 2011), and storm surge generated by Hurricane Katrina in 2005 (Fritz et al., 2007). Furthermore, can be a regular occurrence for locations such as Venice, Italy due to the combined effects of high tides and land subsidence (Comerlati, 2004). Coastal floods/inundations caused by the combination of tides, storm surge, tsunamis, land large ocean waves can cause severe damages to coastal communities.

Coastal flooding/inundation (not only in terms of the water level but also in terms of large current velocities) may cause great loss of live and economic damage to coastal buildings, coastal vegetation, and erosion of shorelines and corals, as well pose great threat to the safety of infrastructure such as nuclear power plants, as in the 2011 Tohoku tsunami (Mimura, 2011).

The effect of global environmental climate change on coasts and small islands in the Asia and Pacific regions is a concern. As many communities in the Asia‐Pacific Region depend on groundwater resources such as freshwater lenses, a possible increased precipitation from coastal storms maybe welcomed, as this will replenish the supply. However, inundation and flooding may negatively impact this supply of fresh groundwater through the intrusion of saltwater lenses. Local environmental changes such as increased land use for economic purposes and decreased space for natural vegetation will also likely increase the intensity of the hazards and increase the consequences. As such coastal communities throughout the Asia‐Pacific region are susceptible to a range of hazards, these may be better understood and predicted though modeling approaches.

6.1 Knowledge

6.1.1 Inundation and overland flow Storm surges are caused by extreme weather events such as strong wind and low atmospheric pressure (Flather et al., 1998), and large tsunamis which can be generated by events such as submarine earthquakes and landslides (Auffret et al., 1982). It is known that local bathymetry and topography (e.g. Cook and Merwade, 2009), sea‐bottom roughness (e.g. Gelfenbaum et al., 2011), and coastal vegetation coverage (Kaiser et al., 2011) can significantly affect the inundation extent and damage caused by flood/inundation events along a specific coastal area. Coral reefs can reduce the inundation extent and shoreline damage from storm surges and tsunamis (Gelfenbaum et al., 2011; McAdoo et al., 2011); however, large storms and tsunamis may also cause damage to the coral reef itself (e.g. Connell et al., 63 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

1997). Even with the large body of literature concerned with coastal inundation there still remain gaps in our current state of knowledge which include:

 The stability of coastal structures under extreme inundation flows;  The amount and fate of debris that can be generated in extreme inundation flows;  Destruction mechanism of corals and coastal vegetation by large storm waves and tsunamis; and  Inundation flows over complex topographies with varying roughness.

6.1.2 Erosion Knowledge concerning sediment entrainment, transport, and deposition on sandy coasts is well established but lacking in coral reef, carbonate material dominated coasts (see Chapter 4). Thus, additional site‐specific variables, properties and processes must be taken into account when existing approaches are used for application in coral reef associated coasts and islands. Settings and dynamics of coral reef associated coasts and islands are unique, and are often influenced by anthropogenic processes as well. Therefore there is no blanket approach for examining coastal erosion at a local scale. In addition, there are a variety of impacts associated with erosion and accretion. To this extent, gaps in knowledge concerning rates of erosion (and accretion, occasionally) encompass the following issues:

 Erosion rates of materials with varying properties (size, porosity, shape);  Role of local initial conditions;  Added value of higher resolution information;  Influence of uplift and subsidence on erosive (and accretionary) processes.

6.1.3 Saline water intrusion The amount of water contained by a freshwater a lens is determined by the size of the island, the amount of rainfall, rates of water withdrawal, the permeability of the rock beneath the island, and salt mixing due to storm‐ or tide‐induced pressure through processes of inundation (Karl et al., 2009; White et al., 2007). Freshwater includes groundwater, surface water, and rainwater, but because islands are small and surrounded by oceans, these resources are limited. Generally, the smaller the island, the smaller and more vulnerable are its water resources (Underwood et al., 1992).

The landscape on high volcanic islands is conducive to the formation and persistence of freshwater streams and the development of soils that can support large and diverse plant and animal populations. In contrast, the low carbonate islands are small and of low elevation. Due to their low relief these islands do not generate orographic rain, and thus the amount of rainfall on low islands is close to that for the surrounding ocean. The atolls generally lack the freshwater and fertile soils that are characteristic of volcanic islands and they have limited terrestrial resources. Low islands are especially prone to drought, but their varied coral reef and lagoon environments support rich marine ecosystems (Keener, 2012). Because high islands have more land and freshwater resources than low islands do, they have more long‐term options for responding to coastal hazards such as erosion, inundation and

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resulting saltwater intrusion. At only a few feet above sea level, low islands are also more vulnerable to sea‐level rise, wave over‐wash, and saltwater intrusion.

