Modelling software general

1. The near field plume dispersion modelling software shall be capable of simulating the near field characteristics of the sewage plume discharged from a rosette type multiport outfall diffuser under various discharge flow rates and loads, current speeds and ambient water quality and stratification profiles.

2. For simulating far field water quality conditions, the modelling software shall be fully 3-dimensional capable of accurately simulating the stratified condition, salinity transport, and effect of wind and tide within the model area.

3. The far field modelling software shall consist of hydrodynamic, water quality and particle dispersion modules. These modules shall have proven record of successful applications locally and overseas.

4. The hydrodynamic and water quality modules shall be strictly mass conserved at all levels.

Model details – Calibration & Validation

1. No field data collection is required for model calibration for this study. However, the models shall be properly calibrated and validated before its use in this study, with the field data collected by:

· Hydraulic and Water Quality Studies in Victoria Harbour (1987)

· Port and Airport Development Strategy - Enhancement of WAHMO Mathematical Models (1990)

· Strategic Sewage Disposal Scheme Stage II - Oceanic Outfall, Oceanographic Surveys and Modelling (1992)

· Update on Cumulative Water Quality and Hydrological Effect of Coastal Developments and Upgrading of Assessment Tool (1998)

· Environmental and Engineering Feasibility Assessment Studies in relation to the Way Forward of the Harbour Area Treatment Scheme (2004)

· EPD’s routine monitoring data

· Tidal data from HK Observatory, Macau and relevant Mainland Authorities.

2. Tidal data shall be calibrated and validated in both frequency and time domain manner.

3. For the purpose of calibration and validation, the hydrodynamic model shall run for not less than 15 days of real sequence of tide (excluding model spin up) in both dry and wet seasons with due consideration of the time required for minimizing the effect of initial condition and convergence of results.

4. In general the hydrodynamic models shall be calibrated to the following criteria:

Criteria Level of fitness

with field data

· tidal elevation (rms) < 8 %

· maxi. phase error at HW and LW < 20 minutes

· maxi. current speed deviation < 30 %

· maximum phase error at peak speed < 20 minutes

· maximum direction error at peak speed < 15 degrees

· maximum salinity deviation < 2.5 ppt

Model details – Simulation

1. The near field model shall be used to simulate the characteristics of the sewage plume in the vicinity of the submarine outfall to determine the initial dilution, plume dimensions, rise height, merging and trapping in various flow conditions and the optimal diffuser configuration. These results shall be used, where appropriate, as inputs to the far field models. Subject to the findings of the near field plume dispersion modelling, the far field impact of any significant level of pollutants, including residual chlorine, chlorination by-products and excess sulphite, remaining outside the zone of initial dilution, shall be simulated and assessed to ascertain its impact on the receiving water system(s) and sensitive receivers.

2. The water quality modelling results shall be qualitatively explainable, and any identifiable trend and variations in water quality shall be reproduced by the model. The water quality model shall simulate and take account of the interaction of dissolved oxygen, phytoplankton, organic and inorganic nitrogen, phosphorus, silicate, BOD, temperature, suspended solids, air-water exchange, and benthic processes. It shall also simulate salinity and E. coli. Salinity results simulated by hydrodynamic models and water quality models shall be demonstrated to be consistent.

3. The hydrodynamic and water quality models must cover the tidal excursion of any discharge associated with the Project with a fine grid commensurate with the detail of assessment. In general, the model grid size shall be less than 400m in open waters and less than 75m around sensitive receivers and outfall diffusers. The grid schematization shall be agreed with EPD. A fine grid model can be used for detailed assessment but shall either be dynamically linked to or formed part of the far field model, which cover the Pearl Estuary and the Lema Channel to incorporate the effects of oceanic and Pearl River discharge processes, through nesting or gradual refinement techniques.

Modelling assessment

1. Scenarios to be assessed shall cover all phases of development being considered, including temporary discharges via the emergency outfall. Corresponding pollution load, bathymetry and coastline shall be adopted in the model set up.

2. Mixing zone analyses shall be performed using the near field model. The model shall be run for different combinations of discharge flow rates and loads, current speeds and ambient water quality and stratification profiles to simulate the sewage plume discharging into the receiving water system(s). The results shall be statistically analyzed to determine the spatial and temporal variations of pollutant concentrations in the plume and the extent and sizes of the mixing zones. Critical conditions reflecting the lowest initial dilutions and highest pollutant concentrations shall be identified and assessed with emphasis on bacterial and toxicity contents, e.g. E.coli, total residual chlorine and chlorination by-products.

3. The hydrodynamic models shall be run for (with proper model spin up) at least a real sequence of 15 days spring-neap tidal cycle in both dry season and wet season.

4. For operation phase impact assessment, the water quality model shall run for at least 30 days (excluding the spin-up time) each for the typical wet and dry seasons taking into account the variations in Pearl River discharges, solar radiation, salinity and water temperature profile and wind velocity.

5. For assessment of temporary or emergency discharges if any, the Applicant shall estimate discharge loading, pattern and duration. The worst case scenario of discharge near slack water of neap tide shall be assessed. The simulation period shall cover at least 15 days spring-neap cycle in wet season, and shall be long enough for the receiving water to recover to its original state before such discharges. Detailed methodology shall be agreed with EPD.

6. The results shall be assessed for compliance with the relevant water quality objectives and criteria and the corresponding ecological and human health impacts.

7. The potential impact or improvement of water quality with respect to bacteria content, particularly at the bathing beaches, shall be modelled and assessed.

8. All modelling input data and results shall be submitted in digital media to EPD.

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