DEPARTMENT OF PUBLIC WORKS, N.S.W

Transcription

1 R. & p, [l;1,\i;cl LBRARY No... 0\-0' _- -=-.. DEPARTMENT OF PUBLC WORKS, N.S.W " ; "YDRAULC AND SOLS LABQRATORY.,.....,) S8AE WATE. FLOOD LEVELS..,..... > f!h'..,.rl 204 1, 1171

2 1 '. :,',".1 '-._ : _ -.- _... t.'. '. EPAR'RlENT OF RJBLC WORKS, N.S.W. HARllOURS AND RVERS BRANCH HYDRAULC AND SOLS LABORADRY BRSBANE WA'lER FLOOD LEVELS MAY 1974 Report No J11l ' 1976

3 - i - FO REW ORD During the storms of Ma;y, 1974 urrusually high water levels were reached i:.l Brisbane Water, and flooding occurred. The aim of this report is to bring together all relevant data and to record this incident as fully as possible for reference in theevent of any future engineering problems or developments arising in that area. Acknowledgement is made to Dr. A.D. Alban! of University of i.s.w. whose report "A Hydrological nvestigation of Brisbane Water" was found to contain much useful information; in parti cular, the plan of tidal gradierum, presented in FigUre 4 is extracted from that report. Acknowledgement is also made to Messr s. Foster, Gordon and Lawson, from whose report "The storms of May--June 1974" tidal and meteorological data was drawn including the charts shovm as Figures 3, 5 and 6. Part of the flood level data and most of the bench marks were provided by the Gosford Shire Engineer's Office.

4 -11- SUMMARY All levels in this report are referred to Standard Datum. The flood level reached in Brisbane Water on this occasion was generally about 1.6m, with higher or lower levels being indicated at certain points, as compared with a Mean Higher High Water level of m. t was found that the flooding was predominantly an effect of tide and ocean wave set-up.. These and various minor factors were taken into account in estimating theoretical flood levels, which were generally in good agreement with the observed values.

5 - iii - CONT E NTS Foreword E!fi2 S\1Jl1Dla.rY ntroduction 1 2. Flood Levels 1 3. Contributory Factors 3.1 Tide Atmospheric pressure Set-Up Surface Runoff Wind and Wave set-up in Broken B8\ Wind and Wave set-up wi thin Brisbane Water 3 4. Conclusions 4 i Addendum - Reliability of calculated and observed levels

6 - 1 - BRSBANE WATER FLOOD NVESTGATON 1. ntroduction On the night of May 1974 appreciable flooding occurred in the Brisbane Water area. A number of houses were entered by flood water. boats were thrown ashore or otherwise damaged, and boat sheds were demolished. This incident was associated with a widespread low-pressure system. the centre of which passed near the area on the night or the nooding. The procedure adopted in this study was first to establish the peak fioocr levels that were reached, and then to consider the factors that could have produced these levels. 2. Flood Levels Flood levels at five locations in the area were taken by Gosford Shire Engineer's staff at the time of the flooding. Subsequently, flood levels at a further eight points were established by Manly Laboratory staff. The method used by Manly staff was to obtain indications of peak levels from local residents who properties were affected by the flooding and who were present at the time; the levels so obtained were then levelled in to the nearest bench marks. This data is presented on a general plan of the area (figure 1), and tabulated (figure 2). 3. Contributory Factors The possible factors contributing to the flood levels are: (1) Tide (2) Atmospheric pressure set-up (3) Surface runoff' (4) Wind and Wave set-up in Broken Bay (5) Wind and Wave set-up within Brisbane Tater. t should be noted that the recording stations at Sydney and Newcastle are the nearest points to Brisbane Water for which tidal and meteorological, data are available. 3.1 Tide. The maximum observed tide at Fort Denison on May , was 1.48 metres at 2300 brs, as compared with 1.01m predicted tide, these levels being converted to Standard Datum. The difference of 0.47 metr es is stated

