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BROOKLYN ESTUARY PROCESS STUDY - APPENDICES (VOLUME II OF II) by Water Research Laboratory Manly Hydraulics Laboratory The Ecology Lab Coastal and Marine Geosciences The Centre for Research on Ecological Impacts of Coastal Cities Edited by B M Miller and D Van Senden Technical Report 2002/20 June 2002 (Issued October 2003) WRL TECHNICAL REPORT 2002/20 Appendix A RMA Hydrodynamic and Water Quality Modelling APPENDIX A – RMA HYDRODYNAMIC AND WATER QUALITY MODELLING A1.1 Hydrodynamic Modelling A.1.1.1 Introduction RMA-2 is a two-dimensional finite element hydrodynamic model for depth averaged flow (King, 1998). The shallow water forms of the Navier-Stokes equations are solved in two dimensions to obtain velocities and water surface elevations at each node on the finite element mesh. The model can either be operated in steady-state or dynamic modes. A two-dimensional finite element model was developed for the Brooklyn Estuary study area to gain an understanding into the current hydrodynamics of the area and also in order to model how the hydrodynamics of the system would be altered in the event that the railway causeway from Brooklyn to Long Island was removed. Two flow regimes were examined. These were base flows and the peak flows that would be expected for a 20% AEP flood. A.1.1.2 Previous Hydrodynamic Modelling Hydrodynamic modelling that has previously been carried out on the Hawkesbury River includes a calibrated hydrodynamic RMA2 model (Hawkesbury Model) that was designed to examine flood behaviour on the Lower Hawkesbury River from Sackville to Broken Bay (AWACS, 1997). For the Brooklyn Estuary Processes Study hydrodynamic modelling, boundary conditions were extracted from the flood model. Therefore the hydrographic and tidal inputs of the flood model used for the Lower Hawkesbury River Flood Study were altered to simulate the flow conditions required for this study. A.1.1.2.1 Hawkesbury Model Input Data Hydrograph Data The hydrographs used in the flood study were manipulated so that baseflows and the 20% AEP peak flows were entered at the hydrograph input points (the major creeks that discharge to the Hawkesbury River). These flow rates were determined from the hydrographs used in the Lower Hawkesbury River Flood Study. Tidal Data Tide data was input on the downstream boundary of the Lower Hawkesbury River Flood Study model. This data was generated using the program WXTide32, for Little Patonga. The data generated by WXTide32 was in ISLW and was converted to the Australian Height Datum. A full tidal cycle (spring-neap-spring) was used as an input to the model, which took approximately eighteen days. The two days prior to the tidal cycle were also used to ‘warm up’ the model. Bathymetry The bathymetry used in the flood model was taken from hydrographic surveys conducted from 1978 and 1984 (see AWACS, 1997). A.1.1.1.3 Brooklyn Model Input Data Boundary Conditions As mentioned previously, the flow rates and water levels for the upstream and downstream boundaries of the Brooklyn model respectively, were extracted from the results file of the Hawkesbury Model. The boundaries of the Brooklyn model were beyond that of the Brooklyn Estuary study area boundaries. In order to negate boundary impacts and obtain a true representation of the hydrodynamics within the study area . Hydrographs Two significant creeks (Mooney-Mooney Creek and Mullet Creek) discharge into the Hawkesbury River in the Brooklyn Estuary study area. Baseflows and peak flood flows used as inputs into the model for Mooney Mooney Creek were obtained from the discharge hydrographs used in the Lower Hawkesbury River Flood Study and were estimated for Mullet Creek. No other diffuse source inputs were considered. Bathymetry The 1978 and 1984 data used in the Hawkesbury Model was used to define the bathymetry in the Brooklyn model. However, further definition of the bathymetry of Sandbrook Inlet was needed for the Brooklyn Estuary Processes Study modelling and therefore hydrographic surveys of the inlet from 1975 were incorporated into the bathymetry of the area. Frictions and Eddy Constants Friction losses in the model are determined from Manning’s Equation, and therefore mannings n was an input parameter. The value for mannings n was generally constant across the study area, however Mannings n values were varied at sections of the model that corresponded to real world changes in channel roughness (ie, oyster leases and mangroves). The mannings n value used for the river bed for most of the study area was 0.023, for oyster leases and mangroves a value of 0.150 was used and for areas where seagrasses were present a mannings n value of 0.033 was used. Turbulence parameters used in the model are