The Clarence River catchment is located on the far north coast of NSW is one of the largest on the east coast of Australia with an area of approximately 20,000 square kilometres. The floodplain contains a number of towns such as Grafton, Ulmarra, Maclean and Yamba and covers an area of 500 square kilometres.

This study examines and defines the flood behaviour of the Lower Clarence River from Mountain View (approximately 10km upstream of Grafton) to the ocean outlet at Yamba. The primary objective of the study is to define flood behaviour in the Lower Clarence River floodplain for a full range of flood events under existing catchment floodplain conditions. The events include the 5 year, 20 year, 100 year, 500 year ARI (Average Recurrence Interval) and an extreme flood event. This study may form the basis for subsequent floodplain risk management studies and plans for the entire Lower Clarence River floodplain. Detailed assessment of flood mitigation options and measures may be undertaken as part of any subsequent studies. The following recommendations are made in relation to this revision of the Lower Clarence River Flood Study:

• The 2D flood model of the Lower Clarence River floodplain should form the basis of all future floodplain risk management investigations for the study area;
• The model will require minor refinements in some areas that could be the subject of future, more detailed studies (e.g. Grafton levee overtopping);
• The length of the design flood inflows at Mountain View will have an effect on the flooding behaviour in the Lower Clarence River floodplain. Further analyses should be carried out to determine the sensitivity of flood behaviour to the assumptions regarding hydrograph length.