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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Release decisions were based on the current operating philosophy of
avoiding spills by incorporating changes in the forecasted runoff as
soon as possible, thus preserving future operational flexibility.

The operating objectives were to limit releases from Lake Mead to a
maximum of 40,000 ft3/s and to maintain as nearly as practicable, a
non-fluctuating flow regime below Parker Dam. In the case of Glen
Canyon Dam, an upper limit release of 31,500 ft3/s was observed
until published forecasts indicated a high risk of bypass of the
powerplant, at which time the release was increased to the
powerplant capacity of 33,100 ft3/s. As in actual practice,
bypasses were delayed when the possibility of not needing them
existed.

This analysis showed that the 100-year snowmelt flood can be
routed through Lake Mead without exceeding a release level of
40,000 ft3/s and without exceeding elevation 1,219.6 feet, the
bottom of the exclusive flood control pool. Flood control releases
of 29,500 ft3/s would be initiated on January 1 and gradually
increase to a maximum of 40,000 ft3/s by July 1. The maximum
release would continue through July 15, followed by a gradual
reduction in release levels coinciding with the flood's recession.
A maximum water surface elevation of 1,219.5 feet would occur on
July 19 through July 21.

At Lake Powell the releases from the dam were 31,500 ft3/s from
January 1 through April 30, 33,100 ft3/s from May 1 through June 7,
48,000 ft3/s from June 8 through June 14, and 60,000 ft3/s from June
15 through July 12. After this time, the releases gradually
decreased as the flood passed. The maximum water surface elevation
of 3,699.93 feet occurred on July 8.

Based on the results of the base snowmelt flood routings, beginning
reservoir elevations were determined for all of the major reservoirs
upstream from Hoover Dam for use in routing the flood caused by the
ULDRS event. Table 6.3 presents the starting reservoir elevations
for Lake Mead and Lake Powell based upon the starting date of the
ULDRS event. The upstream reservoirs were considered at the top of
conservation storage.

6.4 GLEN CANYON AND HOOVER PROBABLE MAXIMUM FLOODS

Determination of the probable maximum floods for Hoover and Glen
Canyon Dams involved generating seasonal flood hydrographs by
applying the results of the meteorological investigation. Numerous
combinations of ULDRS centerings and storm separations were
evaluated to determine the most critical seasonal hydrologic
conditions for the dam. The ULDRS flood hydrographs were combined
with the snowmelt antecedant flood to determine the most critical
hydrologic condition at the dams. Results of these analyses
produced PMFs for the critical May through August storm season.