This report presents the results of a detailed technical investigation of flooding in the Scotts Creek catchment and has been sponsored by Willoughby City Council (WCC) and Department of Environment and Climate Change. Figure 1.1 shows the location of the catchment which drains the suburbs of Chatswood and Castle Cove before discharging to Sugarloaf Bay in Middle Harbour. The investigation defined flooding as far downstream as the Eastern Valley Way.

Mathematical models of the catchment and the floodplain were developed using detailed field surveys and interpreted to present a comprehensive picture of flooding under present day conditions. The study objective was to define flood behaviour in the stream in terms of flows, levels and flooding behaviour for floods ranging between 5 and 200 years average recurrence interval (ARI), as well as the PMF.

The investigation involved hydrologic modelling to assess flows in Scotts Creek between its headwaters in the Chatswood CBD area and the Eastern Valley Way. These flows were applied to a hydraulic model of the main arm of Scotts Creek to assess peak water levels and flow patterns. The hydraulic modelling extended from Havilah Street on the eastern side of Chatswood Chase to the Eastern Valley Way.

The trunk drainage system of Scotts Creek to Penshurst Street is denoted Stormwater Catchment 26 and is owned by Sydney Water. It comprises sections of piped drains, as well as covered and open concrete lined stormwater channels. The piped drainage system extends through the Chatswood CBD to the eastern side of Chatswood Chase in Havilah Street (Figure 1.2). Between Havilah Street and Baldry Street, flows on the main arm of Scotts Creek are conveyed in Sydney Water’s covered stormwater channel which follows the route of the original creek and extends as far as the Temple Emanuel School near Chatswood Avenue. An open concrete lined stormwater channel also owned by Sydney Water continues downstream and conveys flows to Penshurst Street. Willoughby City Council’s channel continues downstream of Penshurst Street to the Eastern Valley Way, and comprises sections of rock lined and “pillow concrete” lined trapezoidal channel. The total catchment area at the Eastern Valley Way is 3 km2. The headwaters of the catchment are drained by separate piped trunk drainage systems which drain the northern and southern portions of the catchment. Overland flows from the southern trunk drain are diverted into Chatswood Oval, which functions as an off-line detention basin during periods of heavy rainfall.

Peak flows from the northern portion of the catchment will be influenced by a flood storage/ water harvesting basin which will be excavated adjacent to Ferguson Lane as part of the Chatswood Civic Place project, due for construction in 2009 and situated just upstream of the intersection of that street with Archer Street. The storage characteristics of both Chatswood Oval and the flood storage basin in Ferguson Lane have been included in the hydrologic model of Scotts Creek developed in this study for the estimation of design flows.

The trunk drainage systems meet at the intersection of Archer Street and Victoria Avenue and continue eastwards as separate piped systems along both the northern and southern sides of Victoria Avenue to Havilah Street. The drainage system is of limited hydraulic capacity and has been surcharged, most recently in April 1998, when flooding was experienced in several commercial properties bordering Victoria Avenue.

It was reported that overland flows heading eastwards along Victoria Avenue in the April 1998 flood were captured by the prevailing natural surface grade and flowed into the basement car park of Chatswood Chase via the driveway entrance near the intersection with Havilah Street.

Overland flows heading eastwards down Mills Lane also entered Chatswood Chase. The car park then acted as a detention basin and had a local influence on downstream flows in the creek. To include the effect of this informal detention basin on downstream flows, the car park storage was also incorporated in the hydrologic model.

For the hydraulic modelling, which converted flows into flood levels and velocities, Willoughby City Council decided to focus on the section of the creek downstream of Havilah Street, where there is residential development bordering both sides of the creek and where flooding problems have been experienced in the past.