The purpose of the study was to define flood behaviour in the Peach Tree Creek floodplain, which is located on the right bank of the Nepean. River between the M4 Motorway and the Nepean River/Peach Tree Creek junction (Figure 1.1). Lyall & Macoun Consulting Engineers (LMCE) had previously carried out hydraulic studies of the floodplain to define 1% and 5% annual exceedance probability (AEP) flooding and to investigate proposed filling activities near the Panthers Club. A computer based quasi - two dimensional hydraulic model was used to define flood levels, flow patterns and velocities. A preliminary design was also prepared for a water quality control basin in the lower reaches of Peach Tree Creek just upstream of the Great Western Highway bridge. The results of those investigations were presented in LMCE 1991 (a), and 1991 (b).

In a supplementary study, LMCE 1991 (c), the hydraulic model was used to investigate the effects of measures aimed at reducing flood levels.

The scope of work involved an investigation of flooding within the Peach Tree Creek flood plain for the following design floods:

• 1% AEP
• 1% AEP flood levels in the Nepean River ± 300 mm (as a sensitivity study)
• 0.5% AEP

Stage and discharge hydrographs in the Nepean River over the reach adjacent to the study area were derived from the recent studies carried out by the Water Board and are reproduced in Appendix C.

The work described herein included an assessment of the effects of various combinations of river flooding and local catchment runoff. The results are presented as tabulations of peak flood levels (Tables 4.3 and 4.5), stage and discharge hydrographs (Figures 3.1 to 3.5) and plans showing flood contours, water surface profiles, flow distributions and velocities (Figures 4.1 to 4.3).

The hydraulic model has been configured to allow for the detailed analysis of flood behaviour for the range of floods up to the 0.5% AEP event, including river floods and local catchment floods, either in combination or in isolation. With appropriate modification, the model could be used for the- evaluation of flood management measures in a future Floodplain Management Study. Extension of the model cross sections and some of the weir relationships will be required to model an extreme flood event.