HAWKESBURY NEPEAN ESTUARY SALINE DYNAMICS MODEL CALIBRATION by D R Cox, W L Peirson and J L Davies Technical Report 2003/01 March 2003 THE UNIVERSITY OF NEW SOUTH WALES SCHOOL OF CIVIL AND ENVIRONMENTAL ENGINEERING WATER RESEARCH LABORATORY HAWKESBURY NEPEAN ESTUARY SALINE DYNAMICS MODEL CALIBRATION WRL Technical Report 2003/01 March 2003 by D R Cox, W L Peirson and J L Davies https://doi.org/10.4225/53/58d4a604e8ea6 Water Research Laboratory School of Civil and Environmental Engineering Technical Report No 2003/01 University of New South Wales ABN 57 195 873 179 Report Status Final King Street Date of Issue March 2003 Manly Vale NSW 2093 Australia Telephone: +61 (2) 9949 4488 WRL Project No. 02107 Facsimile: +61 (2) 9949 4188 Project Manager D R Cox Title Hawkesbury Nepean River Saline Dynamics – Model Calibration Author(s) D R Cox, W L Peirson and J L Davies Client Name Hawkesbury Nepean River Management Forum Client Address PO Box 556, Windsor NSW 2756 Client Contact Brian Walters Client Reference The work reported herein was carried out at the Water Research Laboratory, School of Civil and Environmental Engineering, University of New South Wales, acting on behalf of the client. Information published in this report is available for general release only with permission of the Director, Water Research Laboratory, and the client. WRL TECHNICAL REPORT 2003/01 CONTENTS 1. INTRODUCTION 1 2. BACKGROUND 2 2.1 Environmental Flows 2 2.2 The Hawkesbury Nepean River 2 2.2.1 General Characteristics 2 2.2.2 Modifications to the Natural Flow Regime 3 3. NUMERICAL MODELLING 4 3.1 Estuary Model Description 4 3.2 Hydrodynamic Model Calibration 4 3.3 Salinity Model Calibration 7 4. CONCLUSIONS 10 5. REFERENCES 11 - i - WRL TECHNICAL REPORT 2003/01 LIST OF TABLES 1. Summary of Tide Level and Discharge Calibration Data 2. Comparison of Measured and Modelled Tidal Prisms, 30 April 1981 3. Comparison of Measured and Modelled Tidal Prisms, 29 May 1981 4. Comparison of Measured and Modelled Tidal Prisms, 26 June 1981 5. Comparison of Measured and Modelled Tidal Prisms, 20 August 1981 LIST OF FIGURES 1. Hawkesbury Nepean River System 2. Hawkesbury Nepean Estuary Model Mesh 3. Location of Tidal Calibration Data 4. Comparison of Measured and Modelled Tide Levels, 30 April 1981 5. Comparison of Measured and Modelled Tide Levels, 29 May 1981 6. Comparison of Measured and Modelled Tide Levels, 26 June 1981 7. Comparison of Measured and Modelled Tide Levels, 20 August 1981 8. Comparison of Measured and Modelled Tidal Lags, 30 April 1981 9. Comparison of Measured and Modelled Tidal Lags, 29 May 1981 10. Comparison of Measured and Modelled Tidal Lags, 26 June 1981 11. Comparison of Measured and Modelled Tidal Lags, 20 August 1981 12. Major River Inflows to Hawkesbury Nepean, February 1977 – January 1978 13. Comparison of Measured and Modelled Depth Averaged Salinity, Feb 1977 – Jan 1978 - ii - WRL TECHNICAL REPORT 2003/01 1. 1. INTRODUCTION The University of New South Wales, Water Research Laboratory (WRL) was commissioned by the Hawkesbury Nepean River Management Forum to establish a model of the estuary which was suitable for long term studies of salinity, environmental flows, water quality and associated ecological impacts. This report describes the development of the model and its calibration against measured tide levels, tidal discharges and longitudinal salinity structure in the Hawkesbury Nepean estuary. The one dimensional hydrodynamic and water quality model extends from West Head, at the junction of the Hawkesbury Nepean with Brisbane Water and Pittwater, to the upstream tidal limit at Yarramundi. Tide level and tidal flow data obtained by MHL (1988) during four one-day gauging exercises on the Hawkesbury River during 1981 were used to calibrate the hydrodynamic model. The salinity model was calibrated against depth- averaged salinities obtained along the length of the Hawkesbury River by the Electricity Commission at approximately 2 weekly intervals between February 1977 and January 1978. WRL TECHNICAL REPORT 2003/01 2. 2. BACKGROUND 2.1 Environmental Flows Many estuaries in Australia are subject to irregular and non-seasonal rainfall patterns, in which long periods of low rainfall may be followed by significant rainfall events at varying intervals. As a result, the natural estuarine salinity structure may also be highly variable. During periods of high rainfall, salt water is flushed from the estuary by the increased freshwater flow, while during periods of low rainfall, saline water from the estuary mouth is able to penetrate further up the estuary, either as a density current or through tidal mixing (Dyer, 1997). Human activities have greatly modified the freshwater flow regime in many Australian rivers. Any flow storage or diversion structure that alters the amount of freshwater flow to a river system may influence the health of downstream aquatic ecosystems. Examples of anthropogenic influences on river flows include the construction of dams and weirs, withdrawal of water for