Currently, a growing body of literature focuses on the mechanisms behind the intrusion of saline groundwater lenses. In general this phenomenon is site specific, varies from location to location with few general guiding principles and results in a variety of impacts associated with the intrusion of groundwater lenses. It is acknowledged that in principle, this phenomenon could be predicted if data/models were available.

We have limited understanding however concerning intrusion rates, the influence of local setting, form, geology, recharge rates, abstraction, water table, vegetation ; occurrence rates (probability/return period) ; joint probability of extremes (waves, surge, tides, ENSO); intrusion as a function of the interaction between: waves, surge, tides, salinisation, drought, seasonality, precipitation, recharge rates, abstraction, water table, vegetation; and mean sea level distribution and change as an influence on intrusion. To be more specific, little is known about the impacts of wave overwash and saltwater intrusion on freshwater lenses within atoll islets. Field observations from Pukapuka Atoll in the Northern Cook Islands (Terry, 2010) suggest that freshwater lenses may take one to two years to fully recover from cyclone‐induced saltwater overwash. Post (2010) showed that there is a complex interplay of the vertically intruding saltwater plume with the underlying freshwater layer.

Associated with the saltwater overwash event, is the unstable density stratification that develops when saltwater floats atop fresher (and less dense) groundwater, which may lead to the process of free convection as denser saltwater migrates downward by gravity (Post, 2010). Plumes or fingers of saltwater commonly form under these conditions and result in downward rapid transport of saltwater relative to the diffusion process. The exact pattern of the saltwater fingers can never be accurately predicted in complex natural systems.

Moreover, in areas that were affected by the 2004 Indian Ocean tsunami, the presence of open wells was shown to be a potential pathway for seawater infiltration (Illangasekare, 2006). Local depressions in which seawater remained behind after the flood were also found to play a critical role in explaining the observed increase in groundwater salinities (Violette, 2009). One of the management responses was to try to remove the infiltrated seawater by increased pumping, but adverse effects of this approach were noted in some areas, as the salinities in the wells increased as a result of saltwater upconing (Illangasekare, 2006). Groundwater salinities were expected to recover to freshwater levels within a few years after the flooding occurred, but in some areas, salinities persisted longer than anticipated (Illangasekare, 2009), the reasons for this remain unclear.

While it is to be expected that the insights from these post‐2004 tsunami studies have some relevance to atoll islets, the specific hydrogeology of atoll environments warrants further investigation of the effects of seawater overtopping on fresh groundwater lenses, which will be addressed by the proposed knowledge hub.

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Figure 7: Cross section of regional hydrological processes. Precipitation is the source of both surface water, such as streams, and groundwater on Pacific islands. Variations in precipitation and evapotranspiration rates therefore affect both resources. Surface water is important for human use and provides habitats for fragile ecosystems. Groundwater in islands exists as a freshwater lens underlain by saltwater, and on high volcanic islands it may also exist as high‐level groundwater. Groundwater is a principal source of drinking water on high islands (Modified from Izuka (2011))

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Figure 8: Freshwater resource on low carbonate island. Top: conceptual diagram of an atoll illustrating the main anthropogenic, geological, hydrodynamic, and climate change impacts on the atoll ecosystem (from (Garcin et al., 2011)). Inset on left: idealised cross section through an on an atoll rim showing the geomorphology, lithology and sediments, and typical configuration of the freshwater lens (from (Woodroffe, 2008)). The conglomerate platform is a remnant above modern reef flats due to sealevel highstand conditions that persisted until about 2300 years ago for the Tarawa/Gilbert cluster of islands (Dickinson, 2009).

6.2 Data availability

6.2.1 Inundation and overland flow

On inundation, the availability of high‐resolution bathymetric and topographic elevation and roughness (land use) data is extremely important. However, such high‐resolution data are not available in the public domain in many cases where bathymetric and topographic surveys are usually needed before an inundation study can be performed for a given coastal area. For tsunami inundation studies, one of the

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public data sets available in the public domain is the GEBCO1 (The General Bathymetric Chart of the Oceans), which includes both bathymetric and topographic data. GEBCO uses SRTM302 (Shuttle Radar Topography Mission) gridded digital elevation model and various nautical charts to derive relatively consistent bathymetry data however, the accuracy of topographic data still needs to be carefully checked, especially in the areas where dense coastal vegetation or steep slopes exist. Biases with ground truth data of several meters have been observed (Van Ormondt, pers. Comm.) High‐resolution near‐shore bathymetry is in general lacking altogether, as this area is difficult to remote‐sense cheaply.

