[Federal Register Volume 63, Number 197 (Tuesday, October 13, 1998)]
[Notices]
[Pages 54730-54732]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 98-27345]
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DEPARTMENT OF THE INTERIOR
Bureau of Reclamation
4.5 Foot Spillway Gate Extensions, Glen Canyon Dam
AGENCY: Bureau of Reclamation, Interior.
ACTION: Decision to postpone installation.
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SUMMARY: Based upon recommendations from the Adaptive Management Work
Group (AMWG), the Secretary of the Interior has decided to postpone the
permanent installation of the 4.5 foot spillway gate extensions on Glen
Canyon Dam. During this postponement, the operation of the dam, as
stated in the Record of Decision, shall be in accordance with the
Annual Operating Plan (AOP) process and shall not include the
reservation of storage to compensate for space that would have been
created by the installation of the spillway gate extensions.
SUPPLEMENTARY INFORMATION: Since large dam releases have significant
impacts on downstream resources, the Glen Canyon Dam Environmental
Impact Statement (GCDEIS) contained recommendations on restricting the
frequency of large releases above powerplant capacity, citing two
options for controlling such releases. The Record Of Decision (ROD) for
the GCDEIS selected the option of installing spillway gate extensions
rather than the option of providing a greater vacant storage space
buffer to reduce the frequency of powerplant bypasses.
GCDEIS and Grand Canyon Protection Act (GCPA) Conclusions Regarding
Powerplant Bypasses
The majority of the Glen Canyon Environmental Studies (GCES) Phase
1 research work took place in the mid-1980's, when the releases from
Glen Canyon Dam were at an all time high since the construction of the
dam. These flood flows were radically different than historic releases
and caused such large downstream effects that they greatly influenced
the GCES recommendations. On page 83 of the final GCES Phase 1 report,
the first and foremost conclusion was that ``Adverse downstream
consequences are caused primarily by sustained flood releases
significantly greater than powerplant capacity and by fluctuating
releases'', noting the erosive effect of floods on sand deposits and
vegetation. Generally, these conclusions suggested the elimination or
reduction of flood flows.
In the committee report accompanying the GCPA legislation, the
Congress continued this thinking on adverse impacts by stating that
``Flood releases from the dam erode beaches used by recreational
rafters and campers. The river's now reduced sediment loads are
inadequate to replenish beaches, even if flood releases occur once
every twenty years. Flood releases destroy riparian vegetation and
birds.'' The Act did not specify remedial measures, but seemed to imply
that even the aggressive spill avoidance strategy that had been
implemented to reduce spill frequency might be insufficient.
These conclusions produced the GCDEIS decision to reduce the return
period of powerplant bypasses above 45,000 cfs to no more than an
average of 1 in 100 years. The option of installing the spillway gate
extensions was selected as part of the preferred alternative instead of
the option of targeting an additional 750,000 acre-feet of vacant
storage space when the reservoir filled in July. The extensions were
determined to be 4.5 feet in height, in contrast to the 8-foot high
extensions installed during 1983. Additional questions about the need
to reduce the frequency of powerplant bypasses and the desired
magnitude and impacts of sustained high releases during extreme flood
years now provide impetus to re-examine the original decision that an
additional 750,000 acre-feet of vacant storage space is needed through
the installation of the gate extensions.
The Evolution of Understanding Regarding High Releases
Despite the enormous beaches created by the 1983 spill event, the
general thinking at that time was that there was a very limited supply
of sediment below Glen Canyon Dam and that spills destructively moved
much of this sediment out of the Grand Canyon. During the high flow
years of 1984-1986, the main channel sediment storage was likely much
lower than prior to 1983, and the deposition rate during the 1984-1986
spills was lower as a result. Sediment experts then believed that the
river downstream of the dam was in a sediment-starved condition.
Sediment supply thus became one of the primary driving
[[Page 54731]]
forces behind ecological recommendations for changing powerplant
operations.
Based upon continuing research, including evaluation of the Beach
Habitat Building Flow (BHBF), sediment researchers now believe that
flood flows counteract the possible adverse impacts that fluctuations
have on beach erosion, thus rebuilding the deposits that would
eventually slough back into the eddies, regardless of the nature of the
powerplant operations. Some suggested that more frequent floods could
allow higher levels of fluctuations.
The Agreement Contained in the 1996 AOP
With this evolving positive view towards spills, a desire for a
test of the GCDEIS BHBF was expressed by the Transition Work Group
beginning in 1994. The Basin States strongly opposed this request for a
purposeful powerplant bypass because the 1968 Colorado River Basin
Project Act requires avoiding anticipated spills, interpreted as
powerplant bypasses. This opposition created an impasse that blocked
such a test.
