Ecological Sustainability Assessment for the Firth of Thames Shellfish Aquaculture: Hydrodynamic Modelling

This report addresses hydrodynamic modelling of the Firth of Thames to produce a series of spatially resolved timeseries of velocity vectors, temperature and salinity as inputs to biological modelling. The three-dimensional numerical model MIKE3 was used to simulate hydrodynamics in the Firth of Thames. The model included temperature and salinity, and was forced by tides, winds solar radiation and river inputs. The model was calibrated against available environmental data measured during September 1999 and March 2000, during a La Nina period. No environmental data was available to calibrate for El Nino conditions. Tides dominated the instantaneous flow field in the Firth of Thames, with strongest flows in the outer Firth reaching 0.2 and 0.4 ms-1 during neap and spring tides, respectively. Tidal flows were less than 0.05 m s-1 in the shallow southern Firth. Flood tides are stronger on the eastern side of the Firth near Wilson Bay, and ebb tides stronger on the western side. Wind was of secondary importance to the instantaneous currents, but had a dominant influence on time-averaged currents, which show cumulative flow features. When winds approached from the ENE, surface currents were pushed SW, with a time-averaged clockwise circulation in the lower Firth, and deep currents returned toward the north. When winds approached from the WSW, surface currents were pushed NE, with a time-averaged anticlockwise circulation in the lower Firth, and deep currents returned towards the SW. The effects of stratification on de-coupling vertical water 'layers' means that particles near the water surface are likely to remain there and be transported faster by wind-driven flows during summer than in winter. Purpose: Auckland Regional Council, Environment Waikato and the Western Firth Consortium contracted NIWA to undertake an ecological sustainability assessment for aquaculture in the Firth of Thames. This portion of the study aimed to support biological modeling of aquaculture carrying capacity on Firth of Thames.

Numerical simulations using DHI MIKE3 model on finite-difference (regular) grid. Temperature-stratified, forced by tides, winds, solar radiation and freshwater inputs. Simulations of ~1-month duration run for spring (well mixed) and late summer (stratified). Further wind scenarios included strong NE (a la La Nina) and strong SW (a la El Nino). Table of Contents: 1.1 Introduction 2 Modelling approach 2.1 Hydrodynamic model 2.2 Model set-up 2.2.1 Bathymetry 2.2.2 Vertical grid structure 2.2.3 Turbulence closure 2.2.4 Temperature and salinity dispersion 2.2.5 Seabed resistance 2.3 Forcing inputs to the model 2.3.1 Tide 2.3.2 Stratification 2.3.3 Temperature 2.3.4 Salinity 2.3.5 Wind 2.3.6 "Worst-case" winds 2.3.7 Rivers 2.3.8 Heat exchange 2.3.9 Relative humidity 2.3.10 Model output 3 Model calibration 3.1 Tides 3.2 Temperature calibration 3.3 Residual currents 4 Results - Firth of Thames hydrodynamics 4.1 Tidal currents 4.2 Wind-driven currents 4.3 "Worst-case" winds 5 Conclusions 6 References 7 Glossary

Firth of Thames

Field data collected March 1999 to September 2000. Reporting 2005.

Frequency of collection: n/a

Model calibration data was collected during a period of La Nina conditions. No field data was available to calibrate the model for El Nino conditions. Gaps in collection: See report

Report available in .pdf format from Auckland Regional Council website. Summary and link to report included on Environment Waikato website at www.ew.govt.nz/publications Digital Format: See above

Assessment of impact of marine farming activities.

Report freely available from Environment Waikato or Auckland Regional Council. Sensitivity/Confidentiality: Report not confidential. For raw data contact Environment Waikato, Auckland Regional Council or NIWA.

Data collection completed July 2003.