4.5 Foot Spillway Gate Extensions, Glen Canyon Dam

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

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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.

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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