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Sandy beaches as dynamic refugia: Potential barriers to shoreline retreat on the Sunshine Coast, Queensland, Australia
Ocean & Coastal Management 102 (2014) 32e39
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Sandy beaches as dynamic refugia: Potential barriers to shoreline retreat on the Sunshine Coast, Queensland, Australia Ashton J. Berry a, *, Shireen Fahey a, Noel Meyers b a b
University of the Sunshine Coast, Faculty of Science, Health, Education and Engineering, Locked Bag 4, Maroochydore, 4558 Qld, Australia La Trobe University, Faculty of Education, PO Box 199, Bendigo, 3552 Victoria, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Available online
Refugial habitats retain the requisite ecological, physiological, and environmental conditions conducive to the survival of individual species and ecosystems over ecological and climatic timescales. Refugia play an important role in the conservation of ecosystems and species as climates change. Nevertheless, many habitats that function as refugia have yet to be recognised or protected. Sandy beach ecosystems (SBEs) are one such example. SBEs function as dynamic refugia as they retreat landward in response to sea level rise (SLR) over large temporal scales of hundreds to thousands of years. However, increasing coastal urbanisation and development are diminishing this capacity. The authors used Google Earth Pro and the Australian Coastal Smartline mapping tool to determine that 36.79% of SBEs on the Sunshine Coast, Queensland, Australia, are at risk of reduced capacity for landward retreat as sea levels increase under existing Queensland State Government legislation. This equates to the construction and maintenance of a further 32.68 kms of beach revetments in addition to the 4.44 kms already constructed. A window of opportunity exists on the Sunshine Coast, and similarly developed coastal regions, to incorporate alternate adaptation options, including managed retreat, setbacks and ecosystem engineering, into the design and implementation of coastal development policies and plans that integrate opportunities to establish and maintain dynamic refugia. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Refugia represent isolated pockets of formerly more extensive ecosystems. Some combination of biotic, abiotic, or environmental factors may have diminished habitat size (Dawson et al., 2011). Or it may have been the result of historic processes and events (Ricklefs et al., 1999). Irrespective, components of once more extensive systems persisted in refugia, whilst others did not. Where refugia persist, there exists a suite of favourable environmental, biological, physical or climatic factors that facilitate perseverance of individuals and populations (Keppel et al., 2012). Examples are numerous: the migration of the northern hemisphere's tundra habitat in response to changing climates during the Pleistocene (Payette et al., 2002; Skrede et al., 2006). Similarly, in response to the drying of Australia (Turney et al., 2004), the rainforests that once covered much of the continent retreated into significantly
* Corresponding author. Tel.: þ248 2 885 307. E-mail addresses: [email protected] (A.J. Berry), [email protected] (S. Fahey), [email protected] (N. Meyers). http://dx.doi.org/10.1016/j.ocecoaman.2014.08.006 0964-5691/© 2014 Elsevier Ltd. All rights reserved.
smaller pockets along the east coast (Graham et al., 2010). Refugia, however, serve more than one function. At a particular point in time, refugia serve to conserve landforms, habitat, communities, species, and populations (Anderson and Ferree, 2010; Comes and Kadereit, 1998). Taken in the context of deep time, refugia act as biological, ecological, and environmental time capsules (Sedell et al., 1990). As the climatic, biotic, abiotic, or environmental conditions that constrain the boundaries of refugia ameliorate or change, a refugia's legacy may be realised. In response to changing conditions, species occupying refugia may contribute to the dispersal and establishment of propagules into areas of former habitat, or new areas (Ashcroft, 2010; Carvalho et al., 2010). Similarly the concept of refugia can be applied to coastal ecosystems that provide refugia from sea level rise (SLR) over time scales of hundreds to thousands of years. For example, mangrove forests retreat from SLR and form dynamic refugia when individual propagules disperse and establish in uncolonised stretches of riverbanks and creeks (Gilman et al., 2008). Increasing sea levels heighten salinity levels further inland enabling propagules to remain viable and establish in areas where they were previously unable (Cheeseman, 2012; Di Nitto et al., 2008). Mangrove plants
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invade and modify habitats (Lara et al., 2002; Teh et al., 2008), and provide suitable conditions for the progressive colonisation and development of habitats capable of supporting ecosystems and species as sea levels increase (French, 2006). Similar successional pathways exist for sandy beach ecosystems (SBEs) (Feagin et al., 2005). By migrating landward during times of SLR, SBEs may also function as dynamic refugia that maintain specific habitat parameters amenable to beach species, such as benthic infauna, fish, birds, turtles and plants. These species persist by tracking the retreat of their sandy habitats (Fish et al., 2005; Galbraith et al., 2002; Jones et al., 2007). The ability to adapt to SLR via landward migration ensures SBEs maintain structure and function and, consequently, resilience to the impacts of SLR (Berry et al., 2013). Arguably, SBEs are at the forefront of the impacts of anthropogenic and natural cycles of climate change (Defeo et al., 2009). As such, the spatial locations of sandy shorelines are constantly changing (Cann et al., 1999; Fielding et al., 2006; Haslett et al., 2000). SBEs, comprised of wave, intertidal and backshore zones, are formed from loose accumulations of sand that respond dynamically to influencing factors such as waves, winds, and currents (Alejo et al., 2005). The last glacial period resulted in decreasing sea levels where sand accretion exceeded that of erosion and resulted in the progradation of sandy beaches seaward of their pre-glacial position (Cooper and McKenna, 2008a; Davidson-Arnott, 2005; Woodroffe, 2007). Beaches migrate inexorably in response to changing sea levels. As sea levels change the biological inhabitants of the sandy beaches migrate, disperse to new locations, or succumb to drowning or colonisation by other better-adapted individuals to changing environments. Inchworm-like, SBEs migrate through cycles of colonisation, establishment, growth, and dispersal (Brown and McLachlan, 2002; Caldwell and Segall, 2007). As the glacial came to an end, the climate warmed over hundreds and thousands of years (Petit et al., 1999). A warmer earth saw the ice that once covered much of the northern and southern hemispheres melt. Rainfall increased, sea levels began to rise, and the shoreline moved landward. Whether past increases in sea level, many of which occurred over millennia, were sufficient to cause local extinctions of sandy beach species, as well as the migration and evolution of new species able to tolerate the challenging conditions remains unknown. Although logic dictates that such events occurred. What is known is that for refugia to act as reservoirs of biological and geological ecosystem components, the capacity to disperse and establish ahead of advancing seas remains critical. Where there are no impediments to cycles of dispersal and establishment, the landward migration of sandy beach refugia can continue. Natural boundaries to inland migration of SBEs have always existed. For example, substantial bedrock cliffs and headlands will act as barriers to the migration of sandy beach refugia. Without successful colonisation, establishment, and further expansion around these landforms, refugial migration might cease - perhaps permanently. Caught between the incoming sea, and the impenetrable barrier of cliffs, the refugia would drown. The biological distinctiveness of individuals in the drowned beaches would contribute nothing to future generations. In areas of equitable distribution of genetic diversity within populations comprising refugia, perhaps the loss of diversity to future gene pools due to natural boundaries would remain small. Fortunately, the extent of natural barriers to inland migration of SBEs remains small compared to the length of coastlines (Brown and McLachlan, 2002). Bedrock cliffs or areas of high elevation would preclude the landward transport of sediment and subsequent colonisation of that sediment by associated biologicals. The discontinuous nature of bedrock barriers to inland migration