It is known that bottom roughness can greatly affect the energy dissipation of inundation flows and thus inundation extent. Unfortunately, the field and laboratory data available are not sufficient for us to parameterize bottom roughness for various land covers and coral reefs. Information on population distribution on small islands is also lacking for assessment of inundation risks in these areas.

6.2.2 Erosion

A range of data sources are important for the understanding and modeling of erosive processes. Table 8 provides an overview of data needs and understanding of availability within the Asia‐Pacific region. As inundation and saline water intrusion are also discussed in this chapter and their examination of may require similar data sets, these two hazards have been included in the summary table.

6.2.3 Saline water intrusion

A range of data sources may be beneficial to the analysis of saltwater intrusion (see Table 8) starting with basic topographic maps. Currently however, even these basic resources may be limited or nonexistent for small islands, classified, or impacted by datum issues. Furthermore issues of scale, currency and accuracy may impact their usefulness.

Aerial photography/imagery provides alternative data sources but may also be classified or unavailable for a location and captured at inappropriate scale/resolution or are out of date. Satellite imagery (5, 4) may provide a solution but the data’s usability is also influenced by scale/resolution (spectral, spatial and temporal) and can prove costly.

Terrestrial habitat, land use and or land cover may prove useful and whilst terrestrial habitat mapping can be conducted with aerial photography/imagery and satellite imagery, limited to no marine habitat maps exist accept for possibly some US islands. Geologic/Geophysics/Geomorphic maps for the coastal

1 http://www.gebco.net/data_and_products/gridded_bathymetry_data/

2 http://www2.jpl.nasa.gov/srtm/

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and near shore environments also provide a useful data source but tend to only be available for larger islands.

Precise and accurate monitoring of saltwater intrusion would require access to monitoring bores, good vertical and horizontal control and information concerning abstraction/recharge rates.

6.3 Measurement/monitoring techniques

6.3.1 Inundation and overland flow Coastal tide stations and the deployment of pressure transducers are methods commonly used to measure storm surges. For tsunamis, expensive DARTs 3 (Deep‐ocean Assessment and Reporting of Tsunamis) have been deployed mainly in the Pacific Ocean to record changes of surface elevation during a tsunami event. However, these methods do not measure the flow velocity associated with tsunami or storm‐surge flows.

With overland flow the main culprit for loss of life and coastal damages, there is no current, real‐time monitoring system to measure this process when it occurs. Second‐hand information on overland flows is usually derived using the following methods: interviews; broken tree branches and water‐line marks for flow depth; debris lines for inundation distances and runups; the barring of objects which have been pushed over for directions.

Even though attempts have been made to make use of amateur videos for estimation of overland flow velocities, for example, the videos taken during the 2011 Tohoku tsunami (Fritz et al., 2012); have resulted in large errors in the estimation. Low‐cost, systems for real‐time measurement of overland flows are in urgent need to better understand overland inundation flows and effectively mitigate tsunami and storm surge hazards.

For tsunami inundation studies, rupture models are needed to specify initial tsunamis as input to wave propagation and inundation models. Even though various methods are currently being used to monitor movement of tectonic plates, it is still difficult, to provide accurate descriptions of a rupture event (ill et al, 2012). Innovative measurement/monitoring techniques are needed to closely monitor active faults that can potentially generate large tsunamis.

6.3.2 Erosion Techniques for the measurement and monitoring are critical in understanding spatial and temporal dynamics of the rate of coastal erosion and accretion. In regard to this issue, a selection of techniques can be underlined to capture the observed phenomenon in one‐, two‐, and three‐dimensions. We refer to Table 8 for an overview.

3 http://nctr.pmel.noaa.gov/Dart/

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6.3.3 Saline water intrusion Geophysical instrumentation to determine the geometry of the freshwater lens and assist in model conceptualization does exist and would include the use of resistivity surveys, conductivity measurements, topography data, bores and possibly further drilling. For an overview of monitoring techniques, see for example Jousma (2008).

6.4 Modeling capability and needs

6.4.1 Inundation and overland flow Several open source codes are available for modeling coastal inundation caused by tsunamis and storm surges including: COMCOT4, MOST5, XBeach6, TUNAMI‐N27,ADCIRC8, DELFT3D9, COULWAVE10, etc. MOST and COMCOT can simulate tsunami generation, propagation and inundation; XBeach can simulate both inundation/flooding and sediment transport. These codes are based on nonlinear shallow water equations, and effects of different land covers and sea bottom roughness are modeled by either Manning’s coefficients or friction factors. The vertical structure of near‐shore features cannot be described well by these models, especially in areas of complex topography/bathymetry. For tsunami inundation, it has been well documented (e.g. Apotsos et al., 2011) that shallow water equations tend to give runups or flow depths smaller than observed in areas of complex topography.

