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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Figure 2.5 - Schematic Distribution of Mass Rainfall for ULDRS Events

and near the study region were examined for each defined season.
Days separation between storms and the relation of the magnitude of
areal rainfall both prior and subsequent to the main storm were
noted. The resulting relations developed for each season were
combined and are presented in figure 2.6. The relationships
provided in this figure are only applicable for the sequenced events
having individual storms centered on the San Juan, Boulder, or
Pine/Cedar Mountains and having storm area sizes 40,000 square miles
or larger.

Dry-day interval as used here is defined as the number of days of no
rain separating the end of the rainfall from the first storm event
to the beginning of rainfall from the second storm. One or both
sequenced storms are of ULDRS magnitude dependent on season and dry-
day interval chosen. The relationship of secondary storm rainfall
to primary (ULDRS) is independent of ULDRS positioning in the
sequence (i.e., secondary storm rainfall can be placed either prior
or after the ULDRS event).

2 . 7 SEASONAL VARIATION

Major floods in the study basin can occur as the result of melt of
the snowpack that accumulated over the cool season from September
through May. To assess adequately the flood potential for Hoover
and Glen Canyon Dams, it was necessary to define the magnitude of
the ULDRS event for the period from May through October. It is

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