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ensures there remain pathways around these landforms that allow inland migration of coastal and sandy beach refugia. As sea levels increased, landward colonisation, migration, and local extinction of species would have occurred over hundreds to thousands of years. Changes in human land use resulting from rapid growth in coastal populations have produced extensive urbanisation of coastal zones throughout the world (Scherner et al., 2013). Referred to as coastal development in this paper, the spread of urban areas and infrastructure has produced a range of anthropogenic barriers to the inland migration of SBEs (Dugan et al., 2008; Eurosion, 2004). To protect coastal development of high economic value land managers have established groynes, revetments, geotextile sandbags and other forms of armouring designed to maintain shoreline positions and protect the integrity of infrastructure (Bacchiocchi and Airoldi, 2003; El-Raey et al., 1999; U.S. Army Corps of Engineers, 2002). Although the density of coastal development varies, coastal armouring has become the default adaptation option to protect a range of development and infrastructure types and concentrations in coastal communities globally (Dugan et al., 2008; Stancheva et al., 2011). There exists a coastal squeeze between the rising sea levels and coastal developments (Doody, 2004). These developments are increasing in frequency and extent (Abel et al., 2011). Coastal construction and associated armouring may now obstruct SBE's historical inland migration routes. While the limited extent of some constructions results in SBEs and their component species migrating around those constructions, there remains the strong likelihood that the remainder of the beach systems will be isolated, squeezed, and drowned. This may limit the genetic and ecosystem contributions to the retreating shoreline. The long-term implications of such diminishment in genetic and ecological diversity that result from human coastal development may reduce the evolutionary potential of species and environments to increasingly less predictable prevailing conditions (Alejo et al., 2005). Few studies have considered SBEs to act as refugia (Berry et al., 2013). The capacity of sandy beaches to provide the raw materials for inland migration remains equally poorly studied. Identifying refugia represents an important first step in the selection, design, and management of protected areas (Klein et al., 2009; Mackey et al., 2008). However, many habitats that function as refugia are yet to be recognised or protected (Mackey et al., 2002). Whether SBEs can maintain the ability to migrate in the face of rapid urban and coastal development is unknown. This paper explores the constraints on future inland migration of SBEs in a rapidly changing environment. With increasing urbanisation and development of coastal zones, the capacity for sandy beaches to migrate inland has been curtailed (Dugan et al., 2008). The use of hard-engineered structures, such as rock walls, to protect economically high value buildings and infrastructure further restricts the capacity for sandy beaches to retreat in response to SLR. The susceptibility of shoreline attributes to erosion is a critical determinant of the potential for SBEs to migrate and form refugia. Assessing the stability of shoreline attributes can also determine the location of possible migration pathways through which SBEs can migrate. With projected increases in sea level of >1 m by 2 100 (Pfeffer et al., 2008) and >5 m by 2 500 (Jevrejeva et al., 2012), the inclusion of refugia into coastal conservation planning is critical in preserving biodiversity against the impacts of climate change (Keppel et al., 2012; Klein et al., 2009; Loarie et al., 2008; Mackey et al., 2002; Sedell et al., 1990). This is particularly true for sandy beaches. They support threatened ecosystems containing unique species not yet included in conservation plans (Defeo et al., 2009; Schlacher et al., 2007). The purpose of this paper is to (1) identify the extent to which SBEs on the Sunshine Coast, Queensland,
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Australia, have the capacity to adapt to SLR through landward retreat, and (2) identify potential barriers to the landward retreat of SBEs, which may assist coastal managers in the selection, design, and implementation of adaptation options in response to SLR (Alexander et al., 2012). 2. Physical setting This study was conducted in the Sunshine Coast region, of South East Queensland, Australia (see Fig.1). The study location was chosen for being one of the fastest developing coastal regions in Australia (ABS, 2012). With sandy beaches the major attraction, expanding resident and tourist populations are increasing demand for coastal urbanisation and accommodation development. Residential populations are forecast to reach between 439 100 and 516 250 by 2031, an increase of 38.6% and 62.9% (Mason et al., 2012). Tourism contributed $2 324 million to the regional economy in 2010/11 and 8.2 million tourists visited the region in 2012 (Mason et al., 2012). The coast is eastward-facing, exposed, and features wavedominated intermediate sandy beaches (Schlacher et al., 2008). Beaches are discontinuous. Geomorphic features, such as rocky headlands separate many beaches. Large expanses of sandy backshores exist in the form of strand or coastal plains (Ward, 2006). The plains, deposited during higher Holocene sea levels, provide a record of historic beach retreat and progradation in response to fluctuating sea levels (Gharibreza and Motamed, 2006; Hein et al., 2012; Micallef et al., 2013; Rabineau et al., 2006). The Sunshine Coasts sandy beaches and backshore dune systems are formed from quartz sand deposits eroded from granites and sandstones found along the New South Wales coast (Willmott and Stevens, 1988). These sediments are transported 1500 kms north via longshore drift forming extensive beaches and coastal plains in New South Wales and Queensland. Large parabolic dune systems are also formed from this sand in the Cooloola Recreation Area (see Fig. 1) and on sand islands further to the north including Stradbroke and Fraser Islands (Boyd et al., 2004; Tejan-Kella et al., 1990). Human activities impact Sunshine Coast sandy beaches in many ways. For instance, extensive urbanisation of backshore areas, the use of hard- and soft-engineered coastal protection structures, river dredging, beach cleaning, pollution and trampling by pedestrians produce detrimental impacts on these systems. Soft-engineered structures are constructed of materials such as sand that are soft in comparison to rock. In this way adaptation options like beach nourishment are referred to as examples of soft engineering (Berry et al., 2013). Soft materials are used in recognition of the advantages of enabling coastal processes, such as beach erosion and longshore
sand transport, to continue. Soft structures are also more effective at maintaining the aesthetics of sandy beaches for the benefit of tourism. Supporting an abundance of migratory shorebirds, turtle nesting sites and benthic assemblages, sandy beaches of the Sunshine Coast are considered ecologically significant (Ford, 2010; Limpus, 2008; Schlacher et al., 2008). Fig. 1 also illustrates the location of protected areas in the region where state planning laws preclude coastal development. 3. Methods 3.1. Quantifying the capacity for landward migration of SBEs on the Sunshine Coast Google Earth Pro and the Australian Coastal Smartline mapping tool have been used to identify the extent of SBEs on the Sunshine Coast with the capacity to migrate landward as sea levels increase. The capacity for sandy beach retreat is determined by the susceptibility or sensitivity of shoreline attributes to erosion. Often referred to as a coastal sensitivity index, the susceptibility to erosion and, therefore, capacity to retreat can be determined by factors such as erosivity of shoreline material, shoreline slope, the extent of SLR and the level of wave energy experienced (Abuodha and Woodroffe, 2006, 2010). For example, backshores consisting of sand are more susceptible to wave erosion and therefore landward retreat than hard-engineered revetments or bedrock cliffs. Identifying these attributes as a percentage of the total Sunshine Coast shoreline will help to ascertain the capacity for SBEs to retreat. The length of shoreline attributes such as coastal development and infrastructure provide useful indicators of future revetment construction (Hanak and Moreno, 2012). Current Queensland Government legislation in the form of the Sustainable Planning Act (2009) permits the construction of revetments to protect coastal development (see State Development Assessment Provisions, Part C, Module 10 Coastal Protection). The policy allows for revetment construction if erosion presents an immediate threat to property or infrastructure that is not expendable. Under the Coastal Protection and Management Act (1995) expendable property includes picnic tables, barbeques, lookouts, pathways, decks and portable structures. 3.2. Google Earth Pro Using the global mapping tool Google Earth Pro, the length of the following shoreline attributes were measured and expressed as a percentage of total shoreline (Fig. 2):
Fig. 1. Illustrates the location of the Sunshine Coast, Queensland, Australia, and the five protected areas discussed in this paper.
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Sandy Sandy Sandy Sandy Sandy Sandy
beaches; beaches with beaches with beaches with beaches with beaches with
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bedrock cliff backshores; armoured backshores; developed backshores; undeveloped backshores; protected area backshores.
