US Army Corps of EngineersBUILDING STRONG®

Cofferdam Performance Hurricanes Harvey & Irma - 2017Greg HammerSenior Geotechnical EngineerCELRH-DSPC-GS DSMMCX

Third Workshop on Case Histories inDam Safety Risk-Informed Decision Making

2018 USSD Annual ConferenceMiami, Florida03 May, 2018

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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

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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 1110-2-8152, 1995, use risk guidance

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Considerations Risk Implications –

► D/S consequences, Life loss, Project Costs, Site Loss• ACE/ Pfailure for Dam Overtopping• ACE/ Pfailure for Cofferdam • Consequences of Breach of Dam/ Cofferdam

Damage ($) Loss of Life Project site loss – Equipment, Personnel, Sitework

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{ACE – annual chance of exceedence}

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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

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Considerations Design Efficiency & Effectiveness

► Cofferdam reliability – stability/ integrity• Pfailure of structural failure modes

► ACE Overtopping(height) vs Construction Cost► Landside protection?

• Channel considerations – Backwater/ Tailwater ACE/Pfailure of Downstream cofferdam

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Herbert Hoover DikeLake Okeechobee

143 mile perimeter embankment

730 sq mile surface area Approx. 30 miles across Avg. water depth is 9 feet

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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

Herbert Hoover Dike

Flow From SpillwaysHerbert Hoover DikeSpillway

LEGEND

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Herbert Hoover Dike Potential Failure Modes -

► Seepage & Piping along Outlets and Foundation

► Overtopping/ Overwasherosion

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

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Herbert Hoover Dike

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Herbert Hoover Dike

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“SLOSH”

Sea Lake and Overland Surges from Hurricanes

Cofferdam design must consider Wind effects (incldirection) + Pool elevation

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Herbert Hoover Dike

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Herbert Hoover Dike

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Herbert Hoover Dike

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Herbert Hoover Dike

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Herbert Hoover Dike

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north west

east south

NW NE

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Hurricane Jeanne - 2004

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Hurricane Jeanne - 2004

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Herbert Hoover Dike

Hurricane Wilma (Oct 2005) – debris on slope due to surge & runup

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Hurricane Irma

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

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Low pool

Hurricane Irma RWS ~17.2

Record Pool EL ~18.7

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Herbert Hoover Dike

Flooding potential exists from downstream of the dike also, requiring landside cofferdams

Lake Okeechobee

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Herbert Hoover Dike

Lake Okeechobee

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Herbert Hoover DikeCoffer 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

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Addicks

Barker

Addicks & Barker Dams

Houston

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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

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Replace Outlets – Cut & Cover Construction Cofferdam considerations –

► Construction cost versus cofferdam elevation► Consequences versus cofferdam elevation

• Consequences higher for lower cofferdam elevation► ACE pool/ Pfailure versus cofferdam elevation

• ACEpool higher for lower cofferdam elevation

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Addicks & Barker Dams

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Stage Duration

Curves

Addicks & Barker Dams

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• Addicks

Design

Addicks Dam

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Barker Design

Barker Dam

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Addicks Dam

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Cofferdam Acceptance –

31 Jul 2017

Barker Dam

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Flight Date –19 August

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Hurricane Harvey – August 2017

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Addicks & Barker dams; 35-40” rain

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Addicks Dam

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Record Pool @EL 109.09,30 August

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Addicks Dam

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Addicks Dam

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Barker Dam

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Barker Dam

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Record Pool @ EL 101.5630 August

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Barker Dam

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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.

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

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gregory.g.hammer@usace.army.mil

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Questions?