The influence of the El Niño Southern Oscillation on the entrance regime of ICOLLs

Transcription

1 University of Wollongong Research Online Faculty of Science, Medicine & Health - Honours Theses University of Wollongong Thesis Collections 2014 The influence of the El Niño Southern Oscillation on the entrance regime of ICOLLs Sarah Perry University of Wollongong Follow this and additional works at: Recommended Citation Perry, Sarah, The influence of the El Niño Southern Oscillation on the entrance regime of ICOLLs, Bachelor of Environmental Science (Honours), School of Earth & Environmental Sciences, University of Wollongong, Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au

2 The influence of the El Niño Southern Oscillation on the entrance regime of ICOLLs Abstract Intermittently closed and open lakes and lagoons (ICOLLs) are coastal environments that are prominent features of the NSW coastline. On the south coast ICOLLs are often extensively developed environments, resulting in ICOLL processes being of significance not only ecologically but also for the surrounding community. Consequently ICOLLs are the focus of extensive management, including artificial entrance openings. The natural entrance regime of ICOLLs represents a balance between rainfall, stream-flow and wave processes, which are themselves driven by dominant climatological processes including the El Niño Southern Oscillation. Changes in the entrance regime of ICOLLs due to artificial management and increases in the prevalence and severity of rainfall and storm events due to changes in climate are therefore likely to influence ICOLL processes. With a focus on the entrance processes of a number of ICOLLs on the NSW South Coast, this study explores correlation between the El Niño Southern Oscillation (ENSO), and the entrance regime. The ICOLLs included in this study are located in the Shoalhaven City Council, Eurobodalla Shire Council and Bega Valley Shire Council local government areas. The entrance regime of these ICOLLs comprises both natural and artificial openings over the study period from Long-term in-situ data representing the wave climate, ICOLL water level and catchment rainfall was analysed with respect to the Southern Oscillation Index (SOI) to determine if the entrance condition of the ICOLLs is correlated to the El Niño Southern Oscillation through the application of comparative and statistical methodologies. The results of the analysis show that there is no correlation between the ICOLL entrance condition and the El Niño Southern Oscillation for the study ICOLLs. Although the wave climate and catchment rainfall are correlated to the El Niño Southern Oscillation at some sites, there is no correlation between the wave climate, catchment rainfall and the ICOLL entrance condition. The results indicate that overall the El Niño Southern Oscillation is not a direct influence on the entrance regime of these ICOLLs on the NSW south coast. Degree Type Thesis Degree Name Bachelor of Environmental Science (Honours) Department School of Earth & Environmental Sciences Advisor(s) Colin Woodroffe Keywords ICOLL, coastal lake, ENSO, entrance regime This thesis is available at Research Online:

3 Faculty of Science Medicine and Health School of Earth and Environmental Sciences The influence of the El Niño Southern Oscillation on the entrance regime of ICOLLs Sarah Perry This thesis is presented as part of the requirements for the award of the Degree of Bachelors of Environmental Science Advanced (Honours) of the University of Wollongong October 2014

4 The information in this thesis is entirely the result of investigations conducted by the author, unless otherwise acknowledged, and has not been submitted in part or otherwise for any other degree or qualification. Sarah Perry Cover Photo: Burrill Lake entrance open to the ocean 24 September

5 Abstract Intermittently closed and open lakes and lagoons (ICOLLs) are coastal environments that are prominent features of the NSW coastline. On the south coast ICOLLs are often extensively developed environments, resulting in ICOLL processes being of significance not only ecologically but also for the surrounding community. Consequently ICOLLs are the focus of extensive management, including artificial entrance openings. The natural entrance regime of ICOLLs represents a balance between rainfall, stream-flow and wave processes, which are themselves driven by dominant climatological processes including the El Niño Southern Oscillation. Changes in the entrance regime of ICOLLs due to artificial management and increases in the prevalence and severity of rainfall and storm events due to changes in climate are therefore likely to influence ICOLL processes. With a focus on the entrance processes of a number of ICOLLs on the NSW South Coast, this study explores correlation between the El Niño Southern Oscillation (ENSO), and the entrance regime. The ICOLLs included in this study are located in the Shoalhaven City Council, Eurobodalla Shire Council and Bega Valley Shire Council local government areas. The entrance regime of these ICOLLs comprises both natural and artificial openings over the study period from Long-term in-situ data representing the wave climate, ICOLL water level and catchment rainfall was analysed with respect to the Southern Oscillation Index (SOI) to determine if the entrance condition of the ICOLLs is correlated to the El Niño Southern Oscillation through the application of comparative and statistical methodologies. The results of the analysis show that there is no correlation between the ICOLL entrance condition and the El Niño Southern Oscillation for the study ICOLLs. Although the wave climate and catchment rainfall are correlated to the El Niño Southern Oscillation at some sites, there is no correlation between the wave climate, catchment rainfall and the ICOLL entrance condition. The results indicate that overall the El Niño Southern Oscillation is not a direct influence on the entrance regime of these ICOLLs on the NSW south coast. 3

6 Acknowledgements Thank you firstly to my supervisors Colin Woodroffe and Errol McLean, who provided guidance, feedback and assistance throughout the completion of this project. I appreciate the time and attention that you both gave me. Thank you also to my supervisors from the Office of Environment and Heritage Ray Laine and John Murtagh, for your valuable insights and assistance throughout the year. Additional thanks to Ray for your time in accompanying me on field trips to Newcastle and the ICOLLs for the project. I would also like to thank the Office of Environment and Heritage and the Manly Hydraulics Laboratory for supplying data for use in this project. Thank you to Kerrylee Rogers for your assistance with the statistical analysis, your time and problem solving were very much appreciated. Thank you also to Marijka Batterham from the Statistical Consulting Service for your initial advice on the statistical design of the project. Finally, a big thank you to my family and friends for their support and encouragement! 4

7 Table of Contents Abstract... 3 Acknowledgements... 4 Table of Contents... 5 List of Figures... 6 List of Tables... 7 Chapter One: Introduction Study Context Aim and Objectives Thesis Outline Chapter 2: Literature Review ICOLLs ICOLL Processes ICOLL Management Coastal and Catchment Processes El Niño Southern Oscillation Chapter 3: Regional Setting Site Selection Geology, Climate and Wave-climate of the NSW South Coast Characteristics of study ICOLLs Chapter 4: Methods and Results Datasets Representing ENSO: the Southern Oscillation Index Representing coastal processes: wave statistics and water levels The El Niño Southern Oscillation and Interdecadal Pacific Oscillation ICOLL Entrance Condition Entrance Openings: Natural or Artificial? Rainfall, Wave Climate and Storms Rainfall Wave Climate

8 4.4.3 Storms Statistical Analysis Summary of Results Chapter 5: Discussion Chapter 6: Recommendations and Conclusion References Appendix One Appendix Two List of Figures Figure 1: Panoramic photograph of the Lake Durras entrance (25 September 2014)..9 Figure 2: Schematic of a south coast ICOLL..10 Figure 3: Estuary morphologies in New South Wales (adapted from Roy et al. 2001).13 Figure 4: Evolutionary progression of a barrier estuary (based on Roy 1984 adapted from Hopley 2013) 16 Figure 5: Fish kill at Lake Wollumboola due to low water levels (Stephenson 2011)...18 Figure 6: Artificial entrance openings: training walls at Lake Illawarra Entrance (left) (MAP 2009); Excavation of entrance channel at Burrill Lake (right) (Massie date unknown)..19 Figure 7: Schematic of entrance closure through longshore drift (mechanism 1) and onshore sediment transport processes (mechanism 2) (Ranasinghe and Pattiaratchi 2003)..22 Figure 8: Number of storms occurring on the NSW coast based on 60 year analysis ( ) (Anon 1985 in Short and Trenaman 1992) 23 Figure 9: The three phases of the El Niño Southern Oscillation (BOM 2012)...26 Figure 10: Illustration of the major climatic drivers influencing rainfall variability across Australia (Risbey et al. 2009)..27 Figure 11: Seasonal correlation between rainfall and the El Niño Southern Oscillation across Australia (Risbey et al. 2009)..28 Figure 12: Schematic illustration of the response of ICOLL, including entrance condition, inlet sediment and water level, to rainfall events (Woodroffe 2007)...29 Figure 13: Satellite image of ICOLL locations (left) (Google Earth); Geology of the NSW south coast (right) (Geoscience Australia 2012)..32 6

9 Figure 14: Satellite imagery of each ICOLL (Google Earth).34 Figure 15: Underfloor flooding of waterfront property at Burrill Lake (left) (Spurway et al. 2008); Waterfront tourist accommodation at risk of high water levels at Lake Wallaga (right) (25 September 2014).37 Figure 16: Comparison between two indices of the El Niño Southern Oscillation (NOAA 2005).41 Figure 17: MHL water level gauges at Lake Tabourie (left) and Lake Wallaga (right).42 Figure 18: Monthly SOI (blue), with 5 month moving average (black) and IPO phases (red) (January 1991 December 2013) 43 Figure 19: Water level profile (m) for each ICOLL...45 Figure 20: Indicators of entrance condition inferred from water level curve, illustrated on extract of Durras Lake water level (m) from January 2011 December Figure 21: Entrance constriction illustrated by the M 2 profile (m) (black) for Burrill Lake, Lake Tabourie and Lake Durras...48 Figure 22: Entrance condition of ICOLLs and respective phase of the El Niño Southern Oscillation 50 Figure 23: Proportion of natural and artificial openings for all ICOLLs 51 Figure 24: Long-term monthly mean rainfall for ICOLL catchments 52 Figure 25: Mean monthly rainfall (mm) for each ICOLL catchment over the study period..54 Figure 26: MHL Batemans Bay Waverider Buoy (G) location in respect to study ICOLLs (Google Earth)..55 Figure 27: Mean monthly wave power recorded at Batemans Bay ( )...56 Figure 28: Mean monthly maximum wave height recorded at Batemans Bay ( )..56 Figure 29: Peak wave height for storm events from Batemans Bay ( )...57 Figure 30: Yearly mean rainfall (Lake Tabourie) v yearly mean SOI regression plot with linear fit (P = )...62 List of Tables Table 1: Geological Classification of ICOLLs (Roy et al. 2001)...33 Table 2: ICOLL Characteristics..36 Table 3: Description of Datasets.39 Table 4: Bureau of Meteorology defined El Niño Southern Oscillation phases and corresponding phase of the Interdecadal Pacific Oscillation

