Establishment of a hydrological benchmark for mitigating urbanisation impacts at Logan Village, South-east Queensland

![CDATA[Estimation of flow in ungauged catchments is a difficult task as estimated flow cannot be verified directly due to the absence of recorded flood data. This paper presents a case study for the Quinzeh Creek catchment at Logan Village, South-east Queensland which includes detail flood estimation method to develop management strategies for mitigating flooding impact of urbanising upstream Yarrabilba catchment (area 20 km2). Subdivision in 1985 of some areas at Logan Village ensured all the building footprints were above the flood planning standard of 2% Annual Exceedance Probability (AEP) flood event. In 2011, the Logan City Council (LCC) released 1% AEP flood map for three reasons, namely Queensland State's Policy, Queensland Flood Inquiry Commission's recommendations and amalgamation of three different Planning Schemes. Additional properties are predicted to be affected by the 1% AEP flood map, which resulted in an unsatisfied community due to a perceived impact of this new food map on the property values and insurance premiums. The community is also concerned about flooding impacts of urbanising the upstream rural parts of the Yarrabilba catchment. The developer of Yarrabilba is committed to implementing a 'non-worsening' strategy of flood mitigation. However, the accurate flow estimation for pre-developed condition is critical for development of effective mitigation measures for the urbanisation impacts. Therefore, review of flow estimation for the pre-developed case was undertaken to establish a hydrological benchmark for the mitigation measures. The developed hydrologic model was compared with the ARR 1987 Rational Method and the ARR Regional Flood Frequency Estimation (RFFE) Model 2012 (alpha version). It was found that the ARR RFFE Model 2012 provides much higher flood quantile estimates than the rainfall-based method; however, due to lack of observed flood data the results of the adopted methods could not be independently verified. Updated hydrological benchmark along with a proposed local gauging station in Quinzeh Creek is expected to provide a greater confidence in the flood mitigation strategy among the professionals and the local communities. This approach is expected to provide valuable hydrological insights for other similar catchments.]]