8th International Coastal Symposium (Instructions for ...

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Harris and Vila-Concejo

Wave transformation on a coral reef rubble platform. Daniel L. Harris and Ana Vila-Concejo Geocoastal Research Group, School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia [email protected] [email protected]

www.cerf-jcr.org

ABSTRACT Harris, D.L. and Vila-Concejo, A., 2013. Wave transformation on a coral reef rubble platform In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 506-510, ISSN 0749-0208. www.JCRonline.org

Wave transformation across coral reef platforms is the primary process affecting changes in coral reef geomorphology. Transformation regulates the amount of wave energy entering reef systems, however there have been relatively few hydrodynamic assessments conducted on coral reefs when compared to siliciclastic environments with the effects of common geomorphic features like rubble platforms on wave transformation never specifically examined. This study focuses on the changes in wave characteristics across a rubble platform in a high energy environment (One Tree Reef, southern Great Barrier Reef). Wave conditions were measured at five locations over two days along a cross-reef transect from the reef rim to lagoon. Most of the wave energy was dissipated during wave breaking with energy attenuation due to bottom friction a secondary process. Wave energy attenuation was between 60-99% of the offshore wave conditions only during high tide would wave propagation across the reef platform be capable of affecting reef geomorphology. The wave spectrum also changed with the shorter period gravity wave energy (3 – 20 s) almost completely expending during transformation while longer period infragravity waves (20 – 300 s) were capable of propagating across the reef platform. Wave heights were depth limited and primarily controlled by water depth which suggests that water depth over the reef platform and subsequently elevation of the reef platform above mean sea level govern the amount of wave energy transferred across into reef systems, with most of the gravity wave energy removed during propagation over coral rubble platforms. ADDITIONAL INDEX WORDS: One Tree Reef, coral reef hydrodynamics, coral reef morphodynamics

INTRODUCTION The evolution of coral reef sediment formations is governed by the hydrodynamic energy, sediment flux and sea-level variation (Kench and Brander, 2006). Change to each of these boundary conditions affects the morphodynamic evolution of coral reef systems (Cowell and Kench, 2001; Cowell and Thom, 1994; Kench and Brander, 2006; Woodroffe, 2008). The response of reef systems to such changes is poorly understood due to the limited geomorphic research conducted on coral reefs when compared to siliciclastic environments such as beaches and estuaries. Swell wave transformation across reef platforms is a crucial factor when examining geomorphic change in reef environments. Studies examining wave transformation on coral reefs have been conducted on reef platforms with many different geomorphologies such as; confined solely to reef flat (Gourlay, 1993; Gourlay, 1994; Gourlay, 1996; Hardy and Young, 1996), extended reef flats with sediment deposits and shallow or no lagoon (Brander et al., 2004; Kench and Brander, 2006) and reef flat and back-reef sand apron (Kench and Brander, 2006). None have examined wave transformation across wide coral reef rubble platforms. These studies found similar values for wave energy attenuation with up to 99% of wave energy lost due to breaking and propagation across the reef platform. Water depth (h) is the main control that affects the amount of wave energy transferred across reef ____________________ DOI: 10.2112/SI65-086.1 received 07 December 2012; accepted 06 March 2013. © Coastal Education & Research Foundation 2013

platforms. However, some studies have found that attenuation due to bottom friction can be the dominant force in dissipating wave energy (Lowe et al., 2005). Many authors have noted that the amount of energy capable of propagating into the lagoonal or back-reef environment is too small to have any significant effect on reef geomorphology (e.g. Ball et al., 1967; Bayliss-Smith, 1988; Brander et al., 2004; Frith and Mason, 1986; Macintyre et al., 1987; Maragos et al., 1973; Scoffin, 1993). As a result coral reefs are often considered event based systems that only undergo geomorphic change during low frequency, high intensity events (such as storms or cyclones). However, other authors have found that sediment transport and changes to reef geomorphology can occur even when the majority of the wave energy is dissipated during transformation (Harris et al., 2011; Kench, 1998; Kench and Brander, 2006). Rubble platforms are crucial to this debate since a significant amount of energy is required to move rubble (which can have individual weights greater than 20 kg) (Thornborough and Davies, 2011). However, distinct patterns of sorting have also been observed which may suggest regular movement is not event based. Hence, the specific aims of this study are; 1) to examine the wave conditions on coral reef rubble platforms and, 2) to determine the controlling factors governing wave energy transformation across coral reef rubble platforms.

