Ayres Knowledge Center The Floating Bulkhead | Page 2

THE DESIGN CONCEPT
A floating bulkhead is a system of individual caissons that can be placed in the reservoir, pinned together, and floated
into position upstream of a spillway that is to be dewatered. The crane to place each caisson on the reservoir does not
need to be positioned above the bulkhead, as required for a non-floating bulkhead, but rather can be located almost
anywhere on the reservoir. Often, a larger crane can be located off the dam, with the caissons then floated to the
spillway that is to be dewatered. Once the bulkhead is in position upstream of a spillway bay, the lower caissons are
flooded with water, causing the completed bulkhead system to rotate to the vertical position but still be buoyant for final
positioning.
Many dams have multiple spillway bays with similar geometry. Once the gate test or repairs are done on a bay, water in
the lower caissons is displaced with air, and the bulkhead rises to the surface of the reservoir in a horizontal position. It
can then be floated either to the next spillway bay or to the offloading area and removed from the reservoir for storage.
Caissons typically consist of three chambers and are constructed of wide flange members with steel plates inserted
to size the middle compartment. The wide flange members are designed for the widest span at the highest head, and
generally the same size members are used for all caissons. The designer will evaluate changing caisson size or wide flange
members, but often it is most economical to use the same size wide flange member.
Steel shapes, plates, and bars are generally constructed using Grade 50 steel. Differing bulb seals and flat seals are used
depending on the application. Most bulkheads are painted white or light gray, to provide better diver visibility during
installation. The caissons are fitted with valves to allow flooding when sinking them into place and valves through which
compressed air is blown into the caissons to empty the water from them when it is time to remove them from the
spillway. Seals are placed on the caissons to seal against pier noses as well as between each caisson.
Floating bulkheads are designed to either be hinged and installed like a garage door or fixed so that the entire unit, once
the caissons are pinned together, rotates to the vertical position for placement.
An important characteristic of floating bulkheads is that their installations can be customized to be effective even on
spillway bays that do not have a flat, flush surface for placing the bulkhead, including bays where:
• No sill is available for the bulkhead to rest on. The floating bulkhead is held in place by the pressure of the water
against the reservoir side of the bulkhead.
• The pier noses are sloped, tucking into the spillway, requiring lower caissons to be pulled up to seal the bay.
• Extensions are required on the bulkhead to seal gates with complex shapes.
• The spillway bays at the right and left end of a dam do not have a pier nose but rather an abutment that extends
into the reservoir. In these situations, a steel abutment seat is fabricated to position the bulkhead.
Ayres Associates has designed floating bulkheads throughout the United States and Puerto Rico that solve many of these
challenges. Floating bulkheads have spanned openings measuring 22 to 60 feet (6.7 to 18.3 meters) wide and 14 to 47
feet (4.3 to 14.3 meters) high.
Not only can one floating bulkhead system be used at multiple spillway gates, in certain applications, but the same
bulkhead can also be used on multiple dams. Sometimes this requires simple modifications to the design to facilitate use
at other structures. Once assembled on the reservoir, the floating bulkhead can be installed and the spillway dewatered
in as little as one day.
Typical Bulkhead Installations
Fixed Position with Abutment Seat
Dam with Sill for Placing Bulkhead
Hinged Installation with Abutment Seat
MANY SHAPES AND SIZES
Installation with Horizontal Seat
Hinge and Lifting/Mooring Eye
2 | THE FLOATING BULKHEAD
Installation with Bulkhead Extension
Typical Caisson Cross Section
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