The scale ruler function in Google Earth Pro is considered an accurate measuring tool to measure distance of sandy shorelines and their attributes (Harris et al., 2011; Waltham and Connolly, 2011). Fig. 3 represents a Google Earth Pro image used in this analysis (Google, 2012). A line map format was used to capture and record the length in kilometres of shoreline attributes at Noosa Heads, a coastal town of the Sunshine Coast region. Different line types signify different attributes. For example, red delineates the total length of the Sunshine Coast shoreline, yellow the length of sandy beaches with backshores that are neither armoured or developed and blue the length of sandy beaches with armoured backshores.
3.3. Smartline approach to assessing shoreline stability To aid in determining the capacity for Sunshine Coast SBEs to migrate landward, the Australian Coastal Smartline mapping tool was also used. Smartline combines data from over 200 prior mapped data sources in making an assessment of Australia's coastal vulnerability to SLR. Data sources include, for example, the Commonwealth Department of Climate Change, Geoscience Australia, Geological Survey of NSW, Australian Beach Safety and Management Program (Short et al., 1993) and Queensland Mines and Energy (Sharples et al., 2009).
Fig. 3. An example of a Google Earth Pro image used in this analysis. Two lines appear on the image of Noosa main beach 26 230 05.4000 S, 153 05011.2400 E, at 162 m elevation. Lines measure the total length of Sunshine Coast shoreline (top red line), the length of sandy beaches with undeveloped backshores (bottom yellow line), and the length of sandy beaches with armoured backshores (bottom blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Smartline differentiates backshore areas into two zones (Fig. 4). The backshore proximal occurs immediately landward of the intertidal zone and on sandy coasts is commonly represented by a foredune. The backshore distal includes the area 500 m landward
Fig. 2. Four examples of Google Earth Pro shoreline attributes at 299 m elevation (Google, 2012): a) sandy beach with armoured backshore in Noosa 26 230 09.5100 S, 153 050 27.0700 E; b) bedrock cliff backshores in Noosa 26 230 37.9900 S, 153 060 58.6400 E; c) sandy beach with unarmoured developed backshore in Sunshine Beach 26 230 52.8900 S, 153 060 49.0500 E; d) sandy beach with undeveloped protected area backshore in Currimundi Lake Conservation Park 26 450 36.5800 S, 153 080 09.6000 E.
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A.J. Berry et al. / Ocean & Coastal Management 102 (2014) 32e39 Table 1 Shoreline stability based on attribute composition and form (composition, form and stability from Sharples et al., 2009).
Fig. 4. Conceptualisation of a shoreline divided into subtidal, intertidal, backshore proximal and distal zones.
from the mean high water mark (MHWM). A distance of 500 m was chosen arbitrarily as the area where shoreline and backshore attributes can be successfully characterised using the line mapping technique (Sharples et al., 2009). Beyond this area other mapping techniques may be more suitable. Each zone is described in terms of its geomorphic form and composition. For example, materials of hard lithified composition, such as bedrock cliffs, suggest low levels of vulnerability to erosion and reduced potential for shoreline retreat in comparison to soft materials (Sharples et al., 2009). For the purpose of this paper, hard-engineered coastal armouring constructed from boulders, concrete or geotextiles are considered as geomorphic features as they impact beach morphodynamics in the same way as structures formed from bedrock. Smartline maps classify the stability and form of the shoreline through an assessment of their geomorphic features (Sharples et al., 2009). The unconsolidated composition of SBEs results in limited stability. SBEs remain highly susceptible to erosion and, therefore, retreat in comparison to areas high in stability, such as bedrock cliffs and hard-engineered revetment walls (Hanson and Lindh, 1993; Levin et al., 2008). Highly stable attributes with high to moderate slopes in either backshore zones are resistant to erosion. These structures are likely to form barriers to retreat and affect the capacity of SBEs to migrate landward in response to advancing seas. Thus, an assessment of shoreline attributes in intertidal and backshore areas enables confirmation of the capacity for SBEs to retreat landward as sea levels increase.
4. Results Analysis of the Sunshine Coast shoreline identified a range of geomorphic attributes located within intertidal and backshore zones. Intertidal attributes include sandy beaches, incipient dunes, foredunes, revetments and bedrock cliffs. Backshore attributes include bedrock cliffs, revetments, and backdunes, low lying coastal plains, and developed and undeveloped areas (Fig. 2). The stability of shoreline attributes were determined using the Smartline mapping tool and are displayed in Table 1 (Sharples et al., 2009). Shoreline attributes are divided into sandy beaches (erosion prone), incipient dunes/foredunes (erosion prone), backdune/coastal plains (erosion prone), rock cliffs (unlikely to erode or retreat over human timescales) and revetments (unlikely to erode or retreat over human timescales when maintained). The lengths of intertidal attributes are detailed in Table 2. Sandy beaches represent 87.02% of the Sunshine Coast's 100.91 kms of intertidal open-ocean shoreline, while 11.86% are bedrock cliffs. River mouths of lotic systems, such as the Noosa, Maroochy and Mooloolah and Pumicestone Inlet make up the remaining 1.24% of the total shoreline length.
Shoreline attribute
Composition
Form
Stability
Sand beaches
Mostly sandy
Gentle slopes
Unstable (prone to erosion and retreat) Incipient dunes/ Mostly sandy Gentle/moderate Unstable (prone to foredunes slopes erosion and retreat) Backdunes/ Mostly sandy Gentle/moderate Unstable (prone to coastal plains slopes erosion and retreat) Rock cliff Hard lithified b High cliffs Stable (robust and edrock Moderate slopes unlikely to erode or retreat over human timescales Moderate slopes Mostly stable Revetment Hard lithified (Unlikely to erode bedrock boulders or retreat if Concrete maintained. Geotextile Susceptible to sand bags progressive slumping and collapse)
The lengths of backshore geomorphic attributes are detailed in Table 3. Hard-engineered adaptation options, such as rip-rap boulder, concrete and geotextile sandbag revetments currently occupy 4.4% of the total Sunshine Coast shoreline. This percentage results from revetments used to protect coastal development including Noosa main beach (0.72 km), Coolum beach (0.2 km), Mudjimba beach (0.1 km), Maroochydore beach (0.78 km) Mooloolaba beach (1.6 km) and Caloundra beach (1.04 km) (only total length in km are shown in Table 3). The addition of bedrock cliffs (2.63%) to revetments represents a combined total of 7.03% of the Sunshine Coast shoreline that is currently prevented from landward migration due to the stable composition and high to moderate form of these shoreline attributes. The remaining shoreline consists of sandy beaches with backshores susceptible to erosion and retreat. 32.39% of backshores are developed, but have unstable intertidal and backshore attributes. Of the undeveloped backshores, 0.97% is not protected from future development, while 46.64% is protected from development by national parks and conservation reserves. The Cooloola Recreation Area, part of the Great Sandy National Park, includes 20.47 kms; the Noosa National Park consists of 1.12 kms; the Maroochy River and Currimundi Lake Conservation Parks consist of 1.31 kms; and the Bribie Island National Park comprises 14.5 kms (only total length km are shown in Table 3). 5. Discussion and conclusions In this paper, we identify SBEs as refugia for the biological and geological elements required for species to persist in and migrate landward in response to sea level rise (SLR). Prior to human intervention, these natural coastal processes continued largely
Table 2 Length and percentage of intertidal shoreline attributes on the Sunshine Coast, including sandy beaches, bedrock cliffs, river openings and Pumicestone inlet. Intertidal geomorphic attributes Sandy beaches Bedrock cliffs River openings and Pumicestone inlet Sunshine Coast shoreline
Length (km) 87.81 11.86 1.24 100.91
Total sandy beaches (%) 100
Total shoreline (%) 87.02 11.75 1.23 100
A.J. Berry et al. / Ocean & Coastal Management 102 (2014) 32e39 Table 3 Length and percentage of backshore attributes on the Sunshine Coast, including bedrock cliffs, coastal armouring, developed and undeveloped areas. Backshore geomorphic attributes Sandy beaches with bedrock cliff backshores Sandy beaches with armoured backshores Sandy beaches with developed backshores Sandy beaches with undeveloped backshores Sandy beaches with protected area backshores
Length (km)
Total sandy beaches (%)
Total shoreline (%)
2.65
3.02
2.63
4.44
5.06
4.40
32.68
37.22
32.39
0.98
1.12
0.97
47.06
53.60
46.64
