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File: DAM/1 30/000(0309)

14 May 2009

Mr Tim WaldronChief Executive OfficerWide Bay Water Corporation29-31 Ellengowan Street UranganHERVEY BAY, QLD 4655Dear Mr WaldronLenthalls DamBackwater AnalysisAt a meeting with David Wiskar and Peter Care of Wide Bay Water last Tuesday, I washanded a final copy of a report prepared by consultants GHD entitled "Lenthalls DamFlooding, February 2009". This reporl examined the backwater effects Lenthalls bam has onupstream properties during flood events with some emphasis on the Allan property.This report indicated the following water levels would occur at the Allan house on Lot 1 ofSP159814. Whilethe GHD studyapplied high Manning's'n'values, lacceptthata bettercalibration of their model would not alter the basic outcome that the flood immunity of theAllan house is only of the order of a5o/o to 10% AEP flood event. This immunity is furtherexacerbated by any failure of the spillway gates to open 'as destgned' and the fact that thehouse is separated from higher ground by a lower area which would restrict escape at lowerflood levels.

Scenario Feb 0B 50%AEP 20%AEP 1jYoAEP 5%AEP 2To | 1"/,AEP 1 AEPCrest Gatesclosed 28.5 28.93 30.01 30.51 31 .07 3178 | 3239Crest gatesfullyoperational

28.52 29.58 30.1 B 30.7 31 39 31 87I understand thatthe problems with the gate opening are essentially resolved follornrrng thecompletion of the project to replace the gate seals. However, the system has not yet beentested under flood conditions and I require Wide Bay Water to continue to have trainedpersonnel on site during flood operations to ensure gates can be operated manually if they

Queensland 4 AustraliaTelephone + 61 7Facsrmile + 61 7

Website www derm qld gov auAtsN 83 705 537 586

7/27/2019 Sch 1 3 DERM No 146.pdfSettlement offer 20 may 2013 MFP-2205_001.pdf Wide Bay Water Peter Care CEO WB

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fail to operate automatically as designed.Because the flood immunity of the Allan house is well in excess of the 1% AEP floodresumption level applying on many other major storages in Queensland, I would thereforerecommend that Wide Bay Water seriously consider either:

(a) Relocating the Allan house above the 1% AEP flood level at the house; or(b) Resuming those portions of the Allan property below the 1% AEP flood level.I also note that the 1% AEP flood level is also consistent with a risk management approachfor 'natural floodrng' whereby the risk to life of is of the order of 0.001 . Population at Riskand the risk to individuals should not be higher than 10 5 per annum (as spelt out in ourDepartmental Guidelines on Acceptable Flood Capacity for Dams for major upgrades ofexisting dams).I would also rbcommend that the 1Yo AEP flood level to be applied is the level that wouldoccur at the house for the 1% AEP flood event with all the spillway gates jammed in theraised position.Should you have any further enquiries, please do not hesitate to contact me on telephone3224 7636.Yours sincerely

Peter AllenDirector Dam Safety (Water Supply)

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