DEPARTMENT OF PLANNING, INDUSTRY & ENVIRONMENT Form and function of NSW intermittently closed and open lakes and lagoons Implications for entrance management environment.nsw.gov.au © 2021 State of NSW and Department of Planning, Industry and Environment With the exception of photographs, the State of NSW and Department of Planning, Industry and Environment are pleased to allow this material to be reproduced in whole or in part for educational and non-commercial use, provided the meaning is unchanged and its source, publisher and authorship are acknowledged. Specific permission is required for the reproduction of photographs. The Department of Planning, Industry and Environment (DPIE) has compiled this report in good faith, exercising all due care and attention. No representation is made about the accuracy, completeness or suitability of the information in this publication for any particular purpose. DPIE shall not be liable for any damage which may occur to any person or organisation taking action or not on the basis of this publication. Readers should seek appropriate advice when applying the information to their specific needs. All content in this publication is owned by DPIE and is protected by Crown Copyright, unless credited otherwise. It is licensed under the Creative Commons Attribution 4.0 International (CC BY 4.0), subject to the exemptions contained in the licence. The legal code for the licence is available at Creative Commons. DPIE asserts the right to be attributed as author of the original material in the following manner: © State of New South Wales and Department of Planning, Industry and Environment 2021. Cover photo: Lake Brou, Eurobodalla National Park. DPIE. Authors: Angus Ferguson, Danny Wiecek, Michael Hughes, David Hanslow, David Wainwright and Peter Scanes. The authors wish to acknowledge the input and review provided by Raymond Laine, Mitch Tulau, John Murtagh, Cherie Parmenter, Aimee Beardsmore, Michelle Fletcher and Bruce Coates. Published by: Environment, Energy and Science Department of Planning, Industry and Environment Locked Bag 5022, Parramatta NSW 2124 Phone: +61 2 9995 5000 (switchboard) Phone: 1300 361 967 (Environment, Energy and Science enquiries) TTY users: phone 133 677, then ask for 1300 361 967 Speak and listen users: phone 1300 555 727, then ask for 1300 361 967 Email: [email protected] Website: www.environment.nsw.gov.au Report pollution and environmental incidents Environment Line: 131 555 (NSW only) or [email protected] See also www.environment.nsw.gov.au ISBN 978-1-922558-52-7 EES 2021/0150 April 2021 Find out more about your environment at: www.environment.nsw.gov.au Contents About this document 1 NSW ICOLLs – a unique waterway type 2 Conceptual model of NSW ICOLLs 3 Geomorphic evolution 3 Geomorphic classification 3 ICOLL subtype classifications 6 Distribution of ICOLL subtypes in New South Wales 6 Functional zones and habitats of ICOLLs 7 NSW climate – important driver of ICOLL dynamics 10 Rainfall along the NSW coast 10 Seasonal rainfall 10 Interannual variability 11 Longer period climatic cycles 12 ICOLL entrance dynamics 13 Entrance breakout 15 Entrance closure 21 Balance between entrance opening and closure 23 Entrance closure index 23 Closure index of ICOLL subtypes 24 Water exchange due to tides 25 Astronomic tides 25 Effects of sea level anomalies 26 Tidal flushing 26 Training walls 28 Case study – Lake Illawarra training wall 28 ICOLL water quality 31 Natural variation 31 Water quality changes after freshwater inputs 33 Stratification 34 Lateral water quality gradients 34 Water quality monitoring considerations 37 ICOLL habitats 38 Benthic and fringing wetland habitats 38 Pelagic habitats 47 ICOLL biota 48 iii Fish 48 Birds 50 Climate change 52 Hydrodynamic and geomorphic effects of climate change 52 Ecological effects of climate change 53 Entrance management in ICOLLs 53 Overview 53 System-specific management models 54 Public education 57 System-specific guidelines and thresholds 57 Risk assessment of entrance management options 58 References 60 Appendix A – Baseline classification and morphometrics of NSW estuaries 64 List of tables Table 1 ICOLL functional zones and their subenvironments 7 Table 2 Functional zone characteristics of ICOLL subtypes present in New South Wales 9 Table 3 Statistical summary of physico-chemical water quality in NSW ICOLLs, 2007 to 2020 32 Table 4 Risk factors and risk indicators associated with different entrance management scenarios 59 List of figures Figure 1 Stages of infilling in the geomorphic evolution of a NSW ICOLL 4 Figure 2 Statistical summary of morphometrics and hydrology of the different estuary types in New South Wales 5 Figure 3 Distribution of different estuary types among the northern, central and southern regions of the NSW coast 6 Figure 4 Idealised ICOLL showing the distribution of key functional zones and subenvironments 8 Figure 5 Average annual rainfall for each of the 184 estuary catchments along the NSW coast 10 Figure 6 Long-term rainfall statistics for Ballina Airport, showing the effects of two unseasonally high rainfall events during 2018 