unimpeded. Coastal management has yet to successfully integrate these dynamic processes in conceptualising past, present, and future coastlines. Instead, managers implement strategies designed to maintain a fixed coastline (Zanuttigh 2011). For the first time, hard-engineered structures in the form of revetments and groynes have been built to protect high value infrastructure adjacent to highly erodible SBEs (Berry et al., 2013). This phenomenon has spread rapidly through the developed world (Cooper and McKenna, 2008b; Titus et al., 2009b) and more recently areas of the developing world. Concurrently, sea levels are predicted to continue increasing this century and beyond (Church et al., 2011; Nicholls, 2011). The cumulative impact of these factors is predictable. To maintain suitable habitat, SBEs require pathways through which they can migrate landward over hundreds and potentially thousands of years in response to fluctuating sea levels. Paleoshoreline research provides abundant evidence that SBEs migrate landwards in response to SLR (Gharibreza and Motamed, 2006; Micallef et al., 2013). Historic records also allow the location of these pathways to be identified (Cann et al., 1999; Lewis et al., 2008). Yet, the capacity of SBEs to act as dynamic refugia that track sea levels, have attracted scant attention (Berry et al., 2013). Coastal development in low-lying areas often neglects to consider the role of retreating beaches in enabling species to persist during SLR (Fish et al., 2008), particularly in response to coastal armouring. Many SBEs are now trapped between hard-engineered structures and advancing seas. Unable to retreat they become squeezed, and without direct intervention will drown. To evaluate the likely impact of coastal armouring on the potential for future landward migration of SBEs due to SLR, we used the Sunshine Coast region of Queensland as an analogue for many rapidly developing, and increasingly urbanised coastal communities in close proximity to SBEs. 5.1. Capacity for SBEs to retreat The capacity for Sunshine Coast SBEs to adapt to SLR by retreating landward is diminishing. In 2013, 7.03% of SBEs in this region were prevented from retreat due to bedrock cliffs and coastal armouring. However, new armouring continues to be incorporated in local coastal management strategies. Current Queensland State Government legislation, as with much of the developed world (Caldwell and Segall, 2007; Titus et al., 2009a), affords erosion protection to non-expendable development through the use of hard-engineered structures (Abel et al., 2011). On the Sunshine Coast 36.79% of the total shoreline has been developed by non-expendable urbanisation and infrastructure. As 4.4% of these shorelines have already been armoured, 32.39% of these developed areas are yet to be armoured. Although setbacks of between 10 and 200 m provide some protection, developments on
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low-lying and erosion prone coastal plains remain vulnerable to SLR. Further research is required to ensure setbacks can provide adequate buffers to prevent the need for armouring. Setbacks, by themselves will remain insufficient and alternative adaptation options may become necessary. As sea levels increase, coastal development becomes increasingly vulnerable to retreating beaches and the construction of hardengineered structures becomes more likely. If State Government legislation remains unchanged, 32.39% of developed backshores remain likely to become armoured in addition to the 7.03% currently prevented from retreat. A total of 39.42% of the Sunshine Coast's shorelines would therefore be unable to escape SLR. The capacity for squeezed beaches to act as refugia and provide suitable habitat for species to persist becomes critically reduced (Herben and Soderstrom, 1992; Keppel et al., 2012). 5.2. Coastal management implications A window of opportunity exists on the Sunshine Coast, and in other similarly developed coastal regions around the world. The extent of existing development avails coastal managers with the opportunity to incorporate a range of adaptation options, including managed retreat, setbacks and ecosystem engineering into the design and implementation of coastal adaptation plans and strategies (Berry et al., 2013; Fish et al., 2008; Sano et al., 2011). Adopting this approach will integrate opportunities to create conditions conducive to the persistence of dynamic refugia. For example, establishing setbacks to protect backshores that are either undeveloped, sparsely developed, or where infrastructure is expendable, would facilitate the retreat of SBEs without detriment to existing buildings and infrastructure. We hypothesise similar levels of amenity and opportunity exists in other rapidly developing coastal regions. In coastal regions where only small amounts of shoreline are undeveloped a range of adaptation options will need to be applied over the short- and long term (Berry et al., 2013). The protection of sandy beach backshores from development provides a proactive strategy previously proven to be cost-effective in coastal areas, such as the Humber estuary in England (Turner et al., 2007). The potential to conserve and create dynamic refugia provides an additional and necessary element for coastal zone managers as they choose among adaptation options for setbacks or protected areas (Game et al. 2011). Such strategies ensure high priority sandy beaches are selected. However, refugia should be considered in conjunction with other key ecological, social and economic selection criteria. For example, the maintenance of species and ecosystem representation, genetic diversity of keystone species, critical habitats, and longer-term morphological coastal processes represent necessary considerations (Hannah, 2009; Harris et al., 2011; Mackey et al., 2008; Salm et al., 2000) to ensure the resilience of dynamic refugia and the potential for their contribution to future migrations. Compounding the losses of ecosystem resilience, the social and economic cost of constructing revetments represent significant and escalating investments and temporary insurance against rising sea levels (Nicholls, 2011). If coastal managers in this region seek to establish a fixed shoreline an additional 32.68 km of revetment walls will need to be constructed and maintained. Hard-engineered barriers precluding retreat of refugia over such extensive areas of shoreline may prove socially, economically and ecologically unfeasible (McGlashan, 2003). In areas where backshore distals have only been partially or sparsely developed, alternate adaptation options, such as the managed relocation of coastal buildings and infrastructure, provide feasible approaches to maintaining the adaptive capacity of sandy beaches (Abel et al., 2011; Alexander et al., 2012). To what extent the density of coastal development
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excludes the use of managed relocation as an adaptation option is the subject of future investigation by coastal researchers. Where sandy beach backshores are heavily developed or represent a significant investment, maintaining the capacity for sandy beaches to retreat and to form refugia will require a staged approach in the application of adaptation options (Berry et al., 2013). Such an approach includes the design and implementation of managed retreat policies that enable SBEs to retreat over the long-term through the strategic relocation of buildings and infrastructure (Abel et al., 2011; Alexander et al., 2012; McGlashan, 2003). Losses to the future evolutionary potential of SBEs remain poorly quantified because the current status of biological, ecological, and geological aspects of these systems along the coastlines of the world remain understudied. Further research to determine the likely pathways that SBEs retreat through over hundreds and potentially thousands of years will set the context of the challenges faced by coastal and environmental managers. Providing the capacity for SBEs to act as dynamic refugia and to contribute to the conservation and preservation of the genetic and biological diversity represents a new and important additional tool for coastal zone managers. As national parks in Australia preclude development and allow natural processes to continue, SBEs adjacent to these sites retain the capacity to retreat in response to SLR. If coastlines without conservation status become increasingly less able to retreat due to coastal development, the extent that national parks are relied upon to preserve coastal ecosystems will continue to increase. National parks represent a greatly reduced area over which the pool of genetic, ecological, and geological components will be required to migrate as SBEs retreat. The extent to which coastal development restricts the landward retreat of SBEs represents a field of on-going interest for coastal researchers and managers globally.
Acknowledgements Thanks to Geoff Dews and three anonymous reviewers for providing valuable comments on earlier drafts. This paper was developed with salary support from the School of Science, Education, and Engineering at the University of the Sunshine Coast, Queensland, Australia.