Existing numerical models can provide information on overland‐flow velocity, but further verification using field observational and large‐scale laboratory data still need to be completed; there is a need to establish a shared database populated with field data and large‐scale physical tests for extensive model verification. For accurate prediction of inundation in regions of complex topography/bathymetry, CFD models nested within 2D models are needed for of nuclear power plants.

6.4.2 Erosion

4 http://ceeserver.cee.cornell.edu/pll‐group/comcot.htm

5 http://nctr.pmel.noaa.gov/model.html

6 http://oss.deltares.nl/web/xbeach/

7 Tsunami‐N2 was developed by the Disaster Control Research Center of Tohoku University in Japan.

8 http://adcirc.org/

9 http://oss.deltares.nl/web/delft3d

10 http://isec.nacse.org/models/coulwave_description.php

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To date a variety of modeling tools are available, and can be categorized as analytical or process‐based. Empirical‐based model are output‐oriented generally developed through dense observation without a necessarily need to fully understand all involved processes, while process‐based models are commonly developed through the derivation of basic physics and verified using observations. An example of analytical tool is the Digital Shoreline Analysis System11 (DSAS, Thieler et al. (2009)) which performs two‐ dimensional (i.e. planimetric) analysis based on historical maps and provide estimates of rate of changes. DSAS is an‐open source software linked to commercial GIS software, i.e. ArcGIS. Efforts on development of open‐source versions are in demand in order to facilitate intensified studies on the estimating rate of coastal erosion. This should also include tools to handle three‐dimensional problems based on computations using Digital Elevation Models (DEM) to assist volumetric analysis of rate of changes of erosion or accretion.

XBeach (Roelvink et al., 2009) is an example of numerical tool that is based on physical inter‐relation between parameters involved. Beyond the capability in modeling, there is a need in the development and verification of model results for applicability in the coral reef environments. Quantitative measures from verification and model performance must be made available, also to cover variability due to sediment properties, governing forces (e.g. storm, tsunami), and local settings.

6.4.3 Saline Water Intrusion Commonly used groundwater simulation packages include MODFLOW12, SEAWAT13, and SUTRA14. However their application within the Asia‐Pacific is not well documented. Models used for well abstraction, recharge, evapotranspiration, and surface water bodies are also available and could be used to investigate the effects of extracting varying amounts of groundwater under several different pumping and recharge conditions. Numerical models are increasingly used to simulate the processes in groundwater system. Advances in computing has resulted in improvements to modeling the behaviour of the groundwater system under different scenarios and improved confidence in predicted responses. Modeling of atoll freshwater lenses include 2D models undertaken on Tarawa by Alam and Falkland (1997), and Alam et al. (2002) (also Peterson and Gingerich, 1995). It was found from this work that the freshwater zone would initially increase slightly in thickness and volume with sea level rise, as more of the freshwater lens will be within the upper, lower permeability, Holocene sediments. Aquifer parameters from previous 2D groundwater models can be used to guide the development of the 3D models. A three‐dimensional numerical groundwater model capable of simulating density‐dependent flow and transport is SUTRA (Voss, 2002). SUTRA is a well‐documented and widely accepted model code.

11 http://woodshole.er.usgs.gov/project‐pages/dsas/

12 http://water.usgs.gov/nrp/gwsoftware/modflow.html

13 http://water.usgs.gov/ogw/seawat/

14 http://water.usgs.gov/nrp/gwsoftware/sutra.html

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It is recommended that 3D numerical groundwater modeling be undertaken in tandem with the coastal investigations on inundation and erosion. This concurrent running of activities will lead to improved collaboration and product outputs. The concurrent approach allows for better linkages and synergies between the coastal and groundwater investigations and engagement and use of resources.

An output from the coastal inundation modeling will be spatial and temporal variation of saline water over coastal areas. This informs input parameter or boundary condition to the groundwater model. A 3D numerical model of groundwater, rather than 2D, will be better able to incorporate this information and provide insight on both spatial and temporal impacts on the freshwater lens from overtopping:

 The use of 3D numerical modeling of groundwater will better account for the distribution of abstraction galleries and their impact on the lens as well as the potential impact on water quality at specific galleries from both the inundation and abstraction scenarios.

 Optimisation of the abstraction during "normal" operation or under "recovery" operation from a groundwater reserve using existing or additional galleries with regard to location, timing and volumes will be better represented using a 3D numerical model for groundwater than 2D.

