BOARD01480

• Board Meeting Date: 3/10/1998
• Description: WSP Section - Colorado River Basin Issues - Adaptive Management Work Group
• Board Meetings - Doc Type: Memo

.

.

.

.'

, .

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,
storage 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
I 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 volumes of bypasses in
these types of years are very large, 3.4 MAF in 1983 and 1.0 MAP 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. 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 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.

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 500,000 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.

Proposed Recommendation for the Spillway Gate Extensions

As a result of the limited value of the extensions in controlling extremely high runoff years, the
ability to control more cornmon inflow events without the use of the spillways, and the risk
associated with using the extensions for release moderation rather than emergencies, it is
Reclamation's recommendation that the 4.5-foot spillway extensions not be installed. However,
we do recornmend that the original 8-foot extensions continue to be stored at Glen Canyon Dam
for use only in case of dam safety or hydrologic emergencies.