household use, irrigation or industry, sewage treatment plant discharges, changes to the river channel and catchment modifications affecting runoff (such as land clearing and introduction of impervious surfaces such as roads, rooves and paving). These activities may cause persistent changes to the natural salinity structure of an estuary and consequently have implications for the health of aquatic ecosystems. Consideration of the ecological impacts of reduced river flows due to human water usage has led to the concept of “Environmental Flows”. The term “Environmental Flows” describes freshwater flow (typically instream flow) that is maintained solely for environmental reasons, to maintain the health and biodiversity of a particular water-related entity, such as a river or estuary (Peirson et al., 2001, 2002). Environmental flows are essential to the minimisation of negative influences on the health of aquatic ecosystems resulting from human alterations to the flow regime. 2.2 The Hawkesbury Nepean River 2.2.1 General Characteristics The Hawkesbury Nepean estuary (Figure 1) is an example of a tide-dominated, drowned valley estuary (Roy et al., 2001). The river is approximately 300km long and supplies 98% of Sydney’s potable water supply. The limit of saline intrusion (> 0.5 ppt) in the estuary is WRL TECHNICAL REPORT 2003/01 3. usually in the vicinity of the Colo River junction but may move as far upstream as Sackville (SPCC, 1983). 2.2.2 Modifications to the Natural Flow Regime The freshwater flow and hydraulic characteristics of the Hawkesbury Nepean River have been heavily modified by the construction of dams, weirs and other water supply structures in the upper parts of the catchment, and to a lesser extent, the use of water for irrigation and returns to the river from sewage treatment plants. Dams and other major water supply structures impacting on flows in the Hawkesbury-Nepean include: • The Upper Nepean Scheme (a system of weirs, canals, tunnels and aqueducts constructed from the 1860s onwards to divert flows from the Upper Nepean to Sydney, with storage in Prospect Reservoir following its completion in 1888) • Cataract Dam (completed 1907) • Cordeaux Dam (1926) • Avon Dam (1927) • Nepean Dam (1935) • Warragamba Dam (1960) • Six dams constructed between 1907 and 1942 on tributaries of the Grose River, including Lake Medlow, Greaves Creek Dam, the Upper, Middle and Lower Cascade Dams, and Woodford Dam. These dams now serve only the middle and upper reaches of the Blue Mountains, from Faulconbridge to Mt. Victoria. The Shoalhaven Scheme (completed in 1977) allows transfer of water from the Shoalhaven River to the Wingecarribee Reservoir during times of drought, which can then be directed to either Lake Burragorang or Nepean Dam. Water can also be supplied to the Illawarra from Avon Dam and Nepean Dam. Water is extracted for irrigation purposes at numerous locations, the largest volumes of which are taken from the river between the Grose River and Colo River junctions. WRL TECHNICAL REPORT 2003/01 4. 3. NUMERICAL MODELLING 3.1 Estuary Model Description The hydrodynamics and salinity distribution in the Hawkesbury Nepean estuary have been simulated using RMA-2 and RMA-11, both components of the RMA finite element modelling suite. A similar approach was applied to that previously used for salinity studies of the Richmond River by Peirson et al. (1999). Both RMA-2 and RMA-11 were configured using the same one-dimensional mesh, which extends upstream to the tidal limit at Yarramundi, just upstream of the confluence of the Grose and Nepean Rivers (Figure 2). The mesh includes the tidal sections of the other major tributaries of the Hawkesbury River downstream of this point, namely the Colo River, Macdonald River, Mangrove Creek, Berowra Creek, Mooney Mooney Creek, Mullet Creek and Cowan Creek. At its downstream end, the mesh extends to a line between the eastern sides of Middle Head and West Head, at the junction of the Hawkesbury Nepean estuary with Brisbane Water and Pittwater. 3.2 Hydrodynamic Model Calibration Tidal currents and freshwater inflows to the Hawkesbury Nepean estuary were simulated using the hydrodynamic model RMA-2 (King, 1998). An ocean tidal boundary condition was applied at the downstream end of the model and the major freshwater inflows to the system. Streamflow data for the Nepean River at Penrith, the Colo River and Macdonald River were used to define the freshwater inflow at the upstream limits of the model. Gauged flow data was not available for the Grose River for the period of the hydrodynamic model calibration, and HSPF modelled catchment flows from a study by SMEC (2002a, b) were used to define the freshwater input from this source. It would not be expected that smaller sources of freshwater inflow would have much impact on the tidal calibration. Tide level and tidal flow data obtained by MHL (1988) during four one-day gauging exercises on the Hawkesbury River, between April and August 1981, were used to calibrate the hydrodynamic model.