10 Table 5: Large rainfall events and corresponding ICOLL opening events.53 Table 6: Mean seasonal wave direction for Batemans Bay ( ).56 Table 7: Storms and ICOLL opening events...58 Table 8: Storms and ICOLL closure events 59 Table 9: Regression analysis for all test variables against the SOI.61 Table 10: Regression analysis for the controlling variables against ICOLL water level 62 Table 11: Regression analysis for the controlling variables against ICOLL entrance condition...63 Table 12: ANOVA testing controlling variables against the ICOLL water level...63 Table 13: ANOVA testing controlling variables against the ICOLL entrance condition...63 Table 14: Historic record of Burrill Lake entrance condition from Burrill Lake EMP (Spurway et al. 2008)

11 Chapter One: Introduction 1.1 Study Context Estuaries are distinct coastal environments that provide considerable environmental, recreational and commercial amenity. Estuaries and estuarine sub-groups including coastal lakes and lagoons are transition environments, ranging from freshwater river systems upstream to the saline ocean downstream. As the junction between fluvial and marine systems the physical, chemical and biological processes in estuaries are complex, dynamic and highly variable (Haines 2008; Carvalho and Fidélis 2013). This variability gives rise to ecological communities that are equally diverse, with environments ranging from terrestrial riparian communities to mangroves and saline marshes (Roy et al. 2001). Estuaries are host to a variety of aquatic and terrestrial species and provide primary breeding grounds for many of these species, including fish and migratory birds. The visual and recreational appeal of coastal lakes and lagoons in addition to their provision of commercial opportunity has led to significant development around these environments in Australia, such that no other country has a concentration of urban affluence on a coastal zone equivalent to that of the southeast coast of Australia (Roy et al. 2001). Development around estuaries is so prevalent in New South Wales (NSW) that Nadgee Lake, with its waterway and catchment entirely enclosed in a National Park, is the only site that remains completely in its natural state (Haines 2008). At all other estuaries in NSW the presence of human activity, especially when this activity is incompatible with natural processes, places pressure on the estuary and its ability to function naturally (Carvalho and Fidélis 2013). The Healthy Rivers Commission (HRC) independent inquiry into coastal lakes (2002) states that healthier lakes can be achieved through adequate management, stating that management should prioritise the protection of natural values, including holistically managing the coastal lake and its catchment. Figure 1: Panoramic photograph of the Lake Durras entrance (25 September 2014). 9

12 An Intermittently Closed and Open Lake or Lagoon (ICOLL) is a specific type of barrier estuary that periodically rather than permanently connects to the ocean at the entrance (Figure 2). There are a large number of ICOLLs in Australia, the majority of which are situated on the southeast coastline of New South Wales (Haines et al. 2006). ICOLLs are also found internationally, with examples occurring in South Africa, New Zealand and Brazil (Haines 2008). The temporal nature of the ICOLLs connection with the ocean is referred to as the entrance regime and is a function of the relative influence of opposing wave and fluvial processes. These processes themselves are influenced by regional and large-scale climatic conditions, including the El Niño Southern Oscillation (ENSO). Figure 2: Schematic of a south coast ICOLL. Morphology of features including the entrance channel, presence of tidal deposits, size and depth of the central lake are dependent on the characteristics of the particular embayment. Due to importance of estuarine environments for human activities and the proximity of development to the water bodies themselves, the entrance regimes of a large proportion of ICOLLs are artificially managed to maintain certain conditions in the estuary, for example to enable tidal exchange and to mitigate floods. Entrance management occurs on a spectrum, ranging from the less permanent mechanical excavation of entrance channels to the construction of training walls engineered to permanently maintain open conditions (as is apparent at Lake Illawarra, for example). Permanently changing the entrance condition of an 10

13 ICOLL fundamentally alters the natural processes and as a result can alter the ecosystems that are present. Disparities between commercial requirements, community beliefs regarding water quality and natural processes, and science-based ecological principles often lead to conflicting views regarding entrance management. There have been numerous recorded cases where community members take management into their own hands by illegally opening ICOLL entrances. Estuaries, as an important coastal environment, have been the focus of numerous studies. These studies, focused on estuaries both in NSW and elsewhere in the world, cover a large range of estuarine processes including geomorphology (see for example Roy et al. 2001; Haines et al. 2005) ecology (Jones and West 2005; Dye and Barros 2005; Scanes et al. 2011), and specific entrance process studies (Ranasinghe and Pattiaratchi 2003; Baldock et al. 2008; Morris and Turner 2008). A number of these studies have highlighted the possible influence that the climate has on ICOLL processes, including predictions for how these processes will change with future climate change (see for example Haines and Thom 2004). The El Niño Southern Oscillation, specifically its influence in the Australasian region, has also been the focus of a large number of studies. The relationship between the El Niño Southern Oscillation and the variability of rainfall (referred to as an ENSO teleconnection) has been well established (see for example Allan 1988; Power et al. 1999; Verdon and Wyatt 2004; Cai et al. 2011), so too has that of other relevant teleconnections including storms (You and Lord 2008) and the wave climate (Phinn and Hastings 1992; Goodwin 2005). There have been few studies however that focus on linking changes in specific coastal phenomena to the phase of the El Niño Southern Oscillation. One example is that of Ranasinghe et al. (2004), who focuses on the changes in wave direction and subsequent beach rotation, identifying a correlation between the direction of rotation and the phase of ENSO. The purpose of this project is to further associate physical coastal processes to the phase of the El Niño Southern Oscillation. This study will examine the relationship between the processes that give rise to the entrance regime of ICOLLs and the broad-scale climate phenomenon the El Niño Southern Oscillation to determine if there is correlation between the entrance condition and the phase of ENSO. The project will build upon previous studies that detail physical ICOLL and estuary processes and connect this information to research that examines how the El Niño Southern Oscillation 11

14 affects rainfall, storm and wave processes to identifying if there are any temporal patterns in the entrance regime that occur with respect to the broad climatic conditions. Ranasinghe and Pattiaratchi (2003) state that understanding the dominant processes that cause the seasonal closure of ICOLL is a prerequisite to the development of sustainable management solutions. 1.2 Aim and Objectives The aim of this study is to determine if the entrance regimes of ICOLLs on the south coast are correlated to the phase of the El Niño Southern Oscillation. This aim will be achieved through the fulfilment of the following objectives: 1. Establish if there is a relationship between the coastal and catchment processes, the phase of ENSO and the entrance condition, through visual analysis of time-series data that is representative of these parameters. 2. Determine if any apparent relationship between the entrance condition and the El Niño Southern Oscillation is significant, through statistically examining (1) correlation between the phase of ENSO and the coastal and catchment processes, and (2) correlation between the coastal and catchment processes and the entrance condition. It has been hypothesised that the increased rainfall and storms associated with the La Niña phase of the El Niño Southern Oscillation will lead to more open entrance conditions observed at ICOLLs, while the reduced rainfall and drought conditions more prevalent during the El Niño phase will give rise to more closed entrance conditions. 1.3 Thesis Outline Previous studies of ICOLL processes, coastal and catchment processes, the El Niño Southern Oscillation and the legislative context regarding ICOLL management are reviewed in Chapter 2. The regional context of the NSW south coast and specific ICOLL characteristics are described in Chapter 3, with the methods used in the study and the results obtained presented in a combined format in Chapter 4. A detailed discussion of the implications of the results including the limitations associated with the study is given in Chapter 5, followed by the subsequent recommendations and conclusion in Chapter 6. Data specific to but not included in the body of the thesis is presented in the Appendices. 12

15 Chapter 2: Literature Review 2.1 ICOLLs An Intermittently Closed and Open Lake or Lagoon (ICOLL) as introduced by Professor Bruce Thom in 1998, is a specific sub-group of wave-dominated barrier estuaries. ICOLLs are connected to the ocean only periodically, often for short periods of time, unlike estuaries that are permanently open. ICOLLs are also referred to as saline coastal lakes (Roy 1984), intermittent estuaries (Roy et al. 2001) and seasonally open tidal inlets (Ranasinghe and Pattaratchi 2003). In geological terms, an estuary can be defined as: the seaward portion of a drowned valley, receiving sediment from both fluvial and marine sources, containing facies influenced by tide, wave and fluvial processes; the estuary extends from landward limit of tidal facies at its head and to the seaward limit of coastal facies at its mouth (Dalrymple et al. 1992). Another definition put forth by the NSW Government in the Estuary Management Manual (1992, p.31) incorporates the intermittent characteristic of ICOLLs, stating an estuary is any semi-enclosed body of water having an open or intermittently open connection with the ocean, in which water levels vary in a predictable, periodic way in response to the ocean tide at the entrance. Figure 3: Estuary morphologies in New South Wales (adapted from Roy et al. 2001). The influence of tides and waves in shaping estuary morphology decreases as the respective influence of rivers increases from a d. Plate c is representative of a typical ICOLL in closed entrance condition. 13

16 ICOLLs represent one of many estuary morphologies evident on the NSW coast (Figure 3). The morphology of an estuary is a function of geological inheritance, boundary conditions including sea level, and the relative influence of wave, tide and fluvial (river) action (Roy 1984; Roy et al. 2001). In the Roy et al. (2001) classification model, estuaries are grouped with respect to these parameters, ranging from tide dominated embayments and drowned river valleys (Figure 3, a), to river dominated estuaries (Figure 3, d). The NSW coast is wavedominated and so it is the wave-dominated estuaries (Figure 3, b and c) that are the most prevalent. Under the same wave regime, the nature of the embayment and the sediment supply determines the estuary-mouth dynamic that will form (Roy et al. 2001). The bedrock valleys along the New South Wales coast have evolved over millions of years, during this time cycles of excavation and infilling have occurred as a result of changes in sea level due to the respective glacial and interglacial periods (Roy et al. 2001). Sediment supply is limited to local sources, as the NSW south coast is highly compartmentalised due to the headlands that extend either side of embayed bedrock compartments into deep water on the coastal shelf (Roy and Stephens 1980). The availability of sediment enables barriers to form. A barrier is an elongate shore parallel sand body consisting of beach dunes, tidal deltas, berms and spits composed of marine sand (Boyd et al. 1992), which protects the environment behind it from the majority of wave energy (Dalrymple et al. 1992; Baldock et al. 2008). The barrier also creates a restricted entrance through which the exchange of water between the ocean and the estuary occurs, and in the case of ICOLLs, this entrance is periodically closed due to the accumulation of sediment in the entrance berm (Figure 3, c) (Haines 2006). Entrance berms are formed through the process of sediment overwash, in which marine sand that is entrained by waves is deposited during wave run-up, accumulating on the beach face and berm crest (Hanslow et al. 2000; Weir et al. 2006; Baldock et al. 2008). Deposition will continue to occur until the berm height is equal to the maximum height of wave run-up, which is dependent on incident wave conditions and the slope of the beach face (Hanslow et al. 2000). Tides alter the rate of growth and determine whether accretion occurs vertically or horizontally (Weir et al. 2006). These berm building processes act to close the entrance of the ICOLL. 14

