Dispersal of dredging plumes in Tauranga Harbour, New Zealand: A field study

Abstract

Water quality standards associated with dredging have become more stringent, requiring better monitoring and prediction. Here, we describe the dynamics and development of plumes generated during two dredging cycles and how they vary with respect to time and distance from the dredging activity. Backscatter signals were measured using a boat-mounted acoustic Doppler current profiler (ADCP) and total suspended solid concentrations (TSS) were determined from water samples. Results show that background TSS ranged from 7 to 9 mg l‒1 whilst dredging plumes exhibited a vertical gradient of TSS ranging from 9 to 15 mg l‒1 near the surface (0–2 m), and 24 to 70 mg l‒1 near the bottom (10–12 m). ADCP transects conducted during and after dredging showed that the plume dissipated from the dredged area within 1 hour. Final transects (~1 hour after the dredging ended), revealed backscatter signals ranging from background levels to ~1.2 times greater than the background. Based on TSS concentrations and time for plume dispersion, previous studies indicated that a plume with duration of 1 to 2 hours and TSS concentration around 70 mg l‒1 is below the threshold for causing serious impacts to the biota; therefore, only minor effects can be expected for the two dredging plumes monitored.