Lyall bay (NZ) circulation and implications on water quality

As outlined in a previous post, we implemented a high-resolution hydrodynamic domain to drive dispersion modelling of the untreated wastewater continuing to be discharged from the long outfall at Moa Point, Wellington. This also provides useful insights into the circulation within Lyall Bay and implications for water quality.

Lyall Bay can be thought of as two connected systems; the inner Bay, with a mostly clockwise flow regime, and the outer Bay which typically has a counter clockwise circulation pattern. See Figure 1 for the main flow patterns. 

Figure 1. The red cross indicates the outfall location.

During low wave conditions, the inner Lyall Bay water remains quite isolated from the outer Bay and the adjacent coastal circulation and its strong tidal flows. This amount of disconnection has two important effects from a water quality perspective. While untreated sewage from the offshore outfall has no direct path to the inner Bay, any sewage that does enter the inner Bay (for example due to previous weather conditions) is not readily flushed out. See Figure 2 for a typical low wave flow map. 

Figure 2.

During high waves and southerly wind conditions, a very different picture emerges - particularly during the outgoing tide stage. Here, sewage discharge from the outfall connects with a strong onshore directed flow along the west side of the outer Bay, directing sewage toward the inner Bay and the shoreline. During these southerly storms the inner Bay has a distinct clockwise circulation pattern, which means the sewage is distributed all along the beach and only slowly disperses out to the adjacent coast. See Figure 3. 

Figure 3.

During energetic waves events, onshore Stokes drift will further enhance the potential for pollutant connection with Lyall Bay and should be included in dispersion simulations (Figure 4). Our modelling has also shown it can lead to an underestimation of intrusion within the harbour if omitted.

Figure 4.

Overall, this is not a straightforward process — the configuration of these circulation cells is strongly modulated by the interplay of tidal stage, wind, wave energy, and wave incidence. A high-resolution model is therefore valuable to reliably drive the dispersion modelling.

These insights can be applied operationally to help time outfall shutdowns or flow reductions during periods when connection with Lyall Bay is most likely.

We hope these tools serve as a valuable resource for identifying and planning around expected pollution peaks while the issue remains unresolved.