Rock topography causes spatial variation in the wave, current and beach response to sea breeze activity

Abstract

We hypothesized that beach profiles that are perched on natural rock structures would be better protected from waves and currents than profiles that are not fronted by rock. In southwest Western Australia many beaches, such as at Yanchep, are perched on Quaternary limestone. Yanchep Lagoon is fronted by a low-crested limestone reef that partially encloses a coastal lagoon. The spatial variation of waves and currents around the rock structures were quantified during the sea breeze cycle at locations: (1) offshore; (2) 20 m seaward of the reef; (3) inside the lagoon; and (4) in the surf zone. The spatial variation in the beach profile response was measured at two beach profiles: (1) the Exposed Profile that was not fronted directly seaward by outcropping limestone; and (2) the Sheltered Profile which was fronted seaward by submerged limestone at 2 m water depth and that was near the lagoon exit at the end of the limestone reef. The Sheltered Profile had greater volume changes during the cycle of the sea breeze whilst the Exposed Profile recovered more by overnight accretion when wind decreased. The lagoonal current drove the strong response of the Sheltered Profile and may have contributed to the lack of overnight recovery of the beach together with the seaward rock formation impeding onshore sediment transport. The different direction and speed responses of bottom-currents in the surf zone fronting the two profiles reflected the local variation in geology, the influence of the jet exiting the lagoon, and wave refraction around the reef that was measured with GPS drifters and wave-ray tracing using XBeach. Major spatial variation in waves, currents and beachface behavior at this perched beach shows the importance of the local geological setting.

Introduction

Beaches that have underlying rock and/or that are fronted seaward by geological or engineered structures are common world-wide. These beaches can be referred to as perched beaches, defined as being formed by the accumulation of unconsolidated sediment atop a shallow-rock platform; and/or that are landward of an offshore structure. Such structures may consist of limestone, coral, coquina, shell, worm rock, sedimentary rock, clay or rip-rap (Larson and Kraus, 2000). Internationally, different forms of perched beaches have been described including:

• beaches with patchy exposed rock—often beach rock formed through the precipitation of carbonate sediments (Chowdhury et al., 1997, Dickinson, 1999, Rey et al., 2004, Vousdoukas et al., 2007, Vousdoukas et al., 2009);

• shore platforms (Bartrum, 1926, Muñoz-Pérez et al., 1999, Trenhaile, 2004, Valvo et al., 2006);

• inlets with a hard-bottom (Hanson and Militello, 2005);

• offshore breakwaters and reefs, possibly fronted landward by a coastal lagoon (Sanderson and Eliot, 1996, Dean et al., 1997, González et al., 1999, Eversole and Fletcher, 2003, Frihy et al., 2004); and

• seawalls, revetments and bulkheads (Kraus, 1988, Fitzgerald et al., 1994, Kraus and McDougal, 1996).

In southwest Western Australia rocky beaches have been described by Semeniuk and Johnson, 1982, Green, 2008, Doucette, 2009 and Da Silva (2010), and in the northwest beaches associated with coral such as the fringing reef at Ningaloo (Sanderson, 2000) are also an important part of the coast. A simple classification summarizing the above perched beach descriptions is presented in Fig. 1, focusing purely on the cross-shore perched beach profile rather than alongshore variations due to headlands and reefs which influence longshore sediment transport (Sanderson and Eliot, 1999). Perched beaches may also be backed by hard structures such as cliffs, seawalls, streets or buildings (Fitzgerald et al., 1994) which are not considered for the purpose of the simple classification.

The lack of research on perched beaches was mentioned by Hegge et al. (1996), who noted that while a morphodynamic classification of sandy beaches has been established for open-ocean, wave-dominated environments, many natural beaches are in fact inside embayments or landward of protective reefs which provide shelter from waves. It is accepted that the geology of perched beaches will affect the nearshore hydrodynamics and beach morphology, but there has been little research to quantify and understand these differences compared to ‘typical’ sandy beaches without structural constraints (Larson and Kraus, 2000, Stephenson and Thornton, 2005, Naylor et al., 2010). The response of shorelines to submerged structures such as artificial surfing reefs, is still poorly understood (Ranasinghe et al., 2006). With the threat of sea level rise and increased urbanization of the coast, understanding all types of coastal landforms is becoming more important so it is crucial that the mechanisms of geological beach control are identified.