input as eddy coefficients. In this case, scale factors of the element size were entered to determine the eddy coefficients. When this option is used, the relative size of each element is taken into consideration in the determination of turbulence.Values of 0.5 were used as scale factors to determine the eddy coefficients in both the x and y direction. A.1.1.2 Description of Brooklyn Model A.1.1.2.1 Hydraulic Model Schematisation The model developed to simulate the hydrodynamics of the Brooklyn Estuary study area was a two-dimensional finite element numerical model (RMA2) (King, 1998). RMA2 computes a finite element solution of the Reynolds form of the Navier-Stokes equations for turbulent flows. The two-dimensional model is configured such that: • Velocities are depth averaged; • Friction losses in the model are based on Mannings ‘n’; • The model network is constructed by quadrilaterals and triangles; • Within each element velocity and stage characteristics are calculated by a quadrilateral approximation; and • Eddy viscosity coefficients are used to define turbulence characteristics. Two different layouts of the finite element mesh were designed for the study area, one with and one without the railway causeway. The element meshes generated to model the hydrodynamics can be seen in Figure 4.4. As can be seen in these figures the meshes were refined in the region of Sandbrook Inlet to delineate the boundaries of oyster leases, marinas and mangroves. This was done as water movement in Sandbrook Inlet was of particular interest to the study. The impact of oyster leases, marinas and different types of benthic flora on the hydrodynamics in the area were incorporated by changing the friction losses in these areas. RMA-2 Model Calibration The Brooklyn finite element model was calibrated using ADCP data measured by MHL on the 16th October 2001 at four cross sections within the study area. ADCP bottom tracking profiling current meters were used to measure velocities at these sites. Although multiple cross sections were measured, the peak ebb and flood discharges were used to calibrate the model. Depth average velocity data was extracted from the RMA model results file at the where the ADCP data was measured and at the same time period. The depth averaged ADCP readings and the data extracted from the RMA results file were plotted in ArcInfo GIS for comparison. Model parameters used to calibrate the model were the Manning’s bed roughness and the eddy viscosity. These parameters were set at values based upon experience with similar types of models. The Manning’s roughness was increased for areas where mangroves, seagrasses, oyster leases and marinas were present and was 0.023 for the remainder of the model. A.1.2 Water Quality Modelling The water quality of the Brooklyn Estuary study area was modelled in terms of determining the length of time required to flush a conservative pollutant from the system. Flushing times were predicted under low flows and peak flood flows from the 20% AEP flood event and for the causeway/non-causeway option. To simulate the water quality the water quality model RMA11 (King, 1998) was used. RMA-11 is a finite element water quality model is able to use the hydrodynamic results generated by RMA2. Therefore, in this case, as the results used from the hydrodynamic modelling were depth averaged a two-dimensional approximation of water quality was calculated. To simulate the flushing of a conservative pollutant from the system, the model was configured so that the initial concentration of the pollutant was 100mg/L over the entire study area. The water quality at the boundaries of the model was then decreased from 100mg/L to 0mg/L over a short period of time. (Source: AWACS, 1997) Figure WRL COLO RIVER DESIGN FLOOD HYDROGRAPHS Report No. 2002/20 A1 fig1.doc (Source: AWACS, 1997) Figure WRL MACDONALD RIVER DESIGN FLOOD HYDROGRAPHS Report No.2002/20. A2 fig2.doc (Source: AWACS, 1997) Figure WRL SACKVILLE DESIGN FLOOD HYDROGRAPHS Report No.2002/20. A3 fig3.doc Report No.Report 2002/20 332000 334000 336000 338000 WRL FOR TYPICAL PEAK FLOOD AND EBB TIDES EBB AND FLOOD PEAK TYPICAL FOR 6290000 6290000 MODEL CALIBRATION MODEL 6288000 6288000 adcp data model data fig4.mxd Figure A4 6286000 6286000 332000 334000 336000 338000 Velocity (m/s) <0.0 0.0 to 0.10 0.10 to 0.20 0.20 to 0.30 0.30 to 0.40 0.40 to 0.50 0.50 to 0.60 0.60 to 0.70 0.70 to 0.80 0.80 to 0.90 WRL TYPICAL FLOOD TIDE PEAK VELOCITIES WITH BASE FLOWS Figure A5 Report No. 2002/20 WITH AND WITHOUT CAUSEWAY fig5.mxd Velocity (m/s) <0.0 0.0 to 0.10 0.10 to 0.20 0.20 to 0.30 0.30 to 0.40 0.40 to 0.50 0.50 to 0.60 0.60 to 0.70 0.70 to 0.80 0.80 to 0.90 0.90 to 1.0 >1.0 WRL TYPICAL EBB TIDE PEAK VELOCITIES WITH BASE FLOWS Figure A6 Report No.