FLOOD01902

• County: Pitkin
• Community: Aspen
• Title: Drainage Facility Capacity Analysis of City of Aspen
• Date: 9/1/1998
• Prepared For: Aspen
• Prepared By: WRC Engineering, Inc.
• Floodplain - Doc Type: Project

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maintenance problem in the streams and streets in the City, Infiltration from snowmelt may also

saturate the ground, which could cause landslides. Landslides are different than the land flows

discussed in this report, Landslides are governed by different processes and analysis methods than

land flows, and an analysis ofthe landslide potential of Aspen Mountain is beyond the scope of this

project.

Manmade snow should have little effect on the potential for mudflows or mud floods, The exception

would be if a large rain event occurred on snow and the snow would not normally still be there except

for the addition of manmade snow,

A rain on snow event could cause a very serious problem for the City of Aspen, In reviewing Tables

12 and 13, it is apparent that a 2.year storm occurring on snow would be equivalent to about a 25-

year storm occurring in the summer (without snow), and a lO-year storm occurring on snow would

produce almost a 1 DO-year event without snow, It is strongly recommended that ajoint probability

analysis be performed to determine the actual likelihood of a given frequency rainstorm falling on

snow, It would allow the City to balance the level of risk (frequency of occurrence) of a very large

flow event caused by rain on snow against the cost of building remediation projects to prevent damage

to the City and its people,

G. SUGGESTIONS FOR FURTHER REFINEIvlENT OR ANALYSIS

The models used to develop estimates of the magnitude of water floods, mud floods, mudflows,
snowmelt, and rain on snow could be further refmed to provide more accurate estimates of these flows
and water surface elevations, In some cases, these refinements may involve the use of more detailed
and elaborate analysis methodologies and models, and in other cases, more accurate and detailed data
can be obtained and used in the models,

The following items have been identified during the course of this study as areas that could be refmed
to improve the modeling results,

I, Define the soil parameters of each node individually in FLO-2D, Currently, every node
(6,508 nodes) has the same soil properties. According to the SCS soil map and classifications
there are several very different types of soil (Type B and Type D) on the lower reaches of
Aspen Mountain and through the City, The properties provided by the SCS Soil Survey

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