With the benefit of improved software and data sets collected over longer time periods it is expected that a 3D numerical model will be able to provide additional insight into the behaviour of the groundwater system and the impacts of abstraction and current and proposed land use activities as well as the impact of and recovery from an overtopping event. This information may have direct benefits to the management of a freshwater lens and be a catalyst for future discussions on the protection and the sustainability of the lens at a location under a range of scenarios.

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6.5 Summary

Table 8: Summary for coastal hazards

Issue Score Current state Need KNOWLEDGE Inundation and overland Structure stability under extreme flow flow 3 5 Debris production and transport 2 4 Inundation over complex rough topography 3 5

Erosion Erosion rates of materials with varying properties 3 5 Influence of uplift and subsidence on erosive (and accretionary) processes 2 5

Salinity Intrusion Intrusion rates of fresh water 3 5 Joint probability distribution of extreme values of causing factors 3 5 Saline intrusion as complex interaction between hydrodynamics, hydrology, land use and climate ‐ processes and occurrence rate 2 5

DATA AVAILABILITY Historic topographic maps Limited to nonexistent for small islands and may be classified 2 3 Technical issues: Datum, scale, currency, temporal coverage, accuracy Aerial imagery May be classified and has limited coverage (spatial and temporal) 2 4 Technical issues: Scale, resolution, currency Satellite imagery Costly and may subject to poor temporal coverage 4 4 Technical issues: Spatial resolution, spectral resolution (near shore and open ocean) Bathymetry Visible light satellite tech does not work in ‘near shore’ due to environmental complexity, e.g. , waves 1 5 73 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

Issue Score Current state Need Estimation using video data is possible but requires installation of cameras Limited to no LIDAR available Terrestrial habitat mapping Terrestrial can be done with aerial photography/imagery and satellite 1 3 Marine habitat mapping Limited to no marine habitat maps, exception maybe US islands 1 3 Coastal and nearshore Limited to not available geology and geomorphology maps 2 3 May be available for larger islands May be better than topography maps Beach profiles Limited in coverage (temporal and spatial) and accessibility, i.e. transfer of information 1 5 Technical issues: Accuracy, reliability

MEASUREMENT/ MONITORING 1‐D: Coastal profile changes GNSS/GPS system (e.g. in RTK mode) 5 1 Local positioning system, requires geometric control (total station, level) 5 1 2‐D: Shoreline changes Monitoring cameras and video (fixed) 5 3 Unmanned Aerial Vehicle, e.g. aero‐ models (helicopter, airplane), air balloon 5 3 Geotagged photos 4 4 Crowd sourcing

5 3 3‐D: Volumetric changes Inter‐ and supra‐tidal zone:  LIDAR aerial 5 5  Stereo‐pairs aerial photography/imagery 5 2

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Issue Score Current state Need  Autonomous Underwater Vehicle (and other hi‐tech instrument, e.g. wave glider) 1 4 Supra‐ and sub‐tidal zones:  Satellite derived bathymetry

2 5 Boat/jet ski/kayak derived echo sounder 5 3 Aquifer heads and chloride concentrations 5 5

MODELING Inundation and overland Validation of code developments flow 3 5 Tsunami modeling

Improve model outcomes

Erosion (Semi‐)Empirical models 4 5 Process‐based models

Salinity intrusion 3D numerical modeling of groundwater dynamics

3 5

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7 Vulnerability

7.1 Knowledge

7.1.1 Defining vulnerability As defined by the IPCC (2012), “vulnerability refers to the propensity of exposed elements such as human beings, their livelihoods, and assets to suffer adverse effects when impacted by hazard events” (p. 69) and is a function of the “predisposition, susceptibilities, fragilities, weaknesses, deficiencies, or lack of capacities” of the exposed elements (p. 70). Vulnerability to hazards is well documented within the literature and numerous conceptual models and frameworks have been developed to describe its underpinnings and theories (Cutter et al., 2008). Common to all these approaches however, is the notion of vulnerability as a function of socio‐ecological processes, the acknowledgement of the influence of place on vulnerability and use of vulnerability assessment as a pre‐event planning tool.

Despite the numerous approaches to discussing and describing vulnerability, the concept can be grouped into one of two general perspectives: the global environmental change or climate change adaptation perspective where vulnerability is viewed as the susceptibility, fragility or sensitivity of a system; and the hazards perspective where vulnerability is viewed as a lack of resilience or ability to cope with an impact (IPCC, 2012). Whilst the former is often applied to both human and environmental systems, the later takes a more human‐centric approach where vulnerability and resilience are a linked concept.