17 The temporal nature of entrance closures is referred to as the entrance regime. ICOLL entrance regimes are either mostly open or mostly closed; it is rare than an ICOLL will have an entrance regime that is bimodal where the entrance is open for half of the time and closed for the other half (Haines 2006). ICOLLs that are more river-dominated (occurring when the catchment is greater than 100 km 2 in size) and or have entrances that are protected from waves by a headland will be predominantly open, while those with smaller catchments and or exposed entrances will be predominantly closed (Haines 2006). The latter condition is apparent for approximately 70% of ICOLLs on the New South Wales south coast, due to the proximity of the Great Dividing Range catchments in this region are small in size (Haines 2008, p.6). The entrance regime of an ICOLL is dependent on the relative balance between catchment (fluvial) and coastal (waves and tides) processes (Haines and Thom 2007). Coastal processes largely act to close off the entrance through the development and maintenance of the berm. Catchment processes conversely act to open the ICOLL. When the water level within the ICOLL exceeds the height of the entrance berm a breakout occurs in which water overtopping the berm erodes sediment and scours a channel (Haines 2008). Breakouts occur following large rainfall events where catchment inflow is large and occur with a positive feedback dynamic; ongoing scour progressively enlarges the entrance channel allowing more water to discharge from the estuary, which then further enlarges the channel. This will occur until the water level within the lagoon equilibrates with the ocean and tides (Haines 2008). Over time estuaries shallow and decrease in water area due to the progressive infill of sediment at the floodtide delta, fluvial delta and in central basin (Roy 1984). The extent of infill represents the estuaries stage in an evolutionary progression described by Roy (1984) (Figure 4). Holocene sedimentation was initiated following the post-glacial sea level rise approximately 8000 years ago, since then the rate of infill is dependent on site specific topography, sediment supply and relative level of fluvial or marine influence (Roy et al. 2001). Human driven changes such as the clearing of land in the catchment for development or agriculture can increase the rate of sediment supply and increase the rate of estuary infill (Haines 2008). In addition to increasing the rate of infill, increasing the sediment supply especially when the sediment is rich in organic matter and nutrients, can also give rise to water quality issues within the ICOLL, further discussed below. 15

18 Figure 4: Evolutionary progression of a barrier estuary (based on Roy 1984 adapted from Hopley 2013) ICOLL Processes ICOLLs are complex coastal environments; evidence of this is the large array of processes that determine the physical, chemical and biological characteristics of ICOLLs. The ICOLLs morphology, coastal and fluvial characteristics give rise to a number of morphometric parameters including the tidal prism, catchment input (in terms of total runoff, direct rainfall, sediment and pollutant load) and the entrance regime, which in turn influence the hydrodynamic and ecological processes within the ICOLL (Haines et al. 2006). Morphometric parameters are highly variable by nature, leading to fluctuating and highly dynamic conditions within the estuary. It is due to the variable nature of ICOLLs that they are recognised as the most sensitive estuary out of the estuary types in NSW (Haines et al. 2006). 16

19 The intermittent entrance conditions and the tendency for long periods of closure result in ICOLLs having the largest abiotic variability of all estuary types. This subsequently has a constraining effect on biota (Roy et al. 2001). The ecological communities within ICOLLs must be capable of tolerating fluctuations in salinity, nutrients and depth of water. Shallow barrier estuaries are in general well mixed however the total salinity varies due to the relative input of fresh and saline water (Roy et al. 2001). Evaporation from the water surface can reduce the water depth to low levels during prolonged closed periods where little to no catchment input occurs (during drought periods, for example). In extreme conditions this evaporation of large amounts of water can lead to hypersalinity (Haines et al. 2005). Species diversity tends to be low, with euryhaline species occurring in relatively high abundance (Haines et al. 2006). Scanes et al. (2011) conducted a thorough aquatic survey of Nadgee Lake on the far south coast of NSW to establish baseline data that is deemed to be representative of the most natural state. The biota identified at Nadgee Lake included: phytoplankton, zooplankton, 13 species of fish including Australian salmon, sea mullet, bream and garfish, and species of infauna including polychaetes, crustaceans, and molluscs. Aquatic and riparian vegetation is also limited by the abiotic parameters and is restricted to communities that can withstand periodic inundation of saline or brackish water, variable water level and quality (Haines 2006). Communities that are present include mangroves, salt marshes, seagrasses and freshwater wetlands (Haines 2006; Roy et al. 2001). However, the distribution of these species is varied at each ICOLL dependent on local conditions; mangroves are rarely found in ICOLLs that are mostly closed, while seagrasses are limited at ICOLLs that experience rapid changes in water level (Haines 2008). Additionally, an important process in ICOLLs, as in all estuaries, is the cycling of nutrients as part of primary production. Nutrients in sediment or in runoff are sequestered in the bottom sediment or cycled in the water column by bacterial decomposition (Scanes et al. 2007). The release of nutrients from bottom sediment enables phytoplankton and algae to grow on the water surface, providing food for macrophyte species such as fish and prawns (Roy et al. 2001). The maintenance of natural ICOLL processes is essential for the ecosystems to function. 17

20 Figure 5: Fish kill at Lake Wollumboola due to low water levels (Stephenson 2011). ICOLLs are at risk of degradation due to the increasing anthropogenic activity on and around them reducing their capacity to function naturally. The commercial, recreational and scenic amenity offered by ICOLLs has made them attractive to human settlement (Thom 2004) and has resulted in significant residential, commercial and agricultural development in ICOLL catchments and around the lake margins at many sites (Haines 2006). The increased human activity poses a series of risks for the sustainability of ICOLL ecosystems. These impacts include: increased sediment load from land clearing in catchment (Haines 2006; Borrell 2013), algal blooms and eutrophication from excessive nutrient input (Haines 2006), introduction of pollutants including trace metals, organochlorines and acidic groundwater from acid sulphate soils (associated with draining of alluvial plains during development) (Roy et al. 2001). Further impacts relate specifically to the disruption of the ICOLLs entrance regime through the artificial opening of entrances, either periodically through mechanical excavation or permanently through the construction of training walls engineered to maintain an open entrance (as is apparent at Lake Illawarra) (Figure 6). There are a number of motivating factors that lead to the artificial management of ICOLL entrances; foremost is the risk of flooding for low-lying development on the margin of ICOLLs when the entrance is closed and water levels behind the barrier increase (Stephens and Murtagh 2012). Other motivations for opening entrances include alleviating water quality issues and in attempt to enhance marine fish and prawn recruitment (Stephens and Murtagh 2012). Natural, but often unpleasant, odours and the tendency for lake water to be turbid can lead to public pressure to 18

21 open or maintain a permanently open entrance (Haines 2006). Additional pressure to open entrances can arise from community beliefs surrounding the best practice for the ICOLL and or the belief that it will improve surfing conditions (Stephens and Murtagh 2012). In the extreme, community members illegally initiate entrance openings to uphold their ideals about entrance management. Repeatedly opening entrances artificially has a degrading effect on ICOLLs in the long term. This includes altering the structure of ecological communities as the extent of seagrasses, saltmarshes and riparian wetlands is reduced, removing habitat for biota (Jones and West 2005; Haines 2006), and changing the behaviour of the entrance itself as lower water levels have less potential to scour out the entrance channel and as a result entrances rapidly close (Haines 2006). Short term impacts such as mass kills have been observed at ICOLLs following entrance breakouts (Figure 5) (Stephenson 2011). To mitigate impacts, preserve ecosystem integrity and balance conflicting community and land-use perspectives ICOLLs are systematically managed. Figure 6: Artificial entrance openings: training walls at Lake Illawarra Entrance (left) (MAP 2009); Excavation of entrance channel at Burrill Lake (right) (Massie date unknown) ICOLL Management The diverse land-use and development that occurs around ICOLLs, in addition to their value as ecosystems, has led to a series of legislated management policies at all levels of government. ICOLLs are one of the most complex management systems on the coast due to the number of factors that have to be taken into consideration (HRC 2002; Thom 2004; Haines 2008). Specifically, these issues include entrance management, nutrient and sediment load from catchment land use, present ecosystems and their resilience to impacts, and economic and social interests (Thom 2004). Government legislation in Australia has shifted 19

22 in the recent decades to promote ecologically sustainable development (ESD) (Thom 2004). The foundations of ESD are: the conservation of biological diversity and ecological integrity, inter-generational equity, improved valuation, pricing and incentive mechanisms and the application of the precautionary principle. Government policy advocates that these principles are to be used to guide decision-making in all areas that affect the NSW coast (BVSC and ESC 2000). At the state government level there are a number of policies and instruments that govern the management and use of ICOLLs, examples of these policies include (NSW) Coastal Policy 1997, (NSW) Threatened Species and Conservation Act 1995 and the Fisheries and Management Act Development around ICOLLs is restricted based on land zoning in Local Environment Plans (LEPs) and State Environmental Planning Policies (SEPPs) (Haines 2006). Specific to the management of ICOLLs is the (NSW) Estuary Management Policy, which is contained in the (NSW) Coastal Policy 1997 legislation. The objectives of the Policy are: The protection of estuarine habitats and ecosystems in the long-term, including the maintenance of the hydraulic regime of each estuary The preparation and implementation of a balanced long-term management plan for the sustainable use of each estuary and its catchment, defining strategies for: o Conservation of aquatic and other wildlife habitats o Conservation of aesthetic values of estuaries and wetlands o Prevention of further estuary degradation o Repair of damage to the estuarine environment o Sustainable use of estuarine resources, including commercial uses and recreational uses as appropriate (BVSC and ESC 2000) Estuary Management Plans (EMPs) are the practical way in which the goals of the Estuary Management Policy are enforced. Local governments form Estuary Management Committees that design and implement EMPs based on the requirements of each estuary and the concerns of the local community. The requirements of each estuary, such as threatened species, breeding grounds, anthropogenic impacts and or water quality concerns are identified in Estuary Process Studies or Review of Environmental Factors reports. As water quality is a major concern for a number of estuaries, EMPs include water quality objectives (WQOs) and 20

