Swell impact on reef sedimentary processes: A case study of the La Reunion fringing reef

Abstract

Two surface‐sediment sampling campaigns were carried out in November and December 2003, before and after a strong swell event, in the back‐reef area of a microtidal fringing reef on the western coast of La Reunion, Indian Ocean. The spatial distributions of the mean grain size, sorting and skewness parameters are determined, and grain‐size trend analysis is performed to estimate the main sediment transport pathways in the reef. The results of this analysis are compared with hydrodynamic records obtained in the same reef area during fair weather conditions and during swell events. Sediment dynamics inferred from the hydrodynamic records show that significant sediment erosion and transport occur only during swell events and under strongly agitated sea states. Under normal wave conditions, there is a potential for onshore sediment transport from the reef‐flat to the back‐reef, but this transport is episodic and occurs principally during high‐tide stages. Sediment transport trends revealed by the grain‐size trend analysis method show onshore and alongshore low‐energy transport processes that are in agreement with the hydrodynamic records. The grain‐size trend analysis method also provides evidence of an offshore high‐energy transport trend that could be interpreted as a real physical process associated with return flow from the shore to the reef. The impact of swell on the reef sediment dynamics is clearly demonstrated by onshore and alongshore transport. Considering different combinations of the vector transport trends computed through the grain‐size trend analysis approach, more realistic and pertinent results can be obtained by applying an exclusive OR operation (XOR case) on the vectors. The main results presented here highlight a trend towards the accumulation of carbonate sands in the back‐reef area of the fringing reef. These sediments can only be resuspended during extreme events such as storms or tropical cyclones.

Modern fringing reef carbonates from equatorial SE Asia: An integrated environmental, sediment and satellite characterisation study

“…The degree of homogenisation of sediment characteristics across the different fieldand satellite-identifiable environmental facies with evidence for both seaward and landward transportation of grains is again attributed to these same four factors listed directly above (cf. Cordier et al, 2012). Similar features to those described here and a predominance of grain-rudstone sediments across reef flat, crest and slope deposits are also seen regionally in the high-energy, monsooninfluenced carbonate systems of Kepulauan Seribu, offshore Jakarta.…”

Spatial Variability of Sediment Transport Processes Over Intratidal and Subtidal Timescales Within a Fringing Coral Reef System

“…The sediment on the reef flat is coarser than the sediment in the lagoon (Figure a). Such trends in median grain size have also been commonly observed across a number of different reef environments (e.g., Cordier et al, ; Harris et al, ; Morgan & Kench, ). The absence of finer‐grained sediment suggests that at this site, sediment is continually transported off the reef flat into the lagoon, with some sediment likely to exit via the channels; the suspension of sediment of a suitable grain size is source limited, rather than limited by shear stress (the shear stresses are sufficient to suspend the sediment).…”

Unravelling the influence of water depth and wave energy on the facies diversity of shelf carbonates

“…However, as recognized by Immenhauser () and Peters & Loss (), and as evident in the wave height models, the spectrum of wave patterns is intriguingly complex across the varied depositional topography that typically develops atop carbonate platforms and, therefore, there is no such thing as an average fair weather or storm weather wave base that can be applied platform‐wide, although storm events probably remain relevant (e.g. Cordier et al ., ; Madden et al ., ). According to the calculations of Clifton & Dingler (), the 3 m H m0 calculated for the exposed ocean‐facing reef rims of Ras Al‐Qasabah and Al Wajh is sufficient to impart orbital velocities of 1·0 m sec −1 down to water depths of ca 15 m. Behind the reef rim, where H m0 is the range of 1 m, this level of water movement remains at depths of ca 5 m. It should be noted that H m0 describes the mean of the 33% highest waves and so wave sweeping during rare storm events is likely to extend considerably deeper.…”

“…While the long‐term winnowing effects of tides can be a systematic and large force in steering facies organization (Immenhauser, ), spring and neap tidal ranges are only 0·5 m and 0·05 m, respectively, in the northern Red Sea (Sheppard et al ., ; Sultan et al ., ). In such a microtidal regime, sediment redistribution by swell is likely to dominate that occurring due to tides (Cordier et al ., ), and strong tidal currents are unlikely to develop beyond those restricted to the narrow reef passes that connect the lagoons of both Ras Al‐Qasabah and Al Wajh to the open ocean (as captured in the wave height model for Al Wajh, Fig. B) and these are of limited scale as compared to the extent of the overall depositional system.…”