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

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Hewitt’s Creek Flood Study December 2002Wollongong City CouncilAvailable Data• Geomarine (1991) ‘Coastal Process Assessment - Lot 47 DP 737406 Surfers ParadeThirroul’• Gutteridge Haskins & Davey (1989) ‘Hooker Land Development Thirroul, Flood Study’• Gutteridge Haskins & Davey (1989) ‘Hooker Land Development, Thirroul Stage V,Major Constraints Study, Summary.’3.8 DESIGN RAINFALL INTENSITIESIn accordance with current design practice, rainfall intensities for design storm bursts of a givenAverage Recurrence Interval and duration have been calculated from the procedures of AR&R(1987). The six design rainfall stations described in Table 3.15 have been chosen to reflect thevariation of design rainfall across the catchment. It should be noted that these design gaugesare ‘synthetic’ gauges with no physical significance, developed from the data of ARR ‘87 solelyfor the purpose of representing design rainfall across the catchment in subsequent hydrologicmodelling (refer Appendix 2.1 for the plan location of these ‘design’ gauges).TABLE 3.15DESIGN GAUGE LOCATIONSDESIGN GAUGELOCATIONARR #1Bulli Lookout/Kiosk on edge of escarpmentARR #2Bulli Colliery - End of Hobart Street, BulliARR #3Mount Ousley Road above BulliARR #4Hewitts Creek at McCauley’s BeachARR #5Bulli Pass below bend and elbowARR #6End of Fords Road, ThirroulDesign Intensity - Frequency - Duration plots (IFD) for these six ‘design’ gauges are set out inAppendix 13 of the Compendium of Data together with IFD data for the nearby Public Workspluviograph stations.3.9 DESIGN TIDE & OCEAN CONDITIONSBasic tidal statistics for Port Kembla extracted from the Department of Defence (Navy Office)Australian National Tide Tables are reproduced in Table 3.16 below.Datum for the tide recorders in Sydney & Port Hacking is Indian Spring Low Water (ISLW) whiledatum for the Port Kembla recorder is Port Kembla Hydrographic Datum (PKHD). Levels canbe converted to Australian Height Datum (AHD) using the following conversions:RL (AHD) = RL (ISLW) - 0.925 mRL (AHD) = RL (PKHD) - 0.878 mLevels in the following table have been adjusted to AHD using the second of the aboverelationships.TABLE 3.16TIDAL STATISTICS - PORT KEMBLA21
Hewitt’s Creek Flood Study December 2002Wollongong City CouncilAvailable DataVALUE HAT MHHW MLHW MSL MHLW MLLW LATm AHD 1.12 0.62 0.32 -0.08 -0.58 -0.58 -0.88Source : Department of Defence Navy Office (1990)The ‘effective’ or ‘design’ ocean level at the outlet of a creek during a flood is a combination oftide, plus the rise in sea level (setup) adjacent to the shore due to:• The drop in barometric pressure• The surface shear of onshore winds• Wave set up and runupA preliminary estimate of ‘setup’ was prepared by N.S.W. Public Works (1989) for the Mouth ofLittle Lake, a small estuary some 25 km to the south of Hewitts Creek, in which the componentsof setup were as set out in Table 3.17 below.TABLE 3.17ESTIMATED SETUP - WARILLA BEACHSETUP COMPONENT 5 % AEP 2% AEP 1% AEPPRESSURE DROP (m) 0.29 0.31 0.33WIND SHEAR (m) 0.05 0.06 0.07WAVE ACTION (m) 0.61 0.76 0.81TOTAL SETUP (m) 0.95 1.13 1.21Source : NSW Public Works (1989)A ‘design’ ocean level of a given AEP may then be developed by combining an appropriatetidal level with the above setup. This approach does however require consideration of the jointprobability of occurrence of tide and setup. Notwithstanding these difficulties it is reasonable toassume that the 1% AEP design ocean level is likely to be, on a simplistic basis, somewherebetween (HAT + 1.21 = 2.33 m AHD) and (MHHW + 1.21 = 1.83 m AHD).More recent work by the NSW Pubic Works (1992) has quantified the level of Tide-Storm Surgepresent along the mid New South Wales coast, eliminating the need to consider the jointprobability of occurrence of tide and setup. This has in turn permitted more reliable estimates tobe made of elevated ocean conditions adjacent to the shore. The Tide-Storm surge levelrelationship for Sydney and Port Kembla extracted from the above study is reproduced in Table3.18 below. ‘Design’ ocean levels at the shoreline may then be determined by adding anappropriate wave setup/runup component to this level.TABLE 3.18TIDE STORM SURGE LEVELSMID NSW COASTEVENT AEP 5% AEP 2% AEP 1% AEPOCEAN LEVEL m (AHD) 1.40 1.46 1.49Source : NSW Public Works (1992)22
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