23 river flow objectives (RFOs) in compliance with the Australian Water Quality Guidelines for Fresh and Marine Water (Australian and New Zealand Environment Conservation Council, 1992) and subsequent guidelines set by the NSW Environment Protection Agency (EPA). The final EMP consists of management strategies and a schedule of activities (such as remediation tasks) to be undertaken in order to achieve the objectives (BVSC and ESC 2000; Haines 2008). Haines (2008, p.24) asserts that a formal EMP does not ensure that an ICOLL is managed effectively, nor that the goals of the EMP are given due consideration when development applications are assessed by authorities. Furthermore, the adequate formulation and implementation of EMPs are dependent on funding and resource constraints (Haines 2008). The guidelines for entrance management including the thresholds for initiating an artificial entrance opening are detailed in the relevant ICOLLs EMP. Where artificial openings are undertaken for the purposes of flood mitigation a maximum water level is given in meters Australian Height Datum (AHD). This trigger value represents the maximum height that the water level within the ICOLL can reach before encroaching on low-lying development, infrastructure or services (including sewage and septic systems) and is specific to each ICOLL and its surrounding environment. Approximately 50% of all ICOLLs in NSW are artificially managed for flood mitigation purposes (Haines 2008, p.6). The management of ICOLL entrances, indeed ICOLLs in general, should be conducted with as much consideration as possible to natural processes to promote and maintain the natural entrance regime and associated flow-on ecological and physical conditions. 2.2 Coastal and Catchment Processes The coastline of NSW is wave-dominated; the coast is subject to physical processes that are dominated by wave energy rather than those that are dominated by tide energy, for example (Davis and Hayes 1984). Waves are a principal source of energy for erosion and deposition, and are responsible the movement of sediment along coasts. Waves striking the coastline obliquely result in the longshore transport of sand, which acts to form barriers across coastal inlets and embayments (Haines 2006). Ranasinghe and Pattiaratchi (2003) hypothesise that the formation of barriers occurs by two mechanisms (Figure 7). The first mechanism is that of longshore sediment transport mentioned by Haines (2006), in which waves entrain sediment and transport it along the beach in the direction of the wave current (drift-aligned). 21

24 At an estuary entrance, a shoal forms up-drift resulting in the formation of a spit across the entrance. The size and rate of growth of the spit is dependent on the intensity of the longshore drift, eventually the spit will prograde and close off the entrance when the out-flowing current from the estuary is not strong enough to erode and maintain the channel. In the second mechanism, hypothesised by Ranasinghe and Pattiaratchi (2003), longshore transport rates of sediment are small, instead onshore sediment transport occurs due to swell waves breaking parallel to the shore (swash-aligned). Sand that has been previously eroded from the beach during storms and stored offshore is transported back onto the beach, choking the entrance and forming an entrance berm. This second mechanism is more likely to occur on beaches where near-normal wave incidence is apparent. The mechanisms for berm formation illustrate the importance of the incident directions of waves that make up the wave-climate. Figure 7: Schematic of entrance closure through longshore drift (mechanism 1) and onshore sediment transport processes (mechanism 2) (Ranasinghe and Pattiaratchi 2003). The wave climate of NSW is energetic and highly variable, consisting of a moderate east coast swell that is amplified by storm wave generation originating in the Coral and Tasman Seas (Short and Trenaman 1992). Short and Trenaman (1992) undertook a comprehensive analysis of the wave climate of Sydney NSW, identifying a number of seasonal trends apparent over the twenty-year study period. The east coast swell is predominant, with 42% of waves arriving from an easterly direction annually, with peak incidence in March and June. 22

25 Northeast swells occur during summer; 17% of annual waves are from the northeast while waves from the southeast occur during winter, peaking in August. The energy associated with waves also changes seasonally, with wave-power varying in accordance to changes in incident direction: wave power increases from January to June, before decreasing to the annual minimum in December. These temporal changes in wave climate vary in accordance with the meteorological systems that are responsible for generating waves of certain directions. A subtropical anticyclone high-pressure system has a dominant influence on the climate of Sydney; this system varies in latitude seasonally and its position is associated with the presence of mid-latitude cyclones in the Tasman Sea, which in turn influences deepwater wave power (Goodwin 2005). Other meteorological systems of relevance are tropical and east coast cyclones (Short and Trenaman 1992). Each system generates distinctly different wave conditions, as identified by Phinn and Hastings (1992). Tropical cyclones occur in summer, originating in the Coral Sea before tracking south along the Queensland (QLD) and NSW coasts. High energy waves associated with tropical cyclones originate from the north and northeast, accounting for the peak in wave power early in the calendar year. Strong winds, rainfall and large waves from the Tasman Sea are associated with east coast cyclones (east coast low pressure systems). These systems are most prevalent during April to September and contribute to the high degree of variability, producing waves from a southeast origin (Short and Trenaman 1992). A: Anticyclones SS: Southern Lows CL: Continental Lows IT: Inland Lows EL: East Coast Lows TC: Tropical Cyclones The relative influence of each synoptic system in determining the wave climate illustrates that the coastal ocean off NSW and southeast Australia in general is dominated by storm activity (Figure 8) (Short and Trenaman 1992; Roy et al. 2001). Storms have three main effects on ICOLL entrance condition: (1) storms with rainfall increase catchment input into ICOLLs and can lead to entrance openings; (2) large and high energy Figure 8: Number of storms occurring on the NSW coast based on 60 year analysis ( ) (Anon 1985 in Short and Trenaman 1992). storm waves can erode sediment from entrances and beaches; and (3) large 23

26 storm surges can inundate entrances with sediment and associated debris. Rainfall is associated with a number of storm systems including east coast cyclones (Short and Trenaman 1992), and when sufficiently strong, the opening effect of rainfall can overcome the closing effect of storm waves and lead to natural entrance breakouts (Ranasinghe and Pattiaratchi 2003). Where the ICOLL entrance is already open, erosion by swell and storm waves contributes to the scouring of the entrance channel, acting to prolong the duration of time the ICOLL is open (Ranasinghe and Pattiaratchi 2003). Large, high power storm waves can breach entrances in the form of surges during which these large waves deposit sediment and other associated storm wrack, which chokes the entrance (Roy et al. 2001). The morphology of ICOLL entrances (as with all beach landforms) is influenced by the dominant storm and wave climate, as unconsolidated sediment along the coast constantly readjusts respective to wave direction and power (Harley et al. 2010). As discussed above, the exposure to waves of certain incidences will influence the mechanism of barrier formation (Ranasinghe and Pattiaratchi 2003). The location of the ICOLL entrance with respect to the embayment will influence the degree to which certain wave directions affect the entrance. Sediment in beaches undergoes the phenomenon of rotation due to changes in the direction of waves (Ranasinghe et al. 2004). Rotation occurs where sediment that has been eroded from one end of the compartment is deposited at the other end, resulting in net accretion and widening at that end. Changes in wave incidence reverse the direction of sediment transport, eroding from the previously accreting end of the compartment and depositing the sediment on the previously eroded end (Ranasinghe et al. 2004). Both mechanisms of sediment transport contribute to rotation, with two-thirds the result of onshore sediment transport and the remaining third attributed to longshore sediment transport (Harley et al. 2011). How much waves of a certain incident direction affect the entrance conditions of the ICOLL depends on the direction that the beach faces and the location of the entrance along the beach. ICOLLs that are on the accreting end of the coastal compartment will be inundated with sediment and as the beach widens (Haines and Thom 2007). Conversely, at the eroding end there is reduced potential for sediment ingress and deposition in the entrance, increasing the potential for more open entrance conditions (Haines and Thom 2007). Rotation processes have been linked to broad climate systems including the El Niño Southern Oscillation (Ranasinghe et al. 2004; Short et al. 2000; Haines and Thom 2007). This is largely attributed to the inter-annual variation in the 24

27 wave climate, which is also linked to phase changes in the El Niño Southern Oscillation (Phinn and Hastings 1992; Harley et al. 2011) (discussed further below). 2.3 El Niño Southern Oscillation The El Niño Southern Oscillation (ENSO) is a broad-scale atmospheric circulation phenomenon that occurs across the Pacific Ocean. Atmospheric circulation occurs on a global scale, with a number of regional patterns that drive ocean currents, local winds, monsoons and rainfall together influencing regional climate (Allan 1988; BOM 2014). The El Niño Southern Oscillation consists of the temporal changes in the direction and intensity of the Walker Circulation, which is modulated by changes in sea surface temperature (SST) (Allan 1988). Initial research into the El Niño Southern Oscillation described a see-saw effect in atmospheric pressure, apparent between the eastern and western regions of the Pacific Ocean, with an oscillation occurring every 2 7 years (Bjerknes 1969 in Allan 1988). The oscillation effect is dependent on coupled interactions between the atmosphere and the ocean, with each variable being strongly influenced by the boundary conditions imposed by the other (Neelin et al. 1988). The El Niño Southern Oscillation is responsible for driving many interrelated atmospheric and oceanic parameters including precipitation, pressure and temperature (Allan 1988). Termed teleconnections by Bjerknes in 1969 there are apparent correlations between the phase of the El Niño Southern Oscillation and regional weather patterns including rainfall and storm activity (Diaz et al. 2001; You and Lord 2008). These factors influence other environmental processes such as wave climate and subsequent patterns of erosion and deposition (as discussed above). There are three phases to the El Niño Southern Oscillation: the El Niño phase, the La Niña phase and the neutral phase. In the neutral phase, the typical conditions of the Walker Circulation are apparent, in which warm ocean temperatures cause warm air to rise in the western Pacific (eastern Australia) and circulate east across the Pacific Ocean before descending over the cooler ocean in the east (Figure 9). In the Walker Circulation, the trade winds bring warm moist air that rises above northern Australia and Indonesia creating the dominant low-pressure systems associated with rainfall (BOM 2014). In the La Niña phase the Walker Circulation and trade winds are intensified. The ocean thermocline is higher in the eastern Pacific Ocean resulting the upwelling of cool, deep ocean waters that confine warm water to the western Pacific Ocean. These warm waters in addition to the strengthened trade 25

