Warringah Council has identified a need to prepare a drainage and flood mitigation plan for the Dee Why Town Centre. The plan is needed to accompany the Dee Why Town Centre Masterplan and Public Domain Improvements Plan that has been prepared to guide future redevelopment of the town centre. Council recognised that its existing 1D hydraulic model of the Dee Why CBD has limitations and instead decided to upgrade the floodplain model to a1D/2D model to provide a clearer understanding of the flood characteristics in and around Dee Why CBD. In view of Council’s aim to better understand overland flooding of the Dee Why CDB, it was decided to assemble both a SOBEK model and an xpswmm2D model of the study area to allow the estimated overland flood depths, velocities of flow and hazards to be compared and to identify any significant differences in overland flooding within the study area. The SOBEK model was assembled based on a 1m x 1m grid of surface elevations that was created using 12D and the ALS data supplied by Council. Buildings were represented by raising the surface elevations of all buildings within the building outlines around 10 m higher than the surrounding terrain. The xpswmm2D model of the Dee Why CBD was also based on the ALS data supplied by Council. The MapInfo building outline was imported into xpswmm2D and was used to “null” the grid points within the building outlines. The “nulling” of these grid points eliminated them from the computation scheme on the basis that flow was excluded from buildings.

Both models were run for both the 100 yr ARI and PMF events using the local inflow hydrographs obtained from Council’s MIKE-11 model of the Dee Why CBD. Overland flood depths, velocities, velocity x depth and provisional flood hazards in the 100 yr ARI and PMF events were estimated in the Dee Why CBD.

It was concluded that the SOBEK model and xpswmm2D model estimate similar 100 yr ARI and PMF overland flood levels and patterns of flow. It was concluded that around 60% of the estimated 100 yr ARI flood levels adjacent to properties are equal to or lower than the 100 yr ARI flood levels currently adopted by Council. Conversely around 40% of the estimated 100 yr ARI flood levels are higher than the 100 yr ARI flood levels currently adopted by Council.

10% of the estimated 100 yr ARI flood levels adjacent to properties are more than 0.25 m lower than currently adopted by Council. A further 10% of the estimated 100 yr ARI flood levels adjacent to properties are more than 0.33 m higher than currently adopted by Council. It was noted that overland flows in sections of Oaks Avenue, Howard Avenue, Dee Why Parade and Avon Road exceed the velocity x depth criterion for vehicular stability. Similarly the sections of the overland flowpath located above the trunk drainage line between Oaks Avenue and Howard Avenue and between Dee Why Parade and Richmond Avenue also exceed the velocity x depth criterion for pedestrian and vehicular stability.

Similarly these areas were also assessed to have a provisional high hazard rating. It was concluded that around 80% of the estimated PMF flood levels adjacent to properties are equal to or lower than the PMF flood levels currently adopted by Council.

Conversely around 20% of the estimated PMF flood levels are higher than the PMF flood levels currently adopted by Council. 10% of the estimated PMF flood levels adjacent to properties are more than around 0.6 m lower than currently adopted by Council. A further 10% of the estimated PMF flood levels adjacent to properties are more than 0.16 m higher than currently adopted by Council.

It was noted that the sections of Oaks Avenue, Howard Avenue, Dee Why Parade and Avon Road that exceed the velocity x depth criterion for vehicular stability significantly increase in a PMF. Similarly almost all the overland flowpath located above the trunk drainage line between Oaks Avenue and Richmond Avenue also exceed the velocity x depth criterion for pedestrian and vehicular stability in a PMF.

Similarly these areas were also assessed to have a provisional high hazard rating in a PMF.

Study Area

The Study Area, which comprises of the southern upper region of the Dee Why Lagoon South catchment, is presented in Figure 1 in report. The catchment draining to and through the Dee Why CBD is shown in Figure 2 in report. It has a total catchment area of 156 ha at its point of discharge to Dee Why Lagoon. The landuse is largely residential outside of the Dee Why CBD.