Broadwater Assimilative Capacity Study Dye Release

Abstract

This report presents the results of a novel method to measure advection and dispersion at various locations within the Broadwater; a semi-tropical coastal lagoon located between latitudes 27.98-27.75S and longitudes 153.39-153.43E, in the state of Queensland, Australia. This water body is connected to the Pacific Ocean through a heavily modified tidal entrance named The Gold Coast Seaway. The initial proposal is detailed in the Griffith Centre for Coastal Management Report No 65. The study aims to investigate transport processes within the Broadwater through measuring the advection-dispersion coefficients of the estuary, which are a function of characteristics of the water body. Traditionally advection-dispersion models have been verified by monitoring the dispersion characteristics of a highly concentrated tracer dye (Rhodamine WT) placed into the water body to be studied. This methodology has a number of disadvantages: >> The initial bright red dye concentration can cause concern to bystanders leading to undue negative community reaction. >> The dispersion effects of the vessel monitoring the dye concentration levels can be excessive, particularly as small changes made during the early stages of the study can compound to be large effects later. >> Rhodamine dye has a density of 1.03 at 25° C and tends to float on the surface thus not giving true 3D dispersal representation. The proposed system aims to >> Deliver a controlled low-level release of rhodamine dye at 1 metre depth with a density similar to the surrounding water body. >> Establish a dye plume fixed in space and time that can be monitored in near real-time with minimal effect of the recording vessel. >> Present the results to enable verification of the advection co-efficient and dispersion co-efficient used in the MIKE21 Model. To this end, this report describes the methodology used for the dye release experiment and explains a number of initiatives, which were undertaken for more effective data collection and presentation. Final results are condensed into a table giving the rate and concentration of the dye released at source and dye concentrations at various locations measured in-situ. These values can be factored into the Broadwater Assimilative Capacity model to verify the advection co-efficient and dispersion co-efficient being used are correct. The discussion of the advection – dispersion model calibration using these data is presented in a separate report.