28 Figure 9: The three phases of the El Niño Southern Oscillation (BOM 2012). 26

29 winds result in higher than average rainfall across northern Australia and monsoon conditions in Indonesia (BOM 2012). In the El Niño phase the reverse is apparent: the Walker Circulation and trade winds are weakened or reversed depending on the intensity of the El Niño event, causing the thermocline to deepen in the central and eastern Pacific. Cooler ocean temperatures that result in the western Pacific Ocean combined with the absence of the moisture-laden trade winds result in below average rainfall and drought conditions over Australia (Figure 9) (BOM 2012). Figure 10: Illustration of the major climatic drivers influencing rainfall variability across Australia (Risbey et al. 2009). The variability in rainfall across Australia and the association with the El Niño Southern Oscillation has been widely studied (see for example Power et al. 1999; Risbey et al. 2009; Cai et al. 2010). The strength of ENSOs influence on rainfall variability is varied across different seasons and regions of Australia (Risbey et al. 2009). ENSO is also not the only climate variable that is important in determining rainfall patterns in Australia: Risbey et al. (2009) studied the respective level of influence of ENSO, the Southern Annular Mode (SAM), atmospheric blocking, and the Indian Ocean Dipole (IOD) in contributing to rainfall variability (Figure 10). In addition to ENSO, the SAM and atmospheric blocking contribute to rainfall variability on the southeast coast of Australia; however these drivers are all interdependent, with correlations between each of the drivers apparent. The influence of 27

30 ENSO on rainfall variability in the southeast of Australia is strongest during the winter and spring months, however for the coast of NSW specifically there is no correlation apparent during the winter months, indicating the importance of other climate drivers in influencing rainfall patterns at that time (Figure 11) (Risbey et al. 2009). Figure 11: Seasonal correlation between rainfall and the El Niño Southern Oscillation across Australia. Correlations are significant at the 95% level, from data spanning (Risbey et al. 2009). Seasonally, the variability of rainfall and other climate variables is linked to the El Niño Southern Oscillation, broadly however the phase and associated strength of ENSO is influenced by longer-term climate regimes that vary on decadal scales, including the Interdecadal Pacific Oscillation (IPO). The IPO is a similar phenomenon to ENSO however it is principally the result of changes in sea surface temperate confined to the equatorial belt and extra-tropical North Pacific Ocean. When the IPO is in the positive phase SST anomalies over the North Pacific Ocean are negative, as are anomalies in the South Pacific, while SST in the equatorial Pacific are positive (Salinger et al. 2001). In the negative phase the reverse relationship is apparent. The IPO and ENSO operate on different time scales, however the similarities between the two climate systems suggest that the IPO has a modulating effect on 28

31 ENSO and ENSO teleconnections (Salinger et al. 2001; Cai et al. 2010). In a study focusing on southeast QLD, Cai et al. (2010) show that the decline in summer rainfall since 1980 is consistent with the shift in the IPO phase, reinforcing the notion that the ENSO-rainfall relationship is sensitive to the interdecadal condition. Furthermore, correlations between the IPO and ENSO suggest that the positive phase of the IPO is associated with enhanced and more frequent El Niño events (Salinger et al. 2001). Figure 12: Schematic illustration of the response of ICOLL, including entrance condition, inlet sediment and water level, to rainfall events (Woodroffe 2007). Rainfall variability represents only aspect of how the El Niño Southern Oscillation affects ICOLL processes. Changes in the wave climate and subsequent sediment transport regimes including beach rotation and barrier formation has been shown to be largely influenced by storm activity (as discussed above). Correlation between storms and the El Niño Southern Oscillation has been identified, in terms of both frequency and severity (see Phinn and Hastings 1992; Short et al. 2000; You and Lord 2008). Phinn and Hastings (1992) studied the frequency of tropical cyclone activity based on the foundation work by Nicholls (1979, 1984, 1985) who identified correlations between tropical cyclone and activity and ENSO, concluding that tropical cyclones are more active during La Niña. Phinn and Hastings hypothesise that the increased prevalence of tropical cyclones, including their tendency to 29

32 track further south, will result in prolonged conditions of high-energy cyclone generated waves from the arriving from northeast to the south coast. These high-energy conditions increase the potential for erosion. Their conclusion is supported by other studies: for example, Ranasinghe et al. (2004) state that the frequency of storms in La Niña compared to El Niño is approximately double. In addition, You and Lord (2008) reach the same conclusion, further stating that La Niña storms are more severe. The altered wave climate accounts for oscillating periods of erosion and accretion on beaches, affecting the sediment transport regimes that are present. During La Niña phases the prevalence of waves of northeast direction cause erosion of the northern end of beaches, transporting entrained sediment to the southern end. In El Niño phases, where the incident direction of waves is mostly from the south and southeast due to east coast cyclones and low-pressure systems, the opposite effect is apparent, with the southern end of a compartment eroding while the northern end accretes (Ranasinghe et al. 2004; Short et al. 2000). ICOLLs with entrances positioned at the northern entrance of beaches will be more likely to experience erosion of the entrance berm and barrier during La Niña phase, while ICOLLs with entrances at the southern end of beaches will have increased sediment influx (Figure 12). The reverse trend will be apparent during El Niño phases. 30

33 Chapter 3: Regional Setting 3.1 Site Selection ICOLLs on the south coast of New South Wales that are of interest to the NSW Office of Environment and Heritage were selected to be included in this study. The distribution of ICOLLs in NSW is especially concentrated on the south coast between Wollongong and Victoria (Haines 2008). Out of the large number of ICOLLs in NSW 6 sites were selected to be included in this study. These 6 ICOLLs fulfil the following requirements: The entrance condition of the ICOLL has exhibited both open and closed conditions throughout the study period. Sites may exhibit mostly closed or mostly open tendencies (as it is rare that an ICOLL will be bimodal, as previously discussed). The entrance condition may have been altered by artificial openings, however the entrance must not be permanently opened by engineering structures such as training walls as effectively the estuary is no longer exhibiting the fundamental entrance characteristics of an ICOLL. The ICOLL entrances have different orientations, positions on the beach and / or protection by headlands. This will allow for analysis into the affect that these variables have in determining the ICOLLs response to coastal processes including major storms and associated large waves of different incident directions. The Manly Hydraulics Laboratory captures data for the ICOLL. Water level data provided by MHL from the ICOLL stations is the basis of determining entrance condition and is therefore a necessary requirement. A Bureau of Meteorology rainfall station is within close proximity to the ICOLL. Lake Wollumboola, Swan Lake, Burrill Lake, Lake Tabourie, Durras Lake and Wallaga Lake were the 6 sites selected for inclusion in this study. Two additional ICOLLs, Meroo Lake and Lake Nadgee, were proposed for inclusion as reference ICOLLs as their entrances largely exhibit only natural conditions, however the absence of water level data at these sites excludes them from this study. 31

34 3.2 Geology, Climate and Wave-climate of the NSW South Coast The ICOLLs included in this study (hereafter referred to as the ICOLLs) are spread along the south coast of NSW. The northernmost ICOLL Lake Wollumboola (Figure 13, site A) is situated just north of Jervis Bay, approximately 185 km south of Sydney NSW, while the southernmost ICOLL Lake Wallaga is situated just north of Bermagui on the far south coast 470 km south of Sydney (Figure 13, site F). Geologically, this area of NSW consists of the southernmost portion of the Sydney Basin unit and the igneous units of the Lachlan Orogen to the south (Figure 13) (Geoscience Australia 2012). Jervis Bay Ps Mzg Os Dd EOw Batemans Bay Bermagui Legend: (a) Wollumboola (b) Swan (c) Burrill (d) Tabourie (e) Durras (f) Wallaga Dg Kg Geological Units: Ps Permian Sedimentary Mzg Mesozoic Felsic Os Ordovician Intermediate Dd Devonian Mafic EOw Palaeozoic mixed origin Sedimentary Dg Devonian Felsic Kg Cretaceous Felsic Figure 13: Satellite image of ICOLL locations (left) (Google Earth); Geology of the NSW south coast (right) (Geoscience Australia 2012). 32

35 The geologically-based classification by Roy et al. (2001) of each ICOLL is listed in Table 1. The ICOLLs fall into two categories: intermittently closed saline coastal lagoons or wavedominated barrier estuaries. The sites exhibit different levels of sediment infill and therefore different evolutionary maturity; Lake Tabourie is the most evolutionary mature out of the ICOLLs, grouped in the semi-mature phase (see Figure 4). Table 1: Geological Classification of ICOLLs (Roy et al. 2001) ICOLL Group Type Evolution Lake Wollumboola IV: intermittently closed Saline coastal lagoon Intermediate Swan Lake IV: intermittently closed Saline coastal lagoon Youthful Burrill Lake III: wave dominated Barrier estuary Youthful Lake Tabourie IV: intermittently closed Saline coastal lagoon Semi-mature Durras Lake IV: intermittently closed Saline coastal lagoon Intermediate Wallaga Lake III: wave dominated Barrier estuary Intermediate In broad terms, the climate of the NSW south coast is classified as warm-temperate ranging to cool-temperate nearer to the Victorian border (Roy et al. 2001). The average annual rainfall (based on the 30-yr period from ) for the NSW south coast is approximately 1000 mm, decreasing to approximately 800 mm on the far south coast (BOM 2011). Trends in rainfall specific to the catchment of the individual ICOLLs are identified in the analysis (see Chapter Four). The NSW south coast is wave dominated with a predominant east southeast swell. The wave climate for the south coast is much the same as that of Sydney, and therefore the attributes of the wave climate identified for Sydney in the literature including storm frequency, incident direction of waves and the influence of the El Niño Southern Oscillation will have a similar effect on the south coast (see Chapter Two) (Short and Trenaman 1992; Phinn and Hastings 1992; You and Lord 2008). The influence of the wave climate on the ICOLLs is dependent on the exposure of the entrance and the location with respect to the coastal embayment (discussed below). 33

36 3.3 Characteristics of study ICOLLs (a) Lake Wollumboola (b) Swan Lake (c) Burrill Lake (d) Lake Tabourie (e) Lake Durras (f) Wallaga Lake Figure 14: Satellite imagery of each ICOLL. Each ICOLL exhibits a different size, shape and entrance channel morphology. Additionally, the position of the entrance within the embayment and orientation of the embayment is varied across the sites (Google Earth). The ICOLLs all exhibit a range of entrance morphologies, including orientation, channel length and width and the location of the entrance with respect to the embayment (Figure 14). Lake Wollumboola is unique among the study ICOLLs for two reasons. First, the entrance is relatively wide with the main lake separated from the ocean only by the sand barrier and 34

