BOARD01480

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

.

,

e

. Glen Canyon Dam Spillway Gate Extensions
An Analysis of Their Need and Their Impact on Spill Frequency and Duration

Introduction

The two spillways at Glen Canyon Dam are large tunnels bored through the abutment sandstone
on either side of the dam and are controlled by a total of four 40- by 52.5-foot radial gates, two
on each side of the dam. The gates are raised and lowered through a powered cable system
located on the top of the dam. Figures I and 2 are plan and cross section views of the gates. In
contrast to the approximate 45,000 cfs combined capacity of the powerplant and outlet tubes, the
spillways can discharge about 210,000 cfs when fully opened with the reservoir at elevation 3700
feet.

This large release capacity was designed as a critical part of the dam's capability to
accommodate large inflows. Because of the very large surface area of Lake Powell and the long
duration of the spring runoff from snowmelt, most extreme inflow events can be handled through
anticipatory powerplant releases in the months prior to the peak inflow. It was primarily the
large scale general storm events that require the use of the spillways. These and other large
hydrologic events produce inflows that challenge even the large capacity of Lake Powell.
Inflows of the magnitude of the Probable Maximum Flood (PMF) would produce peak inflows
of about 700,000 cfs, resulting in discharges exceeding 200,000 cfs for a 9-day period of time
and in a maximum reservoir elevation of about 3710 feet. Fortunately, the statistical frequency
of such events is extremely rare, perhaps only on the order of once in 10,000 years or longer.

The spillways were also designed to playa role in normal operations during high inflow years
when there was also a large error in the forecasted runoff, such as 1983. Unanticipated extreme
inflows could not be accommodated with the anticipatory powerplant releases cited above.
There simply isn't enough time in those situations to evacuate sufficient reservoir storage space.
Use of the spillways enables the safe passage of the peak inflows without overtopping the dam.

Since large dam releases have significant impacts on downstream resources, previous documents
such as the GCDEIS contained recommendations on restricting the frequency of large releases
above powerplant capacity, citing two options for controlling such releases. The 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.
The Grand Canyon Protection Act (GCPA) also addressed powerplant bypasses and their impact
on the Grand Canyon.

The installation and use of these spillway gate extensions are the focus of this report. As in the
recent Beach Habitat Building Flow (BHBF) triggering criteria report, the report will include
discussions on the GCDEIS conclusions regarding spills, the recent change in thinking about
spills, and the agreement reached between the Secretary of the Interior and the Basin States
contained in the 1996 AOP. Additionally, there will an alternatives and impacts analysis on
various options that exist regarding the spillway gates extensions.

;"