Gibbergunyah Creek Flood Study

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.

Tags: Gibbergunyah Creek
Organization: Wingecarribee Shire Council