37 berm. The other ICOLLs exhibit narrow, often long, entrance channels that separate the main lakes from the respective barriers. Second, Lake Wollumboola is the only ICOLL with a coastline that is orientated north south, with the beach (and therefore entrance) facing directly east. The embayments in which Swan Lake and Durras Lake are located are angled towards the southeast, while the remaining ICOLLs are oriented east-southeast. The orientation of the embayment affects the extent that the entrance (and beach as a whole) will be impacted by waves from certain directions (Haines et al. 2006). Furthermore, the position of the entrance along the beach with respect to headlands also shelters the entrances from waves of certain directions. The entrance at Lake Durras is very protected from waves from the north by the large headland, while the entrance at Lake Wollumboola receives minimal protection from the smaller headland (Figure 14, e and a). Lake Durras and Lake Wollumboola are the only two entrances located at the northern end of the embayments; the remaining ICOLLs are positioned towards the southern end of their respective beaches, receiving minimal protection from the smaller headlands. The entrance at Lake Tabourie is however protected from waves of easterly incidence by a rocky island and adjoining tombolo, situated directly east of the entrance (Figure 14, d). Similarly, offshore and near-shore rocky reefs shelter the entrance at Swan Lake (Figure 14, b) (Spurway et al. 2004). In addition to the entrance and embayment characteristics, each ICOLL also has a distinct set of physical characteristics including waterway area and depth and the size of the catchment, which are listed in Table 2. The morphological characteristics of each site give rise to the hydrological, chemical and ecological properties of the ICOLL (Haines et al. 2006), which are all outlined in the respective Estuary Management Plans that are prepared for each ICOLL (as discussed above). The EMPs for the study ICOLLs identify a number of endangered ecological communities (EECs) that are present at some or all of the ICOLLs including Bangalay Sand Forest, Coastal Saltmarsh, Swamp Oak Forest, and Swamp Sclerophyll Forest. In addition to EECs, some of the ICOLLs are breeding grounds for a number of vulnerable and endangered sea- and shorebird species including the Little Tern, Pied Oystercatcher and Hooded Plover. At Swan Lake endangered amphibian species, specifically the Green and Golden Bellfrogs have also been sighted (Spurway et al. 2004). Other ecological communities present at the ICOLLs include varied species of fish (both commercial and not commercially valuable), prawns, oysters and seagrasses. Invasive species including the algae Caulerpa taxifolia have also been identified at some of the ICOLLs, notably Burrill Lake (Spurway et al. 2008). The ICOLL ecosystems are at risk from 35

38 anthropogenic activities; Spurway et al. (2008) state in the Burrill Lake EMP that pollution and other impacts from increased urban activity on the estuary fringe pose a greater risk to the EECs than altered hydrological regime from changing entrance condition. Table 2: ICOLL Characteristics ICOLL Location Catchment Area (km 2 ) Wollumboola S E Swan S E Burrill S E Tabourie S E Durras S E Wallaga S Estuary Area (km 2 ) Average Depth (m) Maximum water level* (m AHD) E NB: Table adapted from OEH (2012, a f); *Maximum water level at which the entrance is opened artificially to mitigate flood hazard to low-lying assets. The extent to which anthropogenic activity impacts the ICOLLs is dependent on the concentration of urban and agricultural development within the ICOLLs catchment. Visible in Figure 14, the level and nature of development around the fringe of the ICOLL and the entrance is different at each site. The catchment of Durras Lake is the least developed, as the majority of the catchment is enclosed in the Durras Lake National Park, development is limited to tourist accommodation located on the northern side of the entrance channel (Figure 14, d). Areas of the catchments for Lake Wollumboola, Swan Lake, Lake Tabourie and Wallaga Lake are also enclosed in National Parks, with an additional portion of the Wallaga Lake in use as State Forest. Burrill Lake shows the highest concentration of urban development around the ICOLL, specifically around the entrance channel and floodplain (Figure 14, c). Low-lying development is at risk of inundation when water levels within the lake increase under closed entrance conditions; to prevent damage from occurring the critical height before inundation is ascertained through specific flood studies and analysis of previous flood events (Figure 15). If the water level of the ICOLL reaches the critical height, Council or another relevant local authority will conduct artificial entrance openings to lower the water level. Lake Wollumboola, Swan Lake, Burrill Lake and Lake Tabourie are all managed by 36

39 the Shoalhaven City Council, Lake Durras by the Eurobodalla Shire Council, who also jointly manages Lake Wallaga with the Bega Valley Shire Council. The trigger water level for each of the study ICOLLs is given in Table 2. The tendency for the water level to reach this level dictates how frequently the entrance is opened artificially for flood mitigation purposes, for example, Lake Tabourie frequently reaches the trigger water level and is regularly opened to ensure that low-lying development (specifically the septic tanks of residential properties located on the entrance channel margin) is not inundated. The frequency of artificial entrance openings at the study ICOLLs is analysed in this report (see below). Figure 15: Underfloor flooding of waterfront property at Burrill Lake (left) (Spurway et al. 2008); Waterfront tourist accommodation at risk of high water levels at Lake Wallaga (right) (25 September 2014). 37

40 Chapter 4: Methods and Results Two approaches were used to determine if there is correlation between the ICOLL entrance regime and the El Niño Southern Oscillation. First, data analysis utilising graphing techniques was undertaken to identify the relationship between the ICOLL entrance condition and ENSO. This was achieved by examining the relationship between the ICOLL and the controlling coastal and catchment variables, and in turn the relationship between those variables and ENSO. Second, based on these findings, a number of statistical tests were run to determine if the correlations between the ICOLL entrance regime, the forcing variables and ENSO are significant. The methodology applied and the results are presented in this chapter in a combined format. 4.1 Datasets Time-series datasets that are representative of ICOLL entrance condition, catchment and coastal processes and the El Niño Southern Oscillation were used for this analysis. A summary of these datasets is provided in Table 3 followed by a description of the data capture methods for ENSO and the coastal parameters. The data was acquired for the purposes of this project from multiple government sources including the NSW Office of Environment and Heritage, the Manly Hydraulics Laboratory (MHL) and the Australian Bureau of Meteorology. The temporal coverage of each dataset is different: the record for ENSO is accessible from 1876, whereas the in-situ water level recorders for the ICOLLs are much more recent installations, therefore the availability of water level data became the limiting factor on the time period analysed for each ICOLL. The water level record for Lake Wollumboola, Burrill Lake, Lake Tabourie and Wallaga Lake span approximately 20 years from the early 1990 s, however as the stations for Swan Lake and Lake Durras were only installed in 2000 the records at these sites are considerably shorter. These records, as well as those for rainfall, are incomplete and any absent data was left as an empty cell rather than a zero value during pre-processing. Pre-processing of the data in Microsoft Excel entailed adjusting the data for these absent entries to preserve the time-series as well as the creation of monthly, yearly and long-term means for parameters including water level, maximum wave height and rainfall. 38

41 Table 3: Description of Datasets Dataset (units) Purpose Coverage Source Southern Oscillation Index Representative of El Niño Southern 1876 present Australian Bureau of Meteorology Oscillation Interdecadal Pacific Phases of the IPO Power et al.(1999) Oscillation Water Level (m) Entrance condition Lake Wollumboola: Swan Lake: Burrill Lake: Lake Tabourie: Lake Durras: Wallaga Lake: M 2 Tidal Constituent (m) Rainfall (mm) Wave Data H sig (m) H max (m) Direction (degrees) T P1 (seconds) T sig (seconds) Power (kw/m) Entrance condition Burrill Lake: Lake Tabourie: Lake Durras: Surrogate for catchment flows Lake Wollumboola: Swan Lake: Burrill Lake: Lake Tabourie: Lake Durras: Wallaga Lake: Dai (2013) Manly Hydraulics Laboratory Manly Hydraulics Laboratory Australian Bureau of Meteorology Coastal processes 1986 present Manly Hydraulics Laboratory Aerial Photography Entrance condition Lake Wollumboola: Swan Lake: Burrill Lake: Lake Tabourie: Lake Durras: Wallaga Lake: NSW Office of Environment and Heritage NB: Data for Wallaga Lake is incomplete due to time constraints the complete data series could not be accessed and included in the analysis for this reason Wallaga Lake analysis is limited to the 14 year period. 39

42 4.1.1 Representing ENSO: the Southern Oscillation Index The Southern Oscillation Index (SOI) provides an indication of the development and intensity of El Niño and La Niña phases of the El Niño Southern Oscillation. The SOI is derived from the difference in atmospheric pressure observations recorded at Darwin in the Northern Territory of Australia and Tahiti in the South Pacific. The index is positive when surface pressure is low over Australasia and high over the southeastern Pacific (La Niña) and negative when the reverse relationship is apparent (El Niño) (Allan 1988). The Bureau of Meteorology (from whom the data was sourced) calculates the SOI using the formula established by Troup (1965) as follows: SOI = 10 (Pdiff Pdiffav) SD (Pdiff) Equation 1. Where: Pdiff = mean monthly pressure Tahiti the mean monthly pressure Darwin Pdiffav = long term mean Pdiff for the month, and SD (Pdiff) = long term standard deviation of Pdiff for the month (BOM 2012) SOI values calculated in this way are referred to as Troup SOI data to distinguish from other indices. The Bureau of Meteorology measure and derive the SOI daily, however these values fluctuate significantly due to changes in atmospheric pressure in response to short-term weather patterns. To illustrate long-term climate variability, the SOI is presented as a monthly mean value that is plotted with a 5-month moving average trendline. Sustained positive SOI values greater than 7 are indicative of a La Niña phase while sustained negative values less than -7 indicate an El Niño phase (BOM 2014). Interim periods where the monthly SOI values fluctuate or are only mildly positive or negative are representative of a neutral phase in the Walker Circulation. The SOI is not the only index used to measure the El Niño Southern Oscillation. Other indices are most often ocean-based measurements that record sea surface temperature (SST) anomalies as a means to measure the oscillation. Niño-3, Niño-3.4 and Niño-4 as well as the ENSO Modoki index (EMI) are such indices (Risbey et al. 2009). These indicators are usually well correlated however as the indices utilise different variables to reflect the El Niño Southern Oscillation some variation in the record is apparent (Figure 16) (Risbey et al. 2009). The Troup SOI was selected as the index used to illustrate the El Niño Southern Oscillation 40

