Flood Study Report Wellington

This flood study identifies the nature and extent of the flooding problem in Wellington. Estimates of design discharges, velocities and levels for the 5%, 2% 1% and 0.002% AEP events are provided.

Design discharges were calculated using flood frequency analysis. A complex flood frequency procedure was necessary to account for the attenuation of floods by Burrendong Dam and the coincidence of peaks on the Macquarie and Bell Rivers. It is very difficult to estimate the accuracy of the design discharges predicted by the flood frequency analysis. Several factors limit the accuracy of the discharges obtained, including:

• Uncertainty associated with gauging station rating curves and,
• Relatively short length of records for hydrologic data.

Design flood levels and velocities were calculated using a computer based one-dimensional unsteady flow hydraulic model. Flood levels have been quoted to the nearest 0.1 m, the accepted limit of accuracy for hydraulic models. This aspect must be borne in mind when using the results for development purposes.

Flood mitigation options should be considered as part of a floodplain management study. The MIKE 11 hydraulic model developed as part of this study can be used to model flood mitigation options.

Three rock weirs have recently been constructed in the Bell River to control the bed and bank erosion. Over a period the many years, the bed level of the Bell River may rise as sediment is deposited upstream of these weirs. The effect this would have on flood levels should also be considered in the floodplain management study.

Until a floodplain management study and floodplain management plan can be completed, council should formulate a interim flood policy. This interim plan should be based on the levels and velocities contained in this report.