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Sandy beaches as dynamic refugia: Potential barriers to shoreline retreat on the Sunshine Coast, Queensland, Australia Ashton J. Berry a, *, Shireen Fahey a, Noel Meyers b a b
University of the Sunshine Coast, Faculty of Science, Health, Education and Engineering, Locked Bag 4, Maroochydore, 4558 Qld, Australia La Trobe University, Faculty of Education, PO Box 199, Bendigo, 3552 Victoria, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Available online
Refugial habitats retain the requisite ecological, physiological, and environmental conditions conducive to the survival of individual species and ecosystems over ecological and climatic timescales. Refugia play an important role in the conservation of ecosystems and species as climates change. Nevertheless, many habitats that function as refugia have yet to be recognised or protected. Sandy beach ecosystems (SBEs) are one such example. SBEs function as dynamic refugia as they retreat landward in response to sea level rise (SLR) over large temporal scales of hundreds to thousands of years. However, increasing coastal urbanisation and development are diminishing this capacity. The authors used Google Earth Pro and the Australian Coastal Smartline mapping tool to determine that 36.79% of SBEs on the Sunshine Coast, Queensland, Australia, are at risk of reduced capacity for landward retreat as sea levels increase under existing Queensland State Government legislation. This equates to the construction and maintenance of a further 32.68 kms of beach revetments in addition to the 4.44 kms already constructed. A window of opportunity exists on the Sunshine Coast, and similarly developed coastal regions, to incorporate alternate adaptation options, including managed retreat, setbacks and ecosystem engineering, into the design and implementation of coastal development policies and plans that integrate opportunities to establish and maintain dynamic refugia. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Refugia represent isolated pockets of formerly more extensive ecosystems. Some combination of biotic, abiotic, or environmental factors may have diminished habitat size (Dawson et al., 2011). Or it may have been the result of historic processes and events (Ricklefs et al., 1999). Irrespective, components of once more extensive systems persisted in refugia, whilst others did not. Where refugia persist, there exists a suite of favourable environmental, biological, physical or climatic factors that facilitate perseverance of individuals and populations (Keppel et al., 2012). Examples are numerous: the migration of the northern hemisphere's tundra habitat in response to changing climates during the Pleistocene (Payette et al., 2002; Skrede et al., 2006). Similarly, in response to the drying of Australia (Turney et al., 2004), the rainforests that once covered much of the continent retreated into significantly
* Corresponding author. Tel.: þ248 2 885 307. E-mail addresses: [email protected] (A.J. Berry), [email protected] (S. Fahey), [email protected] (N. Meyers). http://dx.doi.org/10.1016/j.ocecoaman.2014.08.006 0964-5691/© 2014 Elsevier Ltd. All rights reserved.
smaller pockets along the east coast (Graham et al., 2010). Refugia, however, serve more than one function. At a particular point in time, refugia serve to conserve landforms, habitat, communities, species, and populations (Anderson and Ferree, 2010; Comes and Kadereit, 1998). Taken in the context of deep time, refugia act as biological, ecological, and environmental time capsules (Sedell et al., 1990). As the climatic, biotic, abiotic, or environmental conditions that constrain the boundaries of refugia ameliorate or change, a refugia's legacy may be realised. In response to changing conditions, species occupying refugia may contribute to the dispersal and establishment of propagules into areas of former habitat, or new areas (Ashcroft, 2010; Carvalho et al., 2010). Similarly the concept of refugia can be applied to coastal ecosystems that provide refugia from sea level rise (SLR) over time scales of hundreds to thousands of years. For example, mangrove forests retreat from SLR and form dynamic refugia when individual propagules disperse and establish in uncolonised stretches of riverbanks and creeks (Gilman et al., 2008). Increasing sea levels heighten salinity levels further inland enabling propagules to remain viable and establish in areas where they were previously unable (Cheeseman, 2012; Di Nitto et al., 2008). Mangrove plants
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invade and modify habitats (Lara et al., 2002; Teh et al., 2008), and provide suitable conditions for the progressive colonisation and development of habitats capable of supporting ecosystems and species as sea levels increase (French, 2006). Similar successional pathways exist for sandy beach ecosystems (SBEs) (Feagin et al., 2005). By migrating landward during times of SLR, SBEs may also function as dynamic refugia that maintain specific habitat parameters amenable to beach species, such as benthic infauna, fish, birds, turtles and plants. These species persist by tracking the retreat of their sandy habitats (Fish et al., 2005; Galbraith et al., 2002; Jones et al., 2007). The ability to adapt to SLR via landward migration ensures SBEs maintain structure and function and, consequently, resilience to the impacts of SLR (Berry et al., 2013). Arguably, SBEs are at the forefront of the impacts of anthropogenic and natural cycles of climate change (Defeo et al., 2009). As such, the spatial locations of sandy shorelines are constantly changing (Cann et al., 1999; Fielding et al., 2006; Haslett et al., 2000). SBEs, comprised of wave, intertidal and backshore zones, are formed from loose accumulations of sand that respond dynamically to influencing factors such as waves, winds, and currents (Alejo et al., 2005). The last glacial period resulted in decreasing sea levels where sand accretion exceeded that of erosion and resulted in the progradation of sandy beaches seaward of their pre-glacial position (Cooper and McKenna, 2008a; Davidson-Arnott, 2005; Woodroffe, 2007). Beaches migrate inexorably in response to changing sea levels. As sea levels change the biological inhabitants of the sandy beaches migrate, disperse to new locations, or succumb to drowning or colonisation by other better-adapted individuals to changing environments. Inchworm-like, SBEs migrate through cycles of colonisation, establishment, growth, and dispersal (Brown and McLachlan, 2002; Caldwell and Segall, 2007). As the glacial came to an end, the climate warmed over hundreds and thousands of years (Petit et al., 1999). A warmer earth saw the ice that once covered much of the northern and southern hemispheres melt. Rainfall increased, sea levels began to rise, and the shoreline moved landward. Whether past increases in sea level, many of which occurred over millennia, were sufficient to cause local extinctions of sandy beach species, as well as the migration and evolution of new species able to tolerate the challenging conditions remains unknown. Although logic dictates that such events occurred. What is known is that for refugia to act as reservoirs of biological and geological ecosystem components, the capacity to disperse and establish ahead of advancing seas remains critical. Where there are no impediments to cycles of dispersal and establishment, the landward migration of sandy beach refugia can continue. Natural boundaries to inland migration of SBEs have always existed. For example, substantial bedrock cliffs and headlands will act as barriers to the migration of sandy beach refugia. Without successful colonisation, establishment, and further expansion around these landforms, refugial migration might cease - perhaps permanently. Caught between the incoming sea, and the impenetrable barrier of cliffs, the refugia would drown. The biological distinctiveness of individuals in the drowned beaches would contribute nothing to future generations. In areas of equitable distribution of genetic diversity within populations comprising refugia, perhaps the loss of diversity to future gene pools due to natural boundaries would remain small. Fortunately, the extent of natural barriers to inland migration of SBEs remains small compared to the length of coastlines (Brown and McLachlan, 2002). Bedrock cliffs or areas of high elevation would preclude the landward transport of sediment and subsequent colonisation of that sediment by associated biologicals. The discontinuous nature of bedrock barriers to inland migration
