Overtopping was calculated for the seawall and revetment sections of shoreline where assets were located behind the structures. The significant investment in these shore protection structures and the development that has occurred behind means that these structures are likely to be maintained indefinitely.  Note, as sea levels increase, the maintenance requirements of these structures will also increase.

Assessing the overtopping gives an indication of which scenario results in overtopping discharges in excess of safe limits, or scenarios under which the structures may fail. For the vertical wall section, these have been calculated for the narrowest lowest sections of beach beneath the structure (i.e. not where sedimentation between the groynes has elevated the beach level), this is where the greatest water depth at the toe of the structure is able to transmit the largest waves (Figure 6-16). This will ensure the most conservative estimate of overtopping discharge (i.e. worst case). Note, the assessment was done on the lower section of the vertical seawall beneath the lower promenade, rather than the upper wall that is flush with street level. The street elevation and upper wall is approximately 1.5 m higher and should be considered in subsequent risk studies. The assessment used the Lonsdale Bight wave and storm-tide conditions (summarised in Table 6-6). 

Figure 6-16    Locations of overtopping calculations at Point Lonsdale (a) vertical seawall in front of township (b) rock revetment near Lawrence Rd (Images: Google)

The lack of complexity of flooding in the very low-lying foreshore area of Fisherman’s Flats inside the Queenscliff Cut meant that a static inundation model was sufficient to identify the hazards. It is noted that the LiDAR used in the assessment was captured prior to the refurbishment of the Queenscliff harbour and marina. BoQ provided design drawings of the developed areas to confirm the new land levels and quay wall elevations are the same or higher than those in the LiDAR to ensure the assessment was still valid. The flow paths into the inundated area are from the Fisherman’s Flats. The extents within this vicinity will need checking in future revisions of this assessment when new datasets become available. The model used the West Channel Pile design storm-tide levels as they are considered to be more representative of the inner Swan Bay and Fisherman’s Flats areas (Table 6-6).

A simple static model was used for this very low and very flat area. No erosion figures have been produced for Swan Bay as the overriding hazard here is inundation.   

The model used a grid size of 4 m in order to capture the required detail. Culverts were incorporated into the model at five locations along the main drainage path identified by the pink dots in Figure 6-17. Key development features such as roads, railway embankments, drains and shore protection structures were checked to ensure they were picked up as topographic features, and were refined where necessary.

The model was run for the 1% AEP storm-tide case in combination with a number of sea-level rise scenarios to determine the extents of inundation that may arise. The model used the West Channel Pile design storm-tide levels as they are representative of the inner Swan Bay area (Table 6-6).

Figure 6-17      Extent of Lakers Cutting model, showing tidal inflow and culvert locations (pink circles)