560f4ce9-1609-4a90-a134-17a870ab0423 urn:uuid en UTF-8 nonGeographicDataset nonGeographicDataset 2026-01-21T16:12:17 creation ISO 19115 2016 Gibbergunyah Creek Flood Study 560f4ce9-1609-4a90-a134-17a870ab0423 The <strong>Flood Study</strong> comprises two volumes: <ul> <li><strong>Volume 1</strong> comprises the report text and appendices</li> <li><strong>Volume 2</strong> contains all accompanying report figures.</li> </ul> Objectives The objectives of the Gibbergunyah Creek Flood Study are: <ul> <li>to review available flood-related information for the Gibbergunyah Creek catchment;</li> <li>to prepare design flow hydrographs describing the spatial and temporal variation in flows across the catchment using a hydrologic computer model;</li> <li>to develop a hydraulic computer model to simulate the passage of flood flows across the Gibbergunyah Creek catchment;</li> <li>to calibrate the hydrologic and hydraulic computer models to reproduce past floods; to use the calibrated computer models to define peak discharges, water levels, depths and velocities for the design 20%, 10%, 5%, 2%, 1% and 0.5% AEP floods, and the Probable Maximum Flood (PMF);</li> <li>to produce maps showing the extent, depth and velocity of floodwaters for the range of design floods; and,</li> <li>to produce maps showing provisional flood hazard and hydraulic categories for the range of design floods.</li> </ul> CONCLUSION This report documents the outcomes of investigations completed to quantify main stream and overland flooding across the Gibbergunyah Creek catchment for a full range of design floods up to and including the PMF. It provides information on design flood discharges, levels, depths, velocities as well as hydraulic categories and provisional estimates of flood hazard. Flood behaviour across the study area was defined using a hydrologic computer model of the Gibbergunyah Creek catchment as well as a two-dimensional hydraulic model incorporating all major watercourses, stormwater pipes and overland flow paths. The hydrologic computer model was developed using the XP-RAFTS software and the hydraulic model was developed using the TUFLOW software. The computer models were calibrated/verified using rainfall data and photographs for floods that occurred in 2005 and 2010. The models were subsequently used to simulate a range of design floods including the 20%, 10%, 5%, 2%, 1% and 0.5% AEP floods as well as the PMF. The following conclusions can be drawn from the results of the investigation: Flooding across the Gibbergunyah Creek catchment can occur as a result of major watercourses overtopping their banks as well as overland flooding when the capacity of the stormwater system is exceeded. Flooding can occur as a result of a variety of different storm durations. However, a storm duration of less than 3 hours typically produces the worst case flooding conditions across most of the study area. That is, relatively short, high intensity rainfall events typically produce the worst case flooding. Large areas of the Gibbergunyah Creek catchment are predicted to be inundated during each of the design floods. The majority of flow during most design floods are predicted to be contained in designated drainage areas (e.g., waterways, swales). A number of roadways and properties are also predicted to be inundated, however, the depths of inundation are typically quite shallow. As a result, most areas are subject to a low provisional flood hazard. The catchment incorporates a range of drainage infrastructure to convey storm/flood waters (e.g., culverts, stormwater pipes). The results of a blockage sensitivity analysis shows that the severity of flooding upstream of these structures can be increased due to blockage. Conversely, no blockage typically increases the severity of flooding downstream of the structures. A number of properties across the catchment are predicted to be inundated during a range of design floods. Most notably, a large section of West Mittagong, sections of Welby and the commercial sections of Easter Mittagong are predicted to be exposed to significant depths of inundation during the design 1% AEP flood. A number of roadway as well as the Main Southern Railway are predicted to be overtopped at several locations during the 1% AEP flood. This would typically render the roadways/railway impassable for up to 2 hours. The Wingecarribee Shire SES/RFS Headquarters located on the corner of Priestly St and Etheridge St at Mittagong is predicted to be inundated during floods as frequent as the 20% AEP event. Detailed floor level information should be collected to determine the susceptibility of the building to over floor flooding. Regardless, it is likely that vehicular access to the headquarters would be difficult during severe flooding with the catchment. New South Wales (NSW81093) 147.0179 147.0179 -32.1618 -32.1618 Gibbergunyah Creek Gibbergunyah Creek Flood Study documentDigital HTML Gibbergunyah Creek Flood Study documentDigital HTML