43 for the purposes of this project because a long-term record of the mean monthly SOI from 1876 present is freely available from the Australian Bureau of Meteorology website. Additionally, a number of authors in the literature support the use of the Troup SOI; Risbey et al. (2009) purport that Troup SOI offers the most consistent record going back in time. Furthermore, because the SOI is based on large-scale surface pressure variation, the index is more related to rainfall processes and has the highest correlation (out of the various indices) with Australian rainfall (Risbey et al. 2009). Figure 16: Comparison between two indices of the El Niño Southern Oscillation. La Niña phases are represented in blue, El Niño phases in red; overall the comparison between the two indices of ENSO for this period show good correlation (NOAA 2005) Representing coastal processes: wave statistics and water levels Wave statistics and observational records provide datasets for the representation and analysis of the wave climate. Such records were previously ascertained through ship logs and manned lighthouses, however Waverider buoys and satellite modelling techniques have enabled comprehensive datasets that describe the wave climate to be produced (Short and Trenaman 1992; Hemer et al. 2007). Although there are numerous datasets sourced from satellite techniques, data from Waverider buoys is deemed to be the most comprehensive and accurate and was chosen for use in this project (Short and Trenaman 1992; Hemer et al. 2007). The Manly Hydraulics Laboratory (MHL) provided wave data collected in-situ from a Directional Waverider buoy stationed at Batemans Bay for use in this project. The Waverider buoys operated by MHL record data in 34-minute bursts, which are transmitted to shore via a 41

44 telemetry network for processing using zero-crossing and spectral analysis (MHL 2011). Directional Waverider buoys also record wave direction in-situ using on-board accelerometers and compass systems to ascertain the orthogonal direction of arriving waves (MHL 2011). Direction has been recorded in this way at Batemans Bay since 2001 when the Waverider buoy was upgraded; prior to this the deepwater wave direction included in the record was estimated (MHL 2012). Figure 17: MHL water level gauges at Lake Tabourie (left) and Lake Wallaga (right). In addition to data on the wave climate, MHL has collected water level and tide data through permanent and semi-permanent gauges at numerous locations on the New South Wales coast (Modra and Hesse 2011). These gauges form a network that enables analysis of tidal and climatic influences on water level (as with this study) (Modra and Hesse 2011). MHL uses five types of data capture systems (radar sensor, electromagnetic wave staff (EWS), vented pressure systems, solid state floatwell, and submersed water level recorders) situated across four types of locations (open ocean bays, offshore ocean, island sites and river entrance sites) to construct the water level records (Figure 17) (MHL 2013; Modra and Hesse 2011). Data is recorded continuously or in bursts and sent via telemetry to shore for processing into 1- minute or 15-minute time series (MHL 2013). 42

45 1/01/1991 1/01/1992 1/01/1993 1/01/1994 1/01/1995 1/01/1996 1/01/1997 1/01/1998 1/01/1999 1/01/2000 1/01/2001 1/01/2002 1/01/2003 1/01/2004 1/01/2005 1/01/2006 1/01/2007 1/01/2008 1/01/2009 1/01/2010 1/01/2011 1/01/2012 1/01/2013 SOI S Perry (2014) 4.2 The El Niño Southern Oscillation and Interdecadal Pacific Oscillation The phases of the El Niño Southern Oscillation were first established to enable differentiation between ICOLL entrance conditions with respect to the ENSO phase. The Australian Bureau of Meteorology states that sustained negative values of the SOI less than -8 are indicative of an El Niño phase, while sustained positive values of the SOI greater than 8 are indicative of a La Niña phase (as above) (BOM 2014). The monthly mean SOI dataset was converted to a running 5-month moving mean and using conditional formatting the periods that fit the BOM criteria were extracted to be classified as El Niño or La Niña respectively. Periods that did not fit either criterion were classified as the neutral phase of the oscillation. The resulting time periods are tabulated in Table 4. As the severity of any particular El Niño or La Niña phase is not solely dependent on the strength of the SOI (instead it is due to the cumulative effect of climatic variables, as previously discussed) historic records from BOM were used to further categorise each phase as weak, moderate or strong. A line graph of the SOI was produced in Excel, illustrating the phases of ENSO, to use in the later stage of the analysis (Figure 18). The Interdecadal Pacific Oscillation was included as an additional dataset to the SOI (Table 4) as it is known to have a modulating effect on the El Niño Southern Oscillation across Australia. The phases of the IPO were ascertained from analyses published by Power et al. (1999) and Dai (2013). One change in IPO phase coincides with the study period (Figure 18) Positive IPO Negative IPO Figure 18: Monthly SOI (blue), with 5 month moving average (black) and IPO phases (red) (January 1991 December 2013). Positive SOI values indicate La Niña phases while negative values indicate El Niño phases. 43

46 Table 4: Bureau of Meteorology defined El Niño Southern Oscillation phases and corresponding phase of the Interdecadal Pacific Oscillation El Niño La Niña IPO Weak Moderate Strong Weak Moderate Strong Phase Positive Negative Over the time period analysed there has been a total of 7 El Niño phases and 4 La Niña phases recorded by the Bureau of Meteorology (Table 4). The 5-month mean trendline in Figure 18 however indicates the presence of an additional El Niño phase between as well two additional La Niña phases between and These periods may not be recognised as phases by BOM for a number of reasons including inconsistencies with the SOI signal or the interaction of other climate variables that are not included in this analysis. As the BOM guidelines regarding the SOI are used to determine the ENSO phases these additional phases will be included in the analysis. 4.3 ICOLL Entrance Condition Water level data was used as the main surrogate for entrance condition. Although there are some records kept by the NSW Office of Environment and Heritage, MHL and Local Councils regarding the timing and nature of ICOLL openings they are incomplete and could not be used as the sole indication of entrance condition for each ICOLL. The water level is continuously recorded by in-situ stations run by MHL (NSW Public Works 2014) and was supplied for this study as a daily mean dataset for each site. Although the water level is not a direct measurement for the entrance condition it can be used to extrapolate the state of the entrance by analysing the data in graph form. A line graph of the water level for each site was produced in Excel (Figure 19). The scale of the x-axis was set to be the same as that used in the plot of the SOI (Figure 18) to allow for comparative analysis. 44

47 1/01/1992 1/01/1993 1/01/1994 1/01/1995 1/01/1996 1/01/1997 1/01/1998 1/01/1999 1/01/2000 1/01/2001 1/01/2002 1/01/2003 1/01/2004 1/01/2005 1/01/2006 1/01/2007 1/01/2008 1/01/2009 1/01/2010 1/01/2011 1/01/2012 1/01/2013 (m) (m) (m) (m) (m) (m) S Perry (2014) Wollumboola Swan Burrill Tabourie 1.5 Durras Wallaga Figure 19: Water level profile (m) for each ICOLL. 45

48 Water Level (m) S Perry (2014) The following parameters (illustrated in Figure 20) were used to infer the entrance condition from the water level line graph: A stable (or relatively stable) line is indicative of a closed entrance (Figure 20, red arrow). The water level will show an increasing trend over a longer period of time while the entrance is closed. This occurs as rainfall and runoff are stored and accumulate in the ICOLL. A highly variable line illustrates short-term changes in water level due to wave and tidal activity (Figure 20, blue arrow). The entrance is therefore open, allowing tidal exchange to occur. Although there is variation, the water level is likely to be low, as water does not accumulate behind the barrier when the entrance is open to the ocean. An abrupt increase, or spike, in the water level (while closed) occurs in response to a large volume of input over a short span of time (Figure 20, purple or green arrow). For example a storm event releasing a significant volume of rainfall over a few hours. Where a rapid decline in water level occurs immediately after a spike this is indicative of a change in the entrance condition as stored water is released from the ICOLL. This suggests that the entrance has opened. The transition from an open condition to closed condition is illustrated by a gradual lessening in the variation in the curve and an overall increase in the water level. Minimal variability = closed entrance High variability = open entrance Artificial Opening (2011) Natural Opening (2012) Figure 20: Indicators of entrance condition inferred from water level curve, illustrated on extract of Durras Lake water level (m) from January 2011 December

49 In addition to water level, M 2 data can be used to illustrate the entrance condition of an ICOLL. The M 2 (principal lunar semi-diurnal constituent) is one of 4 constituents used in the harmonic analysis of ocean tides (Modra and Hesse 2011; MHL 2012). The derivation of tides and specifically the M 2 through harmonic analysis is beyond the scope of this project, however when calculated using short-term (14-30 day periods) analysis the M 2 value for the estuary mouth can be used to illustrate the tidal exchange through the estuary (ICOLL) entrance, thereby indicating the condition of the entrance at the time. The calculation to derive the M 2 values becomes skewed towards 0 when there is no tidal signature for long periods of time, for example where the entrance to the ICOLL is closed. MHL omits M 2 calculations from annual reports where the site in question is non-tidal for more than 50% of the year in order to maintain data integrity (MHL 2012). For this reason M 2 data for the study ICOLLs was only accessible for the ICOLLs that spent the majority of time open (on average more than 180 days per year) (MHL 2012). The M 2 data used in this project was provided by MHL in processed form. Similar to the water level, line graphs from Excel were used to display the data in a similar way to enable comparisons. An M 2 value of 0m is indicative of a closed entrance, with no tidal exchange occurring. A value close to 0m suggests that the entrance is heavily constricted, infilled with sand, allowing only minimal tidal exchange to occur. Respectively high M 2 values indicate that tidal exchange is occurring through the entrance, illustrating that there is minimal entrance constriction apparent and therefore suggesting open entrance conditions. Figure 21 illustrates the results of the M 2 analysis. The water level profiles are displayed alongside the M 2 profiles to illustrate the correlation between the two proxies of entrance condition. Entrance constriction over time is most evident at Lake Durras and Burrill Lake (Figure 21). The profile of the M 2 changes gradually over time, indicating the progressive infill of sediment in the entrance berm. Similarly, from 2001 to 2004 the M 2 profile of Burrill Lake indicates that the entrance channel was infilling during this time before closing for the first time within the data series (since 1991). The entrance condition at Lake Tabourie changes more abruptly, with entrance openings and closures occurring frequently. These changes in the M 2 profile correspond to similar changes in the water level profile (Figure 19) indicating when the changes in the entrance condition of the ICOLLs occurred. As the M 2 is available for only half of the sites, the M 2 was used as a supplementary dataset but not included further in the analysis. 47

