Report 001 rev P2a Sandon Point Flood Study Climate ... - Stockland

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Hewitt’s Creek Flood Study December 2002Wollongong City CouncilAvailable DataEstimates of the elevated ‘design’ ocean level at the shoreline particular to specific sites in theregion have been made by AWACS (1991) at Warilla Beach, by Geomarine (1991) at thesubject site and by the NSW Public Works (1987) at Shoalhaven Heads.Notwithstanding the considerable difference in analytical approach, geometry of the sites andtheir geographic separation, estimates do not differ greatly as may be seen by reference toTable 3.19.TABLE 3.19ESTIMATED ELEVATED ‘DESIGN’ OCEAN LEVELSSOURCE & LOCATION5% AEP 2% AEP 1% AEPNSW PW (1987) HEADS OPEN (m AHD) 2.0 2.1 2.2NSW PW (1987) HEADS CLOSED (m AHD) 2.1 2.2 2.4AWACS (1991) WARILLA BEACH (m AHD) 1.9 2.1 2.2GEOMARINE (1989) THIRROUL (m AHD) 2.4Note :The above estimates exclude consideration of global warming (Greenhouse Effect)The above estimates include the effect of Tide, Wind Shear, Barometric Pressure and Wave Setup.Geomarine have in addition considered what an ‘extreme’ ocean event might mean in regard toelevated ocean level at the subject site and conclude that, in the long term, an extreme eventcould elevate the ‘design’ ocean level at the outlet to 3.7m AHD. This estimate is based on amid to long term estimate of ocean rise due to global warming of 0.3m, added to a 1% AEPocean Tide-Storm Surge level of 2.4m AHD, together with an additional 1m in waverunup/setup, bringing the effective wave runup/setup height to 1.8m in the ‘extreme’ event andthe total elevated ‘design’ ocean level to 3.7m AHD.On the basis of the preceding analysis, ‘design’ elevated ocean levels have been adopted asfollows:TABLE 3.20ADOPTED ‘DESIGN’ OCEAN LEVELSOCEAN CONDITION @ MCCAULEYS BEACHADOPTED LEVEL(m AHD)For an extreme ocean condition including an allowance of 0.3 m forglobal warming. 3.7For a design 1% AEP ocean condition including an allowance of0.3 m for global warming. 2.7For a design 5% AEP ocean condition including an allowance of0.3 m for global warming. 2.4For a conservatively high but ‘likely’ tidal/ocean condition at thepeak of a design flood (MLHW + 0.7 m). 1.0In considering flooding in the outfall reaches of the Creek systems, it is apparent that majorflooding in these lower reaches could occur with no runoff at all in the creek, flooding in theoutfall reach developing in response to a high tide or other elevated ocean condition.23
Hewitt’s Creek Flood Study December 2002Wollongong City CouncilAvailable DataIn recognition of the two mechanisms controlling flooding in an outfall reach, the ‘design’ floodsurface in each outfall reach has been developed from the envelope of;a) flooding due to storm runoff with a conservatively high ‘likely’ ocean level at RL 1.0mAHD; andb) flooding due to an elevated ocean of the specified ARI penetrating the outfall reachwithout attenuation, with negligible stream flow in the creek system.It is noted that the combination of the peak 1% AEP flood discharge with the unattenuated levelof a 1% AEP ocean event (an extremely rare combination) does little to alter flood levelsupstream of the rear dune line, from those obtained by the simpler approach described above.The approach described above has therefore been adopted in this study.24
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Report 001 rev P2a Sandon Point Flood Study Climate ... - Stockland

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