BUYING IN
BULK
Dam repairs made easier with hinged device
By Sarah Bestul
I
t’s something few people think about: What happens
when part of a dam holding back almost 100 feet of
water needs repair? The bottom of a dam doesn’t have a
drain plug, and drawing down an entire reservoir is usually
not an option, so cofferdams are often constructed to
allow the water to be pumped out so that repair crews can
get to work. But what happens when the size of your dam
makes the cost of installing a cofferdam prohibitive, or
there are environmental concerns with the building and
removal of a cofferdam?
There is a solution, and it’s quick to install, portable, and
best of all, reusable: a hinged floating bulkhead.
The Conowingo Dam, owned and operated by Exelon
Generation Company, LLC, is on the Susquehanna River
in Maryland. It was built in 1928 and was designed and
constructed with the ability to pass high-water floods
by opening gates in the dam. Because of the wide
fluctuations of flow in the Susquehanna River, spaces for
50 spillway gates were included in the design. The design
included additional maintenance gates that could be used
to repair the spillway gates “in place.” Over time, these
service gate slots needed repair; however, in addition
to the sheer number of gates, the dam was more than
100 feet high, and cofferdam methods of dewatering
considered by Exelon would have been incredibly
expensive. Exelon contracted with Crofton Diving
Corporation, based in Portsmouth, Virginia, and Ayres
Associates to provide a turn-key solution to the facility’s
dewatering and rehabilitation problem.
This solution was the sectional hinged floating bulkhead,
which was installed to allow access and repair of the
service gate slots. The bulkhead made possible the
simultaneous rehabilitation of both the service gate slots
and the spillway gate slots. Since the delivery of Floating
Bulkhead No. 1 to the Conowingo facility in 2008, Crofton
has participated in ongoing rehabilitation efforts each
year. When this first floating bulkhead proved successful,
Crofton was tasked with the fabrication and delivery of
an additional bulkhead in 2010, effectively doubling their
rehabilitation production rates.
“We couldn’t have provided a better method of dewatering
these service and crest gates slots for rehabilitation,”
said Kurt Feairheller, Crofton Diving Corporation project
manager for all Exelon Conowingo Dam contracts.
Feairheller has overseen the successful installation of the
Conowingo Dam floating bulkheads at over 25 crest gates
to date and has performed nearly all of the diving portion
of the installations from 2008 through 2011. “The method
has proven itself time and again, and I would venture a
guess that the savings to date has more than paid for the
cost of the two sectional bulkheads.”
FROM THE BEGINNING
Ayres Associates’ engineers developed the first hinged
floating bulkhead for Northern States Power Company,
now Xcel Energy, in 1987. That bulkhead is still in use
today; with proper care and maintenance, the floating
bulkheads can last up to 50 years.
To create the bulkhead for Conowingo, Crofton called in
Ayres Associates’ water resources engineer Todd Rudolph,
who has designed hinged floating bulkheads for clients
around the country and Puerto Rico since 1997.
Creating a custom bulkhead is a collaborative process.
“Once the dam owner decides they want one, we discuss
how they want it installed. We get parameters to zero in on
the size,” Rudolph said. “Usually the most important thing
is what water levels they require. In the past, we worked
with dams where the water levels wouldn’t fluctuate,
but now owners want a wider range (of usability). So we
discuss water levels and performance levels upfront. Then
we do a preliminary design, talk about how each caisson
(a water-tight chamber) will be this long, this high, and
this many, so we can get a better handle on cost. Then we
may have a meeting to address comments and address
any concerns, then a final meeting before creating final
drawings and specifications.”
The whole design process takes about three months to
ensure that the floating bulkhead is designed to fit its dam
and meet every need of its owner.
In addition to requiring a water level variation of up to 4
feet, the Conowingo project offered a challenge in that the
upriver wall was not a vertical surface; it actually curves
Crews at the Conowingo Dam move the bulkhead into position.
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