Regardless of the approach to measuring vulnerability, most assessments incorporate one or more permutations of the following dimensions: exposure, sensitivity and adaptive capacity (Adger, 2006). Exposure is generally defined as the degree of stress experienced by a system (human or environmental) and is influenced by the magnitude, duration and frequency of the stressor (Burton et al., 1993; Cutter, 1996). Sensitivity describes the extent to which a system may be impacted by a stressor and adaptive capacity allows for a system to evolve overtime becoming more or less vulnerable to future impacts (Adger, 2006).

7.1.2 Evolution of Vulnerability Human activity has altered the types of stressors acting upon coral reef, in turn influencing a reefs capacity to cope with disturbances (Nystrom et al., 2000). For example, a disturbance that previously triggered the renewal and development of a reef might, under certain circumstances, become an obstacle to development. Furthermore, the implications of changes enacted by reef‐associated human activities, such as fishing and tourism, can be substantial (Nystrom et al., 2000) and may result in increased vulnerabilities when the livelihoods of reef dependent communities are altered (Cinner et al., 2012).

To this end, anthropogenic influence can affect the health of coral habitats rendering them more susceptible to the impact of hazard events and or chronic stressors. At the same time, coral reef associated communities may be more sensitive to the impact of an event or lack the ability to cope with 77 White Paper on Vulnerability of Coral Reef Protected Coastlines in a Changing Environment

coastal hazards in particular, when human well‐being is influenced by a reefs ability to provide important ecosystem services (Smith et al., 2013)

Natural and anthropogenic impacts on coral reef systems that give rise to their vulnerabilities can be categorized temporal as short, medium and long term threats. Short term threats are driven by , destructive fishing practices and . Until recently, the direct and indirect effects of overfishing and pollution from agriculture and land development have been the major drivers of massive and accelerating decreases in the abundance of coral reef species, causing widespread changes in reef ecosystems. In the medium term, reef degradation and loss through sedimentation has also been observed whilst longer term processes, influenced by climate change, have manifested as events eliciting a range of impacts on the system itself that can result in coral mortality(Brown, 1997a).

7.2 Data availability An extensive body of literature discusses appropriate factors, indicators and approaches for measuring vulnerability in social‐ecological systems (Adger, 2006; Birkmann, 2006, 2007; Blaikie, 1994; Boruff et al., 2005; Cardona, 2006; Cutter et al., 2003) In coastal systems, for example, Dolan and Walker (2006) identified that at the community level, assessments of coastal vulnerability to environmental change should include measures of wealth, access to technology, skills availabilities, risk perception, and social capital. At the national level, economic robustness, health, education, infrastructure, governance, population distributions, agriculture dependence, ecological stress and access to technology (Brooks et al., 2005) should be measured. It should be noted however, that the above are but a few examples of indicators used to measure coastal vulnerability providing a solid foundation for future assessments in the region. The ability to measure vulnerability appropriately however, is subject to, among others, data availability (Dolan and Walker, 2006). For social system assessments, census data provides an important starting point for measuring the demographic characteristics of populations, and many nations within the Asia‐Pacific region collect standard statistical measures. However, currency, scale, and geographic and temporal transferability are always a concern. Furthermore, household specific livelihood and preparedness information may not be collected by a census requiring collection through labour intensive household surveys (see Cinner et al., 2013 for an example).

In terms of anthropogenic influences on reef habitats, development along the coast, of fisheries, destructive fishing practices, terrestrial pollution and erosion, and are all measurable drivers (Bryant et al., 1998; Hughes et al., 2007). Population density data and land use/land cover information as well as the location of ports and other point source polluters can be used as surrogates for examining the level of human induced threats on reef systems. Aerial photography/imagery and satellite imagery can be used to map the distribution of land use/land cover types however; the ability to map stressors and their impacts is influenced by resolution and or scale of the input data as well as the degree to which proxy measures represent real world social‐ecological interactions.

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The population density of communities associated with reef lined coasts can be determined using products such as the Gridded Population15 of the World or Landscan16 however; these products are coarse in resolution and are not appropriate for small islands. It is possible to use census data or census data in conjunction with aerial or satellite derived imagery for higher resolution population density measures using dasymetric mapping techniques however, these processes are heavily reliant on the availability of appropriate data.

Measuring the actual impact of anthropogenic influences on a specific reef system requires in situ data collection and may include measures of species diversity, abundance and biomass, and percent cover of various benthic constituents (Graham et al. 2006; Hughes et al., 2007). Two unusually detailed, long‐ term data sets from Island, Australia, and Jamaica demonstrate some of the complexities of measuring multiple stressors and their impacts over an extended period of time (Maina et al., 2008). Whilst these types of data do presumably exist for locations within the region, identifying and negotiating access is labour intensive. In addition, the systematic nature of data collection, data comparability, and the temporal and spatial extent of measures are a concern.

