The Nepean River Flood Study at Penrith was commenced in March 1995 to consolidate previous studies covering this area and establish flooding behaviour under existing floodplain conditions. The study area encompasses that part of the Nepean River and the floodplain from the M4 Motorway bridge to downstream of Mill Dam Falls near Devlins Road, as shown on Figure 1. On the left bank (looking downstream), it includes extensive residential areas in Emu Plains and Emu Heights as well as extractive industries and the prison farm, while on the right bank it features a mixture of residential, recreational, light industrial, extractive industries and commercial areas. The study was complex in that it was required to model a mixture of developed and undeveloped areas. The Penrith Lakes scheme was also a major feature on the right bank floodplain downstream of Victoria Bridge.

The flood study was undertaken using a two dimensional (2D) numerical hydraulic model to determine flood behaviour. The model was calibrated and validated using two historical floods, and then used to predict flood levels for a wide range of design floods. To this end, the modelling examined flooding of this area associated with Nepean River floods having the 10%, 5%, 2%,1%,0.5%,0.2%,0.1% annual exceedance probability (AEP), as well as the probable maximum flood (PMF).

The study identified that the pattern of flooding over the floodplains in the study area generally followed remnant natural watercourses where they remain, such as in the Penrith Lakes and Peach Tree Creek areas, and follow those streets and lined drainage channels which approximate the remnants of natural drainage paths through built up areas, such as Emu Plains and the Penrith CBD.

Flooding (from the Nepean River) of floodplains upstream of Victoria Bridge was found to be finely balanced at approximately the currently estimated 1% AEP Nepean River flood level.

A 10% increase in the adopted 1% AEP flood flow (which is within the typical accuracy of such estimates), raised the Nepean River level by up to 0.5m, which was enough to cause significant spillage into both the Emu Plains and Peach Tree Creek areas at several locations, while a similar decrease resulted in no over bank flow from the Nepean River on Emu Plains. Local catchment flows were found to have little influence on main stream flood levels, however backwater effects from the floodplains produces higher flood levels within the local drainage system.

Between the M4 Motorway and Victoria Bridges, there was a progression of breakouts on the left and right banks which occurred as the floods rose. In general, the Peach Tree Creek floodplain (right bank looking downstream) experienced flooding by waters backing up the creek and was more susceptible to flooding than the Emu Plains area, where the progression of breakouts was found to be generally from upstream to downstream. Backwater flooding in the lower reaches of Peach Tree Creek occurred for frequent events (5% AEP) while the Emu Plains area was impacted only by floods approaching the 1% AEP level and the Penrith CBD was only impacted by floods in excess of the 0.5% AEP.

Downstream of Penrith Weir, flooding in Emu Heights was primarily caused by backwater floods from the Nepean River, while flooding in the Penrith Lakes Scheme area developed from a combination of backwater flooding and over bank breakouts.

For floods greater than the 1% AEP level, the Main Western Railway embankment was a major flow control in the floodplain.

The 1% AEP design flood approximates the onset of flooding at Penrith. The estimate may vary with time as more flood data becomes available and analytical techniques become more advanced. Inevitably, floods larger (less frequent) than the 1% AEP event will occur, resulting in wide spread inundation of the floodplain. The flood of record, which occurred in 1867 with an estimated discharge of 20000 m³/s would have completely inundated the Emu Plains, Peach Tree Creek and Penrith Lakes areas, as well as partially flooding the Penrith CBO and North Penrith areas. The upper limit of flooding, referred to as the probable maximum flood, would exceed the 1% AEP flood level in the Penrith and Emu Plains areas by up to 6.5 metres.

The model as developed, and the results derived from its use in this study, can be used as a basis for a Floodplain Management Study for the Nepean River at Penrith.