Stagnation Pressures

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Learn more about the dangers of cavitation at DamFailures.org

"Concrete linings can fail due to stagnation pressures which are generated at offsets into the flow at open transverse joints or cracks in a concrete chute slab. Stagnation pressures can result in two outcomes which are detrimental to the stability of a concrete chute slab. The stagnation pressures can result in very high uplift pressures due to the conversion of a portion of the velocity head to a static head which can lead to slab jacking and removal of a section of the concrete lining. Failure of the chute slab can result if the uplift generated by stagnation pressures exceeds the resisting forces provided by: the weight of the concrete slab and the water in the chute; anchor bars which attach the chute to the foundation; and reinforcing steel which can allow for load transfer to sections of the chute slab not subjected to stagnation pressures. Case histories where chute slabs failed due to stagnation pressures and slab jacking include the Big Sandy Dam Spillway and the Dickenson Dam Spillway as well as the Whaley Bridge Dam Spillway. The second outcome which can be adverse to the stability of concrete chutes subjected to stagnation pressures is the erosion of foundation material and loss of support for the slab. Offsets into the flow can result in high velocity flow being injected through the slab and into the foundation. If there are unfiltered exits within the foundation (which often is the underdrain system), foundation materials can be removed creating voids and a lack of support for the chute slab. If the void becomes large enough, the slab may not be able to span the void and the slab could fail by collapsing into the void. Case histories where stagnation pressures led to foundation erosion and voids underneath the chute slab are the Hyrum Dam Spillway and the Whaley Bridge Dam Spillway."

Examples

Learn about the impacts of stagnation pressure from the incident at Oroville Dam (DamFailures.org)

Best Practices Resources

Design Standards No. 14: Appurtenant Structures for Dams (Ch. 3: General Spillway Design Considerations), USBR

Hydraulic Design of Spillways (EM 1110-2-1603), USACE

Trainings

On-Demand Webinar: Designing Spillways to Mitigate Failure Modes

Citations:

Revision ID: 7181
Revision Date: 07/11/2023