Historic dam failures and recent incidents. Tracey Williamson Associate Director Water (Dams & Reservoirs) British Dam Society Chairman

Transcription

1 Historic dam failures and recent incidents Tracey Williamson Associate Director Water (Dams & Reservoirs) British Dam Society Chairman 1

2 Agenda 1. Dams in all their beauty 2. Hazards associated with dams 3. History of dam failures around the world & in the UK 4. Links to UK dam safety legislation & guidance 5. Recent near misses & incidents 2

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7 Hazards and dam failure modes Overtopping Internal erosion Instability 7

8 Hazards and dam failure modes Foundation failure & appurtenant works failure 8

9 Hazards and dam failure modes Spillway failure 9

10 History of dam failures around the world Dam Dam type Country Height (m) Reservoir volume (10 6 m 3 ) Date built Date Failure Type Deaths Vega de Tera CMB Spain SF 144 Malpasset CA France FF 421 Vaiont CA Italy L 2600 Baldwin Hills Emb USA IE 5 Frias Emb Argentina OF >42 Teton Emb USA IE 14 Machhu II Emb India OF 2000 Bagauda Emb Nigeria OF 50 Belci Emb Romania OF 25 Gouhou Emb China IE 400 Zeizoun Emb Syria OF 20 Shakidor Emb Pakistan OF >135 Situ Gintung Emb Indonesia IE 100 Dam type: CA = concrete arch, CMB = concrete and masonry buttress, Emb = embankment. Type of failure: IE = internal erosion, FF = foundation failure, OF = overtopping during flood, SF = structural failure on first filling, L = landslide into the reservoir causing overtopping 10

11 History of dam failures around the world Malpasset arch dam failure in France in 1959 (421 deaths) The causes: High uplift pressures following heavy rainfall & a weakness in the left abutment rock Lessons learnt: Appropriate SI and assessment by experts in all areas of dam design 11

12 History of dam failures around the world Vaiont dam overtopping incident in Italy in 1963 (2600 deaths) The causes: Instability of reservoir slopes causing a landslip & 125m high wave over the dam Lessons learnt: Measure pore water pressures & movements at depth as well as at the surface 12

13 History of dam failures in the UK Dam Height (m) Reservoir volume (10 3 m 3 ) Date built Failure Date Type Deaths Tunnel End OF 1 Diggle Moss (Black Moss) OF 6 Whinhill IE 31 Brent (Welsh Harp) OF 2 Glanderston 1842 OF 8 Bold Venture (Darwen) OF 12 Bilberry IE 81 Dale Dyke 29 3, IE 244 Cwm Carne OF 12 Castle Malgwyn 1875 OF 2 Clydach Vale 1910 OF 5 Skelmorlie OF 5 Eigiau & Coedty , Type of failure: IE = internal erosion, FF = foundation failure, OF = overtopping during flood FF OF 16 13

14 History of dam failures in the UK Dale Dyke dam breach in 1864 (244 deaths) The causes: Internal erosion possibly caused by hydraulic fracture of the core Lessons learnt: Designs include wider cores, use of cohesive & compacted fill and placing pipes in tunnels through natural ground 14

15 Links to UK dam safety legislation & guidance Dam failures Dale Dyke (244 dead) in 1864 Incidents of leaks through foundations in the 1870s Developments in legislation & guidance 1864: Designs include specifications for fill to be worked in layers not exceeding 0.23m 1872: Tunnels driven through natural ground Upstream valve control on pipes through dams 1876: First use of concrete cut-off trenches 1879: First use of grouting to seal foundations 1882: Vyrnwy dam designed with a drainage tunnel network to reduce uplift pressures 15

16 History of dam failures in the UK Eigiau & Coedty dam failures in 1925 (16 deaths) The causes: Foundation failure of Eigiau & overtopping failure of Coedty Eigiau dam failure Lessons learnt: Dams need to be designed, supervised and inspected by qualified engineers 16

17 Coedty dam failure Devastation in Dolgarrog

18 Links to UK dam safety legislation & guidance Dam failures Skelmorie (5 dead) & Dolgarrog (16 dead) in 1925 Major slips at 3 dams during 1937 due to high pore pressures caused by faster construction rates Developments in legislation & guidance 1930: Reservoirs (Safety Provisions) Act periodic inspection by a qualified engineer became mandatory 1937: Soil mechanics used in designs for the first time 1940s: Berms added to designs to stabilise slopes 1955: Drainage blankets & instrumentation included in designs to control construction pore pressure 18

19 Recent near misses & dam incidents Since 1925, no loss of life due to dam disasters in the UK However, dams have breached & many recent near misses Average age of dams in the UK is 115 years How will weather extremes impact potential failure modes of the UK s ageing stock of reservoirs? 19