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ensures there remain pathways around these landforms that allow inland migration of coastal and sandy beach refugia. As sea levels increased, landward colonisation, migration, and local extinction of species would have occurred over hundreds to thousands of years. Changes in human land use resulting from rapid growth in coastal populations have produced extensive urbanisation of coastal zones throughout the world (Scherner et al., 2013). Referred to as coastal development in this paper, the spread of urban areas and infrastructure has produced a range of anthropogenic barriers to the inland migration of SBEs (Dugan et al., 2008; Eurosion, 2004). To protect coastal development of high economic value land managers have established groynes, revetments, geotextile sandbags and other forms of armouring designed to maintain shoreline positions and protect the integrity of infrastructure (Bacchiocchi and Airoldi, 2003; El-Raey et al., 1999; U.S. Army Corps of Engineers, 2002). Although the density of coastal development varies, coastal armouring has become the default adaptation option to protect a range of development and infrastructure types and concentrations in coastal communities globally (Dugan et al., 2008; Stancheva et al., 2011). There exists a coastal squeeze between the rising sea levels and coastal developments (Doody, 2004). These developments are increasing in frequency and extent (Abel et al., 2011). Coastal construction and associated armouring may now obstruct SBE's historical inland migration routes. While the limited extent of some constructions results in SBEs and their component species migrating around those constructions, there remains the strong likelihood that the remainder of the beach systems will be isolated, squeezed, and drowned. This may limit the genetic and ecosystem contributions to the retreating shoreline. The long-term implications of such diminishment in genetic and ecological diversity that result from human coastal development may reduce the evolutionary potential of species and environments to increasingly less predictable prevailing conditions (Alejo et al., 2005). Few studies have considered SBEs to act as refugia (Berry et al., 2013). The capacity of sandy beaches to provide the raw materials for inland migration remains equally poorly studied. Identifying refugia represents an important first step in the selection, design, and management of protected areas (Klein et al., 2009; Mackey et al., 2008). However, many habitats that function as refugia are yet to be recognised or protected (Mackey et al., 2002). Whether SBEs can maintain the ability to migrate in the face of rapid urban and coastal development is unknown. This paper explores the constraints on future inland migration of SBEs in a rapidly changing environment. With increasing urbanisation and development of coastal zones, the capacity for sandy beaches to migrate inland has been curtailed (Dugan et al., 2008). The use of hard-engineered structures, such as rock walls, to protect economically high value buildings and infrastructure further restricts the capacity for sandy beaches to retreat in response to SLR. The susceptibility of shoreline attributes to erosion is a critical determinant of the potential for SBEs to migrate and form refugia. Assessing the stability of shoreline attributes can also determine the location of possible migration pathways through which SBEs can migrate. With projected increases in sea level of >1 m by 2 100 (Pfeffer et al., 2008) and >5 m by 2 500 (Jevrejeva et al., 2012), the inclusion of refugia into coastal conservation planning is critical in preserving biodiversity against the impacts of climate change (Keppel et al., 2012; Klein et al., 2009; Loarie et al., 2008; Mackey et al., 2002; Sedell et al., 1990). This is particularly true for sandy beaches. They support threatened ecosystems containing unique species not yet included in conservation plans (Defeo et al., 2009; Schlacher et al., 2007). The purpose of this paper is to (1) identify the extent to which SBEs on the Sunshine Coast, Queensland,
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Australia, have the capacity to adapt to SLR through landward retreat, and (2) identify potential barriers to the landward retreat of SBEs, which may assist coastal managers in the selection, design, and implementation of adaptation options in response to SLR (Alexander et al., 2012). 2. Physical setting This study was conducted in the Sunshine Coast region, of South East Queensland, Australia (see Fig.1). The study location was chosen for being one of the fastest developing coastal regions in Australia (ABS, 2012). With sandy beaches the major attraction, expanding resident and tourist populations are increasing demand for coastal urbanisation and accommodation development. Residential populations are forecast to reach between 439 100 and 516 250 by 2031, an increase of 38.6% and 62.9% (Mason et al., 2012). Tourism contributed $2 324 million to the regional economy in 2010/11 and 8.2 million tourists visited the region in 2012 (Mason et al., 2012). The coast is eastward-facing, exposed, and features wavedominated intermediate sandy beaches (Schlacher et al., 2008). Beaches are discontinuous. Geomorphic features, such as rocky headlands separate many beaches. Large expanses of sandy backshores exist in the form of strand or coastal plains (Ward, 2006). The plains, deposited during higher Holocene sea levels, provide a record of historic beach retreat and progradation in response to fluctuating sea levels (Gharibreza and Motamed, 2006; Hein et al., 2012; Micallef et al., 2013; Rabineau et al., 2006). The Sunshine Coasts sandy beaches and backshore dune systems are formed from quartz sand deposits eroded from granites and sandstones found along the New South Wales coast (Willmott and Stevens, 1988). These sediments are transported 1500 kms north via longshore drift forming extensive beaches and coastal plains in New South Wales and Queensland. Large parabolic dune systems are also formed from this sand in the Cooloola Recreation Area (see Fig. 1) and on sand islands further to the north including Stradbroke and Fraser Islands (Boyd et al., 2004; Tejan-Kella et al., 1990). Human activities impact Sunshine Coast sandy beaches in many ways. For instance, extensive urbanisation of backshore areas, the use of hard- and soft-engineered coastal protection structures, river dredging, beach cleaning, pollution and trampling by pedestrians produce detrimental impacts on these systems. Soft-engineered structures are constructed of materials such as sand that are soft in comparison to rock. In this way adaptation options like beach nourishment are referred to as examples of soft engineering (Berry et al., 2013). Soft materials are used in recognition of the advantages of enabling coastal processes, such as beach erosion and longshore
sand transport, to continue. Soft structures are also more effective at maintaining the aesthetics of sandy beaches for the benefit of tourism. Supporting an abundance of migratory shorebirds, turtle nesting sites and benthic assemblages, sandy beaches of the Sunshine Coast are considered ecologically significant (Ford, 2010; Limpus, 2008; Schlacher et al., 2008). Fig. 1 also illustrates the location of protected areas in the region where state planning laws preclude coastal development. 3. Methods 3.1. Quantifying the capacity for landward migration of SBEs on the Sunshine Coast Google Earth Pro and the Australian Coastal Smartline mapping tool have been used to identify the extent of SBEs on the Sunshine Coast with the capacity to migrate landward as sea levels increase. The capacity for sandy beach retreat is determined by the susceptibility or sensitivity of shoreline attributes to erosion. Often referred to as a coastal sensitivity index, the susceptibility to erosion and, therefore, capacity to retreat can be determined by factors such as erosivity of shoreline material, shoreline slope, the extent of SLR and the level of wave energy experienced (Abuodha and Woodroffe, 2006, 2010). For example, backshores consisting of sand are more susceptible to wave erosion and therefore landward retreat than hard-engineered revetments or bedrock cliffs. Identifying these attributes as a percentage of the total Sunshine Coast shoreline will help to ascertain the capacity for SBEs to retreat. The length of shoreline attributes such as coastal development and infrastructure provide useful indicators of future revetment construction (Hanak and Moreno, 2012). Current Queensland Government legislation in the form of the Sustainable Planning Act (2009) permits the construction of revetments to protect coastal development (see State Development Assessment Provisions, Part C, Module 10 Coastal Protection). The policy allows for revetment construction if erosion presents an immediate threat to property or infrastructure that is not expendable. Under the Coastal Protection and Management Act (1995) expendable property includes picnic tables, barbeques, lookouts, pathways, decks and portable structures. 3.2. Google Earth Pro Using the global mapping tool Google Earth Pro, the length of the following shoreline attributes were measured and expressed as a percentage of total shoreline (Fig. 2):
Fig. 1. Illustrates the location of the Sunshine Coast, Queensland, Australia, and the five protected areas discussed in this paper.
A.J. Berry et al. / Ocean & Coastal Management 102 (2014) 32e39
Sandy Sandy Sandy Sandy Sandy Sandy
beaches; beaches with beaches with beaches with beaches with beaches with
35
bedrock cliff backshores; armoured backshores; developed backshores; undeveloped backshores; protected area backshores.
