4.1.3. Tide

Figure 4 shows tidal water level and current variation over a neap-spring tidal cycle near the site. Note that all current magnitudes are presented as positive. As the outfall location experiences tidal reversals, near-field dilution analysis is performed at the tidal phase when current velocities are at or near slack tide. The following 20 tide phases were considered for the analysis:


#### 4.1.4. Salinity, Temperature and Density

The average seawater salinity and temperature at the outfall area are 35 parts per thousand (ppt) and 25 °C (77 °F), respectively. No density stratification due to salinity is indicated for the coastal area near the outfall. The ambient density of the seawater was estimated based on the ambient total dissolved solids (TDS) concentration and temperature. Based on the methodology outlined in the UNESCO equation of state [20], TDS concentration of 37,600 mg/L (37.6 kg/m3 ) and temperature of 25 °C (77 °F), the seawater density is estimated as 1025.31 kg/m3 .

#### 4.1.5. Ambient Constituent Concentrations

The ambient TDS concentration of 37,600 mg/L corresponds to Stream Number 1, as shown in Figure 1, under maximum flow conditions. The ambient TSS (Total Suspended Solids) concentration was obtained as 5.3 mg/L corresponds to Stream Number 1 under average flow conditions. The ambient phosphorus concentration was obtained from measured data as 0.0135 mg/L, which was taken as a conservative value. The ambient nitrogen concentration was obtained as 0.15 mg/L corresponding to Stream Number 1. This value is estimated as an average of measured data close to the site.

#### *4.2. Diffuser Geometry*

A *Staged Fanned* diffuser type of arrangement is selected to discharge the combined effluent to the marine environment. Sketch of the diffuser is given in Figure 5. The staged fanned type produces a net horizontal momentum flux parallel to the diffuser line and gave the maximum possible dilution when compared to other diffuser types.

**Figure 4.** Variation of tidal water level and current near the site over a neap-spring cycle.

**Figure 5.** Schematic of staged fan diffuser.

The number of ports (36) was selected based on the duckbill exit velocity. For this analysis, the exit velocity was used to determine the number of ports based on the discharge rate of approximately 750 m3 /h. The flow per nozzle is then calculated as total flow divided by the number of ports (36), which equals to about 21 m3 /h. The staged fanned design type was selected to ensure adequate dilution is attained during different flow directions as well as eliminate any effluent plume hitting the surface. In CORMIX, the duckbill type check valves are modelled with steady flow effluent discharge at the design flow rate.

#### *4.3. Effluent Discharge Conditions*

For this analysis, the required dilution values for the following constituents were evaluated: TDS, Total Nitrogen (Ntot), Total Phosphorous (Ptot), Total Suspended Solids (TSS), and Aluminum (Al+).