FIELD SITE One Tree Reef is a lagoonal reef located in the southern Great Barrier Reef (Figure 1). The dominant wind and swell regime is

Journal of Coastal Research, Special Issue No. 65, 2013

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Figure 1. a) The location One Tree Reef shown by the red box in the southern Great Barrier Reef, Queensland (QLD), Australia b) Satellite image of One Tree Reef with the red box showing the cross-reef transect where measurements were taken and c) The five locations where pressure transducers were deployed (P1-5) shown in black with the survey points used construct a cross reef profile in red. from the southeast with an average offshore significant wave height (Hso) of 1.15 m for the region. Tides on the reef are mesotidal and semi-diurnal with a maximum spring tide range greater than 3 metres. At low tide the reef platforms are elevated above the mean water level resulting in ‘ponding’ of the reef lagoon. During this time the water level in the lagoon remains at approximately 1.4 m above the lowest astronomical tide (LAT) (Ludington, 1979). This leads to a significant portion of each tide cycle where wave and tide processes have no effect on the lagoon and back-reef environment. The windward reef margin has a steep reef front with deep water (≈ 60 m) surrounding the reef leading to very little attenuation of deep water wave energy from the south and southeast before breaking on the reef rim (Davies et al., 1976). Under modal conditions the eastern reef platform (Figure 1) receives the highest wave energy on the reef with wave refraction attenuating some the wave energy before reaching the other platforms (Davies et al., 1976). The eastern reef platform transitions from an algal flat near the reef rim to rubble dominated flat that has three main zones distinguished with clasts of different shape and weight getting progressively smaller in the direction of the lagoon. These zones are boulder (roughly round massive

corals) and plate rubble weighing between 10 – 20 kg dominate the region closest to the algal rim (outer flat), boulder and plate rubble weighing between 5 – 10 kg in the mid-flat and branching and stick rubble closest to the lagoon (inner-flat) (Thornborough and Davies, 2011).

METHODS Waves were measured during two days (6-7 April 2012) using Aquistar PT2X pressure transducers (PTs) which logged continuously at 8 Hz. The PTs were deployed as a cross-reef transect at five locations on the eastern reef platform of One Tree Reef (Figure 1). Five locations were on the reef platform with one in the lagoon which was submerged during the entire duration of deployment. The pressure record was low-pass filtered to remove instrument noise and high-pass filtered to remove waves in the infragravity spectrum (> 0.05 Hz). Zero downcrossing analysis was then performed with wave statistics, including significant wave height (Hs) and mean wave period (Tz), assessed for each 15 minute run. Wave height attenuation (At) was determined by calculating At = 100(Hs/Hso). Wave spectrums were produced for each 15 minute run using the Welch method of computing power


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shallower and much of the energy in the gravity wave spectrum is attenuated with infragravity energy becoming the dominant process (Figure 4).

DISCUSSION Wave Transformation and Attenuation

Figure 2. (a) Changes in mean significant wave height and, (b) attenuation in relation to the cross-reef geomorphology and the inner, mid and outer reef flat (designated by the vertical lines). Points are mean Hs and mean attenuation in (a) and (b) respectively, with the platform geomorphology in LAT shown as a black line. spectral densities. The change in wave spectrum density was examined specfically in the frequencies for gravity (0.05-0.33 Hz) and infragravity (0.05-0.0033 Hz). The location of the PTs was surveyed using a Trimble R8 Real-Time Kinematic Global Navigation Satellite System (RTK-GNSS). A profile of the reef platform was also surveyed using the RTK-GNSS. Offshore significant wave height (Hso) and offshore wave direction were obtained from the NOAA Wave Watch III global wave model for the duration of the deployment. During the study period One Tree reef received normal fair weather summer conditions with Hso ≈ 1.9 m and a direction (Diro) of 125˚.

