A major storm event occurred on Warrena Creek catchment at Coonamble during 22-23 December 2007. A 24 hour rainfall of 171 mm was recorded in Coonamble during this storm event. This is the highest 24 hour recording on record since 1886 at the Coonamble Comparison Rain Gauge. The storm caused widespread flooding in Warrena Creek along the Coonamble Levee and flood levels in Warrena Creek were 150mm below the levee crest in places. This flood event was much larger than the floods from the Warrena Creek catchment that were used to calibrate the computer models developed for the “Coonamble Levee – Flood Gradient Sensitivity Modelling Study” project undertaken by Sinclair Knight Merz (SKM) in September 2007 for the Department of Environment and Climate Change (DECC). Given the magnitude of this flooding, DECC engaged SKM to verify the performance of the computer models against this flood event.

The objective of this study was to verify the performance of the hydraulic computer model against the December 2007 event. This report is structured as follows:

• Section 1 – Introduction
• Section 2 – Available Data: Provides details on the additional data that were available since the September 2007 study
• Section 3 – Hydrologic Modelling: Estimation of inflow hydrographs for December 2007 flood event
• Section 4 – Hydraulic Modelling: Estimation of flood levels in the Castlereagh River and in Warrena Creek for the December 2007 flood event
• Section 5 – Conclusions: Key conclusions from the study regarding results on verification of computer models.