The scale ruler function in Google Earth Pro is considered an accurate measuring tool to measure distance of sandy shorelines and their attributes (Harris et al., 2011; Waltham and Connolly, 2011). Fig. 3 represents a Google Earth Pro image used in this analysis (Google, 2012). A line map format was used to capture and record the length in kilometres of shoreline attributes at Noosa Heads, a coastal town of the Sunshine Coast region. Different line types signify different attributes. For example, red delineates the total length of the Sunshine Coast shoreline, yellow the length of sandy beaches with backshores that are neither armoured or developed and blue the length of sandy beaches with armoured backshores.
3.3. Smartline approach to assessing shoreline stability To aid in determining the capacity for Sunshine Coast SBEs to migrate landward, the Australian Coastal Smartline mapping tool was also used. Smartline combines data from over 200 prior mapped data sources in making an assessment of Australia's coastal vulnerability to SLR. Data sources include, for example, the Commonwealth Department of Climate Change, Geoscience Australia, Geological Survey of NSW, Australian Beach Safety and Management Program (Short et al., 1993) and Queensland Mines and Energy (Sharples et al., 2009).
Fig. 3. An example of a Google Earth Pro image used in this analysis. Two lines appear on the image of Noosa main beach 26 230 05.4000 S, 153 05011.2400 E, at 162 m elevation. Lines measure the total length of Sunshine Coast shoreline (top red line), the length of sandy beaches with undeveloped backshores (bottom yellow line), and the length of sandy beaches with armoured backshores (bottom blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Smartline differentiates backshore areas into two zones (Fig. 4). The backshore proximal occurs immediately landward of the intertidal zone and on sandy coasts is commonly represented by a foredune. The backshore distal includes the area 500 m landward
Fig. 2. Four examples of Google Earth Pro shoreline attributes at 299 m elevation (Google, 2012): a) sandy beach with armoured backshore in Noosa 26 230 09.5100 S, 153 050 27.0700 E; b) bedrock cliff backshores in Noosa 26 230 37.9900 S, 153 060 58.6400 E; c) sandy beach with unarmoured developed backshore in Sunshine Beach 26 230 52.8900 S, 153 060 49.0500 E; d) sandy beach with undeveloped protected area backshore in Currimundi Lake Conservation Park 26 450 36.5800 S, 153 080 09.6000 E.
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A.J. Berry et al. / Ocean & Coastal Management 102 (2014) 32e39 Table 1 Shoreline stability based on attribute composition and form (composition, form and stability from Sharples et al., 2009).
Fig. 4. Conceptualisation of a shoreline divided into subtidal, intertidal, backshore proximal and distal zones.
from the mean high water mark (MHWM). A distance of 500 m was chosen arbitrarily as the area where shoreline and backshore attributes can be successfully characterised using the line mapping technique (Sharples et al., 2009). Beyond this area other mapping techniques may be more suitable. Each zone is described in terms of its geomorphic form and composition. For example, materials of hard lithified composition, such as bedrock cliffs, suggest low levels of vulnerability to erosion and reduced potential for shoreline retreat in comparison to soft materials (Sharples et al., 2009). For the purpose of this paper, hard-engineered coastal armouring constructed from boulders, concrete or geotextiles are considered as geomorphic features as they impact beach morphodynamics in the same way as structures formed from bedrock. Smartline maps classify the stability and form of the shoreline through an assessment of their geomorphic features (Sharples et al., 2009). The unconsolidated composition of SBEs results in limited stability. SBEs remain highly susceptible to erosion and, therefore, retreat in comparison to areas high in stability, such as bedrock cliffs and hard-engineered revetment walls (Hanson and Lindh, 1993; Levin et al., 2008). Highly stable attributes with high to moderate slopes in either backshore zones are resistant to erosion. These structures are likely to form barriers to retreat and affect the capacity of SBEs to migrate landward in response to advancing seas. Thus, an assessment of shoreline attributes in intertidal and backshore areas enables confirmation of the capacity for SBEs to retreat landward as sea levels increase.
4. Results Analysis of the Sunshine Coast shoreline identified a range of geomorphic attributes located within intertidal and backshore zones. Intertidal attributes include sandy beaches, incipient dunes, foredunes, revetments and bedrock cliffs. Backshore attributes include bedrock cliffs, revetments, and backdunes, low lying coastal plains, and developed and undeveloped areas (Fig. 2). The stability of shoreline attributes were determined using the Smartline mapping tool and are displayed in Table 1 (Sharples et al., 2009). Shoreline attributes are divided into sandy beaches (erosion prone), incipient dunes/foredunes (erosion prone), backdune/coastal plains (erosion prone), rock cliffs (unlikely to erode or retreat over human timescales) and revetments (unlikely to erode or retreat over human timescales when maintained). The lengths of intertidal attributes are detailed in Table 2. Sandy beaches represent 87.02% of the Sunshine Coast's 100.91 kms of intertidal open-ocean shoreline, while 11.86% are bedrock cliffs. River mouths of lotic systems, such as the Noosa, Maroochy and Mooloolah and Pumicestone Inlet make up the remaining 1.24% of the total shoreline length.
Shoreline attribute
Composition
Form
Stability
Sand beaches
Mostly sandy
Gentle slopes
Unstable (prone to erosion and retreat) Incipient dunes/ Mostly sandy Gentle/moderate Unstable (prone to foredunes slopes erosion and retreat) Backdunes/ Mostly sandy Gentle/moderate Unstable (prone to coastal plains slopes erosion and retreat) Rock cliff Hard lithified b High cliffs Stable (robust and edrock Moderate slopes unlikely to erode or retreat over human timescales Moderate slopes Mostly stable Revetment Hard lithified (Unlikely to erode bedrock boulders or retreat if Concrete maintained. Geotextile Susceptible to sand bags progressive slumping and collapse)
The lengths of backshore geomorphic attributes are detailed in Table 3. Hard-engineered adaptation options, such as rip-rap boulder, concrete and geotextile sandbag revetments currently occupy 4.4% of the total Sunshine Coast shoreline. This percentage results from revetments used to protect coastal development including Noosa main beach (0.72 km), Coolum beach (0.2 km), Mudjimba beach (0.1 km), Maroochydore beach (0.78 km) Mooloolaba beach (1.6 km) and Caloundra beach (1.04 km) (only total length in km are shown in Table 3). The addition of bedrock cliffs (2.63%) to revetments represents a combined total of 7.03% of the Sunshine Coast shoreline that is currently prevented from landward migration due to the stable composition and high to moderate form of these shoreline attributes. The remaining shoreline consists of sandy beaches with backshores susceptible to erosion and retreat. 32.39% of backshores are developed, but have unstable intertidal and backshore attributes. Of the undeveloped backshores, 0.97% is not protected from future development, while 46.64% is protected from development by national parks and conservation reserves. The Cooloola Recreation Area, part of the Great Sandy National Park, includes 20.47 kms; the Noosa National Park consists of 1.12 kms; the Maroochy River and Currimundi Lake Conservation Parks consist of 1.31 kms; and the Bribie Island National Park comprises 14.5 kms (only total length km are shown in Table 3). 5. Discussion and conclusions In this paper, we identify SBEs as refugia for the biological and geological elements required for species to persist in and migrate landward in response to sea level rise (SLR). Prior to human intervention, these natural coastal processes continued largely
Table 2 Length and percentage of intertidal shoreline attributes on the Sunshine Coast, including sandy beaches, bedrock cliffs, river openings and Pumicestone inlet. Intertidal geomorphic attributes Sandy beaches Bedrock cliffs River openings and Pumicestone inlet Sunshine Coast shoreline
Length (km) 87.81 11.86 1.24 100.91
Total sandy beaches (%) 100
Total shoreline (%) 87.02 11.75 1.23 100
A.J. Berry et al. / Ocean & Coastal Management 102 (2014) 32e39 Table 3 Length and percentage of backshore attributes on the Sunshine Coast, including bedrock cliffs, coastal armouring, developed and undeveloped areas. Backshore geomorphic attributes Sandy beaches with bedrock cliff backshores Sandy beaches with armoured backshores Sandy beaches with developed backshores Sandy beaches with undeveloped backshores Sandy beaches with protected area backshores
Length (km)
Total sandy beaches (%)
Total shoreline (%)
2.65
3.02
2.63
4.44
5.06
4.40
32.68
37.22
32.39
0.98
1.12
0.97
47.06
53.60
46.64
