Ecological sustainability assessment for the Firth of Thames shellfish aquaculture: Task 1 - hydrodynamic modelling

TR 2005/05
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Report: TR05/05

Author: Scott Stephens (NIWA)

Abstract

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 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.

Ecological sustainability assessment for the Firth of Thames shellfish aquaculture: Task 1 - hydrodynamic modelling [PDF, 1.2 MB]

Contents
Executive Summary I
1 Introduction and scope of work 2
1.1 Introduction 2
2 Modelling approach 4
2.1 Hydrodynamic model 4
2.2 Model set-up 4
2.2.1 Bathymetry 4
2.2.2 Vertical grid structure 6
2.2.3 Turbulence closure 6
2.2.4 Temperature and salinity dispersion 7
2.2.5 Seabed resistance 7
2.3 Forcing inputs to the model 7
2.3.1 Tide 7
2.3.2 Stratification 8
2.3.3 Temperature 9
2.3.4 Salinity 9
2.3.5 Wind 9
2.3.6 "Worst-case" winds 10
2.3.7 Rivers 13
2.3.8 Heat exchange 13
2.3.9 Relative humidity 14
2.3.10 Model output 14
3 Model calibration 16
3.1 Tides 16
3.2 Temperature calibration 17
3.3 Residual currents 20
4 Results - Firth of Thames hydrodynamics 22
4.1 Tidal currents 22
4.2 Wind-driven currents 24
4.3 "Worst-case" winds 27
5 Conclusions 30
6 References 32
7 Glossary 34