7 - 2 - by Foster, Gordon and Lawson to be due to wind and barometric set-up; these set-up effects were recalculated for the Broken Bay area. Figure 4 shows tidal gradients in Brisbane Water for the normal ran&e of tides, and relates these effects to levels at the Fort Denison gauge. These diagrams are i ncluded for comparison with the flood levels, which are sho plotted to scale on the same figure. n order to obtain an estimated tidal component for the flood levels, the M.H.W.S. gradient was adapted to the predicted Fort. Denison tide of 1.01m by proportionality. P.W.D. plan 210/161 gives an observed level at Koolewong of.52m for & Fort Denison tide of 1.01m. The conesponding level derived from the adapted gradient is.47m; this serves as a least a partial check on the approximate validity of the adapted gradient. following assumptions: 3.2 Atmospheric pressure set-up The barometric set-up was estimated to be 0.14m, using thp. (a) (b) (c) the central low pressure of the cell (see FigUre 3) was 992 mbs. the normal pressure was 1013 mbs. the zone of maximum wind velocity pas sed close to Broken Bay. Brisbane water. 3.3 Surface runoff No source of streamflow data was found for the creeks entering The Bureau of Meterology recorded SOmms of rainfall for t he recording period 9.00am 25th to 9.00 am 26th May, t is believed that most of this rainfall fell late in the day, and that backing-up of sto= drains may have contributed to flooding in some parts of the area. Several local residents reported heavy rain during the night. nflow of water resulting from surface runoff would not have caused any significant rise in level in Brisbane Water. 3.4 Wind and ','{ave set-up in Broken Bay The wind and wave conditions assumed in these calculations are as fo11ows:- (see Figures 5 and 6)

8 - 3 - Wind Southerly, 80 kts Offshore Waves - From South East, 8m. Wave period - Set up was calcule.ted for both a 10 second and a 13 second period. Wind set-up: The fetch between Barenjoey Head and Wagstaff Point was divided into two sections: 0.12m m at depth 2.30Om at 9.2m depth The combined wind set-up over the two sections was calculated to be Wave set-up:.after plotting a refraction diagram for waves approaching :Bl:oken Bay from a south-easterly direction, wave set-up was calculated as follows: For period 13 seconds: breaker height Hb.. 6.4m at 1650m from shore Set-up For period 10 seconds: Sw.. 1. Om breaker height Hb = 5.4m at 600m from shore Set-up SW.. 0.8m The combined wind and wave set-up in the vicini t was taken to be 1.Om. 3.5 Wind and wave set-up within Brisbane Vlater of Wagstaff Point t was found by Dr. Albani that wind direction and intensity varied from place to place within Brisbane Water, and also differed from those recorded in Sydney. This finding is confirmed by the fact that local residents who were interviewed gave highly contradictory accounts of wind directions during the nood. On the eastern side of Point Frederick a south easterly wind was indicated, and this is consistent with the damage that occurred at that point. On the west side of Point Frederick and also at Saratoga, both of which locations would have been sheltered from the south-east, a strong westerly wind was described. However, the abrupt change in wind direction which was recorded at Newcastle may have affected this area and so caused confusion. Wind and wave set-up were calculated for both Gosford and Point Frederick. A southerly wind of 80 Kts is assumed. Gosford Wind set-up: Fetch length (Pelican sland - Gosford) Mean depth (assumed constant) Calculated set -up 5, 100m 7.3m O.2Om

9 - 4 - Wave set-up: n order to obtain the generated wave height and period, use is made of a relationship developed by Bretschneider and others, which is. presented graphically in the Shore Protection Manual 3 (Fig. 3.25). lence Wave height co 2.Om Period T seconds. Mean beach slope m...05 Derived breaker height Hb co 2.2m and set-up Sw co.26m Total set-up at Gosford.46m Point Frederick Wind set-up: Wave set-up: Fetch lell6th (Pelican sland pt. Frederick) Mean depth Calculated set-up Wave height Period T Slope M Breaker height Bb Set-up Sw Total set-up at Point Frederick seconds.33 2.Om 26m 38m 3.00Om 7.3m.12m Set-down at Saratoga Fetch length (Saratoga-Gosford) 4,500m Mean depth 7.3m Set down.18m Conclusions The various effects calculated in section three can be summed, as set out in Figure 7. These calculated flood levels are also plotted in FigUre 4. Of the thirteen flood levels establ ished, ten are in good agreement with calculated values, despite t he fact that much of the data was obtained from untrained observers, at periods over a year after the event.