50 1/01/1992 1/01/1993 1/01/1994 1/01/1995 1/01/1996 1/01/1997 1/01/1998 1/01/1999 1/01/2000 1/01/2001 1/01/2002 1/01/2003 1/01/2004 1/01/2005 1/01/2006 1/01/2007 1/01/2008 1/01/2009 1/01/2010 1/01/2011 1/01/2012 1/01/2013 (m) (m) (m) (m) (m) (m) S Perry (2014) 0.3 M 2 Burrill Water level Burrill M 2 Tabourie Water level Tabourie M 2 Durras Water level Durras Figure 21: Entrance constriction illustrated by the M 2 profile (m) (black) for Burrill Lake, Lake Tabourie and Lake Durras. Water level profiles (m) (blue) correspond well to the M 2 profiles: when the water level is highly variable the M 2 is high indicating open conditions. 48

51 A record of entrance condition for the whole study period was determined from the water level and M 2 profiles (see Appendix One for the full record). The record details: the date of opening, maximum water level, period of time that the entrance was open, and the date of closure. Using the timing of entrance openings and closures, the ICOLL was classified into a binary condition, either open or closed. For use in the statistical analysis this binary was assigned values; open as 2 and closed as 1. An additional line graph was produced in Excel for each ICOLL illustrating the entrance condition as either open or closed. The SOI was included as an additional series to overall illustrate the relative time period that each ICOLL was open and or closed during each phase of the El Niño Southern Oscillation (Figure 22). This figure illustrates a number of patterns regarding the relationship of ICOLL entrances to the El Niño Southern Oscillation. First, all of the ICOLLs were closed for a relative long period of time during the dry conditions associated with the El Niño (Figure 22). This also occurred (with the exception of Burrill Lake) during the strong El Niño period from (Table 4), however the ICOLLs that exhibit mostly closed entrance conditions, in particular Lake Wollumboola and Swan Lake, had already been closed for a number of years. As all of the ICOLLs did not exhibit a closed entrance condition during earlier El Niño years (for example or ) the closed condition could illustrate the influence of the Interdecadal Pacific Oscillation. In the negative phase of the IPO (apparent for the El Niño), El Niño impacts across Australia are strengthened (Power et al. 1999; Salinger et al. 2001). Second, a number of entrance openings occur when the SOI is transitioning from one phase to another. For example, Lake Wollumboola opened in 1994 and 1998 during the transition out of the respective El Niño phases. The same is also evident for Lake Tabourie however due to the number of openings for that ICOLL there is no clear pattern. Although there are some similarities, overall the six ICOLLs exhibit distinctly different entrance regimes across the same period of time. As the El Niño Southern Oscillation is constant across all sites the difference between these ICOLLs and the response to ENSO must therefore be the result of other variables that are unique to each ICOLL. Although this is a logical conclusion it is important to distinguish that the relationship between the El Niño Southern Oscillation and ICOLL entrance condition is not causal. 49

52 1/01/1992 1/01/1993 1/01/1994 1/01/1995 1/01/1996 1/01/1997 1/01/1998 1/01/1999 1/01/2000 1/01/2001 1/01/2002 1/01/2003 1/01/2004 1/01/2005 1/01/2006 1/01/2007 1/01/2008 1/01/2009 1/01/2010 1/01/2011 1/01/2012 1/01/2013 S Perry (2014) + SOI _ O Wollumboola C O Swan C O Burrill C Tabourie O C Durras O C O Wallaga C Figure 22: Entrance condition of ICOLLs and respective phase of the El Niño Southern Oscillation. + = La Niña, - = El Niño, O = open entrance, C = closed entrance. 50

53 4.3.1 Entrance Openings: Natural or Artificial? The ICOLLs in this study are all managed, and as part of that management artificial opening of the entrance by the relevant authority is routinely carried out. Therefore, not all of the opening events in the entrance record occurred naturally. The records held by the NSW Office of Environment and Heritage, MHL and respective local government provided evidence for the nature of opening events and where possible this information was used to classify the nature of the opening. As these records are incomplete the water level plot was used to infer the nature of openings when the record was not available. The water level plot was analysed for Lake Durras, as there was a record for both a natural and artificial opening (Appendix One). As illustrated in Figure 20, a difference in the water level curve was identified for the natural opening and the artificial opening. When the ICOLL opened naturally the response is abrupt, the spike in water level is immediately followed by a rapid decline in water level. In comparison, the peak in water level is followed by a decline in water level that is not as steep, indicating that it occurred more slowly over a longer period of time. This was applied to other water level curves to determine the nature of opening, as the result is based on observation with no supporting evidence these classifications were italicised to be distinguishable in the record (see Appendix One). Figure 23 illustrates the number of entrance openings and the proportion of those openings that are natural or artificial for each ICOLL Artificial Natural Wollumboola Swan Burrill Tabourie Durras Wallaga Figure 23: Proportion of natural and artificial openings for all ICOLLs. Although the nature of entrance openings was identified, neither were adjusted or treated differently in the analysis. The effect of this will be discussed further in the discussion chapter. Proportionally, artificial openings were more common at all sites with the exception of Lake Durras (which had an equal number of natural and artificial openings). 51

54 4.4 Rainfall, Wave Climate and Storms Coastal and catchment variables were included to expand the analysis from the broad-scale El Niño Southern Oscillation to a smaller-scale site-specific analysis. The relative influence that these variables (specifically rainfall, wave climate and storms) will have in determining the ICOLL entrance condition is different for each site. The data for each of the parameters was analysed and then compared against the water level to identify trends in entrance condition, broadly linking to the phase of the El Niño Southern Oscillation. The relationship between the wave climate, rainfall and storms to the phase of the El Niño Southern Oscillation is further explored in greater detail using statistical analysis, which will be discussed later in this chapter Rainfall Rainfall data was acquired from the Bureau of Meteorology as a daily and monthly record to be used as a surrogate for catchment processes. Rainfall is only a surrogate value as the true catchment processes include stream-flow and runoff in addition to direct rainfall (Haines 2008). The monthly long-term average rainfall for each station was also acquired from the Bureau of Meteorology. The long-term mean data was graphed as a combined line graph in Excel (Figure 24) and used to infer annual and seasonal trends in the catchment rainfall. Evident in Figure 24, the long-term mean rainfall exhibits a distinctly seasonal trend. February and March receive the highest rainfall, with the lowest rainfall occurring in July and August. These seasonal rainfall trends are consistent with those experienced across most of Australia (BOM 2014). Wallaga Lake, the southernmost ICOLL in this study, receives (on average) the lowest annual rainfall of the study sites Wollumboola Swan Burrill Tabourie Durras Wallaga 40 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 24: Long-term monthly mean rainfall for ICOLL catchments. 52

55 The monthly mean rainfall for each ICOLL was graphed as a bar graph with time consistent on the x-axis to enable comparisons between data series (Figure 25). The recorded monthly rainfall was compared against the equivalent month in the long-term mean series to enable periods in which the recorded rainfall is above or below average to be identified. These periods indicate the potential effect of the ENSO and or the IPO. Trends in rainfall at each catchment are similar, as evident in Figure 25. This is likely to be due to the close proximity of each of the ICOLLs to one another and therefore exposure to the same weather systems. To undertake initial comparison, the rainfall (Figure 25) and water level (Figure 19) graphs were printed at a large scale and analysed. From this comparative analysis it is evident that when the ICOLL is closed periods of high rainfall correspond to peaks in water level. When the event is large enough the input of rainfall leads to a change in entrance condition. Approximately one third (36%) of entrance opening events (across all sites) occurred following a large influx of rainfall (Table 5). Although the opening events are not being analysed separately based on their nature (natural or artificial) it is important to note that all except one (Lake Tabourie in 2013) of the natural opening events occurred due to a large influx of rainfall. Table 5: Large rainfall events and corresponding ICOLL opening events ICOLL Date Open ENSO Phase (E, L, N) Lake Wollumboola Rainfall (mm) Natural / Artificial 13/04/1994 E 250 Artificial 18/08/1998 L Natural* 23/03/2011 L Natural* 27/06/2013 N 187 Artificial Swan Lake 24/06/2013 N 106 Natural Lake Tabourie 13/ E 96 Artificial 1/09/1996 N 243 Artificial 8/10/1997 E 113 Artificial 22/10/2004 N 142 Artificial 12/10/2012 L 170 Natural* Lake Durras 6/02/2002 N Natural 12/10/2012 N 215 Natural 26/06/2013 N Natural Wallaga Lake 28/09/2000 L Artificial 11/07/2005 N Artificial 12/02/2007 E 94.4 Artificial NB: The rainfall that occurred on the day of opening and 2 days prior to opening were totalled. A large event was anything greater than 90mm in the 3-day period. Openings with an asterisk were inferred from the data and do not have a supporting record. 53

56 1/01/1991 1/01/1992 1/01/1993 1/01/1994 1/01/1995 1/01/1996 1/01/1997 1/01/1998 1/01/1999 1/01/2000 1/01/2001 1/01/2002 1/01/2003 1/01/2004 1/01/2005 1/01/2006 1/01/2007 1/01/2008 1/01/2009 1/01/2010 1/01/2011 1/01/2012 1/01/2013 (mm) (mm) (mm) (mm) (mm) (mm) S Perry (2014) 600 Wollumboola Swan Burrill Tabourie Durras 500 Wallaga Figure 25: Mean monthly rainfall (mm) for each ICOLL catchment over the study period. 54

57 4.4.2 Wave Climate Wave date was provided by the Manly Hydraulics Laboratory to ascertain the wave climate specific to the south coast and the ICOLLs. Data from the Batemans Bay buoy was used as the buoy is in a central location to the ICOLLs (Figure 26). The wave parameters obtained from MHL used in the analysis are the maximum wave height (H max ), wave power and wave direction. The raw data was processed in Excel to obtain daily and monthly mean values. A line graph for both the daily and monthly series of wave height and wave power against time was produced. At large scale, the daily average plot was used to identify peaks in wave conditions and compare against the already ascertained rainfall and water level graphs to identify cross correlations. Periods where increased wave activity was sustained are evident as peaks in the monthly average (Figure s 27 and 28) such as the month of June 2007, represented by the largest peak in both graphs. Legend: (a) Wollumboola (b) Swan (c) Burrill (d) Tabourie (e) Durras (f) Wallaga (g) Batemans Bay Waverider buoy Figure 26: MHL Batemans Bay Waverider Buoy (G) location in respect to study ICOLLs (Google Earth). 55