In terms of reef vulnerability to environmental change, a number of climate related stressors have been linked to reef degradation (such as coral bleaching) and include: increased/decreased sea temperatures, solar radiation, reduced salinity and infection (Brown, 1997b). More specifically, models of coral reef susceptibility to environmental stress have measured changes in sea surface temperatures (SSTs), chlorophyll concentration, photosynthetically active radiation (PAR), surface currents, wind speeds and UV irradiance (Maina et al., 2008).

Sea surface temperature data17 is available globally from the National Oceanic and Atmospheric Administration (NOAA) primarily derived from the Advanced Very High Resolution Radiometer (AVHRR) carried on board NOAA's POES satellites (operating since 1978). MODIS ocean products are available twice weekly at a resolution of approximately 4km by 4km. In addition, the International Comprehensive Ocean‐Atmosphere Data Set (ICOADS) offers extended reconstructed sea surface temperature (ERSST) data from 1800 onward derived from a variety of in situ data sources. Other nations such as the Australian Bureau of meteorology provide their own SST products contributing to the range of available data sets.

Chlorophyll concentration and PAR data are available through the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) project18 derived using the SeaWiFS sensor carried onboard the SeaStar satellite. Daily and fortnightly measures are available for each at a 9km by 9km resolution however, restrictions impact

15 http://sedac.ciesin.columbia.edu/data/collection/gpw‐v3

16 http://www.ornl.gov/sci/landscan/

17 http://www.ospo.noaa.gov/Products/ocean/index.html

18 http://oceancolor.gsfc.nasa.gov/SeaWiFS/BACKGROUND/

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access to these products. An alternative for chlorophyll concentration19 information are the products produced from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Aqua (EOS PM) satellite. Global daily and bi‐monthly data are available with a resolution of 1km by 1km.

The OSCAR (Ocean Surface Current Analysis Real‐time) provides global current estimates20 derived from a variety of sensors with a 0.33 by 0.33 degree resolution every 5 days. The Special Sensor Microwave Imager (SSM/I) and the Special Sensor Microwave Imager Sounder (SSMIS) onboard the Defense Meteorological Satellite Program (DMSP) satellites provide measures of Surface Wind Speed21. Daily, 3 day, weekly and monthly averages are available globally with an o.25 by 0.25 degree resolution. A variety of in situ surface wind observations are available from the network of weather stations throughout the region although access, consistency, reliability and appropriateness of location may influence the usefulness of these data sets.

7.3 Measurement/Monitoring Techniques As Downing et al. (2000) argue vulnerability is not necessarily an ‘observable phenomenon,’ which adds complexity to its measure. Theoretical frameworks and definitions of vulnerability abound, influencing and further complicating the selection of criteria used to quantify vulnerability. However, without a set of measurable criteria, it is unfeasible to systematically compare the vulnerability of location to another (Luers et al., 2003).

Perhaps the most common method of quantifying vulnerability in the global change community is by using a set or composite of proxy indicators (e.g. Kaly et al., 2002; Moss et al., 2002). As Luers et al. (2003) identified “while the indicator approach is valuable for monitoring trends and exploring conceptual frameworks, indices are limited in their application by considerable subjectivity in the selection of variables and their relative , by the availability of data at various scales, and by the difficulty of testing or validating the different metrics” (p. 257). Furthermore, questions arise concerning the extent to which social‐ecological systems can really be explained using a set of indicators or proxy measures.

It has been suggested however, that in social and ecological systems, measurement of vulnerability is possible at two levels (Adger, 1999): individual and collective. In a social system, for example, high levels of vulnerability are observed when livelihoods are impacted from stressors threatening a narrow set of resources which are depended on for economic and social well being. In an ecological system, reduction in biomass of a specific fish species may result in increased algal growth reducing recruitment and increasing mortality of established corals (Hughes et al., 2007).

19 http://www.ospo.noaa.gov/Products/ocean/index.html

20 http://www.oscar.noaa.gov/index.html

21 http://www.remss.com/missions/ssmi

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At the collective level, social vulnerability is determined by the relative distribution of income; access to and diversity of economic assets; and by the operation of extrinsic policies, institutions and processes (Allison and Ellis, 2001). Ecological vulnerability in corals, on the other hand, is influenced by variability in temperature, light absorption abilities, and associations with wind and wave energy to name a few (West and Salm, 2003). What becomes evident however is the complexity involved in measuring the vulnerability of any social‐ecological system, and as the previous examples highlight, factors influencing vulnerability occur at various scales and change through time.

