[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