The Bremer catchment is utilised for many land uses such as agriculture, grazing, mining and urban development. European settlement, and associated land clearing, agriculture, grazing activities and urban development have led to significant increases in sediment loads being exported from the catchment. Current sediment loads are estimated to be 30 times pre-European rates. Landcover change, predominantly due to vegetation removal, including riparian vegetation, has led to increased erosion from the catchment.
The application of Nature-based Solutions across the catchment provides an opportunity to revegetate relevant parts of the catchment and waterways. This will reduce the impacts of flooding in Ipswich and aid in the mitigation of future erosion and pollutant run-off by stabilising soils, capturing pollutant run-off as flow progresses downstream through riparian buffers and reducing river bank and stream erosion.
Water Technology and Griffith University undertook a range of broad scale technical (flood and water quality), economic and ecological assessments of the Bremer catchment to support and inform Nature-based Solution actions being considered by Ipswich City Council.
The study included detailed numerical modelling, technical assessments (encompassing flood hydraulics, catchment hydrology, pollutant export and waterway stabilisation), ecological and economic analyses. These actions saw the application of contemporary software packages and were informed by local data and calibrated to relevant information where available. The results of the modelling and waterway assessments were informed by extensive earlier investigations conducted within the region in recent years.
The fundamental and most significant finding of the project was that all management interventions investigated, using a holistic suite of impact metrics, had benefit cost ratios much higher than those from previous studies - which focussed on single attributes only (e.g. flooding). Importantly, a number of the scenarios had benefit cost ratios that exceed 1, which means that these scenarios should be able to be positively accepted by funding bodies. In particular, a scenario whereby management actions focussed on stabilising the immediate waterway profile of relevant sections of the Bremer River, with an additional 50m of riparian revegetation on either side of the waterway, achieved the second-highest benefit to cost ratio (1.1). This scenario achieved the second-lowest present value total cost, the joint highest avoided flood damage cost and would more than halve sediment and nitrogen export from the catchment.