Wave height on the reef platform was small despite some deployment locations near the reef rim being close to the initial point of breaking, and the offshore wave conditions being moderately energetic throughout the measurements (Hso ≈ 1.9 m). The largest significant wave height was 0.55 m near the reef rim with an average of 0.2 m for the same location. This suggests that there has already been significant attenuation of wave energy within in the first 50 m of wave propagation across the reef platform with most of the energy in the gravity wave spectrum expended in the first 250 - 300 m over the reef platform (Figure 2). Mean attenuation values near the reef rim show that approximately 85% of the wave height has been lost within the first 50 m from the reef rim which indicates that on average only 15% of the wave height propagates onto the reef platform (Figure 2). The majority of wave energy is expended during wave breaking with attenuation due to bottom friction secondary to this process. Once waves are broken the rough coral rubble surface is successful in attenuating almost all of the wave height by the time it reaches the lagoon. Despite the relatively high energy deep water conditions of One Tree Reef (conditions during this study were more energetic than average) the reef platform is relatively calm (mean Hs ≈ 0.12) which appear unlikely to transport large rubble clasts, particularly on the inner reef platform (P5, Figure 2 and 3). Only during high tide when waves can more easily propagate across the reef platform were conditions potentially energetic enough to transport rubble. During this time Hs on the outer reef platform (P1 – 2) ≈ 0.5 m. However, waves were small even during high tide on the inner reef platform (P4-5) ≈ 0.1 m (Figure 3a). This suggests that both larger swell and higher tides would be required to transport rubble in the inner platform.

RESULTS 0.8 P1 P2 P3 P4 P5

Hs (m)

0.6 0.4 0.2

a) 0

Hs (m)

The topography of the reef platform shows an increase in elevation for the first 320 m from the reef rim (between P3 and P4) at a gentle slope with a gradient of 0.007. After that, there is a semi-flat section (the inner flat) where the elevation gently decreases until 475 m. After that there is an abrupt drop into the lagoon. Mean significant wave heights on the reef platform were small (maximum Hs = 0.55 m at PT1 near the reef rim) with wave height getting progressively smaller towards the lagoon (Figure 2 and 3a). Attenuation of deepwater wave height (Hso) was between 65 – 100% with the majority of the wave energy expended during propagation over the first 250 – 300 m of the reef platform (P1 – 4, Figure 2). Wave heights on the reef platform are primarily controlled by depth (h, Figure 2). Figure 3 shows how a linear trend can be fitted to represent how Hs increased as h increased (Figure 2 and 3). The higher reef elevations incurred less wave energy compared to those in lower elevations (Figure 2 and 3). When PTs are in shallow water (h < 0.3) the wave spectrum shifts from gravity dominated spectra, in those locations close to the ocean, and towards infragravity dominated spectra in the areas that are shallower towards the lagoon (Figure 4). This results in peaks in wave period as each location is initially submerged and also during the final stages of water draining off the platform (Figure 5). During propagation over the reef platform depth becomes

1

1.5

2 2.5 Mean Water Level LAT (m)

3

3.5

0.4 0.2

Hs = 0.27h R2 = 0.98

0

0

0.5

1 Depth (m)

b)

1.5

Figure 3. a) Significant wave height on the reef relation to mean water level (LAT) showing the inundation and subsequent increase in wave height deployment locations as the tide rise and b) the between Hs and depth (h).


2

platform in progressive of the five relationship

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Wave Spectrum Transformation P1 P2 P3 P4 P5

Spectral Density (m2s)

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Frequency (Hz)

Figure 4. The wave spectrum at P1-5 when the mean water level was 2.9 m LAT. Peaks in the gravity wave spectrum at P1-3 (around 0.08 Hz) are completely removed once waves have propagated to P4-5.

Wave height vs depth on rubble platforms Once the platform was submerged wave heights increased linearly as depth increased (Figure 3b). Wave height was depth limited with Hs dependent on h (Figure 3b). Significant wave height at all locations on the reef platform could be predicted based solely on depth using Equation 1: Hs = γh

(1)

where γ = 0.27 (R2 = 0.98, n = 342, Figure 3b). This shows that the increase in wave height with depth, as the tide rose, was similar at different locations on the reef platform. It also suggests that reef elevation has a significant effect on the amount of wave energy transferred across reef platforms, with wave height being smaller in those areas with the highest elevation (Figure 2). This is not necessarily found on all reef platforms with Hardy and Young (1996) observing variable values of γ depending on the location on the platform (γ = 0.38 – 0.7) , with γ decreasing as distance from the reef rim increased. Similarly, Brander et al. (2004) observed different values of γ depending on the location on the reef however, this study covered a wider range of geomorphology (including sand flats) extending well into the back-reef lagoon. The similar values of γ at differing locations in this study may be due to the relatively simple geomorphology of the rubble platform with a gentle slope and little to no complex wave refraction in this location of the reef.

