Great Mackerel Beach Floodplain Risk Management Study & Plan

Study area

As previously mentioned, Great Mackerel Beach (Figure 1) is a small coastal community on the western shore of Pittwater. The entrance system at Great Mackerel Beach is termed an ICOLL (Intermittent Open and Closed Lake and Lagoon) as the entrance to the lagoon is periodically closed. Only a small portion of the catchment in the lower valley has been developed. The community consists of only three streets (with approximately 130 properties) and no vehicular traffic. Ross Smith Parade is in the form of a footpath along the Pittwater foreshore, while Monash Avenue and Diggers Crescent are wide grassed areas that provide access to properties.

The upper catchment of Great Mackerel Creek is characterised by steep slopes with an incised creek valley, which changes to a flat floodplain area prior to the outlet into Pittwater. Great Mackerel Beach (approximately 650 m long and crescent shaped) is enclosed between two high headlands. The main flow path through Great Mackerel Beach is Great Mackerel Creek itself, which discharges into Pittwater via a berm at the northern end of the beach (Photos 1 and 2). The creek forms a small lagoon at the northern end of the beach (Photo 3), fed by an intermittent stream which rises to the plateau of the Lambert Peninsula. The lagoon is often open to tidal action and supports a shallow nearshore sandbar (or delta). The state of the entrance (whether open or closed) during a flood plays a significant part in the resulting flood levels. A brief overview of the history of the entrance is provided in Section 4.3.2.

The study area covers only those areas in the community affected by mainstream flooding (Figure 3). This includes 116 houses (on 120 residential lots) which have some flood-affectation in a Probable Maximum Flood (PMF).

Report

The report contains study area, proposed measures in order of priority, the community consultation process, the existing flood environment, the cost of flooding, floodplain risk management measures as will as climate change implications and adaptive strategies.