The upper catchments of the Manilla and Namoi River catchments are located to the northwest of Tamworth in the New England region of NSW. Both rivers drain large catchments (2075 square kilometres and 3075 square kilometres respectively) before they confluence at Manilla, approximately 25 km upstream of Keepit Dam on the Namoi River. The Manilla River headwaters are located near Split Yard Mountain in the Nandewar Range and drains in a north easterly direction to the Plumthorpe area, before turning south-east towards the town of Barraba. Approximately 20km south of Barraba, the Manilla River discharges into Split Rock Dam (storage volume 400 GL). From Split Rock Dam, the Manilla River drains southwards for a further 25km before it confluences with the Namoi River within the town of Manilla.

The primary objective of this study was to define the main-stem flood behaviour under historical conditions and design flood behaviour under existing and future climate conditions in the study area. The study produced information on flood levels, depths, velocities, flows, hydraulic categories, and provisional hazard categories for a full range of design and historical flood events. In addition, the study produced estimates of flood damage. To achieve this objective, the study collected, compiled and reviewed all available relevant data (including survey, aerial photography and satellite imagery). The design events included the 0.5%, 1%, 2%, 10% and 20% AEP events together with the Probable Maximum Flood. In addition, the potential impacts of climate change on flooding were assessed together with a number of other sensitivity analysis.