11 iv Figure 7 Interannual variation in the Southern Oscillation index since 1997 12 Figure 8 Average annual Interdecadal Pacific Oscillation index and cumulative deviation from the mean for annual rainfall totals at Yamba, Newcastle and Eden 13 Figure 9 Conceptual diagram of the processes that control entrance dynamics in ICOLLs 14 Figure 10 Conceptual diagram of the processes that control entrance dynamics in ICOLLs 14 Figure 11 Factors that determine the hydraulic head at the time of entrance breakout in ICOLLs 16 Figure 12 Wonboyn Lake entrance in various conditions 18 Figure 13 Three phases of entrance breakout based on data from Dee Why Lagoon 20 Figure 14 Relationships between berm height, morphometrics and sand grain size 22 Figure 15 Entrance closure indexes 24 Figure 16 Frequency analysis of NSW ICOLLs 24 Figure 17 Close coupling between Wonboyn Lake water levels and mean sea level residual at Eden 26 Figure 18 Modelled effects of an idealised entrance opening on salinity distributions in Wonboyn Lake 27 Figure 19 Lake Illawarra ICOLL 28 Figure 20 Tidal variation at Koonawarra Bay in Lake Illawarra prior to entrance works 29 Figure 21 Tidal variation at Koonawarra Bay in Lake Illawarra after the entrance works 30 Figure 22 Mean annual water level in Lake Illawarra prior to, and after construction of entrance breakwalls 30 Figure 23 Changes in seagrass coverage since construction of breakwalls in Lake Illawarra 31 Figure 24 Water quality statistics for NSW estuaries including the different ICOLL subtypes 33 Figure 25 Conceptual model of key phases in ICOLL water quality 35 Figure 26 Water quality variation in Wonboyn Lake 37 Figure 27 Examples of a) active (mobile) and b) stable shoal habitats 39 Figure 28 Examples of fringing wetland habitats in ICOLLs 41 Figure 29 Example of NSW Government estuarine habitat mapping for Wonboyn River 42 Figure 30 Proportion of waterway area occupied by Ruppia, Zostera, mangroves and saltmarsh in NSW estuaries 42 v Figure 31 Complete drainage of Dee Why Lagoon following entrance breakout 43 Figure 32 Conceptual diagram of the Lake Innes and Lake Cathie system prior to the construction of the artificial channel 45 Figure 33 The Lake Cathie – Lake Innes system 46 Figure 34 ICOLL habitats for waterbirds 51 Figure 35 Monitoring required for assessing ICOLL management options 56 List of photos Photo 1 Tallow Creek in northern New South Wales 2 Photo 2 Mechanical opening of Lake Conjola on 10 February 2020 17 Photo 3 Formation of a weir and plunge pool during an artificial opening of Durras Lake 21 Photo 4 Tuross Lakes entrance showing tidal exchange 25 Photo 6 Fringing habitats and exposed entrance shoals in Lake Wollumboola 38 Photo 7 Tallow Creek and fringing wetland habitats 53 vi Form and function of NSW intermittently closed and open lakes and lagoons: Implications for entrance management About this document This document provides a technical background to the factors controlling entrance dynamics of intermittently closed and open lakes and lagoons (ICOLLs), how entrance dynamics affect water quality and ecology, and the likely consequences of different management interventions. The primary audiences are natural resource and flood risk managers, local and State government authorities and agencies, and water industry professionals. It provides a precis of the characteristic geomorphic, hydrological and ecological features common to ICOLLs, with a particular focus on the interactions between entrance dynamics and these features. While we present conceptual models that convey some of the commonalities among ICOLLs, there is an underlying understanding that these systems are all unique due to their own blend of environmental factors, and consequently each will react a bit differently to outside influences. 1 Form and function of NSW intermittently closed and open lakes and lagoons: Implications for entrance management NSW ICOLLs – a unique waterway type Photo 1 Tallow Creek in northern New South Wales Photo: A Ferguson/DPIE ‘Intermittently closed and open lakes and lagoons’ (ICOLLs) is a generic term for a distinct type of estuary with a tendency for the ocean entrance to close during periods of low freshwater inflow. ICOLLs are found in mid-latitude countries worldwide, including Australia, South Africa, and Mexico (McSweeney et al. 2017), and they account for more than 60% of the 184 estuaries along the NSW coast (Roper et al. 2011). NSW ICOLLs share a set of common morphological and hydrological attributes, however the relative size and expression of these attributes varies greatly among systems (Maher et al. 2011). This variability presents challenges in generating a universal understanding of the hydrological and ecological functions of ICOLLs, and in turn understanding the unique aspects of individual systems. However, the ubiquity of some features does allow a degree of generalisation about responses to external factors. Urban areas along the NSW coast are often situated around ICOLLs, leading to a need for local councils to manage issues such as flooding and community perceptions of ‘poor’ water quality.