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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4.0

ANTECEDENT FLOOD ANALYSIS

4. 1 GENERAL

The antecedent flood is that flood, and associated climatic
conditions, affecting the basin prior to the onset of the Upper
Limit Design Rain Storm (ULDRS). For this study, the antecedent
flood is a 100-year snowmelt event. This flood is not nearly as
large as what might be expected as the probable maximum snowmelt
flood, but the volume is still very large when compared to the
volume of the ULDRS flood event. In order to model operations of
the reservoirs of the Colorado River above Hoover Dam, daily flows
were required for a complete calendar year.

4.2 METHODOLOGY FOR DEVELOPING THE ANTECEDENT FLOODS

A statistical analysis of flood flows for the Colorado River above
Hoover Dam was the basis for developing the snowmelt base flood.
Flooding in the Colorado River Basin is dominated by snowmelt
runoff. The basic data were not adjusted to remove the effects of
rainfall runoff. The assumption was made that the data include
rainfall similar to that which would be found in any year in which
the ULDRS might occur. The 100-year flood volume was determined for
various time intervals, up to and including a full year period.
This information was used to construct an annual daily flow
hydrograph that is hydrologically reasonable. The 100-year flood
volumes for the area above Glen Canyon Dam and between Glen Canyon
Dam and Hoover Dam were also calculated in the same manner as the
100-year inflow to Hoover Dam.

The timing of the peak of the hydrograph was based on observations
of time of occurrence of flood peaks during the years of record.
The shape of the daily flows near the peak of the hydro graph could
not be based on the monthly data base itself, and accordingly was
patterned after the 1984 flood hydrograph. Runoff in 1984 resulted
in a single peaked hydrograph of the same order of magnitude as the
100-year snowmelt flood.

To accommodate the modeling requirements for the flood control and
storage operations of the upstream storage facilities, the inflow
hydro graph was separated into two constituent hydrographs, one for
inflow into Glen Canyon Dam and one for the intervening area between
Glen Canyon Dam and Hoover Dam. A 100-year balanced hydrograph was
calculated for the flows into Glen Canyon Dam, and the remainder of
the flow was the concurrent intervening inflow (i.e., the difference
between the Hoover Dam 100-year inflow and the Glen Canyon Dam
100-year inflow).

The data used for statistical analysis were the undepleted,
unregulated, natural flow data base that was prepared for use with
the Colorado River Simulation System (CRSS). These data consist of
monthly flows for the period from 1906 to 1983. The data base was
augmented by provisional values for 1984 to 1986. Both log-Pearson