unimpeded. Coastal management has yet to successfully integrate these dynamic processes in conceptualising past, present, and future coastlines. Instead, managers implement strategies designed to maintain a fixed coastline (Zanuttigh 2011). For the first time, hard-engineered structures in the form of revetments and groynes have been built to protect high value infrastructure adjacent to highly erodible SBEs (Berry et al., 2013). This phenomenon has spread rapidly through the developed world (Cooper and McKenna, 2008b; Titus et al., 2009b) and more recently areas of the developing world. Concurrently, sea levels are predicted to continue increasing this century and beyond (Church et al., 2011; Nicholls, 2011). The cumulative impact of these factors is predictable. To maintain suitable habitat, SBEs require pathways through which they can migrate landward over hundreds and potentially thousands of years in response to fluctuating sea levels. Paleoshoreline research provides abundant evidence that SBEs migrate landwards in response to SLR (Gharibreza and Motamed, 2006; Micallef et al., 2013). Historic records also allow the location of these pathways to be identified (Cann et al., 1999; Lewis et al., 2008). Yet, the capacity of SBEs to act as dynamic refugia that track sea levels, have attracted scant attention (Berry et al., 2013). Coastal development in low-lying areas often neglects to consider the role of retreating beaches in enabling species to persist during SLR (Fish et al., 2008), particularly in response to coastal armouring. Many SBEs are now trapped between hard-engineered structures and advancing seas. Unable to retreat they become squeezed, and without direct intervention will drown. To evaluate the likely impact of coastal armouring on the potential for future landward migration of SBEs due to SLR, we used the Sunshine Coast region of Queensland as an analogue for many rapidly developing, and increasingly urbanised coastal communities in close proximity to SBEs. 5.1. Capacity for SBEs to retreat The capacity for Sunshine Coast SBEs to adapt to SLR by retreating landward is diminishing. In 2013, 7.03% of SBEs in this region were prevented from retreat due to bedrock cliffs and coastal armouring. However, new armouring continues to be incorporated in local coastal management strategies. Current Queensland State Government legislation, as with much of the developed world (Caldwell and Segall, 2007; Titus et al., 2009a), affords erosion protection to non-expendable development through the use of hard-engineered structures (Abel et al., 2011). On the Sunshine Coast 36.79% of the total shoreline has been developed by non-expendable urbanisation and infrastructure. As 4.4% of these shorelines have already been armoured, 32.39% of these developed areas are yet to be armoured. Although setbacks of between 10 and 200 m provide some protection, developments on
37
low-lying and erosion prone coastal plains remain vulnerable to SLR. Further research is required to ensure setbacks can provide adequate buffers to prevent the need for armouring. Setbacks, by themselves will remain insufficient and alternative adaptation options may become necessary. As sea levels increase, coastal development becomes increasingly vulnerable to retreating beaches and the construction of hardengineered structures becomes more likely. If State Government legislation remains unchanged, 32.39% of developed backshores remain likely to become armoured in addition to the 7.03% currently prevented from retreat. A total of 39.42% of the Sunshine Coast's shorelines would therefore be unable to escape SLR. The capacity for squeezed beaches to act as refugia and provide suitable habitat for species to persist becomes critically reduced (Herben and Soderstrom, 1992; Keppel et al., 2012). 5.2. Coastal management implications A window of opportunity exists on the Sunshine Coast, and in other similarly developed coastal regions around the world. The extent of existing development avails coastal managers with the opportunity to incorporate a range of adaptation options, including managed retreat, setbacks and ecosystem engineering into the design and implementation of coastal adaptation plans and strategies (Berry et al., 2013; Fish et al., 2008; Sano et al., 2011). Adopting this approach will integrate opportunities to create conditions conducive to the persistence of dynamic refugia. For example, establishing setbacks to protect backshores that are either undeveloped, sparsely developed, or where infrastructure is expendable, would facilitate the retreat of SBEs without detriment to existing buildings and infrastructure. We hypothesise similar levels of amenity and opportunity exists in other rapidly developing coastal regions. In coastal regions where only small amounts of shoreline are undeveloped a range of adaptation options will need to be applied over the short- and long term (Berry et al., 2013). The protection of sandy beach backshores from development provides a proactive strategy previously proven to be cost-effective in coastal areas, such as the Humber estuary in England (Turner et al., 2007). The potential to conserve and create dynamic refugia provides an additional and necessary element for coastal zone managers as they choose among adaptation options for setbacks or protected areas (Game et al. 2011). Such strategies ensure high priority sandy beaches are selected. However, refugia should be considered in conjunction with other key ecological, social and economic selection criteria. For example, the maintenance of species and ecosystem representation, genetic diversity of keystone species, critical habitats, and longer-term morphological coastal processes represent necessary considerations (Hannah, 2009; Harris et al., 2011; Mackey et al., 2008; Salm et al., 2000) to ensure the resilience of dynamic refugia and the potential for their contribution to future migrations. Compounding the losses of ecosystem resilience, the social and economic cost of constructing revetments represent significant and escalating investments and temporary insurance against rising sea levels (Nicholls, 2011). If coastal managers in this region seek to establish a fixed shoreline an additional 32.68 km of revetment walls will need to be constructed and maintained. Hard-engineered barriers precluding retreat of refugia over such extensive areas of shoreline may prove socially, economically and ecologically unfeasible (McGlashan, 2003). In areas where backshore distals have only been partially or sparsely developed, alternate adaptation options, such as the managed relocation of coastal buildings and infrastructure, provide feasible approaches to maintaining the adaptive capacity of sandy beaches (Abel et al., 2011; Alexander et al., 2012). To what extent the density of coastal development
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excludes the use of managed relocation as an adaptation option is the subject of future investigation by coastal researchers. Where sandy beach backshores are heavily developed or represent a significant investment, maintaining the capacity for sandy beaches to retreat and to form refugia will require a staged approach in the application of adaptation options (Berry et al., 2013). Such an approach includes the design and implementation of managed retreat policies that enable SBEs to retreat over the long-term through the strategic relocation of buildings and infrastructure (Abel et al., 2011; Alexander et al., 2012; McGlashan, 2003). Losses to the future evolutionary potential of SBEs remain poorly quantified because the current status of biological, ecological, and geological aspects of these systems along the coastlines of the world remain understudied. Further research to determine the likely pathways that SBEs retreat through over hundreds and potentially thousands of years will set the context of the challenges faced by coastal and environmental managers. Providing the capacity for SBEs to act as dynamic refugia and to contribute to the conservation and preservation of the genetic and biological diversity represents a new and important additional tool for coastal zone managers. As national parks in Australia preclude development and allow natural processes to continue, SBEs adjacent to these sites retain the capacity to retreat in response to SLR. If coastlines without conservation status become increasingly less able to retreat due to coastal development, the extent that national parks are relied upon to preserve coastal ecosystems will continue to increase. National parks represent a greatly reduced area over which the pool of genetic, ecological, and geological components will be required to migrate as SBEs retreat. The extent to which coastal development restricts the landward retreat of SBEs represents a field of on-going interest for coastal researchers and managers globally.
Acknowledgements Thanks to Geoff Dews and three anonymous reviewers for providing valuable comments on earlier drafts. This paper was developed with salary support from the School of Science, Education, and Engineering at the University of the Sunshine Coast, Queensland, Australia.
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