Degree Name

Doctor of Philosophy


Faculty of Engineering


The effectiveness of a floodgate modification strategy that involved restoring tidal flushing via modified floodgates to improve surface water quality and combat acid sulphate soil leachate was investigated in a low-lying flood mitigation drain near Berry in southeastern NSW, Australia. Prior to floodgate modifications, extensive soil, groundwater and surface water analysis indicated ubiquitous acidic conditions that were exacerbated by low drain water levels maintained by the one-way floodgates. The floodgates also increased a range of environmental problems including metal flocculation, low dissolved oxygen levels, pyrite oxidation, and acid transport. Temporal data showed that total drain water acidity was highest after rainfall but decreased when buffering agents inherent in brackish creek water penetrated upstream of the floodgate.

Before floodgate modifications were undertaken, tidal restoration techniques and criteria were developed to address surface water quality, hydraulics, and design concerns. First, an ion-association mixing program was developed and calibrated to simulate water quality and reaction kinetics within the drain. Second, hydraulic calculations and GIS mapping techniques indicated that full tidal restoration could be permitted solely within the primary drain. Floodgate design criteria were then developed and two modified floodgates were constructed and installed. One of these designs, the automated 'Environmentally Controlled Smart Gate System', was developed to permit tidal flushing based on real-time environmental parameters while utilising state-of-the-art technologies.

After floodgate modifications, tidal buffering and dilution improved drain water quality during all flow regimes and was most effective during prolonged dry periods. Tidal flushing also decreased the acid reservoir effect, permitted fish passage, reduced soluble iron concentrations and elevated the groundwater table. The impact of tidal forcing on the groundwater table was investigated biogeochemically through multi-port piezometers, while saline intrusion was simulated using 3-Dimensional finite element techniques. These investigations showed that tidal forcing at the study site was limited due to moderate lateral soil hydraulic conductivity values. Furthermore, while calibrated for the study site conditions, the finite element model and tidal buffering techniques can be easily adopted to other low-lying estuarine sites throughout Australia.

The floodgate modification strategy was shown to improve drain water quality, decrease groundwater drawdown, and improve drain hydraulics. However, before future floodgate modifications are undertaken several concerns should be addressed. These concerns and the findings from the entire thesis were summarised within a comprehensive best management practice developed to assist in effectively, efficiently, and safely restoring tidal flushing to additional sites.


Accompanying disc (Appendix F) can be consulted with the hard copy of the thesis in the Archives Collection, call no. is 628.55/24