FLOOD10355

• County: Statewide
• Stream Name: Colorado River
• Title: Colorado River Basin Probable Maximum Floods, Hoover and Glen Canyon Dams
• Date: 9/1/1990
• Prepared By: US Department of the Interior Bureau of Reclaimation
• Floodplain - Doc Type: Floodplain Report/Masterplan

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1.0

INTRODUCTION

1.1 AUTHORITY AND BACKGROUND

The Bureau of Reclamation (Reclamation) Safety Evaluation of
Existing Dams (SEED) program provides the authority for updating
hydrologic studies for Hoover and Glen Canyon Dams. Discussions
with Reclamation's Division of Dam Safety and Division of Water and
Land Technical Services were initiated in January 1986 to begin
activities related to the probable maximum flood (PMF) study. The
need for the study was initially identified by the Division of Dam
Safety when consideration was given to replacing the needle valves
at Hoover Dam. Funds for the study were obligated from several
sources, including Colorado River Storage Project revenues and SEED
funds.

High runoff from a late season melt of the snowpack in the spring
and summer of 1983 resulted in operation of the Hoover Dam
spillways. During this operation, damage to the concrete lining of
the spillways occurred, leading to the assessment of potential
modifications to alleviate the problem. As a part of this analysis,
the adequacy of the hydrologic engineering aspects of the dam were
evaluated. Additional high runoff occurrences in 1984 and 1986 have
kept the flood issues at Hoover Dam in the forefront. Concern has
also been expressed about lesser magnitude floods (e.g., 25-, 50-,
and 100-yr frequency floods) as they may affect the current
operation at Hoover Dam.

That same year, 1983, also contributed a record inflow to Glen
Canyon Dam. These record inflows caused Lake Powell to be at or
above surcharge (elevation 3700 feet, 27,000,000 acre-feet) for
3.5 months. Extensions were added to the spillway gates at the dam
to accommodate the high inflows and resulting high reservoir water
surface elevations. The spillways were operated in the first
non-test situation since Glen Canyon Dam was completed in 1964.
This operation caused severe cavitation in the spillways. After the
flood peak had passed, and the flood danger subsided (July 1983),
repairs on the spillways began. High inflows in the following year
also occurred. The spillways and power and river outlet works, in
combination with reservoir storage, were able to control and pass
these high inflows without damage occurring to the spillways.

The flood hydrology data used as a basis for sizing the dams, the
outlet works capacity, and the allocated flood storage/surcharge
space were found not to conform to the current state-of-the-art with
respect to operational criteria and technical methodologies. 'These
data also do not reflect recent hydrologic and meteorologic data
acquired since the original design was completed. Previous design
flood investigations were crudely developed'from high water marks
left from large historical flood events. This flood investigation
accounts for the effects of upstream basin development and reservoir
regulation, as well as the knowledge gained from many large storms
which have occurred over the basin since the dams were built.