ST LEONARDS COASTAL HAZARD VULNERABILITY ASSESSMENT

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1 Level 12, 333 Collins Street Melbourne VIC 3000 Australia Telephone: Facsimile: WorleyParsons Services Pty Ltd ABN November 2011 Ref: File: Melbourne Alan Grant Corporate Strategy & Property Services City of Greater Geelong PO Box 104 Geelong, 3220 Dear Alan, ST LEONARDS COASTAL HAZARD VULNERABILITY ASSESSMENT WorleyParsons has been engaged by the City of Greater Geelong to undertake a Coastal Hazard Vulnerability Assessment (CHVA) for two adjacent sites at 1 Hopetoun St and 31 Dudley Parade, St Leonards. St Leonards is the most easterly township on the Bellarine Peninsula with development focussed along its coastline. The study site is almost 400m from the coastline and 200m north-east of St Leonards Lake, which has been constructed within the alignment of a St Leonards Creek. The catchment area for this watercourse is approximately 11km 2, and its outlet is at the St Leonards Foreshore. The elevation of the site is between 3m and 3.5m AHD. The site and key locations are shown in Figure 1 (overleaf). Under the Victorian Coastal Strategy 2008 (VCS) new development in this area, including change in land use, needs to consider potential hazards associated with predicted rise in sea level. In accordance with the VCS, coastal hazards considered in this assessment are: Coastal inundation due to elevated coastal water level Coastal erosion / shoreline retreat Literature Review The Victorian Coastal Strategy,2008 (VCS) recognises climate change as a significant factor impacting Victoria s coastline. The VCS directs planning decisions for coastal Victoria to consider the impacts of increased sea levels to 2100 of 0.8m, as well as the combination of effects of tides, storm surges, coastal processes and other local conditions. k:\fraser\environment\projects\ _st leonards coastal hazard va\03_reports\ _en_let_01.doc

2 A report by the CSIRO (The effect of climate change on extreme sea levels in Port Phillip Bay, CSIRO 2009) provides refinement of the VCS estimates in the Port Phillip Bay region. The report considered the implications of four scenarios for sea level rise brought about by climate change around the Port Phillip Bay coastline. Of these, the sea level rise of the IPCC 2007 A1F1 scenario in combination with high wind speed appears the most appropriate to apply in this CHVA, as it predicts a sea level rise of 0.82m for 2100, which is consistent with the level adopted in the Victorian Coastal Strategy. Study Site Figure 1 St Leonards, key locations The CSIRO report provides storm tide levels for the 100 year average recurrence interval (ARI) evaluated by estimating tide (incorporating astronomical tide) and surge heights separately, and then combining these to estimate storm tide and sea level rise water levels for various locations around Port Phillip Bay. Queenscliffe, Geelong, Werribee and Williamstown are the closest locations included in the storm tide predictions. None of these locations provide a perfect match for the coastal conditions at St Leonards. Williamstown levels are adopted for this study because _EN_LET_01.doc 2 22 November 2011

3 they provide conversion to AHD and the levels identified are conservative, being in the higher range compared to other locations. The 100 year return interval storm tide height predicted for 2100 at Williamstown is 2.32m AHD, as shown in Table 1. Table 1 Current and future predicted storm tide heights at Williamstown, 100 yr ARI (m AHD) (CSIRO, 2009) Coastal Hazard Vulnerability Assessment With regard to local conditions, the St Leonards coastline is low-lying and is likely to be impacted from coastal hazards associated with sea level rise. Impacts would arise from combined coastal processes including elevated coastal water levels, net sediment transport and storm induced erosion. The two key coastal hazards in assessing vulnerability are erosion and inundation. Erosion Given the distance from the coastline, with substantial infrastructure and existing development between, the site is not considered to be vulnerable to erosion arising from coastal processes. Inundation There is potential of inundation of sites which are not at the immediate coastline through coinciding storm surge events with overland flooding. In assessing the vulnerability of the site to inundation, the peak oceanic conditions are considered together with peak flood conditions. For the sites in question, this is flooding of St Leonards Creek, together with overland flow generated from the immediate surrounding catchment. Flow Conditions Peak flow rate The peak 100 year ARI flow rate for St Leonards Creek has been established by the City of Greater Geelong as 26m 3 /s. This is based on a catchment area of 11.3km 2, and runoff coefficient of approximately 25% (pers.comm). We are confident with the assumptions made in establishing this as the peak flow rate for the waterway under existing conditions. Overland flow generated in the local catchment around the Dudley Parade and Hopetoun Street sites was estimated to be 2.4m 3 /s for the 100 year ARI event. This is based on a catchment area of 0.06km 2 and runoff coefficient of 40%, and assumes that the pipe drainage network is ineffective due to backing up of the drainage outlet at the lake or through blockage _EN_LET_01.doc 3 22 November 2011

4 Tailwater levels The 100 year ARI storm tide heights discussed above have been used to estimate tail water levels, with an additional allowance for wave setup. Based on experience with similar sites in Port Phillip Bay, we estimate that an allowance of 0.3m for wave setup in additional to storm tide level is conservative. The downstream tailwater levels (coastal water levels) used in the assessment of inundation are shown in Table 2. Peak flood levels Contour data of 0.5m intervals supplied by the City of Greater Geelong was used to set up a HecRas model. HecRas is a one dimensional hydraulic modelling package widely used to establish peak flood conditions. Several cross sections representing the landscape and watercourse of St Leonards Creek upstream of the site of interest and to the ocean outlet were established. A tributary representing Dudley Parade was also input into the model. The model was run for steady state conditions with ocean boundary conditions for each of the 2011, 2030, 2070 and 2100 scenarios. The water surface level results in metres AHD, for each scenario were taken at St Leonards Lake downstream of Dudley Parade, and at the corner of Dudley Parade and Hoptoun Street, and are shown in the table below (locations of results shown in Figure 1). Table 2 Predicted Peak Water Surface Levels Site Water Surface Levels (m AHD) Tailwater Level (coastal water level) St Leonards Lake Cnr Dudley Pde & Hopetoun Street 2.4m 3 /s Conclusions The hydraulic behaviour of the catchment exhibits only minor variation of water surface levels at St Leonards Lake with changes to the tailwater ocean boundary level. When overland flooding of the site subcatchments coincides with raised lake levels brought about by peak ocean conditions, the surface level of overland flooding has been modelled to reach 3.16mAHD. Under these conditions, this level is not controlled by the downstream boundary condition, with the flow rate generated from the immediate local catchment having a stronger influence upon the water surface level at the site. We therefore conclude that, while the site may be vulnerable to localised _EN_LET_01.doc 4 22 November 2011

5 flooding, it is not likely to be vulnerable to coastal erosion or inundation from predicted increased sea levels out to at least Limitations of the analysis The analysis described above is preliminary only, using relatively coarse contour elevation data and simplistic one dimensional hydraulic flow modelling. The assessment provides an overview of likely effects of overland hydraulics in response to increases in peak sea water conditions. However, this assessment has adopted a conservative approach, including the following: Adopting storm tide heights from Williamstown, which are higher than other westerly locations of Port Philip Bay; Adopting 0.3m for wave set up; and Coincidence of the 100 yr flow rate in St Leonards Creek, locally generated overland flows and the 100 year storm tide level is an event of extremely low probability. Please contact me on if you have any queries regarding this report. Yours sincerely WorleyParsons Christian Taylor Lead Coastal Engineer _EN_LET_01.doc 5 22 November 2011