20 Recent dam failure with no loss of life Warmwithens dam failure in 1970 The causes: Internal erosion along the line of a new tunnel Lessons learnt: Tunnelling works through embankment dams need to be carefully designed in terms of understanding changes to stresses & stability. 20

21 Links to UK dam safety legislation & guidance Dam failures Incidents at Lluest Wen in 1969 and Warmwithens in 1970 Construction failure of Carsington dam in 1984 Developments in legislation & guidance 1975: Reservoirs Act : Publication of Floods and Reservoirs Safety 1986: Reservoirs Act 1975 implemented New roles for enforcement, supervision & undertakers 1990s: Publication of further guidance on embankment dams 21

22 Recent near misses & dam incidents Ulley dam spillway failure in 2007 The causes: Masonry blocks plucked out due to turbulence. Overtopping of the spillway walls. Lessons learnt: Spillway designs to ensure sufficient capacity so flows are discharged safely away from the dam 22

23 Links to UK dam safety legislation & guidance Dam failures Failure of stepped masonry spillway at Boltby dam in 2005 and Ulley in 2007 Developments in legislation & guidance 2007: Post-incident reporting system established 2008: The Pitt Report 2010: Floods and Water Management Act 2010: Guide on design of masonry stepped spillways 23

24 Recent near misses & dam incidents Rhymney Bridge spillway failure Plucking of bricks as water flows at high velocity Erosion beneath the slabs Uplift pressure forces 6t slabs to fracture 24

25 25 Repairs commenced immediately in difficult weather conditions

26 26 24hr working using rapid set concrete

27 High volume pumps to assist with emergency draw down Careful management of the res levels to avoid rapid draw down 27

28 Preparations made for an auxiliary spillway as a precaution CCTV patched to Gold Command where all decisions were made Spillway half complete when an extreme storm event was forecast Reservoir rose 13m in 8 hours due to snow thaw and rainfall 28

29 Dam overflowed at in dreadful weather conditions All contingencies in place - sandbags stockpiled, staff & excavators at the ready LRFs notified Evacuation plans ready Gold Command ready The repairs held 29

30 Before After 30

31 Oroville dam spillway failure Located 75 miles north of Sacramento; completed in 1968 At 235 m high, it is the tallest dam in the U.S. 31

32 Timeline of events Early to mid-jan: series of storms; main spillway gates opened 7 th Feb: flows increase; spillway damage discovered; flows stopped 32

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36 8 th to 10 th Feb: reservoir continues to fill 11 th to 12 th Feb: emergency spillway overflows the hillside began eroding uphill, threatening to collapse the concrete lip, causing the top 10m of the reservoir to empty 36

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39 12 th Feb: main spillway gates opened again to reduce flows over the emergency spillway & evacuation commenced 13 th Feb: crews fill eroded hillside with concrete & rocks 39

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41 41 13 th to 26 th Feb: main spillway flows continue erosion

42 42 27 th Feb: with the reservoir lowered, flows stopped down the main spillway to allow assessment of damage

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48 28 th Feb: works commence to clear the debris at the base of the main spillway

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51 Current state of play The 180,000 evacuees have returned home Crews continue to place millions of tons of rocks and concrete in the emergency spillway Crews have cleared debris out of the river channel below the main spillway The Hyatt Power Plant has reopened In the coming months, crews will begin to fix the main spillway So far an estimated $200 million has been spent 51

52 Theories on the causes of the Oroville dam spillway failure Cavitation: If defects were present in the concrete spillway, water flowing over these may have created turbulence that formed bubbles that collapsed with powerful force, breaking apart the concrete Uplift pressures: Hydraulic uplift pressure (due to water below the spillway) may have built up, lifting the slabs 52

53 Theories on the causes of the Oroville dam spillway failure Internal erosion: Sealant between the concrete slabs may have deteriorated, allowing water to seep underneath the spillway The water then may have eroded the soil underneath the spillway The slabs may have then collapsed into the voids Or a combination of these causes? 53

54 Lessons learnt Ongoing and timely maintenance and monitoring of spillway channels Replacement of deteriorating joint sealant between slabs Ensure pressure relief drainage is functioning Maintenance of underdrains Consider undertaking NDT to check for voids underneath spillways Consider employing leakage detection techniques to check for any flow paths underneath spillways 54

55 Acknowledgements I d like to thank Welsh Water for their permission to share the learning from the spillway incident at Rhymney Bridge, and acknowledge their commitment and management of the incident that led to a successful conclusion. 55

56 ANY QUESTIONS?