Cofferdam Performance Hurricanes Harvey & Irma

Transcription

1 Cofferdam Performance Hurricanes Harvey & Irma Greg Hammer Senior Geotechnical Engineer CELRH-DSPC-GS DSMMCX Third Workshop on Case Histories in Dam Safety Risk-Informed Decision Making 2018 USSD Annual Conference Miami, Florida 03 May, 2018 US Army Corps of Engineers BUILDING STRONG

2 Risk Assessment for Cofferdam Design PURPOSE; Cofferdam design considerations for Herbert Hoover Dike (HHD) and Addicks & Barker dams modifications CONTEXT; Risk assessment for design crest elevation SCOPE; Cut & cover construction to replace outlets 2

3 Background Risk applications for Dam Safety initiated in early 80 s Risk has been a part of cofferdam design much longer No agency has clear guidance for cofferdam design USBR 5 times construction period (you hope!) USACE ER , 1995, use risk guidance 3

4 Risk Implications Considerations D/S consequences, Life loss, Project Costs, Site Loss ACE/ P failure for Dam Overtopping ACE/ P failure for Cofferdam Consequences of Breach of Dam/ Cofferdam Damage ($) Loss of Life Project site loss Equipment, Personnel, Sitework {ACE annual chance of exceedence} 4

5 Considerations Environmental Risks Construction period Schedule On time, or extensions needed? Probability of success? Seasonal issues? Winter cold concerns River/ lake icing Rainy season or Hurricanes 5

6 Considerations Design Efficiency & Effectiveness Cofferdam reliability stability/ integrity P failure of structural failure modes ACE Overtopping(height) vs Construction Cost Landside protection? Channel considerations Backwater/ Tailwater ACE/P failure of Downstream cofferdam 6

7 Herbert Hoover Dike Lake Okeechobee 143 mile perimeter embankment 730 sq mile surface area Approx. 30 miles across Avg. water depth is 9 feet 7

8 Herbert Hoover Dike Inflow from Kissimmee Valley River Basin Inflow exceeds discharge capacity One foot of rain on 5600 sq. mile basin can raise pool by 3-4 feet Max discharge yields approx. 0.1 feet drawdown Wind can have a significant effect on the lake levels across the project such that the reported lake elevation is an average of 8 gages LEGEND Flow From Spillways Herbert Hoover Dike Spillway

9 Potential Failure Modes - Herbert Hoover Dike Seepage & Piping along Outlets and Foundation Overtopping/ Overwash erosion Relatively low consequences, which vary by location around perimeter of the lake, seven different consequence zones Combination of life and economic consequences with likelihood of failure considered unacceptable 9

10 Herbert Hoover Dike 10

11 Herbert Hoover Dike SLOSH Cofferdam design must consider Wind effects (incl direction) + Pool elevation Sea Lake and Overland Surges from Hurricanes 11

12 Herbert Hoover Dike 12

13 Herbert Hoover Dike 13

14 Herbert Hoover Dike 14

15 Herbert Hoover Dike 15

16 Herbert Hoover Dike north west east south NW NE 16

17 Hurricane Jeanne

18 Hurricane Jeanne

19 Herbert Hoover Dike Hurricane Wilma (Oct 2005) debris on slope due to surge & runup 19

20 Hurricane Irma 20

21 Lake Okeechobee's water level drops below targeted range, raising water supply concerns (April 2017 headline) 21

22 Record Pool EL ~18.7 Low pool Hurricane Irma RWS ~

23 Herbert Hoover Dike Flooding potential exists from downstream of the dike also, requiring landside cofferdams Lake Okeechobee 23

24 Herbert Hoover Dike Lake Okeechobee 24

25 Herbert Hoover Dike Coffer design has been adjusted over the course of the project based upon; - Evaluation of wind and pool probabilities - Cost considerations - Occurrence of elevated pool events & extended construction periods - Consequence locale considerations 25

26 Addicks & Barker Dams Addicks Barker Houston 26

27 Addicks & Barker Dams Potential Failure Modes - Spillway erosion Abutments lower than dam crests Seepage & Piping along outlets and Foundation Replace outlets Extremely High Incremental Risk City of Houston, Population at Risk ~ 1 M Combination of life and economic consequences with likelihood of failure considered unacceptable 27

28 Addicks & Barker Dams Replace Outlets Cut & Cover Construction Cofferdam considerations Construction cost versus cofferdam elevation Consequences versus cofferdam elevation Consequences higher for lower cofferdam elevation ACE pool / P failure versus cofferdam elevation ACE pool higher for lower cofferdam elevation 28

29 Addicks & Barker Dams Stage Duration Curves 29

30 Addicks Dam Addicks Design 30

31 Barker Dam Barker Design 31

32 Addicks Dam 32

33 Barker Dam Cofferdam Acceptance 31 Jul 2017 Flight Date 19 August 33

34 Hurricane Harvey August 2017 Addicks & Barker dams; rain 34

35 Addicks Dam Record EL , 30 August 35

36 Addicks Dam 36

37 Addicks Dam 37

38 Barker Dam 38

39 Barker Dam Record EL August 39

40 Barker Dam 40

41 Takeaways HHD Team was tracking path of storm to monitor potential impacts to construction zones. In general, the track of Hurricane Irma (along west coast of Florida), resulted primarily in a rain event over the upstream basin. Storm path uncertainty and severity has led to use of higher cofferdam crest elevations. Addicks & Barker dams The occurrence of a relatively remote event validated the decision that the cofferdam would effectively need to replace the dam itself during construction. 41

42 A Little Help from Others Dan Blaydes Scott Shewbridge John Kendall Bobby Van Cleave Calvin Barefoot Timothy Willadsen Almur Whiting 42

43 Questions? 43