Changes in the distribution of wave energy in the wave spectrum occurred during propagation over the reef platform with wave energy dominance shifting to lower frequency and longer periods (Figure 5). Wave energy decreased towards the lagoon with most of the wave energy attenuation occurring in the gravity wave spectrum (3 – 20 s, Figure 5). Wave spectrums near the lagoon were dominated by long period waves with the peak in wave period occurring in the infragravity spectrum (20 – 300 s, Figure 5). Energy in the infragravity spectrum did not specifically increase towards the lagoon but remained relatively consistent at all locations on the platform (Figure 5). The influence of infragravity energy is also greater when h < 0.3 which resulted in peaks in the wave period (Figure 4). Due to their long period and wavelength it is likely that infragravity waves transferred over the shallow environment of reef platform without much attenuation while gravity waves expended most of their energy during breaking and propagation over the reef platform. It is unclear what effect infragravity energy may have on the geomorphology of coral reef environments, however it may be an important force since it is capable of propagating over reef platforms with little attenuation while most of the energy in the gravity wave spectrum is lost. Further work is required to investigate the effect of infragravity waves on the morphodynamics of coral reef platforms, sediment entrainment at coral reef platforms depends on water depth (Harris et al., 2013; Vila-Concejo et al., 2013); therefore infragravity energy could have a modulating effect on sediment entrainment and transport.

CONCLUDING REMARKS Wave conditions on the reef platform were mostly low energy despite the moderately energetic offshore wave conditions. In general wave energy was severely attenuated throughout most of the tidal cycle. Only during high tide were waves capable of propagating across the reef platform with enough energy to potentially affect the reef geomorphology. Most wave attenuation

3.5 3

Depth LAT (m)

0.8

2.5 2 1.5 1 18:00

00:00

06:00

12:00

18:00

00:00

06:00

12:00

30 P1 P2 P3 P4 P5

25

Tz (s)

This result γ = 0.27 is also smaller than other studies on platforms, with maximum γ of 0.55 observed by Nelson (1994) and Gourlay (1993; 1994) and 0.38 to 0.7 by Hardy and Young (1996). The value for γ found in Nelson and Gourlay relates to horizontal topographies in contrast to those with gentle slopes, like in this study, resulting in even higher values of γ up 0.8 (Nelson, 1987). This suggests that the wave heights were smaller at similar depths when compared to that of previous studies. However, Kench and Brander (2006) and Brander (2004) found similar or smaller values for γ. This may indicate that values of γ differ on a reef-byreef basis with no one value of γ offering uniform applicability. A greater spatial distribution of measurements (i.e. along-reef as well as cross-reef) and different tide and offshore wave conditions may produce different results for γ. However, there have been no other studies specifically investigating wave transformations on coral reef rubble platforms, with additional studies potentially finding similar results for γ in such environments.

20 15 10 5 18:00

00:00

06:00

12:00

18:00

00:00

06:00

12:00

Time

Figure 5. a) Water depth (LAT) at locations P1-5 showing truncated tidal cycles and b) the wave period (Tz) at P1-5 showing distinct peaks when during shallow water periods at each location.


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occurred during breaking and within 50 m from the reef rim with wave attenuation due to bottom friction a secondary process. Once the platform was submerged the significant wave height could be predicting based on the ratio between wave height and water depth (γ). While it is likely that values for γ would change if measurements were conducted under different conditions with greater spatial variation, the strong correlation between Hs and h could be used to predict wave height under modal conditions where Hs = 0.27h. These results indicate that reef geomorphology and the elevation of the reef platform is just as important as incident wave energy in governing the amount of energy transferred across coral reef platforms.

ACKNOWLEDGEMENTS Fieldwork was carried out in One Tree Island Research Station, a facility of The University of Sydney, assistance by the station managers, Peter Dalton and Sara Naylor, was greatly appreciated. Harris is funded by a Australian Institute of Nuclear Science and Engineering (AINSE) Postgraduate award and an Australian Postgraduate Award. Vila-Concejo is funding by a Future Fellowship award from the Australian Research Council (FT100100215) and by the return to work grant of the programme Women in Science of the Faculty of Science of the University of Sydney. Thanks to the students who helped collect data in the field, specifically Nicky Wright, Sally Watson, Alice Wilson, Phil Deakin and Dani Skews.

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