It has been suggested that a rocky bottom at a beach will:

• limit fluctuation of beach profiles (Larson and Kraus, 2000, Vousdoukas et al., 2007);

• alter the nearshore hydrodynamics (Cleary et al., 1996, Larson and Kraus, 2000, Vousdoukas et al., 2007);

• change the flow/ pressure distribution in the beach sediment (Larson and Kraus, 2000);

• reduce the porosity of the beach hence reduce water infiltration possibly leading to erosion (Walton and Sensabaugh, 1979, Larson and Kraus, 2000, Vousdoukas et al., 2009);

• alter erosion rates at rock margins causing scouring (Larson and Kraus, 2000) by changing cross-shore and longshore sediment transport (Vousdoukas et al., 2007); and

• decrease sediment availability (Trenhaile, 2004) because if hard-bottom is exposed, the actual sediment transport rate will be less than the potential, which can also affect surrounding areas without hard-bottoms (Hanson and Militello, 2005).

In a study of the 10,685 mainland beaches of Australia, Short (2006) stressed the importance of geological formations to the form and function of perched beaches. It was noted that bedrock and calcarenite play a major role on Australian beaches by forming beach boundaries so that the average beach length is just 1.37 km. Australian rock formations lie along the coast as beachrock, rocks, headlands and islands, inducing wave refraction and attenuation resulting in lower energy beach types. Rocks were found to dominate the intertidal zone of 779 beaches, and coral reefs are located seaward of at least 1430 beaches in northern Australia. These are mostly barrier beaches that are backed by a lagoon with lower energy beaches that are unusually steep, most common in Western Australia.

Perched beaches are especially common in Western Australia. Yanchep Lagoon in the Perth metropolitan area of southwest Western Australia is predominantly a Type 4 (Fig. 1) perched beach that is partly fronted by a lagoon that is enclosed by a calcarenite limestone reef (Fig. 2). The microtidal coast is largely sheltered from swell by offshore reefs made of Quaternary limestone. Therefore, locally produced wind waves and currents from the unusually strong and persistent sea breeze that blows in summer have a dominant effect on coastal processes (Pattiaratchi et al., 1997), making the coast an ideal field location to quantify the response of beaches to wave and current forcing. The generally shore-parallel sea breeze in southwest Western Australia forms due to a combination of local and synoptic systems and plays a key role in the sediment budget of the coast by driving the mean annual littoral drift to the north (Masselink, 1996). The sea breeze increases significant wave height (H_s) at the coast by up to 0.9 m and can increase sediment suspension tenfold (Pattiaratchi et al., 1997). Past research on the effects of sea breeze activity on coastal processes and geomorphology in southwest Western Australia have focused mainly on beach cusps and have largely excluded the overnight recovery phase of beach morphology that occurs at the cessation of the sea breeze (Kempin and Gelfenbaum, 1953, Pattiaratchi et al., 1997, Masselink and Pattiaratchi, 1998a, Masselink and Pattiaratchi, 1998b, Masselink and Pattiaratchi, 1998c, Masselink and Pattiaratchi, 2001a, Masselink and Pattiaratchi, 2001b). Energetic wave conditions combined with equipment constraints have, until now, prevented a complete record of hydrodynamic measurements over the sea breeze cycle

We hypothesized that beach profiles that were perched on rock structures would be better protected from waves and currents than profiles not fronted by rock. To test this, it was important to investigate: (1) temporal variations over the sea breeze cycle; and (2) spatial variations at different areas of the nearshore to indentify how the rock structures influence the waves and currents. Therefore, there were four main objectives of the work, to:

• Quantify temporal variations of waves and currents during the sea breeze cycle at locations:

• offshore;

• 20 m seaward of the limestone reef;

• inside the lagoon; and

• in the surf zone fronting the Exposed Profile and the Sheltered Profile.

• Quantify the response of the beach to the variation in currents and waves induced by the sea breeze cycle at an exposed beach profile and a sheltered beach profile.