FLOOD06619

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Federal Guidelines for Selecting and Accommodating Inflow Design Floods for Dams
• Date: 2/1/1986
• Prepared For: State of Colorado
• Prepared By: FEMA
• Floodplain - Doc Type: Project

series of hydrometeorological reports that have been prepared by the National

Weather Service, These reports provide an adequate source of PMP data for

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most applications. In some cases, site-specific meteorological evaluations

for. the drainage basin being studied may be required.

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The PMFs are developed from various combinations or sequences of events
(rainstorms and snowmelt), depending upon the climatology of the watershed,
The appropriate sequence of meteorologic events for defining the PMF will
vary with climatic region and season, In those watersheds where snowmelt is
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not a consideration, the principal storm produces PMP on the watershed with
storms occurring antecedent and/or subsequent to it. The occurrence of this
sequence of events is called the PMF series and is normally assumed because

of the persistent nature of severe meteorological events in most regions.
The antecedent storm depths and time intervals between storms will vary with
season, type of storm, and region. In those watersheds where snowmelt
contributes significantly in producing floodflows, the PMF may result from

PMP occurring on an appropriate snowpack or appropriate precipitation falling
on a prObable maximum snowpack,

Any combination of events considered should be meteorologically compatible

with the occurrence of the probable maximum event, rainfall or snowmelt.

The magnitude of the antecedent event should be consistent with the defini-
tion of conditions that can reasonably occur,

F. Guidelines for Selecting IDFs

1. Hazard Evaluation

a. General. - The impoundment of water by a dam generally creates

a potential hazard to downstream developments greater than would exist

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