The catchment area to Old Pittwater Road is 11.9 hectares (Figure 1). Downstream of Old Pittwater Road runoff flows through the Warringah Mall Shopping Centre and into Brookvale Creek through Warringah Golf Course. Ultimately the runoff enters Manly Lagoon and discharges into the Pacific Ocean. The catchment area to Old Pittwater Road is entirely comprised of residential urban development, there are no non-residential developments. The catchment has two main tributary branches entering from Ryan Place which join upstream of Brookvale Avenue within No. 21 Ryan Place. In view of the increasing urban development and the need to accurately define the nature and extent of the flood problem, Warringah Council engaged Webb, McKeown & Associates to undertake an Overland Flood Study. The term overland has been used to signify that flooding results from runoff exceeding the capacity of the piped drainage system and/or is of sufficient magnitude to represent a hazard, as opposed to flooding from the overtopping of a natural or artificial defined creek system. The only area where there is some form of defined creek system is immediately downstream of Ryan Place. The remainder of the natural system has been destroyed over time by urban development. Today overland flow resulting from heavy rainfall does not generally follow a defined path but travels between buildings, under or through fences, along driveways, roads, etc., until it reaches Old Pittwater Road. The route may vary depending upon the size of the rainfall event and may change as residents construct fences or buildings. The primary objectives of this Overland Flood Study are:,

This report has been prepared for Warringah Council to define the nature and extent of flood in the Dee Why South Catchment. Flood modelling was completed to define flood behaviour for a range of storm events from 1 year Average Recurrence Interval (ARI) to Probable Maximum Flood (PMF). The modelling shows that significant flows are conveyed in the piped drainage network and overland through roads and properties. Generally the main overland flowpath starts from several branches at Alfred Street to Beverley Job Park. Flows in the open channel at Victor Road and Redman Road combine with overland flows from Mooramba Road, Fisher Road, and Pittwater Road at the intersection of Redman Road and Pittwater Road. Overland flows are then conveyed along several roads and properties to Dee Why Lagoon as well as in the open channels between Pacific Parade / Oaks Avenue and downstream of Dee Why Parade. In a 1% AEP event, the results show that ponding of runoff occurs at several locations with restricted outlet capacity. High pedestrian areas in Dee Why CBD also experience overland flow inundation, particularly along Redman Road, Pittwater Road, Oaks Avenue and Howard Avenue. In a 1% AEP event floodway and high provisional hazard flow conditions along the open channels, on roads, and some properties including in the Dee Why CBD. A series of climate change scenarios were also modelled to evaluate potential impacts from elevated sea levels and increased rainfall intensity. Results showed that the modelled increases to Lagoon level have an impact to flood inundation of low-lying land near the Lagoon. Most properties within the Dee Why South catchment do not show a significant change in inundation extent for the modelled scenarios. Modelled increases in rainfall intensity showed a rise in peak water levels across the catchment, particularly in trapped low points such as on Sturdee Parade and Alfred Street. Council applies land use planning and development controls to manage development within flood prone areas. The flood result and mapping provided in the document will assist Council with future land use planning, development controls and floodplain risk management. The next stage of the floodplain risk management process following the adoption of the Flood Study is the Floodplain Risk Management Study and Plan. This next stage will investigate various floodplain risk management measures and prioritise these measures for implementation.

Mudflat Creek is a 123 hectare catchment which drains to Hardys Bay through the lower Killcare district (refer Figures 1 and 2). The lower section of the catchment is predominantly occupied by urban residential development. A natural escarpment divides the lower section of the catchment from the upper plateau area. This upper plateau is predominantly natural or rural land type with some residential development around Stewart Street, The Scenic Road and Wards Hill Road (Figure 2). In light of reported flooding incidents in the study area, and following on from the Killcare Catchment Drainage Investigation that was completed in 1999 (Reference 1), Gosford City Council engaged Webb, McKeown & Associates to undertake a Flood Study. The primary objectives of this Flood Study are:,

SWMM

The 32km2 Erina Creek catchment is one of the major tributaries entering Brisbane Water at East Gosford (Figure 1). Figure 2 shows a digital terrain image of the study area and Figures 3 to 5 show further detail of the study area. Figure 6 indicates the current land use zones. The Erina Creek Flood Study Review 1990 was completed in 1991 (Reference 1), which utilised information available and current at the time hydrologic and hydraulic models to produce design flood profiles along the lower parts of Erina Creek. The aim of the present study is to update the Erina Creek Flood Study Review 1990 with currently available data, notably the use of airborne laser scanning (ALS) data and more sophisticated hydraulic modelling techniques as well as verification to the June 2007 event. All earlier flood studies of Erina Creek were superseded with completion of the Erina Creek Flood Study Review 1990. WMAwater was engaged by Gosford City Council to undertake the Erina Creek Flood Study Review utilising current technology and data. The information and results obtained from the study will provide a basis for development of targeted stormwater management strategies and a subsequent Floodplain Risk Management Study and Plan. The study was developed in order to meet the primary objective of defining the flood behaviour (50%, 20%, 10%, 5%, 2%, 1%, 0.5%, 0.2% AEP and the Probable Maximum Flood (PMF) design events) along the major tributaries in the Erina Creek catchment. The study area was significantly greater than for the previous 1991 Flood Study. This report details the results and findings of the Flood Study investigations. The key elements include:,

Final Report and Mapping Compendium

TUFLOW

The Mudflat Creek catchment is characterised by a distinct upper and lower section (refer Figure 1). The upper section of the catchment is located in the plateau area of Killcare Heights. This section of the catchment comprises of residential development around Wards Hill Road, The Scenic Road and Stewart Street together with a large proportion of natural bushland or rural type land. From the plateau the catchment slopes very steeply down undeveloped, densely forested slopes to the area bounded by Fraser Road and Hardys Bay. This lower section is relatively flat and low lying. Runoff from the plateau area drains to Fraser Road via two natural gullies. Pipe and overland flow systems convey flows from these natural gullies, through the residential areas to Mudflat Creek. Mudflat Creek then travels through the rear of properties 37-63 Fraser Road before reaching the bridge in Noble Road and its outlet to Hardys Bay. Measures and Comparisons in report are:,

Spatial data only- creekline, culvert locations, RAFTS layout, sub catchments, SOBEK x-sections, catchment boundary.