This report provides the Flood Study component of the floodplain risk management process and details the investigation of flood behaviour associated with the ‘urban’ area of the township of Pallamallawa. It has been undertaken by Lawson & Treloar (L&T) in conjunction with similar studies for other townships in the Moree Plains Shire Council (MPSC) region including Boggabilla, Boomi, Mungindi and Garah. Pallamallawa is situated on the northern fringe of the Gwydir River floodplain. The township is subject to flooding from the Gwydir River as well as from more localised catchment flooding from the North East of the township. This investigation considers township flooding that results from Gwydir River breakout, which has occurred to varying degrees over the years, including as recently as 1998.
The approach undertaken for the Pallamallawa flooding investigation has included the following key work items:
- Identification of areas requiring additional ground survey;
- Site Inspections and Historic Flood Data Gathering;
- Extraction of discharges and water levels from DIPNR’s existing Gwydir River Mike 11 model for use in developing boundary conditions and for calibration of L&T’s finer scale hydraulic modelling.
- Flood frequency analysis to confirm design flows for the 5%, 2% and 1% AEP events.
- Detailed hydraulic modelling of historic flood events for calibration purposes followed by modelling of the design flood events for the 5%, 2% and 1% annual exceedance probability (AEP) events.
