WD.1368 RevC WCD dam break assessment report with appendices

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Waimea Community Dam: Dam Break AnalysisWaimea Water Ltd4.3 Summary of scenariosWhile not all identified failure modes are directly considered, the hydrological conditions identifiedcover the worst case for the range of different mechanisms. A summary of the failure scenarios andcorresponding failure modes for previous analyses is shown in Table 4-2.The assessed dam breach was located above the tallest part of the dam for all three scenarios. Thiswas to give an idea for the worst possible dam breach with the largest volume of water released.Table 4-2: Summary of breach scenarios and failure modes used in each analysis.Assessment By Scenarios Corresponding Failure Mode(s)2009 Tonkin & Taylor Sunny Day Not specified2012 Tonkin and Taylor Rainy Day Not Specified2020 Waimea Water Sunny Day Piping FM01FM02FM09Rainy Day Induced PipingRainy Day Induced OvertoppingConservative check caseFM13WD.1368 16
Waimea Community Dam: Dam Break AnalysisWaimea Water Ltd5. DAM BREACH OUTFLOWThe downstream flood wave created in a failure event is primarily dependent on the outflowhydrograph developed during a breach. These hydrographs were developed in a HEC-RAS unsteadybreach simulation. The model required the dam breach parameters and characteristics as inputs forsimulation. The HEC-RAS program applies a parametric linear breach model that requires inputs ofmaximum breach bottom width, breach slide slopes and development time. This assumes atrapezoidal breach shape.5.1 Dam Breach ParametersThe dam breach parameters important in design are the breach area, shape, and progression time.These breach parameters were estimated using knowledge of the dam’s design, potential failuremodes and use of various parametric equations. A summary of the dam dimensions is shown inTable 5-1.Table 5-1: Summary of dam dimensions used in model.ScenarioCrestLevel (mRSL) 1Reservoirlevel atfailure (mRSL)Damcrestwidth(m)DamupstreamslopeDamdownstreamslopeReservoirstoragevolume atfailure(Mm 3 )Sunny day –PipingFailureRainy Day –PipingFailureRainy day –OvertoppingFailure201.53 197.2 6 1:1.5 1:1.5 13.02201.53 202.53 6 1:1.5 1:1.5 16.61201.53 202.53 6 1:1.5 1:1.5 16.611 Parapet wall crest level at 203.13 for required freeboard.As discussed, the worst position for a dam breach to occur is with the breach centred over thehighest part of the dam with the development base at the foot of the dam. This corresponds withthe greatest water depth and largest breach area for the failure. This breach position was adopted inthe T&T report and the same logic will be used in this assessment. T&T took the base elevation ofthe breach to be at RL 150 m. With updated designs of the dam, and assuming failure will occur atthe base of the plinth, a new breach bottom elevation of 155.1 m was used. This was one of theprominent changes made in the updated assessment.The breach parametric equations are summarized by Wahl (Wahl, 1998). These parameter equationsare based on a large number of historic failures of earth and rockfill dams. Newer regression modelssuch as Froehlich 2008 and Xu and Zhang 2009 were also used in this assessment. The differentformulas provide a range of values for the breach widths, side slopes and development times. Therange for the parameters was estimated using three of the regression equations; Froehlich 1995,Froehlich 2008 and Xu and Zhang 2009. The range of values and the final selected values for theseparameters are shown in Table 5-2. The breach parameters adopted in this design were taken fromWD.1368 17
Page 1 and 2: Waimea Community Dam: DamBreak Anal
Page 3 and 4: Waimea Community Dam: Dam Break Ana
Page 11 and 12: Elevation (m RL)Waimea Community Da
Page 13 and 14: Elevation (m RL)Waimea Community Da
Page 15: Waimea Community Dam: Dam Break Ana
Page 21 and 22: Discharge (m 3 /s)Waimea Community
Page 31 and 32: Discharge (m3/s)Discharge (m 3 /s)W
Page 39 and 40: Table 6-7: Final flood wave arrival
Page 45 and 46: Inundation Depth (m) - Sunny Day Pi
Page 47: WAIMEA COMMUNITY DAMPREPARED BY REV
Page 50 and 51: Inundation Depth (m) - Rainy Day Ov
Page 52: WAIMEA COMMUNITY DAMPREPARED BY REV
Page 55: 24 November 2020Mr Iain LonieEngine

WD.1368 RevC WCD dam break assessment report with appendices

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