Modelling the removal of an earth bund to maximise seawater ingress into a coastal wetland

Weed infestation is a critical management issue for maintaining the natural values of coastal wetlands across the world. Widespread use of herbicides to control weeds has resulted in environmental issues in the past and has led to the search for more natural control methods such as using saline water. This study investigates management options to restore the natural flow regimes between freshwater wetlands and seawater by removing a bund which was built to grow ponded pasture. The study was carried out in the Mungalla wetland in Queensland, which is an important Nywaigi aboriginal community asset for recreation, cultural and economic activities. The study used modelling and monitoring methods to (a) assess how effective the earth bund was in excluding seawater and (b) the extent to which seawater could enter the wetland without the bund. Eleven scenarios were investigated to estimate seawater ingress under different tide, onshore wind, low atmospheric pressure and sea level rise (SLR) conditions. Results show that removal of the bund reinstated periodic tidal ingress into the wetland. Seawater intrusion was also shown to increase when there were onshore winds and/or low atmospheric pressure associated with cyclones. The greatest impact was under future SLR, where large increases in the inundation frequency and extent are likely to cause a shift in the wetland vegetation towards native salt tolerant species. Findings of this study are useful for examining the potential impact of various management interventions that are being considered for wetland system repair. For example, the removal or height adjustment of tidal barriers, dredging of silted streams, removal of weeds from choked streams and reintroduction of tidal flows to control weed infestation, improve water quality and restore natural values to the wetlands.