In the context of coral reefs and associated human populations, a holistic measure of vulnerability should address the vulnerability of reefs to environmental change as well as anthropogenic threats. Furthermore, a comprehensive assessment would include measures of social vulnerability which encapsulate the influence of reef ecosystem degradation on livelihoods and well‐being. Current theoretical frameworks (see Turner et al., 2003) provide a foundation for understanding vulnerability in a single system, but a comprehensive understanding of the complexity of, and interactions between factors that give rise to vulnerabilities within and between social‐ecological systems warrant further inquiry. This would also shed more light on the contested nature of vulnerability and resilience in small island developing states (Pelling and Uitto, 2001; Scheyvens and Momsen, 2008) which are highly dependent upon coral reef systems and are positioned at the forefront of the impacts of global climate change.

7.4 Modeling Capability and Needs As stated previously, the indicator/index approach for assessing vulnerability dominates the literature. Numerous examples of social vulnerability/resilience modeling exist (Cutter et al., 2008; Cutter et al., 2003) as well as those that model the interaction between social and ecological systems in the coastal zone (Boruff et al., 2005; Cinner et al., 2013; Frazier et al., 2010). However, these approaches, especially those concerned with measuring social‐ecological systems are either broad in nature (Boruff et al., 2005) or focus on a specific hazard or single stressor (Cinner et al., 2013; Frazier et al., 2010) and do not capture the complexity of the social ecological system pertaining to coral reefs.

An understanding of the environmental and anthropogenic influences on reef vulnerability exists however, as Munday (2004) highlighted, the actual prediction or modeling of these impacts is a difficult task. In terms of anthropogenic threats to reefs, Bryant et al. (1998) identified a range of stressors that lead to the degradation of reef habitats however predicting when, where and to what extent these impacts will occur requires further investigation.

Corals vulnerable to changing physical conditions such as SLR, warming and acidification, may result in possible alteration of reef ecosystems and again forecasting when and where these shifts will occur is problematic (Hoegh‐Guldberg et al., 2007). For example, McCulloch et al. (2012) demonstrate that scleractinian corals can buffer declining pH to counteract effects of ocean acidification. Not all corals can do this; hence vulnerability is likely to reflect species composition as well as external physical conditions (place vulnerability).

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Whilst there appears to be a cursory understanding of the impact of climate change on reef systems what is less understood are the long‐term impacts on reef fisheries (Graham et al., 2008; Munday et al., 2008). Further complicating this process are anthropogenic threats to fisheries such as overfishing and destructive fishing practices. The impact on reef fishers by environmental factors, anthropogenic stressors or a combination of the two also requires further attention.

Predicting/modeling the vulnerability of the coupled social‐ecological system as it pertains to coral reefs should be a priority if a comparison of vulnerability between locations is sought. Such a model should allow for a measure of vulnerability which accounts for both anthropogenic and environmental stressors on coral reefs and coral reef fisheries. Adding to the complexity of such a model is the ability to capture feedback within and between systems providing for an understanding of how environmental degradation influences the livelihoods of those supported by coral reefs. However, this is difficult given the limited advancement in modeling vulnerabilities of single reef system components.

Whilst challenges exist, this should not dissuade our attention from further understanding vulnerability in this fragile environment. Development of a complex, integrated model will require a multidisciplinary effort from experts in a range of physical and social sciences. This document provides the foundation from which this effort can move forward and is a call for experts to work together to reduce vulnerability and influence the well‐being of humans who rely on coral reef systems for their livelihoods.

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7.5 Summary

Table 9: Summary for coastal vulnarability

Issue Score Current Need State KNOWLEDGE Vulnerability Appropriateness of current methods 3 3 assessment in Asia‐Pacific region

Hazard (stressor) Contribution to vulnerability 1 5 modeling reduction

MEASUREMENT TECHNIQUES Theoretical Framework Describe the complexity social‐ 1 5 ecological coral reef system

Vulnerability Indicators Identification of appropriate 3 4 measures/metrics

DATA AVAILABILITY Social Vulnerability Spatial and temporal resolution, 4 3 comparability, and appropriateness

Ecological Vulnerability Richness, comparability, 1 5 (anthropogenic) accessibility, and appropriateness

Ecological Vulnerability Spatial and temporal resolution, 4 2 (environmental) accessibility

MODELING CAPABILITY Social Vulnerability Appropriateness 4 5 Modeling

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Issue Score Current Need State Ecological Vulnerability Limited understanding 1 5 Modeling (anthropogenic) Ecological Vulnerability Limited understanding 2 5 Modeling (environmental) Coupled System Model Nonexistent 0 5

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