Additional discussions between members of the Transition Work Group
and the Basin States resulted in a proposal for a modification of the
GCDEIS preferred alternative, that of moving BHBF from years of low
reservoir conditions (when spills would not be required for hydrologic
reasons) to years of high reservoir conditions and high inflows. Thus a
BHBF would occur in years when there was an expectation of having a
hydrological induced spill. This agreement was institutionalized in the
1996 AOP for the Colorado River and signed by the Secretary of the
Interior in December 1995. A subsequent BHBF test was conducted in
April 1996, confirming the hypothesis that high flows could rebuild
sandbar deposits. In December 1996, the GCDEIS Record of Decision was
assigned by the Secretary of the Interior and included this
modification to the preferred alternative.
Impacts of Using Spillway Gate Extensions
GCDEIS Expectations Related To Spillway Gate Extensions
The Colorado River Simulation System (CRSS) modeling,which formed
the hydrologic basis for many of the GCDEIS decisions, determined that
bypasses were rare events, and if a small amount of buffer space were
provided, releases greater than 45,000 cfs could be avoided. Since it
uses a monthly time step, the CRSS model could not really estimate the
peak bypass release other than to average the release over the month in
which it occurred. Thus some judgment was used in estimating the
frequency of releases greater than 45,000 cfs.
The Limited Value of the Spillway Gate Extensions
The GCDEIS commitment to install the 4.5-foot extensions would
produce about 750,000 acre-feet of surcharge storage space above the
normal maximum water surface of 3700 feet. While this is a large amount
of reservoir space, it is small in comparison to either average April--
July inflow which is about 7.8 MAF or the 2.1 MAF forecast error term
for June 1 (5 percent exceedence level). A buffer of this size would
affect primarily moderately high years in which bypasses were on the
range of several hundred thousand acre-feet. Such bypasses could be
reduced or eliminated entirely by storing the excess inflow behind the
gate extensions until it could be released through the powerplant.
Inflow volumes of extremely high inflow years such as 1983 or 1984
had return periods of about 1 in 100 years. These are the types of
years which would produce releases in excess of 45,000 cfs, perhaps for
an extended period of time as occurred in 1983. The volume of bypasses
in these types of years are very large, 3.4 MAF in 1983 and 1.0 MAF in
1984. The greatest determining factor in the amount of bypass is the
forecast error associated with high inflow years.
In contrast, moderately high inflow years such as 1985, 1986, and
1995 would cause bypasses of about 100,000 to 800,000 acre-feet using
current operating practices. These bypass volumes could be released
through the outlet tubes in 3 to 25 days, thus limiting total releases
to 45,000 cfs or less. During these types of years, it would be very
unlikely that use of the spillways would be required.
The Need to Reduce the Frequency of Powerplant Bypasses
Current thinking among sediment experts is that, given high flow
conditions resulting from large runoff years, releases above 25,000 cfs
should be preceded by BHBFs. The BHBF should be greater in magnitude
than the highest expected future release. This not only moves sediment
higher on beaches away from future releases, but also coarsens the main
channel bed which reduces future sediment transport. Some sediment
experts believe that there is sufficient regeneration of main channel
sediment supplies to allow BHBFs in all years that such events would be
allowed by the 1996 agreement, even every year if possible. Longer
duration spills may have different effects than the short duration
BHBFs, so additional sediment transport modeling would help clarify the
allowable frequency of such spills.
The Positive Value of the Spillway Gate Extensions
Although the extensions are not required to limit spillway use to
the 1 in 100 year return period cited in the GCDEIS, some limited value
can be gained from their installation during years in which peak
releases would be less than 45,000 cfs. In these cases, if the total
bypass volume was expected to be 750,000 acre-feet or less, then the
entire expected bypasses could be stored behind the extensions and
released later in the summer. This might produce some environmental
benefits by not releasing greater than 30,000 cfs if such releases
would cause ecological harm. However, it would also carry the dam
safety risks associated with purposefully storing more water in the
reservoir than was assumed during the design of the spillways. If an
extremely rare high inflow event occurred, it could conceivably overtop
the dam, even with full use of the spillways.
It appears from this discussion, that only inflow years with a
return period of about 1 in 100 years would force the use of the
spillways and release more than 45,000 cfs. Reclamation believes that
current operating practices under the AOP would initiate high
powerplant releases and bypasses early enough as required to safely
operate the dam, thus meeting the intent of the GCDEIS provision
without requiring either the additional storage buffer or the spillway
gate extensions.
Decision
Based upon the analysis and comments received from the AMWG the
Secretary of the Interior has decided to postpone permanent
installation of the 4.5 foot spillway gate extensions. During the
postponement period, operation of the dam, as stated in the Record of
Decision, shall be in accordance with the AOP process and shall not
include reservation of storage to compensate for that space that would
have been created by the gate extensions. Also, Reclamation will report
annually to the technical Work Group and AMWG on the effect of not
installing the gate extensions on: (1) The probability of meeting BHBF
triggering criteria and (2) the probability of limiting spills greater
than 45,000 cfs to a 1 in 100 frequency.
[[Page 54732]]
Dated: October 6, 1998.
R. Steve Richardson,
Acting Commissioner, Bureau of Reclamation.
[FR Doc. 98-27345 Filed 10-9-98; 8:45 am]
BILLING CODE 4310-94-M
