Development and application of a three-dimensional water quality model in a partially-mixed estuary, Southampton Water, UK
The aim of this research was to develop a 'transportable' water quality model for the Solent and Southampton Water estuarine system, as a part of an effort to examine the effects of human activity and natural processes on estuarine water quality. Dissolved oxygen (DO), as a main indicator of water quality, is influenced by physical, chemical and biological processes, and has been chosen as the core parameter to link the different processes to be modelled. Monthly surveys of DO, planktonic community respiration rates and other water quality parameters (temperature, salinity, chlorophyll, suspended particulate matter, inorganic nutrients etc.) in the Itchen Estuary and Southampton Water were conducted from January 1998 to April 1999. DO data shows that Southampton Water is a relatively healthy estuary, despite receiving considerable loads of oxygen demanding organic sewage effluent discharged from a number of points. A persistent moderate DO sag (DO saturation > 80%) was observed in the upper Itchen Estuary throughout the year. In the lower Itchen Estuary and Southampton Water, the waters were DO saturated during the non-phytoplankton growth season. Surface DO supersaturation was observed during the phytoplankton growth season especially during algal blooms, but no severe DO depletion was detected following the bloom collapse. Community respiration rates maintained a substantial level in the upper Itchen estuary, while in the lower estuary respiration rates were low during the non-phytoplankton season and increased during the phytoplankton growth season. It is suggested that the high winter respiration rate in the upper Itchen Estuary are sustained by inputs from external sources (rivers, sewage and industrial effluents) and that the summer increase in the lower estuary is a consequence of phytoplankton photosynthesis. Nutrients in the Itchen Estuary and Southampton Water show mainly conservative behaviour in a plot of nutrient concentration against salinity. The removal of the nutrients by phytoplankton activity occurred at high salinities during the spring to summer period. A 3-D finite element baroclinic hydrodynamic model with two-equation q2-q2l turbulence closure has been developed including a mass conservation scheme. The model successfully simulated the tides, tidal currents, and estuarine circulation in the Southampton Water and Solent estuarine system. The modelled tidal induced residual currents and water mass transportation in Southampton Water and the Solent have been examined. Model results show the existence of a predominant westward tidal induced residual current in the Solent. The tidal induced residual water mass transport is extremely limited in Southampton Water, except near the entrance to Southampton Water, where it joins the Solent. The estuarine circulation with surface, seaward flowing fresher water and bottom, landward flowing saltier water provides the main mechanism for water mass transport in the model. The short residence time of waters in the estuary estimated from the survey salinity data confirmed how effective the estuarine circulation is for sea water from the Solent to replace the water within Southampton Water. The trapping effect of estuarine circulation is also crucial for the water quality in the estuary. A water quality model has been developed and coupled with the 3-D hydrodynamic model. The water quality model consists of an external (dissolved oxygen-biochemical oxygen demand) model, which models the direct impact of external inputs (riverine discharge, domestic and industrial effluents) to the water quality, and an internal model, which simulates the impact of local estuarine phytoplankton growth on the water quality. DO and dissolved inorganic nutrients are the 'link substances' between the external model and internal model. The integrated water quality model output has been compared against the survey data for 1998, and has been shown to reproduce the spatial and temporal change in oxygen, nutrients, chlorophyll and planktonic respiration in Southampton Water.