10 - 5 - The two levels at Saratoga are approximately half a metre lower than the general level, as compared with a calculated wind set-down of only 0.2 metre. The floud level indicated at Point Frederick is 1.3m above the general level. Here, however, wave action during the stom was such that two boat sheds were demolished and there was also considerable.. ave run-up, so that an inaccurate estimate of water level is understandable. The general level of 1.6m is believed to be the highest level reached in Brisbane V;ater. This condition was brought about by the coincidence of a maxdmum spring tide with a particular combination of wind and wa;ve conditions and storm surge; these in tuxn were the effects of a low pressure system having a particular location, intensity, size, track and speed. Because of the mul tiplici ty of facto:rs involved, it is not possible to estimate a racurrence pe:riod for this event. Refe:rences 1. Alb ani, A.D. A Hyd:rological nvestigation of Brisbane Water Aust. J. mar. Freshwat. Res. 1913, 24, Foster, D.N., Gordon, A.D., Lawson, N.V. The storms of ltay-june, Proc. Second Aust:ralian Conference on Coastal and Ocean Engineering Queensland 1915 P.. 3. Shore Protection Manual U.S. Army Coastal Engineering Research centre, 1913.

11 "' <".' N Point. 1,63 Frederick Clare Brisbane Water.. ' Kilometres FLOOD LEVELS w.r. t. STANDARD DATUM (STORM MAY-JUNE 1974) FG. 1

12 -J>.- "'-'-'-":"'V'- '" _ '9.. "---'.1",," _....., "';-""'r_-!.;..'-'.-,---.. j.;: 'w.{'1'..,."..1orry t, ',, FLOOD SOURCE OF BENCH : '. - DATE LOCATON NOTES \ LEVEL DATA MARK ", PRETTY BEACH Local residents..public Baths 1-45m leveled in by PWD 24M HARDY'S BAY 25M lob Aroluen Drive 1 47 " Various MAY - NOV 1975 ETTALONG BEACH 316 Booker Boy Rd " 26MW SARATOGA 65 View Porade SARATOGA Brooklyn Rd " 4/M Fair /M Agreement " 1 nc. KOOLEWONG 39 Couche Crucenl 1 60 " 82/MG GREEN PONT 6 Orono St " 50/MG Mean of three points in sloping garden PONT FREDERCK 2 90 S/SUAnk Strong SE wind and waves 8 Albany St. " BM 67/MG Set-up effect i difficulty observing flood levels ) j HARDY'S BAY Public Wharf 1 66 Shire Engineer at time BLACKWALL Blockwoll Point 1 60,. ) of flood DAVSTOWN or Central Wharf 1 62 shortly PONT CLARE :'!1 Noonan Point " "T1 after G) GOSFORD 2 04 Top Broadwater N 1. BRSBANE WATER FLOOD LEVELS, J!.b.....t0 ""1 '- :":"", :"'''<if:.,'_.,,..,

13 1500 hrs. 23rd 0900 hn. 26 th Troc.d trom r.port by FOSTER. GORDON and LAWSON 1500hn. 27 th SYNOPTC CHARTS MAY 1974 FG. 3

14 adopted from A H'tlROLOGlCAL NV1!STCOATON OF BRSBANE WATER by A. D. ALBAN Ob.. rv.d Flood Levell o Calculated Flood llv.le PONT FREDERCK -2'90 GOSFOAD GENERAL FLOOD LEVEL o <l'2 '04 01,48 o HARO 8AY -"66 1 ' 60 BLACK WALL PT. OOW, \ KOOLEWONG -"66 o PONT CLARE _ Hi3 (;Rl!:[N PT. o SARATOGA -1 '00... ' " ".,.. ",....: ::::::-::;::i':::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: '0. ' adapt.d gradi.nt, '..., '...,, ', '....,.... ',,...,,., MHWS "" MHW "'T1 o r FT. DENSON == =="''====== T-========:d=========- STA::NDA::RDDATUM.. ==. WAGST4F PT. 1 BOOKER MSL BAY ORANGE GROVE WOYWOY 9!",'llt r,.'" TDAL GRADENT N BRSBANE WATER PONT CLARE

15 :E z o (J) -0 ft1 ft1 o l> Z o o :::0 ft1 () -i C M... ::> '" a z i a... '" /1 a ZC M40 i '" " < '" Z a... " "' 20 '! 180 a 120 i NOTE ' Each plo tted po.,,' rtpres.,"" ttl. rno.imum gust in an hour / /- '"' x... f:i/.' '0.."'0 '. J. l,. X X nit _. A J......"".0.'''- _.(). /1;. _.n \...,v. v ' \ / ""' r.-- cy " '. b- / "--... ' _ 0 / _.--+_ ' ' J-T--' D '\... x \ \-.+.x -'}y. 0{> J /";-x... / _ v-. _ 0.. '.. x " 'o""r:f x x, u +. -'6,.. O K_... l..0 -O il" 78 _(Y....," d '- O,C ;:: ' - +_ () , ""--" - 0 L L- -L J- L- -L._..._.....j._+ _ +-., o " +".. / \ \.. - -t / "- x..... k. "-_"- ()_.. J.T -=?C:: + ;:-::-9:-:. Q 7 -'O'_, J....'r. LEGEND NOBBY'S - NEWCASTLE MASCOT.. 0--U-"1' " -1'.. -<.rc y... U-_o.;.....,.., :-0-<'1 0- _ ,-- ' '1., L oeoo POn' KEMBLA 1200 From 0 report by FOST!R,GORDONond LAWSON STATO" LEVEL 127 S t 70 t

16 t, PEROO OF SPECTRUM PEAK \ BOTANY BAY in_hor., \ \ \ J \ 'O!>PEROD OF SPECTRUM PEAK \ PORT KEMBLA, f2\/.j V /, ""-.. ZERD CROSSNG PEROD,; PORT M:EMBLA o',."or. E (!) W W > <t Z <t U lj.. Z (!) U1 f \ \ BOTANY BAY inshore J.- \, /'- 1../ de.p woter From a report by FOSTER. GORDON and LAWSON O MAY 1974 WAVE CONDTONS FG. 6

17 LOCATON Tidal CALCULATED VALUES Atmospheric Ocean wave nternal Set-up- S Wind Set-up Set-up Total OBSERVED LEVELS PRETTY BEACH 1 45m HARDY'S BAY HARDY'S BAY ETTA LONG BEACH 0'65m 1 19m BLACKWALL PT DAVSTOWN KOOLEWONG SARATOGA SARATOGA PONT CLARE GREEN PONT PONT FREDERCK 0' m Om m GOSFORD COMPARSON OF CALCULATED AND OBSERVED LEVELS AG.7

18 RELHlll1TYOF CALCULATED AND OBSERVED LEVSLS t is conoidered that greater'reliance should be :9laced on the observed rather than the calculated levels for the following reasons: i) The meteorological data on which the set-up calculations are based are assl1med on the basis of records at Sydney and Uewcastle, and the rrave data is based on hindcasting; the calculated set-up values must therefore be recarded as approximate. l ii) The calculated tidal components 2xe 'derived fron a tid81 gradient obtained by extrapolatio: 'me.'::roceso of extraolation from tidal conditiol1o for LH.W.S. must 81ways be regarded as s1tsject, and in addi Hon ::0)]8 uncertainty exists regardine tho tidal gradients given,\1 b.1 t"'- t f cjll, Survey Branch of he Depar C1J.:en,0 Public Works, who were responsible for caj.culctinc the tides from D:!:. Albarii&' data, f01jjld difficl.ll ty j.n relatine tile,later levels to datum. t is also noticeable that the sradients show an ay':<!arent ne(l;'ative slo!'>e in Mean Sea Level beh!een ';ifa[;staff Point and 300ker Bay (See Fig. 4). However, if ::.ny errors in this' component are only of the order of, the cufference in Mean Sea Level i.e 02m, the' 'lould be nec;list\ble compared v'i th other probable errors. iii).:l1"! estim"j.te of the accuracy of the observed levels ma J ' be obtdned from t\"/o pai2:'s of levels, the members of each 1jair being 8.djacent to each ot!1.er but ootained independently. Thus at Ha,rdys Bay levels of 1.47m Md 1.66!l were observed, and at So.ratoca 1.0Om and 1.13m.' The differences between menberz of a pair are O.19m and O.13m res!)ectively, giving a standard devio.tion of approximately O.11m.