Observations of salinity, flushing time and dispersion in the Waitemata Estuary

Abstract

ABSTRACT The physical behaviour of estuaries controls the transport of substances from freshwater to the ocean. We conducted the first investigation of salinity distribution and its response to spring-neap tides and freshwater flow rates in the Waitemata Estuary, and estimated several estuarine parameters. Four surveys were conducted: two surveys were conducted in summer during spring-tide, low flow and neap-tide, high flow conditions and two in June during spring-tide, high flow and neap-tide, low flow conditions. The vertical and longitudinal salinity variations were measured with shipboard surveys and river flow data were provided from gauged stations. We found the Waitemata Estuary can be classified as a well-mixed estuary due to the combination of strong tidal currents and weak river flow. Highwater slack stratification can reach 0.5 psu over 10 m during times of weak tidal mixing (neap tides) and with increased freshwater flows after a storm. The estimated freshwater flushing time varied from a week to a month. Dispersion coefficients varied from 50 to 150 m2 s−1 on average, depending on river flows and the location along the channel. Using the Simpson number and historical river flows, we expect stronger stratification could develop in the upper estuary during high flows, temporarily creating a partially-mixed estuary.