This report is structured as follows:

Section 1 – Introduction

Section 2 – Background:

Explanation of the need for this overland flow study, of Fairfield’s overland flow study program, the situation of this study area, the study objectives, and the history of flooding in the catchment

Section 3 – Available Data:

Overview of the data collection process for this study

Section 4 – Catchment and Stormwater Model Development:

Explanation of the development of the different hydrological and stormwater modelling approaches used in this study

Section 5 – Hydraulic Modelling:

Background to the development of the two-dimensional hydraulic model used in this study

Section 6 – Sensitivity of Hydrological Modelling:

Evaluation of the different hydrological and stormwater modelling approaches and comparison of their results

Section 7 – Flood Mapping Results:

Use of the study modelling to derive flood extent mapping and flood risk precinct mapping

Section 8 – Conclusions:

Key conclusions from the study regarding modelling methodology and results.

Study Area

The study area for Canley Corridor Overland Flood Study is located south west of Orphan School Creek, between the suburbs of Canley Vale and Canley Heights and Cabramatta. The Canley Corridor catchment has a north easterly aspect and drains overland into Orphan School Creek, which is the main tributary of Prospect Creek. The 258 hectare catchment has a range of land uses including residential, commercial and light industrial. The upper and mid catchment area is largely medium density residential. The lower catchment includes a range of commercial and industrial areas, separated from Orphan School Creek by an open space corridor.

Study Objectives

Key objectives of this study are:

• 1) To test the sensitivity of overland flood modelling using different assumptions of the capacity of the existing stormwater drainage system. It will compare the drainage system at different levels of detail, and evaluate the benefit provided by increasingly detailed models. Conclusions from this study will establish the methodology of subsequent studies for the remainder of the LGA catchments
• 2) To identify the major overland flow paths within the Canley Corridor catchment study area. The study area is detailed in Section 2.4 and includes parts of sub-catchments 13, 14, 15 and 16 as identified in the preliminary city-wide Overland Flood Study (SKM, 2004). The Canley Corridor study area is shown in Figure 1-1.

Other objectives of the study are to:

• Identify properties at risk of local overland flooding
• Assess provisional flood hazard to identified properties due to local overland flooding for the 1% AEP (Annual Exceedance Probability) and the PMF (Probable Maximum Flood)
• Carry out field verification of identified overland flow paths at selected locations
• Prepare flood extent and flood risk precinct maps for the study area for the 1% AEP and PMF events
• Produce flood results (flood level, velocity and flow) for the 5, 20, 100 ARI year and PMF events

Conclusions

Hydrologic Modelling Methodology

Comparison of the different hydrologic models indicates that while detailed modelling provides a more precise picture of flooding, this would not necessarily translate into a different assessment of flood risk, or in significantly different planning controls being applied. All three hydrologic modelling approaches (Detailed DRAINS, Limited DRAINS and RAFTS) resulted in similar high flood risk areas being identified along the main overland flow paths, and reproduced the known historical ‘trouble spots’ within the catchment. However the simpler models tended to overestimate flood risk due to coarser representation of flow travel times and artificial concentration of flows at fewer inflow boundaries.

Production of a detailed DRAINS model including all pits and pipes in the catchment was estimated to be four to five times more expensive than production of a limited DRAINS model, and approximately 14 times more expensive than production of a RAFTS model. This reflects the different amount of survey data required and the complexity of the model construction. The preferred approach identified by the study is a limited DRAINS model representing larger sized pipes in the stormwater network. This approach can be as acceptable in determining significant areas of risk as a detailed DRAINS model approach, if the extent of the modelling stormwater network reflects known trouble spots in the catchment, and represents the major elements of the trunk drainage system.

Canley Corridor Flood Behaviour and Risk

Peak flood depths on most properties are less than 0.5 metres, although there are some areas in the upper catchment where depths are between 0.5 and 1.0 metres. Similarly, flow velocities across most properties are generally below 0.5 metres per second, although higher velocities are seen in many streets and across some upper catchment properties.

A “Zone of Significant Flow” has also been identified where it is important that overland flowpaths are kept clear. It contains much of the 100 year ARI extent in the upper catchment, where flowpath blockage caused by fences, large buildings and debris can significantly increase water levels and divert water onto nearby properties.

These maps only represent flooding due to runoff from within the Canley Corridor catchment.

Those parts of the Corridor along the banks of Orphan School Creek may also be at risk from mainstream flooding, generated in the upper Orphan School Creek catchment to the north and west of the Canley Corridor. Mainstream flood extents for Orphan School Creek are reported in the Flood Study for Orphan School Creek, Green Valley Creek and Clear Paddock Creek (Sinclair Knight Merz & Fairfield Consulting Services, 2008). The Canley Corridor Overland Flood Study has been successful in achieving its objectives which were to:

• test the sensitivity of overland flood modelling to different assumptions about the capacity of the stormwater drainage system
• define flood behaviour and identify the major overland flow paths within the Canley Corridor catchment, and
• identify properties at risk of local overland flooding and to prepare flood risk precinct maps.

The study has also:

• established methodologies for modelling and flood risk mapping for future overland flood studies for Fairfield LGA
• provided maps which are more meaningful to Council officers, development proponents and the community
• provided a good foundation from which to prepare the floodplain risk management study and plan, particularly with regards to flood emergency response, as the next step in the floodplain risk management process