Dispersal and mixing of stormwater run-off plumes in the Port of Tauranga, New Zealand

Abstract

Buoyancy differences between two water bodies can often dominate flows such as stormwater discharge or river plumes in the coastal environment. The buoyancy difference usually arises due to differences in salinity, temperature and suspended solids. These flows form plumes or ‘gravity currents’, which can also transport pollutants and nutrients around in the receiving water body. The plume consists of a bulbous head, a mixing region on the tail, and billows behind the head. The form of the head and the plume water properties dictate what kinds of instabilities develop, which in turn influence the degree and manner of mixing that occurs. Additionally, the mixing depends on the local hydrodynamics of the receiving water body. I report observations of the dynamics of a stormwater run-off plume in a strongly tidal estuary, with particular emphasis on investigation of dispersion and dilution processes. The field site for my thesis research is the barrier-enclosed basin of Tauranga Harbour adjacent to the Port of Tauranga wharf in Mount Maunganui. The Port of Tauranga is the largest timber export port in New Zealand. The area has about 20 storm water runoff pipes that discharge into the main tidal channel of the estuary. The log handling produces bark leachates and resin acids, which get discharged during and after rain events. The leachate is responsible for a serious discolouration of the water. Several surveys were undertaken during rain events to measure plume characteristics and these are compared with a similar undertaken during dry conditions. Based on these measurements and visual observations, the plume was estimated to disperse within around four hours as the freshwater was dispersed into a relatively strong tidal flow (maximum speeds of 0.7 m/s). Acoustic Doppler Current Profiler data and conductivity-temperature-depth data indicated that the maximum across-channel extent of the plume was around 120 m and the maximum along channel extent was around 200 m (for the conditions observed). The plume stability decreases with distance from the source. The plume can be classified as a free buoyant jet or upstream intruding plume. This study will provide inputs into the toxicity assessment of the storm runoff, which will be investigated in a separate project using caged arrays of filter-feeding bivalves to determine the cumulative effects of resin acids and leachates on mussels.