Projects
The power of association
The concerns of dam owners extend decades beyond the careful design and construction
of their facilities. Operation, maintenance, inspection, and emergency action plans all address
long-term safety. Here Ayres Associates in the US discuss the role of floating bulkheads in gate
testing and repair, the proper use of rope access and underwater diving in gate inspection,
and the use of 1D and 2D modelling in dam-break inundation studies
DAM OWNERS WHO NEED to complete maintenance
or testing on dam gates often face steep challenges
when determining how to accomplish these tasks
without either draining the reservoir or building an
expensive cofferdam each time rehabilitation or
operational checks are needed. The floating bulkhead
is a practical answer – even when the spillway does
not have an adequate sealing surface, such as a pier
nose with a flush spillway crest, upon which to seat a
bulkhead.
Most dams have some type of gate(s) to pass
water either for normal river flows or flood flows.
Many of these gates are over 50 years old and either
need maintenance or periodic operation to verify
they can be opened during a high-flow event. If the
dam spillway was not constructed with a stop log
system or other mechanism to separate the gate
from the impoundment, the owner’s choices include
constructing a costly cofferdam or bulkhead system
to dewater the gate, or dewatering the entire reservoir,
which is costly to any power-generation role the
dam may play, and may pose environmental and
recreational concerns.
Traditional cofferdam construction is costly, time-
consuming, and not always feasible. Conventional
bulkheads can be massive, requiring significant
cranage positioned above each spillway opening.
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. Once assembled on
the reservoir, the floating bulkhead can be installed
and the spillway dewatered in as little as one day.
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 can be
located basically anywhere on the reservoir. Often a
larger crane can be located off the dam and can be
used to place the caissons onto the reservoir, where
they can then be 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 while remaining buoyant
for final positioning.
Most dams have multiple spillway bays with similar
geometry. Once the gate test or repairs are done on a
particular bay, the bulkhead is raised to the surface by
displacing water in lower caissons with air. It can then
be floated in its horizontal position either to the next
spillway bay or to the offloading area and removed
from the reservoir for storage.
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.
Many shapes and sizes
An important characteristic of floating bulkheads
is that their installations can be customised 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
often do not have a pier nose but an abutment
that extends into the reservoir. In these situations,
an abutment seat is fabricated to position the
bulkhead.
Ayres Associates has designed floating bulkheads
throughout the US, solving many of these challenges.
Floating bulkheads have spanned openings
measuring 6.7-18m wide and 4-14m high.
Not only can one floating bulkhead system be used
at multiple spillway gates; in certain applications,
the same bulkhead can be used on multiple dams.
Sometimes this requires simple modifications to the
design to facilitate use at other structures.
The hollow steal floating bulkhead 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 when it
is time to float them away from the spillway bay.
The hinged floating bulkhead technology was
developed in 1987 by Xcel Energy and Ayres
Associates engineers. That first hinged bulkhead is
still in service today.
From top to bottom: Time
progession on installing
Floating Bulkheads
Recent floating bulkhead projects
The Conowingo Dam on the Susquehanna River
in Maryland presented owner/operator Exelon
Generation Company a tall order when Exelon was
determining what dewatering system to use to
service 52 spillway gates measuring 11.5m wide by
6.7m high. The cofferdam method of dewatering f
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