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Spillways
New spillway at
Panama Canal
The new Gatun spillway will provide flood protection and regulation to prepare the Panama Canal
infrastructure for extreme events. MWH Global has been working to help maximise the effectiveness
of the spillway and ensure the integrity and viability of this important ship canal.
O
ne of the largest and most challenging
engineering projects ever undertaken,
the Panama Canal is an 80km ship
canal that connects the Atlantic and Pacific
Oceans and creates a passage which represents
5% of international trade. The first attempts to
build it date back to 1881 but was not actually
completed until the effort undertaken by the US
between 1904-14. Construction of the canal no
longer made it necessary for ships to sail the
lengthy route around the southernmost tip of
South America and today, some 14,000 ships
annually pass through this Central American
shortcut. Work is now underway to expand the
canal, including a new set of locks that will
allow much larger ships to pass through the
waterway.
The Panama Canal works via a system of
locks with entrance and exit gates. It relies on
Gatun Lake, a manmade fresh water lake that
was created by the construction of the Gatun
dam across the Chagres River to make it
navigable by large vessels and serves as a
reservoir to operate the canal locks. Gatun Lake,
which presently has a maximum operating level
of 26.67m asl and spans 436km2, is part of the
canal’s shipping channel, serves to generate
hydropower, and is a water source for
communities established adjacent to the
waterway both in Panama City and Colon. Its
dam and spillway structures also provide critical
flood protection for the canal and surrounding
region.
In particular, the Gatun spillway is the
primary hydraulic outlet from Gatun Lake to the
Chagres River. Its purpose is to control the lake
level, meet the water supply and navigation
demands and to protect Gatun dam, the Gatun
and Pedro Miguel Locks and other facilities
around the lake shoreline from flooding. The
spillway was designed a century ago at a time
when little information was available detailing
the hydrology and hydraulics of the Chagres
River. Based on this limited information, the
The spillway model
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INTERNATIONAL WATER POWER & DAM CONSTRUCTION
spillway was designed, constructed and placed
into operation in 1913 with a capacity calculated
from historical flood elevations along the river
and a maximum operational lake level of 25.91m.
As demands for water increased, along with the
number of vessels transiting the Panama Canal,
the maximum operating level was raised to
26.67m.
Rationale for spillway modifications
Engineering firm MWH Global is the lead
technical designer for the concurrent expansion
of Panama Canal through a third set of locks,
which will require a planned increase in
maximum operating level of Gatun Lake. Due to
this increase, there will be less storage available
during floods, leading to increased peak
discharge from the spillway during seasonal
rainfall events.
This has placed greater importance on the
need for a new spillway. After almost 100 years
of operation a significant amount of hydrologic
data has been collected for both the Chagres
River and the climatology of the region. This
compilation of data, when combined with the
advancements in hydrologic design criteria and
standards for dams and spillways, has led to a
consensus that the current spillway capacity has
to be upgraded to safely handle the probable
maximum flood (PMF). International standards
classify the Gatun dam as a large dam with a
high hazard, and as such, the spillway must be
re-designed to meet its associated PMF levels.
The need for a new spillway was identified
very soon after the opening of the Panama Canal.
This resulted in the development of several
hydrologic studies on the Panama Canal
Watershed and even a new spillway design as
early as 1945. While new studies and designs
were later developed, on 8 December 2010 the
Panama Canal experienced the largest storm in its
history, which forced the use of the lock culverts
to control unprecedented Gatun Lake flood levels.
This further accentuated the need for a new
spillway, and led the Panama Canal Authority to
decide on the award of a contract for a detailed
design of a new spillway for Gatun Lake.
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Solutions and processes
To help increase the outflow capacity of the
existing spillway system and ensure that it can
handle the PMF, the Panama Canal Authority
(ACP) awarded a US$4.4M contract to MWH
Global in January 2012 to provide preliminary
and final design services for a new spillway. In
short, MWH engineers are addressing the
varying water levels and spillage needs of Gatun
Lake, alongside all the protection works related
to the extreme spilling conditions.
MWH evaluated the existing Gatun spillway
which consists of a curvilinear concrete dam
measuring 225m along the top, with a crest at
5.7m below the present normal lake level. The
spillway was designed so that water converged
at the bottom of the dam from opposite
directions, hitting baffle blocks to dissipate
energy and exiting through a concrete discharge
channel. The spillway dam is controlled by 14
Stoney gates supported by concrete piers, and
has a capacity of 5155m3/sec at the current
maximum operational lake level.
As part of the contract, MWH has been
tasked with identifying at least three plausible
flood management solutions and developing
corresponding conceptual designs, each
including a new spillway and cost analysis. The
preliminary design phase was to determine the
best way to increase outflow including adding
new spillway structures and has entailed:
Research and analysis: MWH conducted a
review and analysis of available background
information from ACP, including data and
reports conducted throughout the history of
the Gatun spillway.
Site investigation and field works: A site
investigation campaign was developed,
including the location and number of new
boring sets and seismic lines based on
proposed structure location and alignment.
50
Field works were also developed and new
information reviewed to determine design
parameters for structural foundations. These
included:
A Lidar survey and a bathymetric survey
were proposed and executed.
A digital terrain model based on
information from ACP and the new one.
A conceptual three-dimensional model of
local geology of Gatun Hill and nearby
surroundings where the new spillway
structures would be located to estimate
overburden, weathered rock and sound
rock layers.
Modelling: A number of mathematical
models and simulations were developed to
test the design before fully implemented.
Utilising data: Data from the various studies
and models as listed above. Several
alternatives were developed, evaluated and
proposed, including the arrangement of the
spilling structures, dissipation units,
approach channels, and feasible flood
management solutions. However, prior to
submitting the proposals to the ACP, MWH
conducted complete modelling to ensure the
feasibility of each solution. Testing included:
A flood routing model for different crest
elevations of the new spilling structures,
in order to evaluate the required width.
A one-dimensional model of the Chagres
River, which was calibrated using historic
flood data to determine tail water levels
downstream of Gatun dam. The model
enabled the design of several alternatives
of protection works.
Testing of spilling structure arrangements
with a one-dimensional model and a
comparison of excavation and concrete
volume for each one of the variants (crest
elevation and width).
A two-dimensional vertical model of the
new spillway structures and a threedimensional model of the approach channel
modeled with OpenFOAM software.
Three-dimensional CFD model of the new
spillway structures to verify the hydraulic
performance of the stilling basins and
energy dissipation units.
A three-dimensional CFD model of the
existing Gatun Spillway developed to
determine the rating curve. The model
enabled evaluation of the performance of
possible improvements to protect the
spillway and the Gatun hydropower
generation plant from the larger
discharges resulting from the higher
maximum operational lake level.
Creation of a two-dimensional model of
the Chagres River from the Gatun Spillway
to the first downstream river bend.
Design adjustments made based on
recent geological data and Peer Review
Board recommendations.
INTERNATIONAL WATER POWER & DAM CONSTRUCTION
Physical modelling: In addition, a 1:40 scale
model of the existing Gatun spillway was
constructed at the Hydraulic Laboratory of
the National Institute of Water (INA) in
Buenos Aires, Argentina. It reproduces a
section of the dam, the existing spillway, the
dissipation area with baffle blocks, the
discharge channel and a short stretch of the
river downstream. It was applied to verify the
rating curves, and to evaluate the
performance of the spillway in several
scenarios, including higher tail water levels
corresponding to prospective future
operational situations.
Structural and mechanical analysis: For
the three design alternatives, structural
stability analysis was performed on the new
spilling structures using finite element and
rigid body analysis. Seismic design
guidelines were also considered. Dimensions
of the structures have been set, along with
the design of mechanical equipment
including tainter gates, actuating
mechanisms, an emergency closure device
and maintenance stoplogs.
Environmental assessment: A preliminary
environmental feasibility assessment to meet
regulations in Panama was also prepared for
every alternative.
MWH has presented several preliminary design
alternatives, along with cost analysis, to the
Panama Canal Authority for their review. ACP
has recently selected the conceptual design
alternative, and MWH will finalise the design
and develop bid documents and drawings for
construction.
The design capacity of the new spillway
structure is close to 7000m3/sec at the highest
lake flood level. The project is slated for
completion in 2013.
New spillway
The new Gatun spillway will provide flood
protection and regulation of Gatun Lake to
prepare the Panama Canal infrastructure for
extreme events. Maximising the effectiveness of
the spillway will also ensure the integrity and
viability of the Panama Canal, not only for the
existing locks but for the third set of locks that
will be designed with water saving basins saving 60% of the fresh water which would
otherwise be used in the locks. These
advancements come at a time when Panama,
much like other countries worldwide, is eager to
conserve its water resources while increasing
the amount of economic trade and
transportation that can pass through its
waterways.
Author information
Fernando Re is a civil engineer for MWH
Global based in Buenos Aires, Argentina.
October 2012