??????This report represents the first stage of floodplain risk management study for the North Parramatta Precinct in the Baulkham Hills Shire Council Local Government Area, which involves identifying flood behaviour through a flood study in accordance with the NSW Governments (2001) "Floodplain Management Manual" .

Based on the brief issued by Baulkham Hills Shire Council, a total of two development scenarios were assessed for the flood study, being:

• Existing development conditions, and
• Ultimate development conditions.

The existing development scenario included modelling the existing stormwater infrastructure within the catchment, adopting current landuse patterns. Because much of the study area has not experienced significant urban re-development, no onsite detention was included in the existing development condition DRAINS model, except for Northmead Reserve.

The ultimate development conditions DRAINS model assumed that all the catchment was developed in accordance with Council's development control plans (DCP's), and all development utilised onsite detention (OSD) in accordance with the Upper Parramatta Catchment River Trust (UPCRT) OSD policy. The use of OSD on these developments limits the peak flows being generated off the catchment for all storms up to the 100 year ARI. It should be noted that the method used to estimate the impact of providing OSD is a simplification of OSD design and operation and was undertaken to assess the likely impact of providing OSD for future development on peak flows from the catchments.

The flood study used the D NS hydrological model and HEC-RAS hydraulic model for estimating peak flows and flood levels. The hydrological model was used to estimate the peak pipe and overland flows within the study area. Pit and pipe survey data was used in D NS to estimate the existing peak flows along overland flow paths for the 5, 10, 20, 100 and 200 year Average Recurrence Interval (ARI) storms and the Probable Maximum Flood (PMF). The hydraulic grade line (HGL) in each pit is also modelled by DRAINS.

The hydrological modelling identified six locations within the catchment where peak overland flows exceed 0.5 m3/s through private property in the 100 year ARI storm. These locations included:

• Northmead Reserve (west) to Anderson Rd sag,
• Downstream of Anderson Rd sag,
• Downstream of Mary Street sag,
• Windsor Rd - Murray St to Darling Mills Creek,
• Downstream of Lennox to Opal Place
• Campbell St Sag to Darling Mills Creek,

The DRAINS model estimated that all the overland flow paths in the catchment would exhibit significant peak flows during the PMF, particularly areas within road reserves for both existing and ultimate development conditions.

The peak flows for the PMF, 20 and 100 year ARI storms were used in the hydraulic model to estimate flood levels within the above areas. The hydraulic model utilised aerial laser survey (ALS), which in addition to limited ground survey, was used to define the overland flow path geometry within these areas. The HEC-RAS modelling was also used to estimate flood extents within the overland flow paths. The flood extents shown on the figures within this report are approximate and should be viewed in conjunction with this report.

It is intended that the floodplain risk management study, which would follow this study, will examine the feasibility of flood mitigation options and planning issues for reducing the flood risk in the six flooding problem areas. The floodplain risk management study will examine the existing, future and continuing flood risk and may provide any of the following flood mitigation options:

• Capital works targeted at reducing overland flows, such as upgrading of the existing stormwater infrastructure in selected areas,
• Capital works targeted at providing safe overland flow paths,
• Voluntary purchase of properties that are likely to be frequently flooded, lie within high hazard areas or that create adverse flooding conditions to other properties,
• Flood proofing of existing dwellings,
• House raising,
• Provision of community based on-site detention to alleviate flooding of properties downstream,
• Reviewing Council' s existing planning controls in relation to flood risk management,
• Utilising planning controls such as rezoning to a more flood compatible land use, and
• Revising and/or confirming flood planning levels in the North Parramatta Precinct.

The floodplain risk management study should also estimate flood damages to examine the benefit/cost of proposed